Quantitative analyses of 18F-FEDAA1106 binding to peripheral benzodiazepine receptors in living human brain.

N-(5-Fluoro-2-phenoxyphenyl)-N-(2-(18)F-fluoroethyl-5-methoxybenzyl)acetamide ((18)F-FEDAA1106) is a potential PET ligand with highly selective and specific binding to peripheral benzodiazepine receptor (PBR). It has been reported that the regional density of PBR in the brain is increased in several neurodegenerative and psychiatric disorders. Thus, a reliable tracer method for evaluating PBR would be of use clinically and for research. To our knowledge, this is the first study to investigate the (18)F-FEDAA1106 binding to PBR in living human brain by PET. We also aimed to evaluate various analytic methods to quantify the density of PBR. METHODS: PET studies with (18)F-FEDAA1106 were performed on 7 healthy men. Volumes of interest (VOIs) were drawn on PET images. In each VOI, binding potential (BP) was calculated by nonlinear least-squares (NLS) fitting based on the 2-tissue compartment model, and the distribution volume (DV) was also estimated by NLS, Logan plot, and multilinear analysis (MA) methods. To estimate errors in calculation of BP and DV, simulation studies were also performed. RESULTS: The DVs estimated with each of the methods were significantly correlated. There was also significant correlation between BP with NLS and DV with NLS, Logan plot, or MA. But the interindividual differences in the distribution volume of the free and nonspecific binding compartment (K(1)/k(2)) were relatively large. In a simulation study, variation of the DV estimated by Logan plot was relatively small, but it was underestimated as the noise increased. By MA, the bias of DV was smaller, but the variation of DV was larger than by Logan plot. Within a 3% noise level, there was almost no difference between Logan plot and MA in both bias and variation. DVs estimated by both Logan plot and MA were underestimated by 10%-20%. Although the variation of DV was larger by NLS than by Logan plot, it was small enough in the noise level of VOI analysis, and the bias of DV was 0%-2%. CONCLUSION: The simulation studies indicated that NLS is a suitable method for the estimation of (18)F-FEDAA1106 binding to PBRs.     

A 18F-MPPF PET normative database of 5-HT1A receptor binding in men and women over aging.

Neurotransmission imaging studies require normative data for the statistical assessment of neurophysiologic dysfunctions. 2'-Methoxyphenyl-(N-2'-pyridinyl)-p-18F-fluoro-benzamidoethylpiperazine (18F-MPPF) is a specific serotonin 5-HT1A antagonist PET tracer recently characterized, modeled, and used for clinical research to explore abnormalities in the serotoninergic system. Our study reports, to our knowledge, the first large normative imaging database of 18F-MPPF binding potential (BP) over aging, for both males and females. METHODS: Fifty-three healthy volunteers (27 females, 26 males; age, 20-70 y) were selected to undergo structural MRI and single-injection 18F-MPPF multiframe dynamic PET. 18F-MPPF BP values were computed using a nonlinear modeling method with tissue reference. The statistical assessment of the effect of age and sex was performed both at the anatomic structure level, using regions of interest drawn manually on individual MR images, and at the voxel level, using normalized BP parametric images in different statistical parametric mapping designs. RESULTS: A negative linear correlation between age and 18F-MPPF binding (3.6% decrease by decade) was found in females but not in males and involved most of the limbic and paralimbic regions; on the other hand, males in their 30s showed decreased binding in most cerebral regions. CONCLUSION: A comparison of males and females revealed higher BP values independent of age in females in the right hemisphere and a different evolution of BP over aging. These results confirm the necessity of a database for further statistical analysis in individuals or groups with pathology.     

Estimation of serotonin transporter parameters with 11C-DASB in healthy humans: reproducibility and comparison of methods.

The aim of the present study was to define the optimal analytic method to derive accurate and reliable serotonin transporter (SERT) receptor parameters with (11)C-3-amino-4-(2-[(dimethylamino)methyl]phenylthio)benzonitrile ((11)C-DASB). METHODS: Nine healthy subjects (5 females, 4 males) underwent two (11)C-DASB PET scans on the same day. Five analytic methods were used to estimate binding parameters in 10 brain regions: compartmental modeling with 1- and 2-tissue compartment models (1TC and 2TC), data-driven estimation of parametric images based on compartmental theory (DEPICT) analysis, graphical analysis, and the simplified reference tissue model (SRTM). Two variations in the fitting procedure of the SRTM method were evaluated: nonlinear optimization and basis function approach. The test/retest variability (VAR) and intraclass correlation coefficient (ICC or reliability) were assessed for 3 outcome measures: distribution volume (V(T)), binding potential (BP), and specific to nonspecific equilibrium partition coefficient (V(3)'). RESULTS: All methods gave similar values across all regions. The variability of V(T) was excellent (< or =10%) in all regions, for the 1TC, 2TC, DEPICT, and graphical approaches. The variability of BP and V(3)' was good in regions of high SERT density and poorer in regions of moderate and lower densities. The ICC of all 3 outcome measures was excellent in all regions. The basis function implementation of SRTM demonstrated improved reliability compared with nonlinear optimization, particularly in moderate and low-binding regions. CONCLUSION: The results of this study indicate that (11)C-DASB can be used to measure SERT parameters with high reliability and low variability in receptor-rich regions of the brain, with somewhat less reliability and increased variability in regions of moderate SERT density and poor reproducibility in low-density regions.     

In vivo imaging of human cerebral nicotinic acetylcholine receptors with 2-18F-fluoro-A-85380 and PET.

2-(18)F-fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine (2-(18)F-fluoro-A-85380) is a PET radioligand that is specific for nicotinic acetylcholine receptors (nAChRs) and has a high affinity for the alpha(4)beta(2) subtype. The purpose of this study was to evaluate different strategies to quantify 2-(18)F-fluoro-A-85380 binding in healthy nonsmoking human volunteers. METHODS: After intravenous injection of 189 +/- 30 MBq (0.8-5.7 nmol) of 2-(18)F-fluoro-A-85380, the first dynamic PET scan was acquired over 150 min. The second 30-min PET scan was performed 60 min later. Time-activity curves were generated from volumes of interest. 2-(18)F-Fluoro-A-85380 volume of distribution (DV) was quantified using compartmental kinetic analysis and Logan graphical analysis. In the kinetic analysis, the 1-tissue compartment model (1TCM) and the 2-tissue (2TCM) compartment model were applied. The most appropriate kinetic model was determined using the Akaike Information Criterion. The effect of reducing the PET study duration on the reliability of the DV values computed by the kinetic and the graphical analyses was evaluated. RESULTS: Time-activity curves were better described by the 2TCM. The DV values ranged from 5.2 +/- 0.5 in the occipital cortex, 6.2 +/- 0.2 in the frontal cortex, and 7.3 +/- 0.4 in the putamen to 15.4 +/- 2.1 in the thalamus. These regional DV values were consistent with the distribution of nAChRs in the human brain. Logan graphical analysis provided slightly lower DV values than those of the 2TCM (from -3.5% in the occipital cortex to -6.6% in the thalamus). The minimal study duration required to obtain stable DV estimates in all regions was similar for the 2 methods: 140 min for the 2TCM and 150 min for the Logan analysis. DV estimates obtained with the 2TCM were more stable than those calculated by the Logan approach for the same scan duration. CONCLUSION: These results show that 2-(18)F-fluoro-A-85380 can be used to assess nAChRs binding in the human brain with PET.     

Simplified quantification of nicotinic receptors with 2[18F]F-A-85380 PET

INTRODUCTION: Neuronal nicotinic acetylcholine receptors (nAChRs), widely distributed in the human brain, are implicated in various neurophysiological processes as well as being particularly affected in neurodegenerative conditions such as Alzheimer's disease. We sought to evaluate a minimally invasive method for quantification of nAChR distribution in the normal human brain, suitable for routine clinical application, using 2[(18)F]F-A-85380 and positron emission tomography (PET). METHODS: Ten normal volunteers (four females and six males, aged 63.40+/-9.22 years) underwent a dynamic 120-min PET scan after injection of 226 MBq 2[(18)F]F-A-85380 along with arterial blood sampling. Regional binding was assessed through standardized uptake value (SUV) and distribution volumes (DV) obtained using both compartmental (DV(2CM)) and graphical analysis (DV(Logan)). A simplified approach to the estimation of DV (DV(simplified)), defined as the region-to-plasma ratio at apparent steady state (90-120 min post injection), was compared with the other quantification approaches. RESULTS: DV(Logan) values were higher than DV(2CM). A strong correlation was observed between DV(simplified), DV(Logan) (r=.94) and DV(2CM) (r=.90) in cortical regions, with lower correlations in thalamus (r=.71 and .82, respectively). Standardized uptake value showed low correlation against DV(Logan) and DV(2CM). CONCLUSION: DV(simplified) determined by the ratio of tissue to metabolite-corrected plasma using a single 90- to 120-min PET acquisition appears acceptable for quantification of cortical nAChR binding with 2[(18)F]F-A-85380 and suitable for clinical application.     

