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

Geometrical calibration and aperture configuration design in multi-pinhole SPECT

A clinical gamma camera can be converted into a high resolution SPECT system for small animal imaging, by replacing the clinical collimator(s) with pinhole collimators. However, for optimal performance, an accurate geometrical calibration is required. If it is assumed that the detector orbit is a true circle, the calibration requires the determination of seven parameters. It has been shown that these can be uniquely determined from a SPECT scan of a phantom consisting of three point sources, if two of the distances between these point sources are known. For multi-pinhole SPECT, two point sources should be sufficient, knowledge of the distance between the point sources is not required. The calibration method has been extended for cases where the orbit deviates from the ideal circle. A second interesting problem in multipinhole SPECT is the optimisation of the collimator design. This requires a measure of image quality, enabling objective comparison between different designs. An efficient analytical method has been developed for that purpose. With this method, it has been shown that the aperture diameter should be slightly smaller than the desired system resolution. We have also found that increased multiplexing (which comes with an apparent increase in system sensitivity) does not lead to reduced variance for a particular target resolution. In practice, avoiding all overlap seems to yield better performance