Reduced dopaminergic transmission has been implicated in the pathophysiology of major depression. The aim of the present study was to measure striatal D(2) receptor availability and amphetamine-induced dopamine release in nonpsychotic,... more
Reduced dopaminergic transmission has been implicated in the pathophysiology of major depression. The aim of the present study was to measure striatal D(2) receptor availability and amphetamine-induced dopamine release in nonpsychotic, unmedicated, unipolar patients during an episode of major depression. The striatal equilibrium specific to nonspecific partition coefficient (V(3)") of the D(2) receptor antagonist [(123)I]IBZM was measured with single photon emission computerized tomography before and after amphetamine administration in 9 depressed subjects and 10 matched healthy control subjects. No significant differences were observed in preamphetamine D(2) receptor availability between depressed patients (0.73 +/- 0.08) and control subjects (0.78 +/- 0.10, p =.23). Amphetamine-induced reduction in [(123)I]IBZM V(3)" (DeltaV(3)") was similar in depressed patients (-9.8 +/- 5.5%) and control subjects (-7.8 +/- 2.5%, p =.32). Amphetamine induced a transient improvement in symptomatology in depressed patients, but this improvement did not correlate with [(123)I]IBZM DeltaV(3)". This study did not replicate previously reported alterations in striatal D(2) receptor density in depressed patients and suggests that stimulant-induced dopamine release is not altered in major depression.
Research Interests: Psychology, Depression, Adolescent, Medicine, Biological Sciences, and 15 moreBiological Psychiatry, Dopamine, Humans, Major Depressive Disorder, Female, Male, Biological, Amphetamine, Partition Coefficient, Middle Aged, Major Depression, Adult, Benzamides, Corpus striatum, and Medical and Health Sciences
An operationalized workflow paradigm is presented and validated with pilot subject data. This approach is reproducible with a high concordance rate between individual readers (kappa 0.73 [confidence interval 0.59-0.87; P=<0.0001])... more
An operationalized workflow paradigm is presented and validated with pilot subject data. This approach is reproducible with a high concordance rate between individual readers (kappa 0.73 [confidence interval 0.59-0.87; P=<0.0001]) using a 5-point scale to assess [(18)F] labeled fluorodeoxyglucose metabolic activity in lymphomatous lesions. These results suggest an operationally practical 5-point scale workflow paradigm for potential use in larger clinical trials evaluating lymphoma therapeutics.
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A total of 95 patients, including 45 people with no evidence of liver disease and 50 documented hepatocellular disease patients, were studied to assess the value of planar and SPECT scintigraphy in the evaluation of hepatocellular... more
A total of 95 patients, including 45 people with no evidence of liver disease and 50 documented hepatocellular disease patients, were studied to assess the value of planar and SPECT scintigraphy in the evaluation of hepatocellular disease. The accuracy and concordance between techniques was assessed using two criteria. Criterion 1, heterogeneity of the tracer in liver, yielded a concordance of 64%, whereas with criterion 2, liver and spleen tracer pattern, concordance was 78%. SPECT was significantly more accurate than planar imaging (P &lt; 0.01), using both criteria. The clinical severity of the hepatocellular disease was further assessed in 72 of the patients and correlated with a liver scan severity rating scale. The SPECT technique showed a good correlation (r = 0.89) with the composite clinical severity score; however, no significant relationship with planar imaging was noted. The results of this study suggest that liver SPECT is a reliable technique for the assessment of hepatocellular disease.
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The purpose of this study was to investigate how the reagent-adding sequence affects 99mTc red blood cell (RBC) labeling efficiency using the UltraTag RBC kit. The package insert for preparation and quality control was followed during the... more
The purpose of this study was to investigate how the reagent-adding sequence affects 99mTc red blood cell (RBC) labeling efficiency using the UltraTag RBC kit. The package insert for preparation and quality control was followed during the experiment. The reagent-adding sequences were then altered during preparation. If acid-citrate dextrose is added before the NaOCl, the labeling efficiency decreases dramatically. However, the order of addition of Na 99mTcO4 and acid-citrate dextrose did not affect the labeling efficiency. Addition of the NaOCl solution before the acid-citrate dextrose solution is imperative for proper labeling efficiency.
