Computed Tomography Overview

Study Notes

CT scanners were first introduced in 1971 with a single detector
The first CT scanner was used for brain studies
CT scanners underwent multiple improvements with an increase of detectors and a decrease in scan time

It provides accurate diagnostic information
It overcomes the limitations of conventional radiography, which suffers from collapsing of 3D structures into a 2D image
Although the resolution of CT is lower, it has extremely good low contrast resolution

A medical imaging technique that combines X-rays and computer processing to generate tomographic slices of the scanned area
Tomos means slice and Graphein means to write
Defined as the imaging of an object by analyzing its slices

The image is represented as a matrix of numbers
A matrix is a two-dimensional array of numbers arranged in rows and columns
Each element in the image matrix represents a three-dimensional volume element called a voxel
The voxel is represented on the image as a two-dimensional element called a pixel (picture element)

The diameter of the area being imaged
For example, 25 cm for a head and 40 cm for an abdomen
CT pixel size is determined by dividing the FOV by the matrix size, which is generally 512 x 512 in CT

The reduction of the intensity of the X-ray beam as it passes through matter
It can be caused by absorption or deflection of photons
Different factors affect attenuation, such as the beam energy and atomic number of the absorber

The numbers in the image matrix
Each pixel has a number that represents the X-ray attenuation in the corresponding voxel
These numbers are assigned different shades of gray on a gray scale to obtain a visual image
Each shade of gray represents the X-ray attenuation within the corresponding voxel
CT numbers are compared to the attenuation value of water and displayed on a scale of units named Hounsfield units (HU)
The range of CT numbers is 2000 HU wide, although some modern scanners have a greater range up to 4000
Each number represents a shade of gray with +1000 (white) and –1000 (black) at either end of the spectrum

The process of using calculated Hounsfield units to make an image
The various radio density amplitudes are mapped to 256 shades of gray
These shades of gray can be distributed over a wide range of HU values or a narrow range
A narrow window is commonly used for evaluation of structures in the image known as contrast compression

To evaluate the abdomen, a liver window can be used
A liver window would have an average of 70 HU, and would extend from -15 HU to +155 HU
70 HU is the average HU value for the liver
The shades of gray are distributed over a narrow range of HU values, allowing for variations in liver tissue to be identified