Identify glioma recurrence and treatment effects with triple-tracer PET/CT.

BACKGROUND: Differential diagnosis of tumour recurrence (TuR) from treatment effects (TrE), mostly induced by radiotherapy and chemotherapy, is still difficult by using conventional computed tomography (CT) or magnetic resonance (MR) imaging. We have investigated the diagnostic performance of PET/CT with 3 tracers, 13N-NH3, 18F-FDOPA, and 18F-FDG, to identify TuR and TrE in glioma patients following treatment. METHODS: Forty-three patients with MR-suspected recurrent glioma were included. The maximum and mean standardized uptake values (SUVmax and SUVmean) of the lesion and the lesion-to-normal grey-matter cortex uptake (L/G) ratio were obtained from each tracer PET/CT. TuR or TrE was determined by histopathology or clinical MR follow-up for at least 6 months. RESULTS: In this cohort, 34 patients were confirmed to have TuR, and 9 patients met the diagnostic standard of TrE. The SUVmax and SUVmean of 13N-NH3 and 18F-FDOPA PET/CT at TuR lesions were significantly higher compared with normal brain tissue (13N-NH3 0.696 ± 0.558, 0.625 ± 0.507 vs 0.486 ± 0.413; 18F-FDOPA 0.455 ± 0.518, 0.415 ± 0.477 vs 0.194 ± 0.203; both P < 0.01), but there was no significant difference in 18F-FDG (6.918 ± 3.190, 6.016 ± 2.807 vs 6.356 ± 3.104, P = 0.290 and 0.493). L/G ratios of 13N-NH3 and 18F-FDOPA were significantly higher in TuR than in TrE group (13N-NH3, 1.573 ± 0.099 vs 1.025 ± 0.128, P = 0.008; 18F-FDOPA, 2.729 ± 0.131 vs 1.514 ± 0.141, P < 0.001). The sensitivity, specificity and AUC (area under the curve) by ROC (receiver operating characteristic) analysis were 57.7%, 100% and 0.803, for 13N-NH3; 84.6%, 100% and 0.938, for 18F-FDOPA; and 80.8%, 100%, and 0.952, for the combination, respectively. CONCLUSION: Our results suggest that although multiple tracer PET/CT may improve differential diagnosis efficacy, for glioma TuR from TrE, 18F-FDOPA PET-CT is the most reliable. The combination of 18F-FDOPA and 13N-NH3 does not increase the diagnostic efficiency, while 18F-FDG is not worthy for differential diagnosis of glioma TuR and TrE.

Impact of left bundle branch block on myocardial perfusion and metabolism: A positron emission tomography study.

BACKGROUND: Better understanding of pathophysiological changes, induced by left bundle branch block (LBBB), may improve patient selection for cardiac resynchronization therapy (CRT). Therefore, we assessed the effect of LBBB on regional glucose metabolism, 13N-NH3-derived absolute and semiquantitative myocardial blood flow (MBF), and their relation in non-ischemic CRT candidates. METHODS: Twenty-five consecutive non-ischemic patients with LBBB underwent 18F-FDG and resting dynamic 13N-NH3 PET/CT prior to CRT implantation. Regional 18F-FDG uptake, absolute MBF, and late 13N-NH3 uptake were analyzed and corresponding septal-to-lateral wall ratios (SLR) were calculated. Segmental analysis was performed to evaluate "reverse mismatch," "mismatch," and "match" patterns, based on late 13N-NH3/18F-FDG uptake ratios. RESULTS: A significantly lower 18F-FDG uptake was observed in the septum compared to the lateral wall (SLR 0.53 ± 0.17). A similar pattern was observed for MBF (SLR 0.68 ± 0.18), whereas late 13N-NH3 uptake showed a homogeneous distribution (SLR 0.96 ± 0.13). 13N-NH3/18F-FDG "mismatch" and "reverse mismatch" segments were predominantly present in the lateral (52%) and septal wall (61%), respectively. CONCLUSIONS: Non-ischemic CRT candidates with LBBB demonstrate lower glucose uptake and absolute MBF in the septum compared to the lateral wall. However, late static 13N-NH3 uptake showed a homogenous distribution, reflecting a composite measure of altered regional MBF and metabolism, induced by LBBB.

Role of quantitative myocardial blood flow and 13N-ammonia washout for viability assessment in ischemic cardiomyopathy.

