Correct answer: B — 10 mAs

The 15% rule states that a 15% increase in kVp roughly doubles receptor exposure, so to keep exposure constant the mAs must be halved: 20 → 10 mAs. This trade lowers patient dose (fewer photons needed, more of them penetrate) at the cost of reduced subject contrast. Option A would be correct only if kVp had been raised by ~30% (two applications of the rule). Option C (a 30% reduction) confuses this with the smaller adjustments used for visible density change on film. Options D and E would over-expose the receptor — E doubles output on top of a doubled beam penetration, quadrupling exposure.

Source: Bushong, Radiologic Science for Technologists (12th ed.), exposure technique factors; Carlton & Adler, Principles of Radiographic Imaging.

Correct answer: B — cathode over the lower (thicker) thoracic spine

Beam intensity is greater at the cathode end of the field because photons exiting toward the anode side are attenuated by the “heel” of the angled anode itself. Placing the more intense cathode end over the thicker lower thorax (abdomen side) and the weaker anode end over the thinner upper thorax evens out receptor exposure along the spine. Option A is exactly backwards. Option C is wrong in both directions — the heel effect is more pronounced at short SID and with large fields, which is why it matters for a 35 × 43 cm spine field. Option D misstates the physics: the anode side is less intense, not more penetrating. Option E states the inverse of the correct pairing.

Source: Bushong, Radiologic Science for Technologists, the anode heel effect; Bontrager & Lampignano, Textbook of Radiographic Positioning.

Correct answer: C — entirely bremsstrahlung below ~69.5 kVp

Tungsten’s K-shell binding energy is about 69.5 keV, so no projectile electron in a 60 kVp beam has enough energy to eject a K-shell electron — no useful K-characteristic X-rays can be produced, and the beam is effectively all bremsstrahlung. (L-characteristic photons at ~8–12 keV are produced but are absorbed by filtration and contribute nothing useful.) Option A is wrong at any diagnostic kVp: even at 80–120 kVp, characteristic radiation contributes only a minority of the beam. Option B overstates the characteristic fraction. Option D reverses reality — filtration removes low-energy photons, not bremsstrahlung as a class. Option E would be the correct description for a beam above ~70 kVp, which is exactly why the stem sets the exposure at 60 kVp.

Source: Bushberg et al., The Essential Physics of Medical Imaging (4th ed.), X-ray production.

Correct answer: B — 2.5 mm Al equivalent, to remove low-energy photons

Regulatory standards (e.g. IEC 60601-1-3, FDA 21 CFR 1020.30) require a minimum total filtration of 2.5 mm aluminium equivalent for tubes operating above 70 kVp. Its purpose is purely dose protection: photons below ~30 keV are almost entirely absorbed in the patient’s skin and superficial tissue, contributing dose but no image. Option A quotes the wrong thickness and the wrong mechanism — filtration acts on the primary beam before the patient; grids and collimation deal with scatter. Option C confuses the aluminium standard with copper added filtration used in some paediatric/fluoroscopy settings. Option D describes apron/leakage shielding requirements, not beam filtration. Option E is backwards: filtration hardens the beam (raises mean energy) and if anything slightly reduces subject contrast.

Source: IEC 60601-1-3; Bushberg et al., The Essential Physics of Medical Imaging, beam filtration.

Correct answer: B — approximately 0.13 mm

Geometric unsharpness = focal spot size × (OID ÷ SOD). Here the source-to-object distance is 100 − 10 = 90 cm, so Ug = 1.2 × (10/90) ≈ 0.13 mm. The formula shows why unsharpness is minimised by a small focal spot, long SID and anatomy close to the receptor. Option A results from dividing by SID² or misplacing a decimal. Option C would arise from using OID/SID = 10/100 with a 6 mm focal spot, or halving incorrectly. Option D simply repeats the focal spot size, forgetting the geometry entirely — the penumbra only equals the focal spot size when OID equals SOD. Option E multiplies instead of taking the ratio.

Source: Bushong, Radiologic Science for Technologists, image quality and geometric factors.

