What you see

Dark bands or streaks between two dense objects, such as across the posterior cranial fossa, between bones, or between pockets of dense contrast or barium, often with brighter streaks next to them. In a uniform object, a slight fall in CT numbers towards the centre (cupping).

Why it happens

The X-ray beam contains a range of energies. Dense material removes the low-energy photons first, so the beam that comes out is harder than the reconstruction assumes and the numbers along those paths come out too low. Scanners correct for an average amount of hardening, which works less well for dense bone, iodine and metal.

Try first

1. Keep dense material out of the slices you need: arms out of the scan range for the chest and abdomen, and patient position or gantry tilt to angle dense structures away from the area of interest where the protocol allows.
2. A higher kV hardens the beam and reduces the artefact, at a cost in soft-tissue contrast. Use it only as your protocol allows.
3. On a dual-energy scan, higher-energy virtual monoenergetic images reduce beam hardening.
4. For streaks from dense contrast in the veins of the chest, see Dense contrast streaks.

Stop and escalate when

Cupping or dark bands appear on the daily water phantom: go to Daily water phantom out of limits.

Can mimic pathology

The artificially low numbers can hide or imitate low-density areas, for example in the brainstem. A related effect, incomplete correction for beam hardening, raises the numbers in small kidney cysts next to enhancing kidney (see Cyst looks enhanced).

Sources: Boas 2012; Geleijns 2014; Kalisz 2016; Katsura 2018

What you see

Bright and dark streaks radiating from metal, such as joint replacements, spinal fixation, dental fillings, clips, wires or ECG leads, with dark bands between metal objects and lost detail around them.

Why it happens

Metal blocks most of the beam (photon starvation) and hardens what gets through (beam hardening); scatter and the sharp metal edges add more streaks. The size, shape, density and orientation of the metal all matter.

Try first

1. Remove external metal from the scan range: jewellery, clothing with metal, and ECG leads that can be moved.
2. For dental metal on head scans, patient position or gantry tilt can angle the metal out of the slices you need, where the protocol allows.
3. Apply the scanner’s metal artefact reduction (MAR) reconstruction if it has one. MAR can be applied after the scan; a dual-energy acquisition has to be chosen before scanning.
4. On a dual-energy scan, high-energy virtual monoenergetic images reduce beam-hardening streaks.
5. A higher kV reduces beam hardening and scatter, as your protocol allows.
6. Send MAR images alongside the standard images, clearly labelled, not instead of them (a general step; follow your reporting protocol).

Stop and escalate when

Streaks still cover the area the clinical question is about: tell the radiologist, who may choose another test.

Can mimic pathology

Streaks can hide a collection, bleeding or a tumour next to the metal. MAR images have shown findings that were not visible on the originals.

Good to know

MAR can blur structures next to the metal and can create new artefacts, so MAR images are read next to the originals.

Sources: Boas 2012; Katsura 2018; Puvanasunthararajah 2021; Geleijns 2014

What you see

Coarse, fine-grained streaks and noise across the widest part of the body, usually side to side through the shoulders or pelvis; worse in large patients and with the arms by the sides.

Why it happens

Along the most attenuating paths so few photons reach the detector that the signal is mostly noise, which the reconstruction spreads into streaks. The thickest path matters more than the average thickness, so arms at the sides make it much worse.

Try first

1. Raise the arms out of the scan range before the localiser (see Cannot raise arms).
2. Use tube current modulation, which raises the current for the most attenuating projections; it is set from the localiser, so take the localiser in the final scan position.
3. If the arms must stay down, arms resting across the front of the abdomen give less noise than arms at the sides.
4. A higher kV or tube current for the problem region, as your protocol allows, reduces the streaks.
5. For review, thicker slices or a softer kernel trade detail for less noise; iterative reconstruction and the scanner’s adaptive noise filtering reduce these streaks where available.

Stop and escalate when

Streaks still obscure the area of interest: ask the radiologist whether a limited repeat with adjusted settings is justified.

Can mimic pathology

Low-contrast edges, such as small liver lesions or bowel wall, can be lost in the noise.

Sources: Boas 2012; Geleijns 2014; Puvanasunthararajah 2021; Kachelriess 2001; Hsieh 1998; Graef 2022

What you see

Blurred edges, doubled outlines, streaks from high-contrast edges, or steps on coronal and sagittal reformats; in the chest and abdomen, a blurred diaphragm and vessels from breathing.

Why it happens

The reconstruction assumes the patient is still during each rotation. Movement makes the projections disagree.

Try first

1. Explain the scan, rehearse the breath-hold, and give the same breath-hold instruction for the localiser and the scan.
2. Make the patient comfortable and support the part with pads or straps so they do not have to work to keep still.
3. Use the shortest rotation time and fastest acquisition the protocol allows.
4. For patients who cannot hold their breath or keep still, see Cannot hold their breath and Children and immobilisation.

Stop and escalate when

Movement still spoils the images: ask the radiologist whether a repeat is justified and over which range.

Can mimic pathology

The IAEA handbook notes that aortic pulsation can cause artefacts that imitate an aortic dissection. At the aortic root, ECG gating reduces motion but does not remove it (Murphy and colleagues).

Sources: Boas 2012; Geleijns 2014; Kalisz 2016; Abbara 2016; Murphy 2018; CAMRT BPG

What you see

Blurred or doubled coronary arteries, steps between blocks of slices on reformats, or bands of different brightness across the heart.

Why it happens

Gated scans image the heart over one or more beats. A fast or irregular rhythm, ectopic beats, or breathing between blocks makes the data misalign.

Try first

1. Before scanning, check the ECG: clean the skin (shave if needed), keep the leads outside the scan field, and reposition them if the trace is noisy or the T wave is as tall as the R wave.
2. Aim for the heart rate your protocol sets; the SCCT target for coronary CTA is below 60 beats per minute, using your department’s rate-control protocol.
3. Rehearse the breath-hold and ask the patient not to strain (no Valsalva manoeuvre).
4. For an irregular or fast rhythm, use the protocol’s retrospective gating or single-beat mode if the scanner has one.
5. After the scan, review the ECG: editing trigger points and removing ectopic beats can often rescue the study.

Stop and escalate when

Steps or blur through the coronary arteries remain after ECG editing: the radiologist decides whether to repeat.

Can mimic pathology

Motion can imitate a narrowing or soft plaque, or hide one.

Sources: Abbara 2016; Pack 2022; Moser 2024; Kalisz 2016

What you see

Full or partial circles, darker or lighter, centred on the centre of rotation; clearest on uniform tissue and on the water phantom.

Why it happens

A miscalibrated or faulty detector element reads wrongly at every angle, which traces a circle. In one reported case a droplet of spilt contrast on the tube aperture caused a ring until it was cleaned.

Try first

1. Look for the same ring on the daily water phantom images.
2. Run an air calibration as the manufacturer directs and repeat the phantom.
3. Inspect the gantry window for spilt contrast and clean it as the manufacturer allows, then rescan.

Stop and escalate when

Rings persist after several air calibrations, or calibration is not a user task on your system: arrange service and tell the medical physicist. The QC team decides whether, and how, the scanner is used until it is fixed.

Can mimic pathology

A subtle ring has imitated a lesion in the pons.

Good to know

Rings are more visible in noisy images, for example in large patients.

Sources: Boas 2012; ACR CT QC 2017; Geleijns 2014; IAEA HHS 19; AAPM Report 74; Jha 2013; Uppot 2018

What you see

Small or thin structures look larger and fainter than they are, edges blur, or a small cyst measures denser than water.

Why it happens

Each voxel shows the average attenuation of everything in it. Thick slices average tissues along the length of the patient.

Try first

1. Reconstruct thinner slices from the same acquisition for small structures; no extra exposure is needed.
2. Review the area in a second plane.
3. For calcium or stents, review with a wider window.

Can mimic pathology

A small kidney mass is too small to characterise when the slice is thicker than half its diameter. Calcium can make a narrowing look worse than it is.

Sources: Geleijns 2014; Abbara 2016; Silverman 2019; Pack 2022

What you see

Alternating dark and light streaks fanning out from sharp, dense edges such as the skull base or ribs, which seem to rotate as you scroll through the slices; with wide detectors, streaks or blur at high-contrast edges along the patient’s length.

Why it happens

In helical scanning each image is interpolated between detector rows that cross the image plane at different angles, which near sharp edges gives the windmill pattern. With many rows, the outer rows see the patient along a cone rather than a flat fan, and the scanner’s cone-beam reconstruction corrects most but not all of this.

Try first

1. Review thicker reconstructions: windmill streaks are most visible on thin slices.
2. Keep the area of interest near the centre of the field of view.
3. Use the protocol’s standard reconstruction, which includes the scanner’s own corrections.

