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INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS

INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS. DAVID B. CHALPIN, MD. ASSISTANT PROFESSOR OF CLINICAL RADIOLOGY LSU HEALTH SCIENCES CENTER NEW ORLEANS, LA. GOALS. TO BECOME FAMILIAR WITH THE BASICS OF IMAGE GENERATION USING X-rays, CT, AND MRI

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INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS

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  1. INTRODUCTION TO IMAGING PHYSICSCAPABILITIES AND LIMITATIONS

  2. DAVID B. CHALPIN, MD ASSISTANT PROFESSOR OF CLINICAL RADIOLOGY LSU HEALTH SCIENCES CENTER NEW ORLEANS, LA

  3. GOALS • TO BECOME FAMILIAR WITH THE BASICS OF IMAGE GENERATION USING X-rays, CT, AND MRI • TO BECOME FAMILIAR WITH THE LIMITATIONS OF IMAGING AS PRACTICALLY APPLIED

  4. TEST-TAKER TOPICS • KNOW THE WISHFULTHINKING PITFALLS! • REVIEW THE “TAKE-HOME” MESSAGES FOR EACH IMAGING MODALITY! (denoted by a RED asterisk - *)

  5. OVERVIEW • RADIOGRAPHY, FLUOROSCOPY, & DSA • COMPUTED TOMOGRAPHY • MAGNETIC RESONANCE IMAGING

  6. SPECTRUM OFE-M RADIATION

  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.

  8. mAs*and kVp* • e-current through filament (expressed in mAsfor milliAmperes) at Cathode generates a proportionate amount of X-Ray photons • kVp= kiloVoltage peak relates to the Voltage potential between the Anode & Cathode and reflects a SPECTRUM of emitted X-ray photon energies

  9. X-Rays – 3 Fates* • Photons can beABSORBED • Photonscan beSCATTERED with some exposing the film degrading the image, akaFOGGING, OR • Photonscan proceed directly through subject to EXPOSE film.

  10. SCATTERING

  11. How reduce X-ray SCATTERING? ASK YOUR PATIENTS TO LOSE WEIGHT?

  12. TO  SCATTERING • COLLIMATION* of X-ray Beam • Use of GRIDS* in cassettes

  13. X-ray Collimation

  14. X-ray GRID Tradeoff Grids require  mAs compared with XR studies done w/o grids

  15. How Improve Spatial Resolution & Decrease Image Distortion?

  16. Center the Area of Interest!

  17. AP versus PA Direction of emitted beam from the X-ray tube PatientCassette • AP = Anterior to Posterior • PA = Posterior to Anterior

  18. PORTABLE X-RAYS • HOWCONVENIENT!! • DECREASED QUALITY (sometimes) due to: limited kVp & mAs,  tube to subject distance, & positioning ROI • Is it FEASIBLE that the patient could have had the X-ray study done in the Radiology Department? If so, ………..

  19. FIRST APHORISM DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!

  20. DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!!* “Bad” can mean Suboptimal QualityOR the study as ordered was NOT dedicated for evaluation of Region or Organ of Interest.

  21. SECOND APHORISM YOU CANNOT CALL WHAT YOU DON’T SEE!* HOWEVER, IF YOU SUSPECT SOMETHING, GET ANOTHER VIEW!!*

  22. DIGITAL/COMPUTED RADIOGRAPHY IMAGES CAN BE MANIPULATED POST-ACQUISITION TO OPTIMIZE VIEWING OF ONE PART OF H&D Curve.