Correct answer: B — 1 µGy

Radiation intensity from a point-like source falls with the square of distance: doubling the distance from 1 m to 2 m reduces intensity by a factor of 2² = 4, so 4 µGy → 1 µGy. This is why “take two steps back” is the single most effective protection action in mobile radiography. Option A would require the distance to increase by a factor of √8. Option C assumes a linear (1/d) fall-off, the most commonly chosen wrong answer. Option D assumes no change, and Option E inverts the law. Note the stem says “ignoring attenuation” — in reality air attenuation is negligible at these distances, so the inverse square estimate is realistic for scatter treated as a point source at the patient.

Source: Bushberg et al., The Essential Physics of Medical Imaging; ICRP 103 protection principles (time, distance, shielding).

Correct answer: C — roughly 35 times (≈0.7 mSv vs ≈0.02 mSv)

Typical adult effective doses are ~0.02 mSv for a PA chest and ~0.7 mSv for an AP abdominal radiograph — a ratio of about 35. The difference is driven by thicker tissue requiring far more mAs, lower kVp techniques, and the radiosensitive abdominal/pelvic organs in the primary beam. Option A wrongly generalises the famously low chest dose to all plain films. Option B underestimates the gap by an order of magnitude. Option D quotes a value in CT territory — 7–10 mSv is a typical abdominal CT, not a radiograph, a classic examiner trap. Option E misapplies tissue weighting: although lung has a high weighting factor, the chest technique deposits so little energy that the effective dose remains tiny.

Source: Mettler et al., Radiology 2008 (effective dose catalogue); Health Physics Society, “Doses from Medical X-Ray Procedures.”

Correct answer: C — approximately double the target exposure

DI = 10 × log₁₀(actual ÷ target exposure index), so each +1 step is a ~26% increase and +3.0 corresponds to 10^0.3 ≈ 2.0 times the target. TG-116 proposed a target range of −0.5 to +0.5 with action limits at ±1; a DI of +3 sits well outside this and signals systematic overexposure. Crucially, the overexposed digital image usually looks good (low noise) — “dose creep” — so the correct response is technique review, not automatic repeat. Option A misreads the logarithmic scale as percent. Option B confuses the DI value with a linear multiplier (DI +3 ≠ 3×; three times target would be DI ≈ +4.8). Option D describes DI −3. Option E is false — DI is deliberately kVp- and vendor-independent, which is its whole point.

Source: AAPM Report 116 (TG-116), “An Exposure Indicator for Digital Radiography”; AAPM TG-232 (Dave et al., Med Phys 2018).

Correct answer: B — remove the grid and reduce the mAs

A small child produces little scatter, so the grid’s contrast benefit is minimal while its dose cost is large: a 12:1 grid demands roughly five times the mAs of a non-grid exposure (Bucky factor ≈ 5). Removing it allows an ~80% dose reduction — by far the biggest single saving available, and standard paediatric practice for small body parts. Option A slightly reduces entrance skin dose via geometry but by only a few percent once mAs is adjusted. Option C shields tissue outside the primary beam, which receives mainly internal scatter that an apron cannot block; it does not change the imaged-region dose. Option D increases dose — lower kVp with doubled mAs deposits more energy in the patient. Option E affects sharpness only, not dose.

Source: Image Gently campaign (paediatric digital radiography); Bushong, Radiologic Science for Technologists, grids.

Correct answer: B — deterministic, threshold-based, severity scales with dose

Skin erythema is the textbook deterministic effect (ICRP now prefers “tissue reaction”): it appears only above a threshold absorbed dose of roughly 2 Gy to the skin, and its severity — from transient reddening through to necrosis — worsens as dose rises. Deterministic effects reflect bulk cell killing, which is why prolonged fluoroscopy is the classic diagnostic-imaging setting for them. Option A describes cancer induction, where probability (not severity) scales with dose under the no-threshold model. Option C is an incoherent hybrid — stochastic effects have no threshold and their severity is independent of dose. Option D describes heritable effects, which are stochastic and have never been demonstrated in humans at diagnostic doses. Option E swaps the definitions: for deterministic effects it is severity, not probability alone, that tracks dose.

Source: ICRP Publication 103; ICRP 118 (tissue reactions, skin thresholds).