Sources: Boas 2012; Geleijns 2014

What you see

Serrated, staircase edges on reformats and 3D images, or alternating stripes of more and less noise (zebra), most marked away from the centre of the field of view; on gated cardiac scans, steps at the joins between blocks.

Why it happens

The same helical interpolation shows up in the reformat plane. On sequential cardiac scans, movement or a change in heart rate between beats misaligns the adjoining blocks.

Try first

1. Centre the area of interest in the field of view.
2. Reconstruct thin source images for reformats.
3. For cardiac scans, control the heart rate and rehearse the breath-hold; use single-beat or wide-coverage mode where available (see Cardiac motion).

Sources: Boas 2012; Moser 2024; Abbara 2016; Pack 2022

What you see

A bright band or rim at the edge of the field of view, a missing or distorted body outline, or CT numbers that are wrong near the edge; arms or a large abdomen extend beyond the scan field.

Why it happens

The measured scan field is smaller than the bore on most scanners. Tissue outside it is measured on some projections and not others, and extended-field reconstructions estimate the missing data.

Try first

1. Centre the patient and bring the arms in or up so the body lies inside the scan field.
2. Use the extended field-of-view reconstruction if the scanner has one, and tell the radiologist that the edge region is estimated.
3. Check the reconstructed field of view has not cropped out anatomy the radiologist needs.

Stop and escalate when

The anatomy of interest lies outside the measured field: ask the radiologist about another position or a larger-bore scanner.

Can mimic pathology

Truncation or cropping can cut pathology out of the image.

Where sources differ

Boas and Fleischmann argue that tissue outside the field does not itself create artefacts inside it and attribute the bright rim to the reconstruction; Fonseca and colleagues found that errors in estimating the missing data can spread into the true scan field. Both agree that the extended region is estimated and its CT numbers are unreliable.

Sources: Boas 2012; Fonseca 2021; Uppot 2018; IAEA HHS 19

What you see

Images look grainier than usual, across the whole scan or in one region, and low-contrast detail is hard to see.

Why it happens

Noise falls as more photons reach the detector. Common causes are a low-dose or wrong protocol, the wrong patient size or age group, an off-centre patient, a localiser taken in the wrong position, thin slices or a sharp kernel.

Try first

1. Check the protocol, the kV, the tube current or noise setting, and that tube current modulation is on.
2. Check the patient is at the isocentre (see Patient off-centre).
3. Check the slice thickness and kernel match the protocol; thicker slices or a softer kernel reduce noise for review.
4. Apply the iterative or deep-learning reconstruction the protocol specifies.

Stop and escalate when

The daily water phantom noise is outside its limits, or every patient is noisy on an unchanged protocol: go to Daily water phantom out of limits.

Can mimic pathology

Noise hides low-contrast lesions, for example in the liver.

Sources: Boas 2012; Geleijns 2014; Kalisz 2016; Kaasalainen 2019; ACR CT QC 2017

What you see

Higher dose or noise than expected, uneven noise across the image, or a tube current or kV that seems wrong for the patient’s size, when the patient lies above or below the isocentre. Studies find patients are most often positioned too low.

Why it happens

The bowtie filter and tube current modulation assume the patient is centred. On a front-to-back localiser, an off-centre patient is magnified or shrunk by the diverging beam, so the scanner misjudges their size and sets the current (and, with automatic kV selection, the kV) wrongly. Tissue away from the centre lies under a thicker part of the bowtie and is noisier.

Try first

1. Set the table height so the middle of the body part is at the isocentre, using the lateral light, before the localiser.
2. Check centring on a lateral localiser, or with the scanner’s centring aid if it has one.
3. Use the localiser direction your protocol specifies; changing it changes how modulation behaves.
4. If the patient or table height moved after the localiser, repeat the localiser before scanning.

Good to know

In phantom studies, surface dose rose by about 18% and 41% when the phantom was 3 and 6 cm below the isocentre, with noise up by 6% and 22% (Toth and colleagues); the size of the effect varies with the scanner, the localiser direction and automatic kV selection.

Sources: Toth 2007; Li 2007; Kaasalainen 2014; Kaasalainen 2019; Habibzadeh 2012; Graef 2022

What you see

Bone looks blurred, or soft tissue is very grainy; lung or bone detail is missing; a series has been sent in only one kernel or window.

Why it happens

Sharp kernels show fine detail with more noise; soft kernels lower noise and blur detail. A bone kernel makes the brain unreadable through noise, and a soft-tissue kernel blurs bone. The window decides which range of CT numbers is visible.

Try first

1. Reconstruct the missing kernel from the raw data.
2. Check the default window for each series; a narrow window exaggerates the size of calcium and stents.

Sources: Geleijns 2014; Abbara 2016; Boas 2012; Pack 2022

What you see

Only thick slices were sent, reformats look blocky, small lesions cannot be characterised, or the reconstructed field of view is too small, off-centre or crops anatomy.

Why it happens

Thicker slices lower noise but average out small structures; reformats need thin source images. A reconstruction field that is too small or off-centre leaves anatomy out.

Try first

1. Reconstruct thin slices from the same acquisition for reformats and small structures.
2. Check the reconstructed field of view covers all the anatomy and is centred on it.
3. Compare the series sent with the protocol’s list before the patient leaves.

Can mimic pathology

A small kidney mass cannot be characterised when the slice is thicker than half its diameter.

Sources: Geleijns 2014; Abbara 2016; Silverman 2019; Uppot 2018

What you see

Arteries poorly opacified on an arterial study, veins already bright, or organs not enhanced in the phase requested.

Why it happens

When contrast arrives and peaks depends on cardiac output, body size, the injection site and rate, and disease, so a fixed delay cannot suit every patient. On fast scanners a short scan can outrun the bolus.

Try first

1. Use bolus tracking or a test bolus as your protocol sets.
2. Check the injection actually ran as planned: the injector record, the cannula site and the volume given.
3. Match the injection duration to the scan time; for very short scans, injection length matters.
4. Use a saline chaser as the protocol sets.
5. On a dual-energy scan, low-energy monoenergetic images can improve poor arterial opacification.

Stop and escalate when

Contrast is not arriving: stop and check for extravasation. A missed phase that could only be repeated with more contrast and dose: the radiologist decides.

Can mimic pathology

Poor opacification can leave a pulmonary embolus or a dissection impossible to exclude.

Sources: Murphy 2018; Abbara 2016; Rau 2025; Bae 2010; ESUR 2025

What you see

Monitoring scans continue without reaching the threshold, the scan starts before contrast arrives, or the arteries are faint or the veins bright despite tracking.

Why it happens

Tracking measures the CT number in a region of interest and starts the scan after a set delay once a threshold is passed. A region on the wrong vessel or on the vessel wall, a threshold that does not suit the kV, an injection that did not run, or slow circulation can all upset it. Bolus tracking can itself be a source of error.

Try first

1. Place the region of interest in the centre of the target vessel, clear of the wall and of streaks (a general step; one study placed it centrally over a third to a half of the lumen).
2. Watch the monitoring images: if no contrast arrives, check the injection site and the injector.
3. Check the threshold suits the kV: a lower kV raises vessel CT numbers.
4. The post-trigger delay must allow for table movement and the breath-hold instruction.
5. If your protocol allows a manual start, start the scan when the vessel is clearly opacified (a general step; protocols differ).
6. For repeated failures or complex anatomy, a test bolus is an alternative, and it also confirms the IV access.

Stop and escalate when

Contrast is not arriving at all: stop the injection and check for extravasation.

Good to know

Published thresholds and delays vary between protocols; use your own protocol’s values.

Sources: Abbara 2016; Murphy 2018; Wu 2024; Rau 2025; ESUR 2025

What you see

Bright and dark streaks from undiluted contrast in the subclavian or brachiocephalic vein or the superior vena cava, crossing the upper mediastinum, aortic arch or right pulmonary artery.

Why it happens

Undiluted contrast in the veins is dense enough to cause beam hardening and streaks.

Try first

1. Follow the contrast with a saline flush (chaser), which pushes the tail of the bolus on and reduces streaks.
2. Inject through the right arm where the protocol allows: from the left arm, contrast crosses the mediastinum in the left brachiocephalic vein and causes more streaks.
3. On a dual-energy scan, higher-energy monoenergetic images reduce these streaks.

Can mimic pathology

Streaks over the pulmonary arteries can look like an embolus or make the study indeterminate.

Sources: Murphy 2018; Abbara 2016; Wang 2025; Boas 2012

What you see

A small kidney cyst measures higher after contrast than before, enough to suggest enhancement; typically a small cyst inside the kidney, scanned at peak kidney enhancement.

