Computed Tomography Basics

Computed Tomography (CT)

• CT scanners use X-rays and computer processing to create cross-sectional images (slices) of the body.
• CT was introduced in 1971, with a single detector, for brain studies.
• CT technology evolved with an increase in the number of detectors and a decrease in scan time.
• The technology is by AHMED JASEM ABASS (MSC of Medical Imaging).

CT Scanners

• An X-ray device for cross-sectional imaging.
• It creates images of slices through the patient.

Why CT?

• Conventional radiography shows collapsing 3D structures as 2D images.
• CT has lower resolution, but excellent low-contrast resolution.
• Enables detecting small changes in tissue type.
• Provides accurate diagnostic information about the distribution of structures within the body.

Tomography

• Non-invasive medical imaging modality.
• Combines X-rays and computer processing to create tomographic slices.
• Tomos = slice; Graphein = to write.
• Definition: imaging of an object by analyzing its slices.
• Different types of CT include: Computerized Axial Tomography (CAT), Spiral CT, Multi-Slice CT.

Matrix

• The CT image is a matrix of numbers.
• Matrix: A two-dimensional array of numbers arranged in rows and columns.
• Each element in the matrix represents a 3D volume element (voxel).
• Voxel is represented as a 2D picture element (pixel).
• Field of view (FOV): The diameter of the body region being imaged.
• Pixel size in CT = FOV / (matrix size).

Pixel vs. Voxel

• Pixel size depends on matrix size and FOV.
• Voxel size depends on FOV, matrix size, and slice thickness.

Attenuation

• Attenuation: Reduction of X-ray beam intensity as it passes through matter.
• Attenuation is caused by absorption or deflection of photons.
• Factors affecting attenuation include beam energy and atomic number of the absorber.

CT Numbers

• CT numbers are values in the image matrix indicating X-ray attenuation.
• Different shades of gray represent varying CT numbers on a gray scale.
• Hounsfield Units (HU): Units for displaying CT numbers; Water = 0 HU.
• Range of CT numbers: approximately -1000 to +3000 HU. (Modern scanners can extend beyond 4000.)

Hounsfield Values (HU)

• Provides a standardized scale for converting CT numbers to gray shades.
• Different tissues have specific HU values. (e.g., air, lung, fat, water, soft tissue, bone).

Windowing

• Process of adjusting CT image display to enhance specific structures.
• Maps Hounsfield units to 256 gray shades.
• Wide window: Overview of structures, spans a broad range of HU values.
• Narrow window: Focusing on a specific structure, centers gray shades around a particular HU value, highlighting variations in tissue composition.

Windowing - Details

• Window level (WL): CT number for the center of the displayed range of numbers.
• Window width (WW): Total range of values selected for display.
• Narrow window enhances inherent contrast (distinguishes subtle differences).
• Window level influences brightness.

Imaging System

• Scanner: Contains the gantry, patient couch, and other components: X-ray tube, generator, filters, collimators, and detectors.
• Patient couch: 450 pounds (204 kg) weight limit, scannable range from head to thigh (162 cm).

Gantry Characteristics

• Tilting range: +/- 30 degrees.
• Aperture: 70 cm, on average.

Explore the fundamentals of computed tomography (CT), including its history, technology, and applications in medical imaging. This quiz covers the evolution of CT scanners and their importance in accurate diagnosis through cross-sectional imaging. Understand how CT improves upon traditional radiography in evaluating complex structures.