Imaging physics is concept-heavy and high-return: a small set of well-drilled principles across CT, MRI, ultrasound, and radiation safety converts reliably into marks. These free samples from the RadioQBank published set come with full explanations and exam pearls.
Question 1 · CBCT · easy
Which of the following correctly describes the CBCT limitation known as 'cone beam artefact' or 'cone angle effect'?
- a. Streaks from metallic implants caused by beam hardening
- b. Image degradation and data inconsistency in the periphery of the reconstructed volume due to the wide cone angle
- c. Signal loss in the centre of the image from overexposure
- d. Blurring of edges caused by patient motion during the gantry rotation
Answer: B. Cone beam artefact arises because CBCT uses a wide cone angle to irradiate a large volume simultaneously. The Feldkamp (FDK) reconstruction algorithm used for CBCT assumes a narrow fan beam — at wide cone angles (>10–15°), this assumption breaks down, causing data inconsistency and image artefacts (shading, distortion) in the peripheral slices of the reconstructed volume, away from the central plane.
Exam pearl: Cone beam artefact = FDK algorithm fails at wide cone angles → peripheral volume degradation; central plane is most accurate in CBCT.
Radiographics 2017 CBCT in IR; Bushberg: Essential Physics of Medical Imaging 3rd ed
Question 2 · Fluoroscopy · easy
All of the following are effective dose reduction strategies in fluoroscopy-guided interventional procedures EXCEPT:
- a. Using electronic magnification instead of geometric magnification
- b. Avoiding steep angulation of the X-ray beam
- c. Positioning the image intensifier/flat panel close to the patient
- d. Routinely acquiring DSA runs instead of using fluoroscopy store cine clips
Answer: D. DSA (Digital Subtraction Angiography) acquisition delivers substantially higher dose than fluoroscopy — typically 5–10 times more dose per frame — because it requires higher mAs to produce low-noise images for subtraction. Routinely acquiring DSA runs when fluoroscopy store cine clips would suffice significantly increases patient dose. DSA should be reserved for clinically necessary diagnostic runs, not used as a default.
Exam pearl: DSA = 5–10× more dose than live fluoroscopy per frame — use sparingly; prefer fluoroscopy store cine for procedural documentation over routine DSA runs.
Kandarpa IR Handbook 5th ed; IAEA Radiation Safety in IR; Radiographics 2012
Question 3 · Ultrasound Artefacts · moderate
A patient undergoing ultrasound examination shows a hyperechoic linear structure with posterior acoustic shadowing and a "ring-down" artifact deep to it. This combination of artifacts is most likely caused by:
- a. Gas bubble (dirty shadowing)
- b. Calculus (clean shadowing)
- c. Foreign body
- d. Reverberation from bowel gas
Answer: A. The combination of a hyperechoic structure with both posterior acoustic shadowing and ring-down artifact is characteristic of gas. Gas produces "dirty" acoustic shadowing — posterior to the gas bubble, the shadow contains low-level echoes and reverberation artifacts (ring-down or comet-tail), unlike the "clean" shadow behind a calculus. The ring-down artifact occurs due to resonance of gas bubbles trapped between fluid layers, producing continuous emission of sound. Clinically, this combination raises concern for pneumobilia, emphysematous infection, or bowel gas.
Exam pearl: Clean shadow = calculus/bone. Dirty shadow + ring-down = gas. Posterior enhancement = fluid/cyst. Edge shadow = curved surface.
Grainger & Allison, 7th ed, Ch 3; RadioGraphics 2009;29(4):1179-1189
Question 4 · CT Physics · hard
Spectral CT with photon-counting detectors (PCD-CT) represents the next generation beyond DECT. What is the fundamental advantage of PCD-CT over conventional energy-integrating detectors (EID)?
- a. PCD eliminates the need for X-ray tubes by using electronic excitation
- b. PCD counts individual photons and records their energy — eliminating electronic noise and enabling multi-energy bins simultaneously
- c. PCD uses lower X-ray energies than EID, reducing patient dose
- d. PCD enables real-time 3D reconstruction without post-processing
Answer: B. Conventional energy-integrating detectors (EID) convert X-rays to light via a scintillator and sum all deposited energy — they do not discriminate photon energies and add electronic noise to signal. Photon-counting detectors (PCD, e.g., CZT or CdTe semiconductors) directly convert each X-ray photon to an electrical pulse proportional to photon energy, count individual photons, and can sort them into multiple energy bins simultaneously. Benefits: (1) eliminates electronic noise → improves SNR especially at low dose, (2) full spectral information in single acquisition, (3) higher spatial resolution (smaller pixels), (4) reduced dose per image quality, (5) K-edge imaging of contrast agents.
Exam pearl: PCD-CT: counts individual photons + energy bins; eliminates electronic noise; Siemens NAEOTOM Alpha = first clinical PCD-CT (2021); enables ultra-high-resolution and multi-material decomposition.
Radiographics 2022;42:1815; Leng S et al. Radiology 2019;290:170
Question 5 · MRI Physics · hard
In MRI, what is 'ZIP' (Zero-filling Interpolation Processing) primarily used for?
- a. Reducing motion artefact by retrospective correction of k-space data
- b. Creating the appearance of higher spatial resolution by adding zeros to the periphery of k-space before Fourier transformation
- c. Reducing T2* decay by mathematical correction of peripheral k-space
- d. Suppressing fat signal by interpolating out-of-phase echoes
Answer: B. Zero-filling interpolation (ZIP) pads the peripheral (high spatial frequency) regions of k-space with zeros before Fourier transformation. This effectively increases the apparent matrix size (interpolated spatial resolution) without acquiring additional data. The result is smoother-appearing images with reduced pixelation (pixelation/blocky appearance is reduced) — particularly useful for thin-slab 3D acquisitions viewed in reformation planes. Important caveat: ZIP increases apparent resolution and reduces aliasing in reformatted images but does NOT add true spatial information (no new high-frequency data is acquired — just interpolated).
Exam pearl: Zero-filling (ZIP): zeros in peripheral k-space → apparent smoother image; interpolated resolution (NOT true resolution improvement).
Grainger & Allison 7th ed; Westbrook C Handbook of MRI Technique 4th ed