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Multislice CT

Spiral CT: Successes and Limits. Spiral CT became standard" of care for body CTMore slices per breath-holdFaster exam specificationSlices reconstructed at arbitrary positions

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Multislice CT

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    1. Multislice CT

    2. Spiral CT: Successes and Limits Spiral CT became “standard” of care for body CT More slices per breath-hold Faster exam specification Slices reconstructed at arbitrary positions & increments Faster Exam Times Image quality and Patient coverage: limited by x-ray tube load (mA and beam-on time) Solution: use beam more efficiently--collect data for more than one slice at a time (multislice CT)

    3. SSCT Detectors MSCT

    4. SSCT Detectors MSCT

    5. Multislice CT Detectors

    6. Multislice Acquisition: GE QX/i

    7. MSCT: Adaptive Array: Siemens/Philips

    8. Multislice Acquisition: GE 8-Channel

    9. QUIZ: When was first the first clinical use of A Multi-slice CT ?

    10. Dual Slice CT (Elscint, 1991)

    11. The First Multislice CT: EMI Mark 1

    12. MSCT: Concepts and Issues Detector Thickness: the Z-axis size of each basic detector element Detector Collimation: total combined size of elements “linked” to acquire individual slices Beam Collimation: total Z-axis x-ray beam width Slice Thickness: determined by detector collimation and not by x-ray beam collimation

    13. Cone Beam Effect: in Single Slice CT

    14. Cone Beam Effect: MSCT

    15. Scatter in MSCT

    16. 2nd (or 3rd) Generation MSCT

    17. 16-Slice (2nd Generation) MSCT

    18. Current Generation Multislice CT

    19. RADIATION DOSES IN MSCT

    20. Matrix Detectors: MSCT Sensitivity and Dose Efficiency (Adapted from Hsieh, Med Phys 28:491-500) In the beginning of the “modern” MSCT scanners, there was considerable concern about radiation doses being significantly higher than for single slice scanners. Although not as severe an issue as it was initially, there are some loss of dose efficiency in MSCT that could result in higher doses. One straightforward example is clear from looking at the sensitivity profile for a 5 mm slice on a 4-slice scanner with 1.25 mm detector elements. A sensitivity profile is analogous to a dose profile, but is related to the dose actually captured by the detectors and contribute to image formation. It is seen in the slice that some dose passing through the patient and reaching the detector array is “lost” because they strike the dead space “dividers” between the detector elements. This loss is ~10% for a detector array with 1.25 mm elements (higher for smaller detector elements) but depends on detector design. In the beginning of the “modern” MSCT scanners, there was considerable concern about radiation doses being significantly higher than for single slice scanners. Although not as severe an issue as it was initially, there are some loss of dose efficiency in MSCT that could result in higher doses. One straightforward example is clear from looking at the sensitivity profile for a 5 mm slice on a 4-slice scanner with 1.25 mm detector elements. A sensitivity profile is analogous to a dose profile, but is related to the dose actually captured by the detectors and contribute to image formation. It is seen in the slice that some dose passing through the patient and reaching the detector array is “lost” because they strike the dead space “dividers” between the detector elements. This loss is ~10% for a detector array with 1.25 mm elements (higher for smaller detector elements) but depends on detector design.

    21. Matrix Detectors: Sensitivity and Dose Efficiency (16 slice)

    22. Sensitivity Profile: 10 mm Slice The loss of x-rays may actually be seen in the image of a slice containing thin aluminum ramps which slope through the slice. The image of the parts are the ramp at Z-axis positions corresponding to the “lost” x-rays are less intense than elsewhere. The loss of x-rays may actually be seen in the image of a slice containing thin aluminum ramps which slope through the slice. The image of the parts are the ramp at Z-axis positions corresponding to the “lost” x-rays are less intense than elsewhere.

