Initial Tests of a Compact Imaging Photomultiplier

Made From Array of 3x3mm2 Hamamatsu MPPC-

SMD Modules

S. Majewski1, J. Proffitt1, J. McKisson1, R.Raylman2, A. Stolin1 and A.G. Weisenberger1

1Radiation Detector & Medical Imaging Group, Thomas Jefferson National Accelerator Facility, Newport News, VA

2Center for Advanced Imaging, Department of Radiology, West Virginia University, Morgantown, WV

Abstract– We built and tested the first small (~18mm square)

prototype compact photomultiplier (PM) consisting of a 4x4

array of individual 3x3mm2 Hamamatsu Multi-Pixel Photon

Counters (MPPC). To reduce the cost and complexity, the

3x3mm2 units were not arranged in the tightest possible

arrangement, but spaced at 5mm (center-to-center). The MPPC-

PM package includes low power on-board input stage amplifiers

for each MPPC element and a sum amplifier placed on additional

distant electronics board. The initial studies were performed with

arrays of LYSO, NaI(Tl) and CsI(Tl) irradiated with Co57

and

Na22

sources. The PM performed best when used with LYSO

arrays exposed to annihilation photons, indicating good prospects

for use in PET applications. Tests in 3 Tesla MRI magnet showed

no detectable impact on operation of the PM. The expected and

confirmed drawback of this device is that SiPM gain is quite

sensitive to temperature (gain decreases with rising temperature),

and maintaining temperature stability will present the main

practical challenge in using this new PM. During the tests the

constantly monitored temperature of the PM was between 28-30

deg C. Rate performance, another potential issue with this device,

was not studied at this time. We are now constructing and

planning next devices with more compact electronics packages

and with larger fields of view. The ultimate goal of the project is

to utilize the MPPC-PM modules as building blocks of larger

devices, with active arrays as large as 5x5cm2 and beyond.

I. INTRODUCTION

Silicon Photomultipliers (SiPMs) are a new type of photon

counting device made up of multiple avalanche photodiodes

(APD) pixels operated in Geiger mode. They have much

higher gain (105-106) than standard APDs. Thus, these devices

could potentially replace standard photomultipliers in a

number of applications. For example, the nuclear physics

program at Jefferson Lab is considering use of SiPMs in

calorimetric detectors placed in tight places of the detection

system apparatus, and in strong (several Tesla) magnetic

fields. The present investigation started as a spin-off of that

development.

After the initial Russian–based development effort [1],

many R&D projects around the world are focusing on the

development and application of SiPMs to biomedical imaging;

initially for small size imagers applicable to small animal

imaging, but also for dedicated human imagers. The number of

technical efforts around the world focusing on development of

different designs of SiPMs, and studies of their properties, as

well as on designing practical imagers, is increasing

dramatically within the last two years, with only few very

recent works referenced here [2-11].

The advantages of SiPMs are their very small size, and

ability to operate in strong magnetic fields. Thus, permitting

their use in multimodality (PET/MRI, SPECT/MRI) imagers,

supplanting the use of APDs in this application [10,11]. One

of the challenges in applying SiPMs to imaging is their small

size. Most individual units range in size from 1x1mm2 to

3x3mm2. In addition, these devices do not have the capability

to provide positional information (as position sensitive

photomultipliers (PSPMTs) do). It is therefore necessary to

produce arrays of the devices to create a unit appropriate for

imaging application. In other words, each SiPMs operates as

an individual anode pad present on PSPMTs.

There are currently a number of companies developing

SiPMs. SensL Corp. [12], Hamamatsu Photonics [13], three

original entities in Russia [14-16], Radiation Monitoring

Devices [17], Zecotek [18], Photonique [19] and other newly

formed companies, as well as an Italian consortium [20] are all

developing SiPMs, under different names. At this time, we are

working with two suppliers of silicon SiPMs: SensL and

Hamamatsu. We acquired samples of SiPM from both

companies. Currently, SensL is more advanced in the

development of small-integrated imaging arrays than

Hamamatsu.

