Voxel-based estimation of kinetic model parameters of the L-[1-(11)C]leucine PET method for determination of regional rates of cerebral protein synthesis: validation and comparison with region-of-interest-based methods

Increased rates of brain protein synthesis during [N1,N2] sleep: L-[1-11C]leucine PET studies in human subjects

During sleep, reduced brain energy demands provide an opportunity for biosynthetic processes like protein synthesis. Sleep is required for some forms of memory consolidation which requires de novo protein synthesis. We measured regional cerebral protein synthesis rates (rCPS) in human subjects to ascertain how rCPS is affected during sleep. Subjects underwent three consecutive L-[1-11C]leucine PET scans with simultaneous polysomnography: 1. rested awake, 2. sleep-deprived awake, 3. sleep. Measured rCPS were similar across the three conditions. Variations in sleep stage times during sleep scans were used to estimate rCPS in sleep stages under the assumption that measured rCPS is the weighted sum of rCPS in each stage, with weights reflecting time and availability of [11C]leucine in that stage. During sleep scans, subjects spent most of the time in N2, N3, and awake and very little time in N1 and REM; rCPS in N1 and REM could not be reliably estimated. When stages N1 and N2 were combined [N1,N2], estimates of rCPS were more robust. In selective regions, estimated rCPS were statistically significantly higher (30-39%) in [N1,N2] compared with N3; estimated rCPS in N3 were similar to values measured in sleep-deprived awake scans. Results indicate increased rates of protein synthesis linked to [N1,N2] sleep.

Decreased rates of cerebral protein synthesis in conscious young adults with fragile X syndrome demonstrated by L-[1-11C]leucine PET

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. Fragile X mental retardation protein, a putative translation suppressor, is significantly reduced in FXS. The prevailing hypothesis is that rates of cerebral protein synthesis (rCPS) are increased by the absence of this regulatory protein. We have previously reported increased rCPS in the Fmr1 knockout mouse model of FXS. To address the hypothesis in human subjects, we measured rCPS in young men with FXS with L-[1-11C]leucine PET. In previous studies we had used sedation during imaging, and we did not find increases in rCPS as had been seen in the mouse model. Since mouse measurements were conducted in awake animals, we considered the possibility that sedation may have confounded our results. In the present study we used a modified and validated PET protocol that made it easier for participants with FXS to undergo the study awake. We compared rCPS in 10 fragile X participants and 16 healthy controls all studied while awake. Contrary to the prevailing hypothesis and findings in Fmr1 knockout mice, results indicate that rCPS in awake participants with FXS are decreased in whole brain and most brain regions by 13-21% compared to healthy controls.

Synthesis of L-[5-11 C]Leucine and L-α-[5-11 C]Methylleucine via Pd0 -mediated 11 C-Methylation and Microfluidic Hydrogenation: Potentiality of Leucine PET Probes for Tumor Imaging

The efficient synthesis of L-[5-¹¹ C]leucine and L-α-[5-¹¹ C]methylleucine has been investigated using a continuous two-step sequence of rapid reactions consisting of Pd⁰ -mediated ¹¹ C-methylation and microfluidic hydrogenation. The synthesis of L-[5-¹¹ C]leucine and L-α-[5-¹¹ C]methylleucine was accomplished within 40 min with a decay-corrected radiochemical yield of 15-38 % based on [¹¹ C]CH₃ I, radiochemical purity of 95-99 %, and chemical purity of 95-99 %. The Pd impurities in the injectable solution measured using inductively coupled plasma mass spectrometry met the international criteria for human use. Positron emission tomography scanning after an intravenous injection of L-[5-¹¹ C]leucine or L-α-[5-¹¹ C]methyl leucine in A431 tumor-bearing mice was performed. As a result, L-α-[5-¹¹ C]methylleucine was found to be a potentially useful probe for visualizing the tumor. Tissue distribution analysis showed that the accumulation value of L-α-[5-¹¹ C]methylleucine in tumor tissue was high [12±3% injected dose/g tissue (%ID/g)].

