Diagnostic accuracy of myocardial perfusion imaging with single photon emission computed tomography and positron emission tomography: a comparison with coronary angiography

Objective The aim of this study was to compare the diagnostic accuracy of myocardial perfusion imaging (MPI) by positron emission tomography (PET) with the diagnostic accuracy of MPI by single photon emission computed tomography (SPECT) in two comparable patient cohorts, using coronary angiography (CA) as the standard of reference. Methods A “SPECT-group” of 80 patients (15 female, 65 male; mean age 60 ± 9 years) and a “PET-group” of 70 patients (14 female, 56 male; mean age 57 ± 10 years) underwent a one day stress/rest examination either with attenuation-corrected ¹³N-ammonia PET or attenuation-corrected ²⁰¹TlCl SPECT or ^99mTc-hexakis-methoxy-isobutyl-isonitril (MIBI) SPECT. PET and SPECT results were semiquantitatively graded using a 6-segment heart model. All patients underwent CA, and stenoses were graded as a diameter reduction ≥50%. Results Coronary findings between both groups did not significantly differ at CA. For the SPECT-group overall sensitivity and specificity for localisation of stenoses was 77% and 84%. Respective values for the PET-group were 97% and 84%. The specificity of MPI by SPECT in the detection of ischemia was 74% and 91% for MPI by PET. The diagnostic accuracy of MPI improves when the individual coronary dominance and previous coronary revascularisations are taken into account. Conclusion MPI by ¹³N-ammonia PET is more sensitive in the detection and localisation of coronary stenoses, and more specific in the detection of ischemia than MPI by ²⁰¹TlCl/^99mMIBI SPECT.     

上尿路尿路上皮癌的（18）F-FDG PET/CT诊断及评价

目的探讨18F-FDG PET/CT在上尿路尿路上皮癌诊断中的应用价值。方法回顾性分析我中心自2000年4月至2010年12月间29例可疑上尿路尿路上皮癌患者的资料,29例患者在介入、手术及其他治疗前均行静脉尿路造影（IVU）、螺旋CT及18F-FDG PET/CT检查。分析3种影像学检查方法的诊断准确性。结果 29例中,18F-FDG PET/CT检查诊断上尿路尿路上皮癌26例（89.7%）,26例均经手术病理证实为上尿路尿路上皮癌。对照手术病理结果,IVU的敏感性和特异性分别为64.0%和75.0%;螺旋CT的敏感性和特异性分别为75.0%和80.0%;而18F-FDGPET/CT的敏感性和特异性分别为92.9%和100%。18F-FDG PET/CT的敏感性高于IVU和螺旋CT（P=0.041vsCT,P=0.036vs IVU）,同时特异性也高于IVU和螺旋CT（P=0.041vs CT,P=0.036vs IVU）。结论 18F-FDG PET/CT在上尿路尿路上皮癌的诊断中其敏感性和特异性均高于IVU和螺旋CT,为一种有用的临床诊断方法。上尿路尿路上皮癌的最后诊断需经手术病理证实。     

Small animal positron emission tomography imaging and in vivo studies of atherosclerosis

Atherosclerosis is a growing health challenge globally, and despite our knowledge of the disease has increased over the last couple of decades, many unanswered questions remain. As molecular imaging can be used to visualize, characterize and measure biological processes at the molecular and cellular levels in living systems, this technology represents an opportunity to investigate some of these questions in vivo. In addition, molecular imaging may be translated into clinical use and eventually pave the way for more personalized treatment regimes in patients. Here, we review the current knowledge obtained from in vivo positron emission tomography studies of atherosclerosis performed in small animals.     

