Simplified Reference Tissue Model Research Articles

Synthesis and Preclinical Evaluation of [11C]AZ11895530 for PET Imaging of the Serotonin 1A Receptor

The serotonin 1A (5-HT1A) receptor is a G-protein-coupled receptor implicated in the pathophysiology of several neuropsychiatric and neurodegenerative disorders. We here report the preparation of two candidate 5-HT1A radioligands, [11C]AZ11132132 ([11C]3) and [11C]AZ11895530 ([11C]4), and their subsequent evaluation in vitro using autoradiography and in vivo using positron emission tomography (PET). Compounds 3 and 4 were radiolabeled at high radiochemical purity (>99%) and high molar activity (>38 GBq/μmol) by heteroatom methylation with [11C]methyl iodide. Autoradiography on whole hemispheres from post-mortem human brain revealed substantial nonspecific binding of [11C]3, while the binding of [11C]4 to brain tissue was consistent with the distribution of 5-HT1A receptors and sensitive to co-incubation with the reference 5-HT1A antagonist WAY-100635 (10 μM). Following intravenous injection of [11C]4 into a cynomolgus monkey, brain radioactivity concentration (Cmax ∼ 2.2 SUV) was high whereafter it decreased rapidly. The regional binding potential (BPND) values were calculated using the simplified reference tissue model with cerebellum as reference region. The values varied between 0.2 and 1.0 for temporal cortex, raphe nuclei, frontal cortex, and hippocampus which is consistent with the known 5-HT1A expression pattern. After pretreatment with WAY100635 (0.5 mg/kg), a homogeneous distribution of radioactivity was observed in non-human primate (NHP) brain. Although [11C]4 fulfilled important criteria for successful in vivo neuroimaging, including good blood-brain-barrier permeability and high specific binding in vitro to human brain tissue, the regional BPND values for [11C]4 in NHP brain were low when compared to those obtained with existing radioligands and thus do not merit further investigation of [11C]4. Evaluation of structurally related analogues is underway in our laboratory to identify improved candidates for clinical imaging.

Binding of the Dual-Action Anti-Parkinsonian Drug AG-0029 to Dopamine D2 and Histamine H3 Receptors: A PET Study in Healthy Rats.

Introduction: Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor dysfunction and a diverse range of nonmotor symptoms. Functional relationships between the dopaminergic and histaminergic systems suggest that dual-action pharmaceuticals like AG-0029 (D2/D3 agonist/H3 antagonist) could ameliorate both the motor and cognitive symptoms of PD. The current study aimed to demonstrate the interaction of AG-0029 with its intended targets in the mammalian brain using positron emission tomography (PET). Methods: Healthy male Wistar rats were scanned with a small-animal PET camera, using either the dopamine D2/D3 receptor ligand [11C]raclopride or the histamine H3 receptor ligand [11C]GSK-189254, before and after treatment with an intravenous, acute, single dose of AG-0029. Dynamic [11C]raclopride PET data (60 min duration) were analyzed using the simplified reference tissue model 2 (SRTM2) with cerebellum as reference tissue and the nondisplaceable binding potential as the outcome parameter. Data from dynamic [11C]GSK-189254 scans (60 min duration) with arterial blood sampling were analyzed using Logan graphical analysis with the volume of distribution (VT) as the outcome parameter. Receptor occupancy was estimated using a Lassen plot. Results: Dopamine D2/3 receptor occupancies in the striatum were 22.6 ± 18.0 and 84.0 ± 3.5% (mean ± SD) after administration of 0.1 and 1 mg/kg AG-0029, respectively. In several brain regions, the VT values of [11C]GSK-189254 were significantly reduced after pretreatment of rats with 1 or 10 mg/kg AG-0029. The H3 receptor occupancies were 11.9 ± 8.5 and 40.3 ± 11.3% for the 1 and 10 mg/kg doses of AG-0029, respectively. Conclusions: Target engagement of AG-0029 as an agonist at dopamine D2/D3 receptors and an antagonist at histamine H3 receptors could be demonstrated in the rat brain with [11C]raclopride and [11C]GSK-189254 PET, respectively. The measured occupancy values reflect the previously reported high (subnanomolar) affinity of AG-0029 to D2/D3 and moderate (submicromolar) affinity to H3 receptors.