Test–retest stability of cerebral A<Subscript>1</Subscript> adenosine receptor quantification using [<Superscript>18</Superscript>F]CPFPX and PET

PURPOSE: The goal of the present study was to evaluate the reproducibility of cerebral A1 adenosine receptor (A1AR) quantification using [18F]CPFPX and PET in a test-retest design. METHODS: Eleven healthy volunteers were studied twice. Eight brain regions ranging from high to low receptor binding were examined. [18F]CPFPX was injected as a bolus with subsequent infusion over 120 min. Various outcome parameters were compared based on either metabolite-corrected venous blood sampling [e.g. apparent equilibrium total distribution volume (DVt')] or a reference region [ratio of specific to non-specific distribution volume (BP2)]. RESULTS: Test-retest variability was low in the outcome measure BP2 (on average 5.9%) and moderate in DVt' (on average 13.2%). Regarding reproducibility, the outcome parameter BP2 showed an intra-class correlation coefficient (ICC) of 0.94 +/- 0.1. For DVt' the between-subject coefficient of variation (%CV) was similar to the within-subject %CV (around 10%), resulting in a poor ICC of 0.06 +/- 0.2. CONCLUSION: Our results suggest that quantification of [18F]CPFPX imaging is reproducible and reliable for PET studies of the cerebral A1AR. Among the outcome parameters the non-invasive measures were of superior test-retest stability over the invasive.     

Use of reference tissue models for quantification of histamine H1 receptors in human brain by using positron emission tomography and [11C]doxepin

The aim of the present study is to evaluate the validity of the simplified reference tissue model (SRTM) and of Logan graphical analysis with reference tissue (LGAR) for quantification of histamine H1 receptors (H1Rs) by using positron emission tomography (PET) with [11C]doxepin. These model-based analytic methods (SRTM and LGAR) are compared to Logan graphical analysis (LGA) and to the one-tissue model (1TM), using complete datasets obtained from 5 healthy volunteers. Since HIR concentration in the cerebellum can be regarded as negligibly small, the cerebellum was selected as the reference tissue in the present study. The comparison of binding potential (BP) values estimated by LGAR and 1TM showed good agreement; on the other hand, SRTM turned out to be unstable concerning parameter estimation in several regions of the brain. By including the results of noise analysis, LGAR became a reliable method for parameter estimation of [11C]doxepin data in the cortical regions.     

Reference and target region modeling of [11C]-(R)-PK11195 brain studies.

PET with [(11)C]-(R)-PK11195 is currently the modality of choice for the in vivo imaging of microglial activation in the human brain. In this work we devised a supervised clustering procedure and a new quantification methodology capable of producing binding potential (BP) estimates quantitatively comparable with those derived from plasma input with robust quantitative implementation at the pixel level. METHODS: The new methodology uses predefined kinetic classes to extract a gray matter reference tissue without specific tracer binding and devoid of spurious signals (in particular, blood pool and muscle). Kinetic classes were derived from an historical database of 12 healthy control subjects and from 3 patients with Huntington's disease. BP estimates were obtained using rank-shaping exponential spectral analysis (RS-ESA) (both plasma and reference input) and the simplified reference tissue model (SRTM). Comparison between plasma- derived BPs and those produced with the new reference methodology was performed using 6 additional healthy control subjects. Reliability of the new methodology was performed on 4 test-retest studies of patients with Alzheimer's disease. RESULTS: The new algorithm selected reference voxels in gray matter tissue avoiding regions with specific binding located, in particular, in the venous and arterial circulation. Using the new reference, BP values obtained using a plasma input and a reference input were in excellent agreement and highly correlated (r = 0.811, P < 10(-5)) when calculated with RS-ESA and less so (r = 0.507, P < 0.005) when SRTM was used. In the production of parametric maps, SRTM was used with the new reference extraction, resulting in test-retest variability (10.6%; mean ICC = 0.878) that was superior to that obtained using the previous unsupervised clustering approach (mean ICC = 0.596). CONCLUSION: Reference region modeling combined with supervised reference tissue extraction produces a robust and reproducible quantitative assessment of [(11)C]-(R)-PK11195 studies in the human brain.     

Reproducibility studies with 11C-DTBZ, a monoamine vesicular transporter inhibitor in healthy human subjects.

