The CT Trainee Manual — From Day One to Competent
A beginner-to-competent curriculum in computed tomography for student and newly rotated radiographers (UK/AU/NZ/IE) and CT technologists (US/CA). Ten sections take you from your first day at the gantry to independently running a routine CT list — protocol selection, contrast timing, dose optimisation and honest image critique included. Written the way a good senior teaches: what to do, why it works, and where beginners go wrong.
1 · The Competency Ladder — an 8-Week Fast Track
Eight weeks is roughly the time it takes a supervised beginner — a radiographer new to CT — to go from “which pedal moves the table?” to reliably running routine head, chest and abdomen lists under distant supervision. The plan below uses entrustment levels borrowed from EPA (Entrustable Professional Activity) frameworks. For every task you are always at one of four rungs:
Level 1 Observe — watch and narrate what is happening. Level 2 Perform with direct supervision — you do it, a senior stands beside you. Level 3 Perform with distant supervision — you do it alone, a senior is immediately available and reviews your work. Level 4 Independent — you do it, teach it, and catch other people’s errors in it.
Every Friday, grade yourself honestly against the “must be able to DO” column. The verbs are deliberate: they are observable behaviours a supervisor can watch and sign, not feelings of familiarity.
Week-by-week expectations
| Week | Focus | What you must be able to DO by Friday (observable) | Sign-off criterion |
|---|---|---|---|
| 1 | Safety, the room, the workflow | Level 1→2. Walk a visitor through the CT suite pointing out the controlled area, warning lights, emergency stop, contrast pump shut-off and crash trolley. State CTDIvol and DLP for three completed scans and say what each number means. Shadow a full patient journey (request → justification → ID → scan → PACS) and narrate every step. Wear your dosimeter correctly. | Supervisor observes a complete narrated patient journey and a correct dose-readout explanation. |
| 2 | Physics + the console | Level 2. On a phantom, change kV, mA, rotation time and pitch and predict — out loud, before scanning — the effect on noise, dose and scan time. Load a protocol, explain each series in it, and plan a topogram with correct centring. Explain why off-centre positioning raises dose and noise. | Three correct predictions in a row on phantom scans; correct topogram plan on first attempt. |
| 3 | Non-contrast scanning | Level 2→3. Perform non-contrast heads, sinuses and CT KUB on cooperative patients with a senior beside you: ID check, positioning at isocentre, topogram, range planning, scan, recon check, PACS send. Run the image-evaluation checklist (Tab 7) on every scan out loud. | Five consecutive non-contrast exams needing no supervisor correction of range or centring. |
| 4 | IV access + contrast basics | Level 2. Cannulate (per local scope of practice), programme the power injector, state flow-rate and gauge requirements for a routine portal-venous study, run a contrast screening interview (eGFR, allergy, metformin) without prompts, and describe extravasation management step by step. | Injector set-up and full screening interview observed and signed; extravasation drill verbalised correctly. |
| 5 | Contrast studies | Level 2→3. Perform abdo-pelvis portal-venous studies end-to-end including bolus tracking: place the ROI, set the trigger, coach the breath-hold, evaluate enhancement adequacy on the images. Explain the difference between bolus tracking and a test bolus and when each is preferred. | Three portal-venous studies with correctly timed enhancement, verified by a senior on PACS. |
| 6 | CTPA + arterial timing | Level 2→3. Perform CTPA including trigger placement on the pulmonary trunk, breathing instruction that avoids Valsalva, and immediate quality judgement of PA opacification (target >250 HU, acceptable >210 HU). Troubleshoot one suboptimal study: name the cause and the fix. | Two diagnostic CTPAs performed with distant supervision; one timing-failure correctly diagnosed (real or case-based). |
| 7 | Trauma & urgent work | Level 2. Run a simulated (or supervised real) trauma pan-scan: rapid ID, arms positioning decision, split-bolus or dual-phase protocol per local policy, scan without delaying resuscitation, flag critical findings to the radiologist immediately. Demonstrate the “scan fast, check fast, communicate fast” loop. | One pan-scan (real or simulated) completed inside the local time target with correct protocol choice. |
| 8 | Consolidation + independence | Level 3→4. Run a half-day routine list with your supervisor observing silently: correct protocol selection ≥95%, dose within local DRLs, honest repeat/rescan decisions, complete documentation. Map remaining gaps against the sign-off checklist (Tab 10) with your mentor. | Silent-observation list completed; sign-off checklist review meeting held and gaps documented. |
How this manual uses learning science
- Retrieval practice. Every content tab ends with a “Quick recall” block and Tab 9 is a full question bank. Answering from memory — before peeking — strengthens retention far more than re-reading. Re-reading feels productive; testing is productive.
