Similarity and robustness of PET and SPECT binding parameters for benzodiazepine receptors

Bolus infusion scheme for the adjustment of steady state [11C]Flumazenil levels in the grey matter and in the blood plasma for neuroreceptor imaging

The use of hybrid PET/MR imaging facilitates the simultaneous investigation of challenge-related changes in ligand binding to neuroreceptors using PET, while concurrently measuring neuroactivation or blood flow with MRI. Having attained a steady state of the PET radiotracer using a bolus-infusion protocol, it is possible to observe alterations in ligand neuroreceptor binding through changes in distribution volumes. Here, we present an iterative procedure for establishing an administration scheme to obtain steady state [¹¹C]flumazenil concentrations in grey matter in the human brain. In order to achieve a steady state in the shortest possible time, the bolus infusion ratio from a previous examination was adapted to fit the subsequent examination. 17 male volunteers were included in the study. Boli and infusions with different weightings were given to the subjects and were characterised by kbol values from 74 ​min down to 42 ​min. Metabolite analysis was used to ascertain the value of unmetabolised flumazenil in the plasma, and PET imaging was used to assess its binding in the grey matter. The flumazenil time-activity curves (TACs) in the brain were decomposed into activity contributions from pure grey and white matter and analysed for 12 ​vol of interest (VOIs). The curves highlighted a large variability in metabolic rates between the subjects, with kbol ​= ​54.3 ​min being a reliable value to provide flumazenil equilibrium conditions in the majority of the VOIs and cases. The distribution volume of flumazenil in all 12 VOIs was determined.

Dynamic image denoising for voxel-wise quantification with Statistical Parametric Mapping in molecular neuroimaging

Purpose

PET and SPECT voxel kinetics are highly noised. To our knowledge, no study has determined the effect of denoising on the ability to detect differences in binding at the voxel level using Statistical Parametric Mapping (SPM).

Methods

In the present study, groups of subject-images with a 10%- and 20%- difference in binding of [¹²³I]iomazenil (IMZ) were simulated. They were denoised with Factor Analysis (FA). Parametric images of binding potential (BP_ND) were produced with the simplified reference tissue model (SRTM) and the Logan non-invasive graphical analysis (LNIGA) and analyzed using SPM to detect group differences. FA was also applied to [¹²³I]IMZ and [¹¹C]flumazenil (FMZ) clinical images (n = 4) and the variance of BP_ND was evaluated.

Results

Estimations from FA-denoised simulated images provided a more favorable bias-precision profile in SRTM and LNIGA quantification. Simulated differences were detected in a higher number of voxels when denoised simulated images were used for voxel-wise estimations, compared to quantification on raw simulated images. Variability of voxel-wise binding estimations on denoised clinical SPECT and PET images was also significantly diminished.

Conclusion

In conclusion, noise removal from dynamic brain SPECT and PET images may optimize voxel-wise BP_ND estimations and detection of biological differences using SPM.

I-123 iomazenil single photon emission computed tomography for detecting loss of neuronal integrity in patients with traumatic brain injury

Background

Traumatic brain injury (TBI) causes brain dysfunction in many patients. Using C-11 flumazenil (FMZ) positron emission tomography (PET), we have detected and reported the loss of neuronal integrity, leading to brain dysfunction in TBI patients. Similarly to FMZ PET, I-123 iomazenil (IMZ) single photon emission computed tomography (SPECT) is widely used to determine the distribution of the benzodiazepine receptor (BZR) in the brain cortex. The purpose of this study is to examine whether IMZ SPECT is as useful as FMZ PET for evaluating the loss of neuronal integrity in TBI patients.

The subjects of this study were seven patients who suffered from neurobehavioral disability. They underwent IMZ SPECT and FMZ PET. Nondisplaceable binding potential (BP_ND) was calculated from FMZ PET images. The uptake of IMZ was evaluated on the basis of lesion-to-pons ratio (LPR). The locations of low uptake levels were visually evaluated both in IMZ SPECT and FMZ PET images. We compared FMZ BP_ND and (LPR-1) of IMZ SPECT.

