Next-generation PET scanners are expected to fulfill very high requirements in terms of spatial, energy and timing resolution. Modern scanner performances are inherently limited by the use of standard photomultiplier tubes. The use of Silicon Photomultiplier (SiPM) matrices is proposed for the construction of a small animal PET system with depth of interaction capabilities. Measurements showing that SiPM matrices are highly ideal for PET applications, have been reported. \ua9 Societ\ue0 Italiana di Fisica

A. D., Guerra

C., Piemonte

Collazuol, Gianmaria

F., Corsi

G. D., Betta

G., Ambrosi

G., Llos\ue0

G., Magazz\ufa

M., Bisogni

S., Marcatili

Next-generation PET scanners are expected to fulfill very high requirements in terms of spatial, energy and timing resolution. Modern scanner performances are inherently limited by the use of standard photomultiplier tubes. The use of Silicon Photomultiplier (SiPM) matrices is proposed for the construction of a small animal PET system with depth of interaction capabilities. Measurements showing that SiPM matrices are highly ideal for PET applications, have been reported. © Società Italiana di Fisica

S. Marcatili

M. Bisogni

G. Llosà

G. Magazzú

G. Ambrosi

F. Corsi

G. D. Betta

C. Piemonte

A. D. Guerra

COLLAZUOL, GIANMARIA

Archivio istituzionale della ricerca - Università di Padova

Detectors for the next-generation PET scanners

Next-generation PET scanners are expected to fulfill very high requirements in terms of spatial, energy and timing resolution. Modern scanner performances are inherently limited by the use of standard photomultiplier tubes. The use of Silicon Photomultiplier (SiPM) matrices is proposed for the construction of a small animal PET system with depth of interaction capabilities. Measurements showing that SiPM matrices are highly ideal for PET applications, have been reported

Marcatili, Sara

Collazuol, G.

Bisogni, M. G.

Llosà, G.

Magazzú, G.

Ambrosi, G.

Corsi, F.

Dalla Betta, G.

Piemonte, C.

Del Guerra, A.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2009-10491-7Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009Detectors for the next-generation PET scannersS. Marcatili(1)(2)(∗), G. Collazuol(3)(2), M. G. Bisogni(2), G. Llosa`(1)(∗∗),G. Magazzu´(2), G. Ambrosi(4), F. Corsi(5), G. Dalla Betta(4)(7),C. Piemonte(8) and A. Del Guerra(1)(1) Dipartimento di Fisica, Universita` di Pisa - Pisa, Italy(2) INFN, Sezione di Pisa - Pisa, Italy(3) Scuola Normale Superiore - Pisa, Italy(4) INFN, Sezione di Perugia - Perugia, Italy(5) DEE-Politecnico di Bari - Bari, Italy(6) INFN, Sezione di Padova - Padova, Italy(7) Universita` di Trento, DISI - Povo di Trento, Italy(8) FBK-irst Divisione Microsistemi - Povo di Trento, Italy(ricevuto il 19 Settembre 2009; pubblicato online il 2 Dicembre 2009)Summary. — Next-generation PET scanners are expected to fulfill very highrequirements in terms of spatial, energy and timing resolution. Modern scannerperformances are inherently limited by the use of standard photomultiplier tubes.The use of Silicon Photomultiplier (SiPM) matrices is proposed for the constructionof a small animal PET system with depth of interaction capabilities. Measure-ments showing that SiPM matrices are highly ideal for PET applications, have beenreported.PACS 29.40.-n – Radiation detectors.1. – Requirements for the next generation PET scannersPositron Emission Thomography is a powerful imaging technique in which positronemitting radionuclides bound to biologically significant molecules are used to investigatemany metabolic processes in vivo. The two antiparallel photons resulting from each singlepositron annihilation in tissue, are detected in time coincidence and used to determinetheir “line of flight”. Thomographic acquisition of these Line Of Responses (LOR) allowsfor the 3-dimensional reconstruction of the map of activity inside the organism underinvestigation.(∗) E-mail: sara.marcatili@pi.infn.it(∗∗) Now at Instituto de Fisica Corpuscola`r, Valencia, Spain.c© Societa` Italiana di Fisica 447448 S. MARCATILI, G. COLLAZUOL, M. G. BISOGNI, ETC.Standard PET detectors are currently based on the well-established technology ofPhoto Multipliers Tube (PMT). PMTs of different types coupled to crystal scintillatorsof various geometries, provide PET modules with different performances. However, inthe last years, the constraints that next-generation PET scanners are expected to fulfill,have posed the basis for a radical renewal of PET technology.Current PET scanners performances are definitely limited by intrinsic features of thedetector employed. Their spatial resolution is strongly influenced by detector’s pitch andparallax errors, and these ultimately depend upon detector layout. A small-pitch or acontinuous detector, in fact, can result in a more accurate position determination andwould make it feasible the sub-millimetric resolution required in small animal imaging.At the same time, a small and compact detector could be assembled in a multi-layermodule capable of measuring the radiation Depth Of Interaction (DOI). In the processof image reconstruction, the assumption that gammas interact at detector surface isgenerally made, resulting in a bad determination of LORs. In addition, Signal to NoiseRatio (SNR) can be improved by using the Time Of Flight (TOF) technique.An intrinsic limitation of Positron Emission Thomography is its poor capability toreveal detailed morfological strucutures. The need for a better identification of anatom-ical contours has lead to the development of hybrid imaging systems composed by aPET scanner associate to a X-ray Computed Tomography (CT) apparatuses. Their useis nowadays widely diffuse in day-to-day clinical practice, even if they present the dis-advantage of significantly contribute to the total dose delivered to the patient. On theother hand, MRI would provide exceptionally high-resolution images and functional in-formation complemental to those obtained from PET without resulting in an additionaldose delivering. However the use of PMT is not straightforward in a magnetic field en-vironment. All these requirements lead to the necessity of an innovative solution. Theuse of a high-granularity, compact, solid-state detector seems to be the right choice, asmany studies are currently indicating [1-3].The Silicon PhotoMultiplier (SiPM) [4] has been demonstrated to be the optimumcandidate [5]. A SiPM consists of Geiger Mode Avalanche PhotoDiodes (GM-APD)arrays connected in parallel through a polysilicon resistor. Each of the microcells per-mits the conversion of light in one photoelectron, thus, by summing up all the con-tribute from the microcells composing the SiPM, a signal proportional to the depositedenergy can be obtained for a low light flux. Current SiPMs are composed of a vari-able number of microcells (from few hundreds to thousands) of different sizes (lineardimensions from 25 to 100μm) and they are now available in detectors of relativelylarge size. SiPM matrices [5] consist of SiPM arrays grown on a common substrateand they have been produced in 4 × 4 or 8 × 8 pixels versions at FBK-irst [6]. Thanksto its typical pitch, ranging from 1 to 1.5mm with a negligible dead area due to thesignal routing, the SiPM matrix can provide high spatial resolution maps when usedfor imaging applications. Moreover, its compactness makes it possible the realizationof multi-layers detector modules capable to provide DOI information, while the verylimited time jitter (about 70 ps FWHM at single photoelectron level in the case of sin-gle SiPM [7]) makes viable the realization of human TOF-PET scanners if a very fastscintillator is adopted. Furthermore, the high intrinsic gain (up to 106) attainable atlow bias voltage (∼ 50V), and the consequent high signal-to-noise ratio makes the useof sophisticate electronics redundant. Being a solid-state detector, SiPM is intrinsi-cally magnetic field insensitive, and hence can be used in