In this work, we present the performance characteristics of the MAMMOCARE PET prototype based on an adaptation of the NU 42008 NEMA standard. MAMMOCARE is a project under the European Commission's 7th Framework programme to develop a breast biopsy system guided by a dedicated breast PET (dbPET) images. The PET system is formed by two rings with twelve detector modules each. The transaxial FOV is 170mm and the axial FOV is 94 mm. The system can separate the detectors up to 60mm in transaxial plane to allow the biopsy needle entrance.The acquisitions are reconstructed using the LMOS algorithm with tubeofresponse (TOR) backprojector, 1 iteration and 16 subsets. The voxel and pixel sizes are (1 x 1 x 1) mm(3) and (1.6 x 1.6) mm(2) respectively. The radial resolution measured is 1.62mm in the center of the FOV and 3.45mm at 50mm off the center in the radial direction using the closed configuration. In the open configuration the resolution reaches 1.85mm and 3.65mm at center and at 50mm offcenter. The adapted recovery coefficients (ARC) are measured for six hot rods inside a cylindrical phantom with a warm background. The ratio between hot and background regions is 10. The ARC values for the closed configuration are 0.32, 0.77 and 0.96 for the inserts with a diameter of 4.5 mm, 8.3mm and 25 mm, respectively. These values decrease to 0.16, 0.52 and 0.77 for the open configuration. The sensitivity measured using an energy window of 250 keV-750 keV is 3.6% and 2.5% for the closed and open configurations respectively. The NEC peak is 141 kcps@ 68MBq and 147 kcps@ 78MBq for closed and open configurations. The performance characteristics measured with the open ring configuration decreases with respect the closed configuration, however the values remain comparable to other dbPETs.

In this work we present the MAMMOCARE prototype, a biopsy guided system based on PET. The system is composed by an examination table where the patient is situated in prone position, a PET detector and a biopsy device. The PET detector is composed by two rings. These rings can be separated mechanically in order to allow the needle insertion. The first acquisition is performed with the closed ring configuration in order to obtain a high quality image to locate the lesion. Then, the software calculates the optimum path for the biopsy and moves the biopsy and PET systems to the desired position. At this point, two compression pallets are used to hold the breast. Then, the PET system opens and the biopsy procedure starts. The images are obtained at several steps to ensure the correct location of the needle during the procedure. The performance of the system is evaluated measuring the spatial resolution and sensitivity according the NEMA standard The uniformity of the reconstructed images is also estimated. The radial resolution is 1.62mm in the center of the FOV and 3.45mm at 50mm off the center in the radial direction using the closed configuration. In the open configuration the resolution reaches 1.85mm at center and 3.65mm at 50mm. The sensitivity using an energy window of 250keV-750keV is 3.6% for the closed configuration and 2.5% for the open configuration. The uniformity measured in the center of the FOV is 14% and 18% for the closed and open configurations respectively.

The radiation sensitive part of the wide-field, small gamma camera Sentinella (TM) 108 consists of a monolithic, 102 x 102 mm(2) 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 +/-0.2E(p) around the photopeak position of Tc-99m, E-p = 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%.

In this paper we describe the overall design of a PET-MR system for head imaging within the framework of the DREAM Project as well as the first detector module tests. The PET system design consists of 4 rings of 16 detector modules each and it is expected to be integrated in a head dedicated radio frequency coil of an MR scanner.The PET modules are based on monolithic LYSO crystals coupled by means of optical devices to an array of 256 Silicon Photomultipliers. These types of crystals allow to preserve the scintillation light distribution and, thus, to recover the exact photon impact position with the proper characterization of such a distribution. Every module contains 4 Application Specific Integrated Circuits (ASICs) which return detailed information of several light statistical momenta. The preliminary tests carried out on this design and controlled by means of ASICs have shown promising results towards the suitability of hybrid PET-MR systems. (C) 2012 Elsevier B.V. All rights reserved.

In this work we present a method to efficiently collect scintillation light when using monolithic scintillator crystals. The acceptance angle of the scintillation light has been reduced by means of optical devices reducing the border effect which typically affects continuous crystals. We have applied this procedure on gamma detectors for PET systems using both position sensitive PMTs and arrays of SiPMs. In the case of using SiPMs, this approach also helps to reduce the photosensor active areaWe evaluated the method using PMTs with a variety of different crystals with thicknesses ranging from 10 to 24 mm. We found that our design allows the use of crystal blocks with a thickness of up to 18 mm without degrading the spatial resolution caused by edge effects and without a significant detriment to the energy resolution. These results were compared with simulated data The first results of monolithic LYSO crystals coupled to an array of 256 SiPMs by means of individual optical light guides are also presented. (C) 2013 Elsevier B.V. All rights reserved

We suggest a symmetric-polar pixellation scheme which makes possible a reduction of the computational cost for expectation maximization (EM) iterative algorithms. The proposed symmetric-polar pixellation allows us to deal with 3D images as a whole problem without dividing the 3D problem into 2D slices approach. Performance evaluation of each approach in terms of stability and image quality is presented. Exhaustive comparisons between all approaches were conducted in a 2D based image reconstruction model. From these 2D approaches, that showing the best performances were finally implemented and evaluated in a 3D based image reconstruction model. Comparison to 3D images reconstructed with FBP is also presented. Although the algorithm is presented in the context of computed tomography (CT) image reconstruction, it can be applied to any other tomographic technique as well, due to the fact that the only requirement is a scanning geometry involving measurements of an object under different projection angles. Real data have been acquired with a small animal (CT) scanner to verify the proposed mathematical description of the CT system.

PURPOSE The authors have developed a trimodal PET∕SPECT∕CT scanner for small animal imaging. The gamma ray subsystems are based on monolithic crystals coupled to multianode photomultiplier tubes (MA-PMTs), while computed tomography (CT) comprises a commercially available microfocus x-ray tube and a CsI scintillator 2D pixelated flat panel x-ray detector. In this study the authors will report on the design and performance evaluation of the multimodal system. METHODS X-ray transmission measurements are performed based on cone-beam geometry. Individual projections were acquired by rotating the x-ray tube and the 2D flat panel detector, thus making possible a transaxial field of view (FOV) of roughly 80 mm in diameter and an axial FOV of 65 mm for the CT system. The single photon emission computed tomography (SPECT) component has a dual head detector geometry mounted on a rotating gantry. The distance between the SPECT module detectors can be varied in order to optimize specific user requirements, including variable FOV. The positron emission tomography (PET) system is made up of eight compact modules forming an octagon with an axial FOV of 40 mm and a transaxial FOV of 80 mm in diameter. The main CT image quality parameters (spatial resolution and uniformity) have been determined. In the case of the SPECT, the tomographic spatial resolution and system sensitivity have been evaluated with a (99m)Tc solution using single-pinhole and multi-pinhole collimators. PET and SPECT images were reconstructed using three-dimensional (3D) maximum likelihood and ordered subset expectation maximization (MLEM and OSEM) algorithms developed by the authors, whereas the CT images were obtained using a 3D based FBP algorithm. RESULTS CT spatial resolution was 85 μm while a uniformity of 2.7% was obtained for a water filled phantom at 45 kV. The SPECT spatial resolution was better than 0.8 mm measured with a Derenzo-like phantom for a FOV of 20 mm using a 1-mm pinhole aperture collimator. The full width at half-maximum PET radial spatial resolution at the center of the field of view was 1.55 mm. The SPECT system sensitivity for a FOV of 20 mm and 15% energy window was 700 cps∕MBq (7.8 × 10(-2)%) using a multi-pinhole equipped with five apertures 1 mm in diameter, whereas the PET absolute sensitivity was 2% for a 350-650 keV energy window and a 5 ns timing window. Several animal images are also presented. CONCLUSIONS The new small animal PET∕SPECT∕CT proposed here exhibits high performance, producing high-quality images suitable for studies with small animals. Monolithic design for PET and SPECT scintillator crystals reduces cost and complexity without significant performance degradation.

In this work we present an innovative algorithm for the reconstruction of PET images based on the List-Mode (LM) technique which improves their spatial resolution compared to results obtained with current MLEM algorithms. This study appears as a part of a large project with the aim of improving diagnosis in early Alzheimer disease stages by means of a newly developed hybrid PET-MR insert. At the present, Alzheimer is the most relevant neurodegenerative disease and the best way to apply an effective treatment is its early diagnosis. The PET device will consist of several monolithic LYSO crystals coupled to SiPM detectors. Monolithic crystals can reduce scanner costs with the advantage to enable implementation of very small virtual pixels in their geometry. This is especially useful for LM reconstruction algorithms, since they do not need a pre-calculated system matrix. We have developed an LM algorithm which has been initially tested with a large aperture (186mm) breast PET system. Such an algorithm instead of using the common lines of response, incorporates a novel calculation of tubes of response. The new approach improves the volumetric spatial resolution about a factor 2 at the border of the field of view when compared with traditionally used MLEM algorithm. Moreover, it has also shown to decrease the image noise, thus increasing the image quality.

The splitting of the field of view (FOV) in polar voxels is proposed in this work in order to obtain an efficient description of a cone-beam computed tomography (CT) scanner. The proposed symmetric-polar pixelation makes it possible to deal with the 3D iterative reconstruction considering a number of projections and voxel sizes typical in CT preclinical imaging.The performance comparison, between the filtered backprojection (FBP) and 3D maximum likelihood expectation maximization (MLEM) reconstruction algorithm for CT, is presented. It is feasible to achieve the hardware spatial resolution limit with the considered pixelation. The image quality achieved with MLEM and FBP have been analyzed. The results obtained with both algorithms in clinical images have been compared too. Although the polar-symmetric pixelation is presented in the context of CT imaging, it can be applied to any other tomographic technique as long as the scan comprises the measurement of an object under several projection angles.

PURPOSE:The authors have developed a small animal Positron emission tomography (PET) scanner based on monolithic LYSO crystals coupled to multi-anode photomultiplier tubes (MA-PMTs). In this study, the authors report on the design, calibration procedure, and performance evaluation of a PET system that the authors have developed using this innovative nonpixelated detector design.METHODS:The scanner is made up of eight compact modules forming an octagon with an axial field of view (FOV) of 40 mm and a transaxial FOV of 80 mm diameter. In order to fully determine its performance, a recently issued National Electrical Manufacturers Association (NEMA) NU-4 protocol, specifically developed for small animal PET scanners, has been followed. By measuring the width of light distribution collected in the MA-PMT the authors are able to determine depth of interaction (DOI), thus making the proper identification of lines of response (LORs) with large incidence angles possible. PET performances are compared with those obtained with currently commercially available small animal PET scanners.RESULTS:At axial center when the point-like source is located at 5 mm from the radial center, the spatial resolution measured was 1.65, 1.80, and 1.86 mm full width at half maximum (FWHM) for radial, tangential, and axial image profiles, respectively. A system scatter fraction of 7.5% (mouse-like phantom) and 13% (rat-like phantom) was obtained, while the maximum noise equivalent count rate (NECR) was 16.9 kcps at 12.7 MBq (0.37 MBq/ml) for mouse-like phantom and 12.8 kcps at 12.4 MBq (0.042 MBq/ml) for rat-like phantom The peak absolute sensitivity in the center of the FOV is 2% for a 30% peak energy window. Several animal images are also presented.CONCLUSIONS:The overall performance of our small animal PET is comparable to that obtained with much more complex crystal pixelated PET systems. Moreover, the new proposed PET produces high-quality images suitable for studies with small animals.

PET systems designed for specific applications require high resolution and sensitivity instrumentation. In dedicated system design smaller ring diameters and deeper crystals are widely used in order to increase the system sensitivity. However, this design increases the parallax error, which degrades the spatial image resolution gradually from the center to the edge of the field-of-view (FOV). Our group has designed a depth of interaction(DOI)-encoding small animal PET system based on monolithic crystals. In this work we investigate the restoration of radial resolution for transaxially off-center sources using the DOI information provided by our system. For this purpose we have designed a support for point like sources adapted to our system geometry that allows a spatial compression and resolution response study. For different point source radial positions along vertical and horizontal axes of a FOV transaxial plane we compare the results obtained by three methods: without DOI information, with the DOI provided by our system and with the assumption that all the γ-rays interact at half depth of the crystal thickness. Results show an improvement of the mean resolution of 10% with the half thickness assumption and a 16% achieved using the DOI provided by the system. Furthermore, a 10% restoration of the resolution uniformity is obtained using the half depth assumption and an 18% restoration using measured DOI.

We present the first results of an MR compatible PET system based on continuous crystals and SiPM detectors, along with a dedicated application-specific integrated circuit (ASIC) for the readout stage. The use of continuous scintillation crystals preserves the spatial distribution of scintillation light for each γ--ray impact, which can be reconstructed with a small number of statistical moments Therefore, it is possible to reduce the number of analogue-to-digital conversion channels. The current ASIC can measure up to 8 moments of the light distribution, providing information about the X and Y photon impact coordinates, photon energy, depth of interaction and other higher moments that can improve the position accuracy of the photon impact. SiPMs exhibit their best performance for reduce active areas, where the dark counts are minimized. An innovate coupling design between crystal and SiPM-array has been made in order to keep the compromise between reduced active areas and efficient collection of the scintillation light. SiPM detectors are also suitable to work in the presence of magnetic fields. Moreover, they could be integrated in a RF coil of the MRI system due to their reduced dimensions [1]. Experimental results show very promising possibilities for the system.

PURPOSE:A breast dedicated positron emission tomography (PET) scanner has been developed based on monolithic LYSO crystals coupled to position sensitive photomultiplier tubes (PSPMTs). In this study, we describe the design of the PET system and report on its performance evaluation.METHODS:MAMMI is a breast PET scanner based on monolithic LYSO crystals. It consists of 12 compact modules with a transaxial field of view (FOV) of 170 mm in diameter and 40 mm axial FOV that translates to cover up to 170 mm. The patient lies down in a prone position that facilitates maximum breast elongation. Quantitative performance analysis of the calculated method for the attenuation correction specifically developed for MAMMI, and based on PET image segmentation, has also been conducted in this evaluation. In order to fully determine the MAMMI prototype's performance, we have adapted the measurements suggested for National Electrical Manufacturers Association (NEMA) NU 2-2007 and NU 4-2008 protocol tests, as they are defined for whole-body and small animal PET scanners, respectively.RESULTS:Spatial resolutions of 1.6, 1.8, and 1.9 mm were measured in the axial, radial, and tangential directions, respectively. A scatter fraction of 20.8% was obtained and the maximum NEC was determined to be 25 kcps at 44 MBq. The average sensitivity of the system was observed to be 1% for an energy window of (250 keV-750 keV) and a maximum absolute sensitivity of 1.8% was measured at the FOV center.CONCLUSIONS:The overall performance of the MAMMI reported on this evaluation quantifies its ability to produce high quality PET images. Spatial resolution values below 3 mm were measured in most of the FOV. Only the radial component of spatial resolution exceeds the 3 mm at radial positions larger than 60 mm. This study emphasizes the need for standardized testing methodologies for dedicated breast PET systems similar to NEMA standards for whole-body and small animal PET scanners.

Whole-body Positron Emission Tomography (PET) scanners are required in order to span large Fields of View (FOV). Therefore, reaching the sensitivity and spatial resolution required for early stage breast tumor detection is not straightforward. MAMMI is a dedicated breast PET scanner with a ring geometry designed to provide PET images with a spatial resolution as high as 1.5 mm, being able to detect small breast tumors ( < 1 cm). The patient lays down in prone position during the scan, thus making possible to image the whole breast, up to regions close to the base of the pectoral without the requirement of breast compression.Attenuation correction (AC) for PET data improves the image quality and the quantitative accuracy of radioactivity distribution determination. In dedicated, high resolution breast cancer scanners, this correction would enhance the proper diagnosis in early disease stages. In whole-body PET scanners, AC is usually taken into account with the use of transmission scans, either by external radioactive rod sources or by Computed Tomography (CT). This considerably increases the radiation dose administered to the patient and time needed for the exploration. In this work we propose a method for breast shape identification by means of PET image segmentation. The breast shape identification will be used for the determination of the AC. For the case of a specific breast PET scanner the procedure we propose should provide AC similar to that obtained by transmission scans as we take advantage of the breast anatomical simplicity. Experimental validation of the proposed approach with a dedicated breast PET prototype is also presented. The main advantage of this method is an important dose reduction since the transmission scan is not required. (c) 2010 Elsevier B.V. All rights reserved.

Objective: To develop a small-animal SPECT system using a low cost commercial portable gamma camera equipped with a pinhole collimator, a continuous scintillation crystal and a position-sensitive photomultiplier tube.Material and methods: The gamma camera was attached to a variable radius system, which enabled us to optimize sensitivity and resolution by adjusting the radius of rotation to the size of the object. To investigate the capability of the SPECT system for small animal imaging, the dependence of resolution and calibration parameters on radius was assessed and acquisitions of small phantoms and mice were carried out.Results: Resolution values, ranging from 1.0 mm for a radius of 21.4 mm and 1.4 mm for a radius of 37.2 mm were obtained, thereby justifying the interest of a variable radius SPECT system.Conclusions: The image quality of phantoms and animals were satisfactory, thus confirming the usefulness of the system for small animal SPECT imaging. (C) 2011 Elsevier Espana, S.L. and SEMNIM. All rights reserved.

In this work we present the effects on the spatial resolution of a small animal Positron Emission Tomography (PET) using continuous crystals when their Point Spread Function (PSF) information is considered in an iterative reconstruction algorithm. The PET system consisted of eight detector modules with slight differences in their detection efficiency and intrinsic resolution. We have experimentally determined the PSF for each detector using a grid of 7×7 uniformly distributed holes. An image reconstruction algorithm including the PSF information has been implemented. The quantitative performance of PSF reconstruction was compared with the one obtained with a standard reconstruction algorithm. This comparative analysis was carried out with a phantom formed by 9×9 22 Na point-like sources. They had 1 mm in diameter and the distance between adjacent centers was 5 mm. We obtained a resolution improvement of up to 13% when the PSF information was considered in the reconstruction process.

The system MAMMI (acronym for MAMmography with Molecular Imaging) is a PET prototype device specifically designed for the detection of breast cancer. It is based on continuous LYSO crystals coupled to Position Sensitive Photomultiplier Tubes (PSPMTs). The scanner consists of twelve compact modules assembled on a ring configuration with an aperture of 186 mm. The scanner transaxial Field of View (FoV) is as large as 170 mm in diameter whereas the axial FoV can cover up to 170 mm recording several frames which are software overlapped. Most of the performance characteristic tests according to the National Electrical Manofacturers Association (NEMA) NU 2-2007 are specially designed to whole body PET scanners and, thus, present a dimensional limitation on a dedicated breast PET. Also, NEMA NU 4-2008 standards cannot be either conducted because are performed for small animal PETs. In this paper, we propose certain changes based on both standards, as are the dimensions of the phantoms and sources. The results showed a spatial resolution at the centre of the transaxial and axial FoVs of 1.90 1.82 and 1.63 mm in the radial, tangential and axial profiles, respectively. The system sensitivity was measured to be, on average and using different line sources and metallic sleeves, 0.77%. When using a 22Na point source, a value of up to 1% was observed. For a specific breast phantom, the scatter fraction was determined to be 6.7% and the peak noise equivalent count rate, 25 kcps at 176 MBq/ml. Note that these measures were carried out wiyh a 50% peak energy window and and a coincidence timing window of 5 ns.

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Summary Background: The treatment of malignant melanoma or sarcomas on a limb using extremity perfusion with tumour necrosis factor (TNF-α) and melphalan can result in a high degree of systemic toxicity if there is any leakage from the isolated blood territory of the limb into the systemic vascular territory. Leakage is currently controlled by using radiotracers and heavy external probes in a procedure that requires continuous manual calculations. The aim of this work was to develop a light, easily transportable system to monitor limb perfusion leakage by controlling systemic blood pool radioactivity with a portable gamma camera adapted for intraoperative use as an external probe, and to initiate its application in the treatment of MM patients. Methods: A special collimator was built for maximal sensitivity. Software for acquisition and data processing in real time was developed. After testing the adequacy of the system, it was used to monitor limb perfusion leakage in 16 patients with malignant melanoma to be treated with perfusion of TNF-α and melphalan. Results: The field of view of the detector system was 13.8 cm, which is appropriate for the monitoring, since the area to be controlled was the precordial zone. The sensitivity of the system was 257 cps/MBq. When the percentage of leakage reaches 10% the associated absolute error is ± 1%. After a mean follow-up period of 12 months, no patients have shown any significant or lasting side-effects. Partial or complete remission of lesions was seen in 9 out of 16 patients (56%) after HILP with TNF-α and melphalan. Conclusion: The detector system together with specially developed software provides a suitable automatic continuous monitoring system of any leakage that may occur during limb perfusion. This technique has been successfully implemented in patients for whom perfusion with TNF-α and melphalan has been indicated.