COMPUTED TOMOGRAPHY (CT)

Represented by Kadriye Doğan

Introduction

Computed tomography (CT) is an imaging method that uses special x-ray equipment to create detailed scans of areas inside the body.

The term “tomography” comes from the Greek words tomos (a slice) and graphein (to write or record).

History

Invented by Godfrey Hounsfield in 1967,UK and announced in 1972.

Allan McLeod Cormack in US, independently invented similar process,and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.

Terms

Attenuation is a reduction of the beam intensity on passing through the body section.

Scan is made up of multiple x-ray attenuation measurements around a section.

Slice is the cross sectional portion of the body.Voxel is a 3D version of the pixels.CT number is a number which represents the x-ray

attenuation.

CT Scan

When X-rays are irradiated on the human body, some of the rays are absorbed and some pass through the body to produce an image.

X-ray detectors absorbs the penetrated X-rays, measures the X-ray amount, and transmits the data to a computer system.

Computer system calculates and analyzes data from each detector in each level, and finally reconstructs multiple, two-dimensional, cross-sectional images.

Components of CT System

The scanning unit is consists of the gantry with tube and detectors.

The patient table The computer system for image reconstruction Monitor and record system

Image Reconstruction-1

X-rays density is measured before and after passing through the body section.

The difference between measurements are recorded as attenuation of section which across the detector.

The computer system converts digital data which comes from detectors to CT numbers of voxels for each section.

Image Reconstruction-2The pixel value is colored

white,black or gray according to the mean

attenuation of the section that it corresponds to the

Hounsfield scale.

Image Reconstruction Algorithms

There are numerous reconstruction algorithms. Filtered Back Projection is most widely used in clinical CT scanners.

1)Back Projection2)Iterative Reconsruction3)Fouier Reconstruction 4)Filtered Back Projection

Back Projection

Back Projection

Iterative Reconstruction

Projection data are predicted based on an assumption about the initial attenuation coefficients of all voxels.

These predicted data are compared to measured projection data.

The voxel attenuation values are modified until an acceptable level of error between the predicted and measured data is achieved.

Iterative Reconstruction

Fourier Reconstruction

Calculate the 1D Fourier transform of all projections.[p(r) = P(k)]

Place P(k) on polar grid to get P(k,) Resample in Cartesian space to get F(kx,ky) Calculate the 2D inverse Fourier transform of

F(kx,ky) to get f(x,y) – image.

Fourier Reconstruction

Fourier Reconstruction

Filtered Back Projection

A problem that is immediately apparent is the blurring that occur in other parts of the reconstructed image.

The combination of back projection and ramp filtering is known as filtered back projection.

Filtered Back ProjectionFiltered Back Projection

Filtered Back Projection