N. Pavón

Performance tests of two portable mini gamma cameras for medical applications.

We have developed two prototypes of portable gamma cameras for medical applications based on a previous prototype designed and tested by our group. These cameras use a CsI(Na) continuous scintillation crystal coupled to the new flat-panel-type multianode position-sensitive photomultiplier tube, H8500 from Hamamatsu Photonics. One of the prototypes, mainly intended for intrasurgical use, has a field of view of 44×44mm2, and weighs 1.2kg. Its intrinsic resolution is better than 1.5mm and its energy resolution is about 13% at 140keV. The second prototype, mainly intended for osteological, renal, mammary, and endocrine (thyroid, parathyroid, and suprarenal) scintigraphies, weighs a total of 2kg. Its average spatial resolution is 2mm; it has a field of view of 95×95mm2, with an energy resolution of about 15% at 140keV. The main advantages of these gamma camera prototypes with respect to those previously reported in the literature are high portability and low weight, with no significant loss of sensitivity and spatial resolution. All the electronic components are packed inside the mini gamma cameras, and no external electronic devices are required. The cameras are only connected through the universal serial bus port to a portable PC. In this paper, we present the design of the cameras and describe the procedures that have led us to choose their configuration together with the most important performance features of the cameras. For one of the prototypes, clinical tests on melanoma patients are presented and images are compared with those obtained with a conventional camera.

Corrected position estimation in PET detector modules with multi-anode PMTs using neural networks

This paper studies the use of Neural Networks (NNs) for estimating the position of impinging photons in gamma ray detector modules for PET cameras based on continuous scintillators and Multi-Anode Photomultiplier Tubes (MA-PMTs). The detector under study is composed of a 49/spl times/49/spl times/10 mm/sup 3/ continuous slab of LSO coupled to a flat panel H8500 MA-PMT. Four digitized signals from a charge division circuit, which collects currents from the 8/spl times/8 anode matrix of the photomultiplier, are used as inputs to the NN, thus reducing drastically the number of electronic channels required. We have simulated the computation of the position for 511 keV gamma photons impacting perpendicularly to the detector surface. Thus, we have performed a thorough analysis of the NN architecture and training procedures in order to achieve the best results in terms of spatial resolution and bias correction. Results obtained using GEANT4 simulation toolkit show a resolution of 1.3 mm/1.9 mm FWHM at the center/edge of the detector and less than 1 mm of systematic error in the position near the edges of the scintillator. The results confirm that NNs can partially model and correct the non-uniform detector response using only the position-weighted signals from a simple 2D DPC circuit. Linearity degradation for oblique incidence is also investigated. Finally, the NN can be implemented in hardware for parallel real time corrected Line-of-Response (LOR) estimation. Results on resources occupancy and throughput in FPGA are presented.

High-speed data acquisition and digital signal Processing system for PET imaging techniques applied to mammography

This paper is framed into the Positron Emission Mammography (PEM) project, whose aim is to develop an innovative gamma ray sensor for early breast cancer diagnosis. Currently, breast cancer is detected using low-energy X-ray screening. However, functional imaging techniques such as PET/FDG could be employed to detect breast cancer and track disease changes with greater sensitivity. Furthermore, a small and less expensive PET camera can be utilized minimizing main problems of whole body PET. To accomplish these objectives, we are developing a new gamma ray sensor based on a newly released photodetector. However, a dedicated PEM detector requires an adequate data acquisition (DAQ) and processing system. The characterization of gamma events needs a free-running analog-to-digital converter (ADC) with sampling rates of more than 50 Ms/s and must achieve event count rates up to 10 MHz. Moreover, comprehensive data processing must be carried out to obtain event parameters necessary for performing the image reconstruction. A new generation digital signal processor (DSP) has been used to comply with these requirements. This device enables us to manage the DAQ system at up to 80 Ms/s and to execute intensive calculi over the detector signals. This paper describes our designed DAQ and processing architecture whose main features are: very high-speed data conversion, multichannel synchronized acquisition with zero dead time, a digital triggering scheme, and high throughput of data with an extensive optimization of the signal processing algorithms.

Detector optimization of a small animal PET camera based on continuous LSO crystals and flat panel PS-PMTs

We have built a PET camera for small animals based on continuous block LSO crystals coupled to a PS-PMT flat panel. When working with continuous crystals, surface treatment is an important factor that strongly determines the main characteristics of the detector module. As a part of the work done for the development of our small animals PET camera, we have investigated the effect of the scintillator crystal surface treatment on the PET detector module performances, in order to optimize crystal configuration. We present the results for spatial resolution, image compression and energy resolution obtained when using different surface treatments in continuous LSO crystals. These results are compared with those obtained from simulations that have been carried out using DETECT2000 package.

Design and Calibration of a Small Animal Pet Scanner Based on Continuous LYSO Crystals and PSPMTs

We report on the design of a small animal PET scanner based on continuous LYSO crystals and position sensitive photomultiplier tubes (PSPMTs), together with the first results from the calibration. The scanner consists of eight compact modules forming an octagon and leaving a port of 110 mm aperture. Each module is made out of a continuous LYSO crystal and a PSPMT, and contains its associated electronics together with its power supply. For each module, five signals are read, summarizing all the information coming out from its 64 anode pads. Therefore, for the whole scanner only 40 signals are digitized. A calibration procedure has been implemented, measuring a spatial resolution of approximately 1.5 mm at the center of the field of view and an energy resolution of 18%. The sensitivity of the system at the center of the field of view, using only 4 modules, is observed to be of about 1%.