OBJECTIVE: Positron emission tomography (PET) integrating assessment of perfusion with 13N-ammonia (NH3) and viability with 18F-fluorodeoxyglucose (FDG) has high accuracy to identify viable, hibernating myocardium. We tested whether quantification of myocardial blood flow (MBF) and washout (k2) can predict myocardial viability using FDG as standard of reference. METHODS: In 180 consecutive patients with ischemic cardiomyopathy, myocardium was categorized on a segment-level into normal, ischemic, hibernating, and scar. From dynamic images, stress MBF, rest MBF, and k2 were derived and myocardial flow reserve (MFR) and volume of distribution (VD) were calculated. RESULTS: Across myocardial tissues, all parameters differed significantly. The area under the curve (AUC) was 0.564 (95% CI 0.527-0.601), 0.635 (0.599-0.671), 0.553 (0.516-0.591), 0.520 (0.482-0.559), and 0.560 (0.522-0.597) for stress MBF, rest MBF, MFR, k2, and VD. The generalized linear mixed model correctly classified 81% of scar as viable, hibernating myocardium. If the threshold of rest MBF to predict viability was set to 0.45 mL·min-1·g-1, sensitivity and specificity were 96% and 12%, respectively. CONCLUSION: Quantitative NH3 PET parameters have low to moderate diagnostic performance to predict viability in ischemic cardiomyopathy. However, if rest MBF falls below 0.45 mL·min-1·g-1, viability testing by FDG-PET may be safely deferred.

Differentiation of suprasellar meningiomas from non-functioning pituitary macroadenomas by 18F-FDG and 13N-Ammonia PET/CT.

BACKGROUND: Differentiation of suprasellar meningiomas (SSMs) from non-functioning pituitary macroadenomas (NFPMAs) is useful for clinical management. We investigated the utility of 13N-ammonia combined with 18F-FDG positron emission tomography (PET)/computed tomography (CT) in distinguishing SSMs from NFPMAs retrospectively. METHODS: Fourteen NFPMA patients and eleven SSM patients with histopathologic diagnosis were included in this study. Every patient underwent both 18F-FDG and 13N-ammonia PET/CT scans. The tumor to gray matter (T/G) ratios were calculated for the evaluation of tumor uptake. RESULTS: The uptake of 18F-FDG was higher in NFPMAs than SSMs, whereas the uptake of 13N-ammonia was obviously lower in NFPMAs than SSMs. The differences of 18F-FDG and 13N-ammonia uptake between the two groups were significant respectively (0.92[0.46] vs 0.59[0.29], P < 0.05, 18F-FDG; 1.58 ± 0.56 vs 2.80 ± 1.45, P < 0.05, 13N-ammonia). Tumor classification demonstrated a high overall accuracy of 96.0% for differential diagnosis. When the two traces were combined, only 1 SSM was misclassified into the NFPMA group. CONCLUSION: SSMs and NFPMAs have different metabolic characteristics on 18F-FDG and 13N-ammonia PET images. The combination of these two tracers can effectively distinguish SSMs from NFPMAs.

Protocolo de un día para la PET/TC con 18F-FDG y 13N-amonio con escala de desacoplamiento de la captación para diferenciar el glioma de bajo grado no tratado de la inflamación / One-day protocol for 18F-FDG and 13N-ammonia PET/CT with uptake decoupling score in differentiating untreated low-grade glioma from inflammation

PROPÓSITO: La identificación precisa de los gliomas de bajo grado (GBG; grados I y II de la Organización Mundial de la Salud) y su diferenciación de las lesiones por inflamación cerebral (BIL) sigue siendo difícil; sin embargo, es esencial para el tratamiento. Este estudio evaluó si un protocolo de un día para la PET/TC con 18F-FDG y 13N-amonio con análisis de desacoplamiento de la captación podría diferenciar los GBG de las BIL. MATERIALES Y MÉTODOS: Veintiocho pacientes con GBG y 16 pacientes con BIL se sometieron a PET/TC con 18F-FDG y 13N-amonio el mismo día antes de cualquier tipo de terapia. La puntuación de desacoplamiento y la relación tumor/tejido normal (T/N) de 18F-FDG y 13N-amonio se calcularon en cada localización. Se utilizó la prueba t de Student para comparar valores, y el análisis de la curva ROC para establecer un valor de corte para la relación T/N y la puntuación de desacoplamiento. Se calculó el área bajo la curva (AUC) para evaluar la eficacia diferencial. RESULTADOS: Se observaron diferencias significativas en la relación T/N de 13N-amonio (p = 0,018) y en la puntuación de desacoplamiento (p = 0,003) entre los GBG y las BIL; sin embargo, la relación T/N de 18F-FDG no mostró ninguna diferencia (p = 0,413). Los valores de corte óptimos para la relación T/N de 18F-FDG, la relación T/N de 13N-amonio y la puntuación de desacoplamiento fueron 0,73, 0,97 y 2,31, respectivamente, con AUC correspondientes de 0,48, 0,68 y 0,77. Los respectivos parámetros de sensibilidad, especificidad y precisión que utilizan estos valores de corte fueron 53,6%, 62,5% y 56,8%, respectivamente, para 18F-FDG; 50,0%, 75,0% y 59,1%, respectivamente, para 13N-amonio; y 60,7%, 93,8% y 72,7%, respectivamente, para la puntuación de desacoplamiento. CONCLUSIONES: La puntuación de desacoplamiento de la captación de 18F-FDG/13N amonio se puede utilizar para discriminar entre GBG y BIL. El uso de un mapa de desacoplamiento de estos dos trazadores puede mejorar el análisis visual y la precisión del diagnóstico

