CABT Imaging Systems 4 — Questions and Answers
Question 1: A patient with a pacemaker is referred for imaging. Which modality poses the greatest safety risk?
- CT with contrast
- Conventional X-ray
- MRI (Correct answer)
- Nuclear medicine bone scan
Correct answer: MRI
The strong static and time-varying magnetic fields in MRI can cause pacemaker malfunction, lead displacement, or device heating, making it the highest-risk modality for most pacemaker patients.
Question 2: In digital subtraction angiography (DSA), the 'mask' image is:
- An image acquired after contrast injection
- A pre-contrast image used to subtract background structures (Correct answer)
- A magnified region of interest image
- A roadmap image overlaid during intervention
Correct answer: A pre-contrast image used to subtract background structures
In DSA, the mask is a pre-contrast fluoroscopic image; subtracting it from post-contrast images eliminates overlying bone and soft tissue, revealing only the contrast-filled vessels.
Question 3: The modulation transfer function (MTF) of an imaging system describes:
- How patient dose varies with image noise
- How spatial frequencies (detail) are preserved from object to image (Correct answer)
- The uniformity of detector response across its area
- The relationship between kVp and image contrast
Correct answer: How spatial frequencies (detail) are preserved from object to image
The MTF quantifies what fraction of the true contrast is preserved for each spatial frequency, characterizing the system's spatial resolution performance.
Question 4: Which parameter most directly controls image noise in a CT acquisition?
- Gantry rotation speed
- Tube current-time product (mAs) (Correct answer)
- Reconstruction filter (kernel)
- Detector collimation width
Correct answer: Tube current-time product (mAs)
mAs (milliampere-seconds) determines the number of X-ray photons reaching the detector; higher mAs reduces quantum noise while increasing patient dose.
Question 5: In echocardiography, the Doppler technique used to measure high-velocity blood flow across a stenotic valve is:
- Color flow Doppler
- Pulsed-wave (PW) Doppler
- Continuous-wave (CW) Doppler (Correct answer)
- Tissue Doppler imaging (TDI)
Correct answer: Continuous-wave (CW) Doppler
Continuous-wave Doppler has no aliasing limit and can accurately measure the high velocities (>2 m/s) encountered across stenotic cardiac valves, unlike pulsed-wave Doppler.
Question 6: A biomedical technologist is performing a daily quality control check on a diagnostic ultrasound system using a tissue-equivalent phantom. Which parameter is being tested when measuring the depth of the last resolvable reflector?
- Axial resolution
- Lateral resolution
- Penetration depth (sensitivity) (Correct answer)
- Dead zone thickness
Correct answer: Penetration depth (sensitivity)
Penetration depth (maximum depth of visualization) is assessed by identifying the deepest reflector that can be consistently resolved, indicating the system's overall sensitivity.
Question 7: The half-value layer (HVL) of an X-ray beam is defined as the thickness of material that reduces beam intensity by:
- 25%
- 50% (Correct answer)
- 75%
- 90%
Correct answer: 50%
The HVL is the thickness of a specified attenuator (typically aluminum) that reduces the X-ray beam intensity to exactly half (50%) of its original value.
CABT Imaging Systems 4
A patient with a pacemaker is referred for imaging.
Which modality poses the greatest safety risk?