Impact of cerebral blood flow and amyloid load on SUVR bias

BackgroundDespite its widespread use, the semi-quantitative standardized uptake value ratio (SUVR) may be biased compared with the distribution volume ratio (DVR). This bias may be partially explained by changes in cerebral blood flow (CBF) and is likely to be also dependent on the extent of the underlying amyloid-β (Aβ) burden. This study aimed to compare SUVR with DVR and to evaluate the effects of underlying Aβ burden and CBF on bias in SUVR in mainly cognitively unimpaired participants. Participants were scanned according to a dual-time window protocol, with either [18F]flutemetamol (N = 90) or [18F]florbetaben (N = 31). The validated basisfunction-based implementation of the two-step simplified reference tissue model was used to derive DVR and R1 parametric images, and SUVR was calculated from 90 to 110 min post-injection, all with the cerebellar grey matter as reference tissue. First, linear regression and Bland–Altman analyses were used to compare (regional) SUVR with DVR. Then, generalized linear models were applied to evaluate whether (bias in) SUVR relative to DVR could be explained by R1 for the global cortical average (GCA), precuneus, posterior cingulate, and orbitofrontal region.ResultsDespite high correlations (GCA: R2 ≥ 0.85), large overestimation and proportional bias of SUVR relative to DVR was observed. Negative associations were observed between both SUVR or SUVRbias and R1, albeit non-significant.ConclusionThe present findings demonstrate that bias in SUVR relative to DVR is strongly related to underlying Aβ burden. Furthermore, in a cohort consisting mainly of cognitively unimpaired individuals, the effect of relative CBF on bias in SUVR appears limited. EudraCT Number: 2018-002277-22, registered on: 25-06-2018.

Quantitation of the A2A Adenosine Receptor Density in the Striatum of Mice and Pigs with [18F]FLUDA by Positron Emission Tomography.

The cerebral expression of the A2A adenosine receptor (A2AAR) is altered in neurodegenerative diseases such as Parkinson’s (PD) and Huntington’s (HD) diseases, making these receptors an attractive diagnostic and therapeutic target. We aimed to further investigate the pharmacokinetic properties in the brain of our recently developed A2AAR–specific antagonist radiotracer [18F]FLUDA. For this purpose, we retrospectively analysed dynamic PET studies of healthy mice and rotenone–treated mice, and conducted dynamic PET studies with healthy pigs. We performed analysis of mouse brain time–activity curves to calculate the mean residence time (MRT) by non–compartmental analysis, and the binding potential (BPND) of [18F]FLUDA using the simplified reference tissue model (SRTM). For the pig studies, we performed a Logan graphical analysis to calculate the radiotracer distribution volume (VT) at baseline and under blocking conditions with tozadenant. The MRT of [18F]FLUDA in the striatum of mice was decreased by 30% after treatment with the A2AAR antagonist istradefylline. Mouse results showed the highest BPND (3.9 to 5.9) in the striatum. SRTM analysis showed a 20% lower A2AAR availability in the rotenone–treated mice compared to the control–aged group. Tozadenant treatment significantly decreased the VT (14.6 vs. 8.5 mL · g−1) and BPND values (1.3 vs. 0.3) in pig striatum. This study confirms the target specificity and a high BPND of [18F]FLUDA in the striatum. We conclude that [18F]FLUDA is a suitable tool for the non–invasive quantitation of altered A2AAR expression in neurodegenerative diseases such as PD and HD, by PET.

An in vivo Pig Model for Testing Novel Positron Emission Tomography Radioligands Targeting Cerebral Protein Aggregates.

Positron emission tomography (PET) has become an essential clinical tool for diagnosing neurodegenerative diseases with abnormal accumulation of proteins like amyloid-β or tau. Despite many attempts, it has not been possible to develop an appropriate radioligand for imaging aggregated α-synuclein in the brain for diagnosing, e.g., Parkinson’s Disease. Access to a large animal model with α-synuclein pathology would critically enable a more translationally appropriate evaluation of novel radioligands. We here establish a pig model with cerebral injections of α-synuclein preformed fibrils or brain homogenate from postmortem human brain tissue from individuals with Alzheimer’s disease (AD) or dementia with Lewy body (DLB) into the pig’s brain, using minimally invasive surgery and validated against saline injections. In the absence of a suitable α-synuclein radioligand, we validated the model with the unselective amyloid-β tracer [11C]PIB, which has a high affinity for β-sheet structures in aggregates. Gadolinium-enhanced MRI confirmed that the blood-brain barrier was intact. A few hours post-injection, pigs were PET scanned with [11C]PIB. Quantification was done with Logan invasive graphical analysis and simplified reference tissue model 2 using the occipital cortex as a reference region. After the scan, we retrieved the brains to confirm successful injection using autoradiography and immunohistochemistry. We found four times higher [11C]PIB uptake in AD-homogenate-injected regions and two times higher uptake in regions injected with α-synuclein-preformed-fibrils compared to saline. The [11C]PIB uptake was the same in non-injected (occipital cortex, cerebellum) and injected (DLB-homogenate, saline) regions. With its large brain and ability to undergo repeated PET scans as well as neurosurgical procedures, the pig provides a robust, cost-effective, and good translational model for assessment of novel radioligands including, but not limited to, proteinopathies.

Validation and test\u2013retest repeatability performance of parametric methods for [11C]UCB-J PET

[11C]UCB-J is a PET radioligand that binds to the presynaptic vesicle glycoprotein 2A. Therefore, [11C]UCB-J PET may serve as an in vivo marker of synaptic integrity. The main objective of this study was to evaluate the quantitative accuracy and the 28-day test–retest repeatability (TRT) of various parametric quantitative methods for dynamic [11C]UCB-J studies in Alzheimer’s disease (AD) patients and healthy controls (HC). Eight HCs and seven AD patients underwent two 60-min dynamic [11C]UCB-J PET scans with arterial sampling over a 28-day interval. Several plasma-input based and reference-region based parametric methods were used to generate parametric images using metabolite corrected plasma activity as input function or white matter semi-ovale as reference region. Different parametric outcomes were compared regionally with corresponding non-linear regression (NLR) estimates. Furthermore, the 28-day TRT was assessed for all parametric methods. Spectral analysis (SA) and Logan graphical analysis showed high correlations with NLR estimates. Receptor parametric mapping (RPM) and simplified reference tissue model 2 (SRTM2) BPND, and reference Logan (RLogan) distribution volume ratio (DVR) regional estimates correlated well with plasma-input derived DVR and SRTM BPND. Among the multilinear reference tissue model (MRTM) methods, MRTM1 had the best correspondence with DVR and SRTM BPND. Among the parametric methods evaluated, spectral analysis (SA) and SRTM2 were the best plasma-input and reference tissue methods, respectively, to obtain quantitatively accurate and repeatable parametric images for dynamic [11C]UCB-J PET.

A reference tissue forward model for improved PET accuracy using within-scan displacement studies

We report a novel forward-model implementation of the full reference tissue model (fFTRM) that addresses the fast-exchange approximation employed by the simplified reference tissue model (SRTM) by incorporating a non-zero dissociation time constant from the specifically bound compartment. The forward computational approach avoided errors associated with noisy and nonorthogonal basis functions using an inverse linear model. Compared to analysis by a multilinear single-compartment reference tissue model (MRTM), fFTRM provided improved accuracy for estimation of binding potentials at early times in the scan, with no worse reproducibility across sessions. To test the model’s ability to identify small focal changes in binding potential using a within-scan challenge, we employed a nonhuman primate model of focal dopamine release elicited by deep brain microstimulation remote to ventral striatum (VST) during imaging by simultaneous PET and fMRI. The new model reported an unambiguously lateralized response in VST consistent with fMRI, whereas the MRTM-derived response was not lateralized and was consistent with simulations of model bias. The proposed model enabled better accuracy in PET [11C]raclopride displacement studies and may also facilitate challenges sooner after injection, thereby recovering some sensitivity lost to radioactive decay of the PET tracer.

Test‐retest variability of relative tracer delivery rate as measured by [11C]PiB

AbstractBackgroundIn Alzheimer’s disease (AD), reductions in cerebral blood flow (CBF) have been reported and these may be used as proxy for measuring treatment efficacy or disease progression. The gold standard technique for measuring CBF is [15O]H2O positron emission tomography (PET). Recent studies, however, have reported high correlations between [11C]PiB PET derived relative tracer delivery rate R 1 and [15O]H2O derived relative CBF, suggesting that [11C]PiB R 1 may be a good proxy of relative CBF. As longitudinal PET studies become more common for measuring disease progression, it is important to know the intrinsic variability of R 1. The purpose of the present study was to determine R 1 test‐retest (TRT) variability.MethodPreviously, TRT variability of [11C]PiB binding potential has been reported for a group of twelve participants (five cognitively unimpaired (CU), one with mild cognitive impairment and six with AD dementia), in which all participants underwent two 90 min [11C]PiB PET scans using a Siemens ECAT EXACT HR+ scanner1. This dataset was reanalysed to also determine TRT variability of [11C]PiB R1. The voxel‐based implementation of the two‐step simplified reference tissue model with cerebellar grey matter as reference tissue was used to generate R1 images. Next, TRT variability was calculated both globally and regionally, and global R1 measures were compared between diagnostic groups using a non‐parametric Mann‐Whitney U test. Next, correlation analysis was used to assess the relationship between TRT variability and regional volume. Finally, regional test and retest R1 measures were compared using Linear Mixed Effect Models and Bland‐Altman analysis.ResultGlobal relative tracer delivery (R1) was significantly lower in AD patients (test R1:0.82±0.04, retest R1:0.82±0.03) compared with CU participants (test R1:0.93±0.04 retest R1:0.91±0.03, p<0.01). TRT variability was low across regions (range:1.5‐5.8%), with a trend effect towards smaller TRT for larger regions (R2=0.14, p=0.09). Furthermore, test and retest measures showed high, significant correlations (R2=0.92, slope=0.98) and negligible bias (0.69±3.07%).ConclusionThe high precision of [11C]PiB R1 suggests suitable applicability for cross‐sectional and longitudinal studies. This finding further demonstrates the utility of dynamic [11C]PiB PET scans for dual‐biomarker imaging in AD research. Reference: Tolboom et al.(2009) Eur J Nucl Med Mol Imaging 36:1629‐38.

