Martin Schain

Age and disease related changes in the translocator protein (TSPO) system in the human brain: Positron emission tomography measurements with [11C]vinpocetine

BACKGROUNDS AND PURPOSE The main objectives of the present study were (i) to measure density changes of activated microglia and the peripheral benzodiazepine receptor/translocator protein (TSPO) system during normal ageing in the human brain with positron emission tomography (PET) using the TSPO molecular imaging biomarker [(11)C]vinpocetine and (ii) to compare the level and pattern of TSPO in Alzheimer (AD) patients with age matched healthy subjects, in order to assess the biomarkers usefulness as a diagnostic imaging marker in normal (ageing) and pathological (AD) up-regulation of microglia. METHODS AND SUBJECTS PET measurements were made in healthy volunteers, aged between 25 and 78 years, and AD patients, aged between 67 and 82 years, using [(11)C]vinpocetine as the tracer. Global and regional quantitative parameters of tracer uptake and binding, including time activity curves (TAC) of standard uptake values (%SUV), binding affinity parameters, intensity spectrum and homogeneity of the uptake distribution were measured and analysed. RESULTS Both %SUV and binding values increased with age linearly in the whole brain and in all brain regions. There were no significant differences between the %SUV values of the AD patients and age matched control subjects. There were, however, significant differences in %SUV values in a large number of brain regions between young subjects and old subjects, as well as young subjects and AD patients. The intensity spectrum analysis and homogeneity analysis of the voxel data show that the homogeneity of the %SUV values decreases with ageing and during the disease, whereas the centre of the intensity spectrum is shifted to higher %SUV values. These data indicate an inhomogeneous up-regulation of the TSPO system during ageing and AD. These changes were significant between the group of young subjects and old subjects, as well as young subjects and AD patients, but not between old subjects and AD patients. CONCLUSIONS The present data indicate that [(11)C]vinpocetine may serve as a molecular imaging biomarker of the activity of the TSPO system and, consequently, of the up-regulation of microglia during ageing and in neuroinflammatory diseases. However, the global and regional brain %SUV values between AD patients and age matched controls are not different from each other. The disease specific changes, measured with [(11)C]vinpocetine in AD, are significantly different from those measured in age matched controls only if the inhomogeneities in the uptake pattern are explored with advanced mathematical techniques. For this reason, PET studies using [(11)C]vinpocetine, as molecular imaging biomarker, can efficiently visualise the activation of microglia and the up-regulation of TSPO during ageing and in diseased brains with the help of an appropriate inhomogeneity analysis of the radioligands brain uptake pattern.

Quantification of serotonin transporter availability with [11C]MADAM--a comparison between the ECAT HRRT and HR systems.

UNLABELLED The High Resolution Research Tomograph (HRRT) is the PET system providing the highest resolution for imaging of the human brain. In this study, the improved quantitative performance of the HRRT was evaluated in comparison with a previously developed lower resolution PET system, the ECAT HR. The radioligand [(11)C]MADAM was chosen for the purpose since it provides a signal for serotonin transporter (5-HTT) binding in cortical and sub-cortical brain regions of different sizes and expressing different 5-HTT densities. A secondary objective was to assess the effect of partial volume effect (PVE) correction on the cross-comparability between the two systems. METHOD Six male control subjects (ages 20-35 yr) were examined twice using the HRRT and the HR system, respectively. Regions of interest (ROIs) included cortical regions (frontal cortex, temporal cortex, insula, anterior cingulate cortex, and hippocampus), sub-cortical regions (caudate, putamen, thalamus, dorsal brainstem and ventral midbrain) and cerebellum. The ROIs were manually delineated on T1-weighted MRI-images and subsequently applied to both HRRT and HR images. Regional binding potential (BP(ND)) values were calculated with the simplified reference tissue model (SRTM) using cerebellum as the reference region. The percent difference in BP(ND) between the systems was calculated for each ROI. In addition, both HRRT and HR data were corrected for PVE using established MRI-based methods described by Meltzer and Müller-Gärtner. The effect of PVE correction (PVEc) on the agreement between the systems was assessed via percent difference calculation and linear regression analysis. RESULTS Quantification with SRTM showed that regional BP(ND) values for [(11)C]MADAM were on average 23% higher for the HRRT than those obtained by the HR system. More specifically, BP(ND) measured with HRRT was 31.1±48.1% higher in neocortical/limbic regions and 14.6±20.9% higher in sub-cortical regions. The effect of PVEc varied between regions. After correction according to Müller-Gärtner, the agreement between systems was best in the neocortical/limbic regions (3.7±22.5%). With the exception of the caudate, in which the agreement was improved by approximately 17% using the Meltzer method, the effect of PVEc in sub-cortical regions was less pronounced. Linear regression analysis showed improved correlation between the two systems after PVEc, particularly in the neocortical/limbic regions. CONCLUSION As expected, BP(ND) values measured with the HRRT were higher than those measured with the HR due to higher resolution and recovery. The difference in BP(ND) between the two systems was approximately 30% in the neocortical/limbic regions. PVEc improved the agreement between the systems in particular for the neocortical/limbic regions. In these regions, the best agreement was found after applying Müller-Gärtners PVEc. The demonstrated agreement provides an opportunity for combining data between the two systems in clinical studies aimed at evaluating receptor/transporter availability in cortical brain regions.

