Positron emission tomography compartmental models

Assessment of Simplified Methods for Quantification of 18F-FDHT Uptake in Patients with Metastatic Castration-Resistant Prostate Cancer

¹⁸F-fluorodihydrotestosterone (¹⁸F-FDHT) PET/CT potentially provides a noninvasive method for assessment of androgen receptor expression in patients with metastatic castration-resistant prostate cancer (mCRPC). The objective of this study was to assess simplified methods for quantifying ¹⁸F-FDHT uptake in mCRPC patients and to assess effects of tumor perfusion on these ¹⁸F-FDHT uptake metrics. Methods: Seventeen mCRPC patients were included in this prospective observational multicenter study. Test and retest 30-min dynamic ¹⁸F-FDHT PET/CT scans with venous blood sampling were performed in 14 patients. In addition, arterial blood sampling and dynamic ¹⁵O-H₂O scans were obtained in a subset of 6 patients. Several simplified methods were assessed: Patlak plots; SUV normalized to body weight (SUV_BW), lean body mass (SUV_LBM), whole blood (SUV_WB), parent plasma activity concentration (SUV_PP), area under the parent plasma curve (SUV_AUC,PP), and area under the whole-blood input curve (SUV_AUC,WB); and SUV_BW corrected for sex hormone-binding globulin levels (SUV_SHBG). Results were correlated with parameters derived from full pharmacokinetic ¹⁸F-FDHT and ¹⁵O-H₂O. Finally, the repeatability of individual quantitative uptake metrics was assessed. Results: Eighty-seven ¹⁸F-FDHT-avid lesions were evaluated. ¹⁸F-FDHT uptake was best described by an irreversible 2-tissue-compartment model. Replacing the continuous metabolite-corrected arterial plasma input function with an image-derived input function in combination with venous sample data provided similar K _i results (R ² = 0.98). Patlak K _i and SUV_AUC,PP showed an excellent correlation (R ² > 0.9). SUV_BW showed a moderate correlation to K _i (R ² = 0.70, presumably due to fast ¹⁸F-FDHT metabolism. When calculating SUV_SHBG, correlation to K _i improved (R ² = 0.88). The repeatability of full kinetic modeling parameters was inferior to that of simplified methods (repeatability coefficients > 36% vs. < 28%, respectively). ¹⁸F-FDHT uptake showed minimal blood flow dependency. Conclusion: ¹⁸F-FDHT kinetics in mCRPC patients are best described by an irreversible 2-tissue-compartment model with blood volume parameter. SUV_AUC,PP showed a near-perfect correlation with the irreversible 2-tissue-compartment model analysis and can be used for accurate quantification of ¹⁸F-FDHT uptake in whole-body PET/CT scans. In addition, SUV_SHBG could potentially be used as an even simpler method to quantify ¹⁸F-FDHT uptake when less complex scanning protocols and accuracy are required.

Imaging histamine H3 receptors with [18 F]FMH3: Test-retest and occupancy studies in the non-human primate

A positron emission tomography (PET) radioligand, [¹⁸ F]FMH3, has been developed to interrogate histamine receptor subtype 3 (H3R), where dysfunction at this site is linked with obesity, sleep abnormality, and cognitive disorders. [¹⁸ F]FMH3 was evaluated for imaging central H3R sites in non-human primates through test-retest (TRT) and dose-receptor occupancy studies with two selective H3R antagonists in order to support clinical investigations. Two adult female baboons underwent [¹⁸ F]FMH3 PET brain scans in the HR+, at repeated baseline (n = 7) and following administration of escalating doses of ABT-239 (0.003-0.1m/kg, n = 4) and ciproxifan (0.5-2.1 mg/kg, n = 7). Volume of distribution (V_T ) in brain regions was estimated using the 2-tissue compartment model. TRT variability of V_T across repeated baseline scans was reported as % coefficient of variation (COV). ABT-239 and ciproxifan occupancy at H3R was estimated using the occupancy plot, and the relationship of occupancy with dose and plasma levels was determined. In baboons, distribution of [¹⁸ F]FMH3 was high in the striatum, intermediate in cortical regions, and low in the brain stem. COV of baseline V_T was 7.0 ± 3.5%, averaged across regions and animals. Dose-dependent effects of ABT-239 and ciproxifan measured the brain. ED50 and EC50, respectively, were 0.011 mg/kg and 0.942 ng/ml for ABT-239 and 0.73 mg/kg and 208.3 ng/ml for ciproxifan. [¹⁸ F]FMH3 demonstrated high TRT reliability and can be used to measure occupancy of H3R-targeted drugs. Validation in non-human primates support [¹⁸ F]FMH3 PET studies toward clinical investigations of H3R.

High Temporal-Resolution Dynamic PET Image Reconstruction Using a New Spatiotemporal Kernel Method

Current clinical dynamic PET has an effective temporal resolution of 5-10 seconds, which can be adequate for traditional compartmental modeling but is inadequate for exploiting the benefit of more advanced tracer kinetic modeling for characterization of diseases (e.g., cancer and heart disease). There is a need to improve dynamic PET to allow fine temporal sampling of 1-2 seconds. However, the reconstruction of these short-time frames from tomographic data is extremely challenging as the count level of each frame is very low and high noise presents in both spatial and temporal domains. Previously, the kernel framework has been developed and demonstrated as a statistically efficient approach to utilizing image prior for low-count PET image reconstruction. Nevertheless, the existing kernel methods mainly explore spatial correlations in the data and only have a limited ability in suppressing temporal noise. In this paper, we propose a new kernel method which extends the previous spatial kernel method to the general spatiotemporal domain. The new kernelized model encodes both spatial and temporal correlations obtained from image prior information and are incorporated into the PET forward projection model to improve themaximumlikelihood(ML) image reconstruction. Computer simulations and an application to real patient scan have shown that the proposed approach can achieve effective noise reduction in both spatial and temporal domains and outperform the spatial kernel method and conventional ML reconstruction method for improving the high temporal-resolution dynamic PET imaging.

Novel Kappa Opioid Receptor Agonist as Improved PET Radiotracer: Development and in Vivo Evaluation

The kappa opioid receptor (KOR) is involved in depression, alcoholism, and drug abuse. The current agonist radiotracer ¹¹C-GR103545 is not ideal for imaging KOR due to its slow tissue kinetics in human. The aim of our project was to develop novel KOR agonist radiotracers with improved imaging properties. A novel compound FEKAP ((( R))-4-(2-(3,4-dichlorophenyl)acetyl)-3-((ethyl(2-fluoroethyl)amino)methyl) piperazine-1-carboxylate) was designed, synthesized, and assayed for in vitro binding affinities. It was then radiolabeled and evaluated in rhesus monkeys. Baseline and blocking scans were conducted on a Focus-220 scanner to assess binding specificity and selectivity. Metabolite-corrected arterial activities over time were measured and used as input functions to analyze the brain regional time-activity curves and derive kinetic and binding parameters with kinetic modeling. FEKAP displayed high KOR binding affinity ( K_i = 0.43 nM) and selectivity (17-fold over mu opioid receptor and 323-fold over delta opioid receptor) in vitro. ¹¹C-FEKAP was prepared in high molar activity (mean of 718 GBq/μmol, n = 19) and >99% radiochemical purity. In monkeys, ¹¹C-FEKAP metabolized fairly fast, with ∼31% of intact parent fraction at 30 min post-injection. In the brain, it exhibited fast and reversible kinetics with good uptake. Pretreatment with the nonselective opioid receptor antagonist naloxone (1 mg/kg) decreased uptake in high binding regions to the level in the cerebellum, and the selective KOR antagonist LY2456302 (0.02 and 0.1 mg/kg) reduced ¹¹C-FEKAP specific binding in a dose-dependent manner. As a measure of specific binding signals, the mean binding potential ( BP_ND) values of ¹¹C-FEKAP derived from the multilinear analysis-1 (MA1) method were greater than 0.5 for all regions, except for the thalamus. The novel KOR agonist tracer ¹¹C-FEKAP demonstrated binding specificity and selectivity in vivo and exhibited attractive properties of fast tissue kinetics and high specific binding.