Results

In the visual assessment, FMZ BP_ND decreased in 11 regions. In IMZ SPECT, low uptake levels were observed in eight of the 11 regions. The rate of concordance between FMZ PET and IMZ SPECT was 72.7%. The mean values IMZ (LPR-1) (1.95 ± 1.01) was significantly lower than that of FMZ BP_ND (2.95 ± 0.80 mL/mL). There was good correlation between FMZ BP_ND and IMZ (LPR-1) (r = 0.80).

Conclusions

IMZ SPECT findings were almost the same as FMZ PET findings in TBI patients. The results indicated that IMZ SPECT is useful for evaluating the loss of neuronal integrity. Because IMZ SPECT can be performed in various facilities, IMZ SPECT may become widely adopted for evaluating the loss of neuronal integrity.

Tobacco smoking interferes with GABAA receptor neuroadaptations during prolonged alcohol withdrawal

Understanding the effects of tobacco smoking on neuroadaptations in GABAA receptor levels over alcohol withdrawal will provide critical insights for the treatment of comorbid alcohol and nicotine dependence. We conducted parallel studies in human subjects and nonhuman primates to investigate the differential effects of tobacco smoking and nicotine on changes in GABAA receptor availability during acute and prolonged alcohol withdrawal. We report that alcohol withdrawal with or without concurrent tobacco smoking/nicotine consumption resulted in significant and robust elevations in GABAA receptor levels over the first week of withdrawal. Over prolonged withdrawal, GABAA receptors returned to control levels in alcohol-dependent nonsmokers, but alcohol-dependent smokers had significant and sustained elevations in GABAA receptors that were associated with craving for alcohol and cigarettes. In nonhuman primates, GABAA receptor levels normalized by 1 mo of abstinence in both groups--that is, those that consumed alcohol alone or the combination of alcohol and nicotine. These data suggest that constituents in tobacco smoke other than nicotine block the recovery of GABAA receptor systems during sustained alcohol abstinence, contributing to alcohol relapse and the perpetuation of smoking.

Graphic plot analysis for estimating binding potential of translocator protein (TSPO) in positron emission tomography studies with [¹⁸F]FEDAA1106

PURPOSE

[(18)F]FEDAA1106 is expected to be used for evaluating the regional density of the peripheral benzodiazepine receptor (also called TSPO) in several neurodegenerative disorders. Regarding the quantification, direct binding potential (BP(ND)) has been reported to be preferable because of the variation of nondisplaceable distribution volume (V(ND)) among individuals. However, the precise calculation of BP(ND) is difficult in small regions or at voxel levels due to noise. Recently, a new graphical analysis (GA) was proposed to estimate V(ND) in a direct way. In this paper, we evaluated two types of GA for reliable quantification of BP(ND) in PET study with [(18)F]FEDAA1106 using computer simulations and human data.

METHODS

In the simulations, time-activity curves were generated with various rate constants and noise levels, and the errors of BP(ND) estimated by GA were analyzed by comparing with true values calculated from rate constants given for the simulations. Thereafter, in a human study with [(18)F]FEDAA1106 for healthy volunteers, BP(ND) was estimated by two types of GA for region-of-interest (ROI) data. Parametric images of BP(ND) were generated by two types of GA with or without wavelet-denoising.

RESULTS

Simulations showed that BP(ND) by GA was well correlated with true values, despite an underestimation. GA reduced unreasonable estimates compared with a conventional nonlinear least-square fitting (NLS), although larger variation of BP(ND) estimates was observed. In a ROI-based analysis of data obtained in a human study, BP(ND)s estimated by GA were well correlated with those generated by NLS, though they were underestimated. Parametric BP(ND) images by GA could be improved with wavelet-denoising.

CONCLUSION

Graphical analysis could provide BP(ND) values with high stability and simple calculation in both ROI-based and voxel-based analyses of [(18)F]FEDAA1106 data.