- Spaced repetition. Tab 10 schedules re-tests at Day 1, 3, 7 and 30 after each week. Reviewing at expanding intervals, just as forgetting begins, roughly doubles long-term retention compared with massed cramming.
- Interleaving. The question groups deliberately mix domains (a dose question next to a timing question next to an artifact question) because mixed practice builds the discrimination skill you actually need on shift — recognising which kind of problem you are looking at.
- The 80/20 high-yield principle. Around 20% of CT knowledge covers 80% of daily practice: five protocols, one timing model, one dose triad (CTDIvol/DLP/SSDE), ten artifacts. This manual front-loads that 20% and tells you explicitly when a topic is long-tail.
Terminology across countries
| Region | Title / register |
|---|---|
| UK / Ireland | Diagnostic radiographer — HCPC (UK) or CORU (IE); IR(ME)R 2017 governs exposures |
| US | CT technologist — ARRT(CT) post-primary certification |
| Australia | Medical radiation practitioner — AHPRA/MRPBA registration |
| New Zealand | Medical imaging technologist (MIT) — NZ MRTB |
| Canada | MRT(R) with CT competency — CAMRT |
This page uses “radiographer”. IV cannulation and contrast administration scope varies by country and employer — always work within your local scheme of work.
Full CT competency sign-off workbook
The complete printable companion to this manual: graded competency checklists for 30+ CT examinations, supervisor sign-off pages mapped to HCPC, ARRT(CT) and MRPBA frameworks, a contrast-incident drill pack and a personal dose-audit log.
- Per-protocol checklists with pass criteria and common-fault prompts
- Structured mentor feedback forms for weeks 1–8
- Printable contrast-reaction and extravasation quick cards
2 · Radiation Safety & Dose
CT delivers most of the population’s medical radiation dose from a minority of examinations. A routine abdo-pelvis CT is roughly 300–500 chest X-rays’ worth of effective dose. That is why justification and optimisation matter more at the CT console than anywhere else in the department — and why the dose readout on your console is not decoration.
The dose triad: CTDIvol, DLP, SSDE
| Quantity | Unit | What it actually tells you |
|---|---|---|
| CTDIvol (volume CT dose index) | mGy | Scanner output, measured in a standard acrylic phantom (16 cm for head protocols, 32 cm for body). It describes the intensity of the beam per slice of scanned volume — not the dose your patient received. Two patients scanned with identical CTDIvol receive very different doses if one is small and one is large. |
| DLP (dose–length product) | mGy·cm | CTDIvol × scan length. The total radiation “budget” of the acquisition. Every extra centimetre of scan range adds directly to DLP — which is why range discipline is the easiest dose saving you own. |
| SSDE (size-specific dose estimate) | mGy | CTDIvol corrected for the patient’s actual size (water-equivalent diameter, AAPM TG-204/220). The closest routine number to real patient dose: small patients receive more than CTDIvol suggests, large patients less. |
| Effective dose (estimate) | mSv | ≈ DLP × region-specific k-coefficient: head ~0.0021, chest ~0.014, abdo-pelvis ~0.015 mSv per mGy·cm. A rough population-risk currency for comparing exams — never an individual dosimetry claim. |
Typical adult values (approximate — your local DRLs prevail)
| Examination | CTDIvol | DLP | Effective dose |
|---|---|---|---|
| Head (non-contrast) | ~50–60 mGy | ~800–1000 mGy·cm | ~1.5–2 mSv |
| Chest (routine) | ~8–12 mGy | ~300–400 mGy·cm | ~4–6 mSv |
| CTPA | ~8–12 mGy | ~300–450 mGy·cm | ~4–6 mSv |
| Abdomen–pelvis (portal venous) | ~10–15 mGy | ~500–700 mGy·cm | ~7–10 mSv |
| CT KUB (low dose) | ~4–8 mGy | ~200–400 mGy·cm | ~2–4 mSv |
| Trauma pan-scan | — | ~1500–2500 mGy·cm | ~15–25 mSv |
Dose optimisation you control every scan
- Scan length. Plan the range to the clinical question — lung apices to costophrenic angles for a chest, not “chin to kidneys just in case”. Every extra centimetre is pure DLP.