Regional rates of brain protein synthesis are unaltered in dexmedetomidine sedated young men with fragile X syndrome: A L-[1-11C]leucine PET study

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. Fragile X mental retardation protein (FMRP), a putative translation suppressor, is absent or significantly reduced in FXS. One prevailing hypothesis is that rates of protein synthesis are increased by the absence of this regulatory protein. In accord with this hypothesis, we have previously reported increased rates of cerebral protein synthesis (rCPS) in the Fmr1 knockout mouse model of FXS and others have reported similar effects in hippocampal slices. To address the hypothesis in human subjects, we applied the L[1-¹¹C]leucine PET method to measure rCPS in adults with FXS and healthy controls. All subjects were males between the ages of 18 and 24 years and free of psychotropic medication. As most fragile X participants were not able to undergo the PET study awake, we used dexmedetomidine for sedation during the imaging studies. We found no differences between rCPS measured during dexmedetomidine-sedation and the awake state in ten healthy controls. In the comparison of rCPS in dexmedetomidine-sedated fragile X participants (n = 9) and healthy controls (n = 14) we found no statistically significant differences. Our results from in vivo measurements in human brain do not support the hypothesis that rCPS are elevated due to the absence of FMRP. This hypothesis is based on findings in animal models and in vitro measurements in human peripheral cells. The absence of a translation suppressor may produce a more complex response in pathways regulating translation than previously thought. We may need to revise our working hypotheses regarding FXS and our thinking about potential therapeutics.

Substitution of venous for arterial blood sampling in the determination of regional rates of cerebral protein synthesis with L-[1-11C]leucine PET: A validation study

We developed and validated a method to estimate input functions for determination of regional rates of cerebral protein synthesis (rCPS) with L-[1-¹¹C]leucine PET without arterial sampling. The method is based on a population-derived input function (PDIF) approach, with venous samples for calibration. Population input functions were constructed from arterial blood data measured in 25 healthy 18-24-year-old males who underwent L-[1-¹¹C]leucine PET scans while awake. To validate the approach, three additional groups of 18-27-year-old males underwent L-[1-¹¹C]leucine PET scans with both arterial and venous blood sampling: 13 awake healthy volunteers, 10 sedated healthy volunteers, and 5 sedated subjects with fragile X syndrome. Rate constants of the L-[1-¹¹C]leucine kinetic model were estimated voxel-wise with measured arterial input functions and with venous-calibrated PDIFs. Venous plasma leucine measurements were used with venous-calibrated PDIFs for rCPS computation. rCPS determined with PDIFs calibrated with 30-60 min venous samples had small errors (RMSE: 4-9%), and no statistically significant differences were found in any group when compared to rCPS determined with arterial input functions. We conclude that in young adult males, PDIFs calibrated with 30-60 min venous samples can be used in place of arterial input functions for determination of rCPS with L-[1-¹¹C]leucine PET.

Improved discrimination between benign and malignant LDCT screening-detected lung nodules with dynamic over static 18F-FDG PET as a function of injected dose

Lung cancer mortality rate can be significantly reduced by up to 20% through routine low-dose computed tomography (LDCT) screening, which, however, has high sensitivity but low specificity, resulting in a high rate of false-positive nodules. Combining PET with CT may provide more accurate diagnosis for indeterminate screening-detected nodules. In this work, we investigated low-dose dynamic 18F-FDG PET in discrimination between benign and malignant nodules using a virtual clinical trial based on patient study with ground truth. Six patients with initial LDCT screening-detected lung nodules received 90 min single-bed PET scans following a 10 mCi FDG injection. Low-dose static and dynamic images were generated from under-sampled list-mode data at various count levels (100%, 50%, 10%, 5%, and 1%). A virtual clinical trial was performed by adding nodule population variability, measurement noise, and static PET acquisition start time variability to the time activity curves (TACs) of the patient data. We used receiver operating characteristic (ROC) analysis to estimate the classification capability of standardized uptake value (SUV) and net uptake constant K i from their simulated benign and malignant distributions. Various scan durations and start times (t *) were investigated in dynamic Patlak analysis to optimize simplified acquisition protocols with a population-based input function (PBIF). The area under curve (AUC) of ROC analysis was higher with increased scan duration and earlier t *. Highly similar results were obtained using PBIF to those using image-derived input function (IDIF). The AUC value for K i using optimized t * and scan duration with 10% dose was higher than that for SUV with 100% dose. Our results suggest that dynamic PET with as little as 1 mCi FDG could provide discrimination between benign and malignant lung nodules with higher than 90% sensitivity and specificity for patients similar to the pilot and simulated population in this study, with LDCT screening-detected indeterminate lung nodules.