Development and In Vivo Evaluation of a κ-Opioid Receptor Agonist as a PET Radiotracer with Superior Imaging Characteristics

Studies have shown κ-opioid receptor (KOR) abnormalities in addictive disorders, other central nervous system diseases, and Alzheimer's disease. We have developed the first set of agonist ¹¹C-GR103545 and antagonist ¹¹C-LY2795050 radiotracers for PET imaging of KOR in humans. Nonetheless, ¹¹C-GR103545 displays protracted uptake kinetics and is not an optimal radiotracer. Here, we report the development and evaluation of ¹¹C-methyl-(R)-4-(2-(3,4-dichlorophenyl)acetyl)-3-((diethylamino)methyl)piperazine-1-carboxylate (¹¹C-EKAP) and its comparison with ¹¹C-GR103545. Methods: EKAP was synthesized and assayed for in vitro binding affinities and then radiolabeled. PET studies were performed on rhesus monkeys. Blocking studies were performed with naloxone and the selective KOR antagonists LY2795050 and LY2456302. Arterial input functions were generated for use in kinetic modeling. Brain TACs were analyzed with multilinear analysis 1 to derive binding parameters. Results: EKAP has high KOR affinity (inhibition constant, 0.28 nM) and good selectivity in vitro. ¹¹C-EKAP was prepared in good radiochemical purity. ¹¹C-EKAP rapidly metabolized in plasma and displayed fast and reversible kinetics in brain, with peak uptake at less than 20 min after injection. Preblocking with naloxone (1 mg/kg) or LY2795050 (0.2 mg/kg) produced 84%-89% receptor occupancy, whereas LY2456302 (0.05 and 0.3 mg/kg) dose-dependently reduced ¹¹C-EKAP-specific binding, thus demonstrating its binding specificity and selectivity in vivo. Mean multilinear analysis 1-derived nondisplaceable binding potential values were 1.74, 1.79, 1.46, 0.80, and 0.77 for cingulate cortex, globus pallidus, insula, striatum, and frontal cortex, respectively, consistent with the known KOR distribution in primate brains. Conclusion: We have successfully developed ¹¹C-EKAP as a KOR agonist tracer with dual attractive imaging properties of fast uptake kinetics and high specific binding in vivo.

Simultaneous estimation and segmentation from projection data in dynamic PET

PURPOSE

Dynamic positron emission tomography (PET) is known for its ability to extract spatiotemporal information of a radio tracer in living tissue. Information of different functional regions based on an accurate reconstruction of the activity images and kinetic parametric images has been widely studied and can be useful in research and clinical setting for diagnosis and other quantitative tasks. In this paper, our purpose is to present a novel framework for estimating the kinetic parametric images directly from the raw measurement data together with a simultaneous segmentation accomplished through kinetic parameters clustering.

METHOD

An iterative framework is proposed to estimate the kinetic parameter image, activity map and do the segmentation simultaneously from the complete dynamic PET projection data. The clustering process is applied to the kinetic parameter variable rather than to the traditional activity distribution so as to achieve accurate discrimination between different functional areas. Prior information such as total variation regularization is incorporated to reduce the noise in the PET images and a sparseness constraint is integrated to guarantee the solution for kinetic parameters due to the over complete dictionary. Alternating direction method of multipliers (ADMM) method is used to solve the optimization problem. The proposed algorithm was validated with experiments on Monte Carlo-simulated phantoms and real patient data. Symbol error rate (SER) was defined to evaluate the performance of clustering. Bias and variance of the reconstruction activity images were calculated based on ground truth. Relative mean square error (MSE) was used to evaluate parametric results quantitatively.

RESULT

In brain phantom experiment, when counting rate is 1 × 10⁶ , the bias (variance) of our method is 0.1270 (0.0281), which is lower than maximum likelihood expectation maximization (MLEM) 0.1637 (0.0410) and direct estimation without segmentation (DE) 0.1511 (0.0326). In the Zubal phantom experiment, our method has the lowest bias (variance) 0.1559 (0.0354) with 1 × 10⁵ counting rate, compared with DE 0.1820 (0.0435) and MLEM 0.3043 (0.0644). As for classification, the SER of our method is 18.87% which is the lowest among MLEM + k-means, DE + k-means, and kinetic spectral clustering (KSC). Brain data with MR reference and real patient results also show that the proposed method can get images with clear structure by visual inspection.

CONCLUSION

In this paper, we presented a joint reconstruction framework for simultaneously estimating the activity distribution, parametric images, and parameter-based segmentation of the ROIs into different functional areas. Total variation regularization is performed on the activity distribution domain to suppress noise and preserve the edges between ROIs. An over complete dictionary for time activity curve basis is constructed. SER, bias, variance, and MSE were calculated to show the effectiveness of the proposed method.

In Vitro Ligand Binding Kinetics Explains the Pharmacokinetics of [18F]FE-PE2I in Dopamine Transporter PET Imaging

Two of the most popular positron emission tomography (PET) tracers, [¹¹C]PE2I and [¹⁸F]FE-PE2I, used to quantify dopamine transporters (DAT), display dissimilar kinetic behavior in in vivo assays. This difference can be explained by comparing values of kinetic rate constants, which characterize interaction of these tracers with DAT sites in vitro. At the same time, this kinetic analysis showed that the overall binding mechanism is similar for these two tracers and includes a fast step of complex formation followed by a slow isomerization step of this complex. Comparison with previous PE2I data revealed that isomerization of the DAT complex with PE2I occurs three times faster than in the case of FE-PE2I, which leads to the slower onset of peak specific binding of the former tracer in the DAT-rich regions. Therefore, ligands with slower isomerization on-rate, including [¹⁸F]FE-PE2I, seem to be better tracers in vivo, and their properties can be predicted in vitro.

Parametric Imaging of [11C]Flumazenil Binding in the Rat Brain

Purpose

This study evaluates the performance of several parametric methods for assessing [¹¹C]flumazenil binding distribution in the rat brain.

Procedures

Dynamic (60 min) positron emission tomography data with metabolite-corrected plasma input function were retrospectively analyzed (male Wistar rats, n = 10). Distribution volume (V_T) images were generated from basis function method (BFM), Logan graphical analysis (Logan), and spectral analysis (SA). Using the pons as pseudo-reference tissue, binding potential (BP_ND and DVR–1) images were obtained from receptor parametric imaging algorithms (RPM and SRTM2) and reference Logan (RLogan). Standardized uptake value images (SUV and SUVR) were also computed for different intervals post-injection. Next, regional averages were extracted from the parametric images, using pre-defined volumes of interest, which were also applied to the regional time-activity curves from the dynamic data. Parametric data were compared to their regional counterparts and to two-tissue compartment model (2TCM)-based values (previously defined as the model of choice for rats). Parameter agreement was assessed by linear regression analysis and Bland-Altman plots.

Results

All parametric methods strongly correlated to their regional counterparts (R² > 0.97) and to the 2TCM values (R² ≥ 0.95). SA and RLogan underestimated V_T and BP_ND (slope of 0.93 and 0.86, respectively), while SUVR-1 overestimated BP_ND (slope higher than 1.07 for all intervals). While BFM and SRTM2 had the smallest bias to 2TCM values (0.05 for both), ratio Bland-Altman plots showed Logan and RLogan displayed relative errors which were comparable between different regions, in contrast with the other methods. Although SUV consistently underestimated V_T, the bias in this method was also constant across regions.

Conclusions

All parametric methods performed well for the analysis of [¹¹C]flumazenil distribution and binding in the rat brain. However, Logan and RLogan slightly outperformed the other methods in terms of precision, providing robust parameter estimation and constant bias. Yet, other methods can be of interest, because they can provide tissue perfusion (i.e., K₁ with BFM and SA), relative flow (i.e., R₁ with RPM and SRTM2), and model order (SA) images.

Disease-related patterns of in vivo pathology in Corticobasal syndrome

PURPOSE

To assess disease-related patterns of in vivo pathology in 11 patients with Corticobasal Syndrome (CBS) compared to 20 healthy controls and 33 mild cognitive impairment (MCI) patients due to Alzheimer's disease.

METHODS

We assessed tau aggregates with [18F]AV1451 PET, amyloid-β depositions with [18F]AV45 PET, and volumetric microstructural changes with MRI. We validated for [18F]AV1451 standardised uptake value ratio (SUVRs) against input functions from arterial metabolites and found that SUVRs and arterial-derived distribution volume ratio (DVRs) provide equally robust measures of [18F]AV1451 binding.

RESULTS

CBS patients showed increases in [18F]AV1451 SUVRs in parietal (P < 0.05) and frontal (P < 0.05) cortices in the affected hemisphere compared to healthy controls and in precentral (P = 0.008) and postcentral (P = 0.034) gyrus in the affected hemisphere compared to MCI patients. Our data were confirmed at the histopathological level in one CBS patient who underwent brain biopsy and showed sparse tau pathology in the parietal cortex co-localizing with increased [18F]AV1451 signal. Cortical and subcortical [18F]AV45 uptake was within normal levels in CBS patients. In parietal and frontal cortices of the most affected hemisphere we found also grey matter loss (P < 0.05), increased mean diffusivity (P < 0.05) and decreased fractional anisotropy (P < 0.05) in CBS patients compared to healthy controls and MCI patients. Grey matter loss and white matter changes in the precentral gyrus of CBS patients were associated with worse motor symptoms.

CONCLUSIONS

Our findings demonstrate disease-related patterns of in vivo tau and microstructural pathology in the absence of amyloid-β, which distinguish CBS from non-affected individuals and MCI patients.