The reproducibility of (+/-)-alpha-[11C] dihydrotetrabenazine (DTBZ) measures in PET was studied in 10 healthy human subjects, aged 22-76 y. METHODS: The scan-to-scan variation of several measures used in PET data analysis was determined, including the radioactivity ratio (target-to-reference), plasma-input Logan total distribution volume (DV), plasma-input Logan Bmax/Kd and tissue-input Logan Bmax/Kd values. RESULTS: The radioactivity ratios, plasma-input Bmax/Kd and tissue-input Bmax/Kd all have higher reliability than plasma-input total DV values. In addition, measures using the occipital cortex as the reference region have higher reliability than the same measures using the cerebellum as the reference region. CONCLUSION: Our results show that DTBZ is a reliable PET tracer that provides reproducible in vivo measurement of striatal vesicular monoamine transporter density. In the selection of reference regions for DTBZ PET data analysis, caution must be exercised in circumstances when DTBZ binding in the occipital cortex or the cerebellum may be altered.     

Simplified quantification of Pittsburgh Compound B amyloid imaging PET studies: a comparative analysis.

PET studies have been performed using the amyloid binding radiotracer Pittsburgh Compound B (PIB). Previous quantitative analyses using arterial blood showed that the Logan graphical analysis using 90 min of emission data (ART90) provided a reliable measure of PIB retention. This work reports on simplified methods of analysis for human PIB imaging. METHODS: PIB PET scans were conducted in 24 subjects (6 Alzheimer's disease [AD], 10 mild cognitive impairment [MCI], 8 controls) with arterial blood sampling. Retest scans were performed on 8 subjects (3 AD, 1 MCI, 4 controls) within 28 d. Data were analyzed over 60 and 90 min using the Logan analysis and (a) metabolite-corrected input functions based on arterial plasma (ART60, ART90), (b) carotid artery time-activity data with a population average metabolite correction (CAR60, CAR90); and (c) cerebellar reference tissue (CER60, CER90). Data also were analyzed using the simplified reference tissue method (SRTM60, SRTM90) and a single-scan method based on late-scan ratios of standardized uptake values (SUVR60, SUVR90). RESULTS: All methods of analysis examined effectively discerned regional differences between AD and control subjects in amyloid-laden cortical regions, although the performance of the simplified methods varied in terms of bias, test-retest variability, intersubject variability, and effect size. CAR90 best agreed with ART90 distribution volume ratio (DVR) measures across brain regions and subject groups and demonstrated satisfactory test-retest variability (+/-7.1% across regions). CER90 and CER60 showed negative biases relative to ART90 in high-DVR subjects but had the lowest test-retest variability. The single-scan SUV-based methods showed the largest effect sizes for AD and control group differences and performed well in terms of intersubject and test-retest variability. CONCLUSION: Of the simplified methods for PIB analysis examined, CAR90 provided DVR measures that were most comparable to ART90; CER90 was the most reproducible and SUVR90 produced the largest effect size. All simplified methods were effective at distinguishing AD and control differences and may be effectively used in the analysis of PIB. SUVR60 data can be obtained with as little as 20 min of PET emission data collection. The relative strengths and limitations of each method must be considered for each experimental design.     

A reduced extracellular serotonin level increases the 5-HT1A PET ligand 18F-MPPF binding in the rat hippocampus.

4,2'-(Methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-fluorobenzamido]ethylpiperazine ((18)F-MPPF) is a radiotracer used in clinical PET studies for the visualization of serotonin-1A (5-HT(1A)) receptors. In a previous study, we demonstrated that a rapid enhancement of extracellular serotonin concentrations influences (18)F-MPPF-specific binding. Because endogenous serotonin is significantly decreased in some pathologies, the aim of this study was to determine whether (18)F-MPPF is sensitive to depletion of this neurotransmitter. METHODS: Using the beta-microprobe, an original beta(+)-sensitive intracerebral probe, and microdialysis, the effect of decreased serotonin on the specific binding of (18)F-MPPF to 5-HT(1A) receptors was investigated in the hippocampus of the anesthetized rat. Extracellular serotonin was pharmacologically decreased in the hippocampus after a single injection of p-ethynylphenylalanine ([p-EPA] 5 mg/kg), a new tryptophan hydroxylase inhibitor. RESULTS: Our results showed that the (18)F-MPPF-specific binding was significantly enhanced after the decrease of extracellular serotonin. These results were confirmed by the (18)F-MPPF distribution in cerebral tissues (hippocampus-to-cerebellum ratio) and by the decrease of the extracellular (18)F-MPPF collected in hippocampal dialysates. CONCLUSION: This study further supports the view that (18)F-MPPF binding potential is increased in the hippocampus if the endogenous serotonin is pharmacologically decreased after a p-EPA injection. This phenomenon will be an additional factor in the interpretation of the results from (18)F-MPPF clinical PET studies.     

