Pancreatic perfusion and its response to glucose as measured by simultaneous PET/MRI

AIMS

Perfusion of the pancreas and the islets of Langerhans is sensitive to physiological stimuli and is dysregulated in metabolic disease. Pancreatic perfusion can be assessed by both positron emission tomography (PET) and magnetic resonance imaging (MRI), but the methods have not been directly compared or benchmarked against the gold-standard microsphere technique.

METHODS

Pigs (n = 4) were examined by [¹⁵O]H₂O PET and intravoxel incoherent motion (IVIM) MRI technique simultaneously using a hybrid PET/MRI scanner. The pancreatic perfusion was measured both at basal conditions and after intravenous (IV) administration of up to 0.5 g/kg glucose.

RESULTS

Pancreatic perfusion increased by 35%, 157%, and 29% after IV 0.5 g/kg glucose compared to during basal conditions, as assessed by [¹⁵O]H₂O PET, IVIM MRI, and microspheres, respectively. There was a correlation between pancreatic perfusion as assessed by [¹⁵O]H₂O PET and IVIM MRI (r = 0.81, R² = 0.65, p < 0.01). The absolute quantification of pancreatic perfusion (ml/min/g) by [¹⁵O]H₂O PET was within a 15% error of margin of the microsphere technique.

CONCLUSION

Pancreatic perfusion by [¹⁵O]H₂O PET was in agreement with the microsphere technique assessment. The IVIM MRI method has the potential to replace [¹⁵O]H₂O PET if the pancreatic perfusion is sufficiently large, but not when absolute quantitation is required.

Energy expenditure of rugby players during a 14-day in-season period, measured using doubly labelled water

Criterion data for total energy expenditure (TEE) in elite rugby are lacking, which prediction equations may not reflect accurately. This study quantified TEE of 27 elite male rugby league (RL) and rugby union (RU) players (U16, U20, U24 age groups) during a 14-day in-season period using doubly labelled water (DLW). Measured TEE was also compared to estimated, using prediction equations. Resting metabolic rate (RMR) was measured using indirect calorimetry, and physical activity level (PAL) estimated (TEE:RMR). Differences in measured TEE were unclear by code and age (RL 4369 ± 979; RU 4365 ± 1122; U16, 4010 ± 744; U20, 4414 ± 688; U24, 4761 ± 1523 Kcal day- 1). Differences in PAL (overall mean 2.0 ± 0.4) were unclear. Very likely differences were observed in RMR by code (RL 2366 ± 296; RU 2123 ± 269 Kcal day- 1). Differences in relative RMR between U20 and U24 were very likely (U16, 27 ± 4; U20, 23 ± 3; U24, 26 ± 5 Kcal kg- 1 day- 1). Differences were observed between measured and estimated TEE, using Schofield, Cunningham and Harris-Benedict equations for U16 (187 ± 614, unclear; - 489 ± 564, likely and - 90 ± 579, unclear Kcal day- 1), U20 (- 449 ± 698, likely; - 785 ± 650, very likely and - 452 ± 684, likely Kcal day- 1) and U24 players (- 428 ± 1292; - 605 ± 1493 and - 461 ± 1314 Kcal day- 1, all unclear). Rugby players have high TEE, which should be acknowledged. Large inter-player variability in TEE was observed demonstrating heterogeneity within groups, thus published equations may not appropriately estimate TEE.

Doubly labelled water assessment of energy expenditure: principle, practice, and promise

The doubly labelled water method for the assessment of energy expenditure was first published in 1955, application in humans started in 1982, and it has become the gold standard for human energy requirement under daily living conditions. The method involves enriching the body water of a subject with heavy hydrogen (2H) and heavy oxygen (18O), and then determining the difference in washout kinetics between both isotopes, being a function of carbon dioxide production. In practice, subjects get a measured amount of doubly labelled water (2H 218 O) to increase background enrichment of body water for 18O of 2000 ppm with at least 180 ppm and background enrichment of body water for 2H of 150 ppm with 120 ppm. Subsequently, the difference between the apparent turnover rates of the hydrogen and oxygen of body water is assessed from blood-, saliva-, or urine samples, collected at the start and end of the observation interval of 1-3 weeks. Samples are analyzed for 18O and 2H with isotope ratio mass spectrometry. The doubly labelled water method is the indicated method to measure energy expenditure in any environment, especially with regard to activity energy expenditure, without interference with the behavior of the subjects. Applications include the assessment of energy requirement from total energy expenditure, validation of dietary assessment methods and validation of physical activity assessment methods with doubly labelled water measured energy expenditure as reference, and studies on body mass regulation with energy expenditure as a determinant of energy balance.