Quantitative analyses of [¹¹C]Ro15-4513 binding to subunits of GABAA/benzodiazepine receptor in the living human brain

BACKGROUND

Gamma-aminobutyric acid (GABA)A/benzodiazepine (BZ) receptor chloride channel consists of several subunits. The diversity of the α subunits results in the various ligand selectivity and functionally different properties of the GABAA/BZ receptor. Although [¹¹C] Ro15-4513 is reported to be a radioligand that has relatively high affinity for α5 subunit-containing GABAA/BZ receptor, it remained to be evaluated fully.

AIM

The aim of this study was to evaluate the quantitative analyses of [¹¹C]Ro15-4513 in the living human brain.

METHODS

Positron emission tomography examinations were performed in eight healthy male volunteers after intravenous injection of [¹¹C]Ro15-4513. Kinetic analysis of data was performed with the two-compartment and three-compartment models using arterial input function. Linear graphical analysis and the simplified reference tissue model analysis (SRTM) were also performed using pons as a reference region. In a simulation study, the effects of noise to the estimation of binding potentials were evaluated.

RESULTS

The accumulation of [¹¹C]Ro15-4513 in the limbic system was relatively higher than in other cortex. The bindings were well described by the three-compartment model in the regions with specific binding. Binding potentials obtained from the graphical method and SRTM correlated well with those obtained from the three-compartment model. In the simulation study, estimated parameters from SRTM were less affected by noise compared with those from the graphical method.

CONCLUSION

The reference tissue methods using pons as a reference region can be used for quantitative analysis of [¹¹C]Ro15-4513 binding. SRTM seemed less susceptible to noise than does graphical analysis.

SPECT imaging of GABA(A)/benzodiazepine receptors and cerebral perfusion in mild cognitive impairment

PURPOSE

The involvement of neocortical and limbic GABA(A)/benzodiazepine (BZD) receptors in Alzheimer's disease (AD) is controversial and mainly reported in advanced stages. The status of these receptors in the very early stages of AD is unclear and has not been explored in vivo. Our aims were to investigate in vivo the integrity of cerebral cortical GABA(A)/BZD receptors in subjects with amnestic mild cognitive impairment (MCI) and to compare possible receptor changes to those in cerebral perfusion.

METHODS

[(123)I]Iomazenil and [(99m)Tc]HMPAO SPECT images were acquired in 16 patients with amnestic MCI and in 14 normal elderly control subjects (only [(123)I]iomazenil imaging in 5, only [(99m)Tc]HMPAO imaging in 4, and both [(123)I]iomazenil and [(99m)Tc]HMPAO imaging in 5). Region of interest (ROI) analysis and voxel-based analysis were performed with cerebellar normalization.

RESULTS

Neither ROI analysis nor voxel-based analysis showed significant [(123)I]iomazenil binding changes in MCI patients compared to control subjects, either as a whole group or when considering only those patients with MCI that converted to AD within 2 years of clinical follow-up. In contrast, the ROI analysis revealed significant hypoperfusion of the precuneus and posterior cingulate cortex in the whole group of MCI patients and in MCI converters as compared to control subjects. Voxel-based analysis showed similar results.

CONCLUSION

These results indicate that in the very early stages of AD, neocortical and limbic neurons/synapses expressing GABA(A)/BZD receptors are essentially preserved. They suggest that in MCI patients functional changes precede neuronal/synaptic loss in neocortical posterior regions and that [(99m)Tc]HMPAO rCBF imaging is more sensitive than [(123)I]iomazenil GABA(A)/BZD receptor imaging in detecting prodromal AD.