  23. WISHFUL THINKING IN RADIOGRAPHY • QUALITY OF PORTABLE STUDIES* • PATIENT THICKNESS & SIZE* • Table Weight limits* • COOPERATIVENESS OF PATIENT*

  24. WISHFUL THINKING IN RADIOGRAPHY • QUALITY OF PORTABLE STUDIES* • PATIENT THICKNESS & SIZE* • Table Weight limits* • COOPERATIVENESS OF PATIENT*

  25. X-ray COMPUTED AXIAL TOMOGRAPHY • aka CAT scan (archaic,) now CT • “STEP AND SHOOT” mode • 1st Gen CT Scanner – 45 min/slice

  26. 2nd Generation CT scanner

  27. %TransmissionSpecial Case For monochromatic Photon energy – log %T α 1/linear attenuation

  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

  29. X-ray AttenuationRevisited *%Transmission of photon energyreceived by detectors is recorded at multiple projections around the subject & the data is reconstructed to create a cross-sectional image

  30. X-ray ATTENUATION µ - the intrinsic X-ray coefficient a function of: • kVp* • Atomic Mass* • electron density*

  31. ATTENUATION VALUE – CT* Hounsfield Units (H.U.)* of sample S = (μS - μH2O) x 1000 μH2O

  32. CT – ADVANTAGES I COMPARED WITH X-rays, U/S, & MRI • Better Soft Tissue ContrastResolutionthan XR & usually Ultrasound (except reproductive organs, in general)* • Along with Fluoroscopy using Barium, CT best for Intestinal Tract Evaluation* (though not so “dynamic” as fluoro.)

  33. CT – ADVANTAGES II • Easier & Quicker than MRI* but not always better tissue contrast resolution • ~BEST for detection & characterization of CALCIFICATION*

  34. CT BEST FOR Calcification e.g. a Bony Sequestrum & Involucrum of Osteomyelitis

  35. CT - DISADVANTAGES • IONIZING RADIATION!!* • EACH SERIES OF IMAGES TOGETHER IS ONLYONE SNAPSHOT IN TIME* • ARTIFACTS: Partial Volume* Scattering (Obesity)* Beam Hardening* Metal Streaking*

  36. PARTIAL VOLUMEEFFECT

  37. EFFECT OF THICK SLICES

  38. BEAM-HARDENING

  39. Metallic streaking

  40. … AND IMAGES DERIVED FROM THOSE w/ ARTIFACTS

  41. 3RD GENERATION CT

  42. HELICAL CT • 3rd GENERATION CT SCANNER + • ADVENT OF SLIP RING TECHNOLOGY TO CREATE HELICAL ACQ’N!

  43. ORIGIN OF MultiDetector CT • TWIN DETECTORconcept done with conventional “STEP & SHOOT” technique • MARRIAGE OF MULTIDETECTOR DESIGN WITH HELICAL DESIGN → MDCT !

  44. THIN SLICESISOTROPIC VOXELS

  45. IV Contrast - TIMING of Image Acquisition • X-ray, U/S, but ESPECIALLY CT & MRI! • CONTRAST ENHANCEMENT PHASES: Arterial; Hepatic Arterial; Portal Venous; Renal Capillary; Renal Excretion, etc.

  46. Hypervascular Metonly seen on Hepatic Arterial phase

  47. RESOLUTION IN IMAGING • THERE ARE 3 COMPETING FORMS OF RESOLUTION: SPATIAL, CONTRAST, AND TEMPORAL!* • 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.

  48. SPATIAL RESOLUTION • Improves with THINNER SLICES But need  mAs to compensate • Improves with choice of reconstruction KERNEL* emphasizing spatial resolution when facilitated by great inherent differences in attenuation within region or organ of interest

  49. CONTRAST RESOLUTION • MAY IMPROVE WITH INHERENT DIFFERENCES IN TISSUE ATTENUATION, e.g. IV contrast • IMPROVES WITH MORE mAs • IMPROVES WITH USE OF SOFT TISSUE KERNEL

  50. TEMPORAL RESOLUTION • IMPROVES BY SCANNING FASTER • Useful for “Freezing” or Evaluating RAPIDLY-MOVING STRUCTURES, e.g. the HEART OR MULTIPHASIC Imaging for assessing Contrast Enhancement over time within Organ(s) or Lesion(s) → Pt.Increased Radiation Dose if using CT

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