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PET studies of the serotonin (5-hydroxytryptamine, or 5-HT) transporter are increasingly using (11)C-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile (DASB). We noted that the percentage of unmetabolized (11)C-DASB is often... more
PET studies of the serotonin (5-hydroxytryptamine, or 5-HT) transporter are increasingly using (11)C-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile (DASB). We noted that the percentage of unmetabolized (11)C-DASB is often lower at 2 min after injection than at 12 min. We hypothesized that this is due to initial "trapping" of the unmetabolized (11)C-DASB compound in the lung, a major 5-HT transporter site and dose-limiting organ. To determine whether binding to an extracranial pool of 5-HT transporters contributes to the lower initial level of unmetabolized (11)C-DASB, we examined the effects of sertraline. Eleven healthy volunteers had 2 (11)C-DASB PET scans on the same day, and 6 of the 11 had a third scan after sertraline administration. The unmetabolized (11)C-DASB fraction was measured in arterial plasma as a function of time and was fit with 2 exponentials with no damping, power function damping, or no damping with the first point removed. Power function ...
Research Interests: Magnetic Resonance Imaging, Positron Emission Tomography, Neuroimmunology, Nuclear, Serotonin, and 15 moreHumans, Serotonin Transporter, Neuroimage, Sulfides, Lung, Membrane transport proteins, Clinical Sciences, Theoretical Models, SERTRALINE, Curve fitting, Ligands, aniline Compounds, Power Function, Psychology and Cognitive Sciences, and Medical and Health Sciences
Research Interests: Life Sciences, Positron Emission Tomography, Serotonin, Brain, Mice, and 15 moreBlood brain barrier, Animals, High Pressure Liquid Chromatography, Shell Half-Life, Serotonin Receptors, Hot Spot, Single Photon Emission Computed Tomography, Piriform Cortex, Autoradiography, High Density Concrete, Biochemistry and cell biology, Brain stem, Tissue distribution, High density, and Pharmacology and pharmaceutical sciences
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Antagonist 5-HT(1A) PET ligands are available, but an agonist ligand would give more information about signal transduction capacity. Synthesis and in vivo evaluation of... more
Antagonist 5-HT(1A) PET ligands are available, but an agonist ligand would give more information about signal transduction capacity. Synthesis and in vivo evaluation of [O-methyl-(11)C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (10), a potential high affinity (K(i) = 1.36 nM) 5-HT(1A) agonist PET tracer is described. Piperazine 10 is a 5-HT(1A) agonist with an EC(50) comparable to serotonin, based on cAMP formation and GTP(gamma)S binding assays. Radiosynthesis of [(11)C]10 has been achieved by reacting 2-{4-[4-(7-hydroxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (9) and [(11)C]CH(3)OTf in 25 +/- 5% (n = 15) yield at the end of synthesis (EOS). The chemical and radiochemical purities of [(11)C]10 were &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;99% with a specific activity 1500 +/- 300 Ci/mmol (n =15). PET studies in anesthetized baboon demonstrate [(11)C]10 specific binding in brain regions rich in 5-HT(1A) receptors. Binding of [(11)C]10 was blocked by WAY100635 and 8-OH-DPAT. The regional brain volumes of distribution (V(T)) of [(11)C]10 in baboon correlate with [(11)C]WAY100635 V(T) in baboons. These data provide evidence that [(11)C]10 is the first promising agonist PET tracer for the 5-HT(1A) receptors.