the assessment of PET/MRIapparatuses.DETECTORS FOR THE NEXT-GENERATION PET SCANNERS 4492. – A SiPM based small animal PET prototypeOne of the goals of the DASiPM (Development and Application of SiPM) Collab-oration is the construction of a small animal PET scanner prototype based on SiPMmatrices and LYSO scintillator crystals. The complete detector will be composed of twohigh spatial resolution heads with DOI capabilities. Three modules consisting on SiPMmatrices of 1.5mm pitch, coupled to continuous LYSO crystals 0.5mm thick are assem-bled in a pile, in 1.2 cm2 area detector heads. Simulation of the prototype, performedvia Geant4 toolkit, predicts a sub-millimetric spatial resolution at the center of the FieldOf View (FOV), for a 18F point source [8].However the use of SiPMs makes it necessary to develop a read-out system capableto manage signals from many channels. This system should also exhibit excellent timingproperties (time jitter below few hundreds of ps) and good spectroscopy capabilitiesnot to degrade SiPM intrinsic performances. The data acquisition should sustain a10 kHz/cm2 rate, which is the maximum expected in a pre-clinical scenario for a smallanimal PET scanner. The front-end should be based on Application Specific IntegratedCircuit (ASIC), so as to manage many channels and keep the system compact at the sametime. Standard ASIC chips are not ideal for SiPM applications because they have beenmainly conceived for PMTs, and, when connected to a SiPM, they generally saturatebecause of its huge intrinsic gain.The read-out system which is currently being developed, is based on an ASIC chipthat handles both the signals pre-amplification and the fast-or trigger generation. Theread-out board, which at the moment mounts a commercial chip of the VA-TA familyby IDEAS, can also house a dedicated ASIC which has been designed at Politecnico ofBari [9]. The whole acquisition [10] is controlled by means of an FPGA (Stratix II) andcommunicates with the PC via an USB interface. A coincidence board will allow thedetection of photons from a single event in a 5 ns coincidence window, for a good noiserejection.3. – SiPM matrices performances in imaging applicationsSiPM matrices performances as detector for PET have been deeply investigated [5]via a read-out system based on MAROC2 chip and developed at (Linear AcceleratorLaboratory) LAL in Paris. Detectors of 4×4 pixels have been coupled to LYSO scintilla-tor crystals of different sizes and designs in order to study their time, energy and spatialresolution capabilities.Continuous slab of LYSO of the same size of the detector and 0.5mm thick have beenused to carry out spectroscopic measurement with a 22Na source. The typical energyresolution is about 15% FWHM, quite good for PET applications. Thanks to SiPMvery low time jitter, the measured time resolution is dominated by LYSO decay timeproperties and detector efficiency. Tests have been performed both with continuous andpixellated crystals. By summing up the contribute of all SiPM channels, a time jitter of1.8 ns (σ) in the former and a 1.3 ns (σ) time resolution in the latter configuration, havebeen measured.Very encouraging results have been achieved for what concerns its spatial resolutioncapabilities. In this case both pixellated scintillators with the same pitch of the detectors,and continuous black slab have been employed. A 22Na point source placed very nearthe detector module has been collimated performing a time coincidence with a LYSOsingle pixel of 1mm2 surface; in this way a light spot has been obtained, that can be450 S. MARCATILI, G. COLLAZUOL, M. G. BISOGNI, ETC.Fig. 1. – Spatial resolution performances of a SiPM matrix coupled to a pixellated LYSO crystal(left) and a continuous black slab of the same material (right). On the right, a slice view of thereconstructed light distribution exhibits a sigma below 0.3mm.used to scan the module moving the source and the crystal pixel contemporaneously. Inthe case of one-to-one coupling, it has been shown that shifts of 0.5mm can be clearlydistinguished in the resulting image (fig. 1 left). Results relative to the black continuousslab module are the most interesting, since it is the solution that will be implemented inthe final scanner. In this case a light spot at 0.5mm from the center of the FOV producesa very narrow light distribution with a sigma smaller than 0.3mm (fig. 1 right).4. – ConclusionsSiPM matrices have been demonstrated to be optimum candidates for the assessmentof PET apparatuses. Their intrinsic features allow for the construction of PET systemswith performances far better than those of the modern scanners. A small animal PETprototype is under development at University of Pisa in the framework of the INFNproject DASiPM. It will provide DOI information and a sub-millimetric spatial resolutionat the center of the FOV as predicted by Geant4 simulations and then confirmed bymeasurements on a smaller version of the final detector module.∗ ∗ ∗This work is partially supported by MUR (Ministero dell’Universita` e della Ricerca)in the framework of the PRIN (Progetti di Rilevante Interesse Nazionale) program 2007and partially by INFN, Scientific National Commission V, in the framework of DASiPMand DASiPM2.REFERENCES[1] Llosa G. et al., IEEE Trans. Nucl. Sci., 55 (2008) 877.[2] Shao Y. et al., IEEE Trans. Nucl. Sci., 47 (2000) 1051.[3] Pichler B. J. et al., J. Nucl. Med., 47 (2006) 639.[4] Piemonte C. et al., NSS Conference Record, 41 (2007) 2089.[5] Llosa G. et al., MIC Conference Record, 2 (2008) 3574.[6] Llosa G. et al., IEEE Trans. Nucl. Sci., 56 (2009) 543.[7] Collazuol G. et al., Nucl. Instrum. Methods A, 581 (2007) 461.[8] Moehrs S. et al., Phys. Med. Biol., 51 (2006) 1113.[9] Corsi F. et al., NSS Conference Record, 15 (2007) 360.[10] Bisogni M. G. et al., NSS Conference Record, 52 (2008) 3574.

Detectors for the next generation PET scanners

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