Dynamic image denoising for voxel-wise quantification with Statistical Parametric Mapping in molecular neuroimaging

Purpose

PET and SPECT voxel kinetics are highly noised. To our knowledge, no study has determined the effect of denoising on the ability to detect differences in binding at the voxel level using Statistical Parametric Mapping (SPM).

Methods

In the present study, groups of subject-images with a 10%- and 20%- difference in binding of [¹²³I]iomazenil (IMZ) were simulated. They were denoised with Factor Analysis (FA). Parametric images of binding potential (BP_ND) were produced with the simplified reference tissue model (SRTM) and the Logan non-invasive graphical analysis (LNIGA) and analyzed using SPM to detect group differences. FA was also applied to [¹²³I]IMZ and [¹¹C]flumazenil (FMZ) clinical images (n = 4) and the variance of BP_ND was evaluated.

Results

Estimations from FA-denoised simulated images provided a more favorable bias-precision profile in SRTM and LNIGA quantification. Simulated differences were detected in a higher number of voxels when denoised simulated images were used for voxel-wise estimations, compared to quantification on raw simulated images. Variability of voxel-wise binding estimations on denoised clinical SPECT and PET images was also significantly diminished.

Conclusion

In conclusion, noise removal from dynamic brain SPECT and PET images may optimize voxel-wise BP_ND estimations and detection of biological differences using SPM.

Rates of cerebral protein synthesis in primary visual cortex during sleep-dependent memory consolidation, a study in human subjects

If protein synthesis during sleep is required for sleep-dependent memory consolidation, we might expect rates of cerebral protein synthesis (rCPS) to increase during sleep in the local brain circuits that support performance on a particular task following training on that task. To measure circuit-specific brain protein synthesis during a daytime nap opportunity, we used the L-[1-(11)C]leucine positron emission tomography (PET) method with simultaneous polysomnography. We trained subjects on the visual texture discrimination task (TDT). This was followed by a nap opportunity during the PET scan, and we retested them later in the day after the scan. The TDT is considered retinotopically specific, so we hypothesized that higher rCPS in primary visual cortex would be observed in the trained hemisphere compared to the untrained hemisphere in subjects who were randomized to a sleep condition. Our results indicate that the changes in rCPS in primary visual cortex depended on whether subjects were in the wakefulness or sleep condition but were independent of the side of the visual field trained. That is, only in the subjects randomized to sleep, rCPS in the right primary visual cortex was higher than the left regardless of side trained. Other brain regions examined were not so affected. In the subjects who slept, performance on the TDT improved similarly regardless of the side trained. Results indicate a regionally selective and sleep-dependent effect that occurs with improved performance on the TDT.

Effects of shortened scanning intervals on calculated regional rates of cerebral protein synthesis determined with the L-[1-11C]leucine PET method

To examine effects of scan duration on estimates of regional rates of cerebral protein synthesis (rCPS), we reanalyzed data from thirty-nine previously reported L-[1-¹¹C]leucine PET studies. Subjects consisted of 12 healthy volunteers studied twice, awake and under propofol sedation, and 15 subjects with fragile X syndrome (FXS) studied once under propofol sedation. All scans were acquired on a high resolution scanner. We used a basis function method for voxelwise estimation of parameters of the kinetic model of L-[1-¹¹C]leucine and rCPS over the interval beginning at the time of tracer injection and ending 30, 45, 60, 75 or 90 min later. For each study and scan interval, regional estimates in nine regions and whole brain were obtained by averaging voxelwise estimates over all voxels in the region. In all three groups rCPS was only slightly affected by scan interval length and was very stable between 60 and 90 min. Furthermore, statistical comparisons of rCPS between awake and sedated healthy volunteers provided almost identical results when they were based on 60 min scan data as when they were based on data from the full 90 min interval. Statistical comparisons between sedated healthy volunteers and sedated subjects with FXS also yielded almost identical results when based on 60 and 90 min scan intervals. We conclude that, under the conditions of our studies, scan duration can be shortened to 60 min without loss of precision.

Impact of tissue kinetic heterogeneity on PET quantification: case study with the L-[1-11C]leucine PET method for cerebral protein synthesis rates

Functional quantification with PET is generally based on modeling that assumes tissue regions are kinetically homogeneous. Even in regions sufficiently small to approach homogeneity, spillover due to resolution limitations of PET scanners may introduce heterogeneous kinetics into measured data. Herein we consider effects of kinetic heterogeneity at the smallest volume accessible, the single image voxel. We used L-[1-¹¹C]leucine PET and compared rates of cerebral protein synthesis (rCPS) estimated voxelwise with methods that do (Spectral Analysis Iterative Filter, SAIF) and do not (Basis Function Method, BFM) allow for kinetic heterogeneity. In high resolution PET data with good counting statistics BFM produced estimates of rCPS comparable to SAIF, but at lower computational cost; thus the simpler, less costly method can be applied. With poorer counting statistics (lower injected radiotracer doses), BFM estimates were more biased. In data smoothed to simulate lower resolution PET, BFM produced estimates of rCPS 9-14% higher than SAIF, overestimation consistent with applying a homogeneous tissue model to kinetically heterogeneous data. Hence with lower resolution data it is necessary to account for kinetic heterogeneity in the analysis. Kinetic heterogeneity may impact analyses of other tracers and scanning protocols differently; assessments should be made on a case by case basis.

A Variational Bayesian inference method for parametric imaging of PET data

In dynamic Positron Emission Tomography (PET) studies, compartmental models provide the richest information on the tracer kinetics of the tissue. Inverting such models at the voxel level is however quite challenging due to the low signal-to-noise ratio of the time activity curves. In this study, we propose the use of a Variational Bayesian (VB) approach to efficiently solve this issue and thus obtain robust quantitative parametric maps. VB was adapted to the non-uniform noise distribution of PET data. Moreover, we propose a novel hierarchical scheme to define the model parameter priors directly from the images in case such information are not available from the literature, as often happens with new PET tracers. VB was initially tested on synthetic data generated using compartmental models of increasing complexity, providing accurate (%bias<2%±2%, root mean square error<15%±5%) parameter estimates. When applied to real data on a paradigmatic set of PET tracers (L-[1-11C]leucine, [11C]WAY100635 and [18F]FDG), VB was able to generate reliable parametric maps even in presence of high noise in the data (unreliable estimates<11%±5%).

Spectral Analysis of Dynamic PET Studies: A Review of 20 Years of Method Developments and Applications

In Positron Emission Tomography (PET), spectral analysis (SA) allows the quantification of dynamic data by relating the radioactivity measured by the scanner in time to the underlying physiological processes of the system under investigation. Among the different approaches for the quantification of PET data, SA is based on the linear solution of the Laplace transform inversion whereas the measured arterial and tissue time-activity curves of a radiotracer are used to calculate the input response function of the tissue. In the recent years SA has been used with a large number of PET tracers in brain and nonbrain applications, demonstrating that it is a very flexible and robust method for PET data analysis. Differently from the most common PET quantification approaches that adopt standard nonlinear estimation of compartmental models or some linear simplifications, SA can be applied without defining any specific model configuration and has demonstrated very good sensitivity to the underlying kinetics. This characteristic makes it useful as an investigative tool especially for the analysis of novel PET tracers. The purpose of this work is to offer an overview of SA, to discuss advantages and limitations of the methodology, and to inform about its applications in the PET field.

Radiosynthesis of Carbon-11 Labeled Puromycin as a Potential PET Candidate for Imaging Protein Synthesis in Vivo

In order to address the limitations associated with the present range of PET radiotracers used for imaging protein synthesis in vivo we have synthesized a candidate PET radiotracer based on Puromycin (3, PURO), a protein synthesis inhibitor. The desmethylPURO 9 precursor for radiolabeling with carbon-11 radioisotope was synthesized in two steps employing EDC/HOBt amide coupling in overall 76% yield. Optimal conditions for radiolabeling were then established via methylation/deprotection sequence. Under these conditions as determined by NMR analysis 9 showed partial stability (ca. 80%) under acidic conditions. Limited evidence of stereochemical stability of 3 was also found. The radiolabeling of intermediate [(11)C]12 was accomplished with up to 57% conversion from [(11)C]iodomethane. An automated method was then developed for high radioactivity radiosynthesis to produce [(11)C]3 ([(11)C]PURO) in 16 ± 6% (n = 3) decay corrected radiochemical yields.

