Radiography Exam Computed Tomography (CT) Basics 2 — Questions and Answers

Question 1: Which artifact in CT appears as alternating bright and dark bands radiating from high-density structures and is caused by insufficient sampling?

Streak artifact (Correct answer)
Beam hardening artifact
Ring artifact
Partial volume artifact

Correct answer: Streak artifact

Streak artifacts appear as bright and dark bands radiating from high-density objects (e.g., metal implants, dense bone) and result from insufficient projection data or beam hardening at those locations.

Streak artifacts are most commonly caused by metal implants, dense bone, or extreme patient motion. They result from the mathematical inconsistencies in projection data when extremely dense materials cause severe beam hardening or photon starvation. Metal artifact reduction (MAR) algorithms, higher kVp, and selecting imaging planes that avoid metal can reduce these artifacts. Streak artifacts can obscure adjacent anatomy and mimic pathology.

Question 2: In multidetector CT (MDCT), the detector array configuration that allows simultaneous acquisition of multiple thin slices per rotation is called:

Multi-row detector array (Correct answer)
Single-row detector array
Xenon gas detector
Scintillation crystal detector

Correct answer: Multi-row detector array

MDCT uses multi-row detector arrays with multiple rows of detectors, allowing simultaneous acquisition of multiple slices per gantry rotation, dramatically increasing scan speed.

Modern MDCT scanners have detector arrays with 16, 64, 128, 256, or more rows. Each row acquires a separate image slice simultaneously. More detector rows allow faster volume coverage, thinner slices, and isotropic voxels for high-quality multiplanar reformations (MPR) and 3D reconstructions. Solid-state scintillation detectors (coupled with photodiodes) have replaced xenon gas detectors in MDCT due to higher efficiency.

Question 3: Which reconstruction method is used in modern CT to reduce image noise and patient dose compared to filtered back projection (FBP)?

Iterative reconstruction (IR) (Correct answer)
Fourier transform reconstruction
Back-projection (unfiltered)
Maximum likelihood estimation

Correct answer: Iterative reconstruction (IR)

Iterative reconstruction (IR) algorithms repeatedly refine the image by comparing the reconstructed image to the original projection data, achieving lower noise at the same or lower dose than FBP.

Iterative reconstruction (IR) — including techniques like ASIR, SAFIRE, AIDR — models the physics of CT acquisition and uses iterative mathematical refinements to reduce noise while preserving image sharpness. Compared to FBP (the traditional method), IR allows 20–60% dose reduction while maintaining diagnostic image quality. Deep learning reconstruction (DLR) is the next generation, offering even greater noise reduction at ultra-low doses.

Question 4: The spatial resolution of a CT system is characterized by its ability to resolve small, high-contrast objects and is quantified using:

Modulation transfer function (MTF) or line pair per centimeter (lp/cm) (Correct answer)
Contrast-to-noise ratio (CNR)
Hounsfield unit accuracy
Dose efficiency

Correct answer: Modulation transfer function (MTF) or line pair per centimeter (lp/cm)

Spatial resolution in CT is measured by MTF (modulation transfer function) or in line pairs per centimeter (lp/cm) using bar phantoms, indicating the finest detail the system can resolve.

The MTF describes how well the CT system transfers object contrast from the patient to the image across different spatial frequencies. CT systems typically resolve 0.4–1.5 mm objects depending on FOV, matrix, kernel, and detector geometry. Spatial resolution is limited by detector size, focal spot size, and reconstruction parameters. Unlike projection radiography, CT spatial resolution is lower but offers superior contrast resolution.

Question 5: Isotropic voxels in CT imaging are desirable because they:

Have equal dimensions in all three planes, enabling high-quality multiplanar reformations in any direction (Correct answer)
Reduce radiation dose by requiring fewer slices
Increase image noise uniformly across the volume
Allow faster scan times with less patient motion

Correct answer: Have equal dimensions in all three planes, enabling high-quality multiplanar reformations in any direction

Isotropic voxels have equal x, y, and z dimensions, so multiplanar reformations (MPR) in coronal, sagittal, or oblique planes have the same resolution as the original axial images.

Isotropic acquisition (e.g., 0.6 mm × 0.6 mm × 0.6 mm voxels) means the spatial resolution is equal in all three dimensions. This enables high-quality MPR in any plane, 3D volume rendering, and curved reformations (e.g., CT angiography). MDCT with thin collimation enables near-isotropic acquisition. Before MDCT, thick axial slices meant coronal/sagittal reformations were degraded by the thick slice (anisotropic voxels).

Question 6: The gantry in a CT scanner houses the x-ray tube and detector array. The typical gantry rotation time for a modern CT scanner is:

0.27–0.5 seconds per rotation (Correct answer)
1–2 seconds per rotation
5–10 seconds per rotation
0.1 seconds per rotation

Correct answer: 0.27–0.5 seconds per rotation

Modern CT scanners achieve gantry rotation times of approximately 0.27–0.5 seconds per 360° rotation, enabling rapid scanning and cardiac CT capability.

Modern MDCT scanners rotate at 0.27–0.5 seconds per rotation, generating centrifugal forces of up to 30g on the tube and detector. Faster rotation enables cardiac CT (requiring <0.35s for good temporal resolution), trauma protocols, and pediatric scanning with less motion artifact. The temporal resolution for a full 360° rotation is half the rotation time for half-scan reconstruction algorithms used in cardiac CT.

Radiography Exam Computed Tomography (CT) Basics 2

Which artifact in CT appears as alternating bright and dark bands radiating from high-density structures and is caused by insufficient sampling?