Transcript
Design of Scintillator Arrays for Dual-End Depth-ofInteraction Encoding Small-Animal PET Detectors
Kent Burr, Adrian Ivan, Don Castleberry, Jim LeBlanc
Detector Technology Lab, GE Research
Challenges in Small-Animal PET
Actual Event Line
PET Scanner
Position Sensitive APD Scintillator block
No DOI
Recon Line
DOI resolution = 5 mm
Shrink the bore and use long crystals Æ Increase sensitivity but … Parallax error Æ Lose resolution
3D Position Information Breaks Inverse Relationship between Sensitivity and Resolution
*animated
Position Sensitive APD* introduction 1000
T = 20°C
Gain
optical photons incident on front deep-diffused surface silicon APD
10000
100 10
A
high resistivity layer on back C B of APD 4 corner contacts on back
x= y=
1 0
(A+B)-(C+D) (A+B+C+D) (A+D)-(B+C) (A+B+C+D)
200
400
600
800
1000 1200 1400 1600
Bias Voltage (V) Gain Temperature Coefficient (%/ °C)
D
0 -5
normal operating range
-10 -15 0
500
1000
1500
Gain
2000
*PSAPDs manufactured by Radiation Monitoring Devices, Inc., Watertown, MA, USA.
2500
3000
z Top PSAPD
Scintillator
scintillator slab 22Na
PMT
source
array Bottom PSAPD
DOI = 0mm (center of crystals)
Top PSAPD pulse height
Depth-of-Interaction Illustration
Bottom PSAPD pulse height
511 keV
*animated
z z z z z Top PSAPDz Top PSAPD
scintillator slab 22Na
PMT
source
Top PSAPD Top PSAPD Scintillator Top PSAPD Top Scintillator Top PSAPD PSAPD Scintillator Scintillator Scintillator Scintillator Scintillator array array array Bottom PSAPD array array array Bottom array Bottom PSAPD PSAPD Bottom PSAPD Bottom PSAPD Bottom Bottom PSAPD PSAPD
Top PSAPD pulse height
Depth-of-Interaction Illustration
Bottom PSAPD pulse height
*animated
Scintillator Array Design scintillator array selection attribute scintillator material
reason
mixed lutetium silicate, MLS high light output, high density & Z, fast decay PSAPD size, resolution, array size, crystal 8x8 array, dimensions 1.65mm x 1.65mm x 22mm sensitivity, crystal cutting capabilities experimental reflector material • Teflon tape • VM2000 (3M Corp.) experimental crystal side surface • P (polished, 7nm rms) roughness • M5 (lapped with 5µm grit, 16nm rms) • M10 (lapped with 10µm grit, 700nm rms) • M20 (lapped with 20µm grit, 1000nm rms)
reflector material
Teflon
VM2000
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)
Scintillator Array Experiments • 4 different surfaces (P, M5, M10, M20)
• 2 reflector materials (Teflon, VM2000)
P
Black
M20
M10
M5
M10
M20
M5
P
M20
M5
P
M20
M10
M5
P
M5
M10
M20
P
M20
M10
P
M5
M10
• 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
VM2000, 12.4°C Teflon, 19.6°C Teflon, 9.8°C
8 6
15 10 VM2000, 12.4°C Teflon, 19.6°C Teflon, 9.8°C
5 0 Polished M5
M10
M20
Surface Treatment
4 2 0
Energy Resolution (%)
10
20
Polished
M5
M10
Surface Treatment
M20
Roughening the surface improves DOI …
Timing Resolution (ns)
DOI Resolution (mm)
Scintillator Array Experiments: Results
6 5 4 3 2
Teflon, 12.1°C
1 0 Polished M5
M10
M20
Surface Treatment
… without degrading energy resolution and timing resolution.
Detector Evolution
*animated
PSAPD
Detector Specifications
• 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
Flood Histogram – 22Na 600
500
400
300
200
100
0
Energy Window: 250-650 keV
DOI Resolution
Normalized Counts
1
0.8
0.6
0.4
0.2
0
-10
-5
0
Depth (mm) Energy Window: 250-650 keV Integrated counts from all crystals.
5
10
Width of Electronically Collimated Beam z
scintillator slab 22Na
source
PMT
Count Rate (Hz)
PMT
15
measurements (left scale) Error Function fit (left scale) Implied Beam Profile (arbitrary vertical scale)
10
FWHM = 2.3 mm
5
0
-3
-2
-1
0
1
Distance (mm)
2
3
DOI Resolution (deconvolved)
Normalized Counts
1
0.8
0.6
0.4
0.2
0
-10
-5
0
Depth (mm) Energy Window: 250-650 keV Integrated counts from all crystals.
5
10
DOI Resolution Map* 1
Crystal Row Number
2 3.2 3 3.1 4 3 5 2.9 6 2.8 7 2.7 8 1
2
3
4
5
6
7
Crystal Column Number
*without deconvolution of beam width
8
2.6
DOI Resolution (FWHM, mm)*
3.3
Energy Resolution Map 19.5
Crystal Row Number
19
2
18.5
3
18
4
17.5
5
17
6
16.5 16
7
15.5
8 1
2
3
4
5
6
7
Crystal Column Number
8
15
Energy Resolution (FWHM, %)
1
Timing Resolution Distribution
Number of Crystals
15
10
5
0 2.5
3
3.5
4
4.5
5
Timing Resolution (FWHM, ns)
5.5
Radiation Damage? (preliminary measurements) Small-animal PET is not a “high radiation” environment. 22Na
14mm x 14mm PSAPDs
source short distance
8mm x 8mm PSAPD (bias to operating voltage for 22 mm long MLS entire exposure, 10°C) crystals
~10 cm bore
4x4 array of 22 mm long MLS crystals
test setup
pre-radiation
small-animal scanner 200
150
100
50
0
80 60 40 20 0
0
20
40
60
80
100
120
140
160
post-radiation
Exposed PSAPD to 2 - 10 years* equivalent accumulated 511 keV flux 300
250
200
150
100
50
0
80 60 40 20 0
0
20
40
60
80
No Negative Impact on Performance Observed *depending on scanner utilization assumptions
100
120
140
160
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)
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 GammaRay 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).
