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Technological advances have improved the assembly process of PET detectors, resulting in quite small mechanical tolerances. However, in high-spatialresolution systems, even submillimetric misalignments of the detectors may lead to a notable degradation of image resolution and artifacts. Therefore, the exact characterization of misalignments is critical for optimum reconstruction quality in such systems. This subject has been widely studied for CT and SPECT scanners based on cone beam geometry, but this is not the case for PET tomographs based on rotating planar detectors. The purpose of this work is to analyze misalignment effects in these systems and to propose a robust and easy-to-implement protocol for geometric characterization. The result of the proposed calibration method, which requires no more than a simple calibration phantom, can then be used to generate a correct 3D-sinogram from the acquired list mode data. This work was partially funded by the AMIT project (CEN-20101014) from the CDTICENIT program, RECAVA-RETIC Network (RD09/0077/00087), projects TEC2010-21619-C04-01 and TEC2011-28972-C02-01 from the Spanish Ministerio de Ciencia e Innovación, PRECISION grant IPT-300000-2010-3, CPAN (CSD-2007-00042@Ingenio2010), MEC (FPA2010-17142) and ARTEMIS program (S2009/DPI-1802) from the Spanish Comunidad de Madrid and EU-ERDF program. Publicado

Positron range limits the spatial resolution of PET images and has a different effect for different isotopes and positron propagation materials. Therefore it is important to consider it during image reconstruction, in order to obtain optimal image quality. Positron range distributions for most common isotopes used in PET in different materials were computed using the Monte Carlo simulations with PeneloPET. The range profiles were introduced into the 3D OSEM image reconstruction software FIRST and employed to blur the image either in the forward projection or in the forward and backward projection. The blurring introduced takes into account the different materials in which the positron propagates. Information on these materials may be obtained, for instance, from a segmentation of a CT image. The results of introducing positron blurring in both forward and backward projection operations was compared to using it only during forward projection. Further, the effect of different shapes of positron range profile in the quality of the reconstructed images with positron range correction was studied. For high positron energy isotopes, the reconstructed images show significant improvement in spatial resolution when positron range is taken into account during reconstruction, compared to reconstructions without positron range modeling. © 2010 Elsevier B.V. All rights reserved., This work has been supported by the MEC (FPA2007-62216), the UCM (Grupos UCM, 910059), the CPAN (Consolider-Ingenio 2010)CSPD-2007-00042, the RECAVA-RETIC network, ARTEMISS2009/DPI-1802, the European Regional Development and ENTEPRASE grant, PSE-300000-2009-5 and the Ministerio de Ciencia e Innovación, Spanish Government.

Super Bialkali (SBA) photocathode is a new technology that improves the spectral response characteristics of position sensitive PMTs, boosting their quantum efficiency up to 35%. In this experiment, two SBA tubes were introduced into a production line of PET detectors mixed with the regular tubes. The detectors were assembled using the standard factory protocols for detector mounting, calibration and testing. We report an evaluation of the improvement introduced by the SBA photocathode comparing the spatial and energy resolution and the depth-of-interaction (DOI) performance of PET detector modules with DOI capabilities.We conclude that the superior performance of the SBA tube may enable the use of arrays with a larger number of crystals of smaller footprint, thus potentially improving the detector intrinsic spatial resolution without degrading the energy resolution or the phoswich (DOI) discrimination capability. This work was supported in part by the CENIT Program (Ministerio de Industria), Comunidad de Madrid program ARTEMIS-P2009/DPI-1802, TEC2007-64731/TCM (Ministerio de Educación y Ciencia), and the RETIC-RECAVA, ISCIII. The contribution of J. Seidel wa supported in whole or in part with federal funds from the National Cancer Institute, National Institutes of Health, under Contract HHSN261200800001E. Publicado

This work reports on the development and performance evaluation of the VrPET/CT, a new multimodality scanner with coplanar geometry for in vivo rodent imaging. The scanner design is based on a partial-ring PET system and a small-animal CT assembled on a rotatory gantry without axial displacement between the geometric centers of both fields of view (FOV). We report on the PET system performance based on the NEMA NU-4 protocol; the performance characteristics of the CT component are not included herein. The accuracy of inter-modality alignment and the imaging capability of the whole system are also evaluated on phantom and animal studies. Tangential spatial resolution of PET images ranged between 1.56 mm at the center of the FOV and 2.46 at a radial offset of 3.5 cm. The radial resolution varies from 1.48 mm to 1.88 mm, and the axial resolution from 2.34 mm to 3.38 mm for the same positions. The energy resolution was 16.5% on average for the entire system. The absolute coincidence sensitivity is 2.2% for a 100–700 keV energy window with a 3.8 ns coincident window. The scatter fraction values for the same settings were 11.45% for a mouse-sized phantom and 23.26% for a rat-sized phantom. The peak noise equivalent count rates were also evaluated for those phantoms obtaining 70.8 kcps at 0.66 MBq/cc and 31.5 kcps at 0.11 MBq/cc, respectively. The accuracy of inter-modality alignment is below half the PET resolution, and the image quality of biological specimens agrees with measured performance parameters. The assessment presented in this study shows that the VrPET/CT system is a good performance small-animal imager, while the cost derived from a partial ring detection system is substantially reduced as compared with a full-ring PET tomograph This project was supported by the CENIT Program (Ministerio de Industria), CIBER CB07/09/0031 (Ministerio de Sanidad y Consumo), TEC2007-64731/TCM and TEC2008- 06715-C02-01 (Ministerio de Educaci´on y Ciencia) Publicado

In this work we show that there is a limit for the maximum resolution achievable with a high resolution PET scanner, as well as for the best signal-to-noise ratio, which are ultimately related to the physical effects involved in the emission and detection of the radiation and thus they cannot be overcome with any particular reconstruction method. These effects prevent the spatial high frequency components of the imaged structures to be recorded by the scanner. Therefore, the information encoded in these high frequencies cannot be recovered by any reconstruction technique. Within this framework, we have determined the maximum resolution achievable for a given acquisition as a function of data statistics and scanner parameters, like the size of the crystals or the inter-crystal scatter. In particular, the noise level in the data as a limitation factor to yield high-resolution images in tomographs with small crystal sizes is outlined. These results have implications regarding how to decide the optimal number of voxels of the reconstructed image or how to design better PET scanners Publicado

Fully 3D iterative tomographic image reconstruction is computationally very demanding. Graphics Processing Unit(GPU) has be enproposed formany years as potential accelerators incomplex scientific problems, but it has not be enuseduntil there cent advances in the programmability of GPU sthat the best available reconstruction code shave started tobe implemented toberunon GPUs. This work presents a GPU based fully 3D PET iterative reconstruction sofware. This new code may reconstruct sinogram data from several commercially available PET scanners. The most important and time consuming part soft he code,the forward and backward projection operations, arebasedonan accurate model of the scanner obtained with the MonteCarlo code PeneloPET and they have been massively parallelize don the GPU.For the PET scanners considered, the GPU based code is more than 70 times faster than asimilar code running on a single core of a fast CPU, obtainingin both cases the same images. The code has been designed to be easily adapted to reconstruct sinograms from any other PET scanner, including scanner prototypes. This work has been supported by MEC(FPA200762216), UCM (Grupos UCM,910059), CPAN(Consolider Ingenio 2010) CSPD 200700042,the RECAVARETIC network, ARTEMISS2009/ DPI 1802, European Regional Development,ENTEPRASE grant, PSE 30000020095 and TEC20080675C0201, Ministerio de Ciencia e Innovación, Spanish Government. Publicado

Technical advances towards high resolution PET imaging try to overcome the inherent physical limitations to spatial resolution. Positrons travel in tissue until they annihilate into the two gamma photons detected. This range is the main detector-independent contribution to PET imaging blurring. To a large extent, it can be remedied during image reconstruction if accurate estimates of positron range are available. However, the existing estimates differ, and the comparison with the scarce experimental data available is not conclusive. In this work we present positron annihilation distributions obtained from Monte Carlo simulations with the PeneloPET simulation toolkit, for several common PET isotopes (18F, 11C, 13N, 15O, 68Ga and 82Rb) in different biological media (cortical bone, soft bone, skin, muscle striated, brain, water, adipose tissue and lung). We compare PeneloPET simulations against experimental data and other simulation results available in the literature. To this end the different positron range representations employed in the literature are related to each other by means of a new parameterization for positron range profiles. Our results are generally consistent with experiments and with most simulations previously reported with differences of less than 20% in the mean and maximum range values. From these results, we conclude that better experimental measurements are needed, especially to disentangle the effect of positronium formation in positron range. Finally, with the aid of PeneloPET, we confirm that scaling approaches can be used to obtain universal, material and isotope independent, positron range profiles, which would considerably simplify range correction. We kindly acknowledge support from Comunidad de Madrid (ARTEMIS S2009/DPI-1802), Spanish Ministry of Science and Innovation (grants FPA2010-17142 and ENTEPRASE, PSE-300000-2009-5), by European Regional Funds, by CDTI under the CENIT Programme (AMITProject) and by CPAN, CSPD-2007-00042@Ingenio2010. Part of the calculations of this work were performed in the “Cluster de Cálculo de Alta Capacidad para Técnicas Físicas” funded in part by UCM and in part by UE under FEDER programme. This is a contribution to the Campus of International Excellence of Moncloa Publicado

Pile-up and dead-time are two main causes of nonlinearity in the response of a PET scanner as a function of activity in the field of view (FOV). For a given scanner and acquisition system, pile-up effects depend on the material and size of the object being imaged and on the distribution of activity inside and outside the FOV, because these factors change the singles-to-coincidences ratio (SCR). Thus, it is difficult to devise an accurate correction that would be valid for any acquisition. In this work, we demonstrate a linear relationship between SCR and effective dead-time, which measures the effects of both dead-time (losses) and pile-up (gains and losses). This relationship allows us to propose a simple method to accurately estimate dead-time and pile-up corrections using only two calibration acquisitions with, respectively, a high and low SCR. The method has been tested with simulations and experimental data for two different scanner geometries: a scanner with large area detectors and no pile-up rejection, and a scanner composed of two full rings of smaller detectors. Our results show that the SCR correction method is accurate within 7%, even for high activities in the FOV, and avoids the bias of the standard single-parameter method. © 2013 Institute of Physics and Engineering in Medicine., This work was partially funded by AMIT project (CEN-20101014) from the CDTICENIT program, CIBERsam (CB07/09/0031), projects TEC2010-21619-C04-01 and TEC2011-28972-C02-01 from Spanish Ministerio de Ciencia e Innovación, Spanish Government (ENTEPRASE Grant, PSE-300000-2009-5), PRECISION grant IPT-300000- 2010-3, CPAN (CSD-2007-00042@Ingenio2010), MEC (FPA2010-17142) and ARTEMIS program (S2009/DPI-1802) from Spanish Comunidad de Madrid and EU-ERDF program.

This work presents a graphics processing unit (GPU)- based implementation of a fully 3-D PET iterative reconstruction code, FIRST (Fast Iterative Reconstruction Software for [PET] Tomography), which was developed by our group. We describe the main steps followed to convert the FIRST code (which can run on several CPUs using the message passing interface [MPI] protocol) into a code where the main time-consuming parts of the reconstruction process (forward and backward projection) are massively parallelized on a GPU. Our objective was to obtain significant acceleration of the reconstruction without compromising the image quality or the flexibility of the CPU implementation. Therefore, we implemented a GPU version using an abstraction layer for the GPU, namely, CUDA C. The code reconstructs images from sinogram data, and with the same System Response Matrix obtained from Monte Carlo simulations than the CPU version. The use of memory was optimized to ensure good performance in the GPU. The code was adapted for the VrPET small-animal PET scanner. The CUDA version is more than 70 times faster than the original code running in a single core of a high-end CPU, with no loss of accuracy. This work was supported in part by AMIT Project funded by CDTI (CENIT Programme), UCM (Grupos UCM, 910059), CPAN (Consolider-Ingenio 2010, CSPD-2007-00042), RECAVA- RETIC network, Comunidad de Madrid (ARTEMIS S2009/DPI-1802), Ministerio de Ciencia e Innovación, Spanish Government (ENTEPRASE grant, PSE-300000-2009-5 and TEC2007-64731/TCM), and European Regional funds. Publicado

Proceedings of: 2011 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC). Valencia, Spain, 23-29 October 2011 Monte Carlo simulations are widely used in positron emission tomography (PET) for optimizing detector design, acquisition protocols, as well as for developing and assessing corrections and reconstruction methods. PeneloPET is a Monte Carlo code for PET simulations which considers detector geometry, acquisition electronics and materials, and source definitions. PeneloPET is based on PENELOPE, a Monte Carlo code for the simulation of the transport in matter of electrons, positrons and photons, with energies up to 1 GeV. In this work we use PeneloPET to simulate the Biograph TruePoint (B-TP), Biograph TruePoint with TrueV (B-TPTV) and Biograph mCT PET/CT scanners. These configurations consist of three (B-TP) and four (B-TPTV and mCT) rings of 48 detector blocks. Each block comprises a 13 × 13 matrix of 4 × 4 × 20 mm3 LSO crystals. Simulations were adjusted to reproduce some experimental results from the actual scanners and validated by comparing their predictions to further experimental results. Sensitivity, spatial resolution, noise equivalent count (NEC) rate and scatter fraction (SF) were estimated. The simulations were then employed to estimate the optimum values of system parameters, such as energy and time coincidence windows and to assess the effect of system modifications (such as number of rings) on performance. This work was supported in part by Comunidad de Madrid (ARTEMIS S2009IDPI 1802), Spanish Ministry of Science and Innovation (ENTEPRASE Grant, PSE 300000 2009 5) and PRECISION grant IPT 300000 2010 3 and european regional funds and CPAN, Centro de Fisica de Particulas, Astroparticulas y Nuclear (CSD 2007 00042@Ingenio2010 12). This study has been (partially) funded by CDTI under the CENIT Programme (AMIT Project). Part of the calculations of this work was performed in the "Cluster de Calculo de Alta Capacidad para Tecnicas Fisicas "funded in part by UCM and in part by UE with European regional funds" Publicado

Proceeding of: 2011 Nuclear Science Symposium and Medical Imaging Conference, Valencia, España, 23-29 October, 2011 In this work, we investigate PET imaging with 68Ga and 66Ga after proton irradiation on a natural zinc foil. The nuclides 68Ga and 66Ga are ideally suited for off line PET monitoring of proton radiotherapy due to their beta decay halflives of 67.71(9) minutes and 9.49(3) hours, respectively, and suitable fl end point energy. The purpose of this work is to explore the feasibility of PET monitoring in hadrontherapy treatments, and to study how the amount of activity and the positron range affect the PET image reconstruction. Profiting from the low energy reaction threshold for production via (p,n) reactions, both 68Ga and 66Ga gallium isotopes have been produced by activation on a natural zinc target by a proton pencil beam. In this way, it is possible to create detailed patterns, such as the Derenzo inspired one employed here. The proton beam was produced by the 5 MV tandetron accelerator at CMAM in Madrid. The energy of this beam (up to 10 MeV) is similar to the residual energy of the protons used for therapy at the distal edge of their path. The activated target was imaged in an ARGUS small animal PETtCT scanner and reconstructed with a fully 3D iterative algorithm, with and without positron range corrections. This work was supported in part by Comunidad de Madrid (ARTEMIS S2009/DPI 1802), Spanish Ministry of Science and Innovation (grants FPA2010 17142 and ENTEPRASE, PSE 300000 2009 5), by European Regional Funds, by CDTI under the CENIT Programme (AMIT Project), UCM (grupos UCM, 910059) and by CPAN, CSPD 2007 00042@lngenio2010. Publicado