Typically SiPM modules come in small units of 1mm2-

3mm2 in size. Therefore, arrays of these devices are needed to

cover the desired active field of view. Figure 1 shows an

example of how to achieve a SiPM photodetector of up to

~5x5cm2 active field of view using hypothetical ~12.5mm

modules composed of tightly spaced 3x3mm2 SiPM units. For

example, the initial imaging module can be composed of an

array of sixteen 3x3mm2 readout pixels/pads arranged in a 4x4

array. These imaging modules should be four-sides buttable

with a dead space on the order of 1mm or less at the edges. By

assembling them in an array, these basic modules can be

arranged to form larger size imaging macro-modules, for

example composed of 4x4 basic modules, with coverage and

readout needs equivalent to the H8500/9500 flat panel PSPMT

(Figure 1). The 3x3mm2 pads can be either read separately or

coarsely with four or all 16 pads connected to one readout

channel, as shown schematically at left center and bottom of

Figure 1, respectively. In this design, an array of 2x2 of these

basic four-sides buttable modules (8x8=64 of 3x3mm2 pads)

forms a ~1” square photodetector, equivalent for example to

Hamamatsu R8520-C12 PSPMT. Next, the concept for a

“plug-in” replacement module for the H8500/H9500 flat PMT

with about ~5x5cm2 active surface can be implemented.

Currently, some of the SiPM devices are not yet

sufficiently sensitive in the blue spectral region of the LaBr3

emission. For example, the sensitivity of devices from SensL

peaks at 520 nm, with poor response at 400nm. In contrast, the

spectral response of Hamamatsu devices has a good match

with the emission of LaBr3 (Figure 2). As many applications

of SiPMs depend in a crucial way on this extension of the

SiPM sensitivity, therefore it is anticipated that similar SiPM

designs from SensL and from other companies will be

available in the near future.

Figure 1. Concept of building larger photodetector modules from

~3x3mm2 SiPM units. One example of the basic module, shown

schematically at left top, is composed of an array of 4x4 3x3mm2

active pads. These basic modules can be read as 16, 4, or 1

individual channels. They can be in turn arranged in arrays to

form SiPM equivalents of PSPMT structures such as Hamamatsu

R8520 (center) and H9500 (at right).

Figure 2. Spectral response efficiency of several designs of

Hamamatsu MPPCs (SiPMs). The last number (before U) denotes

size of the basic Geiger microcell (25, 50 or 100 micron). Please

note good match with LaBr3 emission (Hamamatsu data).

II. MPPC MODULES

Hamamatsu Photonics [13] introduced very recently their

3x3mm2 silicon PM, called MPPC, in a more compact SMD

package (Figure 3). It is possible that tight arrays of these

sensors can now be assembled resulting in a detector with a

large sensor area. Conceivably good quality imaging could be

achieved through the readout on individual 3x3mm2 sensor

elements butted together on a ~4.3 (x) by 4.8 mm (y) pitch.

This could be possible with the use of an optical light guide

window as long as sufficient sampling of the scintillation

signal is possible to obtain strong enough detection signal and

if good light sharing occurs.

Figure 3. Schematic drawing of the new 3x3mm2-MPPC-SMD

SiPM package from Hamamatsu (model S10362-33-050SMD).

(Drawing provided by Hamamatsu Photonics).

III. PM PACKAGE

As mentioned above, useful FOV devices can be obtained

by assembling arrays of smaller basic SiPM modules, such as

the 3x3mm2 MPPC unit from Hamamatsu. A high level of

flexibility of size choices is now available to users, with the

complication of required special design for each device.

However, development of several sizes of basic building

blocks can facilitate this task (Figure 1).

Imaging with SiPM arrays was so far demonstrated in very

small size devices [9], and usually with bulky test board type

electronics, not matching the size of the tested small

photodetector. Our approach was to attempt assembly of a PM

structure built from an array of “large” size (3mmx3mm)

SiPM modules, with compact matching size electronics

occupying space behind the photodetector. In this way, many

modules of this design can be assembled side by side to form a

larger device, as conceptually shown in Figure 1. We have

spaced the 3x3mm2 elements (model S10362-33-050SMD

with 50 micron microcell size) in a 4x4 array with a 5mm

pitch, to cover a larger surface with the same number of

3x3mm2 modules. Figures 4-6 show the construction of the

PM and the photograph in Figure 7 indicates how we

constructed the first small detector module using a matching

size scintillation array. As in our past work with arrays of

PSPMTs, we implemented a light guide “spreader” window to

allow for scintillation light to spread between neighboring

3x3mm2 active elements for proper operation of the center of

gravity algorithm for scintillation arrays with pixel pitch

smaller than the PM active pad pitch (5mm in this case).

