ABR ABR Imaging Physics and Equipment 1 — Questions and Answers
Question 1: What is the primary source of image noise in computed tomography (CT) imaging?
- Quantum noise from limited photon statistics (Correct answer)
- Electronic noise from the detector
- Patient motion
- Scatter radiation
Correct answer: Quantum noise from limited photon statistics
Quantum noise, arising from statistical fluctuations in the number of x-ray photons detected, is the dominant source of image noise in CT.
Question 2: In CT imaging, what does the CT number (Hounsfield unit) of water equal?
- 0 HU (Correct answer)
- 1000 HU
- -1000 HU
- 500 HU
Correct answer: 0 HU
By definition, water has a CT number of 0 Hounsfield units (HU), with air at −1000 HU and dense bone typically above +400 HU.
Question 3: Which parameter most directly controls spatial resolution in a CT scanner?
- Detector aperture and reconstruction kernel (Correct answer)
- mAs setting
- kVp setting
- Pitch factor
Correct answer: Detector aperture and reconstruction kernel
Spatial resolution in CT is primarily determined by the detector aperture size and the reconstruction (convolution) kernel used in filtered back-projection.
Question 4: What is the purpose of the modulation transfer function (MTF) in evaluating imaging system performance?
- Quantifies spatial resolution by measuring how faithfully a system reproduces contrast at different spatial frequencies (Correct answer)
- Measures image noise spectrum
- Evaluates detector uniformity
- Characterizes temporal resolution
Correct answer: Quantifies spatial resolution by measuring how faithfully a system reproduces contrast at different spatial frequencies
The MTF describes how well a system transfers contrast from the object to the image as a function of spatial frequency, with higher MTF values indicating better resolution.
Question 5: In digital radiography, what is the Detective Quantum Efficiency (DQE) a measure of?
- How efficiently a detector converts incident x-ray quanta into useful image signal relative to an ideal detector (Correct answer)
- The fraction of x-rays absorbed by the detector
- The dynamic range of the detector
- The spatial resolution at the detector surface
Correct answer: How efficiently a detector converts incident x-ray quanta into useful image signal relative to an ideal detector
DQE measures the efficiency with which a detector uses the available x-ray quanta for image formation, accounting for both sensitivity and noise added by the detector.
Question 6: What is the half-value layer (HVL) used to describe in diagnostic x-ray imaging?
- The thickness of material that reduces x-ray beam intensity by 50% (Correct answer)
- The depth of maximum dose in tissue
- The layer where scatter is maximum
- The distance over which mAs doubles
Correct answer: The thickness of material that reduces x-ray beam intensity by 50%
The half-value layer (HVL) is the thickness of a specified material required to reduce the x-ray beam intensity to half its original value, and characterizes beam quality (hardness).
ABR ABR Imaging Physics and Equipment 1
What is the primary source of image noise in computed tomography (CT) imaging?