Why it happens

Beam hardening and scatter from the brightly enhancing kidney around it, with partial volume, raise the cyst’s measured CT number.

Try first

1. Measure in the centre of the cyst, as far as possible from enhancing kidney.
2. Use slices no thicker than half the lesion’s diameter, so partial volume is not added. Pseudoenhancement can still occur with thin slices.
3. Tell the radiologist. The Bosniak 2019 update says masses inside the kidney under 1.5 cm next to avidly enhancing tissue can be affected, and may be too small to characterise.
4. Dual-energy imaging reduces the effect but does not remove it.

Can mimic pathology

A simple cyst can look like a solid or cystic tumour.

Where sources differ

Mechanism: Boas and Fleischmann attribute it to beam hardening and scatter; Maki and colleagues found that beam hardening alone could not explain it and pointed to incomplete correction by the scanner. Reported rises are up to about 28 HU.

Sources: Boas 2012; Maki 1999; Birnbaum 2002; Silverman 2019

What you see

Calcified plaque or stent struts look larger than they are, so the lumen looks narrower or darker.

Why it happens

Mainly partial volume from limited resolution, with motion adding to it; worse with denser calcium, smaller vessels, thicker struts and a narrow window.

Try first

1. Review with a wider window.
2. Use the protocol’s sharp or dedicated kernel for stents or heavy calcium.
3. On a dual-energy scan, high-energy monoenergetic images reduce blooming.
4. A low kV exaggerates blooming; follow the protocol for patients with stents or heavy calcification.

Can mimic pathology

Blooming tends to overestimate a narrowing, though heavy calcium can also hide one.

Where sources differ

Why higher energy helps: Pack and colleagues argue that it works by compressing the CT number scale, much like widening the window, and that beam hardening adds little on modern scanners; Murphy and colleagues and the Kalisz review list higher energy as a remedy without that distinction.

Sources: Pack 2022; Murphy 2018; Abbara 2016; Kalisz 2016

What you see

Images look unusually smooth or plastic, with a coarse, blotchy noise texture; small low-contrast lesions or small vessels look blurred.

Why it happens

Iterative reconstruction suppresses noise using a noise model and a preference for smooth images; at high strengths the remaining noise clumps into blotches. Deep-learning methods take their texture from their training data and can, rarely, add or remove small structures.

Try first

1. Check the reconstruction type and strength match the protocol.
2. If the radiologist asks, reconstruct a lower strength or a standard reconstruction from the raw data.
3. Do not lower the dose further because the images look smooth: at very low doses resolution falls and new artefacts appear.

Can mimic pathology

A blotchy texture can imitate pathology or hide small low-contrast lesions; deep-learning reconstruction can add structures that are not there or blur away small ones.

Sources: Koetzier 2023; Boas 2012; Geleijns 2014; Geyer 2015

What you see

The organ of interest is not fully covered, for example the lung bases or apices missing, or the scan runs well beyond the requested range.

Why it happens

The range is planned on the localiser. A different breath between the localiser and the scan, and planning errors, move the boundaries.

Try first

1. Plan from the localiser with the margin your protocol specifies, and give the same breath-hold instruction for the localiser and the scan.
2. Check coverage on the images before the patient leaves the table.
3. If anatomy is missing, ask the radiologist whether to scan only the missing part.
4. Check the reconstructed field of view has not cropped anatomy out.

Good to know

In a large study of low-dose chest CT, the lung bases were cut off in about 4% of scans, and scanning beyond the lungs was common.

Sources: Geleijns 2014; Wienholt 2026; Uppot 2018; Abbara 2016

What you see

On the daily water phantom, the mean CT number or the noise is outside the limits set for your scanner, or the images show rings, streaks, lines or a darker or brighter centre.

Why it happens

The daily water test is sensitive to many problems: calibration drift, a faulty detector element, tube or software faults. The medical physicist sets the action limits for each scanner and protocol.

Try first

1. Double-check the basics: the right phantom, centred at the isocentre with the lights and phantom marks, the right image, region of interest and protocol.
2. Run the air calibrations if the manufacturer recommends them, then repeat the phantom scan.
3. Lines or streaks: inspect the gantry window for spilt contrast, clean it as the manufacturer allows and rescan.
4. Record the results and every action in the QC log, where the QC team and service engineer can find them.

Stop and escalate when

Still outside the limits after a repeat, or rings persist after several air calibrations: tell the medical physicist, who helps decide whether service is needed before clinical scanning. The radiologist decides whether patient studies continue or are postponed.

Where sources differ

How often and how tight. The ACR manual and AAPM Report 74 make water CT number, noise and artefacts a daily technologist test. The IAEA programme (Human Health Series 19) lists them as a monthly radiographer test, with acceptable limits of ±5 HU for CT number, ±25% of baseline for noise and ±10 HU for uniformity. Follow your local QC programme.

Good to know

The ACR manual’s limit for the water mean on the manufacturer’s phantom is 0 ± 5 HU, or the manufacturer’s own specification; noise limits are set by the medical physicist. QC images are checked before the first clinical scan of the day.

Sources: ACR CT QC 2017; IAEA HHS 19; AAPM Report 74; Geleijns 2014

What you see

The scanner asks for a tube warm-up or an air (detector) calibration, the calibration stops with an error, or the start-of-day routine has not been run.

Why it happens

Air calibrations correct for small differences in sensitivity between detector elements; without them, rings appear. They are usually needed once a day, sometimes more often or when the system flags them. Tube warm-up is run as the manufacturer recommends before the daily phantom test.

Try first

1. Run the warm-up and air calibrations as the manufacturer recommends, before the daily phantom and the first patient.
2. Clear the gantry and table as the calibration requires, then repeat it once if it fails, following the on-screen prompts (a general fault-finding step).
3. Check the daily phantom afterwards for rings or streaks.

Stop and escalate when

Calibration fails again, or rings persist after several calibrations, or calibration is not a user task on your system: call service and tell the medical physicist.

Sources: IAEA HHS 19; ACR CT QC 2017; Geleijns 2014

What you see

The console warns that the tube is too hot, enforces a cooling wait, or will not allow the planned tube current, scan length or number of phases.

Why it happens

CT tubes run at high power for long periods, and the heat builds up in the anode and the tube housing. The generator calculates loading and the cooling time needed from the tube’s rating data. Long, high-current or repeated scans can exceed the heat capacity; the room and heat exchanger also need effective air conditioning.

Try first

1. Follow the console prompt and let the tube cool. Repeated high-power exposures without the required cooling time can damage the anode and bearings.
2. If the patient cannot wait, ask the radiologist whether the protocol can be adapted: fewer images, thicker slices, lower tube current or a higher pitch reduce the heat load, at some cost in image quality.
3. Check that the scanner room and equipment room air conditioning are working.

Stop and escalate when

Cooling waits on a normal workload, repeated tube errors, or a cooling-system message: call service and record it in the fault log.

Sources: Nowotny 2014; Geleijns 2014; IAEA HHS 19

What you see

The alignment lights do not match the scan plane or the position set on the localiser, a marker placed at the light appears off the expected slice, or a light is dim or out.

Why it happens

The internal and external lights mark the scan plane for setting up. They are a rough guide; the localiser sets the exact range. Accurate lights matter for biopsy, for centring, for keeping the eyes out of brain scans and for radiotherapy planning.

Try first

1. Set the exact scan range on the localiser rather than relying on the light.
2. Check the lights with a thin marker as your QC programme describes (the IAEA test uses a wire of about 1 mm).
3. If the internal and external lights disagree, find which one matches the scan plane, and report the other.
4. Keep the laser out of the patient’s eyes, and take particular care with unconscious or anaesthetised patients, who have no blink reflex.

Stop and escalate when

The error is beyond your programme’s limit, or the scanner is used for biopsy or radiotherapy planning: report it to service and the medical physicist and record it.

Where sources differ

Tolerance. The ACR manual’s annual physicist test calls for service if the light or localiser position is out by more than 2 mm; IEC uses ±2 mm (as reported by AAPM TG-233); the IAEA programme’s monthly radiographer test accepts ±5 mm, with ±1 mm achievable. Follow your local QC programme.

Sources: IAEA HHS 19; ACR CT QC 2017; AAPM TG-233; AAPM Report 74

What you see

The gantry will not rotate or tilt, the tilt indicator reads wrongly, a gantry error appears, or there is a new grinding, knocking or whining noise.

Why it happens

The gantry rotates the tube and detectors at high speed under strong centrifugal force. Tilt is set and displayed by the gantry, and its indicator is checked in the QC programme. Most multislice scanners allow a tilted gantry only for axial scans, not helical ones (IAEA).

Try first

1. Stop the scan if it is running, note the message, the time and what was happening, and record it in the fault log.
2. Report the fault straight away; do not keep scanning through a new mechanical noise (a general step; your operator manual takes precedence).

