CABT Imaging Systems 2 — Questions and Answers
Question 1: Which artifact in MRI is caused by the difference in resonant frequencies between water and fat protons?
- Chemical shift artifact (Correct answer)
- Gibbs ringing artifact
- Motion artifact
- Susceptibility artifact
Correct answer: Chemical shift artifact
Chemical shift artifact occurs because water and fat protons precess at slightly different frequencies, causing a spatial misregistration at tissue interfaces.
Question 2: In computed tomography, the Hounsfield unit (HU) value for air is approximately:
- 0
- -1000 (Correct answer)
- +1000
- -500
Correct answer: -1000
Air is assigned a Hounsfield unit value of -1000 on the CT attenuation scale, while water is defined as 0 HU.
Question 3: A biomedical technologist notices that an ultrasound image shows a bright echo directly behind a highly reflective structure. This is best described as:
- Acoustic shadowing
- Posterior acoustic enhancement
- Reverberation artifact (Correct answer)
- Mirror image artifact
Correct answer: Reverberation artifact
Reverberation artifact appears as multiple equally spaced bright lines behind a strongly reflective interface due to sound bouncing back and forth.
Question 4: What is the primary purpose of a grid in diagnostic radiography?
- To increase patient dose
- To reduce scattered radiation reaching the detector (Correct answer)
- To collimate the primary beam
- To filter low-energy photons from the tube
Correct answer: To reduce scattered radiation reaching the detector
A radiographic grid is placed between the patient and detector to absorb scattered radiation, improving image contrast.
Question 5: In fluoroscopy, the automatic brightness control (ABC) system primarily adjusts which parameters to maintain image quality?
- Only kVp
- Only mA
- kVp, mA, and pulse width (Correct answer)
- Only detector gain
Correct answer: kVp, mA, and pulse width
ABC systems dynamically adjust kVp, mA, and pulse width to maintain consistent image brightness as the beam passes through varying tissue thicknesses.
Question 6: Which MRI sequence is most sensitive for detecting acute cerebral ischemia within the first few hours?
- T1-weighted imaging
- T2-weighted imaging
- Diffusion-weighted imaging (DWI) (Correct answer)
- FLAIR
Correct answer: Diffusion-weighted imaging (DWI)
DWI detects restricted diffusion of water molecules in acutely infarcted tissue, making it the most sensitive sequence for hyperacute stroke within hours of onset.
Question 7: The spatial resolution of a digital radiographic system is most directly limited by which factor?
- Detector pixel pitch (Correct answer)
- X-ray tube focal spot size only
- kVp setting
- Source-to-image distance
Correct answer: Detector pixel pitch
In digital radiography, the pixel pitch (size of individual detector elements) sets the fundamental limit on spatial resolution through the Nyquist sampling theorem.
CABT Imaging Systems 2
Which artifact in MRI is caused by the difference in resonant frequencies between water and fat protons?