Optimal parametric imaging methods for dual‐time‐window [18F]flutemetamol and [18F]florbetaben PET studies

AbstractBackgroundAccurate quantitative measures of amyloid beta (Aβ) plaques are needed to identify individuals with subthreshold Aβ levels or for measuring small changes over time. To this end, dynamic or dual‐time‐window scanning protocols are required. Optimal pharmacokinetic models for quantifying Aβ burden using both [18F]flutemetamol and [18F]florbetaben dual‐time‐window scans have previously been identified at a region of interest (ROI) level. This study aimed to determine optimal parametric methods for analyses of [18F]flutemetamol and [18F]florbetaben dual‐time‐window scans.MethodForty‐six participants were scanned on a PET/MR scanner using a dual‐time‐window protocol and either [18F]flutemetamol or [18F]florbetaben. The following parametric approaches were used to derive DVR estimates: reference Logan (RLogan), receptor parametric mapping (RPM), two‐step simplified reference tissue model (SRTM2) and multilinear reference tissue models (MRTM0, MRTM1, MRTM2), all with cerebellar grey matter as reference tissue. In addition, a standardized uptake value ratio (SUVR) was calculated for the 90‐110 min post injection interval. All parametric images were assessed visually for possible image artefacts and regional cortical outcome measures were compared with DVR derived using regional RLogan, based on AUC values for differentiating between Aβ‐positive and Aβ‐negative scans, Bland‐Altman plots and linear regression analyses.ResultVisually, RPM and SRTM2 performed best across tracers and, in addition to SUVR, provided the highest AUC values for differentiating between Aβ‐positive and Aβ‐negative scans ([18F]flutemetamol: range AUC=0.96‐0.97, [18F]florbetaben: range AUC=0.83‐0.85). Outcome parameters of most methods were highly correlated with those of the reference method (R2≥0.87), with lowest correlations observed for MRTM2 (R2=0.71‐0.80). Furthermore, bias was low (≤5%) and independent of underlying amyloid burden for MRTM0 and MRTM1. On the other hand, bias in SUVR was high for [18F]flutemetamol (mean bias>10%), but low for [18F]florbetaben (mean bias=1%) and strongly proportional with underlying Aβ burden for both tracers.ConclusionThe best overall compromise was provided by MRTM0, followed closely by SRTM2. SRTM2 is the preferred method for parametric imaging because, in addition to its good performance, it has the advantage of providing a measure of relative perfusion (R1), which is useful for measuring disease progression. “This work has received support from the EU‐EFPIA Innovative Medicines Initiatives 2 Joint Undertaking (grant No 115952).”

Quantification of Metabotropic Glutamate Receptor 5 Availability With Both [11C]ABP688 and [18F]FPEB Positron Emission Tomography in the Sapap3 Knockout Mouse Model for Obsessive-Compulsive–like Behavior

BackgroundThis study provides a first direct comparison between positron emission tomography radioligands targeting the allosteric site of the metabotropic glutamate receptor 5 (mGluR5): [11C]ABP688 and [18F]FPEB. A blocking paradigm was set up to substantiate the common binding site of both radioligands. Second, both radioligands were applied in Sapap3 knockout (KO) mice showing compulsive-like behavior characterized by a lower in vivo mGluR5 availability. MethodsFirst, wild-type mice (n = 7) received four position emission tomography/computed tomography scans: a [11C]ABP688 scan, a [18F]FPEB scan, and two blocking scans using cold FPEB and cold ABP688, respectively. A second experiment compared both radioligands in wild-type (n = 7) and KO (n = 10) mice. The simplified reference tissue model was used to calculate the nondisplaceable binding potential representing the in vivo availability of mGluR5 in the brain. ResultsUsing cold FPEB as a blocking compound for [11C]ABP688 micro–positron emission tomography and vice versa, we observed averaged global reductions in mGluR5 availability of circa 98% for [11C]ABP688 and 82%–96% for [18F]FPEB. For KOs, the [11C]ABP688 nondisplaceable binding potential was on average 25% lower compared with wild-type control mice (p < .0001–.001), while this was about 17% for [18F]FPEB (p < .05). ConclusionsThe current findings substantiate a common binding site and suggest a strong relationship between mGluR5 availability levels measured with both radioligands. In Sapap3 KO mice, a reduced mGluR5 availability could therefore be demonstrated with both radioligands. With [11C]ABP688, higher significance levels were achieved in more brain regions. These findings suggest [11C]ABP688 as a preferable radiotracer to quantify mGluR5 availability, as exemplified here in a model for compulsive-like behavior.

