Compartmental modeling of 11C-HOMADAM binding to the serotonin transporter in the healthy human brain

Test-Retest Reliability of the SERT Imaging Agent 11C-HOMADAM in Healthy Humans

The aim of this study was to measure the test-retest reliability of ¹¹C-N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (¹¹C-HOMADAM) imaging of serotonin transporter (SERT) density in healthy control subjects. Methods: Two female and 2 male volunteers participated in the study, with each undergoing three 90-min ¹¹C-HOMADAM PET scans. Time-activity curves were derived from SERT-rich structures and fit to 2 models: a simplified reference tissue model and a multilinear graphical model. Binding potential, the ratio of specifically bound uptake to nondisplaceable uptake at equilibrium, was calculated from the model parameter estimates. Ninety-five percent confidence intervals and the intraclass correlation coefficient (ICC) were calculated and adjusted for repeated measures. Results: The ICC values ranged from -0.13 in the dorsal raphe to 0.88 in the caudate nucleus. The highest average ICC values were in the striatum, but other regions were sensitive to measurement outliers. Conclusion: Good-to-excellent test-retest reliability was observed for SERT binding in the striatum. The dorsal raphe ICC value was sensitive to a measurement outlier. ¹¹C-HOMADAM binding potential calculated from the simplified reference tissue model and the multilinear graphical model were robust and in good agreement.

A Bayesian spatial temporal mixtures approach to kinetic parametric images in dynamic positron emission tomography

PURPOSE

Estimation of parametric maps is challenging for kinetic models in dynamic positron emission tomography. Since voxel kinetics tend to be spatially contiguous, the authors consider groups of homogeneous voxels together. The authors propose a novel algorithm to identify the groups and estimate kinetic parameters simultaneously. Uncertainty estimates for kinetic parameters are also obtained.

METHODS

Mixture models were used to fit the time activity curves. In order to borrow information from spatially nearby voxels, the Potts model was adopted. A spatial temporal model was built incorporating both spatial and temporal information in the data. Markov chain Monte Carlo was used to carry out parameter estimation. Evaluation and comparisons with existing methods were carried out on cardiac studies using both simulated data sets and a pig study data. One-compartment kinetic modeling was used, in which K1 is the parameter of interest, providing a measure of local perfusion.

RESULTS

Based on simulation experiments, the median standard deviation across all image voxels, of K1 estimates were 0, 0.13, and 0.16 for the proposed spatial mixture models (SMMs), standard curve fitting, and spatial K-means methods, respectively. The corresponding median mean squared biases for K1 were 0.04, 0.06, and 0.06 for abnormal region of interest (ROI); 0.03, 0.03, and 0.04 for normal ROI; and 0.007, 0.02, and 0.05 for the noise region.

CONCLUSIONS

SMM is a fully Bayesian algorithm which determines the optimal number of homogeneous voxel groups, voxel group membership, parameter estimation, and parameter uncertainty estimation simultaneously. The voxel membership can also be used for classification purposes. By borrowing information from spatially nearby voxels, SMM substantially reduces the variability of parameter estimates. In some ROIs, SMM also reduces mean squared bias.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

Self-transcendence trait and its relationship with in vivo serotonin transporter availability in brainstem raphe nuclei: An ultra-high resolution PET-MRI study

Self-transcendence is an inherent human personality trait relating to the experience of spiritual aspects of the self. We examined the relationship between self-transcendence and serotonin transporter (SERT) availability in brainstem raphe nuclei, which are collections of five different serotonergic nuclei with rostro-caudal extension, using ultra-high resolution magnetic resonance imaging (MRI) and positron emission tomography (PET) with (11)C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([(11)C]DASB) to elucidate potential roles of serotonergic neuronal activities in this personality trait. Sixteen healthy subjects completed 7.0T MRI and High Resolution Research Tomograph (HRRT) PET. The regions of interest (ROIs) included the dorsal raphe nucleus (R1), median raphe nucleus (R2), raphe pontis (R3), and the caudal raphe nuclei (R4 and R5). For the estimation of SERT availability, the binding potential (BPND) was derived using the simplified reference tissue model (SRTM2). The Temperament and Character Inventory was used to measure self-transcendence. The analysis revealed that the self-transcendence total score had a significant negative correlation with the [(11)C]DASB BPND in the caudal raphe (R5). The subscale score for spiritual acceptance was significantly negatively correlated with the [(11)C]DASB BPND in the median raphe nucleus (R2). The results indicate that the self-transcendence trait is associated with SERT availability in specific raphe subnuclei, suggesting that the serotonin system may serve as an important biological basis for human self-transcendence. Based on the connections of these nuclei with cortico-limbic and visceral autonomic structures, the functional activity of these nuclei and their related neural circuitry may play a crucial role in the manifestation of self-transcendence.