Quantitative assessment of dynamic 18F-flumethycholine PET and dynamic contrast enhanced MRI in high risk prostate cancer

OBJECTIVE:

To describe dynamic ¹⁸F-flumethycholine PET (dPET) and dynamic contrast enhancement MR (DCE MR) parameters in localized high-risk prostate cancer (PCa), and determine whether these differ from normal prostate. Furthermore, to determine whether a correlation exists between dPET and DCE MR parameters.

METHODS:

41 consenting patients who underwent prostate DCE MR and dPET were included in this institutionally approved study. Intraprostatic lesions on MR were assigned a PI-RADS v2 score, and focal lesions on PET were documented. All lesions were correlated with pathology. Quantitative and semi-quantitative DCE MR and two-tissue compartmental model dPET parameters were determined and tumor-to-normal gland ratios (T/N) for these parameters were calculated. Finally, dPET and DCE MR correlation was estimated using Spearman correlation coefficients.

RESULTS:

There were 46 malignant lesions per standard of reference. On dPET, peripheral zone (PZ) tumors had higher K1 (p < 0.001), and a T/N ratio ≥2 was significant (p < 0.001). On DCE MR, the parameters in, kep, Ktrans and quantitative iAUC were higher for PZ and non-PZ tumors than corresponding normal tissue (p < 0.001); for PZ tumors, a T/N ratio ≥ 1.5 for Ktrans and pei was significant (p = 0.0019 and 0.0026, respectively). Moderate Spearman correlation (0.40 < ρ < 0.59) was found between dPET K1 and DCE MR Ktrans and pei.

CONCLUSION:

In patients with high-risk PCa, quantitative dPET and DCE-MR parameters in primary tumors differ from normal tissue. Only moderate correlation exists between K1 (dPET) and Ktrans and pei (DCE MR). The incremental value of any of these parameters to PI-RADS v2 warrants further investigation.

ADVANCES IN KNOWLEDGE:

Unique quantitative and semi-quantitative FCH PET/MR parameters in PCa differ from normal gland, and should be further investigated to determine their potential contribution to PI-RADS v2 in the detection of clinically significant PCa.

In vivo evaluation of [11C]TMI, a COX-2 selective PET tracer, in baboons

Overexpression of Cyclooxygenase-2 (COX-2) enzyme is associated with the pathogenesis of inflammation, cancers, stroke, arthritis, and neurological disorders. Because of the involvement of COX-2 in these diseases, quantification of COX-2 expression using Positron Emission Tomography (PET) may be a biological marker for early diagnosis, monitoring of disease progression, and an indicator of effective treatment. At present there is no target-specific or validated PET tracer available for in vivo quantification of COX-2. The objective of this study is to evaluate [¹¹C]TMI, a selective COX-2 inhibitor (Ki ≤ 1 nM) in nonhuman primates using PET imaging. PET imaging in baboons showed that [¹¹C]TMI penetrates the blood brain barrier (BBB) and accumulates in brain in a somewhat heterogeneous pattern. Metabolite analyses indicated that [¹¹C]TMI undergoes no significant metabolism of parent tracer in the plasma for baseline scans, however a relative faster metabolism was found for blocking scan. All the tested quantification approaches provide comparable tracer total distribution volume (V_T) estimates in the range of 3.2-7 (mL/cm³). We observed about 25% lower V_T values in blocking studies with meloxicam, a nonselective COX-2 inhibitor, compared to baseline [¹¹C]TMI binding. Our findings indicate that [¹¹C]TMI may be a suitable PET tracer for the quantification of COX-2 in vivo. Further experiments are needed to confirm the potential of this tracer in COX-2 overexpressing models for brain diseases.

Functional Data Analysis of Dynamic PET Data

One application of positron emission tomography (PET), a nuclear imaging technique, in neuroscience involves in vivo estimation of the density of various proteins (often, neuroreceptors) in the brain. PET scanning begins with the injection of a radiolabeled tracer that binds preferentially to the target protein; tracer molecules are then continuously delivered to the brain via the bloodstream. By detecting the radioactive decay of the tracer over time, dynamic PET data are constructed to reflect the concentration of the target protein in the brain at each time. The fundamental problem in the analysis of dynamic PET data involves estimating the impulse response function (IRF), which is necessary for describing the binding behavior of the injected radiotracer. Virtually all existing methods have three common aspects: summarizing the entire IRF with a single scalar measure; modeling each subject separately; and the imposition of parametric restrictions on the IRF. In contrast, we propose a functional data analytic approach that regards each subject's IRF as the basic analysis unit, models multiple subjects simultaneously, and estimates the IRF nonparametrically. We pose our model as a linear mixed effect model in which population level fixed effects and subject-specific random effects are expanded using a B-spline basis. Shrinkage and roughness penalties are incorporated in the model to enforce identifiability and smoothness of the estimated curves, respectively, while monotonicity and non-negativity constraints impose biological information on estimates. We illustrate this approach by applying it to clinical PET data with subjects belonging to three diagnosic groups. We explore differences among groups by means of pointwise confidence intervals of the estimated mean curves based on bootstrap samples.

Hybrid PET/MRI Methodology

The hybrid PET/MR scanner represents the first implementation of the effective integration of two modalities allowing truly synchronous/simultaneous acquisition of their imaging signals. This integration, resulting from the innovation and development of specific hardware components has paved the way for new approaches in the study of neurodegenerative diseases. This chapter will describe the hardware development that has led to the availability of different clinical solutions for PET/MR imaging as well as the still-open technological challenges and opportunities related to the processing and exploitation of the simultaneous acquisition in neurological studies.

Advances in PET Methodology

Recent advances in disease understanding, instrumentation technology, and computationally demanding image analysis approaches are opening new frontiers in the investigation of movement disorders and brain disease in general. A key aspect is the recognition of the need to determine molecular correlates to early functional and metabolic connectivity alterations, which are increasingly recognized as useful signatures of specific clinical disease phenotypes. Such multi-modal approaches are highly likely to provide new information on pathogenic mechanisms and to help the identification of novel therapeutic targets. This chapter describes recent methodological developments in PET starting with a very brief overview of radiotracers relevant to movement disorders while emphasizing the development of instrumentation, algorithms and imaging analysis methods relevant to multi-modal investigation of movement disorders.

In vivo quantification of glial activation in minipigs overexpressing human α-synuclein

Parkinson's disease is characterized by a progressive loss of substantia nigra (SN) dopaminergic neurons and the formation of Lewy bodies containing accumulated alpha-synuclein (α-syn). The pathology of Parkinson's disease is associated with neuroinflammatory microglial activation, which may contribute to the ongoing neurodegeneration. This study investigates the in vivo microglial and dopaminergic response to overexpression of α-syn. We used positron emission tomography (PET) and the 18 kDa translocator protein radioligand, [¹¹ C](R)PK11195, to image brain microglial activation and (+)-α-[¹¹ C]dihydrotetrabenazine ([¹¹ C]DTBZ), to measure vesicular monoamine transporter 2 (VMAT2) availability in Göttingen minipigs following injection with recombinant adeno-associated virus (rAAV) vectors expressing either mutant A53T α-syn or green fluorescent protein (GFP) into the SN (4 rAAV-α-syn, 4 rAAV-GFP, 5 non-injected control minipigs). We performed motor symptom assessment and immunohistochemical examination of tyrosine hydroxylase (TH) and transgene expression. Expression of GFP and α-syn was observed at the SN injection site and in the striatum. We observed no motor symptoms or changes in striatal [¹¹ C]DTBZ binding potential in vivo or striatal or SN TH staining in vitro between the groups. The mean [¹¹ C](R)PK11195 total volume of distribution was significantly higher in the basal ganglia and cortical areas of the α-syn group than the control animals. We conclude that mutant α-syn expression in the SN resulted in microglial activation in multiple sub- and cortical regions, while it did not affect TH stains or VMAT2 availability. Our data suggest that microglial activation constitutes an early response to accumulation of α-syn in the absence of dopamine neuron degeneration.

Comparison of two different methods of image analysis for the assessment of microglial activation in patients with multiple sclerosis using (R)-[N-methyl-carbon-11]PK11195

Chronic active multiple sclerosis (MS) lesions have a rim of activated microglia/macrophages (m/M) leading to ongoing tissue damage, and thus represent a potential treatment target. Activation of this innate immune response in MS has been visualized and quantified using PET imaging with [¹¹C]-(R)-PK11195 (PK). Accurate identification of m/M activation in chronic MS lesions requires the sensitivity to detect lower levels of activity within a small tissue volume. We assessed the ability of kinetic modeling of PK PET data to detect m/M activity in different central nervous system (CNS) tissue regions of varying sizes and in chronic MS lesions. Ten patients with MS underwent a single brain MRI and two PK PET scans 2 hours apart. Volume of interest (VOI) masks were generated for the white matter (WM), cortical gray matter (CGM), and thalamus (TH). The distribution volume (V_T) was calculated with the Logan graphical method (LGM-V_T) utilizing an image-derived input function (IDIF). The binding potential (BP_ND) was calculated with the reference Logan graphical method (RLGM) utilizing a supervised clustering algorithm (SuperPK) to determine the non-specific binding region. Masks of varying volume were created in the CNS to assess the impact of region size on the various metrics among high and low uptake regions. Chronic MS lesions were also evaluated and individual lesion masks were generated. The highest PK uptake occurred the TH and lowest within the WM, as demonstrated by the mean time activity curves. In the TH, both reference and IDIF based methods resulted in estimates that did not significantly depend on VOI size. However, in the WM, the test-retest reliability of BP_ND was significantly lower in the smallest VOI, compared to the estimates of LGM-V_T. These observations were consistent for all chronic MS lesions examined. In this study, we demonstrate that BP_ND and LGM-V_T are both reliable for quantifying m/M activation in regions of high uptake, however with blood input function LGM-V_T is preferred to assess longitudinal m/M activation in regions of relatively low uptake, such as chronic MS lesions.