Depth of interaction measurement in gamma ray imaging detectors with continuous scintillation crystals

A design for an inexpensive depth of interaction (DOI) detector for gamma rays, suitable for nuclear medical applications, especially positron emission tomography (PET), has been developed, studied by simulations and tested experimentally. The detector consists of a continuous LSO-scintillator of dimensions 42/spl times/42/spl times/10 mm/sup 3/ and a new compact large-area (49/spl times/49 mm/sup 2/) position sensitive photo-multiplier (PSPMT) H8500 from Hamamatsu. Since a continuous crystal is used, the scintillation light distribution is not destroyed and its first 3 moments can be used to determine the energy (0th moment), the centroids along the x- and y-direction (1st moments) and the depth of interaction (DOI), which is strongly correlated to the distributions width and thus its standard deviation (2nd moment). The simultaneous computation of these moments allows a three-dimensional reconstruction of the position of interaction of the /spl gamma/-rays within the scintillating crystal and will be realized by a modified position sensitive proportional (PSP) resistor network. No additional photo detectors or scintillating crystals are needed. According to previous Monte Carlo simulations which estimated the influence of Compton scattering for 511 keV /spl gamma/-rays, the transport of the scintillation light within the detector assembly and also the behavior of the modified PSP resistor network, we expect a spatial resolution of /spl lsim/ 2 mm and a DOI resolution of /spl ap/ 5 mm. The first experimental results presented here yield an intrinsic spatial resolution of /spl lsim/ 1.8 mm and 2.6 mm for the x- and y-direction respectively and a DOI resolution /spl lsim/ 1 cm. Further we measured an energy resolution of 12%-18%.

Accurate Simulation Testbench for Nuclear Imaging Systems

Current testbenches for nuclear imaging devices aim to simulate only a single stage of the system at a time. This approach is useful in early design stages where accuracy is not necessary. However, it would be desirable that different tools could be combined to achieve more detailed simulations. In this work, we present a high precision testbench that has been developed to test nuclear imaging systems. Its accuracy lies in the possibility of linking different simulation tools using the right one for each part of the system. High energy events are simulated using Geant4 (High Energy Simulator). Analog and digital electronics are verified using Cadence Spectre and ModelSim. This testbench structure allows testing any physical topology, scintillation crystals, photomultiplier tubes (PMTs), avalanche photodiodes (APDs), with any kind of ASIC, discrete analog and digital electronics, thus reducing the prototyping and design time. New system developments can be easily verified using behavioral and circuital description models for analog and digital electronics. Finally, a dual-head continuous LSO scintillation crystal positron emission tomography (PET) system has been used as an example for evaluation of the testbench.

Correction algorithms for signal reduction in insensitive areas of a small gamma camera

The radiation sensitive part of the wide-field, small gamma camera Sentinella? 108 consists of a monolithic, 102 ? 102 mm2 wide CsI(Na) crystal coupled to four position-sensitive photomultipliers. These are read out via a resistor mesh reducing the 256 PMT channels to four digitized values which allow for an image reconstruction, based on polynomials, with high spatial resolution. The sensor areas between neighboring PMTs suffer from reduced detection efficiency for scintillation light. The corresponding lack of signal amplitude translates to a poor uniformity and additional image compression in the border regions of the images after the first step of reconstruction. These effects cannot be completely eliminated by a global calibration procedure alone. We have developed and evaluated two classes of algorithms to compensate for this reduction of signal amplitude. Corrections are applied for each detected event separately, maintaining the real-time capability of the gamma camera. Algorithm 1 is based on a four-parameter, geometric calculation of the four output signals. This model aims to attack the underlying lack of detection efficiency without further assumptions on the true event distribution, preserving the global polynomial image decompression. Algorithm 2, to the contrary, starts from measured reference distributions of point sources which are used to model local deformations and energy loss without further information on the underlying detector geometry. Here too, a good image quality has been obtained after application of the deduced corrections. These algorithms are applied in the first step of event reconstruction. Within an energy interval of ?0.2Ep around the photopeak position of 99mTc, Ep = 140.5 keV, and comparing several cameras, global uniformities around 5?8% have been obtained with both models. Measured energy resolutions are about 9?14%.

Fully digital trigger and pre-processing electronics for planar positron emission mammography (PEM) scanners

Digital trigger algorithms performed over large sequences of data can be an efficient way to shift processing time from baseline samples or singles to coincident photo multiplier tube pulses. This paper describes a coincidence processing system intended to implement fully digital trigger and pre-processing algorithms for a positron emission tomography scanner dedicated to breast imaging. In order to efficiently address all the emerging issues of high resolution PET detectors, a high performance DSP processor has been embedded into the backbone of the system. Signals from 12 channels of a dual-head PET camera are acquired in free-running sampling while a first stage of FIFO memories, implemented using a Virtex-II Pro FPGA, translate data from a sequential sample-by-sample processing basis to a more efficient block-by-block one. This approach enables us to carry out trigger and pre-processing tasks in parallel. Moreover, the scheme offers additional benefits: intrinsic temporal coherence, zero data acquisition dead time, and a more flexible software approach to pre- and post-processing issues (from pile-ups and scatter correction to pre-reconstruction processing). It also heavily reduces the bandwidth required for the link to the host computer, enabling the use of a high speed USB port. The DAQ system is capable of handling count rates up to 10 Mevents/s, pre-processing the samples on the fly and lastly delivering data to the host computer for image reconstruction.