Stop and escalate when

Any mechanical fault, noise or error that recurs: report it and call service; service and the radiologist decide whether the scanner stays in use, and it is tested after repair. If the tilt indicator is in doubt, do not rely on tilted scans or plan radiotherapy until the physicist has confirmed the gantry returns to vertical.

Sources: Geleijns 2014; IAEA HHS 19; IAEA SSG-46; HSE L121; ACR CT QC 2017

What you see

The table will not move, stops or creeps, the position readout is wrong, or the patient may be near or above the table’s weight limit.

Why it happens

The table must carry heavy patients without bending, and its travel accuracy is tested in the QC programme. Weight and bore limits are specific to each system, and exceeding the manufacturer’s limit can damage the table mechanics.

Try first

1. Check nothing is caught: lines, injector tubing, cables or blankets (a general step). Leave enough tubing slack for the table to travel.
2. Check the patient’s weight against the limit posted for your scanner, and the girth against the opening with the table at scanning height.
3. Note any error message and record the fault.

Stop and escalate when

The table creeps, stops, or travels inaccurately: stop using it and call service, especially before biopsy or radiotherapy planning. A patient above the system’s limit: do not scan on that table; ask the radiologist about another scanner or test.

Sources: ACR CT QC 2017; Geleijns 2014; Uppot 2018; ACR Contrast Media 2026

What you see

The X-ray-on warning light at the room entrance does not light, a door is open while the beam is on, an emergency stop has been pressed or is blocked, or the intercom or view of the patient fails.

Why it happens

Warning lights and signs show when X-rays are being produced; they do not prevent exposure. Doors are kept shut while the beam is on, and the radiographer must be able to see and talk to the patient throughout.

Try first

1. Report a failed warning light, sign or intercom at once and follow your local rules until it is fixed.
2. Keep doors shut when the beam is on, and keep emergency stops reachable and unobstructed.
3. Do not test the emergency off button yourself; it is tested with the supplier’s engineer present.
4. After an emergency stop, record what happened; the scanner needs resetting before the next scan.

Stop and escalate when

Warning lights, communication or the view of the patient do not work: tell your radiation protection adviser or medical physicist; local rules decide whether scanning continues.

Where sources differ

Door interlocks. The IAEA (SSG-46) says door interlocks are not appropriate in diagnostic X-ray rooms, because stopping the beam may mean repeating the procedure, and its CT QC programme says any door interlock should not shut down the scanner when someone enters. The UK Approved Code of Practice describes interlocks for high-dose-rate enclosures. Your local rules decide.

Sources: IAEA SSG-46; IAEA HHS 19; HSE L121; AAPM TG-233; 21 CFR 1020.33

What you see

Before the scan, the console shows a notification or an alert: the planned dose index (CTDIvol or DLP) would exceed a value set on the scanner.

Why it happens

Scanners built to the NEMA XR-25 dose check standard compare the planned dose indices with values the site sets. A notification value applies to each scan in a protocol and flags a higher-than-usual dose; an alert value applies to the whole examination and flags when the accumulated dose index at any position would exceed it, even if no single scan triggered anything. These values are not limits, targets or diagnostic reference levels. The scanner cannot allow for patient size, so a large patient may appropriately exceed a notification value.

Try first

1. Stop and compare the kV, tube current or noise setting, pitch, detector configuration and scan range with the protocol, and correct anything wrong.
2. Check that the protocol matches the patient (adult or child, body part) and that the patient is centred.
3. Notification: if the settings are correct, confirm and proceed, entering a reason if your site asks for one (for example, patient size).
4. Alert: this calls for a closer review. Your name is recorded, a password may be required, and your site procedure says who reviews before proceeding (for example the lead radiographer, the radiologist or the medical physicist).
5. An alert should not stop a medically necessary scan; if a password is set, someone who knows it must always be available.

Stop and escalate when

You cannot find why the dose is higher than expected, or notifications keep firing on one protocol: tell the lead radiographer and the medical physicist; the event log is reviewed.

Good to know

The AAPM publishes starting values for notification values and says each facility adjusts them with a medical physicist; it reports an FDA-suggested alert value of 1000 mGy CTDIvol. In the United States, CT on equipment without the NEMA XR-29 dose-optimisation attributes attracts a Medicare payment reduction.

Sources: AAPM Dose Check 2011; AAPM Notification Slides 2014; CMS MM9250

What you see

The injector will not arm, stops on a pressure or air alarm, shows an error on the contrast or saline head, or will not link to the scanner so the injection and scan cannot start together.

Why it happens

Injectors stop at their pressure limit when something narrows the path: a kink, a closed clamp, a cannula too small for the flow rate, or viscous contrast.

Try first

1. Check the patient first: the cannula site for pain or swelling.
2. Check the line for kinks and closed clamps, and that the cannula size suits the protocol’s flow rate.
3. Re-prime and clear air from the syringe and tubing as the injector’s instructions describe, then turn the syringe so the tubing points downwards. Do not override an air or pressure alarm.
4. Iodinated contrast warmed to body temperature is less viscous, which lowers injection pressure; follow your warming policy (sources differ, below).
5. If the injector will not link to the scanner, follow the injector manual; start manually only as your protocol allows (a general step).

Stop and escalate when

The injector alarms with a clear line and good access, or a head or the link keeps failing: take it out of use and report it.

Where sources differ

Warming iodinated contrast. ESUR (2025) regards warming as widely accepted good practice; RANZCR says warming lowers viscosity and injection pressure but there is not enough data to say it reduces reactions; the ACR (2026) finds the evidence insufficient to recommend routine warming of any agent, noting some, inconsistent, evidence for higher-viscosity agents, and says warming may be considered to lower viscosity and injection pressure in certain higher-risk patients. These statements are about iodinated contrast only.

Good to know

Air detection and dual-head error handling are specific to each injector: your injector’s manual takes precedence.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

The ECG trace is noisy, flat or low, the scanner misses R waves or triggers on T waves, or the heart rate is irregular so triggering is erratic.

Why it happens

Gated scans time the acquisition from the R wave. Poor skin contact, leads in the wrong place, a T wave nearly as tall as the R wave, or an irregular rhythm upset detection.

Try first

1. Clean the skin with alcohol, shave if needed, and use fresh electrodes.
2. Reposition the leads if the baseline is noisy or the T wave is as tall as the R wave; keep them outside the scan field.
3. Check the cable and connections to the monitor (a general step).
4. For an irregular rhythm, use the protocol’s retrospective gating or single-beat option if the scanner has one.
5. After the scan, ECG editing (moving trigger points, removing ectopic beats) can often rescue the study.

Stop and escalate when

The trace stays poor with new electrodes and leads on more than one patient: report the ECG module to service (a general step).

Sources: Abbara 2016; Kalisz 2016; Murphy 2018

What you see

The scan finishes but images do not appear, the reconstruction queue stalls, or the console freezes.

Why it happens

Images are calculated from the raw projection data after acquisition, by reconstruction hardware and software that can stall.

Try first

1. Note the time, the series and any message; photograph the screen if useful.
2. Check the reconstruction or job queue on the console.
3. Ask service before restarting the console or deleting anything while data are held in the queue (a general step).
4. If the patient is still on the table and the images are needed now, ask the radiologist whether to repeat the scan, as a repeat is another exposure.

Stop and escalate when

The queue does not clear: call service.

Sources: Geleijns 2014; Clunie 2014; IAEA SSG-46

What you see

A study is complete on the scanner but missing or incomplete on PACS, the worklist does not show the patient, or details on the images are wrong.

Why it happens

The scanner sends images to a configured destination, which confirms storage (storage commitment) before the scanner deletes its copies. Patient details come from the modality worklist.

Try first

1. Check the send queue and the destination; resend failed series.
2. Compare image counts on the scanner and on PACS.
3. Do not delete the study from the scanner until PACS has confirmed storage.
4. If the worklist is empty, widen the search (date, station) and check the booking; enter details by hand only under your downtime procedure.
5. If details on the images are wrong, follow your correction process, so changes are made by authorised staff with an audit trail.

Stop and escalate when

Studies are still missing after a resend, or a patient identity error is found: contact your PACS or IT team, and report identity errors through your incident process.

Sources: Clunie 2014; DICOM PS3.4

What you see

The identity of the patient on the table needs confirming before scanning: a patient who cannot answer, a child, a common name, a language barrier, or a transfer from another area.

Why it happens

Scanning the wrong patient or the wrong body part is always possible, for example with similar names or patients unsure of their procedure. Several independent ways of confirming identity, and checking the request and the side, prevent it.