    23. Z-Axis Dose Profile for 10 mm slice A second factor may be understood by referring back to the dose profile for the single slice scanner, for which the slice thickness was taken as the profile’s full width at half maximum (FWHM) size. If we were to divide this 10 mm slice into 4 slices scanned by a 4-slice MSCT with 4 x 2.5 mm detector configuration, it is clear the the “outer” two slices would receive considerably less radiation and would thus be considerably noisier. The only way to avoid this is to widen the x-ray beam so that all four slices are uniformly irradiated. Now, however, we waste the entire penumbra (i.e., edge region) of the dose profile, which can no longer contribute to image formation. Depending on what fraction of the total x-ray beam this represents, this represents another 10-30% A second factor may be understood by referring back to the dose profile for the single slice scanner, for which the slice thickness was taken as the profile’s full width at half maximum (FWHM) size. If we were to divide this 10 mm slice into 4 slices scanned by a 4-slice MSCT with 4 x 2.5 mm detector configuration, it is clear the the “outer” two slices would receive considerably less radiation and would thus be considerably noisier. The only way to avoid this is to widen the x-ray beam so that all four slices are uniformly irradiated. Now, however, we waste the entire penumbra (i.e., edge region) of the dose profile, which can no longer contribute to image formation. Depending on what fraction of the total x-ray beam this represents, this represents another 10-30%

    24. Z-Axis Dose Profile for 10 mm slice A second factor may be understood by referring back to the dose profile for the single slice scanner, for which the slice thickness was taken as the profile’s full width at half maximum (FWHM) size. If we were to divide this 10 mm slice into 4 slices scanned by a 4-slice MSCT with 4 x 2.5 mm detector configuration, it is clear the the “outer” two slices would receive considerably less radiation and would thus be considerably noisier. The only way to avoid this is to widen the x-ray beam so that all four slices are uniformly irradiated. Now, however, we waste the entire penumbra (i.e., edge region) of the dose profile, which can no longer contribute to image formation. Depending on what fraction of the total x-ray beam this represents, this represents another 10-30% A second factor may be understood by referring back to the dose profile for the single slice scanner, for which the slice thickness was taken as the profile’s full width at half maximum (FWHM) size. If we were to divide this 10 mm slice into 4 slices scanned by a 4-slice MSCT with 4 x 2.5 mm detector configuration, it is clear the the “outer” two slices would receive considerably less radiation and would thus be considerably noisier. The only way to avoid this is to widen the x-ray beam so that all four slices are uniformly irradiated. Now, however, we waste the entire penumbra (i.e., edge region) of the dose profile, which can no longer contribute to image formation. Depending on what fraction of the total x-ray beam this represents, this represents another 10-30%

    25. Dose Efficiency vs Slice Thickness What fraction of the x-ray beam is “lost” by discarding the penumbra region depends both on colllimator design and total beam width. The size of the penumbra region depends on collimation and focal spot size, and is approximately constant for all usefull beam widths. Thus the penumbra will represent a larger percent loss (perhaps ~30%) for a 4x1.25 mm scan (5 mm total width) than for a 4x2.5 mm acquisition (10 mm total beam width). For a 4x5 mm scan (total width of 20 mm) the loss is even smaller (<10%). As more simultaneous slices are scanned and total beam widths enlarge with future cone beam scanners, the lost penumbra will be of minimal concern. What fraction of the x-ray beam is “lost” by discarding the penumbra region depends both on colllimator design and total beam width. The size of the penumbra region depends on collimation and focal spot size, and is approximately constant for all usefull beam widths. Thus the penumbra will represent a larger percent loss (perhaps ~30%) for a 4x1.25 mm scan (5 mm total width) than for a 4x2.5 mm acquisition (10 mm total beam width). For a 4x5 mm scan (total width of 20 mm) the loss is even smaller (<10%). As more simultaneous slices are scanned and total beam widths enlarge with future cone beam scanners, the lost penumbra will be of minimal concern.

    26. Dose (Phan Center) vs Collimation

    27. MSCT DOSIMETRY MSCT dosimetry evaluated by considering only pitch and total beam width as if it was one “slice”. “Slice thickness” is irrelevant. Practical Problem: 100 mm pencil chamber used until now for CTDI measurements do not capture the full length of the Z dose profile with large Z field-of-view (>10 mm) in MSCT: correction factors will be needed. May make sense today with MSCT to use a shorter dosimeter and scan several slices

    28. MSCT DOSIMETRY (con’t): Issues Reduced Geometric Efficiency in MSCT: Some x-rays hit dividers between detector elements Beam penumbra is wasted: beam widened so all elements see approximately equal intensity For MSCT Helical Scans, Pitch may be < 1 (more on this later)

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