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