PURPOSE: Accurate identification of low-grade gliomas (LGGs; World Health Organization grades I and II) and their differentiation from brain inflammation lesions (BILs) remains difficult; however, it is essential for treatment. This study assessed whether a one-day protocol for voxel-wise 18F-FDG and 13N-ammonia PET/CT with uptake decoupling analysis could differentiate LGGs from BILs. MATERIALS AND METHODS: Twenty-eight patients with LGGs and 16 patients with BILs underwent 18F-FDG and 13N-ammonia PET/CT on the same day before any type of therapy. The decoupling score and tumor-to-normal tissue (T/N) ratio of 18F-FDG and 13N-ammonia were calculated at each location. Student's t-test was used to compare values, and ROC curve analysis was used to establish a cut-off value for the T/N ratio and decoupling score. Area under the curve (AUC) was calculated to evaluate differential efficacy. RESULTS: Significant differences were observed in 13N-ammonia T/N ratio (p = 0.018) and decoupling score (p = 0.003) between LGGs and BILs; however, the 18F-FDG T/N ratio did not show any differences (p = 0.413). Optimal cut-off values for 18F-FDG T/N ratio, 13N-ammonia T/N ratio, and decoupling score were 0.73, 0.97, and 2.31, respectively, with corresponding AUCs of 0.48, 0.68, and 0.77. The respective sensitivity, specificity, and accuracy parameters using these cut-off values were 53.6%, 62.5%, and 56.8%, respectively, for 18F-FDG; 50.0%, 75.0%, and 59.1%, respectively, for 13N-ammonia; and 60.7%, 93.8%, and 72.7%, respectively, for decoupling score. CONCLUSIONS: 18F-FDG/13N-ammonia uptake decoupling score can be used to discriminate between LGGs and BILs. Use of a decoupling map of these two tracers can improve visual analysis and diagnostic accuracy

One-day protocol for 18F-FDG and 13N-ammonia PET/CT with uptake decoupling score in differentiating untreated low-grade glioma from inflammation. / Protocolo de un día para la PET/TC con 18F-FDG y 13N-amonio con escala de desacoplamiento de la captación para diferenciar el glioma de bajo grado no tratado de la inflamación.

PURPOSE: Accurate identification of low-grade gliomas (LGGs; World Health Organization grades I and II) and their differentiation from brain inflammation lesions (BILs) remains difficult; however, it is essential for treatment. This study assessed whether a one-day protocol for voxel-wise 18F-FDG and 13N-ammonia PET/CT with uptake decoupling analysis could differentiate LGGs from BILs. MATERIALS AND METHODS: Twenty-eight patients with LGGs and 16 patients with BILs underwent 18F-FDG and 13N-ammonia PET/CT on the same day before any type of therapy. The decoupling score and tumor-to-normal tissue (T/N) ratio of 18F-FDG and 13N-ammonia were calculated at each location. Student's t-test was used to compare values, and ROC curve analysis was used to establish a cut-off value for the T/N ratio and decoupling score. Area under the curve (AUC) was calculated to evaluate differential efficacy. RESULTS: Significant differences were observed in 13N-ammonia T/N ratio (p=0.018) and decoupling score (p=0.003) between LGGs and BILs; however, the 18F-FDG T/N ratio did not show any differences (p=0.413). Optimal cut-off values for 18F-FDG T/N ratio, 13N-ammonia T/N ratio, and decoupling score were 0.73, 0.97, and 2.31, respectively, with corresponding AUCs of 0.48, 0.68, and 0.77. The respective sensitivity, specificity, and accuracy parameters using these cut-off values were 53.6%, 62.5%, and 56.8%, respectively, for 18F-FDG; 50.0%, 75.0%, and 59.1%, respectively, for 13N-ammonia; and 60.7%, 93.8%, and 72.7%, respectively, for decoupling score. CONCLUSIONS: 18F-FDG/13N-ammonia uptake decoupling score can be used to discriminate between LGGs and BILs. Use of a decoupling map of these two tracers can improve visual analysis and diagnostic accuracy.

