COMPUTED TOMOGRAPHY 1

Transcription

1 COMPUTED TOMOGRAPHY 1

2 Why CT? Conventional X ray picture of a chest 2

3 Introduction Why CT? In a normal X-ray picture, most soft tissue doesn't show up clearly. To focus in on organs, or to examine the blood vessels, doctors must introduce contrast media into the body. Contrast media are liquids that absorb X-rays more effectively than the surrounding tissue. 3

4 Introduction CT scans are special x-ray tests that produce cross-sectional images of the body and the images allow to look at the inside of the body using x-rays and a computer. 4

5 System Components CT Gantry. Computer. Operating console. The patient support table 5

6 System Components 6

7 CT Gantry The CT gantry contains: X-ray tube. Detection system. An other associated mechanisms. Gantry Design:An x-ray tube is mounted within the scanner, opposite a set of detectors. 7

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10 The patient support table The patient support table allows for insertion of the patient into the hole of gantry, which is approximately cm in diameter. Most gantries can be tilted +/- 30 degree in order to obtain oblique slices. 10

11 Computer The computer system plays a central role in CT scanning because without it, there would be no image computation and formation. The computer controls of: x-ray generation gantry and table motion data acquisition image formation display storage. 11

12 Control console Set scan parameters :kvp, ma, scan time, reconstruction filter, etc. Set scan mode:- Digital radiograph, axial or volume Reconstructor Review and archive images Post-processing 12

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15 Detectors Scintillation Detectors: Types of material will scintillate when irradiated; that is, they emit a flash of light immediately in response to absorption of ionizing radiation. The amount of light emitted is proportional to the amount of energy absorbed by the crystal. 15

16 Detectors Early scintillation detector arrays contained scintillation crystal (Sodium iodide NaI) photomultiplier tube. Scintillation crystal coupled to the window of the crystal is a photomultiplier (PM) tube that converts the light flashes from the scintillator into electrical pulses. These detectors could not be packed together and they required a power supply for each photomultiplier. 16

17 They have been replaced with scintillation crystal (cesium iodide CsI). photodiode Photodiodes convert light into electronic signal, are smaller, cheaper and don t require a power supply for each. Approximately 90% of the x-rays incident on the detector are absorbed and contribute to the output signal 17

18 CT Principles CT uses X-rays to generate cross-sectional, images of the body. An x-ray tube is mounted within the scanner, opposite a set of detectors. The detectors receive the x-rays after they pass through the patients' body. 18

19 CT Principles Both the x-ray tube and detectors rotate around the patient, resulting in x-rays from 360 degrees. This scanned data is used by the CT computer to produce two or three-dimensional cross- sectional images. 19

20 History

21 Operational Modes First generation: rotate/translate. Second generation: rotate/translate. Third generation: rotate/rotate. Fourth generation: rotate/stationary. Fifth generation: fixed tubes, fixed detectors

22 First generation: rotate/translate, pencil beam This system used parallel ray geometry - starting at a particular angle. The X-ray tube and detector system translated linearly across the field of view (FOV), acquiring 160 rays across a 24 - cm FOV, for the projection at that particular angle. When the X - ray tube/detector system was finished with its translation, the whole system was rotated and then another translation was used to acquire the next projection set.

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25 First generation The image was not perfect. One advantage of the first - generation CT scanner was that it accepted only a very small pencil ray of X - rays. 25

26 Second generation: rotate/translate, narrow fan beam The next incremental improvement to the CT scanner was the incorporation of linear array of (3-52) detectors. The use of 30 detectors increased the utilization of the X ray beam by 30 times over the single detector used per slice in first - generation systems. A relatively narrow fan angle of 10 degree was used.

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29 Second generation The disadvantages of the fan beam was the increased scatter radiation. This affected the final image. The translation motion of first and second generation CT scanners was a major limitation because at the end of each translation, the X - ray tube/detector system had to be stopped, the whole system rotated and then the translational motion had to be restarted. 29

30 Third generation: rotate/rotate, wide fan beam The motion of third - generation CT is rotate/rotate, referring to the rotation of the X - ray tube and the rotation of the detector array. It was an evolution, because third - generation scanners could deliver scan times shorter than 5 seconds. The number of detectors used in third - generation scanners was increased (up to about 750 detectors).

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33 Third generation The rotate/rotate geometry solved the mechanical problems by removing the translational motion, practical problems remained 33

34 Fourth generation: rotate/stationary: Fourth - generation CT scanners were designed to overcome the problem of electronic drift between the many detectors used in the system. Fourth - generation uses a stationary ring of detectors positioned around the patient. Only the x-ray source rotates with a wide fan beam geometry, while the detectors are stationary.

35 Fourth generation With fourth - generation design, the detectors are removed from the rotating gantry and are placed on a stationary annulus around the patients. Modern fourth - generation CT systems use from 1200 to 4800 individual detectors. 35

36 Diagram of the fourth-generation CT scanner 36

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39 Fourth generation The principal disadvantage of fourth generation CT scanners is patient dose, which is higher than that with other types of scanners. The cost of these units may also be higher because of the large number of detectors and their associated electronics. 39

40 Fifth generation: fixed tubes, fixed detectors. Real time imaging with no moving parts, heart imaging, expensive. Newer versions use 28 image intensifiers (halfcircle) with gantry rotation at 15 RPM. 3-D real time imaging. 40

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43 Fifth generation: fixed tubes, fixed detectors. Real time imaging with no moving parts, heart imaging, expensive. Newer versions use 28 image intensifiers (half- circle) with gantry rotation at 15 RPM. 3-D real time imaging. 43

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45 Benefits 1\ CT scanning is painless, noninvasive and accurate 2\ A major advantage of CT is that it is able to image bone, soft tissue and blood vessels all at the same time 3\ Unlike conventional x-rays, CT scanning provides very detailed images of many types of tissue well as the lungs, bones, and blood vessels 45

46 4 \ CT examinations are fast and simple; in emergency cases, they can reveal internal quickly enough to help save lives 6\ CT is less sensitive to patient movement than MRI 7\ CT can be performed if you have an implanted medical device of any kind, unlike MRI 46

47 8\ No radiation remains in a patient's body after a CT examination 9\ X-rays used in CT scans usually have no side effects 47

48 Risks 1\ The effective radiation dose from this procedure ranges from approximately two to 10 msv, which is about the same as the average person receives from background radiation in three to five years. 48

49 Risks 3\ CT scanning is, in general, not recommended for pregnant women unless medically necessary because of potential risk to the baby. 4\ Children should have a CT study only if it is essential for making a diagnosis and should not have repeated CT studies unless absolutely necessary 49

50 The limitations of CT Scanning of the Body. A person who is very large may not fit into the opening of a conventional CT scanner or may.be over the weight limit for the moving table 50