- Centring at isocentre. Off-centre patients defeat the bowtie filter and mislead the topogram-based tube-current modulation: too high in the gantry and the AEC over-estimates size → dose up; surface dose to the near side rises steeply. Centre with lasers on the mid-coronal plane, every patient.
- Tube current modulation on, and fed a good topogram. CARE Dose4D / Smart mA / DoseRight / Sure Exposure adapt mA to attenuation — but they calculate from the topogram. Arms in the topogram but out of the scan (or vice versa), metal, or wrong centring all corrupt the modulation.
- kV selection. Lower kV (100, 80) raises iodine contrast (closer to the iodine k-edge at 33 keV) and cuts dose in smaller patients — automated kV selection tools do this per-patient. Don’t force 120 kV out of habit.
- Iterative / deep-learning reconstruction permits 30–50% lower dose at equivalent noise. If your protocols were built for FBP and never re-optimised, they are overdosing — ask.
- Phases discipline. The most common CT dose error is not parameters — it is unjustified extra phases. A routine abdomen does not need a non-contrast plus arterial plus venous plus delayed “to be safe”. Per the request, per protocol, no free-styling.
Paediatric CT
- Children are 2–3× more radiosensitive than adults and have longer to express harm. Justification is stricter: could ultrasound or MRI answer the question?
- Size-based protocols, always — weight or diameter bands, never adult defaults “turned down a bit”. Follow Image Gently and your local paediatric protocol book.
- Single phase almost always suffices. Multiphase paediatric CT needs explicit radiologist justification.
- Lower kV (80–100) suits small bodies and boosts iodine signal — paediatric contrast studies often look better at low kV.
- Immobilisation, feed-and-wrap for infants, play specialists, parent in the room (with protection and pregnancy check per local policy) beat sedation; a fast scanner beats all three.
- No routine patient shielding in the scan field — in-plane bismuth/lead shielding distorts modulation and images; modern guidance (AAPM, BIR) has moved away from it. Out-of-plane shielding is mostly comfort; collimation and range discipline do the real work.
Pregnancy, staff and the room
- Pregnancy check before any abdo-pelvic CT in anyone of childbearing potential, exactly as in plain radiography — ask privately, document, escalate if pregnant or unsure. A clinically justified CT can still proceed (e.g., CTPA in pregnancy is routinely justified; fetal dose is low), but that is a documented practitioner decision, not yours alone.
- Nobody stays in the room during a scan unless clinically essential (anaesthetist, parent). Anyone who stays wears lead and stands as far from the gantry as the task allows — scatter near the gantry aperture during a body scan can reach mGy-per-scan levels at close range.
- Your own dose as a CT radiographer should be near background: the console is behind shielding. Non-trivial personal readings almost always mean room-time during scanning (contrast injections started late, holding patients) — review the workflow, not just the badge.
- Know the location and action of the emergency stop (X-ray and table motion) vs the abort/pause — and when each is appropriate. Emergency stop for entrapment or hardware danger; abort for patient distress mid-scan.