Toxicity and radiation dosimetry studies of the serotonin transporter radioligand [(18) F]AFM in rats and monkeys

Background

[¹⁸ F]AFM is a potent and promising PET imaging agent for the serotonin transporter. We carried out an acute toxicity study in rats and radiation dosimetry in monkeys before the translation of the tracer to humans.

Methods

Single- and multiple-dose toxicity studies were conducted in Sprague–Dawley rats. Male and female rats were injected intravenously with AFM tartrate as a single dose of 98.7 or 987 μg/kg (592 or 5,920 μg/m², 100× or 1,000× the proposed human dose of 8 μg, respectively) on day 1 or as five consecutive daily doses of 98.7 μg/kg/day (592 μg /m²/day, 100× human dose, total dose 493.5 μg/kg). PET/CT scans were performed in four Formosan rock monkeys (two males and two females, each monkey scanned twice) using a Siemens BIOGRAPH scanner. After injection of [¹⁸ F]AFM (88.5 ± 20.3 MBq), a low-dose CT scan and a series of eight whole-body PET scans in 3-D mode were performed. Time-activity data of source organs were used to calculate the residence times and estimate the absorbed radiation dose using the OLINDA/EXM software.

Results

In the rats, neither the single dose nor the five daily doses of AFM tartrate produced overt adverse effects clinically. In the monkeys, the radiation doses received by most organs ranged between 8.3 and 39.1 μGy/MBq. The osteogenic cells, red marrow, and lungs received the highest doses of 39.1, 35.4, and 35.1 μGy/MBq, respectively. The effective doses extrapolated to male and female adult humans were 18.0 and 18.3 μSv/MBq, respectively.

Conclusions

Toxicity studies in Sprague–Dawley rats and radiation dosimetry studies in Formosa rock monkeys suggest that [¹⁸ F]AFM is safe for use in human PET imaging studies.

Trial registration

IACUC-12-200.

Serotonin transporter availability in thalamic subregions in schizophrenia: a study using 7.0-T MRI with [(11)C]DASB high-resolution PET

The serotonin transporter (SERT) is an integral protein that provides an index of serotonergic innervation. Until recently, few studies have investigated SERT binding in thalamic subregions in schizophrenia. The purpose of this study was to examine SERT availability in thalamic subdivisions (anterior nucleus, mediodorsal nucleus, and pulvinar) using 7.0-T magnetic resonance imaging (MRI) and high-resolution positron emission tomography (PET) with (11)C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([(11)C]DASB) in schizophrenia. Antipsychotic-free patients with schizophrenia (n=12) and healthy controls (n=15) underwent PET and MRI scans. For SERT availability, the binding potential with respect to non-displaceable compartment (BPND) was derived using the simplified reference tissue model (SRTM2). The analysis revealed that there were no significant differences in SERT availability between the two groups. In patients with schizophrenia, the severity of negative symptoms had a negative correlation with SERT availability in the anterior nucleus of the left thalamus. The present study did not reveal significant differences in SERT availability in thalamic subdivisions between patients with schizophrenia and control subjects. The association of SERT availability in the anterior nucleus with negative symptoms may suggest a role for the anterior thalamic nucleus in the pathophysiology of symptoms of schizophrenia. The ultra-high resolution imaging system could be an important asset for in vivo psychiatric research by combining structural and molecular information.

Recent progress of imaging agents for Parkinson's disease

Parkinson's disease (PD) is a common progressive, neurodegenerative brain disease that is promoted by mitochondrial dysfunction, oxidative stress, protein aggregation and proteasome dysfunction in the brain. Compared with computer tomography (CT) or magnetic resonance imaging (MRI), non-invasive nuclear radiopharmaceuticals have great significance for the early diagnosis of PD due to their high sensitivity and specificity in atypical and preclinical cases. Based on the development of coordination chemistry and chelator design, radionuclides may be delivered to lesions by attaching to PD-related transporters and receptors, such as dopamine, serotonin, and others. In this review, we comprehensively detailed the current achievements in radionuclide imaging in Parkinson's disease.