18F-前列腺特异性膜抗原-1007 PET/CT显像、11C-胆碱PET/CT显像及单光子发射计算机断层成像术骨显像对前列腺癌骨转移的诊断价值比较

目的探讨¹⁸F-前列腺特异性膜抗原(PSMA)-1007标记的正电子发射体层显像融合计算机体层显像技术(PET/CT)显像、11C-胆碱(11C-CHO)PET/CT显像及单光子发射计算机断层成像术(SPECT)骨显像在前列腺癌骨转移诊断上的差异。方法回顾性分析2018年9月至2020年7月北部战区总医院核医学科收治的43例男性前列腺癌患者的临床资料,年龄(77.47±11.87)岁,年龄范围为55~89岁。所有患者行¹⁸F-PSMA-1007 PET/CT显像、11C-CHO PET/CT显像及SPECT骨显像检查。分别统计¹⁸F-PSMA-1007 PET/CT显像、11C-CHO PET/CT及SPECT骨显像诊断骨转移的阳性例数和阴性例数,计算三种方法各自的灵敏度、特异度、准确性、阳性预测值、阴性预测值,绘制受试者工作特征(ROC)曲线,计算并比较各自曲线下面积(AUC),评价三种检查方法对前列腺癌骨转移的诊断效能。结果 43例前列腺癌患者中,27例患者出现骨转移。¹⁸F-PSMA-1007 PET/CT显像示26例患者发生骨转移,漏诊1例;11C-CHO PET/CT显像示24例患者发生骨转移,误诊1例,漏诊4例;SPECT骨显像示23例患者发生骨转移,误诊3例,漏诊7例,三者诊断前列腺癌骨转移的灵敏度、特异度、准确性分别为96.3%(26/27)、100%(16/16)、97.7%(42/43);85.2%(23/27)、93.8%(15/16)、88.4%(38/43);74.1%(20/27)、81.3%(13/16)、76.7%(33/43)。¹⁸F-PSMA-1007 PET/CT显像、11C-CHO PET/CT和SPECT骨显像ROC曲线的AUC和95%CI分别为0.981(0.885~1.000)、0.913(0.763~0.967)、0.777(0.624~0.889)。三种检查的AUC曲线下面积进行两两比较,差异均有统计学意义(P<0.05)。采用Wilcoxon秩和检验对27例患者三种方法检出的骨转移灶的数量进行两两比较发现,¹⁸F-PSMA-1007 PET/CT显像与SPECT骨显像比较,差异有统计学意义(Z=-2.484,P=0.013),¹⁸F-PSMA-1007 PET/CT显像与11C-CHO PET/CT显像比较,差异无统计学意义(Z=-0.160,P=0.873);11C-CHO PET/CT显像与SPECT骨显像比较,差异有统计学意义(Z=-2.085,P=0.037)。结论 PET/CT显像较SPECT骨显像能发现更多的骨转移灶。¹⁸F-PSMA-1007 PET/CT显像对前列腺癌骨转移诊断的灵敏度、特异度及准确性高于其他两种检查方式。在低前列腺特异性抗原(PSA)的情况下,能够精确地对前列腺癌骨转移做出诊断。     

Quantitation of cannabinoid CB1 receptors in healthy human brain using positron emission tomography and an inverse agonist radioligand

[¹¹C]MePPEP is a high affinity, CB₁ receptor-selective, inverse agonist that has been studied in rodents and monkeys. We examined the ability of [¹¹C]MePPEP to quantify CB₁ receptors in human brain as distribution volume calculated with the “gold standard” method of compartmental modeling and compared results with the simple measure of brain uptake. A total of 17 healthy subjects participated in 26 positron emission tomography (PET) scans, with 8 having two PET scans to assess retest variability. After injection of [¹¹C]MePPEP, brain uptake of radioactivity was high (e.g., 3.6 SUV in putamen at  60 min) and washed out very slowly. A two-tissue compartment model yielded values of distribution volume (which is proportional to receptor density) that were both well identified (SE 5%) and stable between 60 and 210 min. The simple measure of brain uptake (average concentration of radioactivity between 40 and 80 min) had good retest variability ( 8%) and moderate intersubject variability (16%, coefficient of variation). In contrast, distribution volume had two-fold greater retest variability ( 15%) and, thus, less precision. In addition, distribution volume had three-fold greater intersubject variability ( 52%). The decreased precision of distribution volume compared to brain uptake was likely due to the slow washout of radioactivity from brain and to noise in measurements of the low concentrations of [¹¹C]MePPEP in plasma. These results suggest that brain uptake can be used for within subject studies (e.g., to measure receptor occupancy by medications) but that distribution volume remains the gold standard for accurate measurements between groups.     

