CABT Imaging Systems 3 — Questions and Answers
Question 1: Which nuclear medicine imaging modality provides functional 3D images by detecting pairs of annihilation photons?
- SPECT
- PET (Correct answer)
- Planar scintigraphy
- Gamma camera imaging
Correct answer: PET
PET (Positron Emission Tomography) detects coincident 511 keV annihilation photons produced when a positron meets an electron, enabling 3D functional imaging.
Question 2: In ultrasound imaging, axial resolution is primarily determined by:
- Transducer frequency and pulse length (Correct answer)
- Beam width at the focal zone
- Transducer aperture size
- Depth of the target structure
Correct answer: Transducer frequency and pulse length
Axial resolution (along the beam direction) depends on the spatial pulse length, which is determined by the transducer frequency and the number of cycles per pulse.
Question 3: A CT system's ability to distinguish between two objects of similar density is described by which performance metric?
- Spatial resolution (MTF)
- Low-contrast detectability (Correct answer)
- Noise power spectrum
- Slice sensitivity profile
Correct answer: Low-contrast detectability
Low-contrast detectability (or contrast resolution) describes the system's ability to differentiate structures with small differences in X-ray attenuation.
Question 4: The ALARA principle as applied to ionizing radiation imaging means:
- Always Limit And Reduce Artifacts
- As Low As Reasonably Achievable (Correct answer)
- Automatic Level And Range Adjustment
- Allowable Limit for Absorbed Radiation Amount
Correct answer: As Low As Reasonably Achievable
ALARA (As Low As Reasonably Achievable) is the radiation protection philosophy of minimizing patient and staff dose while still obtaining diagnostic quality images.
Question 5: In MRI, the T2* relaxation time differs from T2 because T2* also accounts for:
- RF pulse imperfections
- Magnetic field inhomogeneities and susceptibility effects (Correct answer)
- Chemical shift between tissues
- Motion of the patient
Correct answer: Magnetic field inhomogeneities and susceptibility effects
T2* (T2-star) is shorter than T2 because it incorporates both true spin-spin relaxation and additional dephasing from local magnetic field inhomogeneities.
Question 6: Which type of X-ray detector directly converts X-ray photons to electrical charge without an intermediate light conversion step?
- Cesium iodide flat-panel detector
- Amorphous selenium flat-panel detector (Correct answer)
- Gadolinium oxysulfide screen-film system
- Computed radiography (CR) imaging plate
Correct answer: Amorphous selenium flat-panel detector
Amorphous selenium (a-Se) flat-panel detectors are direct conversion detectors; X-ray photons liberate charge carriers directly in the selenium layer without producing light.
Question 7: What is the main clinical advantage of SPECT over planar gamma camera imaging?
- Lower radiation dose to the patient
- Elimination of depth-dependent overlapping structures (Correct answer)
- Faster acquisition time
- Use of non-radioactive tracers
Correct answer: Elimination of depth-dependent overlapping structures
SPECT acquires projections from multiple angles to reconstruct 3D tomographic slices, eliminating the superimposition of overlying and underlying structures that limits planar imaging.
CABT Imaging Systems 3
Which nuclear medicine imaging modality provides functional 3D images by detecting pairs of annihilation photons?