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 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.
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.
BENEFITS AND RISKS
• Unlike other imaging methods CT scanning offers detailed view of many types of tissues , including lungs, bones, soft tissues and blood vessels.
• CT scanning is painless , noninvasive and accurate.
• CT examinations are fast and simple.
• Diagnosis made with the assistance of CT scan eliminate the need for invasive exploratory surgery and surgical biopsy.
• CT scanning can identify both normal and abnormal structures, making it a useful tool to guide radiotherapy , needle biopsies and other minimally producers.
• CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems.
• CT does involve exposure to radiation in the form of X-rays , but benefits of an accurate diagnosis far outweighs the risks .The effective radiation does from this procedure is about 10mv , which is about the same as the average person receives from background radiation in 3 years.
• Special care is taken during X-ray examination to ensure maximum safety for the patient by shielding the abdomen and pelvis being imaged.
• The risk of serious allergic reaction to iodine containing contrast material is rare and radiology departments are well equipped to deal them.
All computed tomography system consists of four major subsystems.
• Scanning System – takes suitable reading for a picture to be reconstructed. This includes x-ray source and detectors.
• Processing Unit – converts these readings into intelligible picture information.
• Viewing System – presents this information in visual form and includes other manipulative aids to assist diagnosis.
• Storage Unit – here picture is stored in digital form.
The purpose of the scanning is to acquire enough information to reconstruct a picture for an accurate diagnosis. In basic scanning process, a collimated x-ray beam passes through the body and its attenuation is detected by a sensor that moves on a gantry along with the x-ray tube. The tube and the detector moves in a straight line.
Inorder to get a clear image, rotation machines have been designed in which only the x-ray source rotates within a full circle of stationary detectors arranged around a patient. The individual detectors are lined up practically without gaps so that the radiation which has penetrated the patient is optimally used. The system permits calibration during scanning, which eliminates the problem of detector drift.
In CT scanners, the highest image quality free from disturbing blurring effects is obtained with the aid of pulsed x-ray radiation. During rotation, high voltage is applied at all times. A grid tube prevents the electron current from striking the anode except when desired allowing the x-rays to be emitted in bursts. As the gantry rotates an electric signal is generated at certain positions of rotating system.
For a good image quality, it is important to have a stable system response and in that detectors play a significant role. There are three types of detectors commonly used in CT scanning. They are xenon gas ionization detector, scintillation crystal and photomultiplier and scintillarc. A good detector is a pre-requisite to obtain optimal image quality, the measuring electronics must have a large dynamic range to backup the detector.
The information received by the computer from the scanning gantry needs processing for reconstructing the pictures. The data from the gantry contains information on the following parameters.
• Positional information-such as which traverse is being performed and how far the scanning frame is along its traverse.
• Absorption information-the values of attenuation coefficient from the detectors.
• Reference information-obtained from the reference detector that monitors the X-ray tube.
• Calibration information-Obtained at the end of each traverse.
The first stage of computation is to analyze and convert all the collected data in to a set of profiles. However the main part is of processing the profiles to convert the information which can be displayed as a picture and used for diagnosis. In general the reconstruction method can be classified in to three major techniques.
• Back projection-which is analogous to graphic reconstruction.
• Iterative methods-which implement some form of algebraic solution.
In most of the CT system the final picture is available on a television type picture tube. The picture is constructed by a number of elements in a square matrix wherein each element has a value representative of the absorption value of the point in the body which it represents. This technique enables to have a much larger dynamic range than the eye can possibly have.
STORING AND DOCUMENTATION
For subsequent processing or evaluation of a CT picture, various methods of storage are used. The picture is stored in the digital form so that the evaluation is convenient on a computer assisted program. For this purpose the data carries generally employed are magnetic disc ,magnetic tape and floppy disc. The magnetic disc normally hold a small number of pictures. So it cannot be employed as a long term storage medium. Most manufactures of CT units use magnetic tape and floppy disc and floppy disc provide medium storage range. For long term storage magnetic tapes are performed.