Biomedical Imaging I

1. Biomedical Imaging I Class 3 – X-Ray CT Instrumentation 9/28/04

2. X-ray computed tomography • Limits of radiography / fluoroscopy • 3D structures are collapsed into 2D image (obscuring of details, loss of one dimension) • Low soft-tissue contrast • Not quantitative • Features of x-ray CT • X-ray imaging modality (same principles of generation, interaction, detection) • Generation of a sliced view of body interior • Computed reconstruction of images • Good soft-tissue contrast

3. Examples of CT images

4. Principle of x-ray CT • In one plane, obtain set of line integrals for multiple view angles • Reconstruct cross-sectional views Linear scan Source Angular scan Object Detector

5. Scanner Design

6. Realization of x-ray CT • Mathematical basis for computed tomography by Radon (1917) • Idea popularized by Allan Cormack at Tufts Univ. (1963) • First practical x-ray CT scanner introduced by Godfrey Hounsfield of EMI Ltd., England (1972)

7. First generation • EMI Mark I (Hounsfield), “pencil beam” or parallel-beam scanner (highly collimated source)  excellent scatter rejection, now outdated • 180 - 240 rotation angle in steps of ~1  • Used for the head • 5-min scan time, 20-min reconstruction • Original resolution: 80  80 pixels (ea. 3  3 mm2), 13-mm slice

8. Second generation • Hybrid system: Fan beam, linear detector array (~30 detectors) • Translation and rotation • Reduced number of view angles  scan time ~30 s • Slightly more complicated reconstruction algorithms because of fan-beam projection

9. Third generation • Wide fan beam covers entire object • 500-700 detectors (ionization chamber or scintillation detector) • No translation required  scan time ~seconds (reduced dose, fewer motion artifacts) • Reconstruction time ~seconds • Pulsed source (reduces heat load & radiation dose)

10. Fourth generation • Stationary detector ring (600 – 4800 scintillation detectors) • Rotating x-ray tube (inside or outside detector ring) • Scan time, reconstruction time ~seconds • Source either inside detector ring or outside (rocking, nutating detectors)

11. Comparison of 3rd and 4th generation • Both designs currently employed, neither can be considered superior • 3rd Generation (GE, Siemens): • Fewer detectors (better match, cheaper) • Good scatter rejection with focused septa • Cumulative detector drift • 4th Generation (Picker, Toshiba): • Less moving parts • Detectors calibrated twice per rotation

12. X-ray CT sources

13. X-ray tubes • Bremsstrahlung x-ray tubes • Fixed anode: oil-cooled • Rotating Anode • Two focal spot sizes (~0.5 mm  1.5 mm and ~1.0 mm  2.5 mm) • Collimator assembly used to control beam (slice) width (~1.0 - 10 mm) • Power: ~120 kV @ 200-500 mA  spectrum ~30 – 120 keV • High frequency generators (5-50 kHz) • Rotating geometry requires slip rings • High voltage slip rings (~120 kV) if generator stationary • Lower voltage slip rings (480 V) if generator on rotary gantry

14. Rotary Gantry X-ray tube • Picker International, Inc.

15. Slip rings • Picker • International, Inc.

16. X-ray Detectors

17. Detector Performance Desired: • High overall efficiency to minimize patient radiation dose (typ. 0.45…0.85) = product of • Geometric efficiency: fraction of detector area sensitive to radiation • Quantum efficiency: fraction of radiation energy deposited • Conversion efficiency: fraction of absorbed radiation contributing to electrical signal • Large dynamic range (ratio of largest to smallest detectable signal) • Stable in time (low drift) • Insensitive to temperature variations

18. Gas ionization chambers I • Measurement of conductivity induced in a gas volume by the ionizing effect of x-rays. • X-rays ionize gas molecules • Ions are drawn to electrodes by electric field • Number of ion pairs N produced  x-ray intensity + - Collimator Anode Ampmeter - - - - + + + + Cathode