In Vivo Brain Imaging, Biodistribution, and Radiation Dosimetry Estimation of [11C]Celecoxib, a COX-2 PET Ligand, in Nonhuman Primates

COX-2 selective inhibitors (COXIBs) are non-steroidal anti-inflammatory drugs (NSAIDs), with fewer side effects compared with non-selective NSAIDs, and are used for the treatment of arthritis, headaches, and other inflammatory diseases of the brain and peripheral tissues. Radiolabeled COXIBs may permit positron emission tomography (PET) imaging of COX-2 localization and activity in diseases, enable monitoring of inflammatory processes, and determine target occupancy of COX-2 activity by NSAIDs, thus, accelerating the development of novel CIXIBs. We synthesized [¹¹C]celecoxib, one of the COXIBs and a prescription drug, and here report its in vivo uptake in the brain, whole body biodistribution, and radiation dosimetry in baboons using PET. Brain imaging experiments were performed in one baboon and whole body PET scans were performed in triplicates in two male baboons using an ECAT ACCEL (Siemens Medical Solutions, Inc. Knoxville) under anesthetic conditions. PET studies in baboons show that [¹¹C]celecoxib penetrates the blood brain barrier (BBB) and accumulates in the brain, followed by a washout of radioactivity. The liver has the highest residence time and the gallbladder is the critical organ for [¹¹C]celecoxib. Organ Level Internal Dose Assessment (OLINDA) estimates indicate that the maximum permissible single study dosage of [¹¹C]celecoxib in humans is 1110 MBq (30 mCi) for both males and females under the 21 CFR 361.1 dose limit for research subjects.

Accelerated PET kinetic maps estimation by analytic fitting method

In this work, we propose and test a new approach for non-linear kinetic parameters' estimation from dynamic PET data. A technique is discussed, to derive an analytical closed-form expression of the compartmental model used for kinetic parameters' evaluation, using an auxiliary parameter set, with the aim of reducing the computational burden and speeding up the fitting of these complex mathematical expressions to noisy TACs. Two alternative algorithms based on numeric calculations are considered and compared to the new proposal. We perform a simulation study aimed at (i) assessing agreement between the proposed method and other conventional ways of implementing compartmental model fitting, and (ii) quantifying the reduction in computational time required for convergence. It results in a speed-up factor of ∼120 when compared to a fully numeric version, or ∼38, with respect to a more conventional implementation, while converging to very similar values for the estimated model parameters. The proposed method is also tested on dynamic 3D PET clinical data of four control subjects. The results obtained supported those of the simulation study, and provided input and promising perspectives for the application of the proposed technique in clinical practice.

Quantification of O-(2-[18F]fluoroethyl)-L-tyrosine kinetics in glioma

BACKGROUND

This study identified the optimal tracer kinetic model for quantification of dynamic O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) positron emission tomography (PET) studies in seven patients with diffuse glioma (four glioblastoma, three lower grade glioma). The performance of more simplified approaches was evaluated by comparison with the optimal compartment model. Additionally, the relationship with cerebral blood flow-determined by [15O]H2O PET-was investigated.

RESULTS

The optimal tracer kinetic model was the reversible two-tissue compartment model. Agreement analysis of binding potential estimates derived from reference tissue input models with the distribution volume ratio (DVR)-1 derived from the plasma input model showed no significant average difference and limits of agreement of - 0.39 and 0.37. Given the range of DVR-1 (- 0.25 to 1.5), these limits are wide. For the simplified methods, the 60-90 min tumour-to-blood ratio to parent plasma concentration yielded the highest correlation with volume of distribution VT as calculated by the plasma input model (r = 0.97). The 60-90 min standardized uptake value (SUV) showed better correlation with VT (r = 0.77) than SUV based on earlier intervals. The 60-90 min SUV ratio to contralateral healthy brain tissue showed moderate agreement with DVR with no significant average difference and limits of agreement of - 0.24 and 0.30. A significant but low correlation was found between VT and CBF in the tumour regions (r = 0.61, p = 0.007).

CONCLUSION

Uptake of [18F]FET was best modelled by a reversible two-tissue compartment model. Reference tissue input models yielded estimates of binding potential which did not correspond well with plasma input-derived DVR-1. In comparison, SUV ratio to contralateral healthy brain tissue showed slightly better performance, if measured at the 60-90 min interval. SUV showed only moderate correlation with VT. VT shows correlation with CBF in tumour.

In vivo quantification of neurofibrillary tangles with [18F]MK-6240

BACKGROUND

Imaging agents capable of quantifying the brain's tau aggregates will allow a more precise staging of Alzheimer's disease (AD). The aim of the present study was to examine the in vitro properties as well as the in vivo kinetics, using gold standard methods, of the novel positron emission tomography (PET) tau imaging agent [¹⁸F]MK-6240.

METHODS

In vitro properties of [¹⁸F]MK-6240 were estimated with autoradiography in postmortem brain tissues of 14 subjects (seven AD patients and seven age-matched controls). In vivo quantification of [¹⁸F]MK-6240 binding was performed in 16 subjects (four AD patients, three mild cognitive impairment patients, six healthy elderly individuals, and three healthy young individuals) who underwent 180-min dynamic scans; six subjects had arterial sampling for metabolite correction. Simplified approaches for [¹⁸F]MK-6240 quantification were validated using full kinetic modeling with metabolite-corrected arterial input function. All participants also underwent amyloid-PET and structural magnetic resonance imaging.

RESULTS

In vitro [¹⁸F]MK-6240 uptake was higher in AD patients than in age-matched controls in brain regions expected to contain tangles such as the hippocampus, whereas no difference was found in the cerebellar gray matter. In vivo, [¹⁸F]MK-6240 displayed favorable kinetics with rapid brain delivery and washout. The cerebellar gray matter had low binding across individuals, showing potential for use as a reference region. A reversible two-tissue compartment model well described the time-activity curves across individuals and brain regions. Distribution volume ratios using the plasma input and standardized uptake value ratios (SUVRs) calculated after the binding approached equilibrium (90 min) were correlated and higher in mild cognitive impairment or AD dementia patients than in controls. Reliability analysis revealed robust SUVRs calculated from 90 to 110 min, while earlier time points provided inaccurate estimates.

CONCLUSIONS

This evaluation shows an [¹⁸F]MK-6240 distribution in concordance with postmortem studies and that simplified quantitative approaches such as the SUVR offer valid estimates of neurofibrillary tangle load 90 min post injection. [¹⁸F]MK-6240 is a promising tau tracer with the potential to be applied in the disease diagnosis and assessment of therapeutic interventions.

The Search for a Subtype-Selective PET Imaging Agent for the GABAA Receptor Complex: Evaluation of the Radiotracer [11C]ADO in Nonhuman Primates