Correct answer: D — 60 mAs

Standard grid conversion (Bucky) factors are: no grid ×1, 5:1 ×2, 6:1 ×3, 8:1 ×4, 12:1 ×5, 16:1 ×6. Moving from no grid to a 12:1 grid therefore requires 12 × 5 = 60 mAs. The grid absorbs both scatter and a large fraction of primary photons, so the technique must compensate — this arithmetic is also why grid removal is such a powerful paediatric dose-saving tool (see Question 9). Options A, B and C apply the factors for 5:1, 6:1 and 8:1 grids respectively — the classic error is using ×4 (the 8:1 factor) for a 12:1 grid. Option E applies the 16:1 factor.

Source: Carlton & Adler, Principles of Radiographic Imaging; Bushong, Radiologic Science for Technologists, grid conversion factors.

Correct answer: B — asymmetric medial clavicle ends

Rotation is assessed by comparing the medial (sternal) ends of the clavicles with the spinous process of the vertebral body between them: on a true PA projection they lie equidistant. Rotation shifts one clavicle closer to the midline than the other, and it also spuriously alters the apparent size of the heart and the relative density of the two lungs — which is why examiners care. Option A is the criterion for inadequate inspiration, not rotation (aim for 9–10 posterior ribs). Option C indicates the shoulders were not rolled forward, a positioning fault but not rotation. Option D indicates motion or breathing during exposure. Option E is a collimation/field-coverage fault.

Source: Bontrager & Lampignano, Textbook of Radiographic Positioning and Related Anatomy, chest evaluation criteria.

Correct answer: C — swimmer’s lateral of the cervicothoracic junction

A trauma lateral cervical spine is inadequate unless the C7–T1 junction is demonstrated — a substantial proportion of cervical injuries occur at the cervicothoracic junction, and it is the classic site of the “missed fracture.” The swimmer’s projection elevates one arm and depresses the other, separating the shoulder shadows so the junction can be seen. Option A is dangerously wrong for exactly this reason. Option B rarely succeeds through two shoulders and stacks repeat dose on a trauma patient. Option D — obliques show the intervertebral foramina and facet alignment but do not solve the junction-coverage problem. Option E skips the fast, available answer; in many trauma pathways CT is the actual escalation, but among these options the swimmer’s view is the established radiographic solution.

Source: Bontrager & Lampignano, trauma cervical spine; RCR/trauma imaging guidance on cervicothoracic junction coverage.

Correct answer: B — ulnar deviation (with cephalad/scaphoid-directed angulation)

Ulnar deviation swings the distal carpal row ulnarward, rotating the scaphoid so its long axis lies closer to parallel with the receptor — the bone is elongated, foreshortening is removed, and the fracture line (typically at the waist) opens up. A proximally angled central ray (or elevating the fist, as in the Ziter view) enhances this further. Option A does the opposite: radial deviation foreshortens the scaphoid and demonstrates the ulnar-side carpals. Option C describes an AP forearm position and rotates the wrist off true PA. Option D (flexion) superimposes the carpal rows. Option E has no role in scaphoid imaging. Remember that a normal initial series does not exclude a scaphoid fracture — clinical suspicion mandates immobilisation and follow-up imaging.

Source: Bontrager & Lampignano, wrist and scaphoid projections; Clark’s Positioning in Radiography.

Correct answer: E — 12.5 cm

Magnification factor M = SID ÷ SOD. The source-to-object distance is 100 − 20 = 80 cm, so M = 100/80 = 1.25 and the image measures 10 × 1.25 = 12.5 cm. This same geometry is why chest radiography uses 180 cm SID — pushing SID up drives M toward 1 and keeps the cardiothoracic ratio honest. Option A assumes no magnification, which would require the object to be in contact with the receptor. Option C results from the common error of computing M = SID ÷ (SID − OID) with OID misread as 10, or from using 100/90. Option D uses M = 1.2 by dividing 120/100. Option B has no valid derivation — it is a plausible-looking rounding trap.

Source: Bushong, Radiologic Science for Technologists, magnification and distortion.

Correct answer: B — abnormal; C2 soft tissue should be ≤ ~7 mm

Widely used adult thresholds on the lateral radiograph are roughly 7 mm at C2 and 21–22 mm at C6/C7 — the mnemonic “7 at 2, 2 (cm) at 6.” Above the larynx the prevertebral space contains only the thin retropharyngeal tissues, so 10 mm at C2 implies swelling or haematoma and should prompt a search for occult fracture or ligamentous injury (usually with CT), even when alignment looks normal. Option A quotes no recognised threshold. Option C is a distractor built on a true confounder — crying, swallowing, flexion and expiration can transiently widen the soft tissues in children — but that caveat does not normalise 10 mm at C2 in an adult. Option D misapplies the lower-cervical value to the upper cervical spine, where the normal width is a third of that. Option E is simply false: this measurement is a core plain-film trauma check.

