MDCT Technology. Kalpana M. Kanal, Ph.D., DABR Assistant Professor Department of Radiology University of Washington Seattle, Washington

Transcription

1 MDCT Technology Kalpana M. Kanal, Ph.D., DABR Assistant Professor Department of Radiology University of Washington Seattle, Washington ACMP Annual Meeting Seattle, WA

2 Educational Objectives Historical Development of CT Why has the frequency of CT increased? Multi-Row Detector CT and Pitch Advantages of Multi-Row Detector CT Automatic Exposure Control in CT Cardiac CT Future of CT Kanal 2

3 Historical Development of CT 64 slice scanners Dual Source CT Slow scan time Fast scan time Dr. Mahesh, John Hopkins, MD, AAPM Handout. Kanal 3

4 Historical Development of CT Seventh Generation CT (late 1990 s 2000) Multi-Row Detector Systems Radiographics 2000: Kalendar WA, Computed Tomography, Second Edition, 2005, pg 58. Kanal 4

5 Historical Development of CT PET/CT Scanners (2001) Kalendar WA, Computed Tomography, 2 nd ed., 2005, pg 74. Kanal 5

6 Historical Development of CT Dual-Source CT (2005) A scanner using two X-ray sources and two detectors at the same time Enables scanning a heart at any heart rate and at the lowest radiation dose possible Kanal 6

7 Fast Scanning, 0.3 sec Kanal 7

8 Why has the frequency of CT exams increased? UNSCEAR 2000 More like 60% New technology Faster scans New exams done with CT More screening exams ICRP Publication 87 (2001) Kanal 8

9 Single-Row Detector versus Multi-Row Detector CT Radiographics 2000: Kanal 9

10 Single-row Detector vs. Multi-row Detector CT Seeram, Computed Tomography, 2 nd ed., 2001, pg 258. Kanal 10

11 Multiple Detector Arrays Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p Kanal 11

12 Slice Width Selection GE 64 MDCT (40 mm detector) 64 x mm 32 x 1.25 mm 16 x 2.5 mm 8 x 5 mm Kanal 12

13 32/40/64 Slice CT Systems Kanal 13

14 Pitch in Multi-slice CT Two pitch definitions seen in MSCT Collimator pitch = table travel x-ray beam width Detector pitch = table travel detector width Collimator pitch = Detector pitch N c.f. Kanal 14

15 Multi-Row Detector CT - Advantages Same acquisition in shorter times (fewer motion artifacts) Seeram, Computed Tomography, 2 nd ed., 2001, pg 263. Kanal 15

16 Multi-Row Detector CT - Advantages Reconstruction in different slice widths Kanal 16

17 Multi-Row Detector CT - Advantages Efficient use of x-ray x beam with increasing slices Reduction of radiation dose Kalendar WA, Computed Tomography, 2 nd ed., 2005, pg 168. Kanal 17

18 Multi-Row Detector CT - Advantages Thinner slices for better z-axis z resolution Isotropic Imaging Improved Spatial Resolution All sides of the voxel have equal dimensions Seeram, Computed Tomography, 2 nd ed., 2001, pg 264. Kanal 18

19 Multi-Row Detector CT - Advantages Better Multiplanar Reconstruction & 3D Display Radiographics 2002: Kanal 19

20 14% 1) 14% 29% 43% 0% Regarding Multiple Detector CT scanners: 1) A scanner with a 16-detector array employs a total of 16x16 detectors 2) Detectors used are the non-radioactive xenon-gas detectors 3) The slice width is determined by width of detectors in slice thickness dimension 4) The slice width selected can be smaller than the physical size of the detector 5) The spatial resolution is worse than that of a single-detector CT scanners Kanal 20

21 Answer The slice width is determined by width of detectors in slice thickness dimension Kanal 21

22 Multi-Row Detector CT - Advantages Cardiac CT 40 mm detector (Pitch ~0.25) 12.5 cm in 5 sec 20 mm detector (Pitch ~0.25) 6.2 cm in 5 sec Kalendar WA, Computed Tomography, 2 nd ed., 2005, Video. Kanal 22

23 For a 16-detector row CT scanner, the x-ray x beam width is 20 mm, detector width is 5 mm and the table travel per rotation is 20 mm. Calculate the detector pitch and collimator pitch? 15% 1. 1, 4 62% 2. 4, 1 15% 3. 5, 20 8% 4. 4, 4 0% 5. 20, 5 Kanal 23

