ARRT CT Physics 3 — Questions and Answers
Question 1: In CT, 'effective mAs' (mAs/pitch) is important because:
- It determines the field of view
- It remains constant regardless of pitch, maintaining image noise consistency (Correct answer)
- It controls the display window
- It sets the rotation time only
Correct answer: It remains constant regardless of pitch, maintaining image noise consistency
Effective mAs = mAs ÷ pitch; keeping this constant as pitch changes maintains consistent image noise regardless of table speed.
Question 2: What is the effect of decreasing detector collimation (thinner slices) on image noise, assuming all other factors remain constant?
- Noise decreases
- Noise is unaffected
- Noise increases (Correct answer)
- Noise is eliminated
Correct answer: Noise increases
Thinner collimation means fewer photons contribute to each voxel, increasing statistical noise in the image.
Question 3: The Hounsfield unit (HU) value of pure water is defined as:
- -1000 HU
- 0 HU (Correct answer)
- +1000 HU
- +500 HU
Correct answer: 0 HU
The CT number scale defines water as 0 HU and air as −1000 HU, with other tissues placed relative to these references.
Question 4: Which reconstruction kernel (filter) should be selected to best visualize lung parenchyma and fine pulmonary detail?
- Soft tissue (smoothing) kernel
- High-resolution (sharp/edge-enhancing) kernel (Correct answer)
- Brain kernel
- Liver kernel
Correct answer: High-resolution (sharp/edge-enhancing) kernel
A high-resolution or edge-enhancing kernel accentuates fine structural detail needed for lung parenchyma evaluation, at the cost of increased noise.
Question 5: In CT dose reporting, CTDI_vol represents:
- Dose at a single point in the patient
- Volume-averaged radiation dose within the scan volume (Correct answer)
- Total energy deposited in the room
- Dose only at the center of the phantom
Correct answer: Volume-averaged radiation dose within the scan volume
CTDI_vol is the volume CT dose index, representing average dose within the irradiated volume; it accounts for pitch in helical scanning.
Question 6: Which of the following correctly describes 'isotropic voxels' in CT?
- Voxels with equal x, y, and z dimensions (Correct answer)
- Voxels that contain only one tissue type
- Voxels with a z-dimension larger than x and y
- Voxels calibrated to HU = 0
Correct answer: Voxels with equal x, y, and z dimensions
Isotropic voxels have equal dimensions in all three planes, enabling high-quality multiplanar reformations without resolution loss.
Question 7: The primary advantage of iterative reconstruction over filtered back projection (FBP) in CT is:
- Faster image processing time
- Reduced image noise at equivalent or lower dose (Correct answer)
- Elimination of all beam hardening artifacts
- Larger field of view capability
Correct answer: Reduced image noise at equivalent or lower dose
Iterative reconstruction algorithms reduce noise by modeling the imaging system and iteratively comparing projection data, allowing lower-dose acquisitions.
ARRT CT Physics 3
In CT, 'effective mAs' (mAs/pitch) is important because: