First Experimental Resultswith the Clear–PEM Detector

Maria C. Abreu, João D. Aguiar, Edgar Albuquerque, Fernando G. Almeida, Pedro Almeida,Pedro Amaral, Pedro Bento, Ricardo Bugalho, Bruno Carriço, Hugo Cordeiro, Miguel Ferreira,

Nuno C. Ferreira, Fernado Gonçalves, Carlos Leong, FilipeLopes, Pedro Lousã, Mónica V. Martins,Nuno Matela, Pedro R. Mendes, Rui Moura, João Nobre, Nuno Oliveira, Catarina Ortigão, Luı́s Peralta,

Joel Rego, Rui Ribeiro, Pedro Rodrigues, Ana I. Santos, Jos´e C. Silva, Manuel M. Silva,Isabel C. Teixeira, João P. Teixeira, Andreia Trindade andJoão Varela

Abstract— First experimental results of the imaging systemClear-PEM for positron emission mammography, under develop-ment within the framework of the Crystal Clear Collaboratio n atCERN, are presented. The quality control procedures of crystalpixels, APD arrays and assembled detector modules are de-scribed. The detector module performance was characterized indetail. Results on measurements of light yield, energy resolution,depth-of-interaction and inter–channel cross-talk are discussed.The status of the development of the front–end electronics andof the data acquisition boards is reported.

I. I NTRODUCTION

Clear–PEM is a Positron Emission Mammography [1], [2](PEM) prototype under development within the framework ofthe Crystal Clear Collaboration [3]. The detector is basedon pixelized LYSO:Ce crystals optically coupled on bothextremities to avalanche photodiodes (APD) and readout bya fast low–noise electronic system. A dedicated digital triggerand data acquisition system is used for on-line selection ofcoincidence events with high efficiency, large bandwidth andnegligible dead-time. The scanner consists of two compactand planar detector heads with adequate dimensions for breast

Manuscript received November 11, 2005.This project is financed by AdI (Innovation Agency) and POSI (Operational

Program for Information Society), Portugal. The work of N. Matela, R. Moura,P. Rodrigues and A. Trindade is supported by Fundação paraa Ciência e aTecnologia (FCT) under grants SFRH/BD/6187/2001, SFRH/BD/12418/2003,SFRH/BD/10187/2002 and SFRH/BD/10198/2002. The work of P.Amaral, R.Bugalho, H. Cordeiro, N. Oliveira, M. V. Martins and C. Ortigão is supportedby AdI.

A. I. Santos is with Hospital Garcia de Orta, Almada, Portugal. P. Almeida,H. Cordeiro, M. V. Martins, N. Matela and N. Oliveira are withUniv. deLisboa, Facul. de Ciências, IBEB – Inst. de Biofı́sica e Eng. Biomédica,Portugal. R. Bugalho and N. C. Ferreira are with IBILI – Inst.Biomédicade Investigação em Luz e Imagem, Facul. de Medicina, Univ.Coimbra,Portugal. J. D. Aguiar, F. G. Almeida, F. Lopes, J. Sampaio and R. Ribeiroare with INEGI. Inst. Eng. Mecânica Gestão Industrial, Porto, Portugal. P.Bento, F. Gonçalves, C. Leong, P. Lousã, J. Nobre, J. Rego,I. C. Teixeiraand J. P. Teixeira are with INESC-ID and INOV, Lisboa, Portugal. M. C.Abreu, B. Carriço, P. R. Mendes are with LIP – Lab. de Instrumentaçãoe Fı́sica Exp. de Partı́culas, Algarve and Facul. de Ciências e TecnologiaUniv. do Algarve, Faro, Portugal. M. Ferreira, R. Moura, C. Ortigão, L.Peralta, R. Ribeiro, P. Rodrigues, J. C. Silva, A. Trindade and J. Varela (e–mail: Joao.Varela@cern.ch) are with Lab. de Instrumentação e Fı́sica Exp.Partı́culas, Lisboa, Portugal. L. Peralta is also with Facul. de Ciências da Univ.de Lisboa, Portugal. R. Ribeiro and F. G. Almeida are also with Facul. Eng. daUniv. do Porto, Portugal. J. Varela is also with CERN, Geneva, Switzerland.F. Gonçalves, I. C. Teixeira, J. P. Teixeira and J. Varela are also with IST -Instituto Superior Técnico, Universidade Técnica de Lisboa, Portugal.

and axilla imaging. A dedicated gantry is being built to allowthe rotation of the detector heads in breast exams as wellas to permit exams of the axilla region. The Clear–PEMscanner is developed with three main guidelines: low randombackground; high sensitivity; and spatial resolution of theorder of 2 mm. The first requirement arises from the factthat the scanner must cope with a large single photon rate.In order to increase the sensitivity the Clear–PEM imagingsystem allows to exploit Compton interactions in the detector.Finally, in order to deliver the required spatial resolutionallover the field-of-view without compromising the sensitivityby restricting the angle of the accepted lines–of–response, thedetector is able to measure the depth-of-interaction (DoI)ofthe incoming photons [4].

