Silicon photomultipliers (SiPMs) are becoming the reference photodetectors in many fields. In medicine they are slowly replacing photomultiplier tubes and avalanche photodiodes in medical imaging and in PET in particular. In this paper a broad overview of the current applications of SiPM in medicine is presented. The major fields where the SiPMs are used, namely PET/MR and hadrontherapy are discussed at length.

The INSIDE project addresses the online dose monitoring in particle therapy issue.The proposed detectors exploit the secondary neutral and charged particles emitted during the treatment by the irradiated volumes.The new detection techniques allow to return and online a measurement of the released dose and the Bragg peak position.

This paper presents a novel multichannel time to digital converter (TDC) specifically designed for the digitization of photon time of flight (TOF) and energy in positron emission tomography (PET) scanners. A coarse-fine architecture based on a counter combined with a delay locked loop (DLL) is implemented using a fully synchronous approach exploiting the pipeline principle and dynamic logic. This makes the design particularly compact and suitable for multichannel applications. The converter is also able to reject the events generated by the dark noise of the photodetectors used in the PET modules. This significantly reduces the communication bandwidth required for reading the TDC outputs. The TDC has been designed in a 65 nm CMOS process and features 8 channels that provide the arrival time information of an event with an LSB of 102 ps. The core occupies an active area of 0.3 mm(2) mm and consumes 230 mW.

The INSIDE collaboration aims to build an on-line hadrontherapy monitoring system, based on a dual-head in-beam PET scanner and a secondary charged particles profiler. In this work preliminary experimental results are presented. The validation of the FLUKA-based Monte Carlo simulation tool is shown together with the expected scanner performances.

Positron emission tomography (PET) is a clinical and research tool for in vivo metabolic imaging. The demand for better image quality entails continuous research to improve PET instrumentation. In clinical applications, PET image quality benefits from the time of flight (TOF) feature. Indeed, by measuring the photons arrival time on the detectors with a resolution less than 100 ps, the annihilation point can be estimated with centimeter resolution. This leads to better noise level, contrast and clarity of detail in the images either using analytical or iterative reconstruction algorithms. This work discusses a silicon photomultiplier (SiPM)-based magnetic-field compatible TOF-PET module with depth of interaction (DOI) correction. The detector features a 3D architecture with two tiles of SiPMs coupled to a single LYSO scintillator on both its faces. The real-time front-end electronics is based on a current-mode ASIC where a low input impedance, fast current buffer allows achieving the required time resolution. A pipelined time to digital converter (TDC) measures and digitizes the arrival time and the energy of the events with a timestamp of 100 ps and 400 ps, respectively. An FPGA clusters the data and evaluates the DOI, with a simulated z resolution of the PET image of 1.4 mm FWHM.

The pixel identification capability is a common problem of detector systems consisting of scintillating matrices coupled to photodetectors. In positron emission tomography (PET) systems, a better pixel identification leads directly to an improvement of the spatial resolution of the system. To gain pixel identification efficiency especially at the peripheral active area, ultra-transmitting (UT) glasses can be inserted between the crystal and PMT array in order to promote light spread and avoid overlap of responses due to the enhanced light spread. Measurements with three different UT glass thicknesses (d=0.7,1.0 and 1.35mm) have been performed in order to study their impact on pixel identification properties compared to direct coupling.

Positron emission tomography (PET) is a valuable technique to monitor in situ and non-invasively the particle range in ion beam therapy exploiting the beta+ activity produced in nuclear interactions along the beam path within the target volume. Due to the high random rates and dead-time losses induced by the particle spills, as of to date data are usually acquired during beam pauses or after the irradiation. We have developed a new PET prototype with a faster photon discrimination component that reduces the front-end dead time, and a modularized acquisition system that parallelizes the sensitive detector area, so as to enable data acquisition also during therapeutic irradiation (full in-beam measurement). The PET system has been able to sustain the single photon count rates and acquire coincidences during the beam, in conditions of sub-clinical beam currents. A study on the paralyzation conditions and dead time losses under different beam currents is presented and the feasibility of a full in-beam PET scanner is discussed.

The importance of a high-quality hybrid imaging, providing morphological and functional information with only one acquisition session, is widely acknowledged by the scientific community. The historical limitations to the quality of PET images are related to the unsatisfactory measurement of the depth of interaction (DOI) in the crystals and of the time of flight (TOF), that cause a parallax error and an unfavorable signal to background condition in the image reconstruction process, respectively. The 4DMPET project is developing a high performance PET block-detector featuring 4D image reconstruction capabilities. The detector module is based on a fast scintillating continuous crystal coupled on both sides to arrays of Silicon PhotoMultipliers (SiPM). The SiPMs collect the scintillation light and provide the trigger signal, the time and the energy released in the crystal at the pixel level. The photon depth of interaction (DOI) is reconstructed by measuring the cluster size asymmetry on the two faces of the crystal, thus obtaining a comparable spatial resolution in the three coordinates and removing the parallax error. The event position along the line of response can be measured with high precision by means of TOF techniques. We discuss the module design concept and the results of the detailed Monte Carlo detector simulation, which inspire the architectural solutions selected for the layout and the front-end The expected resolution for 3D spatial coordinates of the interaction point in the crystal (1 mm) and the TOF (about 110 ps) would provide a substantial improvement of the image quality. 4DMPET aims at building a prototype block detector demonstrating that the proposed layout meets the expected performance and is suitable for designing a detector focused on a specific application.

