BASICS OF COMPUTED TOMOGRAPHY

RAD 323

LEC (1)

Design/layout by E Bashir, CAMS, King Saud University, 2008 - Ref: Seeram, Karthikeyan,

internet

WHAT IS TOMOGRAPHY ?

WHAT IS TOMOGRAPHY ?

“ TOMO ” from Greek, means (cross-section), layer, or cut. Conventional tomography is imaging of a two-dimensional (2D) cross-section in a sagittal or coronal plane of a three dimensional (3D) object oriented parallel to an x-ray film using a large conical beam.

Transverse Axial Tomography (AT) is imaging of transverse x-ray image cross-sections. Lately, it was renamed (CAT), or Computed Axial Tomography after using computers for processing images, then abbreviated to (CT).

Early conventional tomography had some limitations:1. Inefficient x-ray absorption (75% of beam information is wasted and does not contribute to image formation).2. Scatter to primary (S:P) ratio was high. Scatter reduces the subject contrast.

Generally, 50% of detected x-rays producescatter, so that (S = P). Therefore, the subject contrast is reduced by a contrast reduction factor (CRF) of 0.5

CRF = 1 / [ 1 + (S/P) ] = 1 / 1 + 1 = 0.5

Any low contrast tissue masses will be poorly defined because of scatter radiation densities adding to (deteriorating) the image data. 3. Superimposition of (underlying/overlying) structures or tissues led to reduced (conspicuity) and subject contrast. Any lesions are, therefore, not readily identified.

4. Images had less detail and lack clarity. They had not enough diagnostic information.

BASIC PRINCIPLES OF CT

HOW CT WORKS:-CT scanners use (image reconstruction from image projections) in which a (thin slice) (cross-section) of an object is examined from various (positions or angles) by means of a highly collimated x-ray beam.

-Transmitted rays beam were measured by (detectors), then converted into an analogue current signal.

- And again to a (digital) signal (numerical data) by Digital-to-analog (D/A) converters, fed to a computer for data processing by mathematical algorithms, which outputs the reconstructed image as a digital CT image, that will be finally displayed as visual analogue image on the TV screen, or VDU, stored, and recorded.- Hounsfield EMI scanner, (1971), used x- ray transmission attenuation measurements (attenuation coefficients, ) through the head at various positions and angles.

BASIC PRINCIPLES OF CT

-He had determined attenuation differences of (0.5%), that were sufficient to distinguish between the soft tissues.

CT IMAGE FORMATION:CT image is produced in three main steps:

1.THE SCANNING PHASE: Produces data, but not an image.

BASIC PRINCIPLES OF CT

2. THE RECONSTRUCTION PHASE: Processes the acquired data and forms a digital image.

3. DIGITAL-TO-ANALOG (D/A) PHASE: Produces a visible displayed analogue image (shades of gray).

BASIC PRINCIPLES OF CT

BASIC PRINCIPLES OF CT

- CT imaging is the reconstruction of an image from its projections. - A projection at a given angle ( ) is the integral or (summation) of image in the direction specified by that angle.-In other words, projection means data (information) derived from transmitted x-ray energies when an object is irradiated from a specific angle ( ).

BASIC PRINCIPLES OF CT

Simple projection profile of two solid cylinders.

Profile of object represented by f (x , y). d is the distance of AA’ from O.

BASIC PRINCIPLES OF CT