Kathleen C Schmidt

Measurement of regional rates of cerebral protein synthesis with L-[1-11C]leucine and PET with correction for recycling of tissue amino acids: II. Validation in rhesus monkeys.

The confounding effect of recycling of amino acids derived from tissue protein breakdown into the precursor pool for protein synthesis has been an obstacle to adapting in vivo methods for determination of regional rates of cerebral protein synthesis (rCPS) to positron emission tomography (PET). We used a kinetic modeling approach to estimate λ, the fraction of the precursor pool for protein synthesis derived from arterial plasma, and to measure rCPS in three anesthetized adult monkeys dynamically scanned after a bolus injection of L-[1-11C]leucine. In the same animals, λ was directly measured in a steady-state terminal experiment, and values showed excellent agreement with those estimated in the PET studies. In three additional monkeys rCPS was determined with the quantitative autoradiographic L-[1-14C]leucine method. In whole brain and cerebellum, rates of protein synthesis determined with the autoradiographic method were in excellent agreement with those determined with PET, and regional values were in good agreement when differences in spatial resolution of the two methods were taken into account. Low intrasubject variability was found on repeated PET studies. Our results in anesthetized monkey indicate that, by using a kinetic modeling approach to correct for recycling of tissue amino acids, quantitatively accurate and reproducible measurement of rCPS is possible with L-[1-11C]leucine and PET.

Estimation of Component and Parameter Distributions in Spectral Analysis

A method is presented for estimating the distributions of the components and parameters determined with spectral analysis when it is applied to a single data set. The method uses bootstrap resampling to simulate the effect of noise on the computed spectrum and to correct for possible bias in the estimates. A number of bootstrap procedures are reviewed, and one is selected for application to the kinetic analysis of positron emission tomography dynamic studies. The technique is shown to require minimal assumptions about noise in the measurements, and its small sample properties are established through Monte-Carlo simulations. The advantages and limitations of spectral analysis with bootstrap resampling for deriving inferences for tracer kinetic modeling are illustrated through sample analyses of time-activity curves for [18F]fluorodeoxyglucose and [15O]-labeled water.

Which Linear Compartmental Systems Can Be Analyzed by Spectral Analysis of PET Output Data Summed over All Compartments

General linear time-invariant compartmental systems were examined to determine which systems meet the conditions necessary for application of the spectral analysis technique to the sum of the concentrations in all compartments. Spectral analysis can be used to characterize the reversible and irreversible components of the system and to estimate the minimum number of compartments, but it applies only to systems in which the measured data can be expressed as a positively weighted sum of convolution integrals of the input function with an exponential function that has real-valued nonpositive decay constants. The conditions are met by compartmental systems that are strongly connected, have exchange of material with the environment confined to a single compartment, and do not contain cycles, i.e., there is no possibility for material to pass from one compartment through two or more compartments back to the initial compartment. Certain noncyclic systems with traps, systems with cycles that obey a specified loop condition, and noninterconnected collections of such systems also meet the conditions. Dynamic positron emission tomographic data obtained after injection of a radiotracer, the kinetics of which can be described by any model in the class of models identified here, can be appropriately analyzed with the spectral analysis technique.

Measurement of Regional Rates of Cerebral Protein Synthesis with L-[1-11C]Leucine and PET with Correction for Recycling of tissue amino acids: I. Kinetic Modeling Approach

Measurements of regional rates of cerebral protein synthesis (rCPS) require correction for the effect of recycling of tissue amino acids back into the precursor pool for protein synthesis. The fraction of the precursor pool derived from arterial plasma, λ, can be evaluated as the steady-state ratio of the specific activity of leucine in the tissue tRNA-bound fraction to that in arterial plasma. While λ can be directly measured in terminal experiments in animals, an alternative method is required for use with PET. We report a method to estimate λ based on a kinetic model of labeled and unlabeled leucine and labeled CO2 in the tissue. The kinetic model is also used to estimate the amount of labeled protein and rCPS. We measured time courses of [14C]leucine, [14C]protein, and 14CO2 in the blood and brain of anesthetized rats and estimated parameters of the kinetic model from these data. Simulation studies based on the kinetic parameters were then performed to examine the feasibility of this approach for use with L-[1-11C]leucine and PET. λ and rCPS were estimated with low bias, which suggests that PET can be used for quantitative measurement of rCPS with L-[1-11C]leucine and a kinetic modeling approach for correction for recycling of tissue amino acids.