Ion beam therapy is a valuable method for the treatment of deep-seated and radio-resistant tumors thanks to the favorable depth-dose distribution characterized by the Bragg peak. Hadrontherapy facilities take advantage of the specific ion range, resulting in a highly conformal dose in the target volume, while the dose in critical organs is reduced as compared to photon therapy. The necessity to monitor the delivery precision, i.e. the ion range, is unquestionable, thus different approaches have been investigated, such as the detection of prompt photons or annihilation photons of positron emitter nuclei created during the therapeutic treatment. Based on the measurement of the induced β+ activity, our group has developed various in-beam PET prototypes: the one under test is composed by two planar detector heads, each one consisting of four modules with a total active area of 10 × 10 cm2. A single detector module is made of a LYSO crystal matrix coupled to a position sensitive photomultiplier and is read-out by dedicated frontend electronics. A preliminary data taking was performed at the Italian National Centre for Oncological Hadron Therapy (CNAO, Pavia), using proton beams in the energy range of 93–112 MeV impinging on a plastic phantom. The measured activity profiles are presented and compared with the simulated ones based on the Monte Carlo FLUKA package.

Positron emission tomography (PET) is a molecular imaging technique that provides images of physiological processes inside the body. In clinical applications, PET image quality benefits from the time of flight (TOF) feature. This is based on measuring the difference in the arrival times of couples of nearly collinear photons generated during the PET exam on a detector ring with a resolution less than 100 ps. In this scenario, a time to digital converter (TDC) can be used for the read out of the detectors to provide the photon arrival time. If matrices of Silicon Photomultipliers are used as photodetectors, multichannel topology and real time noise rejection capability are strongly required in the design of the converter along with good linearity. We present an innovative TDC architecture which is suitable for TOF PET applications.

The design of a Positron Emission Tomography detection module capable of working inside a Magnetic Resonant Imaging system is the main objective of the 4D-MPET project. Combining the two imaging technologies offers better soft tissue contrast and lower radiation doses by providing both functional and morphological information at the same time. The proposed detector will feature a three-dimensional architecture based on two tiles of Silicon Photomultipliers coupled to a single LYSO scintillator on both its faces. Silicon Photomultipliers are magnetic-field compatible photo-detectors with a very small size enabling novel detector geometries that allow the measurement of the Depth of Interaction as well as a high detector packing fraction to maximize system sensitivity. Furthermore they can be fabricated using standard silicon technology, have a large gain in the order of 106 and are very fast thus allowing evaluating the Time of Flight. Among the other features of the proposed detection system, the architecture of the innovative readout electronics will be also described which plays a relevant role for the achievement of the desired performance and is based on custom integrated circuits. Simulation results of the whole system show good performance in terms of time and spatial resolution: a timestamp of 100 ps is the ultimate performance achievable with the use of a double threshold technique along with fast electronics. Time over threshold is exploited to provide the energy information with a bin size of 400 ps. Moreover, a z resolution of 1.4 mm Full Width at Half Maximum can be achieved. The proposed detector can also be exploited in other tracking applications, such as High Energy Physics and Astrophysics.

Silicon Photomultipliers are used in many new generation PET block detectors. High granularity pixel SiPMs allow a high precision measurement of the photon interaction coordinates along the crystal surface. In order to further improve the resolution it is necessary to measure the photon Depth of Interaction (DOl), so as to reduce the parallax error in the Line of Response reconstruction. An innovative technique for DOl determination is proposed and tested. Measurements are made with a 2 cm × 2 cm × 1 cm LYSO slab with readout on the front and back large sides by means of two 4 × 4 square SiPM pixel matrices of 5 mm pitch. The data acquisition is based on the new BASIC32 chip read out with an FPGA-based system.

Objectives: The reactor produced low-energy beta emitter Lu (T1⁄2 = 6.7 d) is used on a routine basis in the clinical targeted radiotherapy in nuclear oncology. Currently the radionuclide is commercially available in its c.a. and n.c.a form. Here an efficient use of Lu for radiolabeling can be limited by several factors such as quality of the radionuclide and the chemicals or suboptimal parameters of the radiolabeling reaction. Identification of critical factors as well as an interpretation of the results is, however, complicated and limited by the analytical techniques applied. Methods: C.a. and n.c.a. Lu preparations were obtained from different commercial suppliers. The radionuclide has been employed for the systematical preparation of Lu-DOTA-octreotate. Radiolabeling yields and achievable specific activity of the compound have been analyzed using conventional HPLC. For the characterization and better understanding of critical parameters of the process, species identification in the final product (e.g. radiolabeled compound) has been performed by means of LC-ESI-TOF. Isotope and specific activity analysis of the Lu of different origin have been additionally done by means of SF-ICP-MS. Results: We were able to obtain straightforward information about the radiolabeled compound and isotope such as structure, quality and even origin and isotopic composition of the radionuclide used for the reaction. LC-ESI-TOF allows an efficient identification of the different species in the final product such as unlabeled or contaminated precursors, giving worthwhile information about the quality and limiting parameters. N.c.a. Lu was confirmed to be superior for the preparation of radiolabeled compounds of high specific activity. Conclusions: For the first time we demonstrate that LCESI-TOF is a suitable tool for a direct species identification of Lu-radiolabeled peptides. Using a combination of different analytical approaches we could confirm that Lu is available with a high quality and can be used for an efficient preparation of therapeutic agents. Acknowledgements: We thank the team of Isotope and Elemental Analysis of Paul Scherrer Institute (Hot Laboratory Division/ Nuclear Energy and Safety Department) for the performance of SF-ICP-MS measurements.