Jose Manuel Perez

NEMA NU 4-2008 Performance Measurements of Two Commercial Small-Animal PET Scanners: ClearPET and rPET-1

In this work, we compare two commercial positron emission tomography (PET) scanners installed at CIEMAT (Madrid, Spain): the ClearPET and the rPET-1. These systems have significant geometrical differences, such as the axial field of view (110 mm on ClearPET versus 45.6 mm on rPET-1), the configuration of the detectors (whole ring on ClearPET versus one pair of planar blocks on rPET-1) and the use of an axial shift between ClearPET detector modules. We used an assessment procedure that fulfilled the recommendations of the National Electrical Manufacturers Association (NEMA) NU 4-2008 standard. The methodology includes studies of spatial resolution, sensitivity, scatter fraction, count losses and image quality. Our experiments showed a central spatial resolution of 1.5 mm (transaxial), 3.2 mm (axial) for the ClearPET and 1.5 mm (transaxial), 1.6 mm (axial) for the rPET-1, with a small variation across the transverse axis on both scanners (~1 mm). The absolute sensitivity at the centre of the field of view was 4.7% for the ClearPET and 1.0% for the rPET-1. The peak noise equivalent counting rate for the mouse-sized phantom was 73.4 kcps reached at 0.51 MBq/mL on the ClearPET and 29.2 kcps at 1.35 MBq/mL on the rPET-1. The recovery coefficients measured using the image quality phantom ranged from 0.11 to 0.89 on the ClearPET and from 0.14 to 0.81 on the rPET-1. The overall performance shows that both the ClearPET and the rPET-1 systems are very suitable for preclinical research and imaging of small animals.

Stability and characteristics of large CZT coplanar electrode detectors

The single polarity charge sensing method based on coplanar grid electrodes has demonstrated promising results on CZT detectors having volumes of 1 cm/sup 3/. This work presents the performance of two coplanar grid CZT detectors having dimensions of 1.5 cm/spl times/1.5 cm/spl times/1.0 cm, among the largest CZT spectrometers that have been reported. About 2.3% and 2.0% FWHM energy resolutions have been obtained on the two detectors at 662 keV gamma-ray energy using basic analog techniques. Improvement can be achieved with depth sensing and digital pulse processing techniques. Additional investigations have studied detector characteristics critical to practical application of these devices. This includes the stability of detector response over a period of several months and the variation of detector performance in environments with high gamma radiation fluxes and at different counting rates.

PET Demonstrator for a Human Brain Scanner Based on Monolithic Detector Blocks

We have implemented and evaluated a positron emission tomography (PET) demonstrator using two monolithic detector blocks operating in coincidence with dedicated application-specific integrated circuit (ASIC) readout. Each detector is composed of a monolithic lutetium yttrium orthosilicate (LYSO) scintillator coupled to a pair of Hamamatsu S8550-02 APD arrays. The front-end electronics of this demonstrator is based on the VATA240 ASIC readout, which sums the charge provided by each row and column of the APD array. The ASIC has been characterized obtaining a noise per row or column less than 2000 electrons rms with the APD at its inputs and a good linear response in the range from 5 fC to 30 fC. We have acquired energy spectra of 22Na and 137Cs radioactive sources, achieving energy resolutions between 13.2% and 14.1% full width at half maximum (FWHM) at 511 keV. We have estimated the interaction position over the surface of the monolithic blocks using Neural Networks (NN) position determining algorithms, obtaining spatial resolutions at the detector level down to 2.1 mm FWHM. By using this detector technology and electronics we have achieved images of point sources with spatial resolutions as good as 2.1 mm FWHM for filtered back projection (FBP) reconstructions methods with single slice rebinning (SSRB). Based on the results obtained with this demonstrator, we are developing a PET insert for existing magnetic resonance imaging (MRI) equipment, to be installed in a collaborating hospital and used for clinical PET-MRI of the human brain.

Front End ASIC design for SiPM readout

A Front End ASIC for the readout of Silicon Photo-Multipliers is presented with the following features: wide dynamic range, high speed, multi channel, low input impedance current preamplifier, low power (7mW per channel), DC coupled input with common mode voltage control and separated timing and charge signal output. A detailed description of the SiPM modeling and parameter extraction is also included allowing the emulation of the signal generated by different commercial devices in the design simulation stage. Current prototype is the first step for a more complex mixed signal design including more channels, analog processing and digital outputs, thus reducing power consumption and increasing integration. This prototype includes basic blocks for 3 channels with: preamplifier with two separate signal paths and fast current discriminator with digital output.