The myriad physiological functions of γ-amino butyric acid (GABA) are mediated by the GABA-benzodiazepine receptor complex comprising of the GABA_A, GABA_B, and GABA_C groups. The various GABA_A subunits with region-specific distributions in the brain subserve different functional and physiological roles. For example, the sedative and anticonvulsive effects of classical benzodiazepines are attributed to the α₁ subunit, and the α₂ and α₃ subunits mediate the anxiolytic effect. To optimize pharmacotherapies with improved efficacy and devoid of undesirable side effects for the treatment of anxiety disorders, subtype-selective imaging radiotracers are required to assess target engagement at GABA sites and determine the dose–receptor occupancy relationships. The goal of this work was to characterize, in nonhuman primates, the in vivo binding profile of a novel positron emission tomography (PET) radiotracer, [¹¹C]ADO, which has been indicated to have functional selectivity for the GABA_A α₂/α₃ subunits. High specific activity [¹¹C]ADO was administrated to 3 rhesus monkeys, and PET scans of 120-minute duration were performed on the Focus-220 scanner. In the blood, [¹¹C]ADO metabolized at a fairly rapid rate, with ∼36% of the parent tracer remaining at 30 minutes postinjection. Uptake levels of [¹¹C]ADO in the brain were high (peak standardized uptake value of ∼3.0) and consistent with GABA_A distribution, with highest activity levels in cortical areas, intermediate levels in cerebellum and thalamus, and lowest uptake in striatal regions and amygdala. Tissue kinetics was fast, with peak uptake in all brain regions within 20 minutes of tracer injection. The one-tissue compartment model provided good fits to regional time–activity curves and reliable measurement of kinetic parameters. The absolute test–retest variability of regional distribution volumes (V_T) was low, ranging from 4.5% to 8.7%. Pretreatment with flumazenil (a subtype nonselective ligand, 0.2 mg/kg, intravenous [IV], n = 1), Ro15-4513 (an α₅-selective ligand, 0.03 mg/kg, IV, n = 2), and zolpidem (an α₁-selective ligand, 1.7 mg/kg, IV, n = 1) led to blockade of [¹¹C]ADO binding by 96.5%, 52.5%, and 76.5%, respectively, indicating the in vivo binding specificity of the radiotracer. Using the nondisplaceable volume of distribution (V_ND) determined from the blocking studies, specific binding signals, as measured by values of regional binding potential (BP_ND), ranged from 0.6 to 4.4, which are comparable to those of [¹¹C]flumazenil. In conclusion, [¹¹C]ADO was demonstrated to be a specific radiotracer for the GABA_A receptors with several favorable properties: high brain uptake, fast tissue kinetics, and high levels of specific binding in nonhuman primates. However, subtype selectivity in vivo is not obvious for the radiotracer, and thus, the search for subtype-selective GABA_A radiotracers continues.

Quantification of Tau Load Using [18F]AV1451 PET

Purpose

The tau tracer [¹⁸F]AV1451, also known as flortaucipir, is a promising ligand for imaging tau accumulation in Alzheimer’s disease (AD). Most of the previous studies have quantified tau load using standardized uptake value ratios (SUVr) derived from a static [¹⁸F]AV1451 scan. SUVr may, however, be flow dependent and, especially for longitudinal studies, should be validated against a fully quantitative approach. The objective of this study was to identify the optimal tracer kinetic model for measuring tau load using [¹⁸F]AV1451.

Procedures

Following intravenous injection of 225 ± 16 MBq [¹⁸F]AV1451, 130 min dynamic PET scans were performed in five biomarker confirmed AD patients and five controls. Arterial blood sampling was performed to obtain a metabolite-corrected plasma input function. Next, regional time–activity curves were generated using PVElab software. These curves were analysed using several pharmacokinetic models.

Results

The reversible single tissue compartment model (1T2k_V_B) was the preferred model for all but one control. For AD patients, however, model preference shifted towards a reversible two tissue compartmental model (2T4k_V_B). The simplified reference tissue model (SRTM) derived binding potential (BP_ND) showed good correlation (AD: r² = 0.87, slope = 1.06; controls: r² = 0.87, slope = 0.86) with indirect plasma input binding (distribution volume ratio-1). Standardized uptake value ratios (80–100 min) correlated well with DVR (r² = 0.93, slope = 1.07) and SRTM-derived BP_ND (r² = 0.84, slope = 0.95). In addition, regional differences in tracer binding between subject groups in different tau-specific regions were observed.

Conclusions

Model preference of [¹⁸F]AV1451 appears to depend on subject status and, in particular, V_T. The relationship between model preference and V_T suggests that (higher) tau load may be reflected by a second tissue compartment. Nevertheless, consistent results can be obtained using a 2T4k_V_B model. In addition, SRTM can be used to derive BP_ND.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-017-1080-z) contains supplementary material, which is available to authorized users.

Non-invasive kinetic modelling of PET tracers with radiometabolites using a constrained simultaneous estimation method: evaluation with 11C-SB201745

Background

Kinetic analysis of dynamic PET data requires an accurate knowledge of available PET tracer concentration within blood plasma over time, known as the arterial input function (AIF). The gold standard method used to measure the AIF requires serial arterial blood sampling over the course of the PET scan, which is an invasive procedure and makes this method less practical in clinical settings. Traditional image-derived methods are limited to specific tracers and are not accurate if metabolites are present in the plasma.

Results

In this work, we utilise an image-derived whole blood curve measurement to reduce the computational complexity of the simultaneous estimation method (SIME), which is capable of estimating the AIF directly from tissue time activity curves (TACs). This method was applied to data obtained from a serotonin receptor study (¹¹C-SB207145) and estimated parameter results are compared to results obtained using the original SIME and gold standard AIFs derived from arterial samples. Reproducibility of the method was assessed using test-retest data. It was shown that the incorporation of image-derived information increased the accuracy of total volume of distribution (V _T) estimates, averaged across all regions, by 40% and non-displaceable binding potential (BP _ND) estimates by 16% compared to the original SIME. Particular improvements were observed in K₁ parameter estimates. BP _ND estimates, based on the proposed method and the gold standard arterial sample-derived AIF, were not significantly different (P=0.7).

Conclusions

The results of this work indicate that the proposed method with prior AIF information obtained from a partial volume corrected image-derived whole blood curve, and modelled parent fraction, has the potential to be used as an alternative non-invasive method to perform kinetic analysis of tracers with metabolite products.

Improved quantitation and reproducibility in multi-PET/CT lung studies by combining CT information

Background

Matched attenuation maps are vital for obtaining accurate and reproducible kinetic and static parameter estimates from PET data. With increased interest in PET/CT imaging of diffuse lung diseases for assessing disease progression and treatment effectiveness, understanding the extent of the effect of respiratory motion and establishing methods for correction are becoming more important. In a previous study, we have shown that using the wrong attenuation map leads to large errors due to density mismatches in the lung, especially in dynamic PET scans. Here, we extend this work to the case where the study is sub-divided into several scans, e.g. for patient comfort, each with its own CT (cine-CT and ‘snap shot’ CT). A method to combine multi-CT information into a combined-CT has then been developed, which averages the CT information from each study section to produce composite CT images with the lung density more representative of that in the PET data. This combined-CT was applied to nine patients with idiopathic pulmonary fibrosis, imaged with dynamic ¹⁸F-FDG PET/CT to determine the improvement in the precision of the parameter estimates.

Results

Using XCAT simulations, errors in the influx rate constant were found to be as high as 60% in multi-PET/CT studies. Analysis of patient data identified displacements between study sections in the time activity curves, which led to an average standard error in the estimates of the influx rate constant of 53% with conventional methods. This reduced to within 5% after use of combined-CTs for attenuation correction of the study sections.

Conclusions

Use of combined-CTs to reconstruct the sections of a multi-PET/CT study, as opposed to using the individually acquired CTs at each study stage, produces more precise parameter estimates and may improve discrimination between diseased and normal lung.

A light-fluence-independent method for the quantitative analysis of dynamic contrast-enhanced multispectral optoacoustic tomography (DCE MSOT)

MultiSpectral Optoacoustic Tomography (MSOT) is an emerging imaging technology that allows for data acquisition at high spatial and temporal resolution. These imaging characteristics are advantageous for Dynamic Contrast Enhanced (DCE) imaging that can assess the combination of vascular flow and permeability. However, the quantitative analysis of DCE MSOT data has not been possible due to complications caused by wavelength-dependent light attenuation and variability in light fluence at different anatomical locations. In this work we present a new method for the quantitative analysis of DCE MSOT data that is not biased by light fluence. We have named this method the two-compartment linear standard model (2C-LSM) for DCE MSOT.

4D-PET reconstruction using a spline-residue model with spatial and temporal roughness penalties

4D reconstruction of dynamic positron emission tomography (dPET) data can improve the signal-to-noise ratio in reconstructed image sequences by fitting smooth temporal functions to the voxel time-activity-curves (TACs) during the reconstruction, though the optimal choice of function remains an open question. We propose a spline-residue model, which describes TACs as weighted sums of convolutions of the arterial input function with cubic B-spline basis functions. Convolution with the input function constrains the spline-residue model at early time-points, potentially enhancing noise suppression in early time-frames, while still allowing a wide range of TAC descriptions over the entire imaged time-course, thus limiting bias. Spline-residue based 4D-reconstruction is compared to that of a conventional (non-4D) maximum a posteriori (MAP) algorithm, and to 4D-reconstructions based on adaptive-knot cubic B-splines, the spectral model and an irreversible two-tissue compartment ('2C3K') model. 4D reconstructions were carried out using a nested-MAP algorithm including spatial and temporal roughness penalties. The algorithms were tested using Monte-Carlo simulated scanner data, generated for a digital thoracic phantom with uptake kinetics based on a dynamic [18F]-Fluromisonidazole scan of a non-small cell lung cancer patient. For every algorithm, parametric maps were calculated by fitting each voxel TAC within a sub-region of the reconstructed images with the 2C3K model. Compared to conventional MAP reconstruction, spline-residue-based 4D reconstruction achieved  >50% improvements for five of the eight combinations of the four kinetics parameters for which parametric maps were created with the bias and noise measures used to analyse them, and produced better results for 5/8 combinations than any of the other reconstruction algorithms studied, while spectral model-based 4D reconstruction produced the best results for 2/8. 2C3K model-based 4D reconstruction generated the most biased parametric maps. Inclusion of a temporal roughness penalty function improved the performance of 4D reconstruction based on the cubic B-spline, spectral and spline-residue models.