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Sixteen patients with documented AIDS cholangitis who underwent Tc-99m DISIDA hepatobiliary scintigraphy were retrospectively reviewed to assess the spectrum of changes observed in this disease. AIDS cholangitis was documented by either... more
Sixteen patients with documented AIDS cholangitis who underwent Tc-99m DISIDA hepatobiliary scintigraphy were retrospectively reviewed to assess the spectrum of changes observed in this disease. AIDS cholangitis was documented by either ERCP with aspiration/biopsy or the presence of typical sonographic/CT abnormalities with positive stool culture and a minimum of 6 months follow-up. Images were graded as regards parenchymal function (blood pool clearance, peak parenchymal activity, and degree of parenchymal retention), gallbladder visualization, presence of ductal dilatation, and time of intestinal activity. Three patterns were identified: 1) focal ductal dilatation with focal narrowing and focal or diffuse parenchymal retention; 2) ductal dilatation, without narrowing, and diffuse parenchymal retention; and 3) severe diffuse parenchymal retention with or without ductal abnormality. All 16 studies revealed abnormal parenchymal retention. Gallbladder nonvisualization was demonstrated in nine studies and delayed in two. The hepatobiliary scan is a very sensitive technique for evaluating AIDS cholangitis. Although a spectrum of findings may be observed, parenchymal retention with some degree of ductal abnormality is the most commonly observed pattern.
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Research Interests: Schizophrenia, Principal Component Analysis, Magnetic Resonance Imaging, Prefrontal Cortex, Humans, and 11 moreFemale, Male, The, Single Photon Emission Computed Tomography, Adult, Short Term Memory, Age Factors, Memory Disorders, Predictive value of tests, Dopamine antagonists, and Neuropsychological Tests
A general one-pot method has been developed for the conversion of an aryl thiol moiety masked as the butyrate ester to the corresponding 11C-labeled methylsulfone group. The potential of this methodology has been demonstrated by the... more
A general one-pot method has been developed for the conversion of an aryl thiol moiety masked as the butyrate ester to the corresponding 11C-labeled methylsulfone group. The potential of this methodology has been demonstrated by the successful radiosynthesis of carbon-11 analogues of several highly selective cyclooxygenase-2 (COX-2) inhibitors such as Rofecoxib, Etoricoxib, and 3-(4-methylsulfonylphenyl)-4-phenyl-5-trifluoromethyl isoxazole in high yield. The chemical and radiochemical purities obtained for the 11C-labeled COX-2 inhibitors are &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;99% with a specific activity &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;1000 Ci/mmol.
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ABSTRACT OBJECTIVE AND IMPORTANCE: Focal neurological deficits after carotid endarterectomy may result from ischemia or hyperperfusion. The usefulness of single photon emission computed tomography (SPECT) for differentiating between these... more
ABSTRACT OBJECTIVE AND IMPORTANCE: Focal neurological deficits after carotid endarterectomy may result from ischemia or hyperperfusion. The usefulness of single photon emission computed tomography (SPECT) for differentiating between these two mechanisms has not been previously emphasized. CLINICAL PRESENTATION: An 83-year-old man experienced dysarthria and left-sided weakness immediately after undergoing endarterectomy of the right internal carotid artery. The results of computed tomography of the head were normal, and transcranial Doppler sonography showed symmetrically elevated velocities in both middle cerebral arteries. On the ist postoperative day, the patient&#39;s deficits worsened in parallel with spontaneous increases in blood pressure, and blood pressure reduction with labetalol resulted in clinical improvement. INTERVENTION: On the 2nd postoperative day, technetium-99-hexametazime SPECT demonstrated markedly increased flow in the right basal ganglia and inferior frontal cortex, confirming the diagnosis of cerebral hyperperfusion. The patient&#39;s deficits continued to improve with antihypertensive therapy, and SPECT performed 7 and 48 days after surgery showed gradual normalization of the focal hyperemia. CONCLUSION: SPECT can be used to diagnose and monitor cerebral hyperperfusion after carotid endarterectomy and may be of particular value for differentiating hyperperfusion from ischemia when characteristic computed tomographic and transcranial Doppler sonographic findings are absent.
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... Authors: Abi-Dargham A.; Kegeles L.; Malaspina D.; Clark S.; Gorman J.; Mann J.; van Heertum R.; Laruelle M ... The feasibility of this method in humans was tested by comparing [ 123 ]IBZM binding potential (BP) before and after... more
... Authors: Abi-Dargham A.; Kegeles L.; Malaspina D.; Clark S.; Gorman J.; Mann J.; van Heertum R.; Laruelle M ... The feasibility of this method in humans was tested by comparing [ 123 ]IBZM binding potential (BP) before and after administration of alpha-methyl-para-tyrosine (4 gm ...
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Abstract A 40-year-old man was examined for fever and chronic cough. The admission chest radiograph showed dextrocardia and bilateral hilar adenopathy. A Ga-67 scan showed abdominal situs inversus (with a left-sided liver) and abnormal... more
Abstract A 40-year-old man was examined for fever and chronic cough. The admission chest radiograph showed dextrocardia and bilateral hilar adenopathy. A Ga-67 scan showed abdominal situs inversus (with a left-sided liver) and abnormal uptake in the pulmonary ...
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Research Interests: Evolution, Pathophysiology, Stroke, Brain, Humans, and 16 moreHemodynamics, Female, Male, Clinical Sciences, Aged, Middle Aged, Single Photon Emission Computed Tomography, Adult, Blood Flow, Intracerebral Hemorrhage, X ray Computed Tomography, Cerebral Blood Flow, Cerebral Hemorrhage, Neurosciences, Brain Edema, and Acute Disease
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Multiscale adaptive histogram equalization (MAHE), a wavelet-based algorithm, was investigated as a method of automatic simultaneous display of the full dynamic contrast range of a computed tomographic image. Interpretation times were... more
Multiscale adaptive histogram equalization (MAHE), a wavelet-based algorithm, was investigated as a method of automatic simultaneous display of the full dynamic contrast range of a computed tomographic image. Interpretation times were significantly lower for MAHE-enhanced images compared with those for conventionally displayed images. Diagnostic accuracy, however, was insufficient in this pilot study to allow recommendation of MAHE as a replacement for conventional window display.
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ABSTRACT Abstract1‐Methyl‐4‐[4‐(7‐chloro‐4‐quninolyl‐[3‐14C]amino)‐benzoyl]piperazine (V) was prepared for pharmacokinetic and pharmacodynamic evaluation. The synthesis of V was accomplished first by a modified Claisen ester condensation... more
ABSTRACT Abstract1‐Methyl‐4‐[4‐(7‐chloro‐4‐quninolyl‐[3‐14C]amino)‐benzoyl]piperazine (V) was prepared for pharmacokinetic and pharmacodynamic evaluation. The synthesis of V was accomplished first by a modified Claisen ester condensation reaction of diethyl‐[2‐14C]‐malonate, triethyl orthoformate, and acetic anhydride in the presence of ZnCl2 to form ethyl ethoxymethylene‐[2‐14C]‐malonate (I), which was further condensed with m‐chloroaniline to yield 7‐chloro‐4‐hydroxy‐3‐[14C]‐quinoline‐ethylcarboxylate (II). Saponification of II yielded the corresponding carboxylic acid (III). Decarboxylation of the acid group upon heating and a substitution of the 4‐OH group of compound III into chlorine atom with POCl3 produced 4,7‐dichloro‐[3‐14C]‐quinoline (IV), which then was reacted with 1‐methyl‐4‐(pamino‐benzoyl)piperazine to form the title compound V. There were a total of five steps of reaction, with a overall yield of 34.3%. The specific activity was 1.74 mCi/mmol.
