Computed Tomography Ultimate Flashcards

What is the primary purpose of retrospective gating in cardiac CT?

Answer: To reconstruct images at any phase of the cardiac cycle

Retrospective gating records ECG data throughout the scan and allows reconstruction at any desired cardiac phase after acquisition.

Which artifact appears as a dark band between two high-density objects on CT images?

Answer: Beam hardening artifact

Beam hardening causes dark streaks or bands between dense objects like bone or metal because lower-energy photons are preferentially absorbed.

What does a CT number of -1000 HU represent?

Answer: Air

Air has a CT attenuation value of approximately -1000 Hounsfield Units, the lowest end of the standard CT scale.

In CT, what is the 'matrix size' most commonly used in clinical scanners?

Answer: 512 x 512

Clinical CT scanners most commonly use a 512 x 512 matrix, balancing spatial resolution with acceptable processing time and storage requirements.

What is the main advantage of using iterative reconstruction over filtered back projection (FBP)?

Answer: Reduced image noise at lower radiation doses

Iterative reconstruction algorithms reduce noise more effectively than FBP, enabling diagnostic-quality images at significantly lower radiation doses.

Which organ is typically used as a reference when setting window level for abdominal CT?

Answer: Liver

The liver is commonly used as the window level reference in abdominal CT because it has a consistent, predictable attenuation value of approximately 50-70 HU.

What is the term for the minimum detectable difference in attenuation between two adjacent structures in CT?

Answer: Contrast resolution

Contrast resolution refers to the ability to distinguish between tissues with similar attenuation values, and it is primarily affected by noise and radiation dose.