We have deliberately spaced the individual 3x3mm2

MPPCs at a larger than minimum possible pitch, to investigate

the potential cost advantage with acceptable performance

degradation. In Figure 4 we show the front picture of the mini

PM obtained in this way. The geometrical surface coverage of

36% is obtained in the case of 5mm center-to-center spacing of

3x3mm2 square active elements.

Figure 4. Left: Demonstration of an array of 3x3mm

2-MPPC-

SMD SiPMs arranged in the tightest possible configuration with

4.35mm and 3.85mm vertical and horizontal center-to-center

separation, respectively. Right: Executed first array mounted on

a PC board with increased separation between the individual

elements to ~5mm center-to-center. Active photocathode size is

~18x18mm2

with external PM size of ~19.5x19.5mm2.

Figure 5. Side view of the photodetector board with MPPC

elements (at right), light guide (center) and scintillator array (at

left). Because of the high S/N ratio of SiPMs, in the most compact

version of the detector there is no need for the on-board

electronics. The thickness/size of the detector is mostly defined by

the gamma sensor element (scintillator array is seen covered with

reflective material).

IV. TEST SETUP

A. Electronics system

In this first implementation Hamamatsu recommended bias

and coupling circuit was implemented. All 3x3mm2 MPPC

units were operated under the same positive bias voltage,

adjusted between + 68.8V to 69.1V and supplied by the

Mesytec MHV-4 power supply module [21]. The MPPCs were

operated in the gain region from ~105 to ~106.

To compensate for gain variations between individual 16

channels operating at the same bias, a tunable-gain 16 channel

NIM standard post-amplifier Phillips Scientific [22] Model

778 was used. After adjusting their relative signals to a similar

level, the individual signals were sent to a FPGA ADC DAQ

module. This flexible ADC system operated in a pulse

integrating mode with the width of the integrating gate

adjusted depending on the type of the tested scintillator, from

150 nsec to 1000 nsec. The sum of all the outputs from 16

amplifier channels was used to produce the TTL trigger to the

ADC system, first by sending it to a Phillips Scientific Model

730 discriminator and then to a Phillips Scientific Model 794

Gate/Delay Generator. The sum pulse was available from the

PM package, but we also produced another sum signal in the

NIM crate by summing the amplified individual MPPC signals

using two Phillips Scientific Model 744 Quad Linear Gate

modules.

Figure 6. Picture of the prototype PM package with an array of

3x3mm2 MPPCs in front, and 16 front-end amplifiers and

connector plus small profile multi wire cable. This first package

was designed for modification flexibility and reliability and not

optimized for compactness. Output signals, bias voltage, and +/-

2.5Volt power supply to power on-board amplifiers were all

transmitted via this flexible cable.

Figure 7. Scintillator array, optical light spreader window, and

the PM shown before (left) and during (right) assembly. LYSO

array of 10mm long pixels placed at a 1.675mm pitch, from

Proteus, is also shown in these pictures. Optical window was

2.85mm thick.

Two types of on-board amplifiers were used in the initial

evaluations (Table 1).

Property Amplifier 1

Amplifier 2

Type Voltage Feedback Current Feedback

Voltage Gain ~10 ~8

Input Impedance 1kOhm 50Ohm

Bandwidth

(@Gain = 1)

130MHz 350MHz

Static Power

Consumption

@ +/- 2.5V supplies

12.5mW/channel

(0.2W for 16ch.)

8.5mW/channel

(0.136W for 16ch.)

Table 1. Two amplifiers used in the initial evaluation.

An example of the sum pulse using the slower amplifier

version (Amplifier 1) is shown in Figure 8 and an example of a

single output using faster amplifiers (Amplifier 2) is shown in

Figure 9. A slower version with better S/N ratio produced

shaped pulses with about 50 nsec rise-time. The second faster

version resulted in pulses with a rise-time of about 10 nsec.

