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COMPUTER TOMOGRAPHY(CT) INTRODUCTION: A new method of forming images from X-rays was developed and introduced into clinical use by British Physicist Godfrey Hounsfield and is referred as computerised Axial tomography or computer tomography.


PRINCIPLE: Measurements are taken from the transmitted X-rays through the body and contain information on all the constituents of the body in the path of the X-ray beam. By using multidirectional scanning of the object, multiple data are calculated. The mathematical basis for producing an image of the cross section of these bodies is that if one measures the total attenuation along rows and columns of a matrix, one can compute that attenuation of the matrix elements at the intersections of the rows and columns.

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The number of mathematical operations necessary to yield clinically applicable and accurate images is so large that a computer is essential to do them. Computer performs the calculations and obtains an information. This information can be presented in a conventional raster form and from these results a 2 D picture (slice) can be obtained..


MATHEMATICAL BASIS OF IMAGE CONSTRUCTION A simple calculation called Back Projection Reconstruction can illustrate how the attenuation values along the surface of a transverse slice can be computed from the externally measured attenuation factors. Even though the computer creates images at different angles and solves higher order matrices a 2 x 2 matrix is taken into account and the following illustration demonstrates the manner of analysis involved in the calculations.

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STEP 1: Suppose the actual attenuation values, normalised to zero, are represented by a 2 x 2 matrix, 2 0 1 3 Each number in the matrix represents the attenuation of the space where it is located. Here “0” is a measure of the attenuation in the upper right hand corner of the matrix

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STEP 2 (first estimate) The attenuation values are measured from the outside as those seen along the rows giving the sums 2 and 4. using these as the first estimate we have attenuation numbers, 2 2 4 4

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STEP 3 (second estimate) The second estimate is obtained from the values measured along the columns giving the sums 3 and 3. Thus we have 3 3 3 3 Now add this matrix to the first estimate to get the second estimate as

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2 2 3 3 5 5 4 4 3 3 7 7 STEP 4 (third estimate) A third estimate can be obtained from the values measured along the north east diagonal giving the following matrix: 2 1 1 3 + =

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Add this to the second estimate to get the third estimate as 5 5 2 1 7 6 7 7 1 3 8 10 STEP 5 (fourth estimate) A fourth estimate can be obtained from the values measured along the north west diagonal giving the following matrix: 5 0 1 5 + =

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Add this to the second estimate to get the third estimate as 7 6 5 0 12 6 8 10 1 5 9 15 STEP 6 (final image) Normalize the fourth estimate to zero by subtracting 6 from each element: 6 0 3 9 + =

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Then divide this by 3 to yield final image 2 0 1 3 The final matrix is the same as the first one. The numbers in the matrix correspond to the attenuations of locations on a tissue slice having the same spatial relationship as the matrix numbers. It is seen that the final image has the same attenuation values as the actual transverse slice but the values are obtained from external measurements of attenuation along using CT. The computer does similar calculation in a large scale and finds the matrix values.


BLOCK DIAGRAM FOR A COMPUTER TOMOGRAPHY SCANNER Timing kV+mA control Detector scanner Tube Position control High-voltage supply Dedicated microcomp Output unit And storage Camera CRT P P Control bus Data bus

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The timing, anode voltage (kV) and beam current (mA) are controlled by a computer through a control bus. The high voltage d.c power supply drives an X-ray tube that can be mechanically rotated along the circumference of a gantry. The patient is lying in a tube through the centre of the gantry. The X-rays pass through the patient and are partially absorbed and the remaining X-ray photons impinge upon several of as many as 1000 radiation detectors fixed around the circumference of the gantry. The detector response is directly related to the number of photons impinging on it and so to tissue density since a greater proportion of X-rays passing through the dense tissues are absorbed than that are absorbed by the less dense tissues.

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When they strike the detector, the X-ray photons are converted to scintillations. The computer sensed the position of the X-ray tube and samples the output of the detector along a diameter line opposite to the X-ray tube. A calculation based on data obtained from a complete scan is made by the computer. The output unit then produces a visual image of the transverse plane cross-section of the patient on the cathode ray tube. It can also be photographed with a camera to produce a hard copy record. The present day CT machines can obtain slices in 1-2 seconds in high resolution and 5-10 seconds in precision modes.

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DATA PRESENTATION: Most scanners now present the data obtained as an analog display of each tissue slice on a cathode ray tube. Presentation is usually in the form of a gray scale in which whiteness is proportional to the X-ray attenuation coefficient of tissues at each point of the scan. Thus radio opaque materials appear white and radioluscent tissue appears black. The range can be varied by changing the gate or window width (W) at will so that tissues within a wide range of densities or a narrow range can be evaluated.

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The central point or level (Mean-L) can also be varied. The Hounsfield scale is an arbitrary one with air at 1000 units and water at 0 units as fixed points. The numerical value assigned to the attenuation coefficient bears a linear relationship to the electron density of the tissue concerned. In addition to the analog display the machine can also produce a digital printout.

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SCAN ARTIFACTS: With such complicated apparatus using X-rays, sophisticated photon recording systems and computer programming there are many sources of error which can produce artifacts. These can be classified into 4 types: Noise Motion artifacts Artifacts due to high differential absorption in adjacent tissues. Technical errors and computer artifacts.


APPLICATIONS OF CT CENTRAL NERVOUS SYSTEM: CT has replaced the diagnostic techniques like cisternography and ventriculography. CT stereotaxy is another innovation for diagnostic and therapeutic procedures in brain without open surgery. Injuries It detects small bone injuries, the presence or absence, location and extent of bleeding and damge to the brain and ventricular system.

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Vascular Lesions: CT scan is immensely helpful in detecting arteriovenous malformations like angiomas and aneurysms before catastrophic bleeding occurs due to its rupture. Hemorrhage inside brain of non-traumatic causes, cerebral thrombosis are emergencies requiring CT imaging. In Oncology: CT is an accepted first line investigation for primary malignant lesions, differential diagnosis with other benign lesions and for detecting metastatic disease where surgical removal of solitary metastasis is made feasible.

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In Degenerative disease: Degenerative diseases like cerebral atrophy, helminthic infestations of brain and chronic inflammatory diseases like tuberculomas can be detected using CT scans. ORTHOPEDICS AND BONE TUMOURS: CT scan is used for designing custom made prosthesis for limb conserving or preserving surgery both in bone tumors and in traumatic fractures.

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Assessing the lesion with reference to various components (medulla, cortex, soft tissue compartment). Dimensions of bone for resection and for making prosthesis. The assessment of whether patient is suitable for prosthesis or not depends mainly on CT imaging. Thorax: In the screening of high risk group (chronic smokers) for early detection of lung cancer. In differential diagnosis of solitary pulmonary noduly whether it is malignant or non-malignant.

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Abdomen and pelvis: CT imaging has emerged as an important diagnostic tool both for diagnostic and therapy. CT guided fine needle aspiration for cytology even in inaccessible parts. Evaluation of lymphnodal status with reference to their size can be easily done. Subdiaphragmatic space and peritoneal cavity lesions can be evaluated without the interference from bowel gas. Assessment of calculi for lithotripsy with reference to size, number and density is possible with CT.

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Neck: CT is useful in assessing malignant lesions of thyroid and larynx for planning surgical approach. Radiotherapy planning: Modern radio therapeutic centers routinely use treatment planning system. This involves CT section as the first stage and the image is transferred to treatment planning system for marking out the area of radiotherapy and for dose distribution. During the course of radiotherapy, repeated CT studies can be done and volume of tissue being irradiated can be reduced thereby saving unwanted radiation to normal tissue.

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