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Research Interests: Image Analysis, Adolescent, Humans, Child, Attenuation Correction, and 17 moreFemale, Male, Sarcoma, Statistical Significance, Differential Diagnosis, Cost effectiveness, Clinical Sciences, Aged, Middle Aged, Single Photon Emission Computed Tomography, Adult, Radiopharmaceuticals, Sensitivity and Specificity, Whole Body, Region of Interest, Standardized Uptake Value (SUV), and Iterative Reconstruction
Synthesis and in vivo evaluation of 2-{4-[4-(3-methoxyphenyl)piperazin-1-yl]-butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (5 or MMT), a high affinity and selective serotonin 5-HT1AR agonist PET tracer, are described. GTPγS assay shows... more
Synthesis and in vivo evaluation of 2-{4-[4-(3-methoxyphenyl)piperazin-1-yl]-butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (5 or MMT), a high affinity and selective serotonin 5-HT1AR agonist PET tracer, are described. GTPγS assay shows that MMT is an agonist with an EC50 comparable to 5-HT. Radiolabeling of 5 was achieved in 30% yield (EOS) from desmethyl-MMT (4) with >99% chemical and radiochemical purities and a specific activity >1000 Ci/mmol. PET studies in baboon show that [11C]5 penetrates the blood–brain barrier but, because of low specific binding and fast clearance of radioactivity it is not a suitable PET tracer for the in vivo quantification of 5-HT1AR.
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ABSTRACT A convenient synthesis of [N-methyl-11C]-3-[(6-dimethylamino)pyridin-3-yl]-2,5-dimethyl-N,N-dipropylpyrazolo[1,5-a]pyrimidine-7-amine (R121920), a highly selective CRF1 antagonist has been developed as a potential PET ligand. 3 -... more
ABSTRACT A convenient synthesis of [N-methyl-11C]-3-[(6-dimethylamino)pyridin-3-yl]-2,5-dimethyl-N,N-dipropylpyrazolo[1,5-a]pyrimidine-7-amine (R121920), a highly selective CRF1 antagonist has been developed as a potential PET ligand. 3 - [(6 - methylamino)pyridin - 3 - yl]-2,5-dimethyl-N,N-dipropylpyrazolo [1,5-a]pyrimidine-7-amine (7), the precursor for radiolabelling was synthesized through a novel palladium catalyzed Suzuki coupling of aryl bromide 5 with heteroaryl boronate ester 4. The requisite boronate ester 4 was synthesized in four steps from 2-amino-4-bromopyridine in 50% overall yield. Although the synthesis of cold R121920 proceeded in 93% yield by sodium hexamethyl-disilazide (NaHMDS) mediated N-methylation of the desmethylamine 7 at −78°C, the attempted radiosynthesis under various conditions using conventional bases were not successful. However, the radiolabeling of [11C]R121920 was successfully carried out with [11C]MeOTf in acetone at −20°C in the absence of added basic reagents. The radiotracer was purified by RP-HPLC followed by RP-solid phase extraction. The yield of the reaction was 5% (at EOB) and the specific activity was &gt;1000 Ci/mmol (at EOB) with a radiochemical purity &gt;99%. Copyright © 2003 John Wiley &amp; Sons, Ltd.
