Computed Tomography Test Computed Tomography (CT) 4 — Questions and Answers
Question 1: What is the recommended contrast injection rate for a CT pulmonary angiography (CTPA) exam?
- 0.5–1 mL/s
- 1–2 mL/s
- 3–5 mL/s (Correct answer)
- 8–10 mL/s
Correct answer: 3–5 mL/s
CTPA typically requires a brisk injection rate of 3–5 mL/s to achieve adequate opacification of the pulmonary arteries within the narrow bolus window.
Question 2: Which CT scan parameter, when increased, reduces partial volume averaging artifact?
- Increasing kVp
- Decreasing slice thickness (Correct answer)
- Increasing pitch
- Increasing reconstruction interval
Correct answer: Decreasing slice thickness
Thinner slices reduce the volume of tissue averaged within each voxel, minimizing partial volume artifact and improving detection of small lesions.
Question 3: A ring artifact in CT is most commonly caused by:
- Patient motion during scanning
- Malfunction of one or more detector elements (Correct answer)
- High-density metallic implants
- Incorrect window/level settings
Correct answer: Malfunction of one or more detector elements
Ring artifacts result from a miscalibrated or malfunctioning detector element that consistently misreads attenuation values, creating a characteristic ring or arc pattern.
Question 4: In CT colonography (virtual colonoscopy), what is the standard patient preparation requirement?
- IV contrast injection only
- Bowel cleansing and colonic insufflation with CO₂ or air (Correct answer)
- Drinking 1 liter of barium suspension
- Fasting for 2 hours before the exam
Correct answer: Bowel cleansing and colonic insufflation with CO₂ or air
CT colonography requires bowel prep to clear stool and distension of the colon with gas to allow detection of polyps and mucosal lesions.
Question 5: What is the formula for CT number (Hounsfield unit)?
- HU = (μ_tissue / μ_water) × 1000
- HU = (μ_tissue − μ_water) / μ_water × 1000 (Correct answer)
- HU = (μ_water − μ_tissue) × 1000
- HU = μ_tissue × 1000
Correct answer: HU = (μ_tissue − μ_water) / μ_water × 1000
The Hounsfield unit is calculated as the difference between tissue and water attenuation divided by water attenuation, multiplied by 1000, placing water at 0 HU.
Question 6: Which post-processing technique in CT creates a three-dimensional display by showing only the highest attenuation voxel along each line of sight?
- Multiplanar reformation (MPR)
- Minimum intensity projection (MinIP)
- Maximum intensity projection (MIP) (Correct answer)
- Volume rendering (VR)
Correct answer: Maximum intensity projection (MIP)
Maximum intensity projection (MIP) projects the highest HU value along each ray through the data, making it ideal for visualizing contrast-enhanced vessels and calcifications.
Question 7: What is the significance of the CT dose index (CTDI) metric?
- It measures the effective dose to the patient in millisieverts
- It quantifies the radiation output of a CT scanner for a standardized scan protocol (Correct answer)
- It calculates the contrast-to-noise ratio of an image
- It measures the absorbed dose to the skin surface only
Correct answer: It quantifies the radiation output of a CT scanner for a standardized scan protocol
CTDI measures the scanner's radiation output in a standardized phantom and is used to characterize and compare dose output across protocols and scanners.
Computed Tomography Test Computed Tomography (CT) 4
What is the recommended contrast injection rate for a CT pulmonary angiography (CTPA) exam?