Computerized Axial Tomography How it Works

Computerized Axial Tomography How it Works:

Computerized Axial Tomography (CAT) is the medical imaging method employing tomography created by computer processing. It is also known as computed tomography. In CT imaging X-rays are used in conjunction with computing algorithms to image the body. In computed tomography a volume of data is produced. This data can be processed and analyzed using different techniques and the various bodily structures can be demonstrated based on their ability to block X-rays. Tomography is derived from Greek words namely ‘tomos’ meaning 'slice' and ‘graphen' meaning ‘to write'. So by using CAT technique we can produce clear 2-D or 3-D cross sectional images of deep internal organs.

Scanning components and principle of operation

CT scanning can be the best choice in diagnosing some urgent conditions such as clots in the arteries of lungs, tearing of aortic wall, appendicitis, obstructing kidney stones etc. ln CAT otherwise called as Computerized Axial Tomography X-ray slice data is generated using an X-ray source that rotates around the object. X-ray sensors are placed on the opposite side of the circle from the X-ray source. The earliest sensors were the scintillation detectors with photomultiplier tubes. But nowadays we use scintillation systems based on photodiodes instead of photo multipliers and modern scintillation materials with more desirable characteristics. In Computerized Axial Tomography, a computer generated cross-sectional image (tomogram) is produced with the effect of an X-ray generator and X-ray detector in a ring shaped apparatus. The tomographic reconstruction is done using proper computational algorithms. The main scanning components are illustrated in the block in figure. It is relatively a new method of forming images from X-rays. Here measurements are taken by passing X-rays through the body and contain information from almost all parts of body. Many data scans are taken progressively from the object and they are combined together by the mathematical procedures known as tomographic reconstruction. The data are arranged in a memory, and each data point is convolved with its neighbors according with a seed algorithm using Fast Fourier Transform techniques. This data can then be displayed, photographed, or used as input for further processing, such as multi-planer reconstruction.
CAT scanner block diagram
1. X-ray source: The X-ray generating tube generates and direct-rays towards the subject. During, examination) a low dose of X-ray beam passes through the body around 360 degrees in a rotating table. In CAT, measurements are taken from the transmitted X-rays through the body. These contain information on all parts of the body in the path of X-ray beam. The radiation dose for a particular study depends on many factors such as volume scanned, number and types of scan sequences, the desired resolution of image and the image quality. The intensity of X-rays can be regulated by controlling then anode voltage, beam current etc.

2. Rotary table: The subject is placed in a rotary table. During the process, the tissues absorb small amount of radiation depending on the intensity of X-ray beam. There are two types of CT scanners. They are body scanners and head scanners. As the name indicates, the body scanners scan the whole body while the head scanners scan only the head. CT exposes the patient to more ionizing radiation than a radiograph. The main issue with CT examinations is how to reduce the radiation dose during examinations. We can reduce the radiation dose up to an extent but reducing the radiation dose can affect the image quality. But several methods can be used to reduce the radiation dose. One such method is using software technology. Here the software works as a fitter that reduces random noise.

3. X-ray detector: The X-ray detectors are placed in a ring shaped apparatus which rotate around the patient. They detect the X-ray beam intensity of each tissue and feed it to a computer. Actually the rays coming out of the detector are converted into electronic signals due to light sensitive nature of the detectors. Multidirectional scanning detectors are used to obtain more accurate tomogram. Detectors such as Spiral multi-detector can used which utilizes 8, 16 or 64 detectors during continuous motion of patient through the radiation beam to obtain much clearer images. MDCT (Multi Detector CT) scanners give high resolution and image quality. Nowadays faster scanning times and improved resolution have increased the accuracy of CT scanning.

4. Reconstruction of tomogram : CT produces a group of data which can be manipulated and processes to demonstrate various bodily structures based on their ability to block the X-ray beam. It is called ‘windowing’ technique. The reconstruction of tomogram is done by using a suitable computational algorithm using a computer. By using a computer the image can be produced in a television screen. This is called tomogram and it can provide a very accurate cross sectional view of any area of the body. To reconstruct the image, a number of mathematical operations have to be done and for this we use different computational tools.

Multiplaner reconstruction (MPR) is the simplest tomographic reconstruction method. Digital geometry imaging systems are used to generate a three-dimensional image of the inside of an object. Modern software allows reconstruction of the tomogram in many planes so that any plane can be selected to display an anatomical structure. This may be useful for visualizing the structure of extremely small elements of body such as bronchi.

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