Characterization of an APP/tau rat model of Alzheimer\u2019s disease by positron emission tomography and immunofluorescent labeling

BackgroundTo better understand the etiology and pathomechanisms of Alzheimer’s disease, several transgenic animal models that overexpress human tau or human amyloid-beta (Aβ) have been developed. In the present study, we generated a novel transgenic rat model by cross-breeding amyloid precursor protein (APP) rats with tau rats. We characterized this model by performing positron emission tomography scans combined with immunofluorescent labeling and cerebrospinal fluid analyses.MethodsAPP/Tau rats were generated by cross-breeding male McGill-R-Thy1-APP transgenic rats with female hTau-40/P301L transgenic rats. APP/Tau double transgenic rats and non-transgenic (ntg) littermates aged 7, 13, and 21 months were subjected to dynamic [11C] PiB scan and dynamic [18F]THK-5317 scans. For regional brain analysis, a template was generated from anatomical MR images of selected animals, which was co-registered with the PET images. Regional analysis was performed by application of the simplified reference tissue model ([11C]PiB data), whereas [18F]THK-5317 data were analyzed using a 2-tissue compartment model and Logan graphical analysis. In addition, immunofluorescent labeling (tau, amyloid) and cerebrospinal fluid analyses were performed.Results[11C]PiB binding potential (BPND) and [18F]THK-5317 volume of distribution (VT) showed an increase with age in several brain regions in the APP/Tau group but not in the ntg control group. Immunohistochemical analysis of brain slices of PET-scanned animals revealed a positive correlation between Aβ labeling and [11C]PiB regional BPND. Tau staining yielded a trend towards higher levels in the cortex and hippocampus of APP/Tau rats compared with ntg littermates, but without reaching statistical significance. No correlation was found between tau immunofluorescence labeling results and the respective [18F]THK-5317 VT values.ConclusionsWe thoroughly characterized a novel APP/Tau rat model using combined PET imaging and immunofluorescence analysis. We observed an age-related increase in [11C]PiB and [18F]THK-5317 binding in several brain regions in the APP/Tau group but not in the ntg group. Although we were able to reveal a positive correlation between amyloid labeling and [11C]PiB regional brain uptake, we observed relatively low human tau and amyloid fibril expression levels and a somewhat unstable brain pathology which questions the utility of this animal model for further studies.

Association Between Lack of Insight and Prefrontal Serotonin Transporter Availability in Antipsychotic-Free Patients with Schizophrenia: A High-Resolution PET Study with [11C]DASB.

BackgroundPrevious studies suggested a link between serotonergic neurotransmission and impaired insight in schizophrenia. In this study, we examined the relationship between serotonin transporter (SERT) availability in regions of the prefrontal cortex (dorsolateral, ventrolateral, ventromedial, and orbitofrontal cortices) and insight deficits in antipsychotic-free patients with schizophrenia using high-resolution positron emission tomography (PET) with [11C]DASB.MethodsNineteen patients underwent [11C]DASB PET and 7-Tesla magnetic resonance imaging scans. To assess SERT availability, the binding potential with respect to non-displaceable compartment (BPND) was derived using the simplified reference tissue model. Patients’ level of insight was assessed using the Insight and Treatment Attitude Questionnaire (ITAQ). The relationship between ITAQ scores and [11C]DASB BPND values was examined using the region-of-interest (ROI)- and voxel-based analyses with relevant variables as covariates. The prefrontal cortex and its four subregions were selected as a priori ROIs since the prefrontal cortex has been implicated as the critical neuroanatomical substrate of impaired insight in schizophrenia.ResultsThe ROI-based analysis revealed that the ITAQ illness insight dimension had significant negative correlations with the [11C]DASB BPND in the left dorsolateral, left orbitofrontal, and bilateral ventrolateral prefrontal cortices. The ITAQ treatment insight dimension had significant negative correlations with the [11C]DASB BPND in the bilateral dorsolateral, left orbitofrontal, and bilateral ventrolateral prefrontal cortices. The ITAQ total score showed significant negative correlations with the [11C]DASB BPND in the bilateral prefrontal cortex and three subregions (dorsolateral, ventrolateral, and orbitofrontal cortices). A supplementary voxel-based analysis corroborated a significant negative association between the ITAQ score and the [11C]DASB BPND in the prefrontal cortices.ConclusionOur study provides in vivo evidence of significant negative correlations between insight deficits and prefrontal SERT availability in patients with schizophrenia, suggesting significant involvement of prefrontal serotonergic signaling in impaired insight, one of the core symptoms of schizophrenia.