Altered Cerebral Protein Synthesis in Fragile X Syndrome: Studies in Human Subjects and Knockout Mice

Dysregulated protein synthesis is thought to be a core phenotype of fragile X syndrome (FXS). In a mouse model (Fmr1 knockout (KO)) of FXS, rates of cerebral protein synthesis (rCPS) are increased in selective brain regions. We hypothesized that rCPS are also increased in FXS subjects. We measured rCPS with the L-[1-11C]leucine positron emission tomography (PET) method in whole brain and 10 regions in 15 FXS subjects who, because of their impairments, were studied under deep sedation with propofol. We compared results with those of 12 age-matched controls studied both awake and sedated. In controls, we found no differences in rCPS between awake and propofol sedation. Contrary to our hypothesis, FXS subjects under propofol sedation had reduced rCPS in whole brain, cerebellum, and cortex compared with sedated controls. To investigate whether propofol could have a disparate effect in FXS subjects masking usually elevated rCPS, we measured rCPS in C57Bl/6 wild-type (WT) and KO mice awake or under propofol sedation. Propofol decreased rCPS substantially in most regions examined in KO mice, but in WT mice caused few discrete changes. Propofol acts by decreasing neuronal activity either directly or by increasing inhibitory synaptic activity. Our results suggest that changes in synaptic signaling can correct increased rCPS in FXS.

A spectral analysis approach for determination of regional rates of cerebral protein synthesis with the L-[1-11C]leucine PET method

A spectral analysis approach was used to estimate kinetic model parameters of the L-[1-11C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS) in predefined regions of interest (ROIs). Unlike analyses based on the assumption that tissue ROIs are kinetically homogeneous, spectral analysis allows for heterogeneity within a region. To improve estimation performance, a new approach was developed—spectral analysis with iterative filter (SAIF). In simulation SAIF produced low bias, low variance estimates of the influx rate constant for leucine (K1), blood volume fraction (V b ), fraction of unlabeled leucine in the tissue precursor pool for protein synthesis derived from arterial plasma (λ), and rCPS. Simulation of normal count rate studies showed that SAIF applied to ROI time-activity curves (TACs) performed comparably to the basis function method (BFM) applied to voxel TACs when voxelwise estimates were averaged over all voxels in the ROI. At low count rates, however, SAIF performed better. In measured L-[1-11C]leucine PET data, there was good agreement between ROI-based SAIF estimates and average voxelwise BFM estimates of K1, V b , λ, and rCPS. We conclude that SAIF sufficiently addresses the problem of tissue heterogeneity in ROI data and provides a valid tool for estimation of rCPS, even in low count rate studies.

Regional Rates of Cerebral Protein Synthesis Measured with l-[1-11C]Leucine and PET in Conscious, Young Adult Men: Normal Values, Variability, and Reproducibility

We report regional rates of cerebral protein synthesis (rCPS) measured with the fully quantitative l-[1-11C]leucine positron emission tomography (PET) method. The method accounts for the fraction (Λ) of unlabeled amino acids in the precursor pool for protein synthesis derived from arterial plasma; the remainder (1-Λ) comes from tissue proteolysis. We determined rCPS and Λ in 18 regions and whole brain in 10 healthy men (21 to 24 years). Subjects underwent two 90-min dynamic PET studies with arterial blood sampling at least 2 weeks apart. Rates of cerebral protein synthesis varied regionally and ranged from 0.97 ± 0.70 to 2.25 ± 0.20 nmol/g per min. Values of rCPS were in good agreement between the two PET studies. Mean differences in rCPS between studies ranged from 9% in cortical regions to 15% in white matter. The Λ value was comparatively more uniform across regions, ranging from 0.63 ± 0.03 to 0.79 ± 0.02. Mean differences in Λ between studies were 2% to 8%. Intersubject variability in rCPS was on average 6% in cortical areas, 9% in subcortical regions, and 12% in white matter; intersubject variability in Λ was 2% to 8%. Our data indicate that in human subjects low variance and highly reproducible measures of rCPS can be made with the l-[1-11C]leucine PET method.