The potential of a radiosensitive intracerebral probe to monitor 18F-MPPF binding in mouse hippocampus in vivo

As mouse imaging has become more challenging in preclinical research, efforts have been made to develop dedicated PET systems. Although these systems are currently used for the study of physiopathologic murine models, they present some drawbacks for brain studies, including a low temporal resolution that limits the pharmacokinetic study of radiotracers. The aim of this study was to demonstrate the ability of a radiosensitive intracerebral probe to measure the binding of a radiotracer in the mouse brain in vivo. METHODS: The potential of a probe 0.25 mm in diameter for pharmacokinetic studies was assessed. First, Monte Carlo simulations followed by experimental studies were used to evaluate the detection volume and sensitivity of the probe and its adequacy for the size of loci in the mouse brain. Second, ex vivo autoradiography of 5-hydroxytryptamine receptor 1A (5-HT(1A)) receptors in the mouse brain was performed with the PET radiotracer 2'-methoxyphenyl-(N-2'-pyridinyl)-p-(18)F-fluorobenzamidoethylpiperazine ((18)F-MPPF). Finally, the binding kinetics of (18)F-MPPF were measured in vivo in both the hippocampus and the cerebellum of mice. RESULTS: Both the simulations and the experimental studies demonstrated the feasibility of using small probes to measure radioactive concentrations in specific regions of the mouse brain. Ex vivo autoradiography showed a heterogeneous distribution of (18)F-MPPF consistent with the known distribution of 5-HT(1A) in the mouse brain. Finally, the time-activity curves obtained in vivo were reproducible and validated the capacity of the new probe to accurately measure (18)F-MPPF kinetics in the mouse hippocampus. CONCLUSION: Our results demonstrate the ability of the tested radiosensitive intracerebral probe to monitor binding of PET radiotracers in anesthetized mice in vivo, with high temporal resolution suited for compartmental modeling.     

Optimal duration of PET studies with 18F-fluoroethyl-diprenorphine.

The tracer 6-O-(2-(18)F-fluoroethyl)-6-O-desmethyldiprenorphine (18F-FDPN) provides enhanced flexibility to PET studies of the opioidergic system because the label has a longer half-life than the label of 11C-diprenorphine. Here we evaluated the ideal length of PET studies with 18F-FDPN. METHODS: 18F-FDPN binding kinetics were quantified with protocols of different lengths by use of a 1-tissue or a 2-tissue compartment model for different volumes of interest. Furthermore, the effects of scanning duration were assessed by parametric analyses. RESULTS: A 90-min protocol resulted in less than 10% bias in distribution volume (DV) relative to the full-length protocol. Correlation analyses of the DV estimates for the full-length protocol and the shortened protocols showed good replication of DV estimates for regions with both low and high levels of binding at schedules of up to 90 min. CONCLUSION: Data sampling in dynamic 18F-FDPN PET acquisitions should not be shorter than 90 min to maintain reliable estimates of DV.     

Characterization of the SPECT 5-HT2A receptor ligand 123I-R91150 in healthy volunteers: Part 1--pseudoequilibrium interval and quantification methods.

