LEC (1) Design/layout by E Bashir, CAMS, King Saud University, Ref: Seeram, Karthikeyan, internet

 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 produce Generally, 50% of detected x-rays produce

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.

-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.

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).

- 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. - 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 (  ).

Simple projection profile of two solid cylinders.

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