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CT Quality and Safety

CT Quality and Safety. Name the 4 factors affecting image quality?. Factors Affecting Image Quality. Four main factors contributing to image quality are: Spatial resolution, Contrast resolution, Noise, and Artifacts Diagnostic imaging has superior spatial resolution compared to CT

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CT Quality and Safety

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  1. CT Quality and Safety

  2. Name the 4 factors affecting image quality?

  3. Factors Affecting Image Quality • Four main factors contributing to image quality are: Spatial resolution, Contrast resolution, Noise, and Artifacts • Diagnostic imaging has superior spatial resolution compared to CT • How do we measure spatial resolution? • What is the range for plane film?

  4. CR & DR 4000 x 4000 image only as good a monitor* 525 vs 1000 line more pixels = more memory needed to store resolution dependent on pixel size CR 2 - 5 lp/mm RAD 3-6 lp/mm DR 3 - 5 lp/mm CT 8 to 32 lp/cm IMAGE APPEARS SHARPER BECAUSE CONTRAST CAN BE ADJUSTED BY THE COMPUTER – (DIFFERENCES IN DENSITY) Image Resolution –(how sharply is the image seen)

  5. CT exam Width Center(level) Brain 190 50 Skull 3500 500 Orbits 1200 50 Abdomen400 35 CT exam Width Center(level) Liver 175 45 Mediastinum325 50 Lung 2000 500 Spinal cord 400 50 Spine 2200 400 Merrill's pg 313 TABLE 31-2 Typical window settings

  6. Technologist Determines Image Quality Factors • The technologist choice of slice thickness, focal spot size, display FOV, technique selection, pitch and reconstruction algorithm will effect the resultant image quality

  7. Spatial Resolution • The amount of blurring in an image. • Affecting technical factors: focal spot size, slice thickness, display FOV, matrix and reconstruction algorithm. • Detector width is the most significant factor • MTF/limiting factor

  8. Contrast Resolution • The ability to differentiate between differences in density with in an image. • Tissues with density differences of less than 0.5% can be distinguished with CT. • What contributes to this?

  9. Contrast Resolution • Affecting technical factors: Slice thickness, reconstruction algorithm, image display and x-ray beam energy • Patient size and detector sensitivity also have a direct effect

  10. Noise • Random variations in photon detection • Affecting technical factors: Matrix size, slice thickness, x-ray beam energy and algorithm • Scatter and patient size also contribute

  11. Artifacts • Unwanted densities with in an image, not representing actual patient anatomy.

  12. Data Acquisition • Of the 3 major system components where is the data acquisition selected? • Scan time: usually preselected as part of the programmed protocol, but can be altered. Alterations may change image quality

  13. Scan Diameter • Scan FOV – area being scanned • Display FOV – area to be displayed on the monitor.

  14. Changes to the FOV

  15. Dynamic scanning • After contrast injection different structures enhance at different rates. • As contrast moves through the different phases; arterial, venous, equilibrium, delay, additional imaging can be diagnostically useful.

  16. Axial, Helical/Spiral,Multi detector row CT, Dual source • Axial = point and shoot • Helical = continuous tube rotation and table movement. High speed volume scanning • Multi detector = 2 to 320 slices per revolution • Dual source = two tubes, two sets of detectors

  17. Axial Scanning

  18. Helical Scanning

  19. Pitch

  20. Pitch = 1.5

  21. Pitch = 0

  22. SPIRAL/HELICAL CT • Helical CT = Continuous Tube and Table movement • Pitch • .5 = more information more radiation • 2 = less information less radiation

  23. How do we obtain images between slices when the pitch is higher than 1?

  24. Multi-detector Imaging (MDCT)collects more raw data

  25. This multi-detector array contains 64 rows of 1824 individual detectors, each 0.6 mm wide (116,736 detectors)

  26. Beam Geometry • Fan – The traditional beam geometry, it is opened along the width of the patient. Tube is rotated to the z-axis. • Multi-slice (cone) – The beam has to be “opened” up, head to foot, to cover the additional detectors.

  27. Filtration for CT beam

  28. What if you wanted thinner more detailed images?

  29. THREE-DIMENSIONAL IMAGING • A post-processing technique that is applied to the raw data to create realistic images of the surface anatomy to be visualized • Maximum Intensity Projection (MIP), Shaded surface display (SSD), Volume rendering (VR)

  30. Most common filming devices • Filming …..ha ha! Laser printers • PACS, DVD, Opitical Disk?

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