N. Marino

An innovative detection module concept for PET

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.

A new PET prototype for proton therapy: comparison of data and Monte Carlo simulations

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.

A Multichannel and Compact Time to Digital Converter for Time of Flight Positron Emission Tomography

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 mm2 and consumes 230 mW.

SiPM-based PET module with depth of interaction

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.

TDC-based readout electronics for real-time acquisition of high resolution PET bio-images

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.

Integrated Front-end Electronics for Silicon PhotoMultiplier Readout in Medical Imaging Applications

The 4D-MPET project aims to design a positron emission tomography detection module capable of working inside a magnetic resonant imaging system. 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. 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. Based on custom integrated circuits, the readout electronics include an innovative current mode front-end coupled to a novel time to digital converter. The former, implemented in AMS 0.35 \(\upmu \)m SiGe-BiCMOS technology, features a very low input impedance (17 \(\Omega \)) current buffer and a large bandwidth (1 GHz), which lead to a time resolution of \(\sim \)100 ps FWHM. The time to digital converter exploits the combination of a submicron technology (UMC 65 nm LLLVT) together with a systolic topology so as to work at a high frequency of 2.5 GHz. This yields to a nominal time resolution of 29 ps (\(\sigma )\) whereas the photon energy is evaluated with a bin size of 400 ps by using a time over threshold technique. Finally, the depth of interaction measurement is performed by an external FPGA with a simulated spatial resolution of 1.3 mm FWHM along the z coordinate.

272 AN INNOVATIVE PET DETECTOR CONCEPT FOR TOF-INBEAM PET DOSIMETRY IN HADRON-THERAPY

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.