Preliminary clinical assessment of the relationship between tumor alphavbeta3 integrin and perfusion in patients studied with [(18)F]fluciclatide kinetics and [ (15)O]H 2O PET

BACKGROUND

[(18)F]fluciclatide, a peptide ligand with high affinity for αvβ3/αvβ5 integrins, is a proposed biomarker of tumor angiogenesis. The study rationale was to perform a preliminary evaluation of the relationship between tumor [(18)F]fluciclatide uptake and perfusion by [(15)O]H2O PET.

METHODS

Patients with non-small cell lung cancer and melanoma underwent dynamic imaging with arterial sampling following injection of [(15)O]H2O and [(18)F]fluciclatide. Quantification was performed using a one-tissue compartmental model for [(15)O]H2O and a two-tissue model for [(18)F]fluciclatide at volume-of-interest level, and SUV at voxel level.

RESULTS

Tumor binding potential (k 3/k 4 ratio) of [(18)F]fluciclatide tumor was 5.39 ± 1.46, consistent with previous studies in breast cancer metastases. Voxel-by-voxel maps of [(18)F]fluciclatide delivery strongly correlated with [(15)O]H2O-based perfusion (p < 10(-4) tumor, 1,794 ± 1,331 voxels). Interestingly, this correlation was lost when retention of [(18)F]fluciclatide at late time-points was compared with perfusion (p > 0.15).

CONCLUSIONS

Our study suggests tumor [(18)F]fluciclatide retention is unrelated to tumor perfusion, supporting use of late (60-min) imaging protocols in patients.

Positron emission tomography imaging with 18F-labeled ZHER2:2891 affibody for detection of HER2 expression and pharmacodynamic response to HER2-modulating therapies

PURPOSE

Expression of HER2 has profound implications on treatment strategies in various types of cancer. We investigated the specificity of radiolabeled HER2-targeting ZHER2:2891 Affibody, [(18)F]GE-226, for positron emission tomography (PET) imaging.

EXPERIMENTAL DESIGN

Intrinsic cellular [(18)F]GE-226 uptake and tumor-specific tracer binding were assessed in cells and xenografts with and without drug treatment. Specificity was further determined by comparing tumor localization of a fluorescently labeled analogue with DAKO HercepTest.

RESULTS

[(18)F]GE-226 uptake was 11- to 67-fold higher in 10 HER2-positive versus HER2-negative cell lines in vitro independent of lineage. Uptake in HER2-positive xenografts was rapid with net irreversible binding kinetics making possible the distinction of HER2-negative [MCF7 and MCF7-p95HER2: NUV60 (%ID/mL) 6.1 ± 0.7; Ki (mL/cm(3)/min) 0.0069 ± 0.0014] from HER2-positive tumors (NUV60 and Ki: MCF7-HER2, 10.9 ± 1.5 and 0.015 ± 0.0035; MDA-MB-361, 18.2 ± 3.4 and 0.025 ± 0.0052; SKOV-3, 18.7 ± 2.4 and 0.036 ± 0.0065) within 1 hour. Tumor uptake correlated with HER2 expression determined by ELISA (r(2) = 0.78), and a fluorophore-labeled tracer analogue colocalized with HER2 expression. Tracer uptake was not influenced by short-term or continuous treatment with trastuzumab in keeping with differential epitope binding, but reflected HER2 degradation by short-term NVP-AUY922 treatment in SKOV-3 xenografts (NUV60: 13.5 ± 2.1 %ID/mL vs. 9.0 ± 0.9 %ID/mL for vehicle or drug, respectively).

CONCLUSIONS

[(18)F]GE-226 binds with high specificity to HER2 independent of cell lineage. The tracer has potential utility for HER2 detection, irrespective of prior trastuzumab treatment, and to discern HSP90 inhibitor-mediated HER2 degradation.