Evaluation of [18F]-JNJ-64326067-AAA tau PET tracer in humans

The [18F]-JNJ-64326067-AAA ([18F]-JNJ-067) tau tracer was evaluated in healthy older controls (HCs), mild cognitive impairment (MCI), Alzheimer’s disease (AD), and progressive supranuclear palsy (PSP) participants. Seventeen subjects (4 HCs, 5 MCIs, 5 ADs, and 3 PSPs) received a [11C]-PIB amyloid PET scan, and a tau [18F]-JNJ-067 PET scan 0-90 minutes post-injection. Only MCIs and ADs were amyloid positive. The simplified reference tissue model, Logan graphical analysis distribution volume ratio, and SUVR were evaluated for quantification. The [18F]-JNJ-067 tau signal relative to the reference region continued to increase to 90 min, indicating the tracer had not reached steady state. There was no significant difference in any bilateral ROIs for MCIs or PSPs relative to HCs; AD participants showed elevated tracer relative to controls in most cortical ROIs (P < 0.05). Only AD participants showed elevated retention in the entorhinal cortex. There was off-target signal in the putamen, pallidum, thalamus, midbrain, superior cerebellar gray, and white matter. [18F]-JNJ-067 significantly correlated (p < 0.05) with Mini-Mental State Exam in entorhinal cortex and temporal meta regions. There is clear binding of [18F]-JNJ-067 in AD participants. Lack of binding in HCs, MCIs and PSPs suggests [18F]-JNJ-067 may not bind to low levels of AD-related tau or 4 R tau.

Measurement of changes in endogenous serotonin level by positron emission tomography with [18F]altanserin.

Positron emission tomography (PET) has been used to investigate changes in the concentration of endogenous neurotransmitters. Recently, this technique has been applied to the imaging of serotonin2A receptors using [18F]altanserin. In these measurements, a reduction in binding potential (BP) suggests an increase in endogenous serotonin levels caused by pharmacological or cognitive stimulations, and the sensitivity of BP reduction depends on the characteristics of [18F]altanserin. In this study, we evaluated an analytical method for estimating the changes in endogenous serotonin levels based on PET scans with [18F]altanserin at baseline and stimulated states and validated it using simulations and small animal PET studies. First, in the simulations, the time-activity curves at baseline and the stimulated states were generated using an extended compartment model including the competition for the receptors between the administered [18F]altanserin and endogenous serotonin. In the stimulated state, the magnitude and onset of the endogenous serotonin elevation were altered to varying degrees. In these time-activity curves, BP was estimated using the simplified reference tissue model (SRTM), and the reduction in BP was evaluated by comparison with that of the baseline state. Next, the proposed method was applied to mouse PET studies. Endogenous serotonin levels were elevated by treatment with selective serotonin reuptake inhibitors (SSRIs), and PET studies were performed twice, once with and once without treatment. In both scans, BP was estimated using the SRTM with the cerebellum as a reference region, and the reduction in BP after SSRI treatment was evaluated. In the simulations, the BP estimate of the stimulated state was smaller than that of the baseline state, and their reduction was related to the amount of change in the serotonin concentration. BP reduction was also affected by the onset of serotonin elevation. In the mouse studies, the BP of the cerebral cortex decreased in the scans with SSRI treatment. The reduction in BP estimated using the SRTM from [18F]altanserin-PET studies at baseline and in stimulated states can detect changes in the binding conditions of serotonin2A receptors. This may be useful for investigating the elevation of endogenous serotonin levels caused by stimulations.

Functional Analysis of Brain Imaging Suggests Changes in the Availability of mGluR5 and Altered Connectivity in the Cerebral Cortex of Long-Term Abstaining Males with Alcohol Dependence: A Preliminary Study.

Direct in vivo evidence of altered metabotropic glutamate receptor-5 (mGluR5) availability in alcohol-related disorders is lacking. We performed [11C]ABP688 positron emission tomography (PET) and resting-state functional magnetic resonance imaging (rs-fMRI) in prolonged abstinent subjects with alcohol dependence to examine alterations of mGluR5 availability, and to investigate their functional significance relating to neural systems-level changes. Twelve prolonged abstinent male subjects with alcohol dependence (median abstinence duration: six months) and ten healthy male controls underwent [11C]ABP688 PET imaging and 3-Tesla MRI. For mGluR5 availability, binding potential (BPND) was calculated using the simplified reference tissue model with cerebellar gray matter as the reference region. The initial region-of-interest (ROI)-based analysis yielded no significant group differences in mGluR5 availability. The voxel-based analysis revealed significantly lower [11C]ABP688 BPND in the middle temporal and inferior parietal cortices, and higher BPND in the superior temporal cortex in the alcohol dependence group compared with controls. Functional connectivity analysis of the rs-fMRI data employed seed regions identified from the quantitative [11C]ABP688 PET analysis, which revealed significantly altered functional connectivity from the inferior parietal cortex seed to the occipital pole and dorsal visual cortex in the alcohol dependence group compared with the control group. To our knowledge, this is the first report on the combined analysis of mGluR5 PET imaging and rs-fMRI in subjects with alcohol dependence. These preliminary results suggest the possibility of region-specific alterations of mGluR5 availability in vivo and related functional connectivity perturbations in prolonged abstinent subjects.

