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Imaging physics and limitations

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Imaging physics and limitations

  1. 1. INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS
  2. 3. GOALS <ul><li>TO BECOME FAMILIAR WITH THE BASICS OF IMAGE GENERATION USING X-rays, CT, AND MRI </li></ul><ul><li>TO BECOME FAMILIAR WITH THE LIMITATIONS OF IMAGING AS PRACTICALLY APPLIED </li></ul>
  3. 4. TEST-TAKER TOPICS <ul><li>KNOW THE WISHFUL THINKING PITFALLS! </li></ul><ul><li>REVIEW THE “TAKE-HOME” MESSAGES FOR EACH IMAGING MODALITY! </li></ul><ul><li>(denoted by a RED asterisk - * ) </li></ul>
  4. 5. OVERVIEW <ul><li>RADIOGRAPHY, FLUOROSCOPY, & DSA </li></ul><ul><li>COMPUTED TOMOGRAPHY </li></ul><ul><li>MAGNETIC RESONANCE IMAGING </li></ul>
  5. 6. SPECTRUM OF E-M RADIATION
  6. 7. GENERATION OF X-Rays VACUUM TUBE Electric current is passed through a filament, leading to e - emission, then striking target (W or Mb), leading to X-ray emission.
  7. 8. mAs * and kVp * <ul><li>e - current through filament (expressed in mAs for milliAmperes) at Cathode generates a proportionate amount of X-Ray photons </li></ul><ul><li>kVp = kiloVoltage peak relates to the Voltage potential between the Anode & Cathode and reflects a SPECTRUM of emitted X-ray photon energies </li></ul>
  8. 9. X-Rays – 3 Fates * <ul><li>Photons can be ABSORBED </li></ul><ul><li>Photons can be SCATTERED with some exposing the film  degrading the image, aka FOGGING , OR </li></ul><ul><li>Photons can proceed directly through subject to EXPOSE film. </li></ul>
  9. 10. SCATTERING
  10. 11. How reduce X-ray SCATTERING ? ASK YOUR PATIENTS TO LOSE WEIGHT?
  11. 12. TO  SCATTERING <ul><li>COLLIMATION * of X-ray Beam </li></ul><ul><li>Use of GRIDS * in cassettes </li></ul>
  12. 13. X-ray Collimation
  13. 14. X-ray GRID Tradeoff Grids require  mAs compared with XR studies done w/o grids
  14. 15. How Improve Spatial Resolution & Decrease Image Distortion?
  15. 16. Center the Area of Interest!
  16. 17. AP versus PA <ul><li>Direction of emitted beam from the X-ray tube  Patient  Cassette </li></ul><ul><li> </li></ul><ul><li>AP = Anterior to Posterior </li></ul><ul><li>PA = Posterior to Anterior </li></ul>
  17. 18. PORTABLE X-RAYS <ul><li>HOW CONVENIENT!! </li></ul><ul><li>DECREASED QUALITY (sometimes) due to: limited kVp & mAs,  tube to subject distance, & positioning ROI </li></ul><ul><li>Is it FEASIBLE that the patient could have had the X-ray study done in the Radiology Department? If so, ……….. </li></ul>
  18. 19. FIRST APHORISM DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!
  19. 20. DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!! * “ Bad” can mean Suboptimal Quality OR the study as ordered was NOT dedicated for evaluation of Region or Organ of Interest.
  20. 21. SECOND APHORISM YOU CANNOT CALL WHAT YOU DON’T SEE! * HOWEVER, IF YOU SUSPECT SOMETHING, GET ANOTHER VIEW!! *
  21. 22. DIGITAL/COMPUTED RADIOGRAPHY IMAGES CAN BE MANIPULATED POST-ACQUISITION TO OPTIMIZE VIEWING OF ONE PART OF H&D Curve.