Source: Wholey et al. and subsequent trauma series (e.g. Herr et al., Am J Emerg Med 1998); Raby, Accident & Emergency Radiology: A Survival Guide.

Correct answer: A — lung markings continuing beyond the line

A true pneumothorax displaces the lung medially, so the space peripheral to the visceral pleural line is air only — no vessels can be seen beyond it. A skin fold, by contrast, is a soft-tissue interface superimposed on aerated lung, so vascular markings continue lateral to it; folds also tend to be broad edges (a density gradient that fades) rather than the crisp thin white line of visceral pleura, and they may extend beyond the rib cage. Option B actually favours pneumothorax — the pleural line is thin and sharp, whereas a fold is an edge, not a line. Option C is common to both. Option D only helps exclude tension pneumothorax; most pneumothoraces cause no shift. Option E is unreliable in either direction — when doubt persists, an expiratory or lateral decubitus film, or ultrasound/CT, settles it.

Source: Goodman, Felson’s Principles of Chest Roentgenology.

Correct answer: A — no routine follow-up

Fleischner 2017 recommends no routine follow-up for a single solid nodule under 6 mm in a low-risk patient, because the malignancy risk is well under 1%. This patient — young, never-smoker, no known malignancy, benign nodule morphology — is the definition of low risk. Option B is the recommendation for a solid 6–8 mm nodule in a low-risk patient. Option C describes the pathway for nodules larger than 8 mm. Option D is wrong on modality: Fleischner follow-up is CT-based, since radiographs cannot reliably characterise or measure small nodules. Option E confuses incidental-nodule management with lung cancer screening programmes for high-risk smokers. Also remember the guideline’s scope: it does not apply to patients under 35, immunocompromised patients, or those with known cancer.

Source: MacMahon et al., “Guidelines for Management of Incidental Pulmonary Nodules,” Radiology 2017 (Fleischner Society).

Correct answer: B — moiré artifact from grid–sampling frequency interaction

When a stationary grid’s lead-strip frequency is close to (but not matched with) the detector’s sampling frequency — or the grid lines run parallel to the CR plate-reader scan lines — the two periodic patterns beat against each other, producing the characteristic wavy moiré pattern. The fixes are to use a moving (Bucky) grid, a grid frequency matched to the detector with strips perpendicular to the scan direction, or software grid-line suppression. Option A, grid cut-off, causes a loss of exposure (uniform or one-sided lightening), not a wave pattern. Option C, incomplete erasure, produces a ghost of the previous examination superimposed on the new one. Option D blurs anatomy rather than creating a periodic pattern. Option E, backscatter, classically projects the cassette hinge or electronics as a faint image, not fine waves.

Source: Shetty et al., “Computed Radiography Image Artifacts Revisited,” AJR 2011; Bushberg et al., digital radiography artifacts.

Correct answer: C — erect chest radiograph after time upright

Free intraperitoneal gas rises to lie beneath the diaphragm, where the erect chest projection profiles it as a crescent of lucency — classically able to show volumes of only a few millilitres, provided the patient has been sitting or standing for 5–10 minutes so the gas can migrate. The erect chest also uses a near-horizontal beam through the thinnest tissue at the diaphragm, maximising conspicuity. Option A is the least sensitive common view: on a supine film free gas collects anteriorly and reveals itself only indirectly (Rigler’s sign, falciform ligament visibility), typically needing much larger volumes. Option B puts gas against the anterior abdominal wall, away from any profiled interface. Option D is the correct alternative for patients who cannot sit up — but done “immediately,” without redistribution time, it loses sensitivity, and a left lateral decubitus (right side up) is generally preferred. Option E is not an abdominal gas examination at all. Beware the mimic: Chilaiditi (colonic interposition) shows haustrated bowel, not a clean gas crescent.

Source: Miller & Nelson, “The Roentgenologic Demonstration of Tiny Amounts of Free Intraperitoneal Gas,” AJR 1971; Raby, Accident & Emergency Radiology.