CT Scanner Engineering and Image Quality

Study Notes

CT Scanner Generations

First Generation: Single pencil-like x-ray beam, tube rotates around the patient, long scan duration (25-30 mins).
Second Generation: Multiple beams (up to 30), fan-shaped x-ray beam, translate-rotate movements, average scan duration under 90 seconds.
Third Generation: Numerous detectors (originally 288, now over 700), fan-shaped beam, rotate-rotate movements, significantly faster scans (around 5 seconds).
Fourth Generation: Over 2000 fixed detectors in an outer ring, rotate-fixed movements, very quick scans taking just a few seconds.

Sequential and Spiral CT Scanning

Sequential Scanning: Uses a stop-and-shoot principle where the table is stationary during slice acquisition, resulting in longer scan times.
Spiral CT: Allows for continuous rotation of the tube-detector system, enabling quicker data acquisition and improved imaging speed.

Artefacts in CT Imaging

Definition: Artefacts are discrepancies between expected and actual CT numbers in images, measured in HU units.
Common Artefacts: Beam hardening, Partial Volume Effect (PVE), detector issues, metal interference, and patient motion.

Detailed Artefact Types

Beam Hardening: Results from low-energy x-rays being absorbed by dense structures, causing misinterpretation of attenuation values. Can be mitigated by pre-hardening the beam or using compensatory algorithms.
Partial Volume Effect: Occurs when a pixel encompasses multiple tissue types, blurring distinctions—higher resolution or patient repositioning may help.
Ring Artefacts: Arise from uncalibrated or misaligned detectors; these can distort the image significantly.
Metal Artefacts: Streaking effects caused by metallic materials obstructing x-ray projection data, removable where possible.
Patient Motion: Motion blur caused by patient movement during scanning; mitigated by shorter scan times or asking patients to hold their breath.

Image Quality Factors

Impact of Detector Size: Image quality is determined by detector size, angular projections, and focal spot size.
Radiation Dose Considerations: Higher slice thickness can lower radiation dose but may compromise axial resolution.

Importance of Understanding Artefacts

Misinterpretation of CT artefacts can mimic pathological conditions or result in poor diagnostic quality; thus, recognizing the cause is essential.
Classifications include patient-related artefacts (motion, transient events), physics-related artefacts (beam hardening, noise), and hardware-related artefacts (ring effects, field issues).

Emerging Technologies

Electron Beam CT: Specialized for cardiac imaging utilizing stationary tungsten rings and electron beams instead of rotating tubes.
Dual-Source CT: Features two x-ray tubes at right angles, enhancing image quality and precision in imaging.

Summary of Key Principles

Knowledge of CT scanning principles, including artefact management, is vital for accurate imaging and diagnosis.

Description

This quiz covers the technical aspects of CT scanners, including engineering principles, image quality factors, and artifact identification. It also explores the evolution of CT scanners across different generations.