Diurnal and seasonal variation of the brain serotonin system in healthy male subjects

The mammalian circadian clock underlies both diurnal and seasonal changes in physiology, and its function is thought to be disturbed in both seasonal and non-seasonal depression. In humans, molecular imaging studies have reported seasonal changes in the serotonin system. Despite the role of the circadian clock in generating seasonal physiological changes, however, diurnal variation of serotonin receptors and transporters has never been directly studied in humans. We used positron emission tomography to examine diurnal and seasonal changes in the serotonin 5-HT1A receptor and serotonin transporter in two large cohorts of healthy male subjects, employing a cross-sectional design. In 56 subjects measured with [(11)C]WAY-100635, we observed diurnal increases in the availability of 5-HT1A receptors in the cortex. In 40 subjects measured with [(11)C]MADAM, a decrease in 5-HTT was observed in the midbrain across the day. We also found seasonal changes in the 5-HT1A receptor in serotonin projection regions, with higher availability on days with a longer duration of daylight. Our observation that serotonin receptor and transporter levels may change across the day in humans is corroborated by experimental research in rodents. These findings have important implications for understanding the relationship between the circadian and serotonin systems in both the healthy brain and in affective disorders, as well as for the design of future molecular imaging studies.

Evaluation of Two Automated Methods for PET Region of Interest Analysis

Manual definition of regions of interest (ROIs) has been considered the reference standard method in PET data evaluation. The method is labor-intensive, prone to rater bias and may show low reproducibility. Automated template-based methods for ROI definition may overcome these limitations. The aim of this study was to validate the two automated methods FreeSurfer and the AAL template for definition of ROIs for the PET data analysis. PET data obtained using the radioligands [11C]AZD2184 (amyloid-β radioligand) and [11C]AZ10419369 (5-HT1B receptor radioligand) were evaluated. PET measurements acquired on one high and one lower resolution PET system were included. Outcome measures obtained using automated methods were compared to those obtained using manual ROIs, using linear regression analysis, intraclass correlation coefficients, and repeated measures ANOVA. ROIs provided by the automatic methods were larger than the manually delineated regions, which in some cases introduced biased estimates of the outcome measures. However, with the exception of the caudate, both AAL and FreeSurfer generally provided outcome measures that were in good agreement to those obtained from manually delineated ROIs, as long as the manually defined cerebellum was used as a reference region. Both AAL and FreeSurfer can be used for quantification of PET data, with similar accuracy in the estimates of outcome measures. Thus, the choice of method could be based upon necessity of fast analysis as provided by AAL, or more detailed ROIs and measures of cortical thickness as provided by FreeSurfer.

In vivo evidence of a functional association between immune cells in blood and brain in healthy human subjects.

Microglia, the resident macrophages in the central nervous system, are thought to be maintained by a local self-renewal mechanism. Although preclinical and in vitro studies have suggested that the brain may contain immune cells also from peripheral origin, the functional association between immune cells in the periphery and brain at physiological conditions is poorly understood. We examined 32 healthy individuals using positron emission tomography (PET) and [(11)C]PBR28, a radioligand for the 18-kDa translocator protein (TSPO) which is expressed both in brain microglia and blood immune cells. In 26 individuals, two measurements were performed with varying time intervals. In a subgroup of 19 individuals, of which 12 had repeat examinations, leukocyte numbers in blood was measured on each day of PET measurements. All individuals were genotyped for TSPO polymorphism and categorized as high, mixed, and low affinity binders. We assessed TSPO binding expressed as total distribution volume of [(11)C]PBR28 in brain and in blood cells. TSPO binding in brain was strongly and positively correlated to binding in blood cells both at baseline and when analyzing change between two PET examinations. Furthermore, there was a significant correlation between change of leukocyte numbers and change in TSPO binding in brain, and a trend-level correlation to change in TSPO binding in blood cells. These in vivo findings indicate an association between immunological cells in blood and brain via intact BBB, suggesting a functional interaction between these two compartments, such as interchange of peripherally derived cells or a common regulatory mechanism. Measurement of radioligand binding in blood cells may be a way to control for peripheral immune function in PET studies using TSPO as a marker of brain immune activation.