De Novo Glutamine Synthesis: Importance for the Proliferation of Glioma Cells and Potentials for Its Detection With 13N-Ammonia.

PURPOSE: The aim of this study was to investigate the role of de novo glutamine (Gln) synthesis in the proliferation of C6 glioma cells and its detection with (13)N-ammonia. METHODS: Chronic Gln-deprived C6 glioma (0.06C6) cells were established. The proliferation rates of C6 and 0.06C6 cells were measured under the conditions of Gln deprivation along with or without the addition of ammonia or glutamine synthetase (GS) inhibitor. (13)N-ammonia uptake was assessed in C6 cells by gamma counting and in rats with C6 and 0.06C6 xenografts by micro-positron emission tomography (PET) scanning. The expression of GS in C6 cells and xenografts was assessed by Western blotting and immunohistochemistry, respectively. RESULTS: The Gln-deprived C6 cells showed decreased proliferation ability but had a significant increase in GS expression. Furthermore, we found that low concentration of ammonia was sufficient to maintain the proliferation of Gln-deprived C6 cells, and (13)N-ammonia uptake in C6 cells showed Gln-dependent decrease, whereas inhibition of GS markedly reduced the proliferation of C6 cells as well as the uptake of (13)N-ammoina. Additionally, microPET/computed tomography exhibited that subcutaneous 0.06C6 xenografts had higher (13)N-ammonia uptake and GS expression in contrast to C6 xenografts. CONCLUSION: De novo Gln synthesis through ammonia-glutamate reaction plays an important role in the proliferation of C6 cells. (13)N-ammonia can be a potential metabolic PET tracer for Gln-dependent tumors.

The reproducibility of time-of-flight PET and conventional PET for the quantification of myocardial blood flow and coronary flow reserve with (13)N-ammonia.

BACKGROUND: This study aimed to validate the reproducibility of quantitative analysis using time-of-flight (TOF) and conventional PET with (13)N-ammonia ((13)N-NH3). METHODS AND RESULTS: Phantom images were reconstructed with and without TOF, and recovery coefficients (RCs) and the percent contrast of each sphere over the percent background variability were assessed. In the clinical study, 21 subjects underwent dynamic (13)N-NH3 PET scanning under stress and rest conditions. The dynamic acquisition images and intra- and inter-observer reproducibility of myocardial blood flow (MBF) and coronary flow reserve (CFR) were compared between reconstructions (with and without TOF). In the phantom study, RCs and the percent contrast of each sphere over the percent background variability was improved with TOF. In the clinical study, the noise of blood pool and myocardial images with TOF was less than that without TOF. Territorial and global intra- and inter-observer reproducibility of MBF and CFR values was excellent. Although segmental intra- and inter-observer reproducibility was excellent, there were larger variations in apex and the segment near the right ventricle (RV) without TOF. These variations became inconspicuous with TOF. CONCLUSION: Visual image quality, RCs, and percent contrast over percent background variability with TOF were better than that without TOF. Excellent correlations and good agreements in quantitative values were observed. TOF improved the variation of segmental values.

Validation of pixel-wise parametric mapping of myocardial blood flow with ¹³NH3 PET in patients with hypertrophic cardiomyopathy.