This value still has some amplifier contribution and we feel

more improvement is possible.

Figure 8. Oscilloscope picture of summed and amplified output

using slower variant (#1) of on-board amplifiers. The vertical

scale is 200 mV per division and horizontal is 80 nsec per division.

Figure 9. Oscilloscope picture of one channel output using faster

variant (#2) of on-board amplifiers. Respective scales are 20 mV

and 20 nsec per division. Pulse rise time ~< 10 nsec.

B. Data acquisition hardware and software

We used an in-house developed multichannel DAQ system

[23,24] with each channel operating as an independent

acquisition unit consisting of traditional analog pulse

processing, FPGA analog control and FPGA digital

processing. We used Kmax platform [25] with a collection of

C and Java utilities developed for the data acquisition system.

Java extensions to Kmax were developed to interface Kmax to

the DAQ hardware. Kmax acquires raw data, calculates

centroids, applies corrections, and displays raw and corrected

images in real time [26]. In determining the centroids the

software applies an empirically determined weighted

correction factor to the post amplified signal from each MPPC.

It is in this manner that the obtained two dimensional image

from the array of MPPCs is rendered as distortion free as

possible.

All images shown were acquired with Kmax and displayed

also with Kmax or ImageJ public domain imaging software.

V. IMAGING TESTS

Several different scintillator arrays were used in these

initial trials.

A. LYSO Arrays

Three LYSO arrays from Proteus [27] were tested with the

PM. The first was made of 11x11 10mm long pixels placed at

a 1.675mm pitch.

Figure 10 shows pixel map and five ROI regions around

individual pixels seen in the pixel map. Energy measurements

on a sample of 5 selected pixels (identified in Figure 11)

produced the following values for FWHM energy resolution

@511 keV: 16.5%, 15.3%, 17.4%, 18.8%, and 19.5%.

Figure 10. Pixel map obtained with the 1.675mm pitch LYSO

array. At right there are shown five ROI regions selected around

individual scintillation pixels to measure energy spectra from

these pixels.

The second array used in the PM evaluations, from

Proteus, was made from 10mm thick LYSO pixels placed at

1.5mm pitch. The array was larger than the PM (~1”x~2”).

Pixel map and a profile through a pixel column are shown in

Figure 12. In addition, we performed similar energy

measurements on a sample of 5 selected pixels, resulting in the

following values for FWHM energy resolution @511 keV:

12.4%, 14.6%, 14.4%, 15.1%, and 14.2%.

The third LYSO array, also from Proteus, was only 1mm

thick, and made of 10x10 pixels spaced at 1.0mm. Figure 13

shows the obtained pixel map and example of a one-pixel

energy spectrum. The separation of the individual pixels @511

keV was marginal.

B. NaI(Tl) array

A NaI(Tl) array of 6mm long and spaced at 1.5mm pixels

from Saint Gobain Crystals [28] was tested with a 57Co source

(122 keV gammas). The array was mounted inside a hermetic

package behind a 2mm thick glass window. Figure 14 shows

pixel map and an example of a single pixel energy spectrum.

Again, all scintillator elements are discernable, with the pixels

at the edges somewhat obscured due to optical effects.

FWHM energy resolution values of 27.5%, 28.4%, 27.8%,

26.9%, and 27.8% were measured @122 keV for five selected

representative scintillation pixels, across the PM active field of

view (with none selected at the PM edge).

Figure 12. Pixel map obtained with the 1.5mm pitch LYSO array.

At right a vertical profile through the central column of pixels is

shown.

Figure 13. Pixel map obtained with the 10x10 array of 1mm thick

LYSO pixels spaced at a 1.0mm pitch, from Proteus. Optical

window thickness was adjusted at 3.3mm. At right energy

spectrum measured from one of the pixels obtained with a 22

Na

source is shown. Energy resolution @511 keV photopeak was

measured as 16.8% FWHM.

Figure 14. Pixel map obtained with the NaI(Tl) array (an energy

spectrum from a representative scintillator element is also shown,

energy resolution= 26.9% FWHM @122keV).