Quick recall — 5 questions before you leave this tab
1. Your console shows CTDIvol 12 mGy for two patients, one 45 kg and one 120 kg. Who received the higher actual dose, and which metric shows it?
The 45 kg patient — CTDIvol is phantom-referenced scanner output; SSDE corrects for patient size and will be higher than CTDIvol for the small patient.
2. DLP = ?
CTDIvol × scan length (mGy·cm) — the total acquisition dose budget.
3. Name three things that corrupt tube-current modulation.
Off-centre positioning, arms positioned differently in topogram vs scan, metal/contrast in the topogram (also wrong topogram direction per vendor rules).
4. Why does low kV improve contrast-enhanced studies?
Mean beam energy moves closer to iodine’s 33 keV k-edge → more photoelectric interactions in iodine → higher iodine attenuation/HU per mg of contrast.
5. The commonest avoidable dose error in CT practice?
Unjustified additional phases — extra acquisitions cost far more dose than any parameter tweak saves.
3 · Physics You Actually Need
You don’t need Fourier maths. You need a working model good enough to predict what happens to noise, dose, artifacts and scan time when you change something at the console. That model has five parts: helical acquisition, the exposure triad (kV/mA/rotation), pitch, reconstruction, and Hounsfield units with windowing.
Helical (spiral) acquisition
- The tube and detector array rotate continuously (slip-ring technology) while the table moves through the gantry — the beam traces a helix around the patient. Modern scanners rotate in 0.25–0.5 s and carry 16–320 detector rows.
- Because no single slice is ever measured “straight on”, the scanner interpolates data to reconstruct each axial plane. That’s why helical data can be re-reconstructed at any slice thickness and interval after the scan — thin recon costs nothing extra in dose, only in noise per slice.
- Axial (sequential) mode — step-and-shoot — survives where the table must be still: some head protocols, ECG-gated cardiac (prospective), and interventional guidance.
- Detector collimation (e.g., 64 × 0.6 mm) sets the thinnest slice you can ever reconstruct from the raw data. Acquire thin, reconstruct whatever you need.
kV, mA, rotation time
- kV sets beam energy: penetration, contrast behaviour, and dose (output scales ~kV²). 120 kV is the historical default; 100/80 kV boost iodine contrast and cut dose in smaller patients; 140 kV punches through metal and very large patients at a contrast cost.
- mA × rotation time = mAs: photon quantity, hence noise. Noise ∝ 1/√mAs — to halve noise you must quadruple mAs (and dose). This is why chasing beautiful low-noise images is expensive, and why “diagnostic, not beautiful” is the professional standard.
- Rotation time is your motion-freezing lever: faster rotation = less motion blur and shorter breath-holds, at the price of fewer photons per rotation (the tube has an mA ceiling).
- Effective mAs (Siemens) = mAs ÷ pitch — noise-relevant exposure per slice after accounting for table speed.
Pitch
pitch = table feed per rotation ÷ total beam collimation.- Pitch 1: contiguous coverage. Pitch >1: gaps filled by interpolation — faster coverage, less dose (on most systems), slightly more helical artifact and effective slice broadening. Pitch <1: overlap — better z-resolution and cardiac gating, more dose.
- Typical: routine body 0.8–1.4; CTPA ~1–1.5 (speed matters); retrospective cardiac ~0.2–0.4.
- Caveat: with automatic exposure control, raising pitch often makes the system raise mA to hold image quality — so “pitch up = dose down” is only guaranteed with fixed mA. Know which mode your protocol uses.
Reconstruction — from raw data to images
- Filtered back projection (FBP): the classical fast method — project the attenuation profiles back across the image grid with a sharpening filter. Predictable, noisy at low dose.
- Iterative reconstruction (IR) (ASiR/ASiR-V, SAFIRE/ADMIRE, iDose⁴/IMR, AIDR 3D): repeatedly refines the image against models of noise and scanner geometry — 30–50% dose reduction at matched noise, with a characteristic “plastic” texture at high strengths.