Try first

1. Ask the patient to state their full name, date of birth and what they are here for, including the site and side, rather than asking “Are you …?”. Match the answers to the request and the identity band.
2. For a patient who cannot answer, use their representative, the identity band or the accompanying staff, as your policy describes.
3. Immediately before scanning, stop and confirm the right patient, examination and site or side (an internal time-out when you work alone).
4. Check the patient details and side markers on the images before they are released.

Stop and escalate when

Any mismatch: do not start until it is resolved, and report it through your incident process.

Sources: ACSQHC CT/MRI protocol; IAEA SSG-46; ARPANSA RPS C-5; ORS C1 2024; IR(ME)R 2017

What you see

Images carry the wrong left and right, or reformats look mirrored, because the position selected at the console (head or feet first, supine or prone) does not match how the patient lies.

Why it happens

The patient can lie head first or feet first, supine or prone, and the position is usually recorded with the scan data. Schmidt and Odland describe two cases of mirror-image coronal scans: orientation depends on whether the patient is prone or supine, so it relies on the radiographer’s input, and in the largely symmetrical head and neck a mislabelling may not be noticed.

Try first

1. Before the localiser, check that the position selected at the console matches how the patient is lying (a general step).
2. Before images are released, check the patient details and side markers against the patient and the request.
3. If the position was entered wrongly, do not alter images yourself: tell the radiologist and correct the record through your authorised correction process.

Stop and escalate when

Images with wrong labels have already been released: tell the radiologist and the referrer straight away (a general step) and report it through your incident process.

Sources: Geleijns 2014; Schmidt 2004; ACSQHC CT/MRI protocol; Clunie 2014

What you see

The patient might be pregnant, is not sure, or the screening answer is unclear before a CT that includes the abdomen or pelvis; or a pregnant patient is booked.

Why it happens

Pregnancy status is established before exposures that could give a significant dose to an embryo or fetus, mainly when the abdomen or pelvis is in the beam. A patient may not know she is pregnant. The ACR-SPR practice parameter says CT outside the abdomen and pelvis (with the possible exception of the hip) generally does not need verification.

Try first

1. Ask every patient of childbearing potential, privately where possible, using your department’s questions or form, immediately before the scan. A status written on the request is not enough on its own. Record the question and the answer.
2. If pregnancy cannot be excluded for a scan that includes the abdomen or pelvis, tell the radiologist before scanning; a pregnancy test or a later appointment may be chosen.
3. If the patient is pregnant and the scan is justified, the radiologist and medical physicist adapt the protocol: only the phases and range needed, at the lowest suitable dose.
4. A critically urgent scan goes ahead without waiting; record the circumstances and who directed it.
5. Iodinated contrast, if the radiologist decides it is needed: the ACR does not withhold it when needed, while ESUR gives it in pregnancy only in exceptional circumstances. ESUR and RANZCR ask for the newborn’s thyroid check, which routine newborn screening (the heel-prick test in Australia and New Zealand) provides; the ACR needs nothing beyond routine screening.
6. Follow your local policy on shielding: the ACR-SPR no longer recommends shielding the pelvis during CT of other regions, as it can increase internal scatter; the IAEA (2018) still says to shield radiosensitive organs when appropriate.

Stop and escalate when

A positive test, or she says she is or may be pregnant: the radiologist decides before scanning.

Where sources differ

UK (IR(ME)R 2017; HPA 2009 and RCR 2026 guidance): the operator asks about pregnancy and breastfeeding as the employer’s procedure defines; for locally defined high-dose procedures (for example above about 10 mGy fetal dose, such as CT of the lower abdomen and pelvis), HPA gives two options: book within the first ten days of the cycle, or book normally and rebook patients in the second half of the cycle in whom pregnancy cannot be excluded. Australia (ARPANSA RPS C-5): establish status before any procedure likely to give more than 1 mSv to the uterus; if a pregnant patient proceeds, a medical physicist estimates the fetal dose. New Zealand (ORS C1): ascertain status before any procedure that could give a significant fetal dose; signs ask patients to tell staff.

Sources: ACR-SPR Pregnancy PP; HPA RCE-9 2009; RCR IR(ME)R 2026; IR(ME)R 2017; ARPANSA RPS C-5; ORS C1 2024; IAEA SSG-46; ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

The patient has an implanted or worn electronic device in or near the scan range: pacemaker or ICD, neurostimulator, insulin pump, or a continuous glucose sensor.

Why it happens

When the X-ray beam passes directly over a device, brief interference is common in laboratory testing (transient oversensing, mostly inhibited pacing, in 17 of 20 pacemakers and defibrillators at typical doses), but the FDA says clinically significant problems, such as unintended neurostimulator shocks or insulin pump faults, are rare. Interference happens only when the device is in the direct beam; localisers are not linked to interference. A device should not stop a medically necessary CT.

Try first

1. Ask about devices, and use the localiser to see where the device sits relative to the scan range.
2. Keep the device out of the scan range if possible, after confirming with the radiologist that the anatomy needed is still covered.
3. Worn devices: most manufacturers recommend removing them and leaving them outside the room. Decide with the patient, as pausing an insulin pump has its own risk: check glucose before and after, and remind them to restart it.
4. Neurostimulator: if your local policy and the device team allow it to be switched off, the patient switches it off before the scan and back on afterwards.
5. If the device must be in the beam: use the lowest dose consistent with diagnostic quality and a lower dose rate (the FDA suggests a longer rotation time or lower pitch, if appropriate). Avoid dwelling over the device for more than several seconds (CT perfusion, CT fluoroscopy) unless it is off or the scan is urgent and monitored.
6. Be ready to stop the scan for symptoms: light-headedness, palpitations, pain, shocks or device alarms.

Stop and escalate when

Symptoms or a device alarm during the scan: stop, assess the patient and get medical help. Afterwards the patient checks the device is on with the right settings; a suspected malfunction is reported, and the device team checks it at the next scheduled visit or sooner.

Where sources differ

Manufacturers’ labels differ. Medtronic (MiniMed 780G, US guide) says remove the pump and sensor before entering a CT room; Tandem says do not expose the pump or CGM to CT; Insulet says remove the Omnipod before CT; Dexcom says do not wear the G6 for CT and, for the G7, avoid the sensor in the scanned area; Abbott says FreeStyle Libre 2 and 3 sensors can be worn during CT in the United States, while some of its other regional pages still say remove. New Zealand and Australian labels for these devices could not be verified, so check the local labelling for the patient’s device.

Sources: FDA CT devices; FDA PPHN 2008; McCollough 2007; Medtronic MiniMed 780G UG; Tandem ISI; Insulet Omnipod ISI; Dexcom FAQ; Abbott FreeStyle Libre

What you see

Cannulation is difficult, only a small or distal vein is available, the protocol flow rate looks high for the cannula, or the patient has a central line, PICC or port.

Why it happens

Power injection needs a flexible plastic cannula in a vein large enough for the flow rate. Small, fragile or distal veins (hand, wrist, foot, ankle) and patients who cannot report pain carry a higher risk of extravasation.

Try first

1. Use a flexible plastic cannula rather than a metal needle or butterfly. An antecubital or large forearm vein is preferred.
2. Match the flow rate to the cannula size as your protocol sets it, and reduce the rate for hand or wrist veins where feasible.
3. Confirm placement with backflow (not always seen) and a saline test flush, watching the site where you can.
4. Leave enough slack in the tubing for the table to move without pulling on the line.
5. Central lines, PICCs and ports: power inject only devices labelled for it, within the manufacturer’s flow-rate limit. Small-bore lines inserted in the arm are not power injected unless the manufacturer permits it, because they can break.
6. Ask the patient to tell you at once about pain or swelling during the injection.

Stop and escalate when

Pain or swelling on the test flush or during injection: stop and go to Contrast extravasation. No access suits the protocol’s flow rate: ask the radiologist whether to change the protocol.

Where sources differ

Cannula size and flow rate. The two relevant chapters of the ACR manual give different figures for the same small cannula; ESUR and RANZCR give no figures and say to match the cannula to the vein and the planned flow rate. Follow your department’s cannula and injector policy.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

Contrast is requested for a patient with kidney disease, diabetes, a single kidney, dialysis or metformin, or there is no recent kidney function result.

Why it happens

A fall in kidney function after contrast can have many causes; injury caused by intravenous contrast itself is now thought to be rare. The risk concentrates in patients with acute kidney injury or a low eGFR.