PURPOSE: Transmural abnormalities in myocardial blood flow (MBF) are important causes of ischaemia in patients with left ventricular (LV) hypertrophy. The study aimed to test whether pixel-wise parametric mapping of (13)NH3 MBF can reveal transmural abnormalities in patients with hypertrophic cardiomyopathy (HCM). METHODS: We submitted 11 HCM patients and 9 age-matched controls with physiological LV hypertrophy to rest and stress (dipyridamole) (13)NH3 PET. We measured MBF using a compartmental model, and obtained rest and stress parametric maps. Pixel MBF values were reorganized to obtain subendocardial and subepicardial MBF of LV segments. RESULTS: MBF at rest was higher in the subendocardial than in the subepicardial layer: 0.78 ± 0.19 vs. 0.60 ± 0.18 mL/min/g in HCM patients; 0.92 ± 0.24 vs. 0.75 ± 0.24 mL/min/g in controls (both p < 0.0001). Transmural perfusion gradient (TPG = subendocardial MBF/subepicardial MBF) at rest was similar: 1.35 ± 0.31 in HCM patients; 1.28 ± 0.27 in controls (NS). During stress, controls maintained higher subendocardial MBF: 2.44 ± 0.54 vs. 1.96 ± 0.67 mL/min/g tissue (p < 0.0001), with a TPG of 1.33 ± 0.35 (NS vs. rest). In HCM patients, the difference between subendocardial and subepicardial MBF was reduced (1.46 ± 0.48 vs. 1.36 ± 0.48 mL/min/g tissue, p < 0.01) and TPG decreased to 1.11 ± 0.34 (p < 0.0001 vs. rest and vs. controls). In HCM patients 8 of 176 segments had subendocardial MBF less than -2 × SD of the mean, versus none of 144 segments in controls (p < 0.01). CONCLUSION: Pixel-wise parametric mapping of (13)NH3 MBF enables the identification of transmural abnormalities in patients with HCM.

Synthesis of (13) C2 (15) N2 -labeled anti-inflammatory and cytoprotective tricyclic bis(cyanoenone) ([(13) C2 (15) N2 ]-TBE-31) as an internal standard for quantification by stable isotope dilution LC-MS method.

Tricyclic bis(cyanoenone), TBE-31, one of the most potent activators of the Keap1/Nrf2/antioxidant response element pathway, has been developed as a new anti-inflammatory and cytoprotective agent. (13) C2 (15) N2 -labeled TBE-31 ([(13) C2 (15) N2 ]-TBE-31), which has two (13) C and two (15) N atoms in two cyano groups, was designed to develop a method for quantification of cell, tissue, and plasma levels of TBE-31 that involves chromatography/mass spectrometry coupled with the use of a stable isotope-labeled internal standard. [(13) C2 (15) N2 ]-TBE-31 was successfully synthesized in four steps from a previously reported intermediate, which is prepared in 11 steps from cyclohexanone, by introduction of two (13) C atoms with ethyl [(13) C]formate and two (15) N atoms with hydroxyl[(15) N]amine. The stable isotope dilution liquid chromatography-mass spectrometry method for quantification of TBE-31 was successfully developed using [(13) C2 (15) N2 ]-TBE-31 to compensate for any variables encountered during sample processing and analysis.

Clinical evaluation of iterative reconstruction (ordered-subset expectation maximization) in dynamic positron emission tomography: quantitative effects on kinetic modeling with N-13 ammonia in healthy subjects.

BACKGROUND: The purpose of this study was to investigate the quantitative properties of ordered-subset expectation maximization (OSEM) on kinetic modeling with nitrogen 13 ammonia compared with filtered backprojection (FBP) in healthy subjects. METHODS AND RESULTS: Cardiac N-13 ammonia positron emission tomography (PET) studies from 20 normal volunteers at rest and during dipyridamole stimulation were analyzed. Image data were reconstructed with either FBP or OSEM. FBP- and OSEM-derived input functions and tissue curves were compared together with the myocardial blood flow and spillover values. The late area under the OSEM input functions during dipyridamole is overestimated by 30% (P < .0001) relative to FBP. Conversely, the area under the late part of the OSEM tissue curves is underestimated by 20% (P < .0001) compared with FBP during both rest and dipyridamole. These differences in tissue and input functions cause the resting myocardial blood flow to be underestimated by 15% (P < .0001). During dipyridamole, the OSEM flow is underestimated by 25% (P < .0001) relative to FBP, causing the myocardial flow reserve to be underestimated by 10% (P < .0001). Large inter-regional differences in FBP and OSEM flow values were observed with a flow underestimation of 45% (rest/dipyridamole) in the septum and of 5% (rest) and 15% (dipyridamole) in the lateral myocardial wall. CONCLUSIONS: OSEM reconstruction of myocardial perfusion images with N-13 ammonia and PET produces high-quality images for visual interpretation. However, compared with FBP, OSEM is associated with substantial underestimation of perfusion on quantitative imaging. Our findings indicate that OSEM should be used with precaution in clinical PET studies.

Microbial degradation of the biocide polyhexamethylene biguanide: isolation and characterization of enrichment consortia and determination of degradation by measurement of stable isotope incorporation into DNA.