In vitro and in vivo studies of 17O NMR sensitivity at 9.4 and 16.4 T

In vivo 17O magnetic resonance spectroscopy has been successfully applied for imaging the cerebral metabolic rate of oxygen consumption through the detection of metabolically produced H2(17)O from the inhaled 17O-labeled oxygen in animals at high field. In this study, we compared the 17O sensitivity for detecting natural abundance H2(17)O signals from a phantom solution and rat brains at 9.4 and 16.4 T. The 17O signal-to-noise ratio measured at 16.4 T was 2.9- and 2.7-2.8-fold higher than that at 9.4 T for the phantom and rat brain studies, respectively. Similarly, three-dimensional 17O magnetic resonance spectroscopy imaging data showed a more than 2.7-fold higher signal-to-noise ratio in the central rat brain region at 16.4 T than that at 9.4 T. The substantial 17O signal-to-noise ratio gain at ultrahigh field significantly improved the reliability for imaging the cerebral metabolic rate of oxygen consumption and will provide an opportunity for in vivo assessment of altered oxidative metabolism associated with brain functions and neurological diseases.

Simultaneous and noninvasive imaging of cerebral oxygen metabolic rate, blood flow and oxygen extraction fraction in stroke mice

Many brain diseases have been linked to abnormal oxygen metabolism and blood perfusion; nevertheless, there is still a lack of robust diagnostic tools for directly imaging cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF), as well as the oxygen extraction fraction (OEF) that reflects the balance between CMRO(2) and CBF. This study employed the recently developed in vivo (17)O MR spectroscopic imaging to simultaneously assess CMRO(2), CBF and OEF in the brain using a preclinical middle cerebral arterial occlusion mouse model with a brief inhalation of (17)O-labeled oxygen gas. The results demonstrated high sensitivity and reliability of the noninvasive (17)O-MR approach for rapidly imaging CMRO(2), CBF and OEF abnormalities in the ischemic cortex of the MCAO mouse brain. It was found that in the ischemic brain regions both CMRO(2) and CBF were substantially lower than that of intact brain regions, even for the mildly damaged brain regions that were unable to be clearly identified by the conventional MRI. In contrast, OEF was higher in the MCAO affected brain regions. This study demonstrates a promising (17)O MRI technique for imaging abnormal oxygen metabolism and perfusion in the diseased brain regions. This (17)O MRI technique is advantageous because of its robustness, simplicity, noninvasiveness and reliability: features that are essential to potentially translate it to human patients for early diagnosis and monitoring of treatment efficacy.

Global application of stable hydrogen and oxygen isotopes to wildlife forensics

Stable isotopes are being increasingly used in wildlife forensics as means of determining the origin and movement of animals. The heavy isotope content of precipitated water and snow (deltaD(p), delta(18)O(p)) varies widely and systematically across the globe, providing a label that is incorporated through diet into animal tissue. As a result, these isotopes are potentially ideal tracers of geographic origin. The hydrogen and oxygen isotope tracer method has excellent potential where (1) spatial variation of precipitation isotopes exist, and (2) strong, mechanistic relationships link precipitation and isotope ratios in biological tissue. Here, we present a method for interpolation of precipitation isotope values and use it to create global basemaps of growing-season (GS) and mean annual (MA) deltaD(p) and delta(18)O(p). The use of these maps for forensic application is demonstrated using previously published isotope data for bird feathers (deltaD(f)) in North America and Europe. The precipitation maps show that the greatest potential for applying hydrogen and oxygen isotope forensics exists in mid- to high-latitude continental regions, where strong spatial isotope gradients exist. We demonstrate that deltaD(f)/deltaD(p) relationships have significant predictive power both in North America and Europe, and show how zones of confidence for the assignment of origin can be described using these predictive relationships. Our analysis focuses on wildlife forensics, but the maps and approaches presented here will be equally applicable to criminal forensic studies involving biological materials. These maps are available in GIS format at http://www.waterisotopes.org.

Oxygen and nitrogen isotopic fractionations during human respiration

Both oxygen and nitrogen isotope compositions (delta18O and delta15N) of exhaled air from 10 individuals were measured. Results show linear relations between isotope variation and the fraction of O2 used during the respiration process. The isotopic influence of physiological parameters such as smoking habits, age, haemoglobin count, oxygen fixation rate or physical exercise was assessed. Among them, only smoking habits do not have any effect on delta18O. Delta15N differences between inhaled and exhaled air may indicate an active (but minor) role for nitrogen during the human respiration process. Nevertheless, nitrogen fractionation is homogenous among all the individuals, which is coherent with the fact that nitrogen metabolism is controlled by the intestinal bacterial activity.

Box-modeling of bone and tooth phosphate oxygen isotope compositions as a function of environmental and physiological parameters

A time-dependent box model is developed to calculate oxygen isotope compositions of bone phosphate as a function of environmental and physiological parameters. Input and output oxygen fluxes related to body water and bone reservoirs are scaled to the body mass. The oxygen fluxes are evaluated by stoichiometric scaling to the calcium accretion and resorption rates, assuming a pure hydroxylapatite composition for the bone and tooth mineral. The model shows how the diet composition, body mass, ambient relative humidity and temperature may control the oxygen isotope composition of bone phosphate. The model also computes how bones and teeth record short-term variations in relative humidity, air temperature and delta18O of drinking water, depending on body mass. The documented diversity of oxygen isotope fractionation equations for vertebrates is accounted for by our model when for each specimen the physiological and diet parameters are adjusted in the living range of environmental conditions.

Centile reference charts for total energy expenditure in infants from 1 to 12 months

BACKGROUND

A knowledge of energy expenditure in infancy is required for the estimation of recommended daily amounts of food energy, for designing artificial infant feeds, and as a reference standard for studies of energy metabolism in disease states.

OBJECTIVES

The objectives of this study were to construct centile reference charts for total energy expenditure (TEE) in infants across the first year of life.

METHODS

Repeated measures of TEE using the doubly labeled water technique were made in 162 infants at 1.5, 3, 6, 9 and 12 months. In total, 322 TEE measurements were obtained. The LMS method with maximum penalized likelihood was used to construct the centile reference charts. Centiles were constructed for TEE expressed as MJ/day and also expressed relative to body weight (BW) and fat-free mass (FFM).

RESULTS

TEE increased with age and was 1.40,1.86, 2.64, 3.07 and 3.65 MJ/day at 1.5, 3, 6, 9 and 12 months, respectively. The standard deviations were 0.43, 0.47, 0.52,0.66 and 0.88, respectively. TEE in MJ/kg increased from 0.29 to 0.36 and in MJ/day/kg FFM from 0.36 to 0.48.

CONCLUSIONS

We have presented centile reference charts for TEE expressed as MJ/day and expressed relative to BW and FFM in infants across the first year of life. There was a wide variation or biological scatter in TEE values seen at all ages. We suggest that these centile charts may be used to assess and possibly quantify abnormal energy metabolism in disease states in infants.

Cerebellar representation of the eyeblink response as revealed by PET

Blink reflex-related areas in the human cerebellum were studied using [15O]H2O PET in eight healthy subjects. Eyeblinks were evoked by airpuff stimulation and recorded via surface electrodes. PET analysis revealed a cerebellar increase of regional cerebral blood flow in vermal lobule VI and a small ipsilateral paravermal extension in the eyeblink condition compared to rest. This activation partly overlapped with areas found in recent fMRI and animal studies investigating the cerebellar involvement in control of the unconditioned and acquisition of the conditioned eyeblink response. In conclusion, vermal lobule VI extending to ipsilateral paravermal areas appears to be involved in control of the unconditioned eyeblink response.

Comparison of microsphere-equivalent blood flow (15O-water PET) and relative perfusion (99mTc-tetrofosmin SPECT) in myocardium showing metabolism-perfusion mismatch

Myocardial perfusion imaging with (99m)Tc-tetrofosmin is based on the assumption of a linear correlation between myocardial blood flow (MBF) and tracer uptake. However, it is known that (99m)Tc-tetrofosmin uptake is directly related to energy-dependent transport processes, such as Na(+)/H(+) ion channel activity, as well as cellular and mitochondrial membrane potentials. Therefore, cellular alterations that affect these energy-dependent transport processes ought to influence (99m)Tc-tetrofosmin uptake independently of blood flow. Because metabolism ((18)F-FDG)-perfusion ((99m)Tc-tetrofosmin) mismatch myocardium (MPMM) reflects impaired but viable myocardium showing cellular alterations, MPMM was chosen to quantify the blood flow-independent effect of cellular alterations on (99m)Tc-tetrofosmin uptake. Therefore, we compared microsphere-equivalent MBF (MBF_micr; (15)O-water PET) and (99m)Tc-tetrofosmin uptake in MPMM and in "normal" myocardium.