24 Answer 4, 1 Pitch d = 20 = 4 = Detector pitch 5 Pitch x = 20 = 1 = Collimator pitch 20 Kanal 24

25 Dose Reduction Current Modulation or AEC in CT Modern CT scanners are capable of modulating the ma (current) during the scan Rationale: it takes fewer photons (lower ma) to penetrate thinner tissue, and more x-ray x photons (higher ma) to penetrate thicker projections through the body Benefits Consistent image quality Dose reduction potential Reduced tube loading Extended scan runs Reduction in photon starvation artifact Kanal 25

26 Automatic Exposure Control (AEC) in CT Patient size AEC Patient size AEC: higher ma used for larger patients MHRA Report 05016, February Kanal 26

27 Automatic Exposure Control (AEC) Z-axis AEC Z-axis AEC: higher ma used at more attenuating positions along the z-axis z (patient) MHRA Report 05016, February 2005 Radiographics, 2006, 26: Kanal 27

28 Automatic Exposure Control (AEC) in CT Angular AEC Angular AEC: the degree of modulation depends on the asymmetry of the patient MHRA Report 05016, February 2005 Radiology, 2004, 233: Kanal 28

29 Dose Reduction Cardiac ECG dose modulation It is possible to vary the ma based on R-R R R interval phase % This can lead to a dose decrease of up to 50% in low stable heart rates 70% 80% interval used for image reconstruction 550 ma dose not given to patient 200 Kanal 29 t

30 Dose Reduction - ECG dose modulation HR BPM (One Segment) 70 Exposure CTDI (mgy) Segment mode, no ECG Coverage during scan Heart rate (BPM) 240 o Paul Kinahan and Kelly Branch, UWMC Kanal 30

31 Dose Reduction - ECG dose modulation HR BPM (One Segment), TR 175 msec Exposure C TD I (m G y) Heart rate (BPM) 42% decrease in dose Segment mode, no ECG Segment mode [65-85%], w/ecg Segment mode [70-80%], w/ecg Needed coverage during scan 240 o Paul Kinahan and Kelly Branch, UWMC Kanal 31

32 Regarding automatic tube current modulation in CT: 0% 1. It gives inconsistent image quality 0% 2. It increases the scan time for the exam 0% 100% 0% 3. It generates more heat in the x-ray x tube 4. The tube current is automatically adjusted according to patient size 5. It causes an increase in photon starvation artifact Kanal 32

33 Answer The tube current is automatically adjusted according to patient size Kanal 33

34 Reducing Cardiac Dose Prospective Gating No functional information Kanal Mahesh, et al. Radiographics, 2007; 27:

35 Retrospective Gating Kanal Mahesh, et al. Radiographics, 2007; 27:

36 Prospective vs. Retrospective Gating University of Washington Study Effective Dose (msv) Retrospective Gating 26.4 ±6.2 Prospective Gating 6.2 ± % lower radiation dose with prospective study This study concluded that the 64 channel cardiac CT performed with prospective ECG gating may have similar subjective image quality and substantially lower patient radiation dose compared to retrospective ECG gating Kanal Shuman, et al. submitted for publication, 2007

37 Regarding prospective ECG triggering for cardiac CT exams 0% 1. Projection data has to be collected through out the cardiac cycle 0% 100% 0% 0% 2. The temporal resolution is about 350 ms 3. The radiation dose is reduced compared to retrospective ECG gating 4. Functional information of the heart is available with this exam 5. Increase in motion artifacts occurs Kanal 37

38 Answer The radiation dose is reduced compared to retrospective ECG gating Kanal 38

39 Future of CT Kalendar WA, Computed Tomography, 2 nd ed., 2005, pg 235. Kanal 39

40 Future of CT Other Developments Kanal Kalendar WA, Computed Tomography, 2nd ed., 2005, pg

41 TAKE HOME POINTS MDCT scanners are becoming common and have several advantages over SDCT CT utilization has increased significantly AEC or Automatic Tube Current Modulation and ECG modulation available on modern scanners which helps reduce dose Cardiac CT is becoming popular with MDCT scanners Always keep ALARA in mind! Kanal 41

42 THANK YOU Kanal 42