- Deep-learning reconstruction (DLIR) (TrueFidelity, AiCE, Precise Image): neural networks trained on high-dose data restore low-dose images — better texture than IR, further dose head-room. Your protocol will specify a strength level; changing it changes noise, not the raw data.
- Kernels (filters): soft/standard kernels smooth noise for soft tissue (brain, abdomen); sharp/bone/lung kernels enhance edges at the price of noise. Wrong kernel = “why does this brain look like sandpaper” or “why is this lung so mushy”. Recon jobs usually pair kernel + window + slice thickness per series.
- Slice thickness trade: thin slices (0.6–1.25 mm) → better detail and MPRs, more noise per slice; thick (3–5 mm) → smoother, faster to read. Standard reporting sets are typically 2–5 mm with thin sets kept for reformats.
- MPR / MIP / VR: multiplanar reformats (coronal/sagittal — routine on nearly everything now), maximum-intensity projections (vessels, nodules), volume rendering (surgical planning, fractures). Know which reformats each local protocol expects you to build and send.
Hounsfield units — the ruler
HU = 1000 × (μ − μ_water) / μ_water. Water 0, air −1000, by definition.
| Tissue | Typical HU |
|---|---|
| Air | −1000 |
| Lung | −900 to −500 |
| Fat | −100 to −50 |
| Water / simple fluid / CSF | 0 to +15 |
| Soft tissue / muscle | +30 to +60 |
| Unclotted blood | +30 to +45 |
| Acute clot / haemorrhage | +50 to +75 |
| Enhancing tissue / opacified vessel | +90 to +400+ |
| Trabecular → cortical bone | +300 → +1500+ |
| Metal | >+2000 (scale-capped) |
Learn to drop an ROI and read the number: “is this cyst fluid or solid?” is an HU question before it is anyone’s opinion.
Window & level — the display
Level (centre) = the HU shown as mid-grey. Width = the HU range spread across the grey scale; everything outside is pure black/white. Narrow window = high displayed contrast.
| Window | W / L (typical) | For |
|---|---|---|
| Brain | 80 / 40 | Grey–white differentiation, infarct, bleed |
| Stroke (narrow) | ~35 / 35 | Subtle early infarct |
| Subdural | 130–300 / 50–100 | Thin bleeds against skull |
| Lung | 1500 / −600 | Parenchyma, pneumothorax |
| Mediastinum / soft tissue | 350–400 / 40–50 | Vessels, nodes, organs |
| Liver | 150 / 60–70 | Subtle lesions |
| Bone | 2000–2500 / 300–500 | Cortex, fractures |
Windowing changes the display only — never the data. An image can never be “ruined” by windowing, but a finding can be missed by reviewing in only one window.
Quick recall — 5 questions before you leave this tab
1. Noise is too high on a large patient. You double the mAs. How much does noise fall, and dose rise?
Noise falls by √2 (~30%); dose doubles. Noise ∝ 1/√mAs, dose ∝ mAs.
2. Define pitch, and give the trade-off of pitch 1.4 vs 0.8.
Table feed per rotation ÷ total collimation. 1.4: faster, less dose (at fixed mA), more interpolation/helical artifact; 0.8: overlap, better z-resolution, more dose.
3. A cerebral “lesion” measures −70 HU on ROI. What is it?
Fat density — think lipoma/dermoid, not tumour or blood.
4. Why can you reconstruct 1 mm slices from a helical abdomen scanned “at 5 mm” without rescanning?
Helical raw data is continuous; slice thickness is a reconstruction choice down to the acquired detector collimation.
5. Which recon kernel and window pair for reviewing lungs?
Sharp/lung kernel, lung window ~W1500/L−600.
4 · The Console & Protocols
A CT protocol is a recipe with four courses: the topogram, one or more acquisitions, the contrast programme, and the reconstruction jobs. Competence means being able to open any protocol on your scanner and narrate what each line does — and spot when the recipe doesn’t fit the patient in front of you.