With the aim of characterizing radioiodinated 4-amino-N-1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]5-iodo-2-methoxybenzamide ((123)I-R91150) as a SPECT ligand for subtype 2A of the 5-hydroxytryptamine receptor (5-HT(2A)), tracer kinetic compartmental analyses were compared with the tissue ratio method (TR). The pseudoequilibrium interval after a single bolus injection was identified, and a reference database of specific uptake ratio (SUR) values was obtained. Within-scan and between-subject variability was also assessed. METHODS: Nineteen healthy men (mean age +/- SD, 24.4 +/- 3.3 y) were included and separated into 2 groups. Dynamic scans with venous blood sampling from 0 to 470 min after a single bolus injection of (123)I-R91150 was completed for 7 of the 9 subjects included in group A, and in one of them compartmental modeling was performed with an arterial blood input function using 1-tissue-compartment (1TC) and 2-tissue-compartment (2TC) models. Binding potential (BP) using the simplified reference tissue model (SRTM) (BP(SRTM)) and SUR values using TR over time were also calculated. The 10 remaining subjects (group B) underwent a single scan at pseudoequilibrium with the aim of improving the precision of mean normal SUR estimates. Regions of interest in cortical regions and basal ganglia for specific uptake, and in cerebellum for nonspecific uptake, were manually drawn on each subject's MR images and translated to the corresponding SPECT slices after coregistration. RESULTS: The 1TC model correlated well with the 2TC model (BP(2TC) = 1.04.BP(1TC) - 0.01, R(2) = 0.98), and both methods correlated with BP(SRTM) and SUR with little bias (BP(1TC) = 1.10 BP(SRTM) + 0.03, R(2) = 0.98; BP(2TC) = 1.15 BP(SRTM) + 0.01, R(2) = 0.98; BP(SRTM) = 0.99 SUR(mean) + 0.01, R(2) = 0.98). SUR values stabilized from 180 min after injection in most cortical regions, ranging from 0.51 +/- 0.10 in the orbitofrontal region to 0.27 +/- 0.09 in the parietal region. Within-scan and between-subject variability among regions ranged from 10% to 14.8%, and from 18.3% to 35.4%, respectively. CONCLUSION: (123)I-R91150 distribution agrees with autoradiography results, showing highly specific binding in cortical regions. The correlations found among 1TC, 2TC, SRTM, and TR outcome measurements support the use of TR for quantification of 5-HT(2A) receptor binding with (123)I-R91150 SPECT and a simple protocol avoiding arterial blood sampling and serial scanning over time.     

Reproducibility of [11 C]FLB 457 binding in extrastriatal regions

Extrastriatal D2 dopamine receptors represent an important target of research into the pathophysiology and pharmacotherapy of psychiatric disorders. The high affinity radioligand [11C]FLB 457 makes possible the measurement of low concentrations of D2 receptors in extrastriatal regions using positron emission tomography (PET). The aim of this study was to assess the test/retest variability and reliability of [11C]FLB 457 binding using a reference tissue model. Eight healthy male subjects (aged 20-33 years) underwent two [11C]FLB 457 PET examinations. Radioactivity in the cerebellum was used as the reference. The binding potentials (BPs) for five cortical regions of interest (ROIs) were calculated using the reference tissue model. The BP was also calculated for each pixel in the form of parametric images. Reproducibility was assessed both for the ROI method and for the parametric images. The test/retest reproducibility for [11C]FLB 457 binding was good, with a mean variability ranging from 4.5% for the thalamus to 15.5% for the hippocampus. The parametric images also demonstrated good reproducibility. These results support the suitability of using [11C]FLB 457 for the quantitative evaluation of extrastriatal D2 receptors and for protocols requiring repeated measurements in the same individual.     

Validation of an extracerebral reference region approach for the quantification of brain nicotinic acetylcholine receptors in squirrel monkeys with PET and 2-18F-fluoro-A-85380.

The aim of the present study was to explore the applicability of an extracerebral reference region for the quantification of cerebral receptors with PET. METHODS: Male squirrel monkeys underwent quantitative PET studies of cerebral nicotinic acetylcholine receptors (nAChRs) with 2-(18)F-fluoro-A-85380 (2-FA). Data from dynamic PET scans were analyzed with various compartment- and non-compartment-based models, including a simplified reference tissue model (SRTM). Nondisplaceable volume-of-distribution (VDnd) values were determined in regions of interest after the blockade of 2-FA-specific binding by nicotine infusion. Binding potential values, estimated with the cerebellum and muscle as reference regions, were compared and the reproducibility of measurements was determined. RESULTS: One- and 2-tissue-compartment modeling and linear graphic analysis provided similar total volume-of-distribution (VD(T)) values for each studied region. VD(T) values were high in the thalamus, intermediate in the cortex and midbrain, and low in the cerebellum and muscle, consistent with the distribution pattern of nAChR containing alpha(4) and beta(2) receptor subunits (alpha(4)beta(2)*). The administration of nicotine at 2 mg/kg/d via an osmotic pump resulted in a nearly complete saturation of 2-FA-specific binding and led to very small changes in volumes of distribution in the cerebellum and muscle (-9% +/- 4% [mean +/- SEM] and 0% +/- 6%, respectively), suggesting limited specific binding of the radioligand in these areas. VD(T) measured in muscle in 15 monkeys was reasonably constant (3.0 +/- 0.2, with a coefficient of variation of 8%). VDnd in studied brain regions exceeded VD(T) in muscles by a factor of 1.3. With this factor and with muscle as a reference region, BP* values calculated for studied brain regions with the SRTM were in good agreement with those obtained with the cerebellum as a reference region. Significant correlations were observed between BP* values estimated with these 2 approaches. The reproducibilities of BP* measurements obtained with the 2 methods were comparable, with coefficients of variation of less than 11% and 13% for the thalamus and the cortex, respectively. CONCLUSION: These results suggest that the accurate quantification of nAChRs can be performed with 2-FA and a reference region outside the brain, providing a novel approach for the quantification of brain receptors when no suitable cerebral reference region is available.     