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β-Cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a... more
β-Cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a reserve capacity of insulin secretion and, overall, assessments of insulin and blood glucose levels remain poor measures of BCM, β-cell function and progression
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Research Interests: Brain Imaging, Dementia, Epilepsy, Brain, Absence epilepsy, and 12 moreHumans, Vascular dementia, Temporal Lobe Epilepsy, Differential Diagnosis, Clinical Sciences, Single Photon Emission Computed Tomography, Cysteine, Radiopharmaceuticals, AIDS Dementia Complex, Alzheimer Disease, Predictive value of tests, and Cognition disorders
Research Interests: Psychopharmacology, Depression, Schizophrenia, Mild Cognitive Impairment, Neuropsychopharmacology, and 22 morePositron Emission Tomography, PET, Brain Mapping, Humans, Toxicity, Sex Education, Female, Male, Follow-up studies, Apolipoprotein E, Aged, Analysis of Variance, Parahippocampal Gyrus, Cognitive Decline, Inferior Parietal Lobule, Alzheimer Disease, Clinical Application, Generalized estimating equation, Cognition disorders, Case Control Studies, Severity of Illness Index, and Neuropsychological Tests
β-Cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a... more
β-Cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a reserve capacity of insulin secretion and, overall, assessments of insulin and blood glucose levels remain poor measures of BCM, β-cell function and progression of diabetes. Thus, novel noninvasive determinations of BCM are needed to provide a quantitative endpoint for novel therapies of diabetes, islet regeneration and transplantation. Built on previous gene expression studies, we tested the hypothesis that the targeting of vesicular monoamine transporter 2 (VMAT2), which is expressed by β cells, with [11C]dihydrotetrabenazine ([11C]DTBZ), a radioligand specific for VMAT2, and the use of positron emission tomography (PET) can provide a measure of BCM. In this report, we demonstrate decreased radioligand uptake within the pancreas of Lewis rats with streptozotocin-induced diabetes relative to their euglycemic historical controls. These studies suggest that quantitation of VMAT2 expression in β cells with the use of [11C]DTBZ and PET represents a method for noninvasive longitudinal estimates of changes in BCM that may be useful in the study and treatment of diabetes.
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Cerebral SPECT imaging has the potential to make an important contribution to clinical psychiatry. Cerebral SPECT scanning, stimulated by the work with PET, is readily available and much less expensive than PET. This paper reports a case... more
Cerebral SPECT imaging has the potential to make an important contribution to clinical psychiatry. Cerebral SPECT scanning, stimulated by the work with PET, is readily available and much less expensive than PET. This paper reports a case demonstrating the potential value of cerebral SPECT scanning with I-123 IMP, specifically in the serial evaluation of a schizophrenic patient with auditory hallucinations. The initial scan revealed focal areas of increased uptake in the caudate nuclei of the basal ganglia, and in the right temporal lobe. After pharmacological treatment with clinical improvement, the follow-up SPECT scan demonstrated significant improvement in the distribution of the radiopharmaceutical.
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The serotonin2A (5-HT2A) receptor is implicated in the pathophysiology of schizophrenia and mood disorders, and in vivo studies of this receptor would be of value in studying the pathophysiology of these disorders and in measuring the... more
The serotonin2A (5-HT2A) receptor is implicated in the pathophysiology of schizophrenia and mood disorders, and in vivo studies of this receptor would be of value in studying the pathophysiology of these disorders and in measuring the relationship of clinical response to receptor occupancy for 5-HT2A antagonists such as atypical antipsychotics. Therefore, (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-methoxy)-phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine (MPM) (13), a selective and high-affinity (K(i)=0.79 nM) 5HT2A antagonist, has been radiolabeled with carbon-11 by O-methylation of the corresponding desmethyl analogue (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-hydroxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine (12) with [11C]methyltriflate in order to determine the suitability of [11C]MPM to quantify 5-HT2A in living brain using PET. Desmethyl-MPM 12 and standard MPM were prepared, starting from 3-hydroxymethylphenol (2), in excellent yield. The yield obtained for radiolabeling was 40+/-5% (EOB), and the total synthesis time was 30 min at EOS. PET studies with [11C]MPM in baboon showed a distribution in the brain consistent with the known distribution of 5-HT2A receptors. The time-activity curves for the high-binding regions peaked at approximately 45 min after injection. Blocking studies with M100907 demonstrated not only 38-57% blocking of tracer binding in brain regions known to have 5-HT2A receptors but also 38% blocking in cerebellum, which has a low 5-HT2A receptor concentration. Although [11C]MPM exhibits appropriate kinetics in baboon for imaging 5-HT2A receptors, its specific binding in cerebellum and higher proportion of nonspecific binding limit its usefulness for the in vivo quantification of 5-HT2A receptors with PET.