Arterial input function derived from pairwise correlations between PET-image voxels

A metabolite corrected arterial input function is a prerequisite for quantification of positron emission tomography (PET) data by compartmental analysis. This quantitative approach is also necessary for radioligands without suitable reference regions in brain. The measurement is laborious and requires cannulation of a peripheral artery, a procedure that can be associated with patient discomfort and potential adverse events. A non invasive procedure for obtaining the arterial input function is thus preferable. In this study, we present a novel method to obtain image-derived input functions (IDIFs). The method is based on calculation of the Pearson correlation coefficient between the time-activity curves of voxel pairs in the PET image to localize voxels displaying blood-like behavior. The method was evaluated using data obtained in human studies with the radioligands [ 11 C]flumazenil and [ 11 C]AZ10419369, and its performance was compared with three previously published methods. The distribution volumes (VT) obtained using IDIFs were compared with those obtained using traditional arterial measurements. Overall, the agreement in VT was good (~3% difference) for input functions obtained using the pairwise correlation approach. This approach performed similarly or even better than the other methods, and could be considered in applied clinical studies. Applications to other radioligands are needed for further verification.

Patterns of age related changes for phosphodiesterase type-10A in comparison with dopamine D 2/3 receptors and sub-cortical volumes in the human basal ganglia: A PET study with 18 F-MNI-659 and 11 C-raclopride with correction for partial volume effect

ABSTRACT Phosphodiesterase 10A enzyme (PDE10A) is an important striatal target that has been shown to be affected in patients with neurodegenerative disorders, particularly Huntingtons disease (HD). PDE10A is expressed on striatal neurones in basal ganglia where other known molecular targets are enriched such as dopamine D2/3 receptors (D2/3 R). The aim of this study was to examine the availability of PDE10A enzyme in relation with age and gender and to compare those changes with those related to D2/3 R and volumes in different regions of the basal ganglia. As a secondary objective we examined the relative distribution of D2/3 R and PDE10A enzyme in the striatum and globus pallidus. Forty control subjects (20F/20M; age: 44±11y, age range 27–69) from an ongoing positron emission tomography (PET) study in HD gene expansion carriers were included. Subjects were examined with PET using the high‐resolution research tomograph (HRRT) and with 3T magnetic resonance imaging (MRI). The PDE10A radioligand 18F‐MNI‐659 and D2/3 R radioligand 11C‐raclopride were used. The outcome measure was the binding potential (BPND) estimated with the two‐tissue compartment model (18F‐MNI‐659) and the simplified reference tissue model (11C‐raclopride) using the cerebellum as reference region. The PET data were corrected for partial volume effects. In the striatum, PDE10A availability showed a significant age‐related decline that was larger compared to the age‐related decline of D2/3 R availability and to the age‐related decline of volumes measured with MRI. In the globus pallidus, a less pronounced decline of PDE10A availability was observed, whereas D2/3 R availability and volumes seemed to be rather stable with aging. The distribution of the PDE10A enzyme was different from the distribution of D2/3 R, with higher availability in the globus pallidus. These results indicate that aging is associated with a considerable physiological reduction of the availability of PDE10A enzyme in the striatum. Moreover as result of the analysis, in the striatum for both the molecular targets, we observed a gender effect with higher BPND the female group. Highlights18F‐MNI‐659 binding is a measure of Phosphodiestrease 10A enzyme availability.We examined the effect of age on 18F‐MNI‐659 binding in 40 healthy controls.Age related changes were also evaluated for 11C‐Raclopride and structural volumes.Patterns of molecular age related changes were evaluated with PVEc.We found an evident association between age and striatal 18F‐MNI‐659 binding.