Figure 15. Pixel map obtained with the CSI(Tl) array (an energy

spectrum from a representative scintillator element is also shown,

energy resolution= 34.4% FWHM @122keV).

C. CsI(Tl) array

A CsI(Tl) array from Hilger Crystals [29] was made from

4mm thick pixels spaced at a 1.0mm pitch. It was irradiated

with 122 keV gamma rays from a 57Co source. Signal

integration gate of 1000 nsec was used in the study of this slow

Figure 11. Energy spectra measured with

22Na source for the

selected five pixels as shown in Figure 10, from ROI-1 (top) to

ROI-5 (bottom). The 511 and 1274 keV photopeaks are present.

Energy resolution @511keV photopeak is: 16.5%, 15.3%,

17.4%, 18.8%, and 19.5% FWHM, for respective pixels from

ROI-1 to ROI-5.

scintillator. A 2.85mm thick acrylic spreader window was

placed between the array and the PM. Figure 15 shows pixel

maps and example of a single pixel energy spectrum from this

array. FWHM energy resolution values of 32.0%, 32.7%,

34.4%, 33.8%, and 32.8% were measured @122 keV for

similarly selected 5 representative scintillation pixels, across

the PM active field of view (none was selected at the PM

edge). The pixel separation in some regions of the PM was

marginal at this initial test. The effect of scintillation light cone

signal under-sampling is evident from the non-uniform pixel

distribution in the pixel map. A very similar effect was

observed with the 1mm LYSO array (Figure 13). In the next

planned series of measurements the optical coupling will be re-

optimized. Specifically, we will investigate wet optical

coupling as opposed to dry coupling used throughout these

initial studies.

The results of the above initial studies with three of the

most common scintillators in Nuclear Medicine Imaging:

LYSO, NaI(Tl) and CsI(Tl), confirm that with added light

guide (spreader) window, the 5mm granularity of the

constructed PM structure is adequate to efficiently sample

scintillation light to achieve high response uniformity and

pixel separation, but for pixel pitches larger than 1mm.

D. Optical coupling studies

After the first series of measurements with different

scintillator arrays with no special measures to increase light

collection, we performed a short pilot study to re-optimize

collection of scintillation light the by the active PM elements.

To this effect we placed a net of 2mm wide reflector strips

made of white Millipore paper. Also, different edge treatments

of the window light spreader were tested with no treatment

(rough cut), polished, and white and black paint finish.

At this time, our optimized design has Millipore reflectors

and rough-cut UV acrylic window edges. Further studies are

planned. In addition to improved response uniformity across

the PM surface, and better energy resolution, a much improved

visibility of the outer pixels was obtained.

Figure 16. Left: Pixel map of 11x11 LYSO pixel array of

1.675mm pixels @511 keV using improved optical coupling with

Millipore reflectors. Right: detail of the left upper corner region

with four selected corner pixels. Energy resolution over selected

ROI regions was between 13.0% and 13.9%. Signal amplitude

from the very edge pixel was lowest in the detector and lower by

~35% compared to central pixels. Corner pixels exhibited

following FWHM energy resolution @511 keV: 19.4% (the very

corner pixel), 17.0% (right top corner pixel) , 16.9% (left lower

corner pixel), 19.4% (inner corner pixel).

Figure 16 shows the improved pixel map of the 1.675mm

LYSO array (to be compared with Figure 10). Energy

resolution in the major part of the PM surface improved to

~13.5% FWHM @511 keV, while the corner pixels (the worst

case scenario) had energy resolution between 17.0 and 19.4%.

VI. TESTS IN MRI FIELD

A. MPPC studies in MRI magnet

We have first tested two individual 3x3mm2 MPPC units

mounted at 90 degrees to each other on a test board with

amplifiers (Figure 17). Two units were used to investigate

simultaneously if field direction affected operation of the

MPPC device, with one set of measurements having the face

of the first module perpendicular to the magnetic flux lines and

one set with the flux lines parallel to the second module’s face.

These were 3x3mm2 Hamamatsu S10362-33-050C MPPCs, in

different packaging but with the similar structure to the model

used to construct our imaging PM. The effects of a magnetic

field on the operation of the MPPC were assessed by placing

the board with two MPPCs in magnetic fields with strength of

0, 0.5, 1.0, 1.5, 2.0 and 3.0T.