Coronary flow reserve: measurement with transthoracic Doppler echocardiography is reproducible and comparable with positron emission tomography

Detection of early vascular changes indicated by lowered coronary flow reserve (CFR) would allow early treatment and prevention of atherosclerosis. The purpose of this study was to test whether it is possible to reproducibly measure CFR with transthoracic Doppler echocardiography (TTE) in healthy volunteers. We measured CFR using dipyridamole infusion in ten healthy male volunteers with two methods: TTE and positron emission tomography (PET) with oxygen‐15‐labelled water (group A). However, CFR was assessed twice with TTE in eight healthy male volunteers (group B) to study the reproducibility of this method. We compared CFRs obtained using TTE flow measurements in the left anterior descending coronary artery (LAD) and PET flow measurements in the corresponding myocardial area. Coronary flow in LAD could be measured in all subjects using TTE. By TTE, an average CFR based on peak diastolic flow velocity (PDV) was 2·72 ± 1·16, mean diastolic flow velocity (MDV) 2·56 ± 1·06 and velocity time integral (VTI) 1·87 ± 0·49. The results were reproducible in two repeated TTE studies (coefficient of variation in MDV 6·1 ± 4·3%, n=8). By PET, CFR was 2·52 ± 0·84. CFR assessed by TTE correlated closely with that measured by PET (MDV r=0·942, P<0·001; PDV r=0·912, P<0·002 and VTI r=0·888, P<0·006) and intraclass correlation was 0·929 (MDV) and tolerance limits for differences of CFRs was ?0·78 to 0·72. We show that CFR measured by TTE has an excellent correlation with CFR measured by PET. We also found that TTE measurements of CFR were highly reproducible.     

Liver metastases from colorectal cancer: imaging with superparamagnetic iron oxide (SPIO)-enhanced MR imaging,computed tomography and positron emission tomography

The literature about superparamagnetic iron oxide-enhanced MR imaging, computed tomography (CT) and PET (positron emission tomography using fluorine-18 labelled fluoro-deoxy-glucose) in detection of liver metastases (LM) from colorectal cancer is reviewed in this update. Special emphasis is given to studies with surgical standard of reference allowing for the lesion-by-lesion sensitivity to be determined. Based on the review, it is concluded that state-of-the-art anatomical imaging, e.g., SPIO-enhanced MR imaging and multidetector CT (MDCT), must be considered more sensitive than PET in detection of individual LM, due to technical developments in MR imaging, such as liver specific contrast agents, modern sequences and high performance gradients, and in modern MDCT have increased the performance of these modalities. MR imaging with a liver specific contrast agent is recommended for the preoperative evaluation before liver surgery for LM because of high sensitivity and better discrimination between small LM and cysts compared to MDCT. PET or PET/CT can be used for detection of extra-hepatic tumor before liver surgery.     

Effect of radiolabeled metabolite elimination from the brain on the accuracy of cerebral enzyme activity estimation using positron emission tomography with substrate tracers

Cerebral enzyme activity can be quantified using positron emission tomography (PET) in conjunction with a radiolabeled enzyme substrate. We investigated the relationship between the elimination rate (k(el)) of tracer metabolites from the brain and the precision of target enzyme activity estimation (k(3)). An initial simulation study indicated that the precision of k(3) estimates was highly dependent on k(el), and was characterized by several kinetic parameters including the ratio of k(el) and the efflux rate (k(2)) of authentic tracer (β≡k(el)/k(2)). The optimal tracer condition for high sensitivity was found to be β<0.1. To verify the simulation results, we performed a PET study with a single monkey using two PET tracers, N-[(18)F]fluoroethylpiperidin-4-ylmethyl acetate ([(18)F]FEP-4MA) and N-[(11)C]methylpiperidin-4-yl acetate ([(11)C]MP4A). Both of these substrate type tracers were developed for measuring cerebral acetylcholinesterase activity. There was good retention of the radioactive metabolite of [(11)C]MP4A in the brain (k(el)=0.0036±0.0013 min(-1), β=0.028), whereas that of [(18)F]FEP-4MA was eliminated from the brain (k(el)=0.012±0.0010 min(-1), β=0.085). A non-linear least square analysis for simultaneous estimation of all parameters showed that the precision of the k(3) estimate for [(18)F]FEP-4MA was as high (7.4%) as that for [(11)C]MP4A (10%). These results indicate that tracers with metabolites that are eliminated from the brain at a slow rate (β<0.1) may be useful for the quantitative measurement of target enzyme activity.     