  22. 23. WISHFUL THINKING IN RADIOGRAPHY <ul><li>QUALITY OF PORTABLE STUDIES * </li></ul><ul><li>PATIENT THICKNESS & SIZE * </li></ul><ul><li>Table Weight limits * </li></ul><ul><li>COOPERATIVENESS OF PATIENT * </li></ul>
  23. 24. WISHFUL THINKING IN RADIOGRAPHY <ul><li>QUALITY OF PORTABLE STUDIES * </li></ul><ul><li>PATIENT THICKNESS & SIZE * </li></ul><ul><li>Table Weight limits * </li></ul><ul><li>COOPERATIVENESS OF PATIENT * </li></ul>
  24. 25. X-ray COMPUTED AXIAL TOMOGRAPHY <ul><li>aka CAT scan (archaic,) now CT </li></ul><ul><li>“ STEP AND SHOOT ” mode </li></ul><ul><li>1 st Gen CT Scanner – 45 min/slice </li></ul>
  25. 26. 2 nd Generation CT scanner
  26. 27. %Transmission Special Case For monochromatic Photon energy – log %T α 1/linear attenuation
  27. 28. What data generates an image as a slice? The %Transmission of Photon energy received by detectors is recorded at multiple projections around the subject & the data is then reconstructed to create a cross-sectional image
  28. 29. X-ray Attenuation Revisited * %Transmission of photon energy received by detectors is recorded at multiple projections around the subject & the data is reconstructed to create a cross-sectional image
  29. 30. X-ray ATTENUATION <ul><li>µ - the intrinsic X-ray coefficient </li></ul><ul><li>a function of: </li></ul><ul><li>kVp * </li></ul><ul><li>Atomic Mass * </li></ul><ul><li>electron density * </li></ul>
  30. 31. ATTENUATION VALUE – CT * Hounsfield Units (H.U.) * of sample S = ( μ S - μ H2O ) x 1000 μ H2O
  31. 32. CT – ADVANTAGES I <ul><li>COMPARED WITH X-rays, U/S, & MRI </li></ul><ul><li>Better Soft Tissue Contrast Resolution than XR & usually Ultrasound (except reproductive organs, in general) * </li></ul><ul><li>Along with Fluoroscopy using Barium, CT best for Intestinal Tract Evaluation * (though not so “dynamic” as fluoro.) </li></ul>
  32. 33. CT – ADVANTAGES II <ul><li>Easier & Quicker than MRI * but not always better tissue contrast resolution </li></ul><ul><li>~BEST for detection & characterization of CALCIFICATION * </li></ul>
  33. 34. CT BEST FOR Calcification e.g. a Bony Sequestrum & Involucrum of Osteomyelitis
  34. 35. CT - DISADVANTAGES <ul><li>IONIZING RADIATION!! * </li></ul><ul><li>EACH SERIES OF IMAGES TOGETHER IS ONLY ONE SNAPSHOT IN TIME * </li></ul><ul><li>ARTIFACTS : Partial Volume * </li></ul><ul><li>Scattering (Obesity) * </li></ul><ul><li>Beam Hardening * </li></ul><ul><li>Metal Streaking * </li></ul>
  35. 36. PARTIAL VOLUME EFFECT
  36. 37. EFFECT OF THICK SLICES
  37. 38. BEAM-HARDENING
  38. 39. Metallic streaking
  39. 40. … AND IMAGES DERIVED FROM THOSE w/ ARTIFACTS
  40. 41. 3 RD GENERATION CT
  41. 42. HELICAL CT <ul><li>3 rd GENERATION CT SCANNER + </li></ul><ul><li>ADVENT OF </li></ul><ul><li>SLIP RING TECHNOLOGY TO CREATE HELICAL ACQ’N! </li></ul>
  42. 43. ORIGIN OF MultiDetector CT <ul><li>TWIN DETECTOR concept done with conventional “STEP & SHOOT” technique </li></ul><ul><li>MARRIAGE OF MULTIDETECTOR DESIGN WITH HELICAL DESIGN </li></ul><ul><li>-> MDCT ! </li></ul>
  43. 44. THIN SLICES  ISOTROPIC VOXELS
  44. 45. IV Contrast - TIMING of Image Acquisition <ul><li>X-ray, U/S, but ESPECIALLY CT & MRI! </li></ul><ul><li>CONTRAST ENHANCEMENT PHASES : </li></ul><ul><li>Arterial; Hepatic Arterial; </li></ul><ul><li>Portal Venous; Renal Capillary; </li></ul><ul><li>Renal Excretion, etc. </li></ul>
  45. 46. Hypervascular Met only seen on Hepatic Arterial phase
  46. 47. RESOLUTION IN IMAGING <ul><li>THERE ARE 3 COMPETING FORMS OF RESOLUTION: SPATIAL, CONTRAST, AND TEMPORAL! * </li></ul><ul><li>SUCH “COMPETITION” IS GREATEST IN MRI, WHILE IN CT IT CAN BE TRADED OFF THROUGH CHOICE OF A RECONSTRUCTION KERNEL BUT ESCALATED BY HIGHER RAD’N DOSE & USE OF IV CONTRAST . </li></ul>
  47. 48. SPATIAL RESOLUTION <ul><li>Improves with THINNER SLICES </li></ul><ul><li>But need  mAs to compensate </li></ul><ul><li>Improves with choice of reconstruction KERNEL * emphasizing spatial resolution when facilitated by great inherent differences in attenuation within region or organ of interest </li></ul>
  48. 49. CONTRAST RESOLUTION <ul><li>MAY IMPROVE WITH INHERENT DIFFERENCES IN TISSUE ATTENUATION, e.g. IV contrast </li></ul><ul><li>IMPROVES WITH MORE mAs </li></ul><ul><li>IMPROVES WITH USE OF SOFT TISSUE KERNEL </li></ul>