Proceeding of: 2011 Nuclear Science Symposium and Medical Imaging Conference, Valencia, España, 23-29 October, 2011 Quantification of tracer kinetics is often accomplished from time-activity curves of a region of interest of dynamic PET images. The choice of reconstruction method may affect the timeactivity curves and hence the estimated kinetic parameters. Several studies have shown that statistical-iterative methods, due to non-negativity constrains, may exhibit a quantification bias in low activity regions and thus these methods, in spite of the better image quality they provide, are seldom used to estimate kinetic constants. By means of realistic dynamic simulations, we have investigated the quantitative properties of statistical-iterative (OS EM, both 2D and 3D) and FBP reconstruction methods and the accuracy of the kinetic parameters derived from images reconstructed with each algorithm. We focus on the procedure to fit kinetic constants to data. Our results show that, with appropriate measures to account for quantification bias, iterative reconstructions may be suited to derive kinetic constants from dynamic PET acquisitions. This work was supported from Comunidad de Madrid (ARTEMIS Spanish Ministry of Science and Innovation (ENTEPRASE Grant, PSE-300000-2009-5) and PRECISION grant IPT-300000-2010-3, UCM (Grupos UCM, 910059), and european regional funds and CPAN, Centro de Fisica de Particulas, Astroparticulas y Nuclear (CSD-2007-00042@Ingenio2010-12). This study has been (partially) funded by CDTI under the CENIT Programme (AM IT Project). Part of the calculations of this work was performed in the "Cluster de Calculo de Alta Capacidad para Tecnicas Fisicas" funded in part by UCM and in part by UE with European regional funds" Publicado

Proceedings of: 2011 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC). Valencia, Spain, 23-29 October 2011 he count rate in a PET scanner as a function of the activity in the Field of View (FOV) has a non-linear contribution coming from deadtime, pile-up and random coincidences. These effects must be estimated accurately and corrected in order to perform quantitative PET studies. For a given scanner and acquisition system, the relative importance of deadtime and pile-up effects still depends on the size and materials of the objects being imaged. These facts difficult to devise a universal correction method that yields accurate results for any kind of acquisition. In this work we show that, in a PET scanner, there is a linear relationship between the effective deadtime for coincidences, tau, (which takes into account deadtime and pile-up losses and gains within the energy window) and the Singles to Coincidences Ratio (SCRm) measured by the scanner. This relation has been recovered both in simulations and real data. This allows us to devise a simple method which, requiring only two calibration acquisitions for each energy window, one with high SCRm and one with low SCRm, is able to estimate accurately deadtime and pile up corrections for any other acquisition performed in the same scanner. Simulations show that corrected count rates are accurate within 5%, even when high activities are present in the FOV. This work was supported from Comunidad de Madrid (ARTEMIS S2009/DPI 1802), Spanish Ministry of Science and Innovation (ENTEPRASE Grant, PSE 300000 2009 5) and PRECISION grant IPT 300000 2010 3, UCM (Grupos UCM, 910059), and european regional funds and CPAN, Centro de Fisica de Particulas, Astroparticulas y Nuclear (CSD 2007 00042@lngenio2010 12). This study has been (partially) funded by CDTI under the CENIT Programme (AMIT Project). Part of the calculations of this work was performed in the "Cluster de Calculo de Alta Capacidad para Tecnicas Fisicas" funded in part by UCM and in part by UE with European regional funds". Publicado

Proceeding of: 2010 IEEE Nuclear Science Symposium, Medical Imaging Conference and 17th Room Temperature Semiconductor Detector Workshop (IEEE), Knoxville, Tennessee, USA, October 30 - November 6, 2010 Positron range depends on the materials in which positron propagates and on positron emitter isotope. As positron range limits the spatial resolution of PET images, good quantitative estimates of it should be included in any realistic simulation of PET acquisitions. In this work we compare positron range estimates obtained with PeneloPET to previous available simulations and experimental data. PeneloPET was used to simulate the positron range of 18F, 11C, 13N, 15O, 68Ga and 82Rb in the following tissues: cortical bone, soft bone, skin, muscle, brain, water, adipose tissue and lung. The 3D and 1D annihilation Point Spread Functions (aPSF) were calculated for each isotope-material combination. We have studied with more detail the 3D aPSF (radial distributions) and the cumulative fraction of annihilation events. These aPSF distributions were also studied for non-uniform media. Results obtained were consistent with other results previously reported in the literature as well as with experimentally measured data. This work has been supported by MEC (FPA2007-62216), UCM (Grupos UCM, 910059), CPAN (Consolider- Ingenio 2010) CSPD-2007-00042, European Regional Development and ENTEPRASE grant, PSE-300000-2009-5, and the CENIT program project AMIT, Ministerio de Ciencia e Innovación, Spanish Government. Part of the calculations of this work were performed in the “Clúster de Cálculo de Alta Capacidad para Técnicas Físicas” funded in part by UCM and in part by UE under FEDER programme”. Publicado

The multi pixel photon counter (MPPC) or silicon photomultiplier (SiPM), recently introduced as a solid state photodetector, consists of an array of Geiger mode photodiodes (microcells). It is a promising device for PET due to its potential for high photon detection efficiency (PDE) and its foreseeable immunity to magnetic fields. It is also easy to use with simple read outs, has a high gain and a small size. In this work we evaluate the in field performance of three 1x1mm2 (with 100, 400 and 1600 microcells, respectively) and one 6x6mm2 (arranged as a 2x2 array) Hamamatsu MPPCs for their use in PET imaging. We examine the dependence of the energy resolution and the gain of these devices on the temperature and reverse bias voltage, when coupled to LYSO scintillator crystals under conditions that one would find in a PET system. We find that the 400 and 1600 microcells models and the 2x2 array are suitable for small size crystals, like those employed in high resolution small animal scanners. We have confirmed the good performance of these devices up to magnetic fields of 7 T as well as their suitability for performing PET acquisitions in the presence of fast switching gradients and high duty radiofrequency MRI sequences This work was supported in part by the CDTEAM project of the CENIT program, and by Ministerio de Ciencia e Innovacio´n under projects FPA2007 62216TEC2008 06715 C02 01, UCM (Grupos UCM; 910059), CPAN (Consolider Ingenio 2010) CSPD 2007 00042 projects, and project SENSORCZT S 0505/MAT/000279, Comunidad de Madrid. Part of the computations of this work were done at the ‘‘High Capacity Cluster for Physical Techniques’’ of UCM, funded in part by the UE under the FEDER program and in part by UCM Publicado

Proceeding of: 2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), Orlando, Florida, 25-31 October 2009 A CUDA implementation of the existing software FIRST (Fast Iterative Reconstruction Software for (PET) Tomography) is presented. This implementation uses consumer graphics processing units (GPUs) to accelerate the compute-intensive parts of the reconstruction: forward and backward projection. FIRST was originally developed in FORTRAN, and it has been migrated to C language to be used with NVIDIA C for CUDA, as well as for a straightforward implementation and performance comparison between the C versions of the code running on the CPU and on the GPU. We measured the execution time of the CUDA version compared to the fastest available CPU. The CUDA implementation includes a loop re-ordering and an optimized memory allocation, which improves even more the performance of the reconstruction on the GPUs. This work was supported in part by MEC (FPA2007-62216), CDTEAM (Programa CENIT, Ministerio de Industria), UCM (Grupos UCM, 910059), CPAN (Consolider-Ingenio 2010) CSPD-2007-00042 and the RECAVA-RETIC network

Proceeding of: 2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), Orlando, Florida, 25-31 October 2009 Positron range limits the spatial resolution of PET images. It has a different effect for different isotopes and propagation materials, therefore it is important to consider it during image reconstruction, in order to obtain the best image quality. Positron range distribution was computed using Monte Carlo simulations with PeneloPET. The simulation models positron trajectories and computes the spatial distribution of the annihilation coordinates for the most common isotopes used in PET: 18F, 11C, 13N, 15O, 68Ga and 82Rb. Range profiles are computed for different positron propagation materials, obtaining one kernel profile for each isotope-material combination. These range kernels were introduced in FIRST, a 3D-OSEM image reconstruction software, and employed to blur the object during forward projection. The blurring introduced takes into account the material in which the positron is annihilated, obtained for instance from a CT image. In this way, different positron range corrections for each material in the phantom are considered. We compare resolution and noise properties of the images reconstructed with and without positron range modelling. For this purpose, acquisitions of an Image Quality phantom filled with different isotopes have been simulated for the ARGUS small animal PET scanner. This work has been supported in part by MEC (FPA2007 62216), CDTEAM (Programa CENIT, Ministerio de Industria), UCM (Grupos UCM, 910059), CPAN (Consolider Ingenio 2010) CSPD 2007 00042 and the RECAVA RETIC network. Part of the calculations of this work were performed in the “Clúster de Cálculo de Alta Capacidad para Técnicas Físicas” funded in part by UCM and in part by UE under FEDER programme”.

Monte Carlo simulations play an important role in positron emission tomography (PET) imaging, as an essential tool for the research and development of new scanners and for advanced image reconstruction. PeneloPET, a PET-dedicated Monte Carlo tool, is presented and validated in this work. PeneloPET is based on PENELOPE, a Monte Carlo code for the simulation of the transport in matter of electrons, positrons and photons, with energies from a few hundred eV to 1 GeV. PENELOPE is robust, fast and very accurate, but it may be unfriendly to people not acquainted with the FORTRAN programming language. PeneloPET is an easy-to-use application which allows comprehensive simulations of PET systems within PENELOPE. Complex and realistic simulations can be set by modifying a few simple input text files. Different levels of output data are available for analysis, from sinogram and lines-of-response (LORs) histogramming to fully detailed list mode. These data can be further exploited with the preferred programming language, including ROOT. PeneloPET simulates PET systems based on crystal array blocks coupled to photodetectors and allows the user to define radioactive sources, detectors, shielding and other parts of the scanner. The acquisition chain is simulated in high level detail; for instance, the electronic processing can include pile-up rejection mechanisms and time stamping of events, if desired. This paper describes PeneloPET and shows the results of extensive validations and comparisons of simulations against real measurements from commercial acquisition systems. PeneloPET is being extensively employed to improve the image quality of commercial PET systems and for the development of new ones We acknowledge support from MEC (FPA2007-07393), CDTEAM (CENIT-Ingenio 2010) Ministerio de Industria, Spain, CPAN (Consolider-Ingenio 2010) CSPD-2007-00042 projects and the RECAVA-RETIC network. Part of the computations of this work were done at the ‘High Capacity Cluster for Physical Techniques’ of the Faculty for Physical Sciences of the UCM, funded in part by the UE under the FEDER program and in part by UCM. J L Herraiz and E Vicente acknowledge support from UCM-FPU and CSIC-JAE pre-doctoral fellowships Publicado

Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008 The multi-pixel photon counter (MPPC) or silicon photo-multiplier (SiPM), recently introduced as a solid-state photodetector, consists of an array of Geiger-mode photodiodes(microcells). is a promising device for PET thanks to its potential for high photon detection efficiency (PDE) and immunity to high magnetic fields. is also very easy to use, with simple electronic read-out, high gain and small size. In this work we evaluate the performance of three 1 x 1 mm2 and one 6 x 6 mm2 (2 x 2 array) SiPMs offered by Hamamatsu for their use in PET. We examine the dependence of the energy resolution and the gain of these devices on the thermal and reverse bias when coupled to LYSO scintillator crystals. We find that the 400 and 1600 microcells models and the 2 x 2 array are suitable for small size crystals, like those employed in high resolution small animal scanners. The good performance of these devices up to 7 Tesla has also been confirmed. This work was supported in part by the MEC (FPA2007-07393), CDTEAM (CENIT-Ingenio 2010) Ministerio de Industria, Spain, UCM (Grupos UCM: 910059), CPAN (ConsoliderIngenio 2010) CSPD-2007-00042 projects, and the RECAVA-RETIC network.

Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008 This work reports on the development and evaluation of the PET component of a PETtCT system for small-animal in-vivo imaging. The PET and CT subsystems are assembled in a rotary gantry in such a way that the center of rotation for both imaging modalities is mechanically aligned. The PET scanner configuration is based on 2 detector modules, each of which consist of 2 flat-panel type PS-PMTs (Hamamatsu, H8500) and 2 (30 x 30 elements) LYSO arrays. The dimension of the crystal matrix elements are 1.4 x 1.4 mm2 in cross section and 12 mm in depth. The VrPET detector modules are positioned in opposite sides of a 140 mm diameter ring, providing a transaxial field of view of 86.6 mm diameter and an axial field of view of 45.6 mm. The experimental results obtained in the performance tests are transaxial resolution of 1.5 mm FWHM in the CFOV, and the axial resolution of 2.3 mm FWHM. The absolute coincidence sensitivity is 2.22 % for a coincidence window of 6 ns (100-700 keY). The imaging capability of the PET unit is demonstrated on phantom and animal studies. This work is partially funded by the CD-TEAM project, CENIT Program, Spanish Ministerio de Industria and with grants from the Ministerio de Educaci6n y Ciencia, Projects TEC2007-6473I and TEC2008-06715-C02-01 Publicado

Proceeding of: IEEE Nuclear Science Symposium Conference. NNS 2008, Dresden, Germany, Oct. 19-25, 2008 Super Bialkali (SBA) and Ultra Bialkali (UBA) photocathodes are new technologies that improve the spectral response characteristics of position sensitive PMTs, boosting their quantum efficiency up to 35% and 43% (typ.) respectively [ref Hamamatsu]. Two SBA tubes were introduced into a production line of PET detectors mixed with the regular ones. The detectors were assembled on the same day and by the same operator using the standard factory protocols for detector mounting, calibration and testing. In this work we are reporting our evaluation of the achieved improvement by comparing the spatial and energy resolutions and the depth of-interaction performance of a PET detector modules with DOI capabilities assembled using the regular and the SBA versions of the same PS-PMT. We conclude that the superior performance of the SBA tube may enable the use of arrays with a larger number of crystals of smaller footprint, thus improving the detector intrinsic spatial resolution without degrading the energy resolution or the phoswich discrimination capability This work was supported in part by the CENIT Programme (Ministerio de Industria), CIBER CB07/09/0031, RETIC-RECAVA (Ministerio de Sanidad y Consumo), and TEC2007-64731/TCM (Ministerio de Educación y Ciencia)

Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008 Accurate and reliable quantitative analysis of PET images are necessary for pre-clinical studies. To derive quantitative information from PET images, determination of the calibration curve, that is, the relationship between the pixel values in the reconstructed image and radiotracer concentrations is required. In a typical PET acquisition, several effects, such as random counts and pile-up contributions make this relationship nonlinear. We find that for PET detectors based upon relatively large PMTs like the Hamamatsu H8500, pile up may become the dominant nonlinear effect, with random contributions playing a minor role. We confirm this by means of detailed simulations of small and large cylinders in the rPET small animal scanner as well as with real acquisitions. The simulations allow us to study the impact of pile-up as a source of nonlinearity in the calibration curve of this commercially available small animal PET scanner. We compare the results obtained from images for both real and simulated data. The results show that for the activities considered in this study the quantitative results can be affected by pile-up by more than 20%. We find that pile-up, which shifts counts to the center of the FOV and attenuation, which removes activity from the center of the FOV, may cancel each other for moderate activity values. This would cause quantification errors if attenuation corrections were attempted for acquisitions without pile-up corrections. The pile-up correction software improves the linearity of the calibration curve, extending the range of activity values for which a linear calibration curve can be reliably applied. This work was supported in part by the MEC (FPA2007-07393), CDTEAM (CENIT-Ingenio 2010) Ministerio de Industria, Spain, CPAN (Consolider-Ingenio 2010) CSPD-2007-00042 projects, and the RECAVA-RETIC network. E. Vicente acknowledges support from a CSIC-JAE predoctoral fellowship.

Proceeding of: 2008 IEEE Nuclear Science Symposium Conference Record (NSS '08), Dresden, Germany, 19-25 Oct. 2008 We present a method to compensate for missing projection data in positron emission tomography (PET), which may result from gaps between adjacent detectors or from malfunctioning detectors. To prevent artifacts in the reconstruction when using Fourier rebinning (FORE) or analytical reconstruction algorithms, filling the data gaps is required. This new approach for sinogram data interpolation prior to reconstruction is based on a simple iterative freq uency selective signal extrapolation method initially proposed and successfully applied for error concealment in image and video communication. In this work the method has been improved taking advantage of well known restrictions in the allowed region of frequencies of the sinograms. We compare quantitatively the results of this technique with previously proposed gap filling procedures in both the sinograms and in their reconstructed images. The proposed technique outperforms those methods without requiring too much computational time. This work was supported in part by the MEC (FPA2007-07393), CDTEAM (CENIT-Ingenio 2010) Ministerio de Industria, Spain, CPAN (Consolider-Ingenio 2010) CSPD-2007-00042 projects, and the RECAVA-RETIC network. Vicente acknowledges support from a CSIC-JAE predoctoral fellowship.

Proceeding of: 2006 IEEE Nuclear Science Symposium Conference Record, San Diego, CA, Oct. 29 - Nov. 1, 2006 In this communication we show the dependence of the normalization of 3D small animal PET scanners on the calibrating source geometry and non-uniformity of the activity distribution inside it. The potential impact on the reconstructed images of deviations from the ideal activity is discussed. Results from simulated and real data for a pair of block detectors suggest that planar or annular sources can be equally suitable for a normalization procedure if their geometry and their intersections with the system lines-of-response (LORs) are properly taken into account. When comparing an annulus with a centered planar source, both filled with the same total activity, the planar source produces twice the count rate with a 92% of trues versus 66% of trues for the annulus. Scatter and randoms are lower by factors of 3 and 4 respectively in the planar case. We conclude that, although planar and annular sources can be suitable for a normalization procedure if their geometries are properly taken into account, planar source normalization results are better than those of an annulus, particularly if the normalization does not consider the effect of scatter correction, more significant for the annulus source than for the planar one This work has been partially funded by projects PI052583 and PI052583 (Ministerio de Sanidad), BFM2003-04147-C02-01 (Ministerio de Edcucación y Ciencia) and CD-TEAM (Ministerio de Industria, CENIT program), Spain

Proceeding of: 2007 IEEE Nuclear Science Symposium Conference Record (NSS'07), Honolulu, Hawaii, USA, Oct. 27 - Nov. 3, 2007 Tomographic Data Consistency Conditions (TDCC) are frequently employed to improve the quality of PET data. However, most of these consistency conditions were derived from X-ray computerized tomography (CT) and their validity for other imaging modalities has not been well established. For instance, it is well known from (X-ray) CT data that the sum of the projection data from one view of the parallel-beam projections is a constant independent of the view-angle. This consistency condition is based on well-known mathematical properties of the Radon transform and yields good results when employed in noise removal or sinogram restoration. But this consistency condition assumes that emission and detection of radiation occur within a thin (ideally with zero width) line-ofresponse (LOR), with a flat probability distribution of the detection (in PET) or absorption (X-ray) along such LOR. This assumption, being valid for CT, is not realistic for PET acquisitions. Thus, TDCC for PET should be revised in order to check their validity with more realistic detection models. In this work we review the main differences between PET and CT data and study whether these consistency conditions should be modified in order to take into account the dependence of the probabilities on the distance to the center of the line-of-response. Results from simulations are also presented to illustrate the importance of these effects. They indicate that some consistency conditions can be violated at the 10% level. This work has been partially funded by UCM grant. Part of the computations of this work were done at the 'High capacity cluster for physical techniques' of the Faculty for Physical Sciences of the UCM, funded in part by the UE under the FEDER program and in part by the UCM. This work has been partially funded by CD-TEAM, program CENIT, Ministerio de Industria, Spain.

Proceeding of: 2006 IEEE Nuclear Science Symposium Conference Record, San Diego, CA, Oct. 29 - Nov. 1, 2006 We present an optimal and robust technique for the restoration of positron emission tomography (PET) data. It is based on an iterative deconvolution of Fourier Rebinned (FORE) sinograms employing the EM-ML algorithm regularized with MAP. The deconvolution kernel is related to the System Response Matrix (SRM) and the axial point spread function(PSF) caused by FORE. This method is able to deblur the acquired data whitout the introduction of additional noise and enhancing the quality (resolution, contrast) of the images reconstructed using FBP. Support from UCM grant. J.M. Udias acknowledges support from MEC (FPA2006- 07393) and CENIT (Ministerio de Industria, Spain). Part of the computations of this work were done at the “High capacity cluster for physical techniques” of the Faculty for Physical Sciences of UCM, funded in part by the UE under the FEDER program and in part by the UCM.

Small animal PET scanners require high spatial resolution and good sensitivity. To reconstruct high-resolution images in 3D-PET, iterative methods, such as OSEM, are superior to analytical reconstruction algorithms, although their high computational cost is still a serious drawback. The higher performance of modern computers could make iterative image reconstruction fast enough to be viable, provided we are able to deal with the large number of probability coefficients for the system response matrix in high-resolution PET scanners, which is a difficult task that prevents the algorithms from reaching peak computing performance. Considering all possible axial and in-plane symmetries, as well as certain quasi-symmetries, we have been able to reduce the memory requirements to store the system response matrix (SRM) well below 1 GB, which allows us to keep the whole response matrix of the system inside RAM of ordinary industry-standard computers, so that the reconstruction algorithm can achieve near peak performance. The elements of the SRM are stored as cubic spline profiles and matched to voxel size during reconstruction. In this way, the advantages of on-the-fly calculation and of fully stored SRM are combined. The on-the-fly part of the calculation (matching the profile functions to voxel size) of the SRM accounts for 10 to 30% of the reconstruction time, depending on the number of voxels chosen. We tested our approach with real data from a commercial small animal PET scanner. The results (image quality and reconstruction time) show that the proposed technique is a feasible solution., 19 pages, 10 figures

Proceeding of: 2005 IEEE Nuclear Science Symposium Conference Record, Puerto Rico, 23-29 Oct. 2005 Small animal positron emission tomography (PET) scanners are being increasingly used as a basic measurement tool in modern biomedical research. The new designs and technologies of these scanners and the modern reconstruction methods have allowed to reach high spatial resolution and sensitivity. Despite their successes, some important issues remain to be addressed in high resolution PET imaging. First, iterative reconstruction methods like maximum likelihood-expectation maximization (MLEM) are known to recover resolution, but also to create noisy images and edge artifacts if some kind of regularization is not imposed. Second, the limit of resolution achievable by iterative methods on high resolution scanners is not quantitatively understood. Third, the use of regularization methods like Sieves or maximum a posteriori (MAP) requires them determination of the optimal values of several adjustable parameter that may be object-dependent. In this work we review these problems in high resolution PET and establish that the origin of them is more related with the physical effects involved in the emission and detection of the radiation during the acquisition than with the kind of iterative reconstruction method chosen. These physical effects (positron range, non-collinearity, scatter inside the object and inside the detector materials) cause that the tube of response (TOR) that connects the voxels with a line of response (LOR) is rather thick. This implies that the higher frequencies of the patient organ structures are not recorded by the scanner and therefore cannot be recovered during the reconstruction. As iterations grow, MLEM algorithms try to recover higher frequencies in the image. Once that a certain critic frequency is reached, this only maximizes high frequency noise. Using frequency response analyses techniques, we determine the maximum achievable resolution, before edge artifacts spoil the quality of the image, for a particular scanner as a function of the thickness of the TOR, and independently of the reconstruction method employed. With the same techniques, we can deduce well defined stopping criteria for reconstructions methods. Also, criteria for the highest number of subsets which should be used and how the design of the scanners can be optimized when statistical reconstruction methods are employed, is established. This work was supported in part by the UCM, Fundación para la investigación biomédica del Hospital Gregorio Marañon”, support from MEC project (BFM2003 04147 C02 01).

Proceeding of: 2006 IEEE Nuclear Science Symposium Conference Record, San Diego, California, Oct. 29 – Nov. 4, 2006 PENELOPE is a Monte Carlo code that simulates the transport in matter of electrons, positrons and photons with energies from a few hundred of eV to 1 GeV. It is robust, fast and very accurate, but it may be unfriendly for people not acquainted with the FORTRAN programming language. We have developed a tookit (‘PeneloPET’) to prepare simulations of PET and SPECT within PENELOPE. Sophisticated simulations can be setup by modifying just a few simple input files. The output data can be generated at different levels of detail and can be analyzed afterwards with the preferred programming language or tools. In this work, we present the features of PeneloPET as well as validations against other dedicated PET simulation programs and two real scanners. Support from MEC (FPA2006-07393) and CDTEAM (CENIT-Ingenio 2010) Ministerio de Industria, Spain, projects. The numerical computations of this work were done at the “High capacity cluster for physical techniques'' of the Faculty for Physical Sciences of the UCM, funded in part by the UE under the FEDER programme and in part by the UCM. Parts of this work were performed under the HGGMUCM contract "Modelización de escáneres PET de alta resolución y reconstrucción estadística de imágenes PET en animales pequeños".

Proceeding of: Nuclear Science Symposium Conference Record, 2004 IEEE, Rome ,16-22 Oct. 2004 Small animal PET scanners require high spatial resolution (< 1 mm) and good sensitivity. To obtain high resolution images, iterative reconstruction methods, like OSEM, applied to image reconstruction in three-dimensional (3D) positron emission tomography (PET), have superior performance over analytical reconstruction algorithms like FBP. However, the high computational cost of iterative methods remains a serious drawback to their development and clinical routine use. The increase in performance of current computers should make iterative image reconstruction fast enough to attain clinical viability. However, dealing with the large number of probability coefficients for the response of the system in high-resolution PET scanners becomes a difficult task that prevents the algorithms from reaching peak performance. Taking into account all possible axial, in-plane and other symmetries, we have reduced the storage needs what allows us to store the whole response of the system in dynamic memory of ordinary industry standard computers, so that the reconstruction algorithm can achieve near peak performance.

Proceeding of: 2005 IEEE Nuclear Science Symposium Conference Record, Puerto Rico, 23-29 Oct. 2005 A new reconstruction strategy is proposed for 3D images acquired with rotating PET scanners. This method is based on the decomposition of the System Response Matrix (SRM) into a projection and a rotation components that are inverted independently. We present the rationale of the new reconstruction procedure as well as some examples to compare with analytical 3D reconstruction (3DRP) and 3D statistical reconstruction (3D-OSEM) Part of this work is founded by the IM3 network (G03/185 Ministerio de Sanidad), with grants from the Ministerio de Educación y Ciencia, project TEC2004 07052 C02 01, and Ministerio de Industria, Turismo y Comercio project FIT 330101 2004 3