Kalman Nagy

Performance Evaluation of the Small-Animal nanoScan PET/MRI System

nanoScan is a high-resolution integrated system for consecutive PET and MR imaging of small laboratory animals. We evaluated the performance of the system, using the NEMA NU 4-2008 protocol for the PET component and the NEMA MS 1-2007, MS 2-2008, and MS 3-2007 standards for the MR imaging component. Methods: The imaging system uses magnetically shielded position-sensitive photomultiplier tubes and a compact 1-T permanent-magnet MR imaging platform. Spatial resolution, sensitivity, counting rate capabilities, and image quality parameters were evaluated in accordance with the aforementioned NEMA standards. Further in vivo evaluation experiments complement the physical validation results. Results: The spatial resolution of the PET system enabled the 0.8-mm rods of a Derenzo phantom to be resolved. With point source and 2-dimensional filtered backprojection reconstruction, the resolution varied from 1.50 to 2.01 mm in full width at half maximum in the radial direction and from 1.32 to 1.65 mm in the tangential direction within the radius of 25 mm. Peak absolute sensitivity was 8.41%. Scatter fraction was 17.3% and 34.0%, and maximum noise-equivalent counting rate was 406 and 119 kcps in the mouselike and ratlike phantom, respectively. The image quality test found a nonuniformity of 3.52% and a spillover ratio of 6.2% and 5.8% in water and air, respectively. In testing of the MR imaging component, artifact-free images with high signal-to-noise ratio were recorded. Geometric distortion was below 5%, and image uniformity was at least 94.5% and 96.6% for the 60- and 35-mm radiofrequency coils, respectively. Conclusion: The nanoScan integrated small-animal PET/MR imaging system has excellent spatial resolution and sensitivity. The performance characteristics of the PET and the MR imaging components are not compromised as a result of their integration onto a single platform. Because of its combination of features and performance parameters, the system provides crucial advantages for preclinical imaging studies over existing PET/CT systems, especially in neurologic and oncologic research.

Experimental Evaluation of a SiPM-Based Scintillation Detector for MR-Compatible SPECT Systems

In the present work we briefly describe the architecture of a photo-detection module, designed in the framework of the INSERT (INtegrated SPECT/MRI for Enhanced Stratification in Radio-chemoTherapy) project, supported by the European Community. We focus on two main elements of the module: the SiPM photo-detector unit and the multi-channel ASIC. These two components have been investigated with dedicated and independent setups to assess preliminary performance of INSERT architecture. In details, we designed a 25.30 mm ×25.85 mm tile, comprising 9 pixels, each one with an 8 mm ×8 mm active area. We developed an Anger camera to characterize the tile coupled to a CsI:Tl scintillator (6 mm thick). We measured an average spatial resolution (FWHM) of 2 mm in the central region of the Field of View and a 15.3% energy resolution using a 57Co source (122 keV), when the tile is cooled down to 0 ° C to reduce the impact of the dark count rate. Furthermore, we developed ANGUS, a 36-channels 0.35 μm CMOS technology ASIC designed to cope with input capacitance up to 5 nF, typical of large area SiPM pixels. The spectroscopic capability of single readout channels were evaluated by coupling an 8 mm ×8 mm pixel with a cylindrical CsI:Tl scintillator (8 mm diameter, 10 mm thickness). Energy resolution at room temperature provided values between 13% and 13.5% (FWHM) at the 122 keV line for the nine pixels.

Development of clinical simultaneous SPECT/MRI

There is increasing clinical use of combined positron emission tomography and MRI, but to date there has been no clinical system developed capable of simultaneous single-photon emission computed tomography (SPECT) and MRI. There has been development of preclinical systems, but there are several challenges faced by researchers who are developing a clinical prototype including the need for the system to be compact and stationary with MRI-compatible components. The limited work in this area is described with specific reference to the Integrated SPECT/MRI for Enhanced stratification in Radio-chemo Therapy (INSERT) project, which is at an advanced stage of developing a clinical prototype. Issues of SPECT/MRI compatibility are outlined and the clinical appeal of such a system is discussed, especially in the management of brain tumour treatment.

Development of a high-resolution detection module for the INSERT SPECT/MRI system

A new multi-modality imaging tool is under development in the framework of the INSERT (Integrated SPECT/MRI for Enhanced Stratification in Radio-chemo Therapy) project, supported by the European Community. The final goal is to develop a custom SPECT apparatus that can be used as an insert for commercially available MRI systems. INSERT is expected to offer more effective and earlier diagnosis with potentially better outcome in survival for the treatment of brain tumors, primarily glioma. Two SPECT prototypes are being developed, one dedicated to preclinical imaging (7 and 9.4 T), the second one dedicated to clinical imaging (3 T). n nThe fundamental unit is a 5 cm x 5 cm gamma camera, based on the well-established Anger architecture with a continuous CsI:Tl scintillator readout by an array of silicon photomultipliers (SiPMs). The photodetector matrix will be composed by 12x12 SiPMs (FBK), each one with an active area of 4x4 mm2, for an overall field of view of 50.40x51.70 mm2, considering also insensitive areas between different detectors. In order to reduce complexity and costs the 144 channels are shortcut in group of 4 and readout by a custom-designed 36 channels ASIC. Each electronic channel features a fast current conveyor stage, followed by an RC filter with selectable peaking times and the electronics necessary to provide an appropriate output for the data acquisition system. n nPreliminary Monte Carlo simulations suggest a spatial resolution between 0.8 and 1 mm and an energy resolution between 11% and 15% (140 keV), depending on the dark count rate of the SiPM technology (100-500 kHz/mm2). Experimental measurements are under development to confirm these results. For example, a single 4x4 SiPM (FBK, RGB-HD), coupled to a CsI:Tl scintillator, has been readout by a single channel version of the ASIC, providing an energy resolution close to 12% at 122 keV at room temperature.

A SiPM-based detection module for SPECT/MRI systems

In the present work we present the development of a Silicon PhotoMultiplier (SiPM)-based detection module for hybrid SPECT/MRI instruments. The module is designed for preclinical SPECT systems for mouse and rat brain imaging, but can also be exploited for clinical SPECT brain scanners. The gamma-ray detection module is designed on the well-established Anger architecture, with a continuous 5 cm χ 5 cm CsI:Tl scintillator read by an array of SiPMs (RGB-HD with 25 μm SPAD cells) from Fondazione Bruno Kessler. The current signals are conveyed to a 36-channel ASIC realized in 0.35 μm CMOS technology and digitized by an external data acquisition system. An operative temperature of 0° C is mandatory to reduce the dark count rate of the SiPM array and to enhance the final performance of the detector in terms of energy and spatial resolution. For this purpose, an MRI-compatible heat sink is realized with a plastic material (Coolpolymer D5506) with a glycol-water mixture as cooling fluid. Gamma-ray measurements with Co-57 (122 keV) have provided an energy resolution better than 14% and an average intrinsic spatial resolution below 1.0 mm.

Development of an MRI-compatible cooling unit for SPECT/MRI detection modules

A new multi-modality imaging tool is under development in the framework of the INSERT (Integrated SPECT/MRI for Enhanced Stratification in Radio-chemo Therapy) project, supported by the European Community. The final goal is to develop a custom SPECT apparatus that can be used as an insert for commercially available MRI systems. INSERT is expected to offer more effective and earlier diagnosis with potentially better outcome in survival for the treatment of brain tumors, primarily glioma. Two SPECT prototypes are being developed, one dedicated to preclinical imaging (7 and 9.4 T), the second one dedicated to clinical imaging (3 T). The fundamental unit is a 5 cm × 5 cm gamma camera, based on the well-established Anger architecture with a continuous CsI:Tl scintillator readout by an array of silicon photomultipliers (SiPMs). In order to improve the noise performance of the overall SPECT system, the SiPM arrays need to operate at temperatures around 0 °C to reduce their Dark Count Rate (DCR). Thereby, the detection modules require mild to moderate cooling essential to achieve the final goals of energy and spatial resolutions. However, the presence of static magnetic field B0, rapidly switching gradients and radio-frequency pulses in the bore of the MRI machine, make use of traditional cooling blocks with bulky metallic components unsuitable. In this work we describe the study of a MRI-compatible cooling unit, thermal simulations of their expected performance and the results of a test prototype.

Automated body-lung-air material map segmentation from pre-clinical MRI images for PET attenuation correction in Tera-Tomo 3D PET reconstruction engine of nanoScan PET/MRI system

The aim of current work was to automatically derive a body-lung-air three-valued material map from MRI images for pre-clinical PET attenuation correction. Our goal was also to measure the effect of attenuation on corresponding pre-clinical PET images. n n12 mouse and 9 rat PET/MRI images were acquired by nanoScan PET/MRI (Mediso). Each MRI image was filtered with edge-preserving Non-Local Means filter. n nTwo segmentation stages were determined: the first stage separated the body and the air. The gravitational center of each disjunctive region determined by histogram-derived value ranges of the body and the air were inputs of two independent fuzzy-based affinity map generation steps. The fuzzy maps were merged with simple maximum into an initial binary body-air material map. In the second stage the gravitational center of disjunctive air regions laid inside the body mask were the seed points of the third fuzzy affinity map generation. Three fuzzy maps were merged with maximum operation into a three-valued mask. n nThe three-valued material map was given as input for the Tera-Tomo 3D PET reconstruction engine to serve as the base for attenuation correction. n nMouse and rat material map segmentation result were validated by two experienced pre-clinical researchers who manually drawn the ideal mask boundaries. The ratio of voxel misclassification was measured as 2.23% (mouse) and 1.87% (rat). Quantitative ROI measurements shown 1.3 - 2.7% AC and NAC PET differences. n nAn automated and robust segmentation method was designed to provide a material map for pre-clinical PET attenuation correction. Although current work operated with PET/MRI images, the same method can be applied for nanoScan SPECT/MRI acquisitions. Results show that on the level of real quantitative measurements, attenuation correction of PET makes sense. As the next step of our future work we will investigate the effects of AC on pre-clinical nanoScan SPECT/MRI images.