# Presentation: 691762

## A Compact  16-Module Camera Using 64-Pixel CsI(Tl)/Si PIN Photodiode Imaging Module

- W.-S. Choong1, G.J. Gruber1, W.W. Moses1, S.E. Derenzo1, S.E. Holland1, M. Pedrali-Noy1, B. Krieger1, E. Mandelli1, G. Meddeler1, N.W. Wang1, C.S. Tindall1,  and E.K. Witt2

- 1Lawrence Berkeley National Laboratory, Berkeley, CA 94720 2Capintec, Inc, Ramsey, NJ 07446

## Motivation

- Bulky
- Long imaging distance
- Intrinsic resolution ~ 3.5 mm
- Significant dead area around the periphery
-  E/E ~ 9% FWHM

- Compact
- Short imaging distance
- Intrinsic resolution ~ 2 - 3 mm
- Small dead area around the periphery
-  E/E < 15% FWHM

**Anger Camera**

**Compact Camera**

## Approaches to Compact Cameras

**Discrete scintillator/photodiode camera****: **
**	****gamma rays interact in an array of optically-isolated scintillation crystals that are coupled 1-to-1 to a photodiode array**
**Solid-state camera****: **
**	****gamma rays interact directly with a pixellated  solid-state detector (e.g. CdZnTe)**
**Position-sensitive photomultiplier tube camera****: **
**	****gamma rays interact in one or more scintillation crystals which are then read out by PSPMT.**

## LBNL Low-Noise Photodiode Arrays

- Low leakage current
-  	(<50 pA/pixel @ 50 V)
- 3 pF/pixel capacitance
- 80% quantum efficiency for 540 nm light
- 98.5% good pixel yield during fabrication

- 64 3x3 mm2 pixels (edge and corner pixels are 30% and 50% smaller area respectively)

## Photodiode Fabrication

**Made on 300 ********m thick, high resistivity (>10 k********-cm), n-type silicon**
**Low leakage current is achieved by depositing a thick layer of phosphorus-doped polysilicon to getter impurity atoms**
**p+ junctions are fabricated by ion-implanting boron**
**n+ back-side contact is made by re-depositing a thin layer of polysilicon after etching off the thick polysilicon gettering layer**
**A layer of indium tin oxide (ITO) is deposited on the back-side to serve as an anti-reflective (AR) coating optimized for 540 nm light (the peak output of CsI(Tl) scintillator crystal)**

- Reference: S.E. Holland, N.W. Wang, and W.W. Moses, “Development of low noise,    back-side illuminated silicon photodiode arrays,” *IEEE Transactions on **	**Nuclear Science*, vol. 41, pp. 443-447, 1997

## PETRIC Readout IC

- Input: 64 photodiode signals plus control signals that modify ASIC behavior
- Front end: 64 channels of low-noise charge-sensitive preamplifiers and adjustable shaper amplifiers
- “Winner Take All” (WTA): reduces the 64 amplified, shaped signals to a single “winner” channel (the one with the greatest amplitude)
- Output: the 1 analog “winner” plus 6 digital bits identifying the winning pixel

- Reference: M. Pedrali-Noy, G.J. Gruber, B. Krieger, E. Mandelli, G. Meddeler, W.W. Moses, et al., “PETRIC-A Positron Emission Tomograph Readout Integrated Circuit,” *IEEE Transactions on Nuclear Science*, vol. 48, pp. 479-484, 2001

## Components of Module

**CsI(Tl) array**

**Low-noise Si PIN photodiode array**

**Ceramic PCB**

**Getek PCB**

**PETRIC**

**Surface mount capacitors**

**Surface mount connectors**

**EMF shield for PETRIC **

## Complete 64-Pixel Module

- PETRIC readout: 64 front end channels followed by “Winner Take All” circuitry that selects the channel with the greatest amplitude
- Custom printed circuit boards: provide electrical connections to the photodiodes and the PETRIC
- Custom Si PIN photodiodes: very low leakage current (<50 pA/pixel) => low electronic noise
- CsI(Tl) crystals: commercial 3 x 3 x 5 mm3 crystal array

