Basics of ct lecture 2
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The document discusses the basics of computed tomography (CT) scanning. It explains that after measurements are taken by detectors, mathematical reconstruction techniques are used to convert the data into a CT image. The most common technique is filtered back projection. It also describes how window width and level are used to display CT images by mapping CT numbers to grayscale shades. The document provides advantages of CT over conventional radiography and gives a brief history of the technology.
Basics of ct lecture 2
- 1. تعلم تكن مالم وعلمكتعلم تكن مالم وعلمك ال فضل وكانال فضل وكان عظيما عليكعظيما عليك
- 2. Basics Of Computed Tomography (CT) Lecture Two GGamalamal FFathallaathalla MM.. MMahdalyahdaly [email protected][email protected]
- 3. Image ReconstructionImage Reconstruction After enough transmission measurementsAfter enough transmission measurements done by the detectors, they are sent todone by the detectors, they are sent to ADC computer for processing.ADC computer for processing. The computer uses special mathematicalThe computer uses special mathematical techniques to reconstruct the CT image intechniques to reconstruct the CT image in definite number of steps calleddefinite number of steps called reconstruction algorithms.reconstruction algorithms.
- 4. Tomographic Reconstruction After preprocessing the raw data, a CT reconstruction algorithm Filtered back projection is most widely used in clinical CT scanne The backprojection method builds up the CT image in the compu During backprojection, the μ value for each ray is in essence smeared along the same path i Areas of high attenuation reinforce each other and areas of low a
- 6. CT Image Matrix The rows and columns comprise Matrix sizes are 256x256, 512 x 5 The technologist selects the field Pixel size = FOV/matrix size. The gray scale range for each pix Spatial resolution improves with a
- 7. CT number of bone can be calculated:CT number of bone can be calculated:
- 8. CT number for water can be calculatedCT number for water can be calculated
- 9. Distribution of CT numbers on the Hounsfield and EMI scales
- 10. DisplayDisplay Whilst the range of CT numbers recognized byWhilst the range of CT numbers recognized by the computer is 2000, the human eye cannotthe computer is 2000, the human eye cannot accurately distinguish between 2000 differentaccurately distinguish between 2000 different shades of grey. Therefore to allow the observershades of grey. Therefore to allow the observer to interpret the image, only a limited number ofto interpret the image, only a limited number of CT numbers are displayed.CT numbers are displayed. A clinically useful grey scale is achieved byA clinically useful grey scale is achieved by setting the WL and WW on the computersetting the WL and WW on the computer console to a suitable range of CT numbers,console to a suitable range of CT numbers, depending on the tissue being studied.depending on the tissue being studied.
- 11. Grey Scaling The CT reconstruction process results in a 2D matrix of floating poin These numbers correspond to the average linear attenuation coeffic The CT images are normalized and truncated to integer values that CT numbers are nothing but rescaled linear attenuation coefficients Rescaling these values to definite shades of grey produces the Gre
- 12. Window width (WW) and Window Level (WL)Window width (WW) and Window Level (WL) Window widthWindow width (WW):(WW): Represents the CT numbers of all the tissues of interest andRepresents the CT numbers of all the tissues of interest and these are displayed as various shades of grey. Tissues withthese are displayed as various shades of grey. Tissues with CT numbers outside this range are displayed as either blackCT numbers outside this range are displayed as either black or white.or white. Window LevelWindow Level (WL):(WL): Represents the central numbers of all the CT numbersRepresents the central numbers of all the CT numbers covered by the window width.covered by the window width. ** NB: Both the WL and WW can be set independently on the** NB: Both the WL and WW can be set independently on the monitor and their respective settings affect the final displayedmonitor and their respective settings affect the final displayed image.image.
- 14. Image ManipulationImage Manipulation Image is initially displayed on a cathode ray tubeImage is initially displayed on a cathode ray tube (screen) but can also manipulated (modified to suit the(screen) but can also manipulated (modified to suit the diagnostic purposes) in the following ways:diagnostic purposes) in the following ways: ** Changing the filter, performing MPR,MIP,3D….etc.** Changing the filter, performing MPR,MIP,3D….etc. ** Storage on CD, DVD, FD, Film…etc.** Storage on CD, DVD, FD, Film…etc. ** Electronic transmission of text data and images from** Electronic transmission of text data and images from CT to other devices such as laser printers, diagnosticCT to other devices such as laser printers, diagnostic workstations, PACS/ RIS systems.workstations, PACS/ RIS systems.
- 15. Advantages of CT overAdvantages of CT over Conventional radiographyConventional radiography Better contrast resolution.Better contrast resolution. No tissue superimposition.No tissue superimposition. Less scattered radiation.Less scattered radiation. Measuring subtle density differences.Measuring subtle density differences. 3D imaging.3D imaging. Bone mineral Densitometry.Bone mineral Densitometry. Measuring tissue perfusion.Measuring tissue perfusion.
- 16. Just 4 history: The 1Just 4 history: The 1stst generation head scannergeneration head scanner
- 18. CT Reconstruction AlgorithmsCT Reconstruction Algorithms 1- Back-projection.1- Back-projection. 2- Iterative technique.2- Iterative technique. 3- Analytical method:3- Analytical method: A- Filtered back-projection.A- Filtered back-projection. B- Fourier transformation.B- Fourier transformation.
- 19. Raw data:Raw data: All measurements obtained from the detectors.All measurements obtained from the detectors. Convolution:Convolution: Process of applying a mathematicalProcess of applying a mathematical formula (filter function) to an attenuation profile.formula (filter function) to an attenuation profile. Back projection:Back projection: Process of converting the data fromProcess of converting the data from attenuation profile to matrix. This process involvesattenuation profile to matrix. This process involves multiplication of overlapping portions of the filtermultiplication of overlapping portions of the filter function and detector response curve selectively tofunction and detector response curve selectively to produce a third function that is used for imageproduce a third function that is used for image reconstruction.reconstruction.
- 20. processing Reformatted raw data Image reconstruction algorithm Reconstructed image of CT numbers Convolution with filter Image storage, display, recording, archiving Back projection Of convoluted data
- 21. whilst an optimal WW of 1500 and WL of –600 arewhilst an optimal WW of 1500 and WL of –600 are used to assess the lung fields (mostly air)used to assess the lung fields (mostly air) Here the WW will range from +750 to –750, but according to the WL - 600 the WW will be +150 to -1350
- 22. Any Question???. Take your Tiiime!!! . Again Any Question???. Otherwise, I’m going to ask!!!. Should I Ask???.
- 23. Have a nice dayHave a nice day
Editor's Notes
- #10: Distribution of CT numbers on the Hounsfield and EMI