CT Physics V.G.Wimalasena Principal School of radiography

Slice / Cut The cross sectional portion of the body which is scanned for the production of CT image is called a slice. The slice has width and therefore volume. The width is determined by the width of the x-ray beam

Matrix The image is represented as a MATRIX of numbers. Matrix :- A two dimensional array of numbers arranged in rows and columns. Each number represents the value of the image at that location

VOXEL Each individual element or number in the image matrix represents a three dimensional volume element in the object, called a VOXEL

PIXEL The VOXEL is represented in the image as a two-dimensional element called PIXEL - (picture element)

CT numbers The numbers in the image matrix are called CT numbers. Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object

Visual image & Gray Scale To obtain a visual image, the CT numbers are assigned different shades of gray on a gray scale. Each shade of gray represents the x-ray attenuation within the corresponding voxel

CT Image

Phases of CT imaging Scanning the patient Data Acquisition Tube or tube and detector move Multiple attenuation measurements are taken around the object Image reconstruction Image Display Image archival (recording)

Scanning the patient - Evolution of CT scanners 1st Generation 2nd Generation 3rd Generation 4th Generation

DATA ACQUISITION Basic components X-ray tube Collimators Detector/s Collimated x-ray beam traverses the object and enters the detector. The attenuation is determined from the difference between incident intensity and transmitted intensity X-ray tube Collimators Detector

Translate rotate system In the basic CT system the x-ray tube and detector are translated linearly so that the beam scans the object (eg. A disc of uniform density)

Ray, Ray sum, View & Attenuation Profile Ray – Imaginary line between Tube & Detector Ray Sum – Attenuation along a Ray View – The set of ray sums in one direction The attenuation for each ray sum when plotted as function of its position is called an attenuation profile Ray sums Ray View Attenuation profile

Attenuation profile of different structures Attenuation of objects with different densities will change the attenuation profile Object with low attenuation Object with high attenuation

Attenuation profile at different positions In a translate –rotate CT, after a view is recorded, the tube and detector rotate a small angle and the entire process is repeated until many views have been recorded for the same slice

Views & attenuation profiles for a slice 1 2 4 3

Image reconstruction The image is created by reflecting the attenuation profiles back in the same direction they were obtained This process is called BACK PROJECTION BACK PROJECTION

Drawbacks in Back projection (Artifacts - Star shape & streaks) The resultant image closely resembles the original object But it shows star shaped patterns around objects and streaks These are called ‘Star’ and ‘streak’ artifacts

Formation of Star artifact and streaks Consider a scan of a single high density object suspended in air

The attenuation profile for this object has a single impulse signal

The back projections take the form of a stripe through the center of the object

Back projections are crated for each profile

Addition of the attenuation profiles create an image with star and streak artifacts Final back projection To be continued – CT Complementary 3