In the current year we have completed and published our refined data analysis methods for estimation of rates of cerebral protein synthesis (rCPS) with L-1-C-11leucine and PET. We also completed validation and publication of the impact of a simplified patient scanning protocol. (1) Refinement of kinetic model analysis methods. Quantification with PET tracers utilizes compartmental models that generally assume homogeneity for each tissue region-of-interest (ROI) with respect to relevant physiological variables. The kinetic model of the C-11leucine PET method was originally applied to ROI data with the assumption that each ROI is homogeneous with respect to concentrations of amino acids, blood flow, transport and metabolism of amino acids, and rCPS. Due to the limited spatial resolution of PET, however, most regions contain a kinetically heterogeneous mixture of tissue components; this may bias estimation results. We have been refining our analyses to minimize effects of tissue heterogeneity on estimates of kinetic parameters and rCPS. We reasoned that a substantial reduction in the volume of a tissue region should reduce the impact of heterogeneity. We developed a method to apply the homogeneous tissue model to analysis of PET data at the voxel level, the Basis Function Method (BFM). We found that voxel-level BFM estimates of rCPS averaged over a ROI were substantially less biased than estimates based on direct fitting of the ROI time-activity curve (TAC) with a homogeneous tissue model. Model fits of the TACs showed that effects of tissue heterogeneity had been reduced, but not eliminated. We developed a second approach that explicitly takes heterogeneity within a ROI into account, spectral analysis with an iterative filter (SAIF). When applied to ROI-level data, SAIF-ROI produced low bias, low variance estimates of rCPS. It performed comparably to the voxel-level BFM method when count rates are normal, but at low count rates it performed better. Although SAIF allows for heterogeneity in a ROI, it does require an assumed constraint on the relationship among the kinetic parameters within the heterogeneous tissues. This has the most impact when kinetics of the various tissues within the ROI are most dissimilar. Under the premise that the dissimilarity among the tissues would be less at the voxel level, we extended the SAIF method for analysis of voxel-level data. In normal count rate studies, rCPS estimated with SAIF-voxel was approximately 5-15% higher than with SAIF-ROI analysis; inter-subject variability was comparable. Based on simulation studies we conclude that the difference is predominantly due to underestimation of rCPS with SAIF-ROI, i.e., the performance SAIF-voxel is better. As a final step, we investigated which data analysis method is optimal under various PET scanning conditions. When scanning with the high resolution research tomograph (HRRT), we found that voxel-level BFM and voxel-level SAIF analyses yielded comparable estimates of rCPS at count rates typical of our studies, but when the injected dose of leucine is unusually low, as when an unavoidable delay in injection occurs, low count rates lead to more biased voxel-level BFM estimates of rCPS. When scanner resolution is lower, however, voxel-level SAIF performs better irrespective of injected dose. We conclude that studies on the high resolution PET scanner, with standard injected doses of C-11leucine, can be analyzed with the computationally simpler voxel-level BFM. (2) Simplification of scanning protocols. One of our goals is to be able to study subjects with intellectual disabilities without the need for sedation, so any changes in the procedure that make the study less onerous for participants may help us to achieve that goal. The C-11leucine method was originally validated with a protocol that requires arterial blood sampling and a 90 min scan time. We have tested the effects of two changes to the protocol on computed rCPS: 1) shortening the scan duration and 2) substitution of venous for arterial sampling. (a) We have completed our investigation of the effects of shortening the scan duration. We reanalyzed thirty-nine scans in three groups of participants with the voxel-level Basis Function Method (BFM) that is based on a homogeneous tissue model. These scans were acquired on the high resolution scanner and participants were administered doses of C-11leucine that resulted good counting statistics. In these groups we found that rCPS values estimated on a 60 min scanning interval were almost identical to those estimated with the full 90 minutes of acquired data. Furthermore, statistical comparisons of the groups yielded the same results on the 60 and 90 min estimation intervals. This demonstrates that the scanning interval can be reduced to 60 min when counting statistics are good and BFM is used in the analysis. (b) We examined the effects of substituting venous for arterial blood sampling. We use population-derived arterial input functions calibrated by each participant's own measured venous data. Analyses of 22 scans of three groups of participants support the feasibility of utilizing venous in place of arterial blood sampling. We compared rCPS values determined with measured arterial blood data with rCPS based on venous-calibrated population input functions; we found no statistically significant differences in rCPS in either whole brain or any of the 16 regions examined in any group. We conclude that venous-calibrated population-derived input functions can replace arterial blood input functions. Taken together these validation studies demonstrate that the simplified scanning protocol for the C-11leucine PET method, i.e., a scan time of 60 minutes and use of venous-calibrated population input functions, can be used in C-11leucine studies on the high resolution research tomograph without compromising accuracy of rCPS measurements. This simplified scanning protocol will be employed in all future studies.

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U.S. National Institute of Mental Health
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