Sparsity Constrained Mixture Modeling for the Estimation of Kinetic Parameters in Dynamic PET

The estimation and analysis of kinetic parameters in dynamic positron emission tomography (PET) is frequently confounded by tissue heterogeneity and partial volume effects. We propose a new constrained model of dynamic PET to address these limitations. The proposed formulation incorporates an explicit mixture model in which each image voxel is represented as a mixture of different pure tissue types with distinct temporal dynamics. We use Cramér-Rao lower bounds to demonstrate that the use of prior information is important to stabilize parameter estimation with this model. As a result, we propose a constrained formulation of the estimation problem that we solve using a two-stage algorithm. In the first stage, a sparse signal processing method is applied to estimate the rate parameters for the different tissue compartments from the noisy PET time series. In the second stage, tissue fractions and the linear parameters of different time activity curves are estimated using a combination of spatial-regularity and fractional mixture constraints. A block coordinate descent algorithm is combined with a manifold search to robustly estimate these parameters. The method is evaluated with both simulated and experimental dynamic PET data.

Tracer kinetic modeling of [(11)C]AFM, a new PET imaging agent for the serotonin transporter

[(11)C]AFM, or [(11)C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine, is a new positron emission tomography (PET) radioligand with high affinity and selectivity for the serotonin transporter (SERT). The purpose of this study was to determine the most appropriate kinetic model to quantify [(11)C]AFM binding in the healthy human brain. Positron emission tomography data and arterial input functions were acquired from 10 subjects. Compartmental modeling and the multilinear analysis-1(MA1) method were tested using the arterial input functions. The one-tissue model showed a lack of fit in low-binding regions, and the two-tissue model failed to estimate parameters reliably. Regional time-activity curves were well described by MA1. The rank order of [(11)C]AFM binding potential (BPND) matched well with the known regional SERT densities. For routine use of [(11)C]AFM, several noninvasive methods for quantification of regional binding were evaluated, including simplified reference tissue models (SRTM and SRTM2), and multilinear reference tissue models (MRTM and MRTM2). The best methods for region of interest (ROI) analysis were MA1, MRTM2, and SRTM2, with fixed population kinetic values ( or b') for the reference methods. The MA1 and MRTM2 methods were best for parametric imaging. These results showed that [(11)C]AFM is a suitable PET radioligand to image and quantify SERT in humans.

PET imaging of the brain serotonin transporters (SERT) with N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (4-[18F]-ADAM) in humans: a preliminary study

PURPOSE

The aim of this study was to assess the feasibility of using 4-[(18)F]-ADAM as a brain SERT imaging agent in humans.

METHODS

Enrolled in the study were 19 healthy Taiwanese subjects (11 men, 8 women; age 33 ± 9 years). The PET data were semiquantitatively analyzed and expressed as specific uptake ratios (SUR) and distribution volume ratios (DVR) using the software package PMOD. The SUR and DVR of 4-[(18)F]-ADAM in the raphe nucleus (RN), midbrain (MB), thalamus (TH), striatum (STR) and prefrontal cortex (PFC) were determined using the cerebellum (CB) as the reference region.

RESULTS

4-[(18)F]-ADAM bound to known SERT-rich regions in human brain. The order of the regional brain uptake was MB (RN) > TH > STR > PFC > CB. The DVR (n = 4, t* = 60 min) in the RN, TH, STR and PFC were 3.00 ± 0.50, 2.25 ± 0.45, 2.05 ± 0.31 and 1.40 ± 0.13, respectively. The optimal time for imaging brain SERT with 4-[(18)F]-ADAM was 120-140 min after injection. At the optimal imaging time, the SURs (n = 15) in the MB, TH, STR, and PFC were 2.25 ± 0.20, 2.28 ± 0.20, 2.12 ± 0.18 and 1.47 ± 0.14, respectively. There were no significant differences in SERT availability between men and women (p < 0.05).

CONCLUSION

The results of this study showed that 4-[(18)F]-ADAM was safe for human studies and its distribution in human brain appeared to correlate well with the known distribution of SERT in the human brain. In addition, it had high specific binding and a reasonable optimal time for imaging brain SERT in humans. Thus, 4-[(18)F]-ADAM may be feasible for assessing the status of brain SERT in humans.

Optimal scanning time window for 18F-FP-(+)-DTBZ (18F-AV-133) summed uptake measurements

(18)F-9-fluoropropyl-(+)-dihydrotetrabenazine ((18)F-AV-133) is a novel positron emission tomography tracer for imaging the vesicular monoamine transporter II in dopaminergic neuron degeneration, which might be indicative for Parkinson's disease (PD) and other parkinsonism. Studies were performed to optimize the imaging time window for calculating standardized uptake value ratio (SUVR) with correlation to distribution volume ratio (DVR) and in differentiating PD from normal controls (NCs).