In vivo imaging biomarkers of neuroinflammation in the development and assessment of stroke therapies - towards clinical translation

Modulation of the inflammatory microenvironment after stroke opens a new avenue for the development of novel neurorestorative therapies in stroke. Understanding the spatio-temporal profile of (neuro-)inflammatory imaging biomarkers in detail thereby represents a crucial factor in the development and application of immunomodulatory therapies. The early integration of quantitative molecular imaging biomarkers in stroke drug development may provide key information about (i) early diagnosis and follow-up, (ii) spatio-temporal drug-target engagement (pharmacodynamic biomarker), (iii) differentiation of responders and non-responders in the patient cohort (inclusion/exclusion criteria; predictive biomarkers), and (iv) the mechanism of action. The use of targeted imaging biomarkers for may thus allow clinicians to decipher the profile of patient-specific inflammatory activity and the development of patient-tailored strategies for immunomodulatory and neuro-restorative therapies in stroke. Here, we highlight the recent developments in preclinical and clinical molecular imaging biomarkers of neuroinflammation (endothelial markers, microglia, MMPs, cell labeling, future developments) in stroke and outline how imaging biomarkers can be used in overcoming current translational roadblocks and attrition in order to advance new immunomodulatory compounds within the clinical pipeline.

Voxel level quantification of [11C]CURB, a radioligand for Fatty Acid Amide Hydrolase, using high resolution positron emission tomography

[11C]CURB is a novel irreversible radioligand for imaging fatty acid amide hydrolase in the human brain. In the present work, we validate an algorithm for generating parametric map images of [11C]CURB acquired with a high resolution research tomograph (HRRT) scanner. This algorithm applies the basis function method on an irreversible two-tissue compartment model (k4 = 0) with arterial input function, i.e., BAFPIC. Monte Carlo simulations are employed to assess bias and variability of the binding macroparameters (Ki and λk3) as a function of the voxel noise level and the range of basis functions. The results show that for a [11C]CURB time activity curve with noise levels corresponding to a voxel of an image acquired with the HRRT and reconstructed with the filtered back projection algorithm, the implementation of BAFPIC requires the use of a constant vascular fraction of tissue (5%) and a cutoff for slow frequencies (0.06 min-1). With these settings, BAFPIC maintains the probabilistic distributions of the binding macroparameters with approximately Gaussian shape and minimizes the bias and variability for large physiological ranges of the rate constants of [11C]CURB. BAFPIC reduces the variability of Ki to a third of that given by Patlak plot, the standard graphical method for irreversible radioligands. Application to real data demonstrated an excellent correlation between region of interest and BAFPIC parametric data and agreed with the simulations results. Therefore, BAFPIC with a constant vascular fraction can be used to generate parametric maps of [11C]CURB images acquired with an HRRT provided that the limits of the basis functions are carefully selected.

Reference tissue models in the assessment of 11 C-DTBZ binding to the VMAT2 in rat striatum: A test-retest reproducibility study

Dopaminergic PET imaging is a useful tool to assess the dopaminergic integrity and to follow-up longitudinal studies. The aim of this study was to evaluate the reliability and reproducibility of different reference tissue-based methods to determine the non-displaceable binding potential (BP_ND ) as a quantitative measure of ¹¹ C-DTBZ binding to the VMAT2 in rat striatum using cerebellum as reference region. Eight healthy Wistar rats underwent two microPET scans at the age of 12 (test) and 20 weeks (retest). BP_ND was determined using the simplified reference tissue model, Logan reference tissue model, and multilinear reference tissue models (MRTMo and MRTM2). Additionally, a striatal-to-cerebellar-ratio (SCR) analysis was performed. The reproducibility between the two scans was assessed using the interclass correlation coefficients (ICC) and the variability index. Repeatability indices showed acceptable ICC = 0.66 (SCR) to excellent ICC = 0.98 (MRTM2) reliability for this study and a variability ranging from 12.26% (SCR) to 3.28% (MRTM2). To the best of our knowledge, this is the first report on longitudinal studies for ¹¹ C-DTBZ in rats using reference tissue methods. Excellent intersubject and intrasubject reproducibility was obtained with the multilinear reference MRTM2, suggesting this as the best method to compare longitudinal studies, whereas the SCR method had poor reliability. Logan method, however, is a method simple to compute that shows accurate reproducibility with a reasonable level of inter- and intra-subject variability allowing crossover studies to follow-up the uptake of ¹¹ C-DTBZ in rat striatum.

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.

Longitudinal uncoupling of cerebral perfusion, glucose metabolism, and tau deposition in Alzheimer's disease

INTRODUCTION

Cross-sectional findings using the tau tracer [¹⁸F]THK5317 (THK5317) have shown that [¹⁸F]fluorodeoxyglucose (FDG) positron emission tomography (PET) data can be approximated using perfusion measures (early-frame standardized uptake value ratio; ratio of tracer delivery in target to reference regions). In this way, a single PET study can provide both functional and molecular information.

METHODS

We included 16 patients with Alzheimer's disease who completed follow-up THK5317 and FDG studies 17 months after baseline investigations. Linear mixed-effects models and annual percentage change maps were used to examine longitudinal change.

RESULTS

Limited spatial overlap was observed between areas showing declines in THK5317 perfusion measures and FDG. Minimal overlap was seen between areas showing functional change and those showing increased retention of THK5317.

DISCUSSION

Our findings suggest a spatiotemporal offset between functional changes and tau pathology and a partial uncoupling between perfusion and metabolism, possibly as a function of Alzheimer's disease severity.

Quantitative PET Imaging in Drug Development: Estimation of Target Occupancy

Positron emission tomography, an imaging tool using radiolabeled tracers in humans and preclinical species, has been widely used in recent years in drug development, particularly in the central nervous system. One important goal of PET in drug development is assessing the occupancy of various molecular targets (e.g., receptors, transporters, enzymes) by exogenous drugs. The current linear mathematical approaches used to determine occupancy using PET imaging experiments are presented. These algorithms use results from multiple regions with different target content in two scans, a baseline (pre-drug) scan and a post-drug scan. New mathematical estimation approaches to determine target occupancy, using maximum likelihood, are presented. A major challenge in these methods is the proper definition of the covariance matrix of the regional binding measures, accounting for different variance of the individual regional measures and their nonzero covariance, factors that have been ignored by conventional methods. The novel methods are compared to standard methods using simulation and real human occupancy data. The simulation data showed the expected reduction in variance and bias using the proper maximum likelihood methods, when the assumptions of the estimation method matched those in simulation. Between-method differences for data from human occupancy studies were less obvious, in part due to small dataset sizes. These maximum likelihood methods form the basis for development of improved PET covariance models, in order to minimize bias and variance in PET occupancy studies.

Pharmacokinetics, biodistribution and brain retention of a bispecific antibody-based PET radioligand for imaging of amyloid-β

Monoclonal antibodies (mAbs) have not been used as positron emission tomography (PET) ligands for in vivo imaging of the brain because of their limited passage across the blood-brain barrier (BBB). However, due to their high affinity and specificity, mAbs may be an attractive option for brain PET if their brain distribution can be facilitated. In the present study, a F(ab’)₂ fragment of the amyloid-beta (Aβ) protofibril selective mAb158 was chemically conjugated to the transferrin receptor (TfR) antibody 8D3 to enable TfR mediated transcytosis across the BBB. The generated bispecific protein, 8D3-F(ab’)₂-h158, was subsequently radiolabeled and used for microPET imaging of Aβ pathology in two mouse models of AD. [¹²⁴I]8D3-F(ab’)₂-h158 was distributed across the BBB several fold more than unmodified mAbs in general and its accumulation in the brain reflected disease progression, while its concentration in blood and other organs remained stable across all age groups studied. Cerebellum was largely devoid of 8D3-F(ab’)₂-h158 in young and middle aged mice, while mice older than 18 months also showed some accumulation in cerebellum. In a longer perspective, the use of bispecific antibodies as PET ligands may enable in vivo ‘immunohistochemistry’ also of other proteins in the brain for which PET radioligands are lacking.

Model selection criteria for dynamic brain PET studies

Background

 Several criteria exist to identify the optimal model for quantification of tracer kinetics. The purpose of this study was to evaluate the correspondence in kinetic model preference identification for brain PET studies among five model selection criteria: Akaike Information Criterion (AIC), AIC unbiased (AIC_C), model selection criterion (MSC), Schwartz Criterion (SC), and F-test.

