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Kent Burr, Adrian Ivan, Don Castleberry, Jim LeBlanc Detector Technology Lab, GE Research

Design of Scintillator Arrays for Dual-End Depth-of-Interaction Encoding Small-Animal PET Detectors. Kent Burr, Adrian Ivan, Don Castleberry, Jim LeBlanc Detector Technology Lab, GE Research. PET Scanner. Position Sensitive APD. Actual Event Line. Scintillator block. No DOI.

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Kent Burr, Adrian Ivan, Don Castleberry, Jim LeBlanc Detector Technology Lab, GE Research

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  1. Design of Scintillator Arrays for Dual-End Depth-of-Interaction Encoding Small-Animal PET Detectors Kent Burr, Adrian Ivan, Don Castleberry, Jim LeBlanc Detector Technology Lab, GE Research

  2. PET Scanner Position Sensitive APD Actual Event Line Scintillator block No DOI DOI resolution = 5 mm Recon Line 3D Position Information Breaks Inverse Relationship between Sensitivity and Resolution Challenges in Small-Animal PET Shrink the bore and use long crystals  Increase sensitivity but … Parallax error  Lose resolution *animated

  3. 10000 optical photons incident on front surface T = 20°C 1000 deep-diffused silicon APD Gain 100 10 0 D 1 A 0 200 400 600 800 1000 1200 1400 1600 -5 Bias Voltage (V) Gain Temperature Coefficient (%/ °C) high resistivity layer on back of APD -10 B C normal operating range -15 4 corner contacts on back 0 500 1000 1500 2000 2500 3000 Gain y = x = (A+D)-(B+C) (A+B)-(C+D) (A+B+C+D) (A+B+C+D) Position Sensitive APD* introduction *PSAPDs manufactured by Radiation Monitoring Devices, Inc., Watertown, MA, USA.

  4. z Top PSAPD Scintillator array scintillator slab 22Na source Bottom PSAPD PMT Top PSAPD pulse height 511 keV Bottom PSAPD pulse height Depth-of-Interaction Illustration DOI = 0mm (center of crystals) *animated

  5. z z z z z z z Top PSAPD Top PSAPD Top PSAPD Top PSAPD Top PSAPD Top PSAPD Top PSAPD Scintillator array Scintillator array Scintillator array Scintillator array Scintillator array Scintillator array Scintillator array scintillator slab scintillator slab scintillator slab scintillator slab scintillator slab scintillator slab scintillator slab 22Na source 22Na source 22Na source 22Na source 22Na source 22Na source 22Na source Bottom PSAPD Bottom PSAPD Bottom PSAPD Bottom PSAPD Bottom PSAPD Bottom PSAPD Bottom PSAPD Top PSAPD pulse height Top PSAPD pulse height Top PSAPD pulse height Top PSAPD pulse height Top PSAPD pulse height Top PSAPD pulse height Top PSAPD pulse height PMT PMT PMT PMT PMT PMT PMT Bottom PSAPD pulse height Bottom PSAPD pulse height Bottom PSAPD pulse height Bottom PSAPD pulse height Bottom PSAPD pulse height Bottom PSAPD pulse height Bottom PSAPD pulse height Depth-of-Interaction Illustration *animated

  6. scintillator array attribute scintillator material array size, crystal dimensions reflector material crystal side surface roughness • selection • mixed lutetium silicate, MLS • 8x8 array,1.65mm x 1.65mm x 22mm • Teflon tape • VM2000(3M Corp.) • P(polished, 7nm rms) • M5(lapped with 5m grit, 16nm rms) • M10(lapped with 10m grit, 700nm rms) • M20(lapped with 20m grit, 1000nm rms) reason high light output,high density & Z,fast decay PSAPD size, resolution, sensitivity, crystal cutting capabilities experimental experimental Scintillator Array Design

  7. reflector material Teflon VM2000

  8. reflector material 1mm • Teflon • best light collection • time-consuming to assemble • well-known problems with reproducibility, shrinkage, wicking, etc. • VM2000 • laser cut film • much faster assembly • improved reproducibility • reduced dead-space • somewhat reduced light collection efficiency(~75% of Teflon)

  9. Scintillator Array Experiments Black P M20 M10 M5 M10 M20 M5 P M20 M5 P M20 M10 M5 P M5 M10 M20 P M20 M10 P M5 M10 • 4 different surfaces (P, M5, M10, M20) • 2 reflector materials(Teflon, VM2000) • 5x5 array of crystals (parallel data acquisition under identical conditions) • 1.9mm x 1.9mm x 20mm MLS crystals • 14mm x14mm PSAPD • arrangement chosen so that: • each row and column have at least one of each crystal type • central 3x3 of array has at least two crystals of each type • “black” crystal used for unambiguous identification of crystals • each corner crystal has different surface • each 2x2 in corner has one crystal of each type, except for corner that has “black” crystal

  10. Scintillator Array Experiments: Results 20 15 10 VM2000, 12.4°C Energy Resolution (%) 10 Teflon, 19.6°C 8 Teflon, 9.8°C VM2000, 12.4°C 5 Teflon, 19.6°C 6 DOI Resolution (mm) Teflon, 9.8°C 0 Polished M5 M10 M20 Surface Treatment 4 6 2 5 4 Timing Resolution (ns) 0 Polished M5 M10 M20 3 Surface Treatment Teflon, 12.1°C 2 1 0 Polished M5 M10 M20 Surface Treatment Roughening the surface improves DOI … … without degrading energy resolution and timing resolution.

