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fluoroscopy
- 2. Reasons for Subtraction: Conspicuity
- 4. Reasons for Subtraction: Contrast
- 7. Digital Fluoro Equipment Differences: Electronic aperture for proper light levels to camera at both DSA and fluoroscopic exposure rates Digital Processor Higher quality and stability components
- 8. Digital Fluoroscopy Equipment (con’t) Camera Differences: Low electronic noise (to not add to QM) Must be low lag Usually high resolution (10242) today CCD more common today
- 9. Image Digitization Two Steps: Spatial Digitization: – Generate matrix of pixels (rows and columns) Analog-to-Digital Conversion – Generate digital number from analog signal measurement for each pixel
- 10. Spatial Digitization-how many pixels?
- 11. Spatial Digitization (continued) How many pixels? Acquire via TV camera: one matrix row per TV line (eg-512 rows if 525-line TV) Measure each TV line 512 times for 512 pixels/row Newer systems: 1024x1024 (hi-res)
- 13. Digitization Errors and Noise
- 14. Iodine Brightness versus Intensity
- 15. Subtraction Problem: Intensity artifact
- 16. Iodine Brightness & X-ray Intensity Pixel value proportional to x-ray intensity ‘N’ at image intensifier (a= constant of propor): For mask: M =aN For Enhanced Image: E = aNe-µd Subtracted: M-L = aN - aNe-µd = aN(1- e-µd) aNµd Where: µ = attenuation of iodinated vessel d = projected thickness of vessel
- 17. Logarithmic Conversion Pixel value proportional to x-ray intensity ‘N’ at image intensifier (a= constant of propor): For mask: M =Log(aN) For Enhanced Image: E = Log(aNe-µd) = Log(aN) - µd Subtracted: M-L = Log(aN) - Log(aN) + µd M-L = µd
- 18. Logarithmic Conversion: no artifact
- 19. Noise in DSA
- 20. Noise In DSA Noise is limiting factor in DSA image quality: if not, Iodine concentration could be reduced (and iodine reaction is serious consideration) Noise is worse in subtraction image than in either original image (~1.41 times higher) Increasing mAs reduce quantum mottle only. Other noise sources may become apparent, Frame-averaging reduces all sources of noise equally--preferred
- 21. DSA: Mask Mode Subtraction
- 22. DSA: Time Interval Subtraction
- 23. MOTION IN ANGIOGRAPHY Exams uncorrupted by motion is a main reason DSA rapidly replaced film SA: Methods: Pixel shifting: “move” mask relative to image by fractions of a pixel until “good subtraction found – Usually fixes motion at least over area of interest Remasking: – Select image acquired after motion (or at end of run) to use as mask instead. Last Resort: Reacquire: know quickly if needed Ideal: Take mask and run image at same time (??)
- 24. Absorption by Tissue, Bone and Iodine
- 25. Dual Energy Subtraction Dual collimator filters rapidly rotated in/out Acquire image with iodine filter (energy just below 33 keV) Acquire image with cerium filter (energy just above 33 keV) Subtract-only Iodine