II. T HE CLEAR–PEM DETECTORMODULE

The basic detector module of the Clear-PEM scanner iscomposed by a 32 2×2×20 mm3 LYSO:Ce pixels, readoutat both ends by Hamamatsu APD S8550–01 (4×8 pixels,2 sub-arrays of 2× 8 pixels each) for depth–of–interactionmeasurement [4]. The components of the detector module arehoused and sealed in a dedicated plastic mechanical assembly.Twenty-four detector modules are mechanical fixed and elec-trically connected to front and back electronics PCBs forminga supermodule with about 4×14 cm2. Four supermodulesare mounted in a detector head. In total the Clear–PEMscanner has 192 detector modules and 6144 crystals, coveringa 16.2×14.1 cm2 Field–of–View (FoV) [4] with a packingfraction of 52% – Fig. 1.

The quality control of the detector modules components,individual crystal pixels and APDs, is already in progress usingdedicated experimental setups which are described in detail inSection III. After that, the detector modules are assembledaccording to a developed and rigorous procedure in order toguarantee the precise positioning and matching between thecrystal and the APDs. Finally, they are connected to the PCBs– Fig. 2. The production of the first 24 is concluded and arebeing assembled on a reduced channel version of the Clear–PEM scanner.

A. Crystals and APDs Quality Control

All the required 6300 LYSO:Ce crystal pixels have beenreceived and quality control of about 10% performed in a

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Fig. 1. Schematic representation of the Clear–PEM scanner.

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Fig. 2. (a) Hamamatsu APD S8550-01 sample. (b) LYSO:Ce 4×8 crystalmatrix. (c) Two assembled detector modules already connected to the front-end PCB.

MiniACCOS machine at CERN, which allows the measure-ment of individual crystal pixels with large throughput [5].The evaluated parameters were the light yield and energyresolution. The crystal surface roughness, a critical parameterfor DoI capability, was also measured on a sampling basis.

The production of 400s S8550–01 APD arrays by Hama-matsu Photonics Inc. and its delivery to the Clear–PEM projectwas concluded during the first semester of 2005. Currenly,about 190 APDs were tested in terms of bias voltage for gain(M ) 50, 100 and 200; dark current (Id/M ) ratio per sub–array; gain gradient (dM/dV ) per sub–array; relative gain ofindividual pixels. Results are presented in Fig. 3.

The mechanical parameters that characterize the Hama-matsu S8550-01 were also measured. Results are presentedin Fig. 4. It was found that for this new S8550 APD arraysub-version (01), all the measured parameters are within thetolerance limits agreed with Hamamatsu Photonics. Theselimits were imposed by demanding requirements that havearise from front–end electromechanics integration issues.

III. D ETECTORMODULE CHARACTERIZATION

The detector module performance was characterized bydetailed experimental measurements. Light yield, energy res-olution, depth-of-interaction resolution and inter-crystal cross-talk were measured in a dedicated setup (discrete chargeamplifiers Cremat CR–101, 12–bit peak sensing CAEN V785ADCs and CAEN V1718 VME–USB controller). One inchNaI(Tl) scintillator was used in coincidence with the modulefor electronic collimation.

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A. Light yield, APD gain and Energy Resolution

An analysis in the 511 keV photopeak region, correspondingto the sum of top and bottom APD pixel channels, wasconducted. Relative gain and energy resolution, taking into ac-count the contribution of light yield variation, light collectionefficiency, APD quantum efficiency and gain were evaluated -Fig. 5. The r.m.s. relative dispersion of the gain measured in623 pixels is of the order of 18%, with a bias voltage settingfor an APD gainM = 50. The average energy resolutionis 18% and the r.m.s. dispersion is 3%. These results wereobtained with a22Na flood irradiation.

B. DoI Measurements

The capability to measure the depth-of-interaction coor-dinate was assessed in the first 24 detector modules. Lightcollection asymmetry:

Asymetry =(APDtop − APDbottom)

(APDtop + APDbottom)(1)

was calculated and two reference parameters evaluated: 1)slope defined as the asymmetry variation per unit length; 2)DoI resolution estimated by the FHWM of asymmetry peakover slope. The results are shown in Fig. 6 for events in the 511keV photopeak. The average value of the slope is 5%/mm andthe relative dispersion (r.m.s.) is 12%, measured in a sample

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Fig. 5. (a) Relative gain distribution of 623 detector crystals and (b) averageenergy resolution of the first batch of 24 detector modules, obtained with a22Na flood irradiation.

of 623 crystal pixels. The average DoI resolution is 2 mm (notcorrected for the≈1 mm beam width).