The studies were done at the West Virginia University’s

Health Sciences Center. Specifically, the unit was placed at

various distances from the isocenter of a 3T MRI system (GE

Medical Systems, Inc.). The magnetic field strength was

monitored with a three-axis Hall probe. At each magnetic

field strength, data were acquired from the module while it

was irradiated with a 40µCi 18F source.

Figure 17. Test board with two orthogonally mounted MPPCs

dry-coupled to 3x3x10mm GSO crystals, seen wrapped with white

Teflon tape. The board was inserted into magnet bore with its

horizontal (left to right) axis aligned with the magnet bore axis.

The second sensor was orthogonal to the magnetic field

throughout the tests.

No effects from the magnetic field were noticed on the

performance of the two sensors (including the input stage

amplifiers). Both MPPC outputs were nearly identical and

constant as the field was changed. The small amplitude

variations shown in Figure 18 were associated with

temperature variations of the sensors and board. The

measurements were not performed in the order of rising field,

but from low value of 0.4 Tesla went to 2 Tesla and 3 Tesla,

and then to intermediate values of 1.5 and 1.0 Tesla, and this

explains why the peak amplitude seen in the plot is not

smoothly changing with the magnetic field value.

B. PM studies in MRI magnet

In the second part of the studies, our small imaging PM

was tested outside and inside the 3 Tesla MRI magnet in two

series of measurements, first with PM axis aligned with the

magnet bore axis, and second with PM axis perpendicular to

the magnet axis (transverse). LYSO array of 11x11 pixels

spaced at 1.675mm was used in this test. As before, Hall probe

was used to measure magnetic field at the sensor position.

Figure 19 show examples of pixel maps obtained at zero

field and at 3 Tesla for both axial and transverse orientations

of the PM relative to the axis of the magnet (and the direction

of the prevailing field).

Figure 18. Left: Energy spectrum from one of the sensors

obtained with a 137

Cs source (662 keV gammas) at 3T. Right:

Measured position of the 662 keV photo peak for the two

orthogonal MPPCs versus magnetic field.

Figure 19. Three pixel map images obtained at zero field (left), 3

Tesla axial orientation (center), and 3 Tesla transverse

orientation (right). The white line is drawn below a row of pixels

selected for more precise local analysis of any potential changes in

the measured pixel positions.

When overlapping the 2D pixel maps, no changes in

positions of all pixels were observed up to the maximum field

of 3 Tesla (at the center of the magnet bore). To more

precisely analyze potential position shifts, plots of one of the

crystal rows (as marked in Figure 19) at all the measured

magnetic field values of: 0, 0.5, 1.0, 1.5, 2.0, and 3.0 Tesla

were overlaid on top of each other (Figures 20 and 21). Within

statistical fluctuations, all the plots are exactly the same.

In summary, no effects from the magnetic field up to 3

Tesla on the operation of the PM were observed. The observed

small amplitude change (increase) was correlated with the

temperature change during the duration of the test (temperature

of the PM was decreasing). If the observed gain change is

entirely explained by the temperature change, then the

variation of temperature by ~1 deg. C produced gain change of

about 15%.

This test demonstrated again how immune SiPM-based

devices are to high magnetic fields and underscored the

importance of maintaining a controlled temperature

environment. Compared to APD-based devices used in MRI

fields for simultaneous PET/MRI imaging [30-32], the tested

PM showed similar or better operational immunity to strong

magnetic fields. However, the same type of extensive testing

needs to be still performed to entirely validate compatibility of

the SiPM solution to the simultaneous MRI/PET imager.

Figure 20. Profile plots for row of crystals indicated in Figure 19

for 0.0, 0.5, 1.0, 1.5, 2.0, and 3.0 Tesla magnetic field oriented in

the axial direction.

Figure 21. Profile plots for row of crystals indicated in Figure 19

for 0, 0.5, 1.0, 1.5, 2.0, and 3.0 Tesla magnetic field oriented in the

transverse direction.

Figure 22. Dependence of 511 keV photopeak position for five

selected scintillation pixels on magnetic field value and

temperature, in the axial field case. The small amplitude changes

(increase) with time can be explained by PM temperature getting

lower with time.