FlexToT - Current mode ASIC for readout of common cathode SiPM arrays

A front end application specific integrated circuit (ASIC) for the readout of common cathode Silicon Photo-Multipliers arrays is presented with the following features: wide dynamic range, high speed, multi channel, low input impedance current amplifier, low power (≈10mW per channel), common cathode connection, directly coupled input with common mode voltage control and separated timing and charge signal output.A 16 channel prototype with 16 independent outputs for energy and pile-up detection and a single fast timing output is described. The low jitter current mode processing together with a configurable differential current mode logic (CML) output provides a timing signal suitable for Time of Flight (TOF) applications, such as TOF-PET (Positron Emission Tomography). Each channel delivers a digital output of a Time Over Threshold (TOT) type with a pulse width proportional to peak current (charge) input. The current mode input stage features a novel double feedback; a low speed feedback loop keeps input node voltage constant while a higher speed feedback loop keeps input impedance low. Dedicated circuitry allows SiPM high over-voltage operation, thus maximizing Photon Detection Efficiency (PDE) and timing resolution. Design was submitted in June 2012 in Austria Microsystems (AMS) 0.35 μm HBT BiCMOS technology and is under test.

Design and prototyping of a human brain PET scanner based on monolithic scintillators

We are developing a PET insert for existing MRI equipment to be used in clinical PET/MR studies of the human brain. Previous results have demonstrated that our detector concept, based on monolithic scintillator crystals coupled to magnetically-compatible APD matrices with a dedicated ASIC front-end, is suitable for this application. In this work we present the final design of our PET scanner and report on the characterization of a prototype demonstrator used to validate the coincidence processing and data readout architecture.

Optimization of a monolithic detector block design for a prototype human brain PET scanner

We are presently developing a novel PET scanner for human brain functional imaging based on monolithic scintillator crystals read by APD matrices, capable of being inserted into an MRI system. In this work we report on the detailed study that has been made of the design of the detector blocks, aiming at defining the most suitable geometrical and readout configuration for optimizing the overall performance of the entire scanner. Both parallel and trapezoidal geometries have been simulated, featuring two layers of active scintillator material with different or similar thickness and APD readout on the front or back side. Results of this study indicate that a trapezoidal geometry with equal thickness of both layers is the best solution for improving the expected scanner performance.

A dedicated ASIC front-end readout for the monolithic detector blocks of the BrainPET prototype

In this work we present experimental results obtained with a PET prototype module based on monolithic block detectors of LYSO:Ce coupled to 2 Hamamatsu S8550-02 APD arrays and a dedicated ASIC front end readout. This module has been designed to be compatible with strong magnnetic fields and will be used on a research prototype for PET/MRI human brain imaging. The front-end electronics presented is based on a new ASIC: the VATA240. This ASIC implements the sum of the signals provided by the 8 pixels of each column and row of the detector array, composed of two APD modules. Using these 16 summed channels instead of the output of the 64 single pixels. the signal to noise ratio of the overall detector response is enhanced. To estimate the impinging photon direction we use Neural Networks (NN). The use of the summed channels also reduces the complexity of the NN algorithms implemented. The characterization of the VATA240 ASIC and the adjustment of key parameters have been carried out, and both the spatial and energy resolution with an electronically-collimated 22Na radioactive source have been experimentally evaluated.

Evaluation of monolithic detector blocks for high- sensitivity PET imaging of the human brain

We propose and evaluate an improved design at the level of PET detector blocks based on monolithic crystals that will eventually be used on a research prototype for human brain PET/MRI imaging - the BrainPET scanner. These new detector blocks, when compared with pixilated designs, feature simpler mechanics, lower cost, larger sensitive volume, better energy and spatial resolutions, all of which contribute to improvements in PET detector technology. Moreover, the magnetic compatibility of all the materials composing the block makes it suitable for operation inside an MRI scanner. Results from both experimental data and Monte Carlo simulations allow an evaluation of the performance of the detector blocks, illustrating their potential for high-sensitivity PET imaging of the human brain.

DIGITAL IMPLEMENTATION OF FILTERS FOR NUCLEAR APPLICATIONS USING THE DISCRETE WAVELET TRANSFORM

Abstract This paper presents a novel digital pulse processing technique based on fast implementations of a modern signal analysis method known as the wavelet transform (WT). From the point of view of standard nuclear filtering, the whole analysis may be understood as the action of a bank of gaussian shapers. The algorithm permits the evaluation of relevant parameters on each pulse and, making use of this information, a spectral improvement is achieved in the response of HgI 2 detectors constructed in our laboratories. As the performance of these detectors is mainly limited by the hole trapping phenomenon, the introduction of a charge loss correction making use of the WT has been considered. In this work, the pulse processing has been carried out by transferring the digital recorded pulses to a computer where a software version of the algorithm is performed.