Low activity [11C]raclopride kinetic modeling in the mouse brain using the spatiotemporal kernel method

Depending on the molar activity of the tracer, the maximal allowable injected activity in mouse brain PET studies can be extremely low in order to avoid receptor saturation. Therefore, a high level of noise can be present in the image. We investigate several dynamic PET reconstruction methods in reduced counts, or equivalently in reduced injected activity, data exemplified in [11C]raclopride BP ND and quantification using the simplified reference tissue model (SRTM). We compared independent frame reconstruction (IFR), post-reconstruction HYPR denoising (IFR + HYPR), direct reconstruction using the SRTM model (DIR-SRTM), and the spatial (KERS) and spatiotemporal kernel reconstruction (KERST). Additionally, HYPR denoising of the frames used as features for the calculation of the spatial kernel matrix, was investigated (KERS-HYPR and KERST-HYPR). In vivo data of 11 mice, was used to generate list-mode data for five reduced count levels corresponding to reductions by a factor 4, 8, 12, 16 and 32 (equivalently 2.07, 1.04, 0.691, 0.518, and 0.260 MBq). Correlation of regional BP ND and values (reduced versus full counts reconstructions) was high (r > 0.94) for all methods, with KERS-HYPR and KERST-HYPR reaching the highest correlation (r > 0.96). Among methods with regularization, DIR-SRTM showed the largest variability in BP ND (Bland–Altman SD from 3.0% to 12%), while IFR showed it for (5.1%–14.6%). KERST and KERST-HYPR were the only methods with Bland–Altman bias and SD below 5% for noise level up to a reduction factor of 16. At the voxel level, BP ND and correlation was gradually decreased with increasing noise, with the largest correlation (BP ND r > 0.88, r > 0.62) for KERS-HYPR and KERST-HYPR. The spatial and the spatiotemporal kernel methods performed similarly, while using only temporal regularization with direct reconstruction showed more variability. Although R1 values present noise, using the spatiotemporal kernel reconstruction, accurate estimates of binding potential could be obtained with mouse injected activities as low as 0.26–0.518 MBq. This is desirable in order to maintain the tracer kinetics principle in mouse studies.

Test-Retest Variability of Relative Tracer Delivery Rate as Measured by [11C]PiB

PurposeModerate-to-high correlations have been reported between the [11C]PiB PET-derived relative tracer delivery rate R1 and relative CBF as measured using [15O]H2O PET, supporting its use as a proxy of relative CBF. As longitudinal PET studies become more common for measuring treatment efficacy or disease progression, it is important to know the intrinsic variability of R1. The purpose of the present study was to determine this through a retrospective data analysis.ProceduresTest-retest data belonging to twelve participants, who underwent two 90 min [11C]PiB PET scans, were retrospectively included. The voxel-based implementation of the two-step simplified reference tissue model with cerebellar grey matter as reference tissue was used to compute R1 images. Next, test-retest variability was calculated, and test and retest R1 measures were compared using linear mixed effect models and a Bland-Altman analysis.ResultsTest-retest variability was low across regions (max. 5.8 %), and test and retest measures showed high, significant correlations (R2=0.92, slope=0.98) and a negligible bias (0.69±3.07 %).ConclusionsIn conclusion, the high precision of [11C]PiB R1 suggests suitable applicability for cross-sectional and longitudinal studies.

Cerebral grey matter density is associated with neuroreceptor and neurotransporter availability: A combined PET and MRI study

Positron emission tomography (PET) can be used for in vivo measurement of specific neuroreceptors and transporters using radioligands, while voxel-based morphometric analysis of magnetic resonance images allows automated estimation of local grey matter densities. However, it is not known how regional neuroreceptor or transporter densities are reflected in grey matter densities. Here, we analyzed brain scans retrospectively from 328 subjects and compared grey matter density estimates with neuroreceptor and transporter availabilities. µ-opioid receptors (MORs) were measured with [11C]carfentanil (162 scans), dopamine D2 receptors with [11C]raclopride (92 scans) and serotonin transporters (SERT) with [11C]MADAM (74 scans). The PET data were modelled with simplified reference tissue model. Voxel-wise correlations between binding potential and grey matter density images were computed. Regional binding of all the used radiotracers was associated with grey matter density in region and ligand-specific manner independently of subjects’ age or sex. These data show that grey matter density and MOR and D2R neuroreceptor / SERT availability are correlated, with effect sizes (r2) ranging from 0.04 to 0.69. This suggests that future studies comparing PET outcome measure different groups (such as patients and controls) should also analyze interactive effects of grey matter density and PET outcome measures.