  11. Detector Evolution *animated

  12. Detector Specifications • PSAPD • 14 mm x 14 mm active area • operated at 10°C and -1630V • gain ~950 • leakage current ~1µA • Scintillator array • 8x8 MLS • 1.65 mm x 1.65 mm x 22.00 mm • 1.75 mm pitch • 3M VM2000 reflective film • rough side surfaces, polished ends • coupled to PSAPD using Cargille Meltmount • Electronics • corner contacts: low noise JFET input wide bandwidth transimpedance amplifier with a 100 kohm transimpedance gain • trigger and energy signal from analog sum of corner contacts

  13. 600 500 400 300 200 100 0 Flood Histogram – 22Na Energy Window: 250-650 keV

  14. 1 0.8 0.6 0.4 0.2 0 -10 -5 0 5 10 Depth (mm) DOI Resolution Normalized Counts Energy Window: 250-650 keV Integrated counts from all crystals.

  15. z scintillator slab 15 22Na source measurements(left scale) PMT Error Function fit (left scale) PMT Implied Beam Profile (arbitrary vertical scale) 10 Count Rate (Hz) FWHM = 2.3 mm 5 0 -3 -2 -1 0 1 2 3 Distance (mm) Width of Electronically Collimated Beam

  16. 1 0.8 0.6 0.4 0.2 0 -10 -5 0 5 10 Depth (mm) DOI Resolution (deconvolved) Normalized Counts Energy Window: 250-650 keV Integrated counts from all crystals.

  17. 1 3.3 2 3.2 3 3.1 4 Crystal Row Number 3 DOI Resolution (FWHM, mm)* 5 2.9 6 2.8 7 2.7 8 2.6 1 2 3 4 5 6 7 8 Crystal Column Number DOI Resolution Map* *without deconvolution of beam width

  18. 19.5 1 19 2 18.5 3 18 4 17.5 Crystal Row Number Energy Resolution (FWHM, %) 17 5 16.5 6 16 7 15.5 8 15 1 2 3 4 5 6 7 8 Crystal Column Number Energy Resolution Map

  19. 15 10 Number of Crystals 5 0 2.5 3 3.5 4 4.5 5 5.5 Timing Resolution (FWHM, ns) Timing Resolution Distribution

  20. Radiation Damage? (preliminary measurements) 22Na source 14mm x 14mm PSAPDs short distance 8mm x 8mm PSAPD 300 ~10 cm bore 4x4 array of 22 mm long MLS crystals (bias to operating voltage for entire exposure, 10°C) 200 250 22 mm long MLS crystals test setup 200 150 150 100 100 small-animal scanner 50 50 0 0 80 80 60 60 40 40 20 20 0 0 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 pre-radiation post-radiation No Negative Impact on Performance Observed Small-animal PET is not a “high radiation” environment. Exposed PSAPD to 2 - 10 years* equivalent accumulated 511 keV flux *depending on scanner utilization assumptions

  21. Conclusion • Demonstrated High-Res DOI PET detector with: • 8x8 array of 1.65 mm x 1.65 mm x 22.00 mm crystals, with surface treatments chosen to optimize DOI resolution • Minimal dead-space within array • Compact front-end electronics • No radiation damage effects observed • Tileable design • Excellent performance • DOI resolution of <3 mm FWHM • Energy resolution of ~16% FWHM • Timing resolution of ~4 ns FWHM (vs. plastic)

  22. References • DOI in PET • L. R. MacDonald and M. Dahlbom, "Parallax Correction in PET Using Depth of Interaction Information," IEEE Trans. Nucl. Sci., vol. 45, no. 4, pp. 2232 – 2237, Aug. 1998. • Y. Shao, R. M. Manjeshwar, F. P. Jansen, P. N. Kumar, A. F. Chatziioannou, “Simulation Studies for a High Resolution and High Sensitivity Small Animal PET with DOI Detection Capability,” presented at IEEE Medical Imaging Conference, M6-11, Portland, OR, Oct. 2003. • dual-end readout • W. W. Moses, S. E. Derenzo, "Design Studies for a PET Detector Module Using a PIN Photodiode to Measure Depth of Interaction," IEEE Trans. Nucl. Sci., vol. 41, pp. 1441 – 1445, Aug. 1994. • Y. Shao, K. Meadors, R. W. Silverman, R. Farrell, L. Cirignano, R. Grazioso, K. S. Shah, S. R. Cherry, "Dual APD Array Readout of LSO Crystals: Optimization of Crystal Surface Treatment," IEEE Trans. Nucl. Sci., vol. 49, no. 3, pp. 649 – 654, June 2002. • PSAPDs • K. S. Shah, R. Farrell, R. Grazioso, E. S. Harmon, E. Karplus, "Position-Sensitive Avalanche Photodiodes for Gamma-Ray Imaging," IEEE Trans. Nucl. Sci., vol. 49, no. 4, pp. 1687 – 1692, Aug. 2002. • MLS • C. M. Pepin, P. Berard, R. Lecomte, "Comparison of LSO, LGSO and MLS Scintillators," in Proc. IEEE Nuclear Science Symposium, vol. 1, San Diego, CA, Nov. 2001, pp. 124 – 128. • VM2000 • R. S. Miyaoka, S. G. Kohlmyer, T. K. Lewellen, "Performance Characteristics of Micro Crystal Element (MiCE) Detectors," IEEE Trans. Nucl. Sci., vol. 48, no. 4, pp. 1403 – 1407, Aug. 2001. Acknowledgment • K. Shah and R. Farrell (RMD). • T. Lewellen, R. Miyaoka, and M. Janes (U. Wash).

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