19. Gas ionization chambers II • Usually filled with Xenon (high Z) under pressure (up to 30 atm) to optimize efficiency • Cheap • Excellent stability • Large dynamic range • High spatial resolution • Low efficiency

20. Scintillation detectors • Scintillating material (phosphor) converts x-ray energy into flashes of visible light • Light is measured using photomultiplier tube (PMT) or photo diode (PD) • Scintillation materials: • For PMT: NaI(Tl), BGO • For PD: CdWO4, CsI, rare earth oxides • Scintillation material thick enough to provide quantum efficiency ~ 100% Scintillator Electric signal PD / PMT Collimator

21. Photomultiplier tubes (PMT) • External photoelectric effect converts light intensity into current of free electrons • Electrostatic acceleration of secondary electrons • Cascade of secondary electron emission and multiplication on dynodes • Signal amplification G = N typ. ~106 (N: no. of dynodes, : gain per dynode ~4)

22. Photodiode • Photons create electrons-hole pairs in semiconductor (photoelectric effect) • Direct conversion of visible photons into electric energy • Generation of photocurrent (~0.5 A / 1 Wopt) requires precision amplifier Packaging in x-Ray CT Detector

23. Advanced Applications

24. 5th Generation scanners • Exploring the temporal dimension • Especially important in cardiovascular (CV) imaging because of fast moving structures • Fast slice acquisition • Triggering on cardiac cycle • High repetition rate

25. Imatron • No moving parts • Electromagnetically swept electron beam • 50 ms (single slice) or 100 ms (multi-slice) scan time  imaging of beating heart • Developed 1979 at UCSF (Boyd et al.), licensed to Imatron, Inc.

26. Imatron front view

27. Imatron as marketed by GE

28. Continuous volume scanning (CVS) Step volume scanning (SVS) Single slice sequence (100 ms)

29. Multi slice sequence (50 ms) • 8 cm axial coverage

30. Triggered acquisition • RCA moves at velocities of ~25 – 100 mm/s

31. Aortic stent Colon w/ 7-mm polyp Imaging examples I

32. Imaging examples II • Cardiac wall motion "Sharp, motion-free 50 ms images of the heart throughout one entire heart cycle aid physicians in determining and specifying wall motion anomalies."

33. Axial Scans Obtaining Volumetric (3D) Information

34. Volumetric imaging

35. distance to slice centerbeam with Slice Sensitivity Profile SSP • Defined by variation of relative sensitivity along z in the slice center • Ideally rectangular (stop-and-shoot profile) -1.5 -1 -0.5 0 0.5 1 1.5

36. Spiral CT • Continuous linear motion of patient table during multiple scans • Increased coverage volume / rotation • Pitch: Number of slice thicknesses the table moves during one rotation (typically ~1-2) pitch

37. Helical reconstruction • Projections for one slice do not lie in one plane • Interpolation from data outside the slice plane necessary 1st 2nd 3rd 4th Rotation 1st 2nd 3rd 4th Rotation 0 0 direct data 180 180 complementary data 360 360 -1 0 1 -0.5 0.5 Interpolation: 360 Degree Linear Standard (180 Degree Linear)

38. = Complementary data • Data sets for view angles 180º apart are identical: Detector array = Detector array 180º 360º

39. Spiral CT SSP • Because of interpolation, SSP deviates from square profile • Depending on pitch • Full width at half maximum (FWHM) ~ nominal slice width

40. Multi slice spiral scanning I • Interweaving multiple helices  increased data density • Allows higher pitch (faster scan speed) pitch = 4 x single slice pitch

41. Variable Slice Thickness • Detector elements (~ 1000 scintillator/PD) are multiplexed to vary slice number and thickness • Scan time ~ 0.5 s per rotation

42. Hounsfield units • Assign calibrated values to gray scale of CT images • Based on measurements with the original EMI scanner invented by Hounsfield • Relates the linear attenuation coefficient of a local region  to the linear attenuation coefficient of water,W(Eeff = 70 keV)