Feasibility of 2–deoxy–2–[18F]fluoro–D–glucose– A85380–PET for imaging of human cardiac nicotinic acetylcholine receptors in vivo

Summary Nicotinic acetylcholine receptors mediate the parasympathetic autonomic control of cardiac function. Aim of this study was the assessment of cardiac nicotinic acetylcholine receptor distribution with a novel (α₄β₂) nicotinic acetylcholine receptor PET ligand (2–deoxy–2– [¹⁸F]fluoro–D–glucose–A85380) in humans. Five healthy volunteers without cardiac disease and six patients with either Parkinson's disease or multiple system atrophy without additional overt cardiac disease were evaluated with 2–deoxy–2–[¹⁸F]fluoro–D–glucose–A85380 PET–imaging to assess the cardiac parasympathetic innervation and the putative impact of both disorders. 2–deoxy–2– [¹⁸F]fluoro–D–glucose–A85380 whole body PET–scans were performed on a Siemens PET/CT biograph^TM 75.4 min±6.7 after i.v. injection of 371.2±58.1 MBq. Average count rate density of left ventricle ROI's and a standard ROI in the right lung were measured within three consecutive slices of 10.0 mm thickness. Heart–to–lung ratios were calculated in each volunteer and patient. Tracer uptake in the left ventricle could be measured in all of the five volunteers and the six patients. Heart–to–lung ratios in the volunteer group were not different from patients suffering from Parkinson's disease or MSA (3.2 ± 0.5 vs 3.2 ± 0.8 and 2.96±0.7, mean ± SD), respectively. Human cardiac nicotinic acetylcholine receptors can be visualized and measured by 2–deoxy–2– [¹⁸F]fluoro–D–glucose–A85380 PET scans both in cardiac–healthy subjects and patients suffering from Parkinson's disease or multiple system atrophy. The heart– as well as the lung–tracer uptake was almost constant throughout all subjects leading to a good targetto– background ratio. These first results suggest no impact of either PD or MSA on cardiac nicotinic acetylcholine receptors. An erratum to this article is available at .     

Feasibility of 2–deoxy–2–[18F]fluoro–D–glucose– A85380–PET for imaging of human cardiac nicotinic acetylcholine receptors in vivo

Summary  Nicotinic acetylcholine receptors mediate the parasympathetic autonomic control of cardiac function. Aim of this study was the assessment of cardiac nicotinic acetylcholine receptor distribution with a novel (α₄β₂) nicotinic acetylcholine receptor PET ligand (2–deoxy–2– [¹⁸F]fluoro–D–glucose–A85380) in humans. Five healthy volunteers without cardiac disease and six patients with either Parkinson's disease or multiple system atrophy without additional overt cardiac disease were evaluated with 2–deoxy–2–[¹⁸F]fluoro–D–glucose–A85380 PET–imaging to assess the cardiac parasympathetic innervation and the putative impact of both disorders. 2–deoxy–2– [¹⁸F]fluoro–D–glucose–A85380 whole body PET–scans were performed on a Siemens PET/CT biograph^TM 75.4 min±6.7 after i.v. injection of 371.2±58.1 MBq. Average count rate density of left ventricle ROI's and a standard ROI in the right lung were measured within three consecutive slices of 10.0 mm thickness. Heart–to–lung ratios were calculated in each volunteer and patient. Tracer uptake in the left ventricle could be measured in all of the five volunteers and the six patients. Heart–to–lung ratios in the volunteer group were not different from patients suffering from Parkinson's disease or MSA (3.2 ± 0.5 vs 3.2 ± 0.8 and 2.96±0.7, mean ± SD), respectively. Human cardiac nicotinic acetylcholine receptors can be visualized and measured by 2–deoxy–2– [¹⁸F]fluoro–D–glucose–A85380 PET scans both in cardiac–healthy subjects and patients suffering from Parkinson's disease or multiple system atrophy. The heart– as well as the lung–tracer uptake was almost constant throughout all subjects leading to a good target–to–background ratio. These first results suggest no impact of either PD or MSA on cardiac nicotinic acetylcholine receptors. The Article “Feasibility of 2-deoxy-2-[¹⁸F]fluoro-D-glucose-A85380-PET for imaging of human cardiac nicotinic acetylcholine receptors in vivo” by J. Bucerius et al. (the online version can be found at has been published in issue 95 (2006):105–109. Unfortunately a wrong notation for the tracer was given. Instead of 2-deoxy-2-[¹⁸F]fluoro-D-glucose-A85380 it should be 2-[¹⁸F]-A85380. The publisher apologies for any inconvenience caused by this mistake.     

