2. INTRODUCTION Designed by Godfrey N. Hounsfield to overcome the visual representation challenges in radiography and conventional tomography by collimating the X-ray beam and transmitting it only through small cross-sections of the body
3. G.N.HOUNSFIELD ALLAN M. CORMACK In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology & Medicine with Allan MacLeod Cormack, Physics Professor who developed solutions to mathematical problems involved in CT.
4. Important events Dr. Ledley introduced the whole body CT scanner (ACTA scanner) 1974 G.N. Hounsfield shared the Nobel Prize with Allan MacLeod Cormack 1979 First paper on CT presented to British Institute of Radiology by Hounsfield and Dr. Ambrose 1972 First clinically useful CT head scanner was installed at Atkinson-Morley Hospital (England) 1971 G.N. Hounsfield developed first clinically useful CT head scanner 1969 EVENTS YEAR
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6. Cross-sectional slices Think like looking into a loaf of bread by cutting it into thin slices and then viewing the slices individually.
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9. This is the basic idea of computer aided tomography. In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. Comparison of CT with Conventional Radiography A conventional X-ray image is basically a shadow. Shadows give you an incomplete picture of an object's shape.
78. CT Sinogram The data acquired for one CT slice can be displayed before reconstruction. This type of display is called a Sinogram.
79. What are we measuring? The average linear attenuation coefficient ( µ ), between tube and detectors Attenuation coefficient reflects the degree to which the X-ray intensity is reduced by a material
95. CT NUMBER & HOUNSFIELD UNIT The computer calculates a relationship between the linear attenuation coefficients of the pixel and water which is given as CT number. To image materials with higher than dense bone CT number larger than 1000 should be available CT numbers based on a magnification constant of 1000 are Hounsfield units
Imagine you are standing in front of a wall, holding a pineapple against your chest with your right hand and a banana out to your side with your left hand. Your friend is looking only at the wall, not at you. If there's a lamp in front of you, your friend will see the outline of you holding the banana, but not the pineapple -- the shadow of your torso blocks the pineapple. If the lamp is to your left, your friend will see the outline of the pineapple, but not the banana. In order to know that you are holding a pineapple and a banana, your friend would have to see your shadow in both positions and form a complete mental image. This is the basic idea of computer aided tomography
Na I crystals are Hydroscopic Long after glow Fast scan times negate the use of NaI PMT are fairly big Large number of detectors negate the use of PMT
Na I is replaced with Cd WO4 / Cs I / BGO Most commonly CdWO4 is used PMT is replaced with silicon photodiodes
In III generation CT scanners, the X-ray tube and detectors maintain a fixed relationship, so the beam is always aligned with the long axis of each detector. In IV generation CT scanner, angle at which X-rays hit the detector changes constantly. Obliquely entering X-rays would pass through only a short distance of gas before they hit the wall of the detector. In such a case, the X-rays are absorbed in the detector walls and the information they carry in lost for all time.
