CT: Physics Principles & Equipment Design

Transcription

1 CT: Physics Principles & Equipment Design James Kofler, Ph.D Radiology Mayo Clinic Rochester, MN June 27, 2012

2 Disclosures Nothing to disclose

3 Learning Objectives Understand fundamental concepts of - CT image acquisition - CT image formation Know basic CT scanner components and functions Implementation and implications of multi-slice CT But first

4 Jim s One Minute History of CT Johan Radon mathematically demonstrated reconstruction of an object from multiple projections Allan Cormack introduced the Fourier transform in the calculations. But, FALSE URBAN LEGEND says it all started with

5 Jim s One Minute History of CT

6 Jim s One Minute History of CT

7 Jim s One Minute History of CT Godfrey Hounsfield Nobel Award 1979 (with Cormack) Physiology or Medicine

8 Jim s Mythical One Minute History of CT Maizlin ZV, Vos PM. Do we really need to thank the Beatles for the financing of the development of the computed tomography scanner? J Comput Assist Tomogr Mar-Apr;36(2): No evidence of Beatles CT connection EMI music did not fund EMI Medical Most of the funding came from British Department of Health and Social Security (DHSS)

9 CT Image Basics Image acquisition and reconstruction

10 System Introduction

11 System Introduction

12 Goal Determine attenuation coefs within a section of the body Ray: One x-ray path Ray sum (or line integral): Total attenuation along 1 ray Projection (or view): Set of ray sums at a fixed angle N=N 0 e - t

13 Image Creation

14 Image Creation Call this 0 degrees Backprojection

15 45 Degrees

16 90 Degrees

17 135 Degrees

18 Image Creation 4 projections 16 projections 8 projections 32 projections

19 Why So Blurry? Inherent in back projection scheme Solution: Filter prior to back-projecting Called the Reconstruction Algorithm or Kernel

20 Computer Generated 4 views (filtered) 20 views (filtered) 100 views (filtered) 600 views (filtered) 600 views (not filtered) Images courtesy of Lifeng Yu, Ph.D., Mayo Clinic, Rochester, Minnesota

21 Reconstruction Algorithm Affects frequency and noise in image Can reprocess (if have raw data) no need to re-scan Very Smooth Kernel Very Sharp Kernel

22 A Brief Sidetrack The elusive Sinogram

23 A Brief Sidetrack Sinogram Reconstructed Image Images courtesy of Shuai Leng, Ph.D., Mayo Clinic, Rochester, Minnesota

24 A Common Artifact Bad Detector

25 What We Are Actually Measuring Number of photons after traveling though thickness t N = N o e - t Starting number of photons Linear attenuation coefficient Know N (measured during scan) Know N o (from calibration) Know t (from matrix size and SFOV) Solve for μ

26 Linear Attenuation Coefficient Physical property of the material (z, ) Much more at

27 Linear Attenuation Coefficients Dependence on energy of photons

28 X-Ray Energy Spectra Adapted from Johnson, et. al., Eur Radiol (2007) 17: 1510Ð1517 and from R. Raupach,

29 X-Ray Energy Spectra - Filtration HVL 8-10 mm Al (120 kv) Adapted from Johnson, et. al., Eur Radiol (2007) 17: 1510Ð1517 and from R. Raupach,

30 A Simple Example Lather, rinse, repeat solve for μ s Clinical CT (512 x 512 squares, 1000s of projections)

31 Reconstructions FFT techniques Iterative techniques More information AAPM Virtual Library (view on-line) Terry M. Peters. CT Reconstruction Presented at the 44th AAPM Annual Meeting. Montreal, Quebec, Canada. July 18, 2002.

32 Hounsfield Units CT # (HU) = ( material - water ) water X 1000 Water: 0 HU (by definition) Air: HU Soft Tissue: +30 to +60 HU Fat: -80 to -40 HU Bone: HU WW and WL control displayed range.

33 Hounsfield Units Assigned to pixel values -987 HU 142 HU 12 HU 1010 HU

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35 Which is true regarding the reconstruction algorithm/kernel? 29% 1. Applied after back-projection. 2% 8% 62% 0% 2. Reduces ring artifact. 3. Used only during data acquisition. 4. Affects image noise. 5. Used for scanner calibration.

