21-09-2008, 10:05 AM
Introduction
There are two main limitations of using conventional x-rays to examine internal structures of the body. Firstly superimpositions of the 3-dimensional information onto a single plane make diagnosis confusing and often difficult. Secondly the photographic film usually used for making radiographs has a limited dynamic range and therefore only object that have large variation in the x-ray absorption relative to their surroundings will cause sufficient contrast differences on the film to be distinguished by the eye. Thus the details of bony structures can be seen, it is difficult to discern the shape and composition of soft tissue organ accurately.
CT uses special x-ray equipment to obtain image data from different angles around a body and then shows a cross section of body tissues and organs. i.e., it can show several types of tissue-lung,bone,soft tissue and blood vessel with great clarity. CT of the body is a patient friendly exam that involves little radiation exposure.
Basic Principle
In CT scanning, the image is reconstructed from a large number of absorption profiles taken at regular angular intervals around a slice, each profile being made up from a parallel set of absorption values through the object. ie, CT also passes x-rays through the body of the patient but the detection method is usually electronic in nature, and the data is converted from analog signal to digital impulses in an AD converter. This digital representation of the x-ray intensity is fed in to a computer, which then reconstruct an image.
The method of doing of tomography uses an x-ray detector which translates which translates linearly on a track across the x-ray beam, and when the end of the scan is reached the x-ray tube and the detector are rotated to a new angle and the linear motion is repeated. The latest generation of CT machines use a 'fan-beam' geometry with an array of detectors which simultaneously detect x-rays on a number of different paths through the patient.
CT Scanner
CT scanner is a large square machine with a hole in the centre, something like a doughnut. The patient lies still on a table that can move up/down and slide in to and out from the centre of hole. With in the machine an X-ray tube on a rotating gantry moves around the patient's body to produce the images.
Procedure
In CT the film is replaced by an array of detectors which measures X-ray profile. Inside the scanner, a rotating gantry that has an X-ray tube mounted on one side an arc -shaped detector mounted on opposite side. An X-ray beam is emitted in a fan beam as the rotating frame spins the X-ray tube and detector around the patient. Each time the X-ray tube and detector make a 360 degree rotation and X-ray passes through the patient's body the image of a thin section is acquired. During each rotation the detector records about 1000 images (profiles) of the expanded X-ray beam. Each profile is then reconstructed by a dedicated computer into two time.