Try first

1. Ask your department’s screening questions. The ACR lists a history of kidney disease (including dialysis, kidney surgery or ablation) and metformin, with diabetes optional; RANZCR asks about kidney disease, diabetes and metformin. Age alone is not a reason to test.
2. Check how recent the eGFR must be under your protocol (sources differ, below). In acute kidney injury the eGFR is unreliable.
3. Do not delay emergency imaging, such as for stroke, bleeding or trauma, while waiting for a kidney result.
4. eGFR below 30 mL/min/1.73 m² or acute kidney injury: tell the radiologist. The guidelines treat this as a relative, not absolute, contraindication; the decision weighs benefit against risk, and IV hydration may be ordered.
5. Dialysis: patients who no longer pass urine and have no working transplant are not at risk of kidney injury from contrast; patients who still pass urine are treated as higher risk to protect what function remains. Dialysis is not started or rescheduled because of contrast.
6. Give the dose the protocol sets (sources differ, below).

Stop and escalate when

eGFR below 30, acute kidney injury, a dialysis patient who still passes urine, or no result for a patient your protocol says must have one: the radiologist decides before contrast is given.

Where sources differ

ACR: with a stable eGFR of 45 or more, IV contrast is not an independent risk; at 30 to 44 the risk is rare; below 30 studies disagree, and 30 is the threshold with the best evidence; there is no agreed interval between test and scan. ESUR: risk below 30 for IV contrast, but below 45 for intra-arterial first-pass contrast and for intensive care patients; eGFR within 7 days for inpatients or acute illness and within 3 months for others, with a recheck at 48 hours in patients at risk. RANZCR: above 45 the risk is likely non-existent, 30 to 45 very likely low or non-existent, and the interval between test and scan is clinical judgement. Dose: the ACR advises against cutting the contrast dose to protect the kidneys; ESUR asks for the lowest dose consistent with a diagnostic result.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018; ACR-NKF 2020

What you see

A patient booked for IV contrast takes metformin, on its own or in a combination tablet.

Why it happens

Metformin does not raise the risk of kidney injury after contrast. The concern is a build-up of metformin if kidney function falls afterwards, so the rules depend on the eGFR and on acute kidney injury.

Try first

1. Check the latest eGFR and whether there is acute kidney injury.
2. Follow your protocol’s metformin rule (the guidelines differ in detail, below).
3. If metformin is to be stopped, make sure the patient and the referrer know who rechecks kidney function and when the tablets restart.

Stop and escalate when

eGFR below 30, acute kidney injury, an eGFR that is not known, or an intra-arterial study: ask the radiologist before contrast.

Where sources differ

ACR (chapter last revised 2014): eGFR 30 or more with no acute kidney injury, no need to stop; eGFR below 30, acute kidney injury, or arterial catheter studies that might send emboli to the kidneys, stop at or before the procedure for 48 hours and restart only once kidney function is re-checked and normal. ESUR (2025): continue if eGFR is above 30 with no acute kidney injury; stop from the time of contrast if eGFR is below 30, for any first-pass intra-arterial injection, or in acute kidney injury; check eGFR within 48 hours and restart if it has not changed significantly. RANZCR: for IV contrast, continue above 30; stop for at least 48 hours if eGFR is below 30, not known, or the patient is unwell, and check eGFR before restarting.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018; ACR-NKF 2020

What you see

The patient says they reacted to contrast before, reports an “iodine allergy”, or arrives asking about premedication.

Why it happens

A previous allergic-like reaction to the same class of contrast is the strongest risk factor for another one. Shellfish or skin iodine allergy is not a significant risk factor.

Try first

1. Find out what happened: the agent’s name, the date, the symptoms, how severe it was and how it was treated.
2. Physiologic reactions (warmth, nausea, an altered taste, a vasovagal faint) are not allergic, and a prior physiologic reaction is not a reason for premedication.
3. Tell the radiologist before contrast. Options include another test, a scan without contrast, a different agent, or premedication.
4. If premedication was prescribed, check it was taken as ordered, including the timing your guideline requires (sources differ, below). Doses are on your departmental protocol.
5. Be ready for a reaction whatever was given: premedication does not replace preparedness. Keep the cannula in and observe as your policy directs.
6. Record the exact agent in the patient’s record; “iodine allergy” alone is not enough to guide the next scan.

Stop and escalate when

A previous severe reaction, or a reaction despite premedication (a breakthrough reaction): the radiologist decides whether and how to proceed, ideally with allergy advice.

Where sources differ

A reaction to the other class (gadolinium): the ACR finds no cross-reactivity and does not recommend routine premedication for it; CAR/CSACI counts only a previous reaction to the same type of agent. RANZCR’s screening asks about previous significant allergic reactions to other substances as a risk factor, and ESUR refers patients who have reacted to agents of both classes to an allergy specialist. Premedication. ACR (2026): may be considered after an allergic-like or unknown-type reaction to the same class; regimens shorter than 4 to 5 hours have no evidence of efficacy; switching to a different agent may help more, and both together seem to work best. ESUR (2025): switch agent and refer to an allergist; premedication only for an emergency examination after a severe reaction. CAR/CSACI (2025): recommends against routine steroid premedication; switching to a chemically different agent is the single best protective measure. RANZCR (2018): premedication reduces the likelihood and severity of reactions, but there is no evidence that it reduces deaths from a breakthrough reaction; oral corticosteroid premedication must start at least 6 hours before the contrast study; use a different agent from the one used before.

Sources: ACR Contrast Media 2026; ESUR 2025; CAR/CSACI 2025; RANZCR ICM 2018

What you see

A breastfeeding patient asks whether she needs to stop feeding after the scan.

Why it happens

Less than 1% of the injected contrast reaches breast milk in 24 hours, and less than 1% of that is absorbed by the baby.

Try first

1. Tell the patient that the ACR, ESUR and RANZCR all say breastfeeding can continue.
2. If she still prefers to pause, the ACR says she may stop for 12 to 24 hours and express and discard the milk; there is little value in stopping for longer.

Where sources differ

The ACR offers the 12 to 24 hour pause as the mother’s informed choice; ESUR and RANZCR say feeding continues normally with no need to discard milk.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

The patient has an overactive thyroid, a history of thyroid storm, a toxic nodule or goitre, or is booked for a thyroid scan or radioiodine treatment.

Why it happens

The iodine in contrast can rarely trigger an overactive thyroid in people at risk, and it blocks thyroid uptake of radioactive iodine for weeks.

Try first

1. Ask about thyroid disease and any planned thyroid scan or radioiodine treatment.
2. Known or suspected untreated hyperthyroidism, or thyroid storm now: tell the radiologist before contrast.
3. Radioiodine treatment or a thyroid scan coming up: tell the radiologist and the referrer, because the interference lasts weeks (below).
4. After contrast in a patient at risk, the referrer arranges follow-up; RANZCR says an overactive thyroid usually appears three to six weeks later.

Stop and escalate when

Acute thyroid storm (ACR: avoid iodinated contrast) or manifest hyperthyroidism (ESUR: do not give it): the radiologist decides.

Where sources differ

ACR: a history of hyperthyroidism alone is not a reason to restrict contrast; avoid it in acute thyroid storm; before radioiodine or thyroid imaging, the suggested gap is ideally 3 to 4 weeks for hyperthyroid and 6 weeks for hypothyroid patients. ESUR: do not give iodinated contrast with manifest hyperthyroidism; untreated Graves’ disease and multinodular goitre with autonomy are at risk; it gives no gap before radioiodine. RANZCR: test and treat known or suspected hyperthyroidism before contrast; thyroid isotope uptake is blocked for 8 weeks.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

For a chest or abdomen scan, the patient cannot raise one or both arms above the head: injury, pain, a stiff shoulder, reduced consciousness, or lines and drains in the way.

Why it happens

Arms beside the body add the most attenuating paths through the scan, causing streaks and noise, and tube current modulation raises the dose to get through them. Modulation is set from the localiser, so the arm position at the localiser is what counts.

Try first

1. Raise the arms before the localiser, not after it: raising them after an arms-down localiser did not lower the dose in a trauma study.
2. If both arms cannot go up, one arm up is better than none.
3. If the arms must stay down, rest the forearms crossed over the front of the abdomen rather than at the sides.
4. Keep the arms in the same position for the localiser and the scan; if they moved, repeat the localiser.
5. With a suspected shoulder injury, or in an unstable patient where time matters, arms down is accepted (ESER).

Good to know

In one trauma study with automatic exposure control, effective dose was 18% higher with one arm down and 45% higher with both arms down than with both arms up (Brink and colleagues).

Sources: Brink 2008; Graef 2022; ESER 2020; Boas 2012; IAEA SSG-46

What you see

The patient is breathless, cannot follow instructions or cannot hold their breath for the scan; images of the chest or abdomen show breathing motion.

Why it happens

Breathing during the acquisition blurs and doubles edges. Faster acquisitions leave less time for movement.