Modeling and analysis of PET studies with norepinephrine transporter ligands: the search for a reference region

The development of positron emission tomography (PET) ligands for the norepinephrine transporter (NET) has been slow compared to the development of radiotracers for others systems, such as the dopamine (DAT) or the serotonin transporters (SERT). The main reason for this appears to be the high nonspecific (non-NET) binding exhibited by many of these tracers, which makes the identification of a reference region difficult. With other PET ligands the use of a reference region increases the reproducibility of the outcome measure in test/retest studies. The focus of this work is to identify a suitable reference region or means of normalizing data for the NET ligands investigated. METHODS: We have analyzed the results of PET studies in the baboon brain with labeled reboxetine derivatives (S,S)-[(11)C]O-methyl reboxetine (SS-MRB), (S,S)-[(18)F]fluororeboxetine (SS-FRB) as well as O-[(11)C]nisoxetine and N-[(11)C]nisoxetine (NIS), and, for comparison, the less active (R,R) enantiomers (RR-MRB, RR-FRB) in terms of the distribution volume (DV) using measured arterial input functions. RESULTS: (1) For a given subject, a large variation in DV for successive baseline studies was observed in regions with both high and low NET density. (2) The occipital cortex and the basal ganglia were found to be the regions with the smallest change between baseline (SS-MRB) and pretreatment with cocaine, and were therefore used as a composite reference region for calculation of a distribution volume ratio (DVR). (3) The variability [as measured by the coefficient of variation (CV) = standard deviation/mean] in the distribution volume ratio (DVR) of thalamus (to reference region) was considerably reduced over that of the DV using this composite reference region. (4) Pretreatment with nisoxetine (1.0 mg/kg 10 min prior to tracer) in one study produced (in decreasing order) reductions in thalamus, cerebellum, cingulate and frontal cortex consistent with known NET densities. (5) [(11)C]Nisoxetine had a higher background non-NET binding (DV) than the other tracers reported here with basal ganglia (a non-NET region) higher than thalamus. CONCLUSIONS: The reboxetine derivatives show a lot of promise as tracers for human PET studies of the norepinephrine system. We have identified a strategy for normalizing DVs to a reference region with the understanding that the DVR for these tracers may not be related to the binding potential in the same way as, for example, for the dopamine tracers, since the non-NET binding may differ between the target and nontarget regions. From our baboon studies the average DVR for thalamus (n = 18) for SS-MRB is 1.8; however, the lower limit is most likely less than 1 due to this difference in non-NET binding.     

Quantitative analysis of norepinephrine transporter in the human brain using PET with (S,S)-18F-FMeNER-D2

(S,S)-18F-FMeNER-D2 was recently developed as a radioligand for the measurement of norepinephrine transporter imaging with PET. In this study, a norepinephrine transporter was visualized in the human brain using this radioligand with PET and quantified by several methods. METHODS: PET scans were performed on 10 healthy men after intravenous injection of (S,S)-18F-FMeNER-D2. Binding potential relative to nondisplaceable binding (BP(ND)) was quantified by the indirect kinetic, simplified reference-tissue model (SRTM), multilinear reference-tissue model (MRTM), and ratio methods. The indirect kinetic method was used as the gold standard and was compared with the SRTM method with scan times of 240 and 180 min, the MRTM method with a scan time of 240 min, and the ratio method with a time integration interval of 120-180 min. The caudate was used as reference brain region. RESULTS: Regional radioactivity was highest in the thalamus and lowest in the caudate during PET scanning. BP(ND) values by the indirect kinetic method were 0.54 +/- 0.19 and 0.35 +/- 0.25 in the thalamus and locus coeruleus, respectively. BP(ND) values found by the SRTM, MRTM, and ratio methods agreed with the values demonstrated by the indirect kinetic method (r = 0.81-0.92). CONCLUSION: The regional distribution of (S,S)-18F-FMeNER-D2 in our study agreed with that demonstrated by previous PET and postmortem studies of norepinephrine transporter in the human brain. The ratio method with a time integration interval of 120-180 min will be useful for clinical research of psychiatric disorders for estimation of norepinephrine transporter occupancy by antidepressants without requiring arterial blood sampling and dynamic PET.     