METHODS

Thirteen (18)F-AV-133 positron emission tomography studies were conducted on four NCs (age, 62.3±4.9 years) and nine PD patients (age, 60.8±6.0 years) with Hoehn and Yahr stages 2 to 3. Dynamic images were acquired within 180 min (0-30, 50-140 and 160-180 min) and were rearranged into 14 of 10-min scans. The contralateral striatum was defined as the opposite striatum to the predominantly affected limbs. Volumes of interest (VOIs) of bilateral putamen, caudate nuclei and occipital cortex (OC; as the reference region) were delineated from individual magnetic resonance imaging. SUVRs of striatum to OC were computed from 14 dynamic image sets. The DVRs were computed from Logan graphic analysis by using OC as the input. The performance of SUVR was evaluated based on the correlation of SUVR at each time window to DVR, as well as the Cohen effect size (group mean SUVR difference between PD and NC/standard deviation).

RESULTS

(18)F-AV-133 uptake decreased in PD subjects at bilateral striatum especially at contralateral side with posterior putamen predominant as compared with NC. Consistent higher correlations of SUVRs to DVR for all VOIs were observed at later time window and reached to its maximal value of 0.9917 at 90-100 min. The group mean SUVR differences between NC and PD subjects increased and reached relatively stable values after 90 min. The effect sizes for all VOIs were stable across different time window and with the largest value around 90~120 min.

CONCLUSION

The scanning time of 90-100 min for (18)F-AV-133 is considered as the optimal time window for summed uptake measurements in terms of SUVRs' correlation to DVRs, differential power, stability and clinical feasibility across and between NC and PD patients.

Computerized evaluation of mean residence times in multicompartmental linear system and pharmacokinetics

Deriving mean residence times (MRTs) is an important task both in pharmacokinetics and in multicompartmental linear systems. Taking as starting point the analysis of MRTs in open or closed (Garcia-Meseguer et al., Bull Math Biol 2003, 65, 279) multicompartmental linear systems, we implement a versatile software, using the Visual Basic 6.0 language for MS-Windows, that is easy to use and with a user-friendly format for the input of data and the output of results. For any multicompartmental linear system of up to 512 compartments, whether closed or open, with traps or without traps and with zero input in one or more of the compartments, this software allows the user to obtain the symbolic expressions, in the most simplified form, and/or the numerical values of the MRTs in any of its compartments, in the entire system or in a part of the system. As far as we known from the literature, such a software has not been implemented before. The advantage of the present software is that it reduces on the work time needed and minimizes the human errors that are frequent in compartmental systems even those that are relatively staightforward. The software bioCelTer, along with instructions, can be downloaded from http://oretano.iele-ab.uclm.es/~fgarcia/bioCelTer/.

Biodistribution, toxicity and radiation dosimetry studies of the serotonin transporter radioligand 4-[18F]-ADAM in rats and monkeys

PURPOSE

4-[(18)F]-ADAM is a potent serotonin transport imaging agent. We studied its toxicity in rats and radiation dosimetry in monkeys before human studies are undertaken.

METHODS

Single and multiple-dosage toxicity studies were conducted in Sprague-Dawley rats. Male and female rats were injected intravenously with 4-F-ADAM as a single dose of 1,023.7 microg/kg (1,000 times the human dose) or as five consecutive daily doses of 102.37 microg/kg (100 times the human dose). PET/CT scans were performed in seven Formosa Rock monkeys (four males and three females) using a Siemens Biograph scanner. After injection of 4-[(18)F]-ADAM (182+/-8 MBq), a low dose CT scan and a series of eight whole-body PET scans were performed. Whole-body images were acquired in 3-D mode. Time-activity data of source organs were used to calculate the residence times and estimate the absorbed radiation dose using OLINDA/EXM software.

RESULTS

In the rats neither the single dose nor the five daily doses of 4-F-ADAM produced overt adverse effects clinically. In the monkeys the radiation doses received by most organs ranged between 7.1 and 35.7 microGy/MBq, and the urinary bladder was considered to be the critical organ. The effective doses extrapolated to male and female adult humans were 17.4 and 21.8 microSv/MBq, respectively.

CONCLUSION

Toxicity studies in Sprague-Dawley rats and radiation dosimetry studies in Formosa Rock monkeys suggested that 4-[(18)F]-ADAM is safe for use in human PET imaging studies.