Materials and Methods

Six tracers were evaluated: [¹¹C]FMZ, [¹¹C]GMOM, [¹¹C]PK11195, [¹¹C]Raclopride, [¹⁸F]FDG, and [¹¹C]PHT, including data from five subjects per tracer. Time activity curves (TACs) were analysed using six plasma input models: reversible single-tissue model (1T2k), irreversible two-tissue model (2T3k), and reversible two-tissue model (2T4k), all with and without blood volume fraction parameter (V_B). For each tracer and criterion, the percentage of TACs preferring a certain model was calculated.

Results

For all radiotracers, strong agreement was seen across the model selection criteria. The F-test was considered as the reference, as it is a frequently used hypothesis test. The F-test confirmed the AIC preferred model in 87% of all cases. The strongest (but minimal) disagreement across regional TACs was found when comparing AIC with AIC_C. Despite these regional discrepancies, same preferred kinetic model was obtained using all criteria, with an exception of one FMZ subject.

Conclusion

In conclusion, all five model selection criteria resulted in similar conclusions with only minor differences that did not affect overall model selection.

Electronic supplementary material

The online version of this article (10.1186/s40658-017-0197-0) contains supplementary material, which is available to authorized users.

Molecular Imaging of Tumor Hypoxia: Existing Problems and Their Potential Model-Based Solutions

Molecular imaging of tissue hypoxia generates contrast in hypoxic areas by applying hypoxia-specific tracers in organisms. In cancer tissue, the injected tracer needs to be transported over relatively long distances and accumulates slowly in hypoxic regions. Thus, the signal-to-background ratio of hypoxia imaging is very small and a non-specific accumulation may suppress the real hypoxia-specific signals. In addition, the heterogeneous tumor microenvironment makes the assessment of the tissue oxygenation status more challenging. In this study, the diffusion potential of oxygen and of a hypoxia tracer for 4 different hypoxia subtypes: ischemic acute hypoxia, hypoxemic acute hypoxia, diffusion-limited chronic hypoxia and anemic chronic hypoxia are theoretically assessed. In particular, a reaction-diffusion equation is introduced to quantitatively analyze the interstitial diffusion of the hypoxia tracer [(18)F]FMISO. Imaging analysis strategies are explored based on reaction-diffusion simulations. For hypoxia imaging of low signal-to-background ratio, pharmacokinetic modelling has advantages to extract underlying specific binding signals from non-specific background signals and to improve the assessment of tumor oxygenation. Different pharmacokinetic models are evaluated for the analysis of the hypoxia tracer [(18)F]FMISO and optimal analysis model were identified accordingly. The improvements by model-based methods for the estimation of tumor oxygenation are in agreement with experimental data. The computational modelling offers a tool to explore molecular imaging of hypoxia and pharmacokinetic modelling is encouraged to be employed in the corresponding data analysis.

Quantitative Analysis of [18F]FMISO PET for Tumor Hypoxia: Correlation of Modeling Results with Immunohistochemistry

PURPOSE

Quantitative evaluation of tumor hypoxia based on H-1-(3-[¹⁸F]fluoro-2-hydroxypropyl)-2-nitroimidazole ([¹⁸F]FMISO) positron emission tomography (PET) can deliver important information for treatment planning in radiotherapy. However, the merits and limitations of different analysis methods in revealing the underlying physiological feature are not clear. This study aimed to assess these quantitative analysis methods with the support of immunohistological data.

PROCEDURES

Sixteen nude mice bearing xenografted human squamous cell carcinomas (FaDu or CAL-33) were scanned using 2-h dynamic [¹⁸F]FMISO PET. Tumors were resected and sliced, and the hypoxia marker pimonidazole was immunostained followed by H&E staining. The pimonidazole signal was segmented using a k-means clustering algorithm, and the hypoxic fraction (HF) was calculated as the hypoxic area/viable tumor-tissue-area ratio pooled over three tissue slices from the apical, center, and basal layers. PET images were analyzed using various methods including static analysis [standard uptake value (SUV), tumor-to-blood ratio (T/B), tumor-to-muscle ratio (T/M)] and kinetic modeling (Casciari αk _A , irreversible and reversible two-tissue compartment k ₃, Thorwarth w _A k ₃, Patlak K _i , Logan V _d , Cho K), and correlated with HF.

RESULTS

No significant correlation was found for static analysis. A significant correlation between k ₃ of the irreversible two-tissue compartment model and HF was observed (r = 0.61, p = 0.01). The correlation between HF and αk _A of the Casciari model could be improved through reducing local minima by testing more sets of initial values (r = 0.59, p = 0.02) or by reducing the model complexity by fixing three parameters (r = 0.63, p = 0.0008).

CONCLUSIONS

With support of immunohistochemistry data, this study shows that various analysis methods for [¹⁸F]FMISO PET perform differently for assessment of tumor hypoxia. A better fitting quality does not necessarily mean a higher physiological correlation. Hypoxia PET analysis needs to consider both the mathematical stability and physiological fidelity. Based on the results of this study, preference should be given to the irreversible two-tissue compartment model as well as the Casciari model with reduced parameters.

Evaluation of [18F]Fluorothymidine as a Biomarker for Early Therapy Response in a Mouse Model of Colorectal Cancer

PURPOSE

In oncology, positron emission tomography imaging using dedicated tracers as biomarkers may assist in early evaluation of therapy efficacy. Using 3'-deoxy-3'-[¹⁸F]fluorothymidine ([¹⁸F]FLT), we investigated the early effects of chemotherapeutic treatment on cancer cell proliferation in a BRAF-mutated colorectal cancer xenograft model.

PROCEDURES

Colo205 subcutaneously inoculated animals underwent 90-min dynamic imaging before and 24 h after treatment with vehicle (control), cetuximab (resistant) or irinotecan (sensitive). Total distribution volume was quantified from dynamic data, and standardized uptake values as well as tumor-to-blood ratios were calculated from static images averaged over the last 20 min. In vivo imaging data was correlated with ex vivo proliferation and thymidine metabolism proteins.

RESULTS

All imaging parameters showed a significant post-treatment decrease from [¹⁸F]FLT baseline uptake for the irinotecan group (p ≤ 0.001) as compared with the cetuximab and vehicle group and correlated strongly with each other (p ≤ 0.0001). In vivo data were in agreement with Ki67 staining, showing a significantly lower percentage of Ki67-positive cells in the irinotecan group as compared with other groups (p ≤ 0.0001). Tumor expression of thymidine kinase 1 phosphorylated on serine 13, thymidylate synthase, and thymidine phosphorylase remained unaffected, while thymidine kinase 1 expression was, surprisingly, significantly higher in irinotecan-treated animals (p ≤ 0.01). In contrast, tumor ATP levels were lowest in this group.

CONCLUSIONS

[¹⁸F]FLT positron emission tomography was found to be a suitable biomarker of early tumor response to anti-proliferative treatment, with static imaging not being inferior to full compartmental analysis in our xenograft model. The dynamics of thymidine kinase 1 protein expression and protein activity in low ATP environments merits further investigation.

Revisiting the Logan plot to account for non-negligible blood volume in brain tissue

Background

Reference tissue-based quantification of brain PET data does not typically include correction for signal originating from blood vessels, which is known to result in biased outcome measures. The bias extent depends on the amount of radioactivity in the blood vessels. In this study, we seek to revisit the well-established Logan plot and derive alternative formulations that provide estimation of distribution volume ratios (DVRs) that are corrected for the signal originating from the vasculature.

Results

New expressions for the Logan plot based on arterial input function and reference tissue were derived, which included explicit terms for whole blood radioactivity. The new methods were evaluated using PET data acquired using [¹¹C]raclopride and [¹⁸F]MNI-659. The two-tissue compartment model (2TCM), with which signal originating from blood can be explicitly modeled, was used as a gold standard.

DVR values obtained for [¹¹C]raclopride using the either blood-based or reference tissue-based Logan plot were systematically underestimated compared to 2TCM, and for [¹⁸F]MNI-659, a proportionality bias was observed, i.e., the bias varied across regions. The biases disappeared when optimal blood-signal correction was used for respective tracer, although for the case of [¹⁸F]MNI-659 a small but systematic overestimation of DVR was still observed.

Conclusions

The new method appears to remove the bias introduced due to absence of correction for blood volume in regular graphical analysis and can be considered in clinical studies. Further studies are however required to derive a generic mapping between plasma and whole-blood radioactivity levels.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-017-0314-z) contains supplementary material, which is available to authorized users.

