Computed Tomography Test CT Image Quality and Reconstruction 1 — Questions and Answers
Question 1: In CT, spatial resolution is most commonly measured using which parameter?
- Modulation Transfer Function (MTF) or line pairs per centimeter (lp/cm) (Correct answer)
- Signal-to-noise ratio (SNR)
- Hounsfield Unit (HU) accuracy
- Contrast-to-noise ratio (CNR)
Correct answer: Modulation Transfer Function (MTF) or line pairs per centimeter (lp/cm)
Spatial resolution in CT is quantified by the MTF or high-contrast resolution measured in line pairs per centimeter, describing the system's ability to resolve fine detail.
Question 2: What is the Hounsfield Unit (HU) value of water on a CT scan?
- 0 HU (Correct answer)
- −1000 HU
- +1000 HU
- +100 HU
Correct answer: 0 HU
Water is defined as 0 HU in the Hounsfield scale, which serves as the reference point with air at −1000 HU and dense cortical bone at +1000 HU or greater.
Question 3: Which reconstruction algorithm provides the sharpest images but also the most noise in CT?
- High-frequency (bone/edge) kernel (Correct answer)
- Soft-tissue kernel
- Lung kernel
- Iterative reconstruction
Correct answer: High-frequency (bone/edge) kernel
High-frequency (bone/detail) kernels sharpen edges and fine structures but amplify image noise, making them ideal for bone but not soft tissue evaluation.
Question 4: What does the term 'partial volume effect' describe in CT imaging?
- Averaging of attenuation from different tissues within a single voxel (Correct answer)
- Loss of signal at scan edges
- Artifact from metal implants
- Increased noise at the image periphery
Correct answer: Averaging of attenuation from different tissues within a single voxel
Partial volume averaging occurs when a voxel contains multiple tissue types, and the assigned HU is an average of all tissues present, potentially obscuring small lesions.
Question 5: In CT, increasing the field of view (FOV) while keeping matrix size constant will have what effect on voxel size?
- Voxel size increases, reducing spatial resolution (Correct answer)
- Voxel size decreases, improving spatial resolution
- Voxel size stays the same
- FOV does not affect voxel size
Correct answer: Voxel size increases, reducing spatial resolution
Voxel size = FOV / matrix size; a larger FOV with the same matrix produces larger voxels, lowering spatial resolution.
Question 6: What type of CT artifact appears as dark streaks radiating from dense metallic objects such as hip prostheses?
- Beam hardening and metal streak artifact (Correct answer)
- Ring artifact
- Motion artifact
- Partial volume artifact
Correct answer: Beam hardening and metal streak artifact
Metal objects severely attenuate and preferentially absorb low-energy photons (beam hardening), causing dark streaks that obscure surrounding anatomy.
Computed Tomography Test CT Image Quality and Reconstruction 1
In CT, spatial resolution is most commonly measured using which parameter?