SPECT of serotonin transporters using 123I-ADAM: optimal imaging time after bolus injection and long-term test-retest in healthy volunteers.

(123)I-ADAM (2-([2-([dimethylamino]methyl)phenyl]thio)-5-(123)I-iodophenylamine) has been recently proposed as a new serotonin transporter (SERT) ligand for SPECT. The objective of this study was to characterize (123)I-ADAM in healthy volunteers. (123)I-ADAM distribution in the normal brain, pseudoequilibrium interval after a single injection, normal specific uptake values, and long-term test-retest variability and reliability were investigated. METHODS: Ten healthy volunteers underwent 2 SPECT sessions under the same conditions 47.6 +/- 24.0 d apart. Scans were sequentially acquired from the time of (123)I-ADAM intravenous injection up to 12 h after injection. Regions of interest (ROIs) for cerebellum (C), midbrain, thalamus, striatum, mesial temporal region, and cortex were drawn on MR images and pasted to corresponding SPECT slices after coregistration. Specific uptake ratios (SURs) at pseudoequilibrium and the simplified reference tissue model (SRTM) methods were used for quantification. SURs were obtained as ([region - C]/C) at each time point. Test-retest variability and reliability (intraclass correlation coefficient [ICC]) were calculated. RESULTS: The highest (123)I-ADAM specific uptake was found in the midbrain and thalamus, followed by the striatum and mesial temporal region. Quantification results using SUR and SRTM were correlated with R = 0.93 (test) and R = 0.94 (retest). SURs remained stable in all regions from 4 to 6 h after injection. Using SUR, test-retest variability/ICC were 13% +/- 11%/0.74 in midbrain, 16% +/- 13%/0.63 in thalamus, 19% +/- 18%/0.62 in striatum, and 22% +/- 19%/0.05 in mesial temporal region. CONCLUSION: (123)I-ADAM accumulates in cerebral regions with high known SERT density. The optimal imaging time for (123)I-ADAM SPECT quantification is suggested to be from 4 to 6 h after a single injection. Long-term test-retest variability and reliability found in the midbrain are comparable to that reported with other (123)I-labeled SPECT ligands. These results support the use of (123)I-ADAM SPECT for SERT imaging after a single injection in humans.     

PET imaging of serotonin transporters with [11C]DASB: test-retest reproducibility using a multilinear reference tissue parametric imaging method.

Parametric imaging of serotonin transporters (SERT) with 11C-labeled 3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)benzonitrile ([11C]DASB) PET is a useful data analysis tool. The purpose of this study was to evaluate the reproducibility of measurements of SERT binding potential (BP) and relative blood flow (R1) by a 2-parameter multilinear reference tissue parametric imaging method (MRTM2) for human [11C]DASB studies. METHODS: Eight healthy subjects (3 men, 5 women; age, 26 +/- 9 y) underwent 2 [11C]DASB PET scans separated by 1 h on the same day (dose, 703 +/- 111 MBq). Parametric images of BP and R1 were generated by MRTM2 using the cerebellum as a reference region. The k'2 (clearance rate constant from the reference region) required by MRTM2 was estimated by the 3-parameter MRTM. Reproducibility of BP and R1 measurements was evaluated by calculating bias (100 x (retest - test/test), variability (SD of the bias), and reliability (intraclass correlation coefficient = rho) for several representative regions of interest (ROIs). BP and R1 were estimated for ROI time-activity curves fitted by MRTM2 and were compared with those based on the parametric images. RESULTS: The test-retest (0.066 +/- 0.013/0.06 +/- 0.011 min(-1)) MRTM k'2 reproducibility was excellent with small bias (3%) and variability (6%) and high reliability (0.95). Retest BP values were consistently lower than those of test BP values in all regions (a mean negative bias of approximately 6%; P < 0.001). The test-retest BP variability was relatively small, ranging from 4% to 13%, with rho ranging from 0.44 to 0.85. In contrast to BP, test-retest R1 values were similar with negligible bias of < or =0.1%. The test-retest R1 variability was excellent and smaller than that of BP ranging from 3% to 6%, with rho ranging from 0.58 to 0.95. BP and R1 values estimated by the ROI time-activity curve-fitting method were slightly lower ( approximately 3% and approximately 1%, respectively) than those by the parametric imaging method (P < 0.001). However, the test-retest bias and variability of BP and R1 were very similar for both ROI and parametric methods. CONCLUSION: Our results suggest that [11C]DASB parametric imaging of BP and R1 with the noninvasive MRTM2 method is reproducible and reliable for PET studies of SERT.