Figure 23. Dependence of 511 keV photopeak position for the

same five selected scintillation pixels with magnetic field value

and temperature in the transverse field case. This test followed

the axial field test and temperature was still decreasing, which

again correlates with increasing amplitudes.

VII. SUMMARY

We constructed and tested the first small compact

photomultiplier made of a 4x4 array of individually read

3x3mm2 Hamamatsu Silicon Photomultiplier (SiPM) devices.

Intentionally, the 3x3mm2 units are not arranged in the tightest

possible arrangement, in an effort to lower the cost and

complexity of the device. The package includes low power on-

board input stage amplifiers for each SiPM element and a sum

amplifier placed on an additional but distant electronics board.

The initial studies were performed with LYSO, NaI(Tl) and

CsI(Tl) arrays using 57Co and 22Na sources. Best performance

was obtained with LYSO arrays and a 22Na source, indicating

good prospects for PET applications. Gain of this prototype

PM was found to be quite sensitive to temperature (gain

decreases with raising temperature) and from our pilot studies

maintaining temperature stability will present the main

practical challenge in using this new PM. Rate performance

and operation in the MRI field will be studied next. We are

also constructing and planning next devices with more

compact electronics package and with larger fields of view. In

principle, one has flexibility now to construct different shapes

and sizes, for example to match the sizes of Position Sensitive

PMTs (Figure 1).

VIII. FUTURE WORK

A new more compact electronics package is in preparation

to benefit even more from the very compact structure of the

basic 3x3mm2 SiPM units. We are also building two more PM

structures, one with about 1”x2” field of view for an

application in endorectal prostate PET probe (Figure 24).

The second prototype is planned to be based on a round

LaBr3 module, as depicted in Figure 25, coupled to an array of

uniformly spaced 3x3mm2 Hamamatsu Silicon PMs. The

purpose of this design is to maximize the size of the useful

field of view (within the performance limits) and go beyond

the large granularity designs of the current structures imposed

by ~2.5cm or ~5cm typical imager modularity, due to sizes of

available photomultipliers. The concept of using individual

small photodetector units that can be arbitrarily arranged in

almost any shape (within the restriction of ~4-5mm basic unit

occupancy), offers substantial flexibility in system design. In

addition, the system will also have an advantage of being very

compact, permitting easy portability and/or easy

implementation as an imaging unit of multi-modular system

for SPECT applications or in intraoperative imaging probes.

The optimization of the amplifier choice for both high S/N

and for fast operation will continue in preparation for the

timing studies for application of this new compact PM in PET.

We will also work closely with Hamamatsu to investigate if

the basic 3x3mm2 units can be made even more compact for

higher occupancy.

Figure 24. Example diagram of a small-size prostate PET probe

prototype detector module using the 3x3mm2 detection elements

shown in Figure 3, each assumed to occupy up to~5mm square in

size. In this example an active field of view of over 2.5cm (wide)

by 5cm (high) is covered. The readout of the scintillation array is

obtained by coupling the scintillation array to the photodetector

array via light spreader optical window.

Figure 25. Concept of the Prototype Module #2. Left: Design of

the proposed 6cm diameter 6mm thick LaBr3 scintillator module

from Saint Gobain. Right: Preliminary arrangement of 120 units

of 3x3mm2 SiPM modules from Hamamatsu, covering the 6cm

active diameter LaBr3 detector module shown at left.

IX. ACKNOWLEDGEMENTS

The Jefferson Science Associates (JSA) operates the

Thomas Jefferson National Accelerator Facility for the United

States Department of Energy under contract DE-AC05-

06OR23177. Support for this research came in part from the

DOE Office of Biological and Environmental Research and

from the DOE Office of Nuclear Physics. State of Virginia

provided key funds for the project via internal grant from

Jefferson Science Associates. This work was also supported in

part by the National Cancer Institute (Grant Number R01

CA094196). The authors acknowledge timely assistance of

Hamamatsu Photonics (Tom Bailey and Earl Hergert) in

providing the 3x3mm2 MPPC modules. We also thank Philip

Parkhurst from Proteus for promptly providing us the many

scintillator samples used in this pilot study.

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