In vivo saturation binding of GABA-A receptor ligands to estimate receptor occupancy using liquid chromatography/tandem mass spectrometry

Typically, the dose-occupancy curves for GABA-A receptor ligands are determined using in vivo binding of [3H]flumazenil. This study describes in vivo binding experiments without the use of tracer ligands. Bound and free fractions were measured directly using a highly sensitive LC/MS/MS detection method after in vivo administration of the GABA-A ligands zolpidem, (RS)-zopiclone, L-838417 and flumazenil, to demonstrate affinity and saturation of the filter-retained, membrane-bound fraction. The in vivo binding of flumazenil and L-838417 both saturated around 200 nM, at a similar level to the specific binding of (S)-zopiclone after doses of the racemic zopiclone, using (R)-zopiclone to estimate non-specific binding. This saturable component represented an estimate of benzodiazepine binding sites available on GABA-A receptors in vivo (200 nM). Dose-occupancy curves were constructed to estimate the dose required to achieve 50% occupancy and matched estimates obtained with tracer methods. In contrast to tracer methods, this method is uniquely suitable to the demonstration of stereoselective binding of the (S)-isomer in vivo after doses of racemic zopiclone. These results demonstrate that the LC/MS/MS measurements of total drug concentrations typically used in early drug development can be adapted to provide information about receptor occupancy in vivo.

Comparison of plasma input and reference tissue models for analysing [(11)C]flumazenil studies

A single-tissue compartment model with plasma input is the established method for analysing [(11)C]flumazenil ([(11)C]FMZ) studies. However, arterial cannulation and measurement of metabolites are time-consuming. Therefore, a reference tissue approach is appealing, but this approach has not been fully validated for [(11)C]FMZ. Dynamic [(11)C]FMZ positron emission tomography scans with arterial blood sampling were performed in nine drug-free depressive patients and eight healthy subjects. Regions of interest were defined on co-registered magnetic resonance imaging scans and projected onto dynamic [(11)C]FMZ images. Using a Hill-type metabolite function, single (1T) and reversible two-tissue (2T) compartmental models were compared. Simplified reference tissue model (SRTM) and full reference tissue model (FRTM) were investigated using both pons and (centrum semiovale) white matter as reference tissue. The 2T model provided the best fit in 59% of cases. Two-tissue V(T) values were on average 1.6% higher than 1T V(T) values. Owing to the higher rejection rate of 2T fits (7.3%), the 1T model was selected as plasma input method of choice. SRTM was superior to FRTM, irrespective whether pons or white matter was used as reference tissue. BP(ND) values obtained with SRTM correlated strongly with 1T V(T) (r=0.998 and 0.995 for pons and white matter, respectively). Use of white matter as reference tissue resulted in 5.5% rejected fits, primarily in areas with intermediate receptor density. No fits were rejected using pons as reference tissue. Pons produced 23% higher BP(ND) values than white matter. In conclusion, for most clinical studies, SRTM with pons as reference tissue can be used for quantifying [(11)C]FMZ binding.

A comparison of recent parametric neuroreceptor mapping approaches based on measurements with the high affinity PET radioligands [11C]FLB 457 and [11C]WAY 100635

In positron emission tomography (PET) studies, the detailed mapping of neuroreceptor binding is a trade-off between parametric accuracy and spatial precision. Logan's graphical approach is a straightforward way to quickly obtain binding potential values at the voxel level but it has been shown to have a noise-dependent negative bias. More recently suggested approaches claim to improve parametric accuracy with retained spatial resolution. In the present study, we used PET measurements on regional D2 dopamine and 5-HT1A serotonin receptor binding in man to compare binding potential (BP) estimates of six different parametric imaging approaches to the traditional Logan ROI-based approach which was used as a "gold standard". The parametric imaging approaches included Logan's reference tissue graphical analysis (PILogan), its version recently modified by Varga and Szabo (PIVarga), two versions of the wavelet-based approach, Gunn's basis function method (BFM) and Gunn et al.'s recent compartmental theory-based approach employing basis pursuit strategy for kinetic modeling (called DEPICT). Applicability for practical purposes in basic and clinical research was also considered. The results indicate that the PILogan and PIVarga approaches fail to recover the correct values, the wavelet-based approaches overcome the noise susceptibility of the Logan fit with generally good recovery of BP values, and BFM and DEPICT seem to produce values with a bias dependent on receptor density. Further investigations on this bias and other phenomena revealed fundamental issues regarding the use of BFM and DEPICT on noisy voxel-wise data. In conclusion, the wavelet-based approaches seem to provide the most valid and reliable estimates across regions with a wide range of receptor densities. Furthermore, the results support the use of receptor parametric imaging in applied studies in basic or clinical research.