Voxel-Based Estimation of Kinetic Model Parameters of the l-[1-11C]Leucine PET Method for Determination of Regional Rates of Cerebral Protein Synthesis: Validation and Comparison with Region-of-Interest-Based Methods

We adapted and validated a basis function method (BFM) to estimate at the voxel level parameters of the kinetic model of the l-[1-11C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS). In simulation at noise levels typical of voxel data, BFM yielded low-bias estimates of rCPS; in measured data, BFM and nonlinear least-squares parameter estimates were in good agreement. We also examined whether there are advantages to using voxel-level estimates averaged over regions of interest (ROIs) in place of estimates obtained by directly fitting ROI time-activity curves (TACs). In both simulated and measured data, fits of ROI TACs were poor, likely because of tissue heterogeneity not taken into account in the kinetic model. In simulation, rCPS determined from fitting ROI TACs was substantially overestimated and BFM-estimated rCPS averaged over all voxels in an ROI was slightly underestimated. In measured data, rCPS determined by regional averaging of voxel estimates was lower than rCPS determined from ROI TACs, consistent with simulation. In both simulated and measured data, intersubject variability of BFM-estimated rCPS averaged over all voxels in a ROI was low. We conclude that voxelwise estimation is preferable to fitting ROI TACs using a homogeneous tissue model.

Propofol anesthesia does not alter regional rates of cerebral protein synthesis measured with L-[1-11C]leucine and PET in healthy male subjects

We report regional rates of cerebral protein synthesis (rCPS) in 10 healthy young males, each studied under two conditions: awake and anesthetized with propofol. We used the quantitative l-[1-11C]leucine positron emission tomography (PET) method to measure rCPS. The method accounts for the fraction (1) of unlabeled leucine in the precursor pool for protein synthesis that is derived from arterial plasma; the remainder comes from proteolysis of tissue proteins. Across 18 regions and whole brain, mean differences in rCPS between studies ranged from 5% to 5% and were within the variability of rCPS in awake studies (coefficient of variation range: 7% to 14%). Similarly, differences in Λ (range: 1% to 4%) were typically within the variability of Λ (coefficient of variation range: 3% to 6%). Intersubject variances and patterns of regional variation were also similar under both conditions. In propofol-anesthetized subjects, rCPS varied regionally from 0.98 ± 0.12 to 2.39 ± 0.23 nmol g−1 min−1 in the corona radiata and in the cerebellum, respectively. Our data indicate that the values, variances, and patterns of regional variation in rCPS and Λ measured by the l-[1-11C]leucine PET method are not significantly altered by anesthesia with propofol.

Use of Spectral Analysis with Iterative Filter for Voxelwise Determination of Regional Rates of Cerebral Protein Synthesis with L-[1-11C]leucine PET

A spectral analysis approach was used to estimate kinetic parameters of the L-[1-11C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS) on a voxel-by-voxel basis. Spectral analysis applies to both heterogeneous and homogeneous tissues; it does not require prior assumptions concerning number of tissue compartments. Parameters estimated with spectral analysis can be strongly affected by noise, but numerical filters improve estimation performance. Spectral analysis with iterative filter (SAIF) was originally developed to improve estimation of leucine kinetic parameters and rCPS in region-of-interest (ROI) data analyses. In the present study, we optimized SAIF for application at the voxel level. In measured L-[1-11C]leucine PET data, voxel-level SAIF parameter estimates averaged over all voxels within a ROI (mean voxel-SAIF) generally agreed well with corresponding estimates derived by applying the originally developed SAIF to ROI time-activity curves (ROI-SAIF). Region-of-interest-SAIF and mean voxel-SAIF estimates of rCPS were highly correlated. Simulations showed that mean voxel-SAIF rCPS estimates were less biased and less variable than ROI-SAIF estimates in the whole brain and cortex; biases were similar in white matter. We conclude that estimation of rCPS with SAIF is improved when the method is applied at voxel level than in ROI analysis.