The topogram (scout / surview / scanogram)
- A low-dose planar acquisition (tube stationary, table moving) used to plan the scan range and to feed the tube-current modulation its attenuation model.
- Cover more than you plan to scan — you cannot extend a range beyond the topogram, and re-running it costs time and (a little) dose. But keep it sane: a head-to-thigh topogram for a sinus CT is sloppy.
- Some vendors want a specific topogram projection (AP, lateral, or both) for accurate modulation — follow the protocol, not habit.
- Look at the topogram before scanning. It already shows you: rotation, arms position, metal, lines and leads in the field, a lung base at an unexpected level. Thirty seconds of topogram reading prevents most range errors.
Anatomy of a protocol
| Element | What it defines | Beginner checkpoints |
|---|---|---|
| Acquisition(s) | Range, direction, kV/mAs or AEC reference, pitch, rotation time, collimation | Does the range match the clinical question? Is AEC on and centred? |
| Contrast programme | Agent concentration, volume, flow rate, saline chaser, phase delays or trigger settings | Cannula gauge/site adequate for the flow rate? Screening complete? |
| Timing method | Fixed delay, bolus tracking (ROI + threshold), or test bolus | ROI on the correct vessel? Threshold per protocol? Breathing instruction planned? |
| Recon jobs | Slice thickness/interval, kernel, window, FOV, reformats (cor/sag/MIP), destination | Are all expected series present and sent — including thin data where policy requires? |
Contrast phases — one timing model to rule them all
After an antecubital injection, contrast arrives in the pulmonary arteries at ~8–12 s, the aorta at ~15–20 s, peaks in arteries ~30 s, in the portal vein and liver parenchyma ~60–80 s, equilibrates by ~3 min, and appears in the collecting systems from ~5 min. Every “phase” is just a scan planted at one of these stations:
| Phase | Typical timing* | What enhances | Classic uses |
|---|---|---|---|
| Pulmonary arterial | ~10–15 s (tracked) | Pulmonary arteries | CTPA |
| Systemic arterial | ~20–35 s (tracked) | Aorta, arteries | CTA, dissection, active bleeding |
| Late arterial | ~35–45 s | Hypervascular lesions, pancreas | HCC, pancreatic protocol |
| Portal venous | ~65–80 s | Liver parenchyma, portal/hepatic veins, bowel wall | The routine abdomen workhorse |
| Nephrographic | ~90–120 s | Renal parenchyma uniformly | Renal mass characterisation |
| Delayed / excretory | 3–15 min | Washout; opacified collecting systems/ureters/bladder | Adrenal washout, urogram, urine leak |
*From injection start; fixed delays assume normal cardiac output — slow circulations (elderly, heart failure) arrive late, which is exactly why tracking beats fixed delays.
Bolus tracking (the routine method)
- Acquire an unenhanced monitoring slice at the reference vessel level (pulmonary trunk for CTPA; aorta at the diaphragm or coeliac axis for body arterial work).
- Place the ROI inside the vessel lumen — not on the wall, not over calcification or a line tip.
- Start the injection; low-dose monitoring scans fire every 1–2 s.
- When the ROI crosses the threshold (commonly 100–150 HU above baseline), the scanner triggers after a short post-trigger delay (~3–10 s: breath-hold instruction + table travel; protocol-set).
- Coach the patient before injection: they will hear the instruction mid-injection and must not take a huge gasp (see the Valsalva trap, Tab 6).
Test bolus (the precision method)
- Inject a small test dose (~10–20 mL contrast + saline chaser) at the planned flow rate.
- Run repeated low-dose scans at the reference level and plot the time–attenuation curve.
- Read the time-to-peak; set the main scan delay = time-to-peak (± offset per protocol).
- Inject the full bolus and scan at the calculated delay.
- Pros: per-patient circulation timing; useful in cardiac CT and poor cardiac output.
