Computed Tomography Test Computed Tomography Science 3 — Questions and Answers
Question 1: In CT scanner design, the gantry aperture refers to:
- The opening through which the patient passes (Correct answer)
- The angle subtended by the detector array
- The collimator slit width
- The focal spot size of the X-ray tube
Correct answer: The opening through which the patient passes
The gantry aperture is the circular opening (typically 70–90 cm in diameter) through which the patient or object is positioned for scanning.
Question 2: What is the primary purpose of the bow-tie (wedge) filter in a CT system?
- To increase tube current at the center of the FOV
- To equalize photon flux across the detector array (Correct answer)
- To reduce table speed during acquisition
- To sharpen the penumbra of the X-ray beam
Correct answer: To equalize photon flux across the detector array
The bow-tie filter attenuates the peripheral portion of the beam more than the center, equalizing photon fluence across the detector and reducing patient dose.
Question 3: During a CT scan, tube current modulation (TCM) adjusts the mA based on:
- Patient heart rate
- Patient attenuation in real time (Correct answer)
- Detector read-out speed
- Gantry rotation speed
Correct answer: Patient attenuation in real time
TCM reduces mA in less-attenuating regions (e.g., lungs) and increases it in denser regions (e.g., shoulders), optimizing dose while maintaining image quality.
Question 4: The 'reconstruction kernel' (convolution filter) in CT primarily affects:
- Slice thickness
- Spatial resolution and noise (Correct answer)
- Gantry rotation time
- Contrast injection timing
Correct answer: Spatial resolution and noise
Sharp kernels enhance spatial resolution but increase noise, while smooth kernels reduce noise at the cost of spatial resolution.
Question 5: Which measure quantifies the overall radiation output of a CT scanner independent of scan length?
- DLP (Dose-Length Product)
- CTDI (CT Dose Index) (Correct answer)
- SSDE (Size-Specific Dose Estimate)
- Effective dose
Correct answer: CTDI (CT Dose Index)
CTDI quantifies the radiation output of the CT scanner per rotation, independent of scan length, and is measured in mGy.
Question 6: In iterative reconstruction (IR), image quality is improved compared to FBP primarily by:
- Increasing tube voltage
- Reducing reconstruction matrix size
- Modeling noise and system physics to suppress artifacts (Correct answer)
- Speeding up gantry rotation
Correct answer: Modeling noise and system physics to suppress artifacts
Iterative reconstruction uses repeated cycles that model photon statistics and system geometry to reduce noise and artifacts, often permitting lower-dose acquisitions.
Question 7: Which scanning mode is most appropriate for imaging moving structures such as the heart in CT?
- Sequential (axial) mode with wide detectors
- Low-pitch helical mode
- Prospective ECG-gated axial mode (Correct answer)
- High-pitch helical mode without gating
Correct answer: Prospective ECG-gated axial mode
Prospective ECG-gated axial acquisition triggers the scan at a specific cardiac phase, freezing motion and substantially reducing radiation dose compared to retrospective gating.
Computed Tomography Test Computed Tomography Science 3
In CT scanner design, the gantry aperture refers to: