Design and performance of a compact and stationary microSPECT system

Purpose: Over the last ten years, there has been an extensive growth in the development of microSPECT imagers. Most of the systems are based on the combination of conventional, relatively large gamma cameras with poor intrinsic spatial resolution and multipinhole collimators working in large magnification mode. Spatial resolutions range from 0.58 to 0.76 mm while peak sensitivities vary from 0.06% to 0.4%. While pushing the limits of performance is of major importance, the authors believe that there is a need for smaller and less complex systems that bring along a reduced cost. While low footprint and low-cost systems can make microSPECT available to more researchers, the ease of operation and calibration and low maintenance cost are additional factors that can facilitate the use of microSPECT in molecular imaging. In this paper, the authors simulate the performance of a microSPECT imager that combines high space-bandwidth detectors and pinholes with truncated projection, resulting in a small and stationary system. Methods: A system optimization algorithm is used to determine the optimal SPECT systems, given our high resolutions detectors and a fixed field-of-view. These optimal system geometries are then used to simulate a Defrise disk phantom and a hot rod phantom. Finally, a MOBY mouse phantom, with realistic concentrations of Tc99m-tetrofosmin is simulated. Results: Results show that the authors can successfully reconstruct a Defrise disk phantom of 24 mm in diameter without any rotating system components or translation of the object. Reconstructed spatial resolution is approximately 800 mu m while the peak sensitivity is 0.23%. Finally, the simulation of the MOBY mouse phantom shows that the authors can accurately reconstruct mouse images. Conclusions: These results show that pinholes with truncated projections can be used in small magnification or minification mode to obtain a compact and stationary microSPECT system. The authors showed that they can reach state-of-the-art system performance and can successfully reconstruct images with realistic noise levels in a preclinical context. Such a system can be useful for dynamic SPECT imaging. 2013 American Association of Physicists in Medicine

Sheep can be used as animal model of regional myocardial remodeling and controllable work

Background: Pacing the right heart has been shown to induce reversible conduction delay and subse­quent asymmetric remodeling of the left ventricle (LV) in dogs and pigs. Both species have disadvantages in animal experiments. Therefore the aim of this study was to develop a more feasible and easy-to-use animal model in sheep. Methods: Dual-chamber (DDD) pacemakers with epicardial leads on the right atrium and right ven­tricular free wall were implanted in 13 sheep. All animals underwent 8 weeks of chronic rapid pacing at 180 bpm. Reported observations were made at 110 bpm. Results: DDD pacing acutely induced a left bundle branch block (LBBB) — like pattern with almost doubling in QRS width and the appearance of a septal flash, indicating mechanical dyssynchrony. Atrial pacing (AAI) resulted in normal ventricular conduction and function. During 8 weeks of rapid DDD pacing, animals developed LV remodeling (confirmed with histology) with septal wall thinning (–30%, p < 0.05), lateral wall thickening (+22%, p < 0.05), LV volume increase (+32%, p < 0.05), decrease of LV ejection fraction (–31%, p < 0.05), and functional mitral regurgitation. After 8 weeks, segmental pressure-strain-loops, representing regional myocardial work, were recorded. Switching from AAI to DDD pacing decreased immediately work in the septum and increased it in the lateral wall (–69 and +41%, respectively, p < 0.05). Global LV stroke work and dP/dtmax decreased (–27% and -25%, respectively, p < 0.05). Conclusions: This study presents the development a new sheep model with an asymmetrically remod­eled LV. Simple pacemaker programing allows direct modulation of regional myocardial function and work. This animal model provides a new and valuable alternative for canine or porcine models and has the potential to become instrumental for investigating regional function and loading conditions on regional LV remodeling

What approach to brain partial volume correction is best for PET/MRI?

Many partial volume correction approaches make use of anatomical information, readily available in PET/MRI systems but it is not clear what approach is best. Seven novel approaches to partial volume correction were evaluated, including several post-reconstruction methods and several reconstruction methods that incorporate anatomical information. These were compared with an MRI-independent approach (reblurred van Cittert ) and uncorrected data. Monte Carlo PET data were generated for activity distributions representing both 18F FDG and amyloid tracer uptake. Post-reconstruction methods provided the best recovery with ideal segmentation but were particularly sensitive to mis-registration. Alternative approaches performed better in maintaining lesion contrast (unseen in MRI) with good noise control. These were also relatively insensitive to mis-registration errors. The choice of method will depend on the specific application and reliability of segmentation and registration algorithms

Assessment of Maximum A Posteriori Image Estimation Algorithms for Reduced Acquisition Time Medical Positron Emission Tomography Data

This study examines the effects of reduced radioactive dosage data collection on positron emission tomography reconstruction reliability and investigates the efficiency of various reconstruction methods. Also, it investigates properties of the reconstructed images under these circumstances and the limitations of the currently used algorithms. The methods are based on maximum likelihood and maximum a posteriori estimation, but no explicit solutions exist and hence iterative schemes are obtained using the expectation-maximisation and one-step-late methods, while greater efficiency is obtained by using an ordered-subset approach. Ten replicate real datasets, from the Hoffman brain phantom collected using a Siemens Biograph mMR scanner, are considered using standard deviation, bias and mean-squared error as quantitative output measures. The variability is very high when low prior parameter values are used but reduces substantially for higher values. However, in contrast, the bias is low for low parameter values and high for high parameter values. For individual reconstructions, low parameter values lead to detail being lost in the noise whereas high values produce unacceptable artefacts at the boundaries between different anatomical regions. Considering the mean-squared error, a balance between bias and variability, still identifies high prior parameter values as giving the best results, but this is in contradiction to visual inspection. These findings demonstrate that when it comes to low counts, variability and bias become significant and are visible in the images, but that improved reconstruction can be achieved by a careful choice of the prior parameter

Targeting murine heart and brain: visualisation conditions for multi-pinhole SPECT with 99mTc- and 123I-labelled probes

The study serves to optimise conditions for multi-pinhole SPECT small animal imaging of (123)I- and (99m)Tc-labelled radiopharmaceuticals with different distributions in murine heart and brain and to investigate detection and dose range thresholds for verification of differences in tracer uptake.A Triad 88/Trionix system with three 6-pinhole collimators was used for investigation of dose requirements for imaging of the dopamine D(2) receptor ligand [(123)I]IBZM and the cerebral perfusion tracer [(99m)Tc]HMPAO (1.2-0.4 MBq/g body weight) in healthy mice. The fatty acid [(123)I]IPPA (0.94 +/- 0.05 MBq/g body weight) and the perfusion tracer [(99m)Tc]sestamibi (3.8 +/- 0.45 MBq/g body weight) were applied to cardiomyopathic mice overexpressing the prostaglandin EP(3) receptor.In vivo imaging and in vitro data revealed 45 kBq total cerebral uptake and 201 kBq cardiac uptake as thresholds for visualisation of striatal [(123)I]IBZM and of cardiac [(99m)Tc]sestamibi using 100 and 150 s acquisition time, respectively. Alterations of maximal cerebral uptake of [(123)I]IBZM by >20% (116 kBq) were verified with the prerequisite of 50% striatal of total uptake. The labelling with [(99m)Tc]sestamibi revealed a 30% lower uptake in cardiomyopathic hearts compared to wild types. [(123)I]IPPA uptake could be visualised at activity doses of 0.8 MBq/g body weight.Multi-pinhole SPECT enables detection of alterations of the cerebral uptake of (123)I- and (99m)Tc-labelled tracers in an appropriate dose range in murine models targeting physiological processes in brain and heart. The thresholds of detection for differences in the tracer uptake determined under the conditions of our experiments well reflect distinctions in molar activity and uptake characteristics of the tracers

High Quality Image Formation of Small Animals Using Emission Tomography (Hoge kwaliteitsbeeldvorming van kleine dieren door middel van emissietomografie)

De voorbije tien jaar zijn zowel SPECT als PET uitgebreid bestudeerd ter verbetering van de kwaliteit van functionele beeldvorming in kleine proefdieren. In dit werk hebben we ons toegespitst op SPECT van kleine dieren, wat veel toepassingsmogelijkheden heeft en de ruimtelijke resolutie van de micro-PET kan overtroeven. Het reconstructieprogramma werd verbeterd door het nauwkeuriger modelleren van het beeldvormingsproces, wat superieure beelden oplevert. Bovendien werd een efficiënte methode ontwikkeld voor beeldkwaliteitsevaluatie, die gevalideerd werd voor enkel- en meervoudige pinhole SPECT. Hiermee werd de invloed van vele ontwerpparameters voor pinholecollimatoren op de reconstructiebeeldkwaliteit onderzocht, de hoeveelheid overlap in multipinhole SPECT projecties incluis. Op basis van die resultaten werd een multipinhole-ontwerp voor muisbeeldvorming geoptimaliseerd, vervaardigd, en getest op een klinische gammacamera uitgerust met twee pinholecollimatoren. Dezelfde methode werd ook toegepast om de eigenschappen van time-of-flight PET te bestuderen. Tot slot werd een overzicht van onze belangrijkste pinhole SPECT toepassingen gegeven ter illustratie van de beeldkwaliteitsevolutie.status: publishe

Noise Propagation in Multipinhole SPECT Calibration

It has been shown previously that multipinhole single photon emission computed tomography (SPECT) calibration only requires a two-point source measurement, without knowledge of the distance between them, to uniquely determine the camera geometry. In this work, we studied how this method propagates measurement noise into errors on the estimated calibration parameters, which in turn will degrade the reconstruction image quality. Furthermore, the effect on the calibration and reconstruction accuracy of fixing either the inter-point or the inter-pinhole distance during calibration is assessed. As expected, including information about the distance between the point sources improved the robustness of the calibration against measurement noise. In addition, it was found that similar robustness and even a superior spatial resolution can be obtained by fixing the distance(s) between the pinhole apertures instead of fixing those between the point sources. Finally, optimal two-point source configurations were determined for calibrating our newly-designed 7-pinhole SPECT system for limited field of view mouse imaging while fixing the inter-pinhole distances.status: publishe

Multi-Pinhole SPECT Calibration: Influence of Data Noise and Systematic Orbit Deviations

The geometry of a single pinhole SPECT system with circular orbit can be uniquely determined from a measurement of three point sources, provided that at least two inter-point distances are known. In contrast, it has been shown mathematically that, for a multi-pinhole SPECT system with circular orbit, only two point sources are needed, and the knowledge of the distance between them is not required. In this paper, we report that this conclusion only holds if the motion of the camera is perfectly circular. In reality, the detector heads systematically slightly deviate from the circular orbit, which may introduce non-negligible bias in the estimated parameters and degrade the reconstructed image. An analytical linear model was extended to estimate the influence of both data noise and systematic deviations on the accuracy of the calibration and on the image quality of the reconstruction. It turns out that applying the knowledge of the distances greatly reduces the reconstruction error, especially in the presence of systematic deviations. In addition, we propose that instead of using the information about the distances between the point sources, it is more straightforward to use the knowledge about the distances between the pinhole apertures during multi-pinhole calibration. The two distance-fixing approaches yield similar reconstruction accuracy. Our theoretical results are supported by reconstruction images of a Jaszczak-type phantom scan.status: publishe