Valentino Orsolini Cencelli

Monolithic integration of near-infrared Ge photodetectors with Si complementary metal–oxide–semiconductor readout electronics

Using a low-temperature technology for polycrystalline Ge deposition, we report on the monolithic integration of an array of near-infrared Ge photodiodes on Si complementary metal–oxide–semiconductor (CMOS) electronics. The integrated microsystem consists of a linear array of 120×120 μm2 pixels, an analog CMOS multiplexer and a transimpedance amplifier. The resulting photoresponse covers the near-infrared up to 1.6 μm.

FPGA based readout electronics for multi anode PSPMT

The gamma cameras built on a LaBr3 crystal and Hamamatsu H8500 that are being developed have a number of channel that can vary from 64 to 256 depending on the number of PSPMTs that are used and, due to the gain differences, channels have to be acquired and corrected individually. In this context, the readout electronics has to be able to acquire and possibly to make computations on a high number of channels at a rate of kiloevents. A possible solution is the use of an FPGA programmed to collect the data and, in parallel, make a preliminary analysis. The high number of pins available on a modern FPGA allows to acquire and to manage all the data coming from the ADCs, driving the control signals and acquiring the data from several device at the same time. The possibility, even on average grade FPGAs, of having clock rate in the 100 MHz range, makes it feasible to make preliminary energy discrimination in order not to overload the communication channel with the control PC that, due to the high number of data, can became the real bottleneck of system.

GEANT4 simulation for modelling the optics of LaBr scintillation imagers

The excellent scintillation properties of the LaBr3:Ce crystals makes their use very attractive in a system of gamma imaging for Single Photon Emission Tomography (SPET) applications. In this work we use GEANT4 simulations, in order to better understand the intrinsic properties of a gamma camera based on LaBr3:Ce crystals, coupled to a Hamamatsu H8500 Multi Anodes Photomultiplier (MA-PMT). All electromagnetic process are simulated, together with the detection of the scintillation light distribution on the anodic plane of a MAPMT. The position linearity response, the spatial and energy resolutions are investigated. The values obtained by Monte Carlo show a good agreement with the experimental results.

Multichannel Readout Electronics for Flat Panel PSPMT

We introduce a new readout system for flat panel PSPMT detectors, based on discrete semiconductors. Due to the high number of simultaneous channels to be acquired, the first and most important component selected is the 8 channel ADC with 8 different track and hold. This approach enables us to acquire each of the requested 64 anodic channels in parallel, at 250 ksps/ch, with a typical resolution of 14 bits. All the necessary 8 ADCs are controlled by an FPGA, and an embedded Linux based CPU, that made the system capable to transfer data trough the Ethernet port or store it in an USB mass storage device like the popular USB pen.

ISPA Front End Integrated Circuit for Gamma Imaging Application

In recent works we presented the gamma0, a chip expressly designed for gamma imaging application based on the ISPA tube (i.e. a vacuum sealed tube with a quartz or crystal window in which electrons, generated by the scintillating light on a photocathode, are accelerated over a 20kV potential to hit a pixelated silicon detector matrix). We now made a new version that includes new features, including a more complete test subsystem. The chip is a matrix of 1024 square pixels 135mum in side, and is designed to be bump bonded to the pin diode matrix. In each pixel, the analog part, includes a Charge Sensitive Amplifier, a shaper, a transconductance amplifier, that converts the voltage output of the shaper to a current and a discriminator. The digital part has a ten event buffer and five configuration flip flops. In this work we present the test of the device with pulses equivalent to 2000 e-, that is well below the 5500 e- that are given in the ISPA tube at a 20kV supply, obtaining a very clear response and a measured noise below 200 e-. We will also present the tests of the integrated energy discrimination system.