C. Inter–Crystal Cross–Talk

Inter-crystal cross-talk was studied by selecting events witha deposited energy in a central crystal (sum energy read fromtop and bottom APD) and recording the energy distribution infirst neighbor channels. As expected, Compton scattered eventsare predominant in the forward direction. This contributionis reduced as the threshold value in the central crystal isincreased. Selecting events near the photopeak of that crystalshows a small fraction of cross-talk – Fig. 7. This cross-talkmay be due to escape x-rays and may have an instrumentalorigin. Analysis is underway to quantify the two possiblecontributions.

IV. ELECTRONICS ANDDATA ACQUISITION SYSTEM

In the Clear–PEM scanner, four front–end electronic blocksare responsible for analogue signal detection, each one directlyconnected to the top or bottom of a crystal plane. Ampli-fiers and multiplexer integrated circuits (ASIC) and the free–sampling analogue to digital converters (ADCs) are the maincomponents of the front–end electronics. After serialization,digital signal cables connect the front–end blocks to the off–detector electronics which are housed in a standard cratesystem. Accepted data is sent to a host computer for a secondlevel of event filtering before image reconstruction.

A. Front–end Electronics

The front-end electronics development has been centered onthe design of a low noise amplifier and multiplexer chip [6]

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Channel 0 Channel 1 Channel 2

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Fig. 7. Energy distribution in central crystal and in its first neighbors asfunction of the energy threshold.

- Tab. I. This chip performs the readout of one side of sixmodules (total of 192 APD pixel channels), amplification,sampling and storage in analogue memories and the selectionof two active channels (192:2 multiplexing) above a commonthreshold. The chip operates in data–driven synchronous mode,such that the output samples have fixed latency relative to theinput pulse. No dedicated trigger signal is generated by thefront-end system, otherwise it would require sophisticated dis-criminators. Instead, the trigger information is extracted fromthe main data flow in the off-detector system. Each analoguedataframe is composed of 10 samples and stored in analoguememories. The dataframe may contain 2 to 4 pre-samples [6].The pre–samples are used by the off-detector electronic systemfor pedestal estimation and correction on an event-by-eventbasis. If three or more channels are found active, i.e. twochannels are already transmitting two dataframes and a requestis made to transmit a third, an error code is produced indicat-ing the occurrence of an overflow condition. The analoguesamples are digitized in the front-end by 10-bit samplingADCs. The digital data are serialized in LVDS bit streamsand transmitted to the off-detector system. The transmissionof 10-sample dataframes per detector pulse to the trigger anddata acquisition system represents a more flexible way to adaptthe trigger algorithms, implemented in programmable logic,to future requirements. A first version (V1) of the front–endASIC implemented in AMS CMOS 0.35µm technology hasbeen received from foundry. The V1 chip – Fig. 8 – is basicallya multi–purposed development version which includes in thesame circuit the following functionalities: 32 input channelamplification circuit with individual access points; two fullyinstrumented channels with amplification and digital controlcircuit, but also with individual access points (microPEMcircuit); a complete 32:2 amplification/multiplexing circuit(miniPEM circuit).

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Recent tests have allowed us to successfully validate theanalog memory and digital controls. The amplifier was foundto required further changes which are being addressed in arevised version of the chip. These developments were com-plemented by the production of a prototype front-end serviceboard which distributes voltage lines (HV and LV) to the front-end chips, ADCs and APDs. This service board provides alsotemperature measurements to the software monitoring system,which is essential to control the APDs gain. The service boardis also responsible for the fanout of clock, synchronization, test

TABLE I

FRONT–END CHIP SPECIFICATIONS.

Main Characteristics

– Pulse amplitude of 1 V for a maximum input charge of 90 fC

– 192 charge amplifiers and comparators

– Pulse peaking time around 30–40 ns

– ENC around 1000 electrons

– 100 MHz maximum sampling frequency

– 2112 analog memory cells

and reset lines to the front–end boards.

