Computed Tomography Test Computed Tomography Ultimate 2 — Questions and Answers
Question 1: What is the primary purpose of retrospective gating in cardiac CT?
- To reduce radiation dose
- To reconstruct images at any phase of the cardiac cycle (Correct answer)
- To eliminate the need for beta-blockers
- To increase spatial resolution
Correct answer: To reconstruct images at any phase of the cardiac cycle
Retrospective gating records ECG data throughout the scan and allows reconstruction at any desired cardiac phase after acquisition.
Question 2: Which artifact appears as a dark band between two high-density objects on CT images?
- Ring artifact
- Beam hardening artifact (Correct answer)
- Motion artifact
- Partial volume artifact
Correct answer: Beam hardening artifact
Beam hardening causes dark streaks or bands between dense objects like bone or metal because lower-energy photons are preferentially absorbed.
Question 3: What does a CT number of -1000 HU represent?
- Bone cortex
- Water
- Air (Correct answer)
- Fat
Correct answer: Air
Air has a CT attenuation value of approximately -1000 Hounsfield Units, the lowest end of the standard CT scale.
Question 4: In CT, what is the 'matrix size' most commonly used in clinical scanners?
- 128 x 128
- 256 x 256
- 512 x 512 (Correct answer)
- 1024 x 1024
Correct answer: 512 x 512
Clinical CT scanners most commonly use a 512 x 512 matrix, balancing spatial resolution with acceptable processing time and storage requirements.
Question 5: What is the main advantage of using iterative reconstruction over filtered back projection (FBP)?
- Faster reconstruction speed
- Reduced image noise at lower radiation doses (Correct answer)
- Improved temporal resolution
- Elimination of all motion artifacts
Correct answer: Reduced image noise at lower radiation doses
Iterative reconstruction algorithms reduce noise more effectively than FBP, enabling diagnostic-quality images at significantly lower radiation doses.
Question 6: Which organ is typically used as a reference when setting window level for abdominal CT?
- Kidney
- Spleen
- Liver (Correct answer)
- Pancreas
Correct answer: Liver
The liver is commonly used as the window level reference in abdominal CT because it has a consistent, predictable attenuation value of approximately 50-70 HU.
Question 7: What is the term for the minimum detectable difference in attenuation between two adjacent structures in CT?
- Spatial resolution
- Temporal resolution
- Contrast resolution (Correct answer)
- Noise equivalent quanta
Correct answer: Contrast resolution
Contrast resolution refers to the ability to distinguish between tissues with similar attenuation values, and it is primarily affected by noise and radiation dose.
Computed Tomography Test Computed Tomography Ultimate 2
What is the primary purpose of retrospective gating in cardiac CT?