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

Shrink the bore and use long crystals Increase sensitivitybut …Parallax error Lose resolution

No DOI

PET Scanner

3D Position Information Breaks Inverse Relationship between Sensitivity and Resolution

Position Sensitive

APDScintillator

block

DOI resolution = 5 mm

Actual Event Line

Recon Line

Challenges in Small-Animal PET

*animated

Position Sensitive APD* introduction

high resistivity layer on back of APD

DA

BC

4 corner contacts on back

deep-diffused silicon APD

optical photons incident on front surface

*PSAPDs manufactured by Radiation Monitoring Devices, Inc., Watertown, MA, USA.

x = (A+B)-(C+D)(A+B+C+D)

y = (A+D)-(B+C)(A+B+C+D)

0 200 400 600 800 1000 1200 1400 16001

10

100

1000

10000

Gain

Bias Voltage (V)

T = 20°C

0 500 1000 1500 2000 2500 3000

-15

-10

-5

0

GainGain

Tem

pera

ture

Coeffi

cien

t (

%/

°C)

normal operating range

Bottom PSAPD pulse heightTop

PS

APD

pu

lse h

eig

ht

511 keV

DOI = 0mm (center of crystals)

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Depth-of-Interaction Illustration

*animated

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Top PSAPD

Scintillator

arrayBottom PSAPD

z

scintillator slab

PMT

22Na source

Bottom PSAPD pulse height

Top

PS

APD

pu

lse h

eig

ht

Depth-of-Interaction Illustration

*animated

Scintillator Array Designscintillator array attributescintillator 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 5m grit, 16nm rms)• M10 (lapped with 10m grit, 700nm rms)• M20 (lapped with 20m grit, 1000nm rms)

reason

high light output,high density & Z,fast decayPSAPD size, resolution, sensitivity, crystal cutting capabilitiesexperimental

experimental

VM2000Teflon

reflector material

reflector material

VM2000• laser cut film• much faster assembly• improved reproducibility• reduced dead-space• somewhat reduced light collection efficiency(~75% of Teflon)

Teflon• best light collection• time-consuming to assemble• well-known problems with reproducibility, shrinkage, wicking, etc.

1mm

Scintillator Array Experiments

Black

P

P

P

P

P

P

M5

M5

M5

M5

M5

M5

M10M10

M10

M10

M10

M10

M20

M20

M20

M20

M20

M20

• 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

Scintillator Array Experiments: Results

Polished M5 M10 M200

2

4

6

8

10

DO

I R

eso

luti

on

(m

m)

Surface Treatment

VM2000, 12.4°CTeflon, 19.6°CTeflon, 9.8°C

0

5

10

15

20

Polished M5 M10 M20En

erg

y R

eso

luti

on

(%

)

Surface Treatment

VM2000, 12.4°CTeflon, 19.6°CTeflon, 9.8°C

1

2

3

4

5

6

0Tim

ing

Reso

luti

on

(n

s)

Polished M5 M10 M20Surface Treatment

Teflon, 12.1°C

Roughening the surface improves DOI …

… without degrading energy resolution and timing resolution.

Detector Evolution

*animated

Detector SpecificationsPSAPD• 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

Energy Window: 250-650 keV

0

100

200

300

400

500

600

DOI Resolution

Energy Window: 250-650 keVIntegrated counts from all crystals.

Nor

mal

ized

Cou

nts

-10 -5 0 5 100

0.2

0.4

0.6

0.8

1

Depth (mm)

Width of Electronically Collimated Beam

-3 -2 -1 0 1 2 30

5

10

15

Distance (mm)

Cou

nt

Rate

(H

z)

FWHM = 2.3 mm

measurements(left scale)

Error Function fit(left scale)Implied Beam Profile

(arbitrary vertical scale)

scintillator slab

PMT

PMT

22Na source

z

DOI Resolution (deconvolved)

Energy Window: 250-650 keVIntegrated counts from all crystals.

Nor

mal

ized

Cou

nts

-10 -5 0 5 100

0.2

0.4

0.6

0.8

1

Depth (mm)

DOI Resolution Map*

DO

I Reso

lutio

n (FW

HM

, mm

)*

2.6

2.7

2.8

2.9

3

3.1

3.2

3.3

Crystal Column Number

Cry

stal R

ow

Nu

mb

er

1 2 3 4 5 6 7 8

1

2

3

4

5

6

7

8

*without deconvolution of beam width

Energy Resolution Map

15

15.5

16

16.5

17

17.5

18

18.5

19

19.5

Crystal Column Number

Cry

stal R

ow

Nu

mb

er

1 2 3 4 5 6 7 8

1

2

3

4

5

6

7

8

En

erg

y R

eso

lutio

n (FW

HM

, %)

Timing Resolution Distribution

2.5 3 3.5 4 4.5 5 5.50

5

10

15

Timing Resolution (FWHM, ns)

Nu

mb

er

of

Cry

stals

Radiation Damage? (preliminary measurements)Small-animal PET is not a “high radiation” environment.

*depending on scanner utilization assumptions

small-animal scanner

14mm x 14mm PSAPDs

~10 cm bore

22 mm long MLS crystals

Exposed PSAPD to 2 - 10 years* equivalent accumulated 511 keV flux

test setup

22Na source

8mm x 8mm PSAPD

4x4 array of 22 mm long MLS crystals

short distance

(bias to operating voltage for entire exposure, 10°C)

0 20 40 60 80 100 120 140 1600

20

40

60

800

50

100

150

200

pre-

radi

atio

n

0 20 40 60 80 100 120 140 1600

20

40

60

800

50

100

150

200

250

300

post

-rad

iatio

n

No Negative Impact on Performance Observed

ConclusionDemonstrated 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 readoutW. 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.

• PSAPDsK. 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.

• MLSC. 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.

• VM2000R. 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).