AIMS: To isolate micro-organisms capable of utilizing polyhexamethylene biguanide (PHMB) as a sole source of nitrogen, and to demonstrate biodegradation of the biocide. METHODS AND RESULTS: Two consortia of bacteria were successfully enriched at the expense of PHMB, using sand from PHMB-treated swimming pools as inoculum. Both consortia were shown to contain bacteria belonging to the genera Sphingomonas, Azospirillum and Mesorhizobium. It was shown that the presence of both Sphingomonas and Azospirillum spp. was required for extensive growth of the consortia. In addition, the Sphingomonads were the only isolates capable of growth in axenic cultures dosed with PHMB. Using a stable isotope (15N)-labelled PHMB, metabolism of the biocide by both consortia was demonstrated. By comparing the level of 15N atom incorporation into bacterial DNA after growth on either 15N-PHMB or 15N-labelled NH4Cl, it was possible to estimate the percentage of PHMB biodegradation. CONCLUSIONS: The microbial metabolism of nitrogen from the biguanide moiety of PHMB has been demonstrated. It was revealed that Sphingomonas and Azospirillum spp. are the principal organisms responsible for growth at the expense of PHMB. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first study to demonstrate the microbial metabolism of PHMB.

Regional pulmonary perfusion, inflation, and ventilation defects in bronchoconstricted patients with asthma.

RATIONALE: Bronchoconstriction in asthma leads to heterogeneous ventilation and the formation of large and contiguous ventilation defects in the lungs. However, the regional adaptations of pulmonary perfusion (Q) to such ventilation defects have not been well studied. METHODS: We used positron emission tomography to assess the intrapulmonary kinetics of intravenously infused tracer nitrogen-13 ((13)NN), and measured the regional distributions of ventilation and perfusion in 11 patients with mild asthma. For each subject, the regional washout kinetics of (13)NN before and during methacholine-induced bronchoconstriction were analyzed. Two regions of interest (ROIs) were defined: one over a spatially contiguous area of high tracer retention (TR) during bronchoconstriction and a second one covering an area of similar size, showing minimal tracer retention (NR). RESULTS: Both ROIs demonstrated heterogeneous washout kinetics, which could be described by a two-compartment model with fast and slow washout rates. We found a systematic reduction in regional Q to the TR ROI during bronchoconstriction and a variable and nonsignificant change in relative Q for NR regions. The reduction in regional Q was associated with an increase in regional gas content of the TR ROI, but its magnitude was greater than that anticipated solely by the change in regional lung inflation. CONCLUSION: During methacholine-induced bronchoconstriction, perfusion to ventilation defects are systematically reduced by a relative increase in regional pulmonary vascular resistance.

Metabolism of [13N]ammonia in rat lung.

Bolus injection of [13N]ammonia into the femoral vein of pentobarbital-anesthetized rats was followed by rapid clearance from the blood and first-pass extraction of nearly 30% by the lungs. Of the label present in the lungs at 6 s after injection (about 27% of the dose), more than 20% was in metabolized form. Of the label present in the lungs at 2 min after injection (about 10% of the dose), 18-25% was in ammonia, about 75% was in glutamine (amide) and less than 1% was in glutamate and aspartate. Thus, despite the presence of significant amounts of glutamate dehydrogenase, the overwhelming route for metabolism of ammonia entering the rat lung in vivo was the glutamine synthetase reaction. Lung tissue that was removed 6 s after intravenous injection of [13N]ammonia and incubated in Krebs-Ringer glucose medium at 37 degrees C for 20 min, showed a significant increase (more than one-third), compared to unincubated lung tissue in the quantity of label in glutamine. Between 6s and 2 min after injection, during which time the total 13N content of the lungs decreased by more than 60%, the maintenance of a quasi-steady state in the concentration of labeled glutamine suggested a short-term balance between formation from extracted ammonia and loss of glutamine into the circulation. Our data support the concept that the lungs are a source of circulating glutamine in the rat. Despite the large fractional extraction of blood-borne [13N]ammonia by the lungs, only minute amounts of tracer (0.2-0.6 ppm of the injected dose) were detected in the expired air within the first 5 min after administration of [13N]ammonia to anesthetized rats, so that pulmonary excretion was not a significant pathway of ammonia elimination. The present findings emphasize the importance of the lungs in the maintenance of whole-body nitrogen homeostasis and suggest the use of [13N]ammonia and 13N-labeled amino acids as non-invasive probes in the study of normal and diseased lung metabolism.