Generalized separable parameter space techniques for fitting 1K-5K serial compartment models

PURPOSE

Kinetic modeling is widely used to analyze dynamic imaging data, estimating kinetic parameters that quantify functional or physiologic processes in vivo. Typical kinetic models give rise to nonlinear solution equations in multiple dimensions, presenting a complex fitting environment. This work generalizes previously described separable nonlinear least-squares techniques for fitting serial compartment models with up to three tissue compartments and five rate parameters.

METHODS

The approach maximally separates the linear and nonlinear aspects of the modeling equations, using a formulation modified from previous basis function methods to avoid a potential mathematical degeneracy. A fast and robust algorithm for solving the linear subproblem with full user-defined constraints is also presented. The generalized separable parameter space technique effectively reduces the dimensionality of the nonlinear fitting problem to one dimension for 2K-3K compartment models, and to two dimensions for 4K-5K models.

RESULTS

Exhaustive search fits, which guarantee identification of the true global minimum fit, required approximately 10 ms for 2K-3K and 1.1 s for 4K-5K models, respectively. The technique is also amenable to fast gradient-descent iterative fitting algorithms, where the reduced dimensionality offers improved convergence properties. The objective function for the separable parameter space nonlinear subproblem was characterized and found to be generally well-behaved with a well-defined global minimum. Separable parameter space fits with the Levenberg-Marquardt algorithm required fewer iterations than comparable fits for conventional model formulations, averaging 1 and 7 ms for 2K-3K and 4K-5K models, respectively. Sensitivity to initial conditions was likewise reduced.

CONCLUSIONS

The separable parameter space techniques described herein generalize previously described techniques to encompass 1K-5K compartment models, enable robust solution of the linear subproblem with full user-defined constraints, and are amenable to rapid and robust fitting using iterative gradient-descent type algorithms.

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-(11)C]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.

The development, past achievements, and future directions of brain PET

The early developments of brain positron emission tomography (PET), including the methodological advances that have driven progress, are outlined. The considerable past achievements of brain PET have been summarized in collaboration with contributing experts in specific clinical applications including cerebrovascular disease, movement disorders, dementia, epilepsy, schizophrenia, addiction, depression and anxiety, brain tumors, drug development, and the normal healthy brain. Despite a history of improving methodology and considerable achievements, brain PET research activity is not growing and appears to have diminished. Assessments of the reasons for decline are presented and strategies proposed for reinvigorating brain PET research. Central to this is widening the access to advanced PET procedures through the introduction of lower cost cyclotron and radiochemistry technologies. The support and expertize of the existing major PET centers, and the recruitment of new biologists, bio-mathematicians and chemists to the field would be important for such a revival. New future applications need to be identified, the scope of targets imaged broadened, and the developed expertize exploited in other areas of medical research. Such reinvigoration of the field would enable PET to continue making significant contributions to advance the understanding of the normal and diseased brain and support the development of advanced treatments.

Imaging of cellular proliferation in liver metastasis by [18F]fluorothymidine positron emission tomography: effect of therapy

Although [(18)F]fluorothymidine positron emission tomography (FLT-PET) permits estimation of tumor thymidine kinase-1 expression, and thus, cell proliferation, high physiological uptake of tracer in liver tissue can limit its utility. We evaluated FLT-PET combined with a temporal-intensity information-based voxel-clustering approach termed kinetic spatial filtering (FLT-PET(KSF)) for detecting drug response in liver metastases. FLT-PET and computed tomography data were collected from patients with confirmed breast or colorectal liver metastases before, and two weeks after the first cycle of chemotherapy. Changes in tumor FLT-PET and FLT-PET(KSF) variables were determined. Visual distinction between tumor and normal liver was seen in FLT-PET(KSF) images. Of the 33 metastases from 20 patients studied, 26 were visible after kinetic filtering. The net irreversible retention of the tracer (Ki; from unfiltered data) in the tumor, correlated strongly with tracer uptake when the imaging variable was an unfiltered average or maximal standardized uptake value, 60 min post-injection (SUV(60,av): r = 0.9, SUV(60,max): r = 0.7; p < 0.0001 for both) and occurrence of high intensity voxels derived from FLT-PET(KSF) (r = 0.7, p < 0.0001). Overall, a significant reduction in the imaging variables was seen in responders compared to non-responders; however, the two week time point selected for imaging was too early to allow prediction of long term clinical benefit from chemotherapy. FLT-PET and FLT-PET(KSF) detected changes in proliferation in liver metastases.