B. Data Acquisition Electronics

The off-detector DAQ system is housed in a 6U crate withtwo dedicated buses (Trigger and Data) implemented in Com-pactPCI backplanes. Two types of electronic boards were de-veloped and produced: Data Acquisition Boards (DAQ Boards,Fig. 9 (a)) and the Trigger and Data Concentrator Board(TGR/DCC Board, Fig. 9 (b)) equipped with 4 and 2 milliongates Xilinx Virtex II FPGAs, respectively. DAQ Boardsperform the initial phase of data reduction/selection (pipelinedata storage, parallel algorithmic processing to extract theamplitude and time of the detector hits) and transmit thepotentially interesting events to the TGR/DCC Board whichperforms the trigger candidate selection. The energy, timeextraction algorithms and their impact on trigger performanceare discussed in more detail in [6], [7]. Two types of coinci-dence triggers operate simultaneously in the TGR/DCC Board:coincidence trigger in which candidate hits with timetagsless than the coincidence window are accepted and randomcoincidence trigger in which timetags before comparison arerandomized. At each trigger, complete dataframes of the iden-tified detector hits are transmitted to the Acquisition PC via aPCI adapter card with a throughput of 400 Mbyte/s. Both typeof boards possess a Built–In–Self–Test [8] which is used fromthe early phase of component testing up to the level of systemtesting after the final detector electronics integration. FPGAsfirmware was developed and bit–level comparisons betweenFPGA VHDL testbenches (compiled and synthesized by ISEProject Navigator 6.2.03i and simulated by ModelSimTM XEII 5.7g) against a C++ high–level FPGA simulation performed.Front-end dataframes were generated from Geant4 simulationsand used as test vectors for the VHDL testbench and C++FPGA simulations. For each front-end dataframe features like,pulse peak search, pedestal computation, energy and timeextraction and trigger candidates selection were evaluated. Aperfect match at bit-level between the outputs of the energyand time extraction algorithms was accomplished.

V. SUMMARY

We have reported on experimental work aiming at the char-acterization of the Clear–PEM detector modules. The qualitycontrol of the LYSO crystal pixels performed with a dedi-cated machine on 512 crystals confirmed the expected lightcollection at the pixel extremities (≈13 photons/keV) with a

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standard deviation of about 12%. Part of this dispersion (≈ 1-2%) is due to the calibration of the measurement equipment.The Hamamatsu S8550-01 APD arrays followed systematiccontrol of mechanical quantities, gain and dark current, gaingradient with bias voltage and pixel relative gain within anarray. All measured arrays are within the specifications ofthe producer. A dedicated setup was used to characterizefully assembled detector modules. The presented results arebased on the measurement of a first batch of 24 modules.We observed a relative dispersion of the individual pixelsintegrated gain (light yield and APD gain) of the order of 18%.This dispersion can be reduced to about 12% by adjusting thebias of the APD arrays (common to 16 pixels). The averageenergy resolution at 511 keV is 18% and the r.m.s. dispersionof this value is 3%. The DoI resolution of the≈600 measuredpixels is about 1.7 mm (corrected for beam width) with asmall dispersion (≈12%). First experimental measurementsshow indications of negligible instrumental cross-talk betweenpixels. We also reported on the status of the development ofelectronics and data acquisition systems. Detailed systemandVHDL simulations have confirmed the expected trigger anddata acquisition performance. A first prototype frontend ASICin 0.35 µm CMOS technology was produced which provedthe viability of the analog sampling and multiplexing schemeused to reduce the amount of data transferred to the backend

system. The trigger and data acquisition boards are now undertest.

ACKNOWLEDGEMENTS

The authors acknowledge the Brussels and Lausanne groupsin the Crystal Clear Collaboration (CCC) for making availablethe front–end electronics they use in the detector modulesquality control systems, the contribution of the CERN groupin CCC to the qualification of the Clear–PEM crystal pixels inthe MiniACOS machine, and finally the contribution of JoanLuyten to the quality control of the APD arrays.

REFERENCES

[1] C. J. Thompson et al., Positron Emission Mammography (PEM): apromising technique to detect breast cancer, IEEE Trans. Nucl. Sci. vol.42, pp.1012-1017, 1995.

[2] W. W. Moses, Positron emission mammography imaging, Nucl. Instrum.Meth. vol. A 525, pp. 249-252, 2004.

[3] P. Lecoq and J. Varela, Clear-PEM, a dedicated PET camerafor mam-mography, Nucl. Instrum. Meth. vol A 486, pp.1-6, 2002.

[4] M. C. Abreu, Design and evaluation of the Clear-PEM scanner forpositron emission mammography, IEEE Trans. Nucl. Sci. (accepted).

[5] J. Trummer et al., Scintillation properties of LuYAP andLYSO crystalsmeasured with MiniACCOS, an automatic crystal quality control systems,IEEE 2005 NSS–MIC Conference, Puerto Rico, USA.

[6] E. Albuquerque et al., The Clear-PEM electronics system, IEEE Trans.Nucl. Sci. (accepted).

[7] P. Rodrigues et al., Performance simulation studies of the Clear-PEMDAQ/Trigger system, in Conference Records of the 14th IEEE–NPSSRT2005, Stockholm, Sweden.

[8] C. Leong et al., Design and test issues of a FPGA based dataacquisitionsystem for medical imaging using PEM, in Conference Recordsof the14th IEEE–NPSS RT2005, Stockholm, Sweden.