Recovery of (15)n in the body, urine, and gas phase of piglets infused intravenously with (15)n L-alanine from 12-72 hours of age.

Previous studies of nitrogen metabolism provided evidence suggesting that nitrogen excretory product(s) not measured by standard methods of analysis escape detection. To determine whether (15)N could be recovered quantitatively in the body, urine, or expired gas, newborn piglets (n = 16; 1.47 +/- 0.27 kg) were infused intravenously with (15)N L-alanine from 12 to 72 h of age at a rate providing 25% of the piglets' resting energy expenditure and a (15)N abundance of 2.3 (n = 4), 2.8 (n = 10), or 3.3 (n = 2) atom percent. To investigate the possibility of gaseous nitrogen excretion, 4 piglets infused with (15)N L-alanine were housed in a closed circuit respiration system initially flushed with an 80% argon:20% O(2) mixture. The gas composition of the system was monitored at 12-h intervals throughout the experiment. Mean total recovery of (15)N was 93.3 +/- 2.8% and was significantly different from 100% (P < 0.001). To determine whether (15)N recovery was altered by metabolism, 2 piglets (1.34 +/- 0.13 kg) were killed 6 min after a bolus i.v. infusion of (15)N L-alanine (97.96 +/- 1.13 atom percent). Mean recovery of (15)N in the bodies of these piglets was 101.5 +/- 1.6% and was not different from 100%. No change in chamber gas (28)N(2) (P = 0.0969) or (29)N(2) (P = 0.08565) over 72 h was evident. The inability to recover 6.7 +/- 2.8% of infused (15)N suggests that a nitrogen-containing excretory product or metabolite may be escaping detection, but the discrepancy cannot be explained by gaseous nitrogen ((28)N(2), (29)N(2), or (30)N(2)) excretion.

Nitrogen-13 ammonia cardiac positron emission tomography in mice: effects of clonidine-induced changes in cardiac work on myocardial perfusion.

We explored the feasibility of imaging myocardial perfusion and of demonstrating the flow changes in response to reduction of cardiac work non-invasively in anesthetized mice using high spatial resolution, dedicated small-animal positron emission tomography (microPET). In 31 C57BL/6 mice anesthetized with pentobarbital or isoflurane, (13)N-ammonia was injected intravenously and images were recorded with microPET from 4 to 20 min. Fifteen mice (group 1) were studied consecutively at baseline (BL) and after reduction of heart rate (HR) with intraperitoneal injection of clonidine (CLN) to investigate effects of CLN-induced reduction of cardiac work on myocardial (13)N-ammonia uptake. Eight mice (group 2) were imaged repeatedly at BL and eight mice (group 3) twice after CLN to examine reproducibility. Total myocardial (13)N-ammonia accumulation was determined from the transaxial images and normalized for injected dose (%ID). HR was 412+/-97 beats/min at BL and 212+/-44 beats/min after CLN (P<0.0001). In group 1, the %ID significantly decreased from 1.50%+/-0.27% at BL to 1.29%+/-0.28% after CLN (P<0.0001). In groups 2 and 3, reproducibility of %ID was good (y=0.96x+0.105, SEE=0.212%, r(2)=0.749, P<0.0001). In conclusion, (13)N-ammonia microPET imaging demonstrated non-invasively a reduction of myocardial perfusion induced by clonidine in mice. We believe this study is of importance as the first report on myocardial perfusion imaging and flow validation in in vivo mouse hearts with a left ventricular size of only 5 mm using (13)N-ammonia and PET. MicroPET will aid in elucidating cardiac pathophysiology in transgenic mice and monitoring effects of gene therapies on myocardial perfusion.

Quantification of regional ventilation-perfusion ratios with PET.