A Theoretical Model for Fast Evaluation of Position Linearity and Spatial Resolution in Gamma Cameras Based on Monolithic Scintillators

In this work, we developed a model that is able to predict in a few seconds the response of a gamma camera based on continuous scintillator in terms of linearity and spatial resolution in the whole field of view (FoV). This model will be useful during the design phase of a SPECT or PET detector in order to predict and optimize gamma camera performance by varying the parameter values of its components (scintillator, light guides, and photodetector). Starting from a model of the scintillation light distribution on the photodetector sensitive surface, a theoretical analysis based on the estimation theory is carried out in order to find the analytical expressions of bias and FWHM related to four interaction position estimation methods: the classical Center of Gravity method (Anger Logic), an enhanced Center of Gravity method, a Mean Square Error fitting method, and the Maximum Likelihood Estimation method. Afterwards, spatial resolution as well as depth of interaction (DOI) distribution effects are evaluated by processing biases and FWHMs at different DOIs. The comparison between the model and GEANT4 Monte Carlo simulations of four different detection systems has been carried out. Our model prediction errors of spatial resolution, in terms of percentage RMSDs with respect to the simulated spatial resolution, are lower than 13.2% in the whole FoV for three estimation methods. The computational time to calculate spatial resolutions with the model in the whole FoV is five order of magnitudes faster than an equivalent standard Monte Carlo simulation.

LaBr 3 (Ce) and NaI(Tl) performance comparison for single photon emission imaging

LaBr3(Ce) crystal showed excellent spectrometric properties mainly due to a very high light output and to an energy resolution slightly affected by the intrinsic factor. Furthermore, the properties of the radiation absorption are very similar to NaI(Tl) ones, hence it offers the potential to replace it in scintillation imaging detectors. Unfortunately, the cost of this crystal is high and it is also affected by a very high hygroscopicity and lower mechanical stability which may compromise its durability. In order to evaluate the real benefits of LaBr3(Ce), we propose an imaging comparison with a second sample of NaI(Tl) by coupling them to a Hamamatsu H8500C multi-anode PMT. The crystal was produced identical in shape by Saint Gobain. In addition a second high QE MA-PMT was also tested (H8500C-Mod8 series PMT). A Geant4 Monte Carlo simulation was also implemented, where the light transport inside the crystals was taken into account. Very close results were obtained for both crystals when coupled to the new high QE MA-PMT and (0.95±0.05) mm spatial resolution -SR resulted, in spite of simulation where SR of LaBr3(Ce) was expected to be 22% better than the NaI(Tl) one. This results seems to be related to an unexpected scintillation light background measured only in the LaBr3(Ce), which limits the advantages would occur due to its highest light yield. Saint-Gobain, the most important manufacturer of LaBr3(Ce) crystals, has established its own choices in the surface treatment so apparently limiting the intrinsic response of this crystal. In conclusion, these results revisit the use of continuous crystals for small-FoV gamma-camera applications with high spatial and energy resolution tanks to the improvement obtained with the new MA-PMT with high quantum efficiency.

Design and characterization of a dual modality (SPET-US) tomographic device

In the last few years, integrated dual-imaging systems have emerged as a new modality for cancer staging with the aim to offer both functional and anatomic information. At moment the prevalent dual modality devices are based on Computer Tomography and Positron Emission Tomography. In this sense, the scientific community is debating about the high effective dose to the patient, representing an indicator of the stochastic risk, especially from Computer Tomography examination. So, a new dual modality imager, based on a Ultrasound probe and a Single Photon Emission Tomography was made in order to combine functional information, from gamma camera with structural one, obtained from the Ultrasound equipment. The Ultrasound probe is the most diffuse anatomical examination device at zero-dose, using a cost-effective and reliable method with few restriction in use. The proposed Single Photon Emission Tomography detector is a compact gamma camera (10×10 cm2 active area), based on LaBr3:Ce scintillation crystal coupled to 4×4 array of Hamamatsu H8500C-MOD8 Multi Anode Photomultiplier, with high spatial and energy resolution performances, equipped with rotating slant-hole collimator. A calibration phantom, made of a Co57 point source inside a water filled box, was utilized to acquire 3D dual modality images. The detector has shown good performances in terms of spatial resolution and localization along z-axis of object of interest. This project was developed by several Italian Universities under an INFN collaboration.