Computed Tomography Basics

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Questions and Answers

Why are CT scans useful for detecting small changes in tissue type?

CT scans have extremely good low contrast resolution, which allows for the detection of small changes in tissue type.

What does the acronym 'CT' stand for?

Computerized Axial Tomography

What is the function of a CT scanner?

A CT scanner is an X-ray device capable of cross-sectional imaging, creating images of slices through the patient.

What is the primary difference between a CT scan and a conventional X-ray?

CT scan creates a 3D image, while conventional X-ray produces a 2D image. Signup and view all the answers

The intensity of X-ray beams remains constant as they travel through matter.

False Signup and view all the answers

What does the term 'attenuation' refer to in the context of CT scans?

Attenuation refers to the reduction in the intensity of an X-ray beam as it traverses matter, caused by absorption or deflection (scattering) of photons. Signup and view all the answers

How are CT numbers related to X-ray attenuation?

Each CT number represents the X-ray attenuation in the corresponding voxel of the object. Signup and view all the answers

What is the Hounsfield unit (HU) scale used for, in the context of CT scans?

The Hounsfield unit (HU) scale is used to display the attenuation value of different tissues relative to water. Signup and view all the answers

What is the general range of Hounsfield units (HU) in a CT scan?

The general range of Hounsfield units (HU) in a CT scan is 2000 HU wide, although modern scanners can have a greater range, up to 4000 HU. Signup and view all the answers

Match the following structures with their respective Hounsfield unit (HU) ranges:

Air = -1000 Bone = +400 to +1000 Water = 0 Soft tissue = +40 to +80 Fat = -60 to -100 Lung = -400 to -600 Signup and view all the answers

What is windowing in the context of CT scans?

Windowing is the process of using calculated Hounsfield units to create a visual image, by mapping tissue densities to shades of gray within a specific range. Signup and view all the answers

What does window width determine in CT image processing?

Window width determines the contrast of the CT image. Signup and view all the answers

What are the two main components of a CT scanner?

The two main components of a CT scanner are the gantry and the patient couch. Signup and view all the answers

What is the typical aperture size of a CT scanner's gantry?

Most CT scanners have a 70 cm aperture. Signup and view all the answers

Study Notes

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.

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Description

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.