- 24 mm

- 24 mm

- 14 mm

- Connectors

- EMF shield

- Capacitors

## 16-Module Camera Design

**  ****A scintimammography imaging camera constructed from tiling 16 **
**    ****individual 64-pixel modules**
**  ****Separate WTA multiplexes signals from up to 16 modules**

**WTA, peak detect, interface I/O**

## Motherboard: Block Diagram

## PETRIC I/O Signals

## Motherboard: Timing Diagram

## Photograph of the 16-Module Camera

**WTA-3**

**Comparator**

**Peak detector**

**Readout timing circuitry**

**Four        64-pixel  module**

**Photodiode bias voltage**

**Cable to NI-DAQ card**

## Pulse Height Spectrum (Tc99m)

- The numbers in parenthesis are the corresponding fwhm energy resolution.

## Energy Resolution

- Inner Pixels

- Edge Pixels

## Energy Resolution(From a High SNR Module)

- Inner Pixels

- Edge Pixels

## Energy Resolution(From a Low SNR Module)

- Inner Pixels

- Edge Pixels

## System Spatial Resolution(with Line Source)

- In Air

- 10 cm Acrylic Block

- 8000 cps/mCi, 23.5 mm thick

- 16,200 cps/mCi, 23.5 mm thick Collimator

_**All-Purpose Collimator**_

_**High-Sensitivity Collimator**_

## System Spatial Resolution

|  | All-Purpose Collimator |  |  | High-Sensitivity Collimator |  |  |
| --- | --- | --- | --- | --- | --- | --- |
|  | In Air | 5 cm Block | 10 cm Block | In Air | 5 cm Block | 10 Block |
| FWHM (mm) | 8.7 | 8.8 | 9.0 | 11.2 | 11.1 | 11.5 |
| FWTM (mm) | 15.8 | 16.0 | 16.5 | 20.4 | 20.3 | 21.0 |

## Noise Sources

- (7% - 10% fwhm)

- (2% - 3% fwhm)

- (8% - 10% fwhm)

## Electronic Noise (w/o PD)

- Module #1

- Module #2

## Electronic Noise (w/ PD)

- Module #1

- Module #2

## Light Collection Inhomogeneity(Measured w/ PMT, Excited by 511 keV)

- Note:All crystals have similar performance if re-wrapped with Teflon tape.

- Center = 147

- Center = 137

- Center = 123

- Center = 157

## Future Directions

**Working with Capintec, Inc under a CRADA (Cooperative Research and Development Agreement) project to build a 25-module and 100-module gamma camera using smaller pixel (2.4 mm x 2.4 mm) photodiode arrays.**
**Design and fabricate a new WTA I.C. with 32 input channels and a peak detect to be used in the gamma camera. **

## Canberra Photodiode Arrays

- LBNL Photodiode

- _Canberra Photodiode_

- 64 2.4x2.4 mm2 pixels (edge and corner pixels are 32% and 54% smaller respectively)

- 19.6 mm

- 19.6 mm

## Canberra Photodiode Arrays: Performance

## 32-Channel WTA IC with Peak Detect

## Conclusions

**Have implemented a compact 16-module gamma camera**
**Have demonstrated the working of the 16-module gamma camera (at least with 4 modules) with good SNR (12.4% and 14.4% on the inner and edge pixel respectively for 140 keV emissions of Tc99m)**
**Have measured system spatial resolution with an all-purpose and high-sensitivity collimator**
**This technology is promising and may soon be available for breast cancer and axillary lymph nodes imaging**
**Working with Capintec, Inc to build a 25-module and 100-module gamma camera using 2.4 mm x 2.4 mm pixel photodiode array**

## Funding

**U.S. Department of Army**
**U.S. Department of Energy**
**National Institute of Health**
**Capintec, Inc**