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-(11)C]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-(11)C]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.

Evaluation of deuterated 18F- and 11C-labeled choline analogs for cancer detection by positron emission tomography

PURPOSE

(11)C-Choline-positron emission tomography (PET) has been exploited to detect the aberrant choline metabolism in tumors. Radiolabeled choline uptake within the imaging time is primarily a function of transport, phosphorylation, and oxidation. Rapid choline oxidation, however, complicates interpretation of PET data. In this study, we investigated the biologic basis of the oxidation of deuterated choline analogs and assessed their specificity in human tumor xenografts.

EXPERIMENTAL DESIGN

(11)C-Choline, (11)C-methyl-[1,2-(2)H(4)]-choline ((11)C-D4-choline), and (18)F-D4-choline were synthesized to permit comparison. Biodistribution, metabolism, small-animal PET studies, and kinetic analysis of tracer uptake were carried out in human colon HCT116 xenograft-bearing mice.

RESULTS

Oxidation of choline analogs to betaine was highest with (11)C-choline, with reduced oxidation observed with (11)C-D4-choline and substantially reduced with (18)F-D4-choline, suggesting that both fluorination and deuteration were important for tracer metabolism. Although all tracers were converted intracellularly to labeled phosphocholine (specific signal), the higher rate constants for intracellular retention (K(i) and k(3)) of (11)C-choline and (11)C-D4-choline, compared with (18)F-D4-choline, were explained by the rapid conversion of the nonfluorinated tracers to betaine within HCT116 tumors. Imaging studies showed that the uptake of (18)F-D4-choline in three tumors with similar radiotracer delivery (K(1)) and choline kinase α expression-HCT116, A375, and PC3-M-were the same, suggesting that (18)F-D4-choline has utility for cancer detection irrespective of histologic type.

CONCLUSION

We have shown here that both deuteration and fluorination combine to provide protection against choline oxidation in vivo. (18)F-D4-choline showed the highest selectivity for phosphorylation and warrants clinical evaluation.

Quantification of receptor-ligand binding with [¹⁸F]fluciclatide in metastatic breast cancer patients

PURPOSE

The purpose of the study was to estimate the receptor-ligand binding of an arginine-glycine-aspartic acid (RGD) peptide in somatic tumours. To this aim, we employed dynamic positron emission tomography (PET) data obtained from breast cancer patients with metastases, studied with the α(v)β(3/5) integrin receptor radioligand [(18)F]fluciclatide.

METHODS

First, compartmental modelling and spectral analysis with arterial input function were performed at the region of interest (ROI) level in healthy lung and liver, and in lung and liver metastases; compartmental modelling was also carried out at the pixel level. The selection of the most appropriate indexes for tumour/healthy tissue differentiation and for estimation of specific binding was then assessed.

RESULTS

The two-tissue reversible model emerged as the best according to the Akaike Information Criterion. Spectral analysis confirmed the reversibility of tracer kinetics. Values of kinetic parameters, estimated as mean from parametric maps, correlated well with those computed from ROI analysis. The volume of distribution V(T) was on average higher in lung metastases than in the healthy lung, but lower in liver metastases than in the healthy liver. In agreement with the expected higher α(v)β(3/5) expression in pathology, k(3) and k(3)/k(4) were both remarkably higher in metastases, which makes them more suitable than V(T) for tumour/healthy tissue differentiation. The ratio k(3)/k(4), in particular, appeared a reasonable measure of specific binding.

CONCLUSION

Besides establishing the best quantitative approaches for the analysis of [(18)F]fluciclatide data, this study indicated that the k(3)/k(4) ratio is a reasonable measure of specific binding, suggesting that this index can be used to estimate α(v)β(3/5) receptor expression in oncology, although further studies are necessary to validate this hypothesis.

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

A spectral analysis approach was used to estimate kinetic model parameters of the L-[1-(11)C]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 (K(1)), blood volume fraction (V(b)), fraction of unlabeled leucine in the tissue precursor pool for protein synthesis derived from arterial plasma (lambda), 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-(11)C]leucine PET data, there was good agreement between ROI-based SAIF estimates and average voxelwise BFM estimates of K(1), V(b), lambda, 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.