In vivo variation in same-day estimates of metabotropic glutamate receptor subtype 5 binding using [11C]ABP688 and [18F]FPEB

Positron emission tomography tracers [11C]ABP688 and [18F]FPEB target the metabotropic glutamate receptor subtype 5 providing quantification of the brain glutamatergic system in vivo. Previous [11C]ABP688 positron emission tomography human test-retest studies indicate that, when performed on the same day, significant binding increases are observed; however, little deviation is reported when scans are >7 days apart. Due to the small cohorts examined previously (eight and five males, respectively), we aimed to replicate the same-day test-retest studies in a larger cohort including both males and females. Results confirmed large within-subject binding differences (ranging from -23% to 108%), suggesting that measurements are greatly affected by study design. We further investigated whether this phenomenon was specific to [11C]ABP688. Using [18F]FPEB and methodology that accounts for residual radioactivity from the test scan, four subjects were scanned twice on the same day. In these subjects, binding estimates increased between 5% and 39% between scans. Consistent with [11C]ABP688, mean absolute test-retest variability was previously reported as <12% when scans were >21 days apart. This replication study and pilot extension to [18F]FPEB suggest that observed within-day binding variation may be due to characteristics of mGluR5; for example, diurnal variation in mGluR5 may affect measurement of this receptor.

Novel 18F-Labeled κ-Opioid Receptor Antagonist as PET Radiotracer: Synthesis and In Vivo Evaluation of 18F-LY2459989 in Nonhuman Primates

The κ-opioid receptor (KOR) has been implicated in depression, addictions, and other central nervous system disorders and, thus, is an important target for drug development. We previously developed several ¹¹C-labeled PET radiotracers for KOR imaging in humans. Here we report the synthesis and evaluation of ¹⁸F-LY2459989 as the first ¹⁸F-labeled KOR antagonist radiotracer in nonhuman primates and its comparison with ¹¹C-LY2459989. Methods: The novel radioligand ¹⁸F-LY2459989 was synthesized by ¹⁸F displacement of a nitro group or an iodonium ylide. PET scans in rhesus monkeys were obtained on a small-animal scanner to assess the pharmacokinetic and in vivo binding properties of the ligand. Metabolite-corrected arterial activity curves were measured and used as input functions in the analysis of brain time-activity curves and the calculation of binding parameters. Results: With the iodonium ylide precursor, ¹⁸F-LY2459989 was prepared at high radiochemical yield (36% ± 7% [mean ± SD]), radiochemical purity (>99%), and mean molar activity (1,175 GBq/μmol; n = 6). In monkeys, ¹⁸F-LY2459989 was metabolized at a moderate rate, with a parent fraction of approximately 35% at 30 min after injection. Fast and reversible kinetics were observed, with a regional peak uptake time of less than 20 min. Pretreatment with the selective KOR antagonist LY2456302 (0.1 mg/kg) decreased the activity level in regions with high levels of binding to that in the cerebellum, thus demonstrating the binding specificity and selectivity of ¹⁸F-LY2459989 in vivo. Regional time-activity curves were well fitted by the multilinear analysis 1 kinetic model to derive reliable estimates of regional distribution volumes. With the cerebellum as the reference region, regional binding potentials were calculated and ranked as follows: cingulate cortex > insula > caudate/putamen > frontal cortex > temporal cortex > thalamus, consistent with the reported KOR distribution in the monkey brain. Conclusion: The evaluation of ¹⁸F-LY2459989 in nonhuman primates demonstrated many attractive imaging properties: fast tissue kinetics, specific and selective binding to the KOR, and high specific binding signals. A side-by-side comparison of ¹⁸F-LY2459989 and ¹¹C-LY2459989 indicated similar kinetic and binding profiles for the 2 radiotracers. Taken together, the results indicated that ¹⁸F-LY2459989 appears to be an excellent PET radiotracer for the imaging and quantification of the KOR in vivo.

Quantification of [11C]Ro15-4513 GABAAα5 specific binding and regional selectivity in humans

[¹¹C]Ro15-4513 has been introduced as a positron emission tomography radioligand to image the GABA_Aα5 receptor subtype thought to be important in learning, memory and addiction. However, the in vivo selectivity of the ligand remains unknown and a full assessment of different analysis approaches has yet to be performed. Using human heterologous competition data, with [¹¹C]Ro15-4513 and the highly selective GABA_Aα5 selective negative allosteric modulator Basmisanil (RG1662), we quantify the GABA_Aα5 selectivity of [¹¹C]Ro15-4513, assess the validity of reference tissues and evaluate the performance of four different kinetic analysis methods. The results show that [¹¹C]Ro15-4513 has high but not complete selectivity for GABA_Aα5, with α5 representing around 60-70% of the specific binding in α5 rich regions. Competition data indicate that the cerebellum and pons are essentially devoid of α5 signal and might be used as reference regions under certain conditions. Off-target non-selective binding to other GABA_A subtypes means that the choice of analysis method and the interpretation of outcome measures must be considered carefully. We discuss the merits of two tissue compartmental model analyses to derive both V_T and V_S, band-pass spectral analysis for estimation of [Formula: see text] and the simplified reference tissue model for estimation of [Formula: see text].

Quantitative analysis of dynamic 18F-FDG PET/CT for measurement of lung inflammation

Background

An inflammatory reaction in the airways and lung parenchyma, comprised mainly of neutrophils and alveolar macrophages, is present in some patients with chronic obstructive pulmonary disease (COPD). Thoracic fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) has been proposed as a promising imaging biomarker to assess this inflammation. We sought to introduce a fully quantitative analysis method and compare this with previously published studies based on the Patlak approach using a dataset comprising ¹⁸F-FDG PET scans from COPD subjects with elevated circulating inflammatory markers (fibrinogen) and matched healthy volunteers (HV). Dynamic ¹⁸F-FDG PET scans were obtained for high-fibrinogen (>2.8 g/l) COPD subjects (N = 10) and never smoking HV (N = 10). Lungs were segmented using co-registered computed tomography images and subregions (upper, middle and lower) were semi-automatically defined. A quantitative analysis approach was developed, which corrects for the presence of air and blood in the lung (qABL method), enabling direct estimation of the metabolic rate of FDG in lung tissue. A normalised Patlak analysis approach was also performed to enable comparison with previously published results. Effect sizes (Hedge’s g) were used to compare HV and COPD groups.

Results

The qABL method detected no difference (Hedge’s g = 0.15 [−0.76 1.04]) in the tissue metabolic rate of FDG in the whole lung between HV (μ = 6.0 ± 1.9 × 10⁻³ ml cm⁻³ min⁻¹) and COPD (μ = 5.7 ± 1.7 × 10⁻³ ml cm⁻³ min⁻¹). However, analysis with the normalised Patlak approach detected a significant difference (Hedge’s g = −1.59 [−2.57 −0.48]) in whole lung between HV (μ = 2.9 ± 0.5 × 10⁻³ ml cm⁻³ min⁻¹) and COPD (μ = 3.9 ± 0.7 × 10⁻³ ml cm⁻³ min⁻¹). The normalised Patlak endpoint was shown to be a composite measure influenced by air volume, blood volume and actual uptake of ¹⁸F-FDG in lung tissue.

Conclusions

We have introduced a quantitative analysis method that provides a direct estimate of the metabolic rate of FDG in lung tissue. This work provides further understanding of the underlying origin of the ¹⁸F-FDG signal in the lung in disease groups and helps interpreting changes following standard or novel therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-017-0291-2) contains supplementary material, which is available to authorized users.

A hybrid deconvolution approach for estimation of in vivo non-displaceable binding for brain PET targets without a reference region

BACKGROUND AND AIM

Estimation of a PET tracer's non-displaceable distribution volume (VND) is required for quantification of specific binding to its target of interest. VND is generally assumed to be comparable brain-wide and is determined either from a reference region devoid of the target, often not available for many tracers and targets, or by imaging each subject before and after blocking the target with another molecule that has high affinity for the target, which is cumbersome and involves additional radiation exposure. Here we propose, and validate for the tracers [11C]DASB and [11C]CUMI-101, a new data-driven hybrid deconvolution approach (HYDECA) that determines VND at the individual level without requiring either a reference region or a blocking study.

METHODS

HYDECA requires the tracer metabolite-corrected concentration curve in blood plasma and uses a singular value decomposition to estimate the impulse response function across several brain regions from measured time activity curves. HYDECA decomposes each region's impulse response function into the sum of a parametric non-displaceable component, which is a function of VND, assumed common across regions, and a nonparametric specific component. These two components differentially contribute to each impulse response function. Different regions show different contributions of the two components, and HYDECA examines data across regions to find a suitable common VND. HYDECA implementation requires determination of two tuning parameters, and we propose two strategies for objectively selecting these parameters for a given tracer: using data from blocking studies, and realistic simulations of the tracer. Using available test-retest data, we compare HYDECA estimates of VND and binding potentials to those obtained based on VND estimated using a purported reference region.

RESULTS

For [11C]DASB and [11C]CUMI-101, we find that regardless of the strategy used to optimize the tuning parameters, HYDECA provides considerably less biased estimates of VND than those obtained, as is commonly done, using a non-ideal reference region. HYDECA test-retest reproducibility is comparable to that obtained using a VND determined from a non-ideal reference region, when considering the binding potentials BPP and BPND.

CONCLUSIONS

HYDECA can provide subject-specific estimates of VND without requiring a blocking study for tracers and targets for which a valid reference region does not exist.