36 Which is true regarding the reconstruction algorithm/kernel? 1. Applied after back-projection. 2. Reduces ring artifact. 3. Used only during data acquisition. 4. Affects image noise. 5. Used for scanner calibration. Reference: Bushberg JT, Seibert JA, Leidholt EM, and Boone JM, The Essential Physics of Medical Imaging, 2 nd edition, Lippincott Wiliams & Wilkins, 2001, ISBN

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38 Which is true regarding CT Numbers? 4% 1. CT # of water depends on the kv. 85% 3% 4% 3% 2. Independent of window/level setting. 3. Range from HU. 4. Equal to linear attenuation coefficient. 5. Scaling factors for back-projection.

39 Which is true regarding CT Numbers? 1. CT # of water depends on the kv. 2. Independent of window/level setting. 3. Range from HU. 4. Equal to linear attenuation coefficient. 5. Scaling factors for back-projection. Reference: Bushberg JT, Seibert JA, Leidholt EM, and Boone JM, The Essential Physics of Medical Imaging, 2 nd edition, Lippincott Wiliams & Wilkins, 2001, ISBN

40 AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation CT Systems

41 CT System Gantry X-ray Tube High Voltage Generator Bow-tie & Flat Filter Pre-Patient Collimation Patient Post-Patient Collimation Operational Control Computer Operator Console Detectors Data Acquisition System (DAS) Image Generation

42 Tube and Generator Demanding on X-ray tube - >500 max ma - Scan times can be 30 sec or more - Heat capacities up to 30 MHU - Water or oil-cooled Generator kw - kv settings: (discrete steps)

43 Bowtie and Flat Filter X-ray source Patient Flat Filter Uniform (AL or Cu) Removes soft x-rays Bowtie Filter Compensates for different path lengths Reduces dose at periphery Different for different apps (body, head, peds, cardiac)

44 Bowtie Filter

45 Collimation Single-Slice Multi-Slice Total Coll. = Slice thickness No. detectors used x detector width

46 Detectors Electrical Signal Output Need very short afterglow Scintillating Material CdWO 4 Yttrium & gadolinium ceramics Others Reflective coating Photodiodes

47 Detectors

48 CT System Gantry X-ray Tube High Voltage Generator Bow-tie & Flat Filter Pre-Patient Collimation Patient Post-Patient Collimation Operational Control Computer Operator Console Detectors Data Acquisition System (DAS) Image Generation

49 Data Acquisition System Amplifies detector signals Integrates detector signals over sampling period - Sample and hold, reset of each sample - Potential for cross-talk (imperfect reset) Analog to digital conversion Want output to be linear to input Output contains electronic and quantum noise - Electronic noise should be small fraction

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51 Which is true regarding the bowtie filter? 67% 1. Reduces patient dose. 13% 14% 4% 3% 2. Uniformly removes low energy x-rays. 3. Applied prior to back-projection. 4. Between the patient and detectors. 5. Same filter used for all scans.

52 Which is true regarding the bowtie filter? 1. Reduces patient dose. 2. Uniformly removes low energy x-rays. 3. Applied prior to back-projection. 4. Between the patient and detectors. 5. Same filter used for all scans. Reference: Hsieh J, Computed Tomography: Principles, Design, Artifacts, and Recent Advances, SPIE The International Society of Optical Engineering, Bellingham, WA, 2003, ISBN

53 AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation CT Implementations: Spiral Scanning

54 Some CT Milestones 1972: First CT scanner (EMI), heads only 1975: First body CT scanner 1989: Spiral CT introduced 1992: Dual Slice CT 1998: 4-Slice CT 2002: 8 and 16-Slice CT 2012: 64, 128, 320-Slice CT

55 Before Spiral Only axial (or sequential) One rotation, stop, increment table, repeat (still can do this) Needed to reverse rotation to unwind cables Data from each rotation in same plane Overlap Contiguous Gaps

56 Slip Rings Metal Contacts Transfer power to gantry Transfer data to and from gantry No need to unwind cables Allow for continuous table motion (spiral scanning) Wolbarst A B, Hendee W R Radiology 2006;238: by Radiological Society of North America

57 Slip rings Contacts are metal blocks (not bristles) Voltage Control signals/data Wolbarst A B, Hendee W R Radiology 2006;238: by Radiological Society of North America

58 Spiral Scanning Continuous acquisition Greatly reduced scan time Data not in same reconstruction plane New Term: Pitch Spiral Data Acquisition Path

59 Pitch Pitch < 1 Pitch = 1 Pitch > 1 The amount of overlap of spiral slices Pitch = Table index per rotation Beam width* *Single slice scanners = slice thickness *Multi-slice scanners = # images per rot n x image width

60 Beam Width Single-Slice Multi-Slice Total = Beam Width Slice thickness No. images per rot n x image width

61 Spiral Reconstruction Incomplete data in any plane Need to interpolate from nearest points Can use full 360 or 180 data Arbitrary reconstruction planes One data point in plane Reconstruction plane

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63 What is the function of the slip ring? 1% 1. Reduces ring artifact. 2% 94% 3% 0% 2. Provides precise table movement. 3. Transfers power to the gantry. 4. Stabilizes unit during gantry rotation. 5. Determines scan field of view.