Try first

1. Explain and practise the breath-hold before the scan, using the same words for the localiser and the scan (a general step for routine scans; the SCCT recommends breath-hold practice for coronary CTA).
2. Identify breathing limits before positioning and adapt where you can.
3. Use the shortest rotation time and fastest acquisition the protocol allows.
4. On a dual-source scanner, a high-pitch acquisition with free breathing has reduced breathing artefacts in breathless patients.
5. If none of this works, scan during quiet breathing and tell the radiologist.

Stop and escalate when

Motion still spoils the region that answers the clinical question: ask the radiologist whether a repeat is justified.

Sources: Geleijns 2014; Abbara 2016; Bauer 2012; Varchetta 2021; Liu 2025; CAMRT BPG

What you see

A baby or young child needs CT, or a child cannot keep still or follow breathing instructions.

Why it happens

Children are more sensitive to radiation and need protocols sized for them: adult protocols are inappropriate for children. Movement is the commonest reason a child’s scan fails.

Try first

1. Use the child-size protocol: kV and tube current adjusted to the child’s size, a single phase (usually enough), and only the region the question needs.
2. Explain the scan to the child and the parent in words they understand, and coach older children through the breath-hold.
3. Immobilise comfortably: straps, swaddling, head or body holders, foam pads, sponges, sandbags or pillows; vacuum mattresses help small children in chest CT.
4. Use the fastest acquisition available; for chest CT in children under five, high-pitch fast protocols are preferred to reduce motion and dose.
5. Avoid having anyone hold the child. If a carer must, they should not be pregnant and should not be a member of staff.
6. Sedation or anaesthesia only under your local policy, with fasting instructions, a suitably trained team, and emergency equipment and drugs for the child’s size.

Stop and escalate when

The child cannot be kept still by these means: the radiologist and the referrer agree a plan, such as sedation, anaesthesia or another test.

Sources: Image Gently CT; NCI Pediatric CT guide; ACR-SPR CT PP; IAEA SSG-46; ESR Essentials paediatric chest 2025

What you see

The patient may be near or above the table’s weight limit, may not fit the bore, extends beyond the scan field, or the images are very noisy.

Why it happens

Weight and bore limits are specific to each system. Once the table is inside the gantry the vertical opening is smaller than the bore. The measured scan field is smaller than the bore, and extended-field reconstructions estimate what lies outside it. More tissue means fewer photons reach the detectors, so noise rises.

Try first

1. Confirm the weight and the girth, and compare them with the limits posted for your scanner; weight and girth matter more than body mass index.
2. Check the fit with the table at scanning height, before the patient is moved onto it.
3. Centre carefully and keep the body within the scan field; use the extended field-of-view reconstruction if available, and tell the radiologist the edge region is estimated.
4. Use automatic tube current modulation; a higher kV, slower rotation or lower pitch may be used as your protocol allows; iterative reconstruction reduces noise.
5. Record the weight and girth in the record so the next booking goes to a suitable scanner.

Stop and escalate when

The patient is above the system’s limits or does not fit: do not scan; ask the radiologist and referrer about a larger scanner or another test.

Sources: Uppot 2018; Uppot 2007; IAEA HHS 19; Boas 2012

What you see

The patient does not share your language, is Deaf or hard of hearing, or cannot understand or respond to instructions such as breath-holds or reporting pain during the injection.

Why it happens

Screening, consent, breath-holds and the injection all depend on two-way communication. Patients who cannot report pain or swelling are at higher risk of extravasation.

Try first

1. Arrange a qualified interpreter before the appointment, in person or by a reliable remote service.
2. Before the scan, agree how the patient will signal (call button, hand signal) and explain that they can be brought out at any time.
3. Keep the patient in view and within talking distance from the control room throughout.
4. For patients who cannot report pain, watch the injection site where you can and take extra care with cannula position.

Stop and escalate when

Screening or consent cannot be completed reliably: ask the radiologist before going ahead.

Sources: CAMRT BPG; IAEA SSG-46; ACR Contrast Media 2026; HDC Code 1996; AAPM TG-233

What you see

The patient calls out, moves, sits up or asks to stop during the scan or injection.

Why it happens

Patients can be told before the scan that they can be brought out at any time. The operator can end the exposure at any point.

Try first

1. Stop the scan and the injection and talk to the patient straight away.
2. Check the cannula site if the injection was running.
3. Find the cause: pain, warmth from the contrast, nausea, anxiety, breathlessness or needing the toilet.
4. Help the patient off the table only when they feel well, using the handling aids you need (see Fall).
5. Agree with the patient and the radiologist whether to continue, and which part is still needed.

Stop and escalate when

Pain or swelling at the cannula, breathlessness, a rash or feeling faint after contrast: go to Extravasation or Contrast reaction.

Sources: CAMRT BPG; 21 CFR 1020.33; IAEA SSG-46; ACR Contrast Media 2026

What you see

During or soon after iodinated contrast: hives or itching, flushing, swelling of the face or throat, hoarseness or stridor, wheeze or breathlessness, low blood pressure with a fast pulse, low blood pressure with a slow pulse, a seizure, or collapse.

Do this now

1. Stop the injection, keep the cannula in, and call for help early. Nearly all life-threatening reactions happen within 20 minutes of the injection.
2. Assess quickly: general appearance, whether the patient can speak and how the voice sounds, breathing, pulse and blood pressure. Give oxygen by mask as your reaction card directs.
3. For a severe reaction (anaphylaxis, stridor, wheeze with low oxygen, shock), call the resuscitation team. Adrenaline is the first-line drug; antihistamines and corticosteroids do not replace it.
4. Lie the patient flat. Let them sit up if breathing is hard, but do not let them stand. Raise the legs if blood pressure is low. Low blood pressure with a slow pulse suggests a vasovagal reaction.
5. A mild reaction can progress: observe the patient with the cannula in until it settles, for at least as long as your policy says.
6. If your policy uses it, blood for serum tryptase is taken as soon as practical and within 4 hours of the start of the reaction (ESUR).
7. Record the exact agent, the symptoms, severity, treatment and outcome; give the patient the agent’s name; report the reaction as your policy requires.

Where sources differ

Route of adrenaline. ESUR, RANZCR, CAR/CSACI and ASCIA give it into the muscle. The ACR also gives intramuscular doses, but its adult table prefers the intravenous route when blood pressure is low, because poorly perfused limbs may not absorb an intramuscular injection. Observation: after a mild reaction, the ACR says at least 20 to 30 minutes and ESUR at least 30 minutes; after a moderate reaction ESUR says 4 to 6 hours after treatment and admission after a life-threatening one; RANZCR says at least 4 to 6 hours after a severe reaction resolves, because biphasic reactions occur. After any injection, RANZCR keeps the cannula in and supervises for at least 15 minutes, or 30 minutes at increased risk. Route, dose and observation time come from your department’s card.

Good to know

Doses are on your department’s card and the site’s contrast reaction emergency card. Delayed skin reactions can appear from about an hour up to a week after the injection: tell the patient to contact their doctor if a rash appears.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018; CAR/CSACI 2025; ASCIA 2026

What you see

Swelling, tightness, stinging or burning pain at the cannula site during or after injection; the injector alarms, or the monitoring scan shows no contrast arriving.

Do this now

1. Stop the injection. ESUR also says stop the scan.
2. Tell the responsible doctor and mark the edge of the swelling (ESUR). A health care provider examines the limb: tenderness, swelling, redness, sensation, finger movement and circulation.
3. Raise the limb above the level of the heart, and apply compresses as your policy directs (sources differ, below).
4. Do not routinely aspirate through the cannula, inject hyaluronidase or steroids, or squeeze the area.
5. Record the site, agent, volume, assessment and advice; tell the referrer about a symptomatic extravasation; record it in your incident system if your policy requires.
6. Observe an outpatient until symptoms improve, and give clear written advice on where and when to seek help (ESUR: a patient leaflet), because a severe injury can develop several hours later.

Stop and escalate when

Get an urgent surgical opinion for severe pain, increasing swelling or pain, reduced capillary refill, a change in sensation, worsening movement, blistering or ulceration. ESUR also asks for a surgical opinion when a mild extravasation does not improve.

Where sources differ

Compresses. The ACR recommendation is cold compresses or ice packs at first rather than warm, although its text notes no clear evidence favouring either; ESUR (2025) uses ice packs; RANZCR allows cold or warm. Volume. The ACR says the volume alone should not trigger a surgical consultation; ESUR (2025) also recommends a surgical opinion for an extravasation of more than 150 mL. Follow your local policy.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

You find that the person scanned was not the patient on the request, the wrong body part, side or examination was scanned, or images were stored under the wrong patient.