- Cons: extra time, extra contrast, and the test dose slightly pre-enhances tissues. For routine work, tracking wins; know both.
Console discipline — every examination
Right patient → right protocol for the indication → screening & consent done → topogram reviewed → range planned → contrast programme checked against the cannula → breathing coached → scan → images checked (coverage, motion, timing) → recons built → PACS send verified → dose recorded.
Selecting the wrong protocol is the CT equivalent of the wrong bucky — everything downstream inherits the error, and contrast you’ve given cannot be ungiven.
Quick recall — 5 questions before you leave this tab
1. What two jobs does the topogram do?
Plans the scan range and feeds the attenuation model for tube-current modulation.
2. Portal venous phase: typical delay and what should visibly enhance?
~65–80 s; liver parenchyma at peak, portal and hepatic veins opacified, bowel wall enhancing.
3. Where does the tracking ROI go for a CTPA, and a typical trigger threshold?
Pulmonary trunk; commonly ~100 HU above baseline (protocol-dependent).
4. When would you prefer a test bolus over tracking?
When individual circulation timing is critical or unreliable — e.g., cardiac CTA, severe cardiac failure.
5. Request says “CT abdomen”; indication is renal colic. Which protocol?
Non-contrast low-dose CT KUB — the indication, not the wording, chooses the protocol; confirm with the radiologist if the request conflicts.
5 · Patient Prep & Positioning
Most CT disasters are decided before the gantry ever spins: the cannula that can’t take 5 mL/s, the eGFR nobody checked, the arms left by the sides, the breath-hold never rehearsed. Preparation is where a good CT radiographer earns their keep.
IV access for power injection
- Site and gauge to match the flow rate: 18–20G in a large antecubital vein for 4–5 mL/s (CTPA, CTA); 20G comfortably runs ~3 mL/s for routine portal-venous work; 22G is a compromise (≤~2–3 mL/s per local policy). Hand veins and tiny cannulas are last resorts with reduced flow rates.
- Test the cannula before connecting the injector: brisk saline flush, no pain, no swelling, good flashback history. A cannula that “just about” flushes will fail at 5 mL/s.
- Central lines and ports: only power-inject through devices explicitly rated “power injectable” (check the device, lumen labelling and local policy) at the rated flow. PICC ratings are printed on the hub.
- Programme the injector per protocol: contrast volume + flow rate + saline chaser (chaser pushes the tail of the bolus through, tightens timing, saves ~10–20 mL of contrast and clears the injection arm veins).
- During injection, watch the patient and the pressure curve — most injectors plot pressure; a spike + patient pain = stop.
Contrast screening — the interview you’ll run 30 times a day
- Renal function: check eGFR per local policy (typically required in patients with known renal disease, diabetes, or other risk factors — many departments use a screening questionnaire to decide who needs a recent result). Common decision points: eGFR ≥45 proceed normally; 30–44 consider volume/alternatives per policy; <30 discuss with the radiologist — the risk of post-contrast acute kidney injury is weighed against clinical need, and modern guidance (ACR/ESUR/RANZCR) is far less restrictive than older habit, especially for IV (vs arterial) contrast.
- Metformin: current practice — continue normally if eGFR ≥30 and no AKI; withhold at the time of contrast and for 48 h (with renal re-check) if eGFR <30, AKI, or arterial procedures per policy. Know your local rule.
- Previous contrast reaction: what agent, what reaction, how treated? Prior moderate/severe reaction → radiologist decision: alternative test, different agent, premedication per policy, and scanning with the team prepared.
- Asthma and significant atopy raise reaction risk modestly — document, don’t cancel; severe uncontrolled asthma warrants discussion.
- Pregnancy and breastfeeding: iodinated contrast in pregnancy is given when justified; breastfeeding may continue normally (agent excretion in milk is negligible — reassure per guidance).
- Thyroid: iodinated contrast can affect thyrotoxic patients and interferes with radioiodine therapy/imaging for weeks — flag known severe hyperthyroidism and upcoming radioiodine treatment.
