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Multi-Slice CT

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Multi-Slice CT

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    1. CT Multi-Slice CT

    2. Third Generation CT Single or Multislice

    3. Single Slice Thickness Determined by Collimation

    4. Single-Slice Detectors Many detectors rotate around patient Single row in z-direction Slice thickness determined by collimation

    5. Single Slice CT: Slice Thickness

    6. Multi-slice CT

    7. What’s Different for Multislice CT?

    8. Multislice CT Multiple rows of detectors Open collimators in “Z” direction

    9. Multi-slice CT Developed in late 1990’s Detector array segmented in z-direction Simultaneous acquisition of multiple slices

    10. Single Slice vs. Multislice Detector

    11. Multi-Slice Detectors Many detectors going around patient Many detector rows in z-direction Slice thickness determined by Collimation electronic detector selection

    12. Multi-slice CT Size & distribution of detectors in non-axial direction similar to previous CT’s Similar spatial & contrast resolution

    13. Distribution of detectors in axial direction varies with manufacturer All detectors same width

    14. Multi-slice CT Uniform Detector Thickness Multiple detectors in axial direction Size must accommodate thinnest slice Detector signals can be used Individually In groups

    15. Detectors vs. Channels # Physical Detectors not necessarily equal to # of possible Slices Maximum # slices limited by Digital Acquisition System (DAS) channels Electronic counters Imaging speed bottleneck How fast data can be received from detector arrays

    16. Detectors vs. Channels Example 16 detectors 4 channels

    17. Multi-Slice Detector Example 16 Detector Rows, 4 Channels

    18. Detectors vs. Channels 4 X 1.25 mm Beam collimated to 4 detector rows 1 detector row per DAS channel

    19. Detectors vs. Channels 4 X 2.5 mm Beam collimated to 8 detector rows 2 detector rows per DAS channel

    20. Detectors vs. Channels 4 X 3.75 mm Beam collimated to 12 detector rows 3 detector rows per DAS channel

    21. Detectors vs. Channels 4 X 5 mm Beam collimated to 16 detector rows 4 detector rows per DAS channel

    22. Capture Efficiency Fraction of detector area that is active detector

    23. Equal-width Detectors Disadvantage Many gaps Gaps are dead space Reduce capture efficiency

    24. Multi-slice CT “Adaptive Array Detectors” Some scanners use detectors of various widths Post-collimators used to partially block wider elements for thinner slices

    25. Variable Width Detectors Center detectors thinner Thicker detectors can function as thinner ones using collimation Thinner detectors can function a thicker one by combining signals

    26. Single Slice Pitch Definition table motion during one rotation Slice Pitch = --------------------------------------- slice thickness

    27. Beam Pitch Defined only for Multi-slice scanners table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness

    28. Beam Pitch Defined only for Multi-slice scanners

    29. CT Beam Pitch

    30. Example

    31. Beam Thickness

    32. Table Speed

    33. Slice Thickness Defined at Rotational Center

    34. Detector Field must be Larger than Slice Thickness at Rotational Center

    35. Beam Divergence More of a Problem for Multi-Slice

    36. Multislice CT Doses Can be 10-30% higher than for single slice units (ICRP #47) Cause Divergent beam Other considerations Tendency to cover more volume (anatomy) Better availability of equipment

    37. Other Reasons for High CT Doses Repeat Exams No adjustment of technique factors for different size patients No adjustment for different areas of body

    38. Multislice CT Advantage? Speed!

    39. Single slice / Multislice Images about the same!

    40. Speed = Power Speed enables new applications

    41. How do we spend our new speed?

    42. Multi-slice CT Imaging Clinical Advantages Same acquisition in shorter time Trauma Peds OR Thinner slices for improved z-direction resolution OR Scan larger volumes in same time

    43. Multi-slice CT Imaging Clinical Advantages Thinner slices Improvement in CTA of neck, aorta, renal vessels Better reconstructions Sagittal, coronal, oblique 3-D Fundamental Trade-off “z” axis resolution vs. image noise

    44. Multi-slice CT Imaging Clinical Advantages Better Contrast Utilization Simplifies timing of contrast bolus Continuous observation of target vessel Can reduce amount of contrast required Possible coverage from aorta to lower extremities Runoff

    45. Multi-slice CT Imaging Clinical Advantages Improved x-ray tube utilization Reduced x-ray tube loading 4 slices acquired with same tube loading previously used for 1 Less need to pause of tube cooling Reduced wear & tear Other anticipated benefits CT endoscopy Diagnosis of pulmonary embolism

    46. Continuous CT Imaging Interventional Procedures Biopsy & drainage Neuro Chest Abdominal Spine Catheter and tube placement Helps operator avoid critical structures near path of biopsy needle Better visualizing of moving structures Respiration Functional CT Brain perfusion

    47. Multi-Slice Compared to Single-slice helical Much Faster No significant image quality differences Equivalent Patient Dose Ref: Willi Kalender, Ph.D Institute of Medical Physics University of Erlanger, Germany

    48. Multi-slice challenges Much more raw data More archival capacity More thin slices possible May make multi-image printing cumbersome May require viewing on-line Requires faster systems Requires faster communications for remote viewing Radiologist legally responsible for all images

    49. Acknowledgement Many drawings obtained from www.impact.org website

    50. The Future More detector channels More speed Flat panel area detectors ???

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