In vivo imaging of adenosine A1 receptors in the human brain with [18F]CPFPX and positron emission tomography

The important roles played by the A(1) adenosine receptor (A(1)AR) in brain physiology and pathology make this receptor a target for in vivo imaging. Here we describe the distribution of A(1)ARs in the living human brain with PET, made possible for the first time by the highly potent and selective A(1)AR antagonist 8-cyclopentyl-3-(3-[(18)F]fluoropropyl)-1-propylxanthine ([(18)F]CPFPX). In vivo data demonstrate a rapid cerebral uptake, peaking at 2.9 +/- 0.6% injected dose/liter at 3.3 +/- 1.3 min, followed by a gradual washout. Consistent with the results of autoradiography, high receptor densities occurred in the putamen and the mediodorsal thalamus. Neocortical regions showed regional differences in [(18)F]CPFPX binding, with high accumulation in temporal > occipital > parietal > frontal lobes and a lower level of binding in the sensorimotor cortex. Ligand accumulation was low in cerebellum, midbrain, and brain stem. Metabolism of [(18)F]CPFPX is rapid outside the central nervous system, but the metabolites do not penetrate the blood-brain barrier. In conclusion, in vivo application of [(18)F]CPFPX, a highly potent and selective PET ligand, for the first time allows the imaging of A(1)ARs in the living human brain.     

Voxel-wise quantification of myocardial blood flow with cardiovascular magnetic resonance: effect of variations in methodology and validation with positron emission tomography

Background

Quantitative assessment of myocardial blood flow (MBF) from cardiovascular magnetic resonance (CMR) perfusion images appears to offer advantages over qualitative assessment. Currently however, clinical translation is lacking, at least in part due to considerable disparity in quantification methodology. The aim of this study was to evaluate the effect of common methodological differences in CMR voxel-wise measurement of MBF, using position emission tomography (PET) as external validation.

Methods

Eighteen subjects, including 9 with significant coronary artery disease (CAD) and 9 healthy volunteers prospectively underwent perfusion CMR. Comparison was made between MBF quantified using: 1. Calculated contrast agent concentration curves (to correct for signal saturation) versus raw signal intensity curves; 2. Mid-ventricular versus basal-ventricular short-axis arterial input function (AIF) extraction; 3. Three different deconvolution approaches; Fermi function parameterization, truncated singular value decomposition (TSVD) and first-order Tikhonov regularization with b-splines. CAD patients also prospectively underwent rubidium-82 PET (median interval 7 days).

Results

MBF was significantly higher when calculated using signal intensity compared to contrast agent concentration curves, and when the AIF was extracted from mid- compared to basal-ventricular images. MBF did not differ significantly between Fermi and Tikhonov, or between Fermi and TVSD deconvolution methods although there was a small difference between TSVD and Tikhonov (0.06 mL/min/g). Agreement between all deconvolution methods was high. MBF derived using each CMR deconvolution method showed a significant linear relationship (p < 0.001) with PET-derived MBF however each method underestimated MBF compared to PET (by 0.19 to 0.35 mL/min/g).

Conclusions

Variations in more complex methodological factors such as deconvolution method have no greater effect on estimated MBF than simple factors such as AIF location and observer variability. Standardization of the quantification process will aid comparison between studies and may help CMR MBF quantification enter clinical use.     

18-F fluorodeoxyglucose positron emission tomography/computed tomography findings of bilateral primary fallopian tube carcinoma and metastasis to the uterus: a case report and literature review

Existing literature on primary carcinoma of the fallopian tube is limited because of the rarity of this disease. We report a patient with intermittent vaginal bleeding and vaginal discharge who underwent transvaginal ultrasound, magnetic resonance imaging, and 18-F-fluorodeoxyglucose positron emission tomography/computed tomography (18-F FDG PET/CT) in our hospital. Ultrasound showed a bilateral fallopian tube mass and a uterine lesion. Magnetic resonance imaging revealed typical sausage-shaped bilateral adnexal masses, but overlooked a small lesion in the uterus in the initial diagnosis. FDG PET/CT findings not only showed bilateral fallopian tube masses and uterine lesions, but also ruled out distant metastasis. Postoperative pathology confirmed bilateral primary high-grade serous adenocarcinoma of the fallopian tube with implants in the uterus. These findings suggest that 18-F FDG PET/CT imaging could be a good approach for the diagnosis and staging of primary carcinoma of the fallopian tube.     

