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CT Scanning

CT Scanning. Dr. Craig Moore Medical Physicist & Radiation Protection Adviser Radiation Physics Service CHH Oncology. Brief History of CT Scanning. First CT Scanner - 1972. Originally called CAT A = axial 80 x 80 resolution 4 min. per rotation 8 grey levels overnight reconstruction.

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CT Scanning

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  1. CT Scanning Dr. Craig Moore Medical Physicist & Radiation Protection Adviser Radiation Physics Service CHH Oncology

  2. Brief History of CT Scanning

  3. First CT Scanner - 1972 • Originally called CAT • A = axial • 80 x 80 resolution • 4 min. per rotation • 8 grey levels • overnight reconstruction

  4. Here and Now • 512 x 512 or 1024 x 1024 resolution • Sub second rotation • 4096 grey levels • 100’s slicesper rotation

  5. Components of a CT Scanner

  6. Principal components of CT scanner • X-ray tube, collimator, and detector array on a rotating gantry • Rotation axis is referred to as Z axis • Fan beam wide enough to cover patient cross-section • Narrower width in the z-axis • Behind the patient is a bank of detectors • Patient lies on a couch that is moved longitudinally through the gantry

  7. X-ray Tube • Tube parallel to patient movement – minimise anode heel effect • X-rays are produced by firing electrons at a metal target – typically tungsten • Capable of producing long exposure times at high mA – get very hot (require heat capacities up to 4MJ and active cooling mechanisms) • Continuous scanning limited to around 90s • Focal spot size typically 0.6 - 1mm • High kVps (80 – 140) and beam heavily filtered (6-10 mm Al filters) to optimise spectrum • Stops attenuation coefficients varying with depth via beam hardening

  8. Collimation and Filtration • Want a monoenergetic beam to avoid beam hardening artefacts • As beam passes through patient low energies are filtered • This results in the apparent reduction of attenuation and CT number of tissues • Computer reconstruction assumes monoenergetic beam • Not possible with X-ray tubes so they are heavily filtered • At least 6 mm aluminium or copper and typically high kVp • Some manufacturers use shaped filters such as bow-tie filters to even out the dose distribution (conform to the shape of an elliptical patient) • Pre-patient collimator is mounted on the X-ray tube • Beam is approx 50cm wide to cover cross section of patient • The size is variable in the z-axis • Multi-slice scanners between 0.5 and 40 mm thick beams • Post patient collimation is not used with multi-slice scanner

  9. Bow Shaping Filters • Body is approx circularly shaped • Shape of beam that reaches detector is therefore non-uniform which can lead to image artefacts • Shaping filters are applied to the beam prior to make the dose distribution more uniform

  10. Detectors • Requirements: • Small enough to allow good spatial resolution • Up to 1000 detectors per scanner • Typically 1.5 mm width but can be a small as 0.5mm • High detection efficiency • Fast response • Wide dynamic range – massive variation in X-ray intensity • Stable and noise free • No afterglow • There needs to be separation between detectors to prevent light crossover • This reduces efficiency from 98% to 80%

  11. Detectors • Modern CT detectors are fabricated as detector modules • A side view shows individual detector elements coupled to photodiodes that are layered on electronics • Spaces in between filled with optical filler to reduce cross talk

  12. Detectors

  13. Generations of CT Scanner

  14. CT Imaging • Conventional radiography suffers from the collapsing of 3D structures onto a 2D image • However, CT scanning has extremely good low contrast resolution, enabling the detection of very small changes in tissue type • Almost true depiction of subject contrast • CT gives accurate diagnostic information about the distribution of structures inside the body • Generation of images in transaxial section • Perpendicular to the axis of rotation of the X-ray tube about the body • Perpendicular to the craniocaudal axis of patient

  15. Number of detectors and projections • Typically, for a 3rd generation scanner: • 650 – 900 detectors • 1000 to 2000 projections per rotation

  16. Image contrast 2:1 Collapse of 3D Data into 2D Plane • Planar imaging • 2D representation of 3D Distribution of Tissue • No depth information • Structures at different depths are superimposed • Loss of contrast Subject Contrast 4:1 X rays