Biological properties of 2′-[18F]fluoroflumazenil for central benzodiazepine receptor imaging

A novel positron emitting agent, 2'-[18F]fluoroflumazenil (fluoroethyl 8-fluoro-5-methyl-6-oxo-5,6-dihydro-4H-benzo-[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate, FFMZ), has been reported for benzodiazepine imaging. In the present study, biological properties of [18F]FFMZ were investigated. Stability tests of [18F]FFMZ in human and rat sera were performed. Biodistribution was investigated in mice and phosphorimages of brains were obtained from rats. A receptor binding assay was performed using rat brain (mixture of cortex and cerebellum) homogenate. A static positron emission tomography (PET) image was obtained from a normal human volunteer. Although [18F]FFMZ was stable in human serum, it was rapidly hydrolyzed in rat serum. The hydrolysis was 39%, 63% and 92% at 10, 30 and 60 min, respectively. According to the biodistribution study in mice, somewhat even distribution (between 2 approximately 3% ID/g) was observed in most organs. Intestinal uptake increased up to 6% ID/g at 1 h due to biliary excretion. Bone uptake slowly increased from 1.5% to 3.5% ID/g at 1 h. High uptakes in the cortex, thalamus and cerebellum, which could be completely blocked by coinjection of cold FMZ, were observed by phosphorimaging study using rats. Determination of Kd value and Bmax using rat brain tissue was performed by Scatchard plotting and found 1.45+/-0.26 nM and 1.08+/-0.03 pmol/mg protein, respectively. The PET image of the normal human volunteer showed high uptake in the following decreasing order: frontal cortex, temporal cortex, occipital cortex, cerebellum, parietal cortex and thalamus. In conclusion, the new FMZ derivative, [18F]FFMZ appears to be a promising PET agent for central benzodiazepine receptor imaging with a convenient labeling procedure and a specific binding property.

Wavelet variance components in image space for spatiotemporal neuroimaging data

Neuroimaging studies place great emphasis on not only the estimation but also the standard error estimates of underlying parameters derived from a temporal model. This allows inferences to be made about the signal estimates and resulting conclusions to be drawn about the underlying data. It can often be advantageous to interrogate temporal models after spatial transformation of the data into the wavelet domain. Wavelet bases provide a multiresolution decomposition of the spatial data dimension and an ensuing reduction in spatial correlation. However, widespread acceptance of these wavelet techniques has been hampered by the limited ability to reconstruct both parametric and error estimates into the image domain after analysis of temporal models in the wavelet domain. This paper introduces a derivation and a fast implementation of a method for the calculation of the variance of the parametric images obtained from wavelet filters. The technique is proposed for a class of estimators that have been shown to be useful in neuroimaging studies. The techniques are demonstrated for both functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) data sets.

Visualization of alpha5 subunit of GABAA/benzodiazepine receptor by 11C Ro15-4513 using positron emission tomography

Although [(11)C]Ro15-4513 and [(11)C]flumazenil both bind to the central benzodiazepine (BZ) receptors, the distributions of the two ligands are not identical in vivo. Moreover, the in vivo pharmacological properties of [(11)C]Ro15-4513 have not been thoroughly examined. In the present study, we examined the pharmacological profile of [(11)C]Ro15-4513 binding in the monkey brain using positron emission tomography (PET). [(11)C]Ro15-4513 showed relatively high accumulation in the anterior cingulate cortex, hippocampus, and insular cortex, with the lowest uptake being observed in the pons. Accumulation in the cerebral cortex was significantly diminished by the BZ antagonist flumazenil (0.1 mg/kg, i.v.), but not that in the pons. Using the pons as a reference region, the specific binding of [(11)C]Ro15-4513 in most of the cerebral cortex including the limbic regions clearly revealed two different affinity sites. On the other hand, specific binding in the occipital cortex and cerebellum showed only a low affinity site. Zolpidem with affinity for alpha1, alpha2, and alpha3 subunits of GABA(A)/BZ receptor fully inhibited [(11)C]Ro15-4513 binding in the occipital cortex and cerebellum, while only about 23% of the binding was blocked in the anterior cingulate cortex. Diazepam with affinity for alpha1, alpha2, alpha3, and alpha5 subunits inhibited the binding in all brain regions. Since Ro15-4513 has relatively high affinity for the alpha5 subunit in vitro, these in vivo bindings of [(11)C]Ro15-4513 can be interpreted as the relatively high accumulation in the fronto-temporal limbic regions representing binding to the GABA(A)/BZ receptor alpha5 subunit.