Relationship between regional 18F-fluorodeoxyglucose and 13N ammonia uptake in normal myocardium assessed by positron emission tomography: patterns of mismatch and effects of aging

Increased regional myocardial ¹⁸F fluorodeoxyglucose (¹⁸FDG) uptake in relation to ¹³N ammonia (¹³NH₃) uptake – i.e. glucose metabolism–blood flow mismatch – appears to be a strong indicator of myocardial viability in patients with ischemic heart disease (IHD) and regionally reduced contractile function. Reference values of regional ¹⁸FDG and ¹³NH₃ uptake have not been determined in healthy subjects with the target age for the development of IHD. We therefore studied healthy middle-aged and old men using positron emission tomography (PET). Twenty-three healthy men aged 51 to 83 years of age were studied. ¹⁸FDG and ¹³NH₃ uptake was quantified in 16 myocardial segments with PET and circumferential profile analysis. The relative ¹⁸FDG/¹³NH₃ uptake was considerably heterogeneous with ¹⁸FDG uptake consistently higher than ¹³NH₃ uptake in the left lateral ventricular wall. This regional mismatch pattern was observed in all subjects, but was most prominent in middle-aged men. The observed age-dependency was the result of a progressive increased in ¹³NH₃ uptake with advancing age in the left ventricular lateral wall. Age-matched reference values of myocardial ¹⁸FDG and ¹³NH₃ uptake appears to be important for the discrimination between physiological and pathological glucose metabolism–blood flow mismatch assessed by PET and circumferential profile analysis.     

PET Mapping of Extrastriatal D2-like Dopamine Receptors in the Human Brain Using an Anatomic Standardization Technique and [C]FLB 457

The Computerized Brain Atlas (CBA) transforms PET images of individual subjects into a standard brain anatomy. We have previously applied this to PET images with [¹¹C]raclopride and confirmed that the D2 dopamine receptors in the striatum can be evaluated accurately with a standard brain anatomy. There is growing evidence that extrastriatal D2 receptors, in spite of their low density, have pathophysiological significance for schizophrenia. We used the CBA to explore the extrastriatal distribution of D2 receptors in 13 healthy subjects using [¹¹C]FLB 457, a substituted benzamide with very high affinity for D2 and D3 receptors. There was good agreement between the specific binding ratios from CBA quantification of standardized images and those from region-of-interest analyses of original images. The highest levels of binding were observed in the putamen and caudate nucleus, followed by the globus pallidus and nucleus accumbens. Besides the basal ganglia, the hypothalamus and nucleus ruber also showed high levels of binding. Intermediate levels were found in the substantia nigra, nucleus subthalami, amygdala, and thalamus. Interestingly, there was very heterogeneous binding among the thalamic nuclei. The anterior and mediodorsal nuclei showed relatively high binding. The cerebral cortices showed lower levels with significant regional differences. Binding was highest in the temporal cortex and hippocampus followed by the anterior cingulate gyrus, and the parietal and frontal cortices, but was lowest in the occipital cortex. The use of CBA for analysis of [¹¹C]FLB 457 binding makes it possible to build a normal database for the extrastriatal D2 receptors in the living human brain. The heterogeneous distribution of D2 receptors provides an attractive opportunity for new research on the pathophysiology and drug treatment of schizophrenia.     

Measurement of the proportion of D2 receptors configured in state of high affinity for agonists in vivo: a positron emission tomography study using [11C]N-propyl-norapomorphine and [11C]raclopride in baboons

Dopamine D2 receptors are configured in interconvertible states of high (D(2 high)) or low (D(2 low)) affinity for agonists. The in vivo proportion of sites in high-affinity state remains poorly documented. Previous studies have established the D2 agonist [11C]N-propyl-norapomorphine (NPA) as a suitable positron emission tomography radiotracer for imaging D(2 high) in the living brain. To elucidate the proportion of D2 receptors configured in D(2 high) states in vivo, imaging studies were conducted in three baboons with both [11C]NPA and the D2 receptor antagonist [11C]raclopride. These studies were performed under noncarrier- and carrier-added conditions, to compare the Bmax of [11C]NPA and [11C]raclopride in the same animals. [11C]raclopride in vivo KD and Bmax were 1.59 +/- 0.28 nM (n = 3) and 27.3 +/- 3.9 nM (n = 3), respectively. The in vivo KD of [11C]NPA was 0.16 +/- 0.01 nM (n = 3), consistent with its affinity for D(2 high) reported in vitro. The maximal density of sites for [11C]NPA was 21.6 +/- 2.8 nM (n = 3), i.e., 79% of the [11C]raclopride Bmax. This result suggested that 79% of D2 receptors are configured as D(2 high) in vivo. This large proportion of D(2 high) sites might explain the vulnerability of D2 radiotracers to competition by endogenous dopamine, and is consistent with a previous report that the in vivo binding of agonist radiotracer [11C]NPA is more vulnerable to competition by endogenous dopamine than that of antagonist radiotracer [11C]raclopride.