Lay out 1. CT in brief 2. Generation types & their features (i) First generation: rotate/translate,pencil beam with one detector
(ii)Second generation: rotate/translate, narrow fan beam, multiple detectors
(iii) Third generation: rotate/rotate, wide fan beam (iv) Forth generation: rotate/stationary, with detector ring (v). Fifth generation: electron beam CT (vi). Sixth generation: helical (or spiral) CT (vii). Seventh generation: multiple detector array 3. Summary 4. References 2
CT: The beginning CT founded in 1970 by Sir Godfrey Hounsfield
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CT combines X radiation and radiation detectors coupled with a computer to create cross sectional image of any part of the body. Equipment Arrangement Radiography v/s Tomography Cross-sectional slices
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CT scanners are categorized into several generations depending on the Type of detectors,
Scanning method etc.
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First Generation Scanners Single detector, single x-ray tube, rotate/translate pencil beam system, rotation angle/step 1° Long scan time 5 min Over night image reconstruction Poor spatial resolution Best scatter rejection Designed only for evaluation of brain The axis of rotation passed through the centre of the patient’s head 9/22/2015
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• Head kept enclosed in a water bath • Two side-by-side detectors • A reference detector
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Second Generation Scanners Linear array of about 30 detectors, single x-ray tube, rotate/translate motion, narrow angle (10 ° ) fan beam rotation angle/step 10° Shortest scan time was about 18 s per slice
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Third Generation Scanners Linear array of about 800 detectors, single x-ray tube, rotate/rotate motion only, wide fan beam (500-550) to cover the entire patient Scan time of newer scanners is about ½ s per slice Can produce ring artifacts
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Fourth Generation Scanners Complete circular array of about 4800 stationary detectors Single x-ray tube rotates with in the circular array of detectors Wide fan beam to cover the entire patient Scan time of newer scanners is about ½ s per slice Designed to address ring artifacts 9/22/2015
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Fifth Generation Scanners
Electron Beam CT (EBCT)
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Fifth Generation Scanners ctd 210° arc array of stationary detectors x-rays are produced from the focal track as high energy electron beam strikes the tungsten target Scan time is about 50 ms per slice Developed for cardiac tomographic imaging
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5th generation: ctd….
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6th Generation CT (Helical) (1990)
Eliminating interscan delays- Slip ring concept
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* Allows the complete elimination of interscan delays * Slip ring passes electrical power to the rotating components (X ray tune and Detectors) without fixed connections. 9/22/2015
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6th Generation CT (Helical) ctd… Patient is transported continuously through gantry
while data are acquired c continuously during several 360 o rotations. By avoiding the time require to translate the patient table, the total scan time required to image the patient can be much shorter. Allow the use of less contrast agent and increase patient throughput In some instances the entire scan be done within a single breath-hold of the patient. 9/22/2015
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*
7th Generation : Multiple detector Array
When using multiple detector arrays, the collimator spacing is wider and more of the X rays that are produced by the tube are used in producing image data. ---Opening up the collimator in a single array scanner increase the slice thickness, reducing spatial resolution in the slice thickness ---With multiple detector array scanners, slice thickness is determined by detector size not by collimator. 9/22/2015
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Advantages of Multiple detector Array
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Summary Generation
Configuration
Detector
Beam
Min scan time
First
Translate rotate
1-2
Pencil thin
2.5min
Second
Translate rotate
3-52
Narrow fan
10sec
Third
Rotate- rotate
256-1000
Fourth
Rotate- fixed
Fifth
Electron beam
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Dis AdV
Wide fan 0.5sec
Better predetector collimation
Ring artifact
600-4800
Wide fan 0.5sec
Avoid ring artifact
Higher Patient Dose (tube close to Pt.)
1284
Wide fan 50ms electron beam
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References 1. “Computer Tomography”, PPT presentation by Moustafa M. Mohamed. 2. “Computer Tomography”, PPT presentation by Lovnish Thakur 3. “Basic & Conceptual Information about Computed Tomography” PPT presentation by by Maulik R. Shah. 4. “ Basic Principles of CT scanning” PPT presentation by NG. Hareesha 9/22/2015
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