Mapping the distribution of serotonin transporter in the human brainstem with high-resolution PET: Validation using postmortem autoradiography data

The human brainstem is a complex structure with several small nuclei and neural pathways of interest in the pathophysiology of central nervous system (CNS) disorders. In common with other monoaminergic systems, serotoninergic neurons originate from a group of nuclei located in the brainstem. The present study was designed to validate a user-independent approach for a detailed in vivo quantification of serotonin transporter (5-HTT) availability in the human brainstem using a template-based approach that consisted of three steps. First, 3T-MR images and parametric binding potential (BPND) [(11)C]MADAM images of ten healthy subjects were used to generate a PET template of 5-HTT availability. In the second step, volumes of interest (VOIs) for different brainstem nuclei were obtained using a method in which VOIs are initially delineated on MRI images using anatomical landmarks and then are finally tailored on the distribution of 5-HTT binding using a thresholding approach applied to the 5-HTT template. In the final step, the VOIs were transformed and applied individually to BPND images of 16 healthy subjects (14M/2F, 20-64years). The in vivo distribution of BPND values obtained with the template-based method were in good agreement with an individual-based approach taken as gold standard. Results were also in agreement with 5-HTT quantification using in vitro binding data obtained with autoradiography (ARG) studies using [(3)H]MADAM. The proposed template-based method can be applied to PET data acquired in several CNS disorders in which serotonin neurons in the brainstem might be affected.

An automated method measures variability in P-glycoprotein and ABCG2 densities across brain regions and brain matter.

Changes in P-glycoprotein and ABCG2 densities may play a role in amyloid-beta accumulation in Alzheimer’s disease. However, previous studies report conflicting results from different brain regions, without correcting for changes in vessel density. We developed an automated method to measure transporter density exclusively within the vascular space, thereby correcting for vessel density. We then examined variability in transporter density across brain regions, matter, and disease using two cohorts of post-mortem brains from Alzheimer’s disease patients and age-matched controls. Changes in transporter density were also investigated in capillaries near plaques and on the mRNA level. P-glycoprotein density varied with brain region and matter, whereas ABCG2 density varied with brain matter. In temporal cortex, P-glycoprotein density was 53% lower in Alzheimer’s disease samples than in controls, and was reduced by 35% in capillaries near plaque deposits within Alzheimer’s disease samples. ABCG2 density was unaffected in Alzheimer’s disease. No differences were detected at the transcript level. Our study indicates that region-specific changes in transporter densities can occur globally and locally near amyloid-beta deposits in Alzheimer’s disease, providing an explanation for conflicting results in the literature. When differences in region and matter are accounted for, changes in density can be reproducibly measured using our automated method.

Optimal Acquisition Time Window and Simplified Quantification of Dopamine Transporter Availability Using 18F-FE-PE2I in Healthy Controls and Parkinson Disease Patients

18F-(E)-N-(3-iodoprop-2-enyl)-2β-carbofluoroethoxy-3β-(4′methylphenyl)nortropane (18F-FE-PE2I) is a newly developed dopamine transporter (DAT) PET radioligand. Full quantification methods rely on dynamic acquisition of 18F-FE-PE2I, but in a clinical setting a simplified protocol is preferable. The aims of this study were to identify the optimal acquisition time window for 18F-FE-PE2I and to validate the specific binding ratio (SBR) as a simplified quantification method. Methods: Ten Parkinson disease (PD) patients and 10 controls were included. Ninety-three-min dynamic PET measurements with 18F-FE-PE2I were conducted using the high-resolution research tomograph (HRRT). The dynamic measurement was also smoothed to the resolution of a clinical PET system (HR). Regions of interest for the caudate, putamen, ventral striatum, substantia nigra (SN), and cerebellum were manually drawn on coregistered MR images. The outcome measure was the SBR, and the gold standard was the binding potential obtained with wavelet-aided parametric imaging (WAPI BPND). The cerebellum was used as a reference region. In a preliminary analysis, SBR was computed for 8 time windows (SBRdyn). Linear regression analysis and Bland–Altman plots were used to select the optimal acquisition time window. An average image from the selected time window was created, from which new SBR values (SBR calculated on the average image on the HRRT and SBR calculated on the average image on the simulated HR images) were calculated and compared with WAPI BPND. The effect size was calculated. Results: SBRdyn values for the time window between 16.5 and 42 min correlated best with WAPI BPND (r2 = 0.98, P < 0.001). Significant correlations (P < 0.001) were observed between SBRHR and WAPI-BPND (r2 = 0.95 in controls and 0.97 in PD patients). In the striatum, SBRHR values were 37% lower than BPND in controls, 29% in PD patients, whereas in the SN the underestimation was 22% in controls and 15% in PD patients. Similar effect sizes for BPND and SBRHR were found in the caudate (0.6), putamen (1.7 and 1.4), ventral striatum (0.7), and SN (0.5 and 0.4). Conclusion: A single 18F-FE-PE2I acquisition between 16.5 and 42 min provides the best outcome measure for simplified DAT quantification. Despite underestimation of the BPND, the SBR can be used in a clinical setting as a valid quantification method for DAT using 18F-FE-PE2I, because it provides differentiation similar to BPND between controls and PD patients.