Do this now

1. Stop any further scanning, check on both patients and confirm who each one is, and tell the radiologist now.
2. Keep the images and the dose report; do not delete anything. Write down what happened and when.
3. Report it through your incident system as your local procedure directs. A medical physicist (medical physics expert) estimates the dose received.
4. The referrer and the patient (or their representative) are told what happened, under your open disclosure or duty of candour policy (sources differ on when, below).
5. Correct the patient records through your authorised correction process.
6. Whether the regulator must be notified depends on your country’s criteria; your employer decides after the preliminary investigation. Do not guess thresholds yourself.

Where sources differ

The frameworks agree on investigating, estimating the dose and informing the referrer and patient; the detail differs. UK (IR(ME)R 2017 regulation 8, CQC guidance): the employer investigates and notifies the enforcing authority of exposures significantly greater than intended, with criteria that differ between England and the other nations; the duty to tell the patient covers clinically significant exposures (not defined in the regulations; RCR guidance helps employers decide), alongside the duty of candour. New Zealand (ORS C1): the licence holder notifies the Director of an overexposure as soon as practicable, investigates with dose estimates, and makes sure the referrer and patient are told of any unintended medical exposure; the HDC expects open disclosure usually within 24 hours. Australia (ARPANSA RPS C-5): the operator reports to the Responsible Person and the radiologist, a medical physicist estimates the dose, the radiologist informs the patient, and reports go to the state or territory regulator as it requires. IAEA SSG-46: investigation with a physicist’s dose estimate, the referrer notified, significant events reported to the regulator.

Sources: IAEA SSG-46; IR(ME)R 2017; CQC SAUE 2024; RCR IR(ME)R 2026; ORS C1 2024; HDC Open Disclosure 2019; ARPANSA RPS C-5; ACSQHC CT/MRI protocol; Clunie 2014

What you see

A scan needs repeating, or more was given than planned: the wrong protocol (such as an adult protocol on a child), wrong settings, scanning beyond the range, a failed injection or bolus tracking, movement, or an equipment failure.

Do this now

1. Before repeating, ask the radiologist whether a repeat is justified and over what range.
2. Find and correct the cause first: settings, centring, protocol, injection or equipment.
3. Record the reason and keep the dose report.
4. Equipment fault, error or unusual behaviour: stop using the equipment until it is put right, record it and report it (see Scanner failure mid-scan).
5. Report wrong settings, wrong protocols and exposures much greater than intended through your incident process; a medical physicist estimates the dose and your employer decides on notification.

Good to know

In the UK, a technical repeat with no procedural, human, systematic or equipment error (images spoiled by movement or contrast extravasation) is not a statutory notification, but events and near misses are still recorded and analysed locally (CQC). Any necessary repeat counts towards the total exposure from an incident.

Sources: CQC SAUE 2024; IR(ME)R 2017; ARPANSA RPS C-5; ORS C1 2024; IAEA SSG-46

What you see

After a CT, it emerges that the patient was pregnant at the time of the scan.

Do this now

1. Tell the radiologist and report it through your incident process: an inadvertent exposure of an embryo or fetus is investigated.
2. A medical physicist estimates the dose to the embryo or fetus; it needs the scan details and the stage of pregnancy.
3. The referrer and the patient are informed, and counselling is given by a radiologist or clinician with the right experience, using the dose estimate.
4. Record how pregnancy status was checked before the scan, and the estimate.
5. Your regional rules decide whether the regulator must be told.

Good to know

For the counselling clinician, not for the radiographer to give: the ACR-SPR practice parameter says a well-managed single-phase CT of the abdomen and pelvis gives the embryo or fetus less than 35 mGy, typically about 10 to 25 mGy, which does not warrant ending the pregnancy; CT away from the abdomen and pelvis gives much less. HPA (UK): after a higher-dose examination done inadvertently, termination would not be justified solely on the basis of the radiation risk to the unborn child.

Sources: IAEA SSG-46; ACR-SPR Pregnancy PP; HPA RCE-9 2009; RCR IR(ME)R 2026; ARPANSA RPS C-5; ORS C1 2024; IR(ME)R 2017

What you see

Air on the images in the veins of the chest, the main pulmonary artery or the right ventricle; with a large volume, sudden breathlessness, air hunger, cough, chest pain, a fast pulse, low blood pressure or wheeze. Air reaching the arterial side can cause neurological signs.

Why it happens

Air left in the syringe or tubing is injected with the contrast. Small volumes are common and usually cause no symptoms; a large volume is rare but can be fatal. Patients with a right-to-left shunt or a pulmonary arteriovenous malformation are at higher risk even from small volumes.

Do this now

1. Stop the injection and call for help and the radiologist.
2. Give 100% oxygen and turn the patient onto their left side (left lateral decubitus), as the ACR manual describes.
3. If the patient arrests, start chest compressions at once and call the resuscitation team.
4. Hyperbaric oxygen has been recommended; the medical team decides on referral.
5. Report the incident and check the injector set-up before the next patient.

Good to know

Prevention, as the ACR manual describes it: clear air from the syringe and pressure tubing, then turn the syringe so the tubing points downwards.

Sources: ACR Contrast Media 2026; ANZCOR Guideline 8 2021

What you see

The patient is unresponsive and not breathing normally, or only gasping.

Do this now

1. Call the resuscitation team on your emergency number and stop the scan.
2. Start chest compressions at once: CPR is started for anyone unresponsive and not breathing normally, even with occasional gasps. Staff trained and willing to give rescue breaths do so.
3. Attach a defibrillator or AED as soon as it arrives and follow its prompts. Minimise pauses in compressions.
4. Bring the table out of the gantry and clear space so the team can reach the patient (a general step; follow your local procedure).
5. If contrast was given and the cause of the arrest is unclear, the ACR suggests assuming it is an allergic-like reaction; if air embolism is possible, see Air embolism.
6. In a visibly pregnant patient, the ACR advises compressions with the patient supine and the uterus displaced manually up and to the left, if enough staff are present.
7. Afterwards, record the times and events and report the incident.

Sources: ANZCOR Guideline 8 2021; ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

The injector stops on a pressure alarm, the tubing or a connector leaks or bursts, or a central line, PICC or port leaks or breaks during injection.

Why it happens

Pressure rises with a small cannula, a high flow rate, viscous contrast, a kink or a blocked line. Central lines that are not rated for power injection can break.

Do this now

1. Stop the injection and check the patient first: the cannula or line site, any swelling, and any symptoms.
2. If the patient has signs of extravasation, go to Contrast extravasation.
3. If a central line, PICC or port may be damaged, do not use it again and tell the radiologist and the team responsible for the line (a general step).
4. Before restarting, find the cause: a kink or closed clamp, a cannula too small for the flow rate, or a central device not rated for power injection at that flow rate.
5. Take a faulty injector or set out of use and report it under your equipment and incident procedures (a general step; your injector manual takes precedence).

Good to know

Warming and central-line rules: see Injector faults and Poor IV access.

Sources: ACR Contrast Media 2026; ESUR 2025; RANZCR ICM 2018

What you see

A patient falls, slips or is lowered to the floor while getting on or off the table, during a transfer, or while walking in the department.

Why it happens

Falls happen in imaging departments, often in CT and MRI areas, and most patients who fall have at least one risk factor; some are injured.

Do this now

1. Call for help, stay with the patient and check whether they are hurt; get a doctor to assess them (a general step; follow your falls procedure).
2. Do not lift the patient alone: use the handling aids and the number of staff your assessment calls for.
3. Record the fall and report it through your incident process, and follow up as your policy requires.
4. Before the next transfer, reassess fall risk: it can change during the examination, for example after sedation.

Good to know

Prevention: assess each patient’s fall risk under your policy, help patients on and off the table, keep the call button within reach, and use slide sheets, slide boards or lifts as needed.

Sources: CAMRT BPG; Abujudeh 2011

What you see

The scan stops partway, an error appears, the tube fails, the table stops or the power goes off while the patient is on the table.

Why it happens

CT scanners end the exposure automatically when an equipment failure affects data collection, and show that this has happened; a manual reset is needed before another scan. A tube fault during acquisition can make the images unusable.

Do this now

1. Check the patient first, and stop any injection that is running.
2. Bring the patient out of the gantry and off the table if the fault prevents scanning (a general step; follow your operator manual if the table is stuck).
3. Note the time, the message, what was scanned and how much contrast was given; record it in the fault log and call service.
4. Ask the radiologist whether a repeat is justified, and on which scanner; an unplanned repeat is a further exposure (see Unplanned repeat).
5. After repair, the scanner is tested before it returns to clinical use.

Good to know

A power failure can lose work in progress unless an uninterruptible power supply is fitted; the IAEA advises one for CT, as a standby generator may not be stable enough.

Sources: 21 CFR 1020.33; IAEA SSG-46; Geleijns 2014; ACR CT QC 2017