UNLABELLED: The topographic matching of alveolar ventilation (V(A)) and perfusion (Q) is the main determinant of gas exchange efficiency of the lung. However, no pulmonary functional imaging technique has been shown to predict whole-lung gas exchange in health and disease. This study aims to present a PET-based method to estimate regional alveolar ventilation-to-perfusion ratios (V(A)/Q) predictive of arterial blood gases. METHODS: The method is based on the regional tracer kinetics of (13)N-nitrogen ((13)NN) after an intravenous bolus injection during a breath-hold period and subsequent washout from the lungs with resumption of breathing. The method takes into account the presence of inter- and intraregional nonuniformities at length scales smaller than the imaging spatial resolution. An algorithm used regional tracer washout to classify regional V(A)Q/ uniformity. Intraregional V(A)/Q mismatch in nonuniform regions was described with a 2-compartment model. Regional V(A)/Q estimates were combined into a whole-lung distribution of V(A)/Q ratios and were used to compute global arterial blood gases. The method was applied to 3-dimensional PET data from anesthetized and mechanically ventilated sheep before and after methacholine bronchoconstriction (n = 3) and pulmonary embolism (n = 3) and after saline lung lavage (n = 3). RESULTS: PET images revealed regional changes in ventilation and perfusion consistent with the different disease models. Quantification of the images using PET-derived V(A)Q/ distributions showed unimodal and narrow distributions in control conditions that became wider and unimodal after pulmonary embolism and saline lung lavage and bimodal after bronchoconstriction. Images of regional gas exchange allowed for visualization of regional gas exchange. Arterial blood gases estimated from the PET-based V(A)/Q distributions closely agreed with measured values (partial pressure of oxygen, arterial [PaO(2)]: r(2) = 0.97, P < 0.001; partial pressure of carbon dioxide, arterial [PaCO(2)]: r(2) = 0.96, P < 0.001). CONCLUSION: Tracer kinetics analysis of PET images after an intravenous injection of (13)NN provides a quantitative assessment of regional V(A)/Q heterogeneity including that corresponding to length scales smaller than the spatial resolution of the imaging method. Quantification of V(A)/Q mismatch obtained with the presented technique is directly related to severity of gas exchange impairment as determined by arterial blood gases.

Evaluation of ammonia metabolism in the skeletal muscles of patients with cirrhosis using N-13 ammonia PET.

OBJECTIVE: Skeletal muscle is said to compensate for the decreased ammonia metabolism in patients with cirrhosis. Branched-chain amino acids (BCAA) are being used as a treatment for hyperammonemia, and are believed to decrease blood ammonia by consumption of BCAA in skeletal muscles. We examined ammonia metabolism of the skeletal muscles in patients with liver cirrhosis after administration of BCAA using 13N-ammonia positron emission tomography (PET). METHODS: The subjects were patients with compensated or decompensated liver cirrhosis. PET studies were performed before and 2 hours after injection of BCAA. Serial dynamic PET scans (2 min x 10 frames) were started simultaneously with 13N-ammonia injection. The standardized uptake value (SUV) of both thighs was calculated. RESULTS: In the patient with compensated liver cirrhosis, there was little difference in the rate of increase in SUV before to after administration of BCAA. However, in the patient with decompensated liver cirrhosis, the rate of increase in SUV after administration was higher than that before administration of BCAA. CONCLUSION: Ammonia metabolism in the muscle of patients with liver cirrhosis could be examined noninvasively under physiological conditions using 13N-ammonia PET. The muscles were found to metabolize ammonia partially, and the role of this contribution to metabolism of ammonia in patients with decompensated liver cirrhosis is particularly important.

Variability of insulin-stimulated myocardial glucose uptake in healthy elderly subjects.

The aim of this study was to assess regional and global variability of insulin-stimulated myocardial glucose uptake in healthy elderly subjects and to evaluate potentially responsible factors. Twenty men with a mean age of 64 years, no history of cardiovascular disease, and normal blood pressure, bicycle exercise test, electrocardiogram and echocardiography were studied [ P(coronary artery disease) <5%]. Whole-body insulin sensitivity and insulin-stimulated myocardial glucose uptake were measured during hyperinsulinaemic euglycaemic glucose clamp with fluorine-18 fluorodeoxyglucose, and myocardial rest and hyperaemic blood flow during dipyridamole infusion were measured with nitrogen-13 ammonia and positron emission tomography in 16 left ventricular myocardial segments. Intra-individual and inter-individual variability of insulin-stimulated myocardial glucose uptake [relative dispersion = (standard deviation/mean)] was 13% and 29% respectively. Although inter-individual variability of glucose uptake and blood flow at rest was of the same magnitude, no correlation was found between these measures. Regional and global insulin-stimulated myocardial glucose uptake correlated linearly with whole-body insulin sensitivity ( r=0.51, P<0.05 and r=0.56, P<0.01). The strongest independent association by multivariate linear regression analysis was found between myocardial glucose uptake and hyperaemic blood flow ( r=0.63, P<0.005). We conclude that in healthy elderly subjects, insulin-stimulated myocardial glucose uptake is homogeneous throughout the left ventricle, but has moderate inter-individual variability. Inter-individual variability of insulin-stimulated myocardial glucose uptake is primarily explained by variability in coronary vascular reactivity and tissue insulin sensitivity.