Recognising contrast reactions
| Grade | Looks like | Your move |
|---|---|---|
| Physiological | Warmth, flushing, metallic taste, transient nausea | Reassure — warn patients beforehand so it doesn’t frighten them |
| Mild allergic-like | Limited urticaria, itch, mild nasal congestion | Observe, keep IV access, inform radiologist; usually no treatment or antihistamine |
| Moderate | Diffuse urticaria, facial swelling without airway threat, bronchospasm responding to treatment | Call the radiologist now, monitor obs, oxygen/treatment per protocol |
| Severe | Laryngeal oedema, severe bronchospasm, hypotension + tachycardia, anaphylaxis | Emergency call. IM adrenaline per protocol. Airway, oxygen, legs up, fluids. Never leave the patient |
Know where the emergency drugs and the call button are in your room before your first contrast injection — not during your first reaction. Most severe reactions begin within 20 minutes of injection: that is what the observation period is for.
Positioning — small choices, big physics
- Isocentre, every patient. Mid-coronal plane on the horizontal laser, midline on the sagittal laser. Off-centring degrades the bowtie filter’s dose shaping, corrupts AEC size estimation, raises surface dose and noise. It is the least glamorous, highest-yield habit in CT.
- Arms up for chest and abdomen (above the head on the arm rest): arms by the sides throw beam-hardening and photon-starvation streaks across the very organs you’re imaging and inflate dose via AEC. Arms down only when injury or compliance forces it — then expect and explain the artifact.
- Arms down (by the sides) for head and neck; shoulders pulled gently caudad for the lower neck if achievable.
- Head first vs feet first, supine vs prone: per protocol — record it correctly on the console; laterality and reformats depend on it being true.
- Remove metal from the scan range before the patient is on the table: necklaces, bras, piercings, dentures (for facial/neck work), hearing aids, ECG dots where allowed. The topogram will show you what you missed.
- Comfort = stillness. Knee bolster for supine comfort, warm blanket, clear time estimate (“about ten minutes, two breath-holds”). An uncomfortable patient moves; a frightened patient breathes.
Breath-hold coaching — the 60-second investment
- Explain first: “You’ll hear the machine say ‘breathe in and hold your breath’. Take a gentle, comfortable breath in — not the biggest breath of your life — and hold still until it says breathe.”
- Rehearse once on the couch before scanning. You learn their compliance, they learn the rhythm, and you time how long they can genuinely hold.
- Same inspiratory depth every phase — a different breath between phases shifts the organs and ruins comparisons and subtraction.
- For CTPA specifically: a gentle inspiration or even a relaxed breath-hold at rest — a huge gasp (Valsalva) sucks unopacified IVC blood into the right heart and can wreck the PA opacification (Tab 6).
- Can’t hold? Shallow quiet breathing plus the fastest available acquisition beats a failed hold; tell the radiologist and note it on the study.
Quick recall — 6 questions before you leave this tab
1. Minimum sensible access for a 5 mL/s CTPA injection?
18–20G in a large antecubital vein, flushing briskly and painlessly — and a power-rated line if using a central device.
2. eGFR is 27 and the referrer wants a contrast-enhanced abdomen. Your move?
Don’t inject; discuss with the radiologist — risk/benefit decision, possible alternatives, hydration, or proceed with documentation if clinically essential.
3. What does the saline chaser achieve?
Pushes the contrast tail through, tightens the bolus (better timing), saves contrast volume, clears arm veins (less streak at the SVC).
4. Why do off-centre patients get more dose and more noise?
The bowtie filter and AEC assume the patient is at isocentre; off-centre, the size estimate and dose shaping are both wrong.
5. First three actions in extravasation?
Stop the injection, assess the site (keep cannula initially per policy), elevate the limb — then document, inform, aftercare, and escalate if severe features.
6. Why must every phase use the same inspiratory depth?
Different breaths displace the diaphragm and organs — lesions “move” between phases and comparisons fail.