  17. Typical CT Image

  18. CT Images • Commonly calculated on 512x512 matrix, but 256x256 and 1024x1024 are also used • Each pixel is more accurately described as a voxel, because it has depth information • The value stored in each voxel is referred to as the CT number which is related to the attenuation of a particular tissue: • CTn = 1000 x (µt - µw)/ µw • Sometimes referred to as Hounsfield Units • Each CT number is assigned a certain shade of grey in the resulting image • CT number represents x-ray attenuation coefficient of the corresponding voxel within the patient

  19. CT Numbers

  20. Image Display • CT image represented by a range of CT numbers from -1000 to + 3000 (ie 4000 levels of grey) • Human eye dose not have the capacity to distinguish so many grey levels • If 4000 shades of grey displayed altogether there would be very little difference between different tissues

  21. Window Width and Level • The appearance of the image on the screen can be changed by altering the window width and level • Window width refers to the range of CT numbers selected for display • This range of CT numbers is centred at a particular level called the window level • e.g. if imaging bone window level should be ~1000 • Can spread a small range of CT numbers over a large range of grayscale values

  22. Window Level –593 Window Width 500 Good contrast in lungs Only see CT numbers +/- 250 around -593 Window Level –12 Window Width 400 Good soft tissue contrast Only see CT numbers +/- 200 around -12

  23. How do we get the images? • Tube and detector rotate smoothly around the patient • X-rays are produced continuously and the detectors sample the X-ray beam approx 1000 times during one rotation • Typically 2 to 4 revolutions per second

  24. In reality not always parallel to detectors • Each voxel is traversed by one or more x-ray beams for every measurement (1000 per rotation) • Number of measurements taken in single axial section depends on • number of detectors • Number of samples per rotation • Assume 800 detectors measured at 0.5° intervals per 360 ° rotation • This is 576,000 measurements • More than needed as we only need 260,000 measurements (512 x 512)

  25. How do we get the picture? • Back Projection • Reverse the process of measurement of projection data to reconstruct image • Each projection if smeared back across the reconstructed image

  26. Back Projection – the basics • Consider cylindrical uniform body with a hole down the centre • A beam passing through this body from one direction will have a transmitted profile in its central region • This single measurement cannot determine the position of the hole other than identifying that it is in the line of the pencil beam passing through the centre of the body • Pixel values along this line are decreased by the amount of attenuation measured • These values are projected back along the field of view • A second projection at 90° provides a second band of grey • This is then projected back across field of view • Progressive projections are shown in the final figure – a star like pattern • We now have an image that looks similar to what we are scanning

  27. Back Projection • Back Project each planar image onto three dimensional image matrix 3 6 3 3 3 6 6 3 3 3 6 3

  28. Back Projection • Back Project each planar image onto three dimensional image matrix 3 6 3 1 2 1 1 2 1 1 2 1

  29. Back Projection • Back Project each planar image onto three dimensional image matrix 3 6 3 3 2 2 1 3 2 1 2 1 2 1 6 3 4 3 3 2 1 2 3 1 2

  30. Back Projection • Back Project each planar image onto three dimensional image matrix 3 6 3 6 3 4 4 3 6 6 6 8 6 6 3 4 4 3 3 6 3

  31. Back Projection • Back Project each planar image onto three dimensional image matrix 3 6 3 6 3 4 4 3 6 6 6 8 6 6 3 4 4 3 3 6 3

  32. Back Projection • More views – better reconstruction • 1/r blurring, even with infinite number of views

  33. Filtered Back Projection • Back projection produces blurred transaxial images • Projection data needs to be filtered before BP • Different filters can be applied for different diagnostic procedures • Smoother filters for viewing soft tissue • Sharp filters for high resolution imaging • After filtration, back projection same as before • Data from neighbouring beams are used • Some data is subtracted • Some data is added • Filters are convolved with the blurred image data in Fourier Space

  34. Filtered Back Projection

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