METHODS

Forty-two patients with severe coronary artery disease, referred for myocardial viability diagnostics, were examined using (18)F-FDG PET and (99m)Tc-tetrofosmin perfusion SPECT. Relative (18)F-FDG and (99m)Tc-tetrofosmin uptake values were calculated using 18 segments per patient. Normal myocardium and MPMM myocardium were classified using a previously validated (99m)Tc-tetrofosmin SPECT/(18)F-FDG PET score. In addition, (15)O-water PET was performed to assess kinetic-modeled MBF (MBF_kin), the water-perfusable tissue fraction (PTF), and the resulting MBF_micr (MBF_kin x PTF), which is comparable to tracer uptake values. (99m)Tc-tetrofosmin uptake and MBF_micr values were calculated for all normal and MPMM segments and averaged within their respective classifications.

RESULTS

Mean relative (99m)Tc-tetrofosmin uptake was 86% +/- 1% in normal myocardium and 56% +/- 1% in MPMM, showing a significant difference (P < 0.001), as was expected from the classification. Contrary to these findings, mean MBF_micr in MPMM myocardium was 0.60 +/- 0.03 mL x min(-1) x mL(-1), which did not significantly differ from normal myocardium (0.64 +/- 0.01 mL x min(-1) x mL(-1)). All values are given as mean +/- SEM.

CONCLUSION

Differences between reduced (99m)Tc-tetrofosmin uptake and the unchanged MBF_micr in MPMM myocardium suggest that the pathophysiologic basis of MPMM is not a blood flow reduction but cellular alterations that affect uptake and retention of (99m)Tc-tetrofosmin independently of blood flow. Therefore, it seems that perfusion deficits in MPMM myocardium are greatly overestimated by (99m)Tc-tetrofosmin and that it tends to give false-positive findings.

18O incorporation in the oxidation of N-methylcarbazole by lignin peroxidase and a model compound: a mechanistic insight into the oxidative N-demethylation of aromatic tertiary amines

Using 18O labelled reactants and/or solvent, the origin of the oxygen in the products of the oxidation of N-methylcarbazole by H2O2 catalysed by lignin peroxidase and a model compound has been determined, so getting important information about the mechanism of the oxidative N-demethylation of aromatic tertiary amines.

Oxidative interactions of synthetic lung epithelial lining fluid with metal-containing particulate matter

Epidemiology studies show association of morbidity and mortality with exposure to ambient air particulate matter (PM). Metals present in PM may catalyze oxidation of important lipids and proteins present in the lining of the respiratory tract. The present study investigated the PM-induced oxidation of human bronchoalveolar lavage (BAL) fluid (BALF) and synthetic lung epithelial lining fluid (sELF) through the measurement of oxygen incorporation and antioxidant depletion assays. Residual oil fly ash (ROFA), an emission source PM that contains approximately 10% by weight of soluble transition metals, was added (0-200 microg/ml) to BALF or sELF and exposed to 20% (18)O(2) (24 degrees C, 4 h). Oxygen incorporation was quantified as excess (18)O in the dried samples after incubation. BALF and diluted sELF yielded similar results. Oxygen incorporation was increased by ROFA addition and was enhanced by ascorbic acid (AA) and mixtures of AA and glutathione (GSH). AA depletion, but not depletion of GSH or uric acid, occurred in parallel with oxygen incorporation. AA became inhibitory to oxygen incorporation when it was present in high enough concentrations that it was not depleted by ROFA. Physiological and higher concentrations of catalase, superoxide dismutase, and glutathione peroxidase had no effect on oxygen incorporation. Both protein and lipid were found to be targets for oxygen incorporation; however, lipid appeared to be necessary for protein oxygen incorporation to occur. Based on these findings, we predict that ROFA would initiate significant oxidation of lung lining fluids after in vivo exposure and that AA, GSH, and lipid concentrations of these fluids are important determinants of this oxidation.