Nuclear medicine applications in molecular imaging

With the emergence of the new field of molecular imaging, there is an increasing demand for development of sensitive and safe novel imaging agents that can be rapidly translated from small animal models into patients. Nuclear medicine and positron emission tomography (PET) techniques have the ability to detect and serially monitor a variety of biologic and pathophysiologic processes, usually with tracer quantities of radiolabeled peptides, drugs, and other molecules at doses free of pharmacologic side effects, unlike the current generation of intravenous agents required for magnetic resonance (MR) and computed tomography (CT) scanning. In this article, we will review a representative sampling of the wide array of radiopharmaceuticals developed specifically for nuclear medicine radionuclide imaging that have been approved for clinical use, and those in pre-clinical trials. We will also review the existing strategies used to select the appropriate biologic markers and targets for radionuclide labeling that have been employed in the development of novel radiotracers and the imaging of small animals with new microSPECT (single photon emission computed tomography) technologies.

Quantitative analysis of benzodiazepine receptor in temporal lobe epilepsy: [(125)I]iomazenil autoradiographic study of surgically resected specimens

PURPOSE

To evaluate the changes of the inhibitory neurotransmitter receptor system related to epileptogenesis by measuring central benzodiazepine receptors (BZDRs) in surgically resected specimens of temporal lobe epilepsy by using [(125)I]iomazenil autoradiography.

METHODS

Surgically resected specimens were obtained from 66 temporal lobe epilepsy patients [51 with mesial temporal lobe epilepsy (MTLE) and 15 with non-MTLE] receiving no BZDs and seven MTLE patients receiving BZDs. BZDR densities in brain sections were measured by using [(125)I]iomazenil autoradiography. Cell densities were measured from cresyl violet-stained sections.

RESULTS

Compared with non-MTLE patients, non-BZD-treated MTLE patients showed remarkable reduction of BZDR density in the pyramidal cell region of cornu ammonis (CA) 1, CA3, and CA4, and a smaller but significant reduction in CA2 and the molecular and granule cell layers of dentate gyrus (mDG). In the MTLE group, the BZDR density in the mDG correlated with that in lateral cortex. Significant correlations between BZDR density and cell density were found in all hippocampal regions. A significant difference in BZDR density/cell-density ratio was observed in CA1 region between MTLE and non-MTLE. BZD-treated patients tended to have lower BZDR densities than did non-BZD-treated patients, although the differences did not reach significance. In all MTLE cases, [(123)I]iomazenil singlephoton emission computed tomography (SPECT) showed decreased BZDR binding in MTL.

CONCLUSIONS

In MTLE, BZDR densities decreased parallel to reduction in cell density in most hippocampal subfields, but BZDR density appeared to decrease in excess of neuron loss in CA1. [(125)I]iomazenil SPECT might be useful for detecting in vivo changes of BZDR density.

When is positron emission tomography really necessary in epilepsy diagnosis?

Positron emission tomography can be used for localization of epileptic foci, and preoperative functional mapping. Rapid improvements in magnetic resonance imaging, however, have restricted the need for positron emission tomography to a minority of patients who have unrevealing magnetic resonance imaging scans. Positron emission tomography will continue to be of value in investigations of the pathophysiology of seizure disorders.