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Rad T 265 CT Lecture

Rad T 265 CT Lecture. History Equipment Image Production/Manipulation. History of CT. 1895 - Roetgen discovers x-rays 1917 - Radon develops recontruction formulas 1963 - Cormack develops mathematics for x- ray absoprtion in tissue 1972 - Housfield demonstrates CT. Dateline. Dateline 2.

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Rad T 265 CT Lecture

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  1. Rad T 265 CT Lecture • History • Equipment • Image Production/Manipulation

  2. History of CT • 1895 - Roetgen discovers x-rays • 1917 - Radon develops recontruction formulas • 1963 - Cormack develops mathematics for x- ray absoprtion in tissue • 1972 - Housfield demonstrates CT Dateline

  3. Dateline 2 • 1975 - first whole body CT • 1979 - Housfield and Cormack win Nobel prize • 1983 - EBCT • 1989 - spiral CT • 1991 - multi-slice CT

  4. Hounsfield’s 1972 CT • Original idea was to move the patient not the beam. • The intent was to produce a homogeneous or monoenergetic beam. • Original scanner used a radioisotope instead of a tube.

  5. Scanner Generations • To date there have been four accepted generations with some consideration as EBCT to be the fourth. • The first fourth generation scanner was unveiled in 1978 four years after the first scanner.

  6. First generation Translate/rotate • Pencil thin beam - highly collimated • Single radiation detector • 180 translations at 1 degree of rotation • One image projection per translation • 5 minutes of scan time per image • Heads only

  7. Second generation Translate/rotate • Fan shaped beam • Multiple detectors - a detector array • 18 translations with 10 degrees between them. • Multiple image projections per translation • 30 second scan time per image • Head and body imager

  8. Third generation Rotate/rotate • Fan beam that covers the entire width of the patient • Several hundred detectors in a curvilinear detector array • Both the source and the detector array move • Hundreds of projections are obtained during each rotation, thereby producing better spatial and contrast resolution. • Scan time is reduced to one second or less per image

  9. Fourth generation Rotate/stationary • Still a fan beam • Thousands of detectors are now used • Thousands of projections are acquired producing better image quality • Sub-second scan times • Various arcs of scanning are possible increasing functionality

  10. Electron beam (EBCT) • Intended for rapid imaging • Scan time less than 100 msec • No tube, instead tungsten rings are used • Four rings allow four slices to be acquired simultaneously • No moving parts

  11. Spiral CT • Third or fourth generation scanners with constant patient movement • Use slip ring technology • Can cover a lot of anatomy in a short period of time

  12. Generation comparison first spiral 300 s <1 s scan time 80x80 1024x1024 matrix 13 mm 1 mm slice th 3 lp/cm 15 lp/cm spatial res

  13. CT image circa 1971

  14. CT Gantry • X-ray source • Detector array • Collimator • High voltage generator

  15. X-ray source • 10,000 rpm anodes • 8 MHU • Tube is parallel the patient to reduce anode heel effect • 200 - 800 mA

  16. Filters • Bow tie filters are used to ‘even out’ the beam intensity at the detectors • Primary purpose is to harden the beam • Reduces artifacts

  17. High Voltage Generation • CT uses a high kVp to minimize photoelectric effect • High kVp allows the maximum number of photons to get to the dectector array • All current scanners use high frequency generators • High frequency generators are much smaller than three phase units allowing for a smaller footprint and less voltage fluctuation

  18. Detector Array • Early scanners used scintillation crystal photomultiplier detectors as a single element • Currently two types of detector arrays • Gas filled • Solid state

  19. Gas filled detectors • Filled with high pressure xenon • Fast response time with no afterglow or lag • 50% dectection efficiency • Can be tightly packed • Less interspacing, fewer lost photons

  20. Gas filled • Ion chambers are approximately 1 mm wide • Geometric efficiency is 90% for the entire array • Total detector efficiency = geometric efficiency x intrinsic efficiency

  21. Solid state detectors • Cadmium tungstate • Scintillator • Material is optically coupled with a photodiode • Nearly 100 % efficiency • Due to design they cannot be tightly packed

  22. Solid state • 80 % total detector efficiency • Automatically recalibrate • Reduced noise • Reduced patient dose • More expensive than gas filled

  23. DAS (data acquisition system) • Amplifies the signal • Converts the analog signal to digital(ADC) • Transmits the signal to the computer Located between the detector array and the computer

  24. Detector arrays • Multiple detector arrays allow for multiple slices to be acquired simultaneously

  25. Two collimators are used in CT • Pre-patient • Controls patient dose • Determines dose profile • Post-patient • Controls slice thickness

  26. Image reconstruction algorithms • Most common process is filtered back projection • Fourier transformation • Analytic • Iterative

  27. Image reconstruction sequence • Data acquisition • Preprocessing • Reformatting and convolution • Image reconstruction • Image display • Post-processing activities

  28. High pass filters • Suppress low spatial frequencies resulting in images with high spatial resolution • Bone • Inner ear • High-res chest

  29. Low pass filters • Suppress high spatial frequencies • Most commonly used filters • Images appear smoother • Less noisy

  30. Image display • Images are displayed on a matrix • Today most are 512 x 512 or 1024 x 1024 • The original matrix was 80 x 80 • The matrix consists of pixels • Pixels represent voxels

  31. Field of View (FoV) • The diameter of the reconstructed image is the FoV

  32. Spatial resolution • Generally, pixel size is the limiting factor in spatial resolution. • The smaller the pixel the higher the spatial resolution. • Pixel size (spatial resolution) is determined by matrix size and FoV.

  33. Post-processing • Post-processing does not increase the amount of information available. It presents the original information in a different format

  34. CT Numbers • This is numerical value assigned to each pixel. • CT numbers are derived from the attenuation coefficient of the tissue in the voxel. • CT numbers are also called Hounsfield units

  35. CT numbers and attenuation coefficients tissue CT number Att Coeff bone 1000 0.46 muscle 50 0.231 white matter 45 0.187 gray matter 40 0.184 blood 20 0.182 CSF 15 0.181 water 0 0.18 fat -100 0.162 lung -200 0.094 air -1000 o.0003

  36. Attenuation • Atomic number • Tissue density • Beam energy

  37. Lambert Beer • I=Ioe-µx • Based on a homogenous beam Attenuation

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