  49. 50. TEMPORAL RESOLUTION <ul><li>IMPROVES BY SCANNING FASTER </li></ul><ul><li>Useful for “Freezing” or Evaluating RAPIDLY-MOVING STRUCTURES , e.g. the HEART OR </li></ul><ul><li>MULTIPHASIC Imaging for assessing Contrast Enhancement over time within Organ(s) or Lesion(s) -> Pt. Increased Radiation Dose if using CT </li></ul>
  50. 51. WISHFUL THINKING IN CT * <ul><li>PATIENT SIZE – WEIGHT LIMIT OF SCANNER TABLE </li></ul><ul><li>PATIENT BODY HABITUS OBESITY ->  SCATTER ; “PRETZEL” CONFIGURATION </li></ul><ul><li>RESIDUAL DENSE GI Contrast </li></ul>
  51. 52. WISHFUL THINKING IN CT * <ul><li>(rhetorical negatives) </li></ul><ul><li>NO INCREASED BEAM HARDENING ARTIFACT AT SHOULDERS & HIPS </li></ul><ul><li>NO EFFECT 2 ° to UE position </li></ul><ul><li>PT. COOPERATION – NO PROB! </li></ul>
  52. 53. MRI 1 <ul><li>CURRENTLY, CLINICAL MRI INVOLVES PRIMARILY HYDROGEN NUCLEI </li></ul><ul><li>1 TESLA = 10,000 gauss </li></ul><ul><li>Earth Magnetic Field Strength = 0.5g </li></ul>
  53. 54. MRI 2 <ul><li>TWO SPIN STATES FOR PROTONS EXIST - PARALLEL TO APPLIED MAIN MAGNETIC FIELD AND ANTIPARALLEL </li></ul><ul><li>THE ANTIPARALLEL STATE HAS A HIGHER ENERGY LEVEL (Q.M.) </li></ul><ul><li>AT EQUILIBRIUM, 100,000 NUCLEI ARE ANTI-// AND 100,001 ARE //. </li></ul>
  54. 55. MRI 3
  55. 56. MRI 4 <ul><li>RF (radiofrequency) Energy added to system, “flipping” protons from parallel to higher energy antiparallel state. </li></ul><ul><li>The excitation frequency required, ω , to “flip” the protons is governed by the LARMOR EQUATION: ω = γ  B o </li></ul>
  56. 57. The NMR Phenomenon
  57. 58. MAGNETIC FIELD GRADIENTS <ul><li>MANIPULATION (OF THE RF ENERGY DEPOSITED) BY MAGNETIC FIELD GRADIENTS IS DONE TO ENCODE SPATIAL INFORMATION </li></ul><ul><li>ADDITIONAL GRADIENTS MAY BE USED TO CREATE IMAGES BASED ON DIFFUSION, DIFFERENCES IN FLOW VELOCITY, etc. </li></ul>
  58. 59. MR Signal Reception <ul><li>When RF turned off, the excess # of protons in antiparallel state returns to the ground state and emit either heat or RF , i.e. the patient is essentially turned into a “little radio station”!! </li></ul>
  59. 60. PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 1 * <ul><li>An RF coil * is used to receive the emitted signal, like an antenna. </li></ul>
  60. 61. PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 2 * <ul><li>The larger the coil used, the greater the volume of coverage. * </li></ul><ul><li>BUT , the Larger the Coil, the Lower the Signal-to-Noise (aka S/N) * </li></ul>
  61. 62. PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 3 * <ul><li>AND , the Further the Region of Interest is from the coil, </li></ul><ul><li>the Lower the S/N !! * </li></ul>
  62. 63. WHAT IS THE SIGNIFICANCE? <ul><li>USE THE SMALLEST POSSIBLE COIL NECESSARY TO SCAN THE REGION & ANSWER THE CLINICAL QUESTION! * </li></ul><ul><li>THUS, STATING THE CLINICAL QUESTION(S) CLEARLY MAY AID NOT ONLY IMAGE INTERPRETATION, BUT MAY DETERMINE HOW THE STUDY IS CONDUCTED !! * </li></ul>
  63. 64. IMAGE CONTRAST POSSIBILITIES Processing of emitted RF signal yields Spatial Information as well as various forms of Image Contrast
  64. 65. Forms of MRI contrast <ul><li>T1 </li></ul><ul><li>T2 </li></ul><ul><li>T2* </li></ul><ul><li>Balanced (“Proton Density”) </li></ul><ul><li>Contrast administration effects </li></ul>
  65. 66. Forms of MRI contrast <ul><li>Selective 1 H excitation or presaturation in lipid, free H 2 O, bound H 2 O, or Si-hyd </li></ul><ul><li>Flow velocity or rate </li></ul><ul><li>Differential [O 2 ] (aka BOLD) </li></ul><ul><li>Diffusion </li></ul><ul><li>Diffusion Tensor </li></ul><ul><li>Multi-nuclear Spectroscopy, e.g. 1 H, 13 C, 19 F, 31 P </li></ul>
  66. 67. MRI 7 * WISHFUL THINKING <ul><li>PATIENTS MUST - </li></ul><ul><li>LIE FLAT! </li></ul><ul><li>BE STILL! </li></ul><ul><li>FIT INSIDE MAGNET! </li></ul><ul><li>Have SAFETY SCREENING Done! </li></ul><ul><li>FOLLOW INSTRUCTIONS (prn) ! </li></ul>
  67. 68. ACKNOWLEDGEMENTS ILLUSTRATIONS COURTESY OF: MRI in Practice , 3 rd ed. Westbrook… Clinical MRI Atlas , 2 nd ed. Runge… Radiologic Physics , 4 th ed. Christenson… Fundamentals of Radiology , LF Squire

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