Parametric imaging of dual-time window [18F]flutemetamol and [18F]florbetaben studies

Optimal pharmacokinetic models for quantifying amyloid beta (Aβ) burden using both [18F]flutemetamol and [18F]florbetaben scans have previously been identified at a region of interest (ROI) level. The purpose of this study was to determine optimal quantitative methods for parametric analyses of [18F]flutemetamol and [18F]florbetaben scans. Forty-six participants were scanned on a PET/MR scanner using a dual-time window protocol and either [18F]flutemetamol (N=24) or [18F]florbetaben (N=22). The following parametric approaches were used to derive DVR estimates: reference Logan (RLogan), receptor parametric mapping (RPM), two-step simplified reference tissue model (SRTM2) and multilinear reference tissue models (MRTM0, MRTM1, MRTM2), all with cerebellar grey matter as reference tissue. In addition, a standardized uptake value ratio (SUVR) was calculated for the 90–110 min post injection interval. All parametric images were assessed visually. Regional outcome measures were compared with those from a validated ROI method, i.e. DVR derived using RLogan. Visually, RPM, and SRTM2 performed best across tracers and, in addition to SUVR, provided highest AUC values for differentiating between Aβ-positive vs Aβ-negative scans ([18F]flutemetamol: range AUC=0.96–0.97 [18F]florbetaben: range AUC=0.83–0.85). Outcome parameters of most methods were highly correlated with the reference method (R2≥0.87), while lowest correlation were observed for MRTM2 (R2=0.71–0.80). Furthermore, bias was low (≤5%) and independent of underlying amyloid burden for MRTM0 and MRTM1. The optimal parametric method differed per evaluated aspect; however, the best compromise across aspects was found for MRTM0 followed by SRTM2, for both tracers. SRTM2 is the preferred method for parametric imaging because, in addition to its good performance, it has the advantage of providing a measure of relative perfusion (R1), which is useful for measuring disease progression.

Spatially constrained kinetic modeling with dual reference tissues improves 18F-flortaucipir PET in studies of Alzheimer disease.

Recent studies have shown that standard compartmental models using plasma input or the cerebellum reference tissue input are generally not reliable for quantifying tau burden in dynamic 18F-flortaucipir PET studies of Alzheimer disease. So far, the optimal reference region for estimating 18F-flortaucipir delivery and specific tau binding has yet to be determined. The objective of the study is to improve 18F-flortaucipir brain tau PET quantification using a spatially constrained kinetic model with dual reference tissues. Participants were classified as either cognitively normal (CN) or cognitively impaired (CI) based on clinical assessment. T1-weighted structural MRI and 105-min dynamic 18F-flortaucipir PET scans were acquired for each participant. Using both a simplified reference tissue model (SRTM2) and Logan plot with either cerebellum gray matter or centrum semiovale (CS) white matter as the reference tissue, we estimated distribution volume ratios (DVRs) and the relative transport rate constant R1 for region of interest-based (ROI) and voxelwise-based analyses. Conventional linear regression (LR) and LR with spatially constrained (LRSC) parametric imaging algorithms were then evaluated. Noise-induced bias in the parametric images was compared to estimates from ROI time activity curve-based kinetic modeling. We finally evaluated standardized uptake value ratios at early phase (SUVREP, 0.7-2.9min) and late phase (SUVRLP, 80-105min) to approximate R1 and DVR, respectively. The percent coefficients of variation of R1 and DVR estimates from SRTM2 with spatially constrained modeling were comparable to those from the Logan plot and SUVRs. The SRTM2 using CS reference tissue with LRSC reduced noise-induced underestimation in the LR generated DVR images to negligible levels (< 1%). Inconsistent overestimation of DVR in the SUVRLP only occurred using the cerebellum reference tissue-based measurements. The CS reference tissue-based DVR and SUVRLP, and cerebellum-based SUVREP and R1 provided higher Cohen's effect size d to detect increased tau deposition and reduced relative tracer transport rate in CI individuals. Using a spatially constrained kinetic model with dual reference tissues significantly improved quantification of relative perfusion and tau binding. Cerebellum and CS are the suggested reference tissues to estimate R1 and DVR, respectively, for dynamic 18F-flortaucipir PET studies. Cerebellum-based SUVREP and CS-based SUVRLP may be used to simplify 18F-flortaucipir PET study.