64 What is the function of the slip ring? 1. Reduces ring artifact. 2. Provides precise table movement. 3. Transfers power to the gantry. 4. Stabilizes unit during gantry rotation. 5. Determines scan field of view. Reference: Wolbarst AB, Hendee WR, Evolving and Experimental Technologies in Medical Imaging, Radiology 2006; 238:16-39.

65 AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation CT Implementations: Multi-Slice CT

66 Detector Differences Single Slice Multi-slice GE CT/i (single slice) GE QX/i (4-slice)

67 Multi-slice Detectors

68 16 Data Channels (24 detector rows) 1.25 mm detectors (4) mm detectors (16) 1.25 mm detectors (4)

69 4 Data Channels (16 detector rows) # Slices Width Detectors 4 x 1.25 mm 4 x 2.50 mm 4 x 3.75 mm 4 x 5.00 mm Can recon down to width of one CHANNEL

70 16 Data Channels (24 detector rows) Number of slices x slice thickness 8 x x x x x x x x 5 2 x x x 5 1 x 1.25 * Doesn t consider recons, not all available in helical

71 Various Configurations (32 mm) (24 mm) (20 mm) (20 mm)

72 Various Configurations mm (38.4 mm) mm (40 mm) mm (160 mm) mm (80 mm)

73 Multi-Slice, Many Choices Not all configurations available under all circumstances. Depends on image thickness, table speed, pitch, axial or helical. Still many choices. Current trend appears to be to limit options.

74 Multi-Slice Advantages over Single Slice Single Slice Limited sampling for interpolation Scan width increases with pitch Image noise independent of pitch Multi-slice Much better sampling Scan width nearly independent of pitch Image noise depends on pitch

75 Multi-Slice Advantages over Single Slice Single Slice Limited scan coverage (per rotation). One recon image thickness. Multi-slice More coverage per rotation (shorter scan times). Many possible recon thicknesses. Thinner images possible.

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77 What is an advantage of single-slice scanners over multi-slice scanners? 3% 1. Shorter scan times. 2% 4% 5% 87% 2. Improved patient coverage. 3. Better sampling. 4. More image thickness options. 5. Easier reconstructions.

78 What is an advantage of single-slice scanners over multi-slice scanners? 1. Shorter scan times. 2. Improved patient coverage. 3. Better sampling. 4. More image thickness options 5. Easier reconstructions. Reference: Bushberg JT, Seibert JA, Leidholt EM, and Boone JM, The Essential Physics of Medical Imaging, 2 nd edition, Lippincott Wiliams & Wilkins, 2001, ISBN

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80 What is true regarding multi-slice detectors? 4% 1. Data channels equals rows of elements. 2. Smallest data channel determines thinnest 90% image width. 1% 3% 2% 3. Most use high pressure xenon gas. 4. Pre-patient collimation determines image thickness. 5. Changing image thickness implies rescanning the patient.

81 What is an advantage of single-slice scanners over multi-slice scanners? 1. Data channels equals rows of elements. 2. Smallest data channel determines thinnest image width. 3. Most use high pressure xenon gas. 4. Pre-patient collimation determines image thickness. 5. Changing image thickness implies rescanning the patient. Reference: Hsieh J, Computed Tomography: Principles, Design, Artifacts, and Recent Advances, SPIE The International Society of Optical Engineering, Bellingham, WA, 2003, ISBN

82 Some Typical Specifications kv settings: (discrete steps) ma Range: ma Rotation times: sec Recon rate: 4 40 images/sec Bore opening: cm (80-90 cm wide bore ) Max. couch weight: kg ( lbs) Generator power rating: kw Much much more info at

83 Detailed Specifications & Measurements

84 Scanners in Motion

85 The Scanner Story (parts 1 and 2) Transfer from cine film of a documentary by EMI covering the development and early years of medical x-ray CT scanning

86 Thank You!