Estimation of cerebral oxygen utilization rate by single-bolus 15O2 inhalation and dynamic positron emission tomography

Quantification of brain oxygen extraction and metabolism with [15O]-gas PET: A technical review in the era of PET/MRI

Oxygen extraction fraction (OEF) and the cerebral metabolic rate of oxygen (CMRO₂) are key cerebral physiological parameters to identify at-risk cerebrovascular patients and understand brain health and function. PET imaging with [¹⁵O]-oxygen tracers, either through continuous or bolus inhalation, provides non-invasive assessment of OEF and CMRO₂. Numerous tracer delivery, PET acquisition, and kinetic modeling approaches have been adopted to map brain oxygenation. The purpose of this technical review is to critically evaluate different methods for [¹⁵O]-gas PET and its impact on the accuracy and reproducibility of OEF and CMRO₂ measurements. We perform a meta-analysis of brain oxygenation PET studies in healthy volunteers and compare between continuous and bolus inhalation techniques. We also describe OEF metrics that have been used to detect hemodynamic impairment in cerebrovascular disease. For these patients, advanced techniques to accelerate the PET scans and potential synthesis with MRI to avoid arterial blood sampling would facilitate broader use of [¹⁵O]-oxygen PET for brain physiological assessment.

Rapid quantitative CBF and CMRO(2) measurements from a single PET scan with sequential administration of dual (15)O-labeled tracers

Positron emission tomography (PET) with (15)O tracers provides essential information in patients with cerebral vascular disorders, such as cerebral blood flow (CBF), oxygen extraction fraction (OEF), and metabolic rate of oxygen (CMRO(2)). However, most of techniques require an additional C(15)O scan for compensating cerebral blood volume (CBV). We aimed to establish a technique to calculate all functional images only from a single dynamic PET scan, without losing accuracy or statistical certainties. The technique was an extension of previous dual-tracer autoradiography (DARG) approach, but based on the basis function method (DBFM), thus estimating all functional parametric images from a single session of dynamic scan acquired during the sequential administration of H(2)(15)O and (15)O(2). Validity was tested on six monkeys by comparing global OEF by PET with those by arteriovenous blood sampling, and tested feasibility on young healthy subjects. The mean DBFM-derived global OEF was 0.57±0.06 in monkeys, in an agreement with that by the arteriovenous method (0.54±0.06). Image quality was similar and no significant differences were seen from DARG; 3.57%±6.44% and 3.84%±3.42% for CBF, and -2.79%±11.2% and -6.68%±10.5% for CMRO(2). A simulation study demonstrated similar error propagation between DBFM and DARG. The DBFM method enables accurate assessment of CBF and CMRO(2) without additional CBV scan within significantly shortened examination period, in clinical settings.

Omission of [(15)O]CO scan for PET CMRO(2) examination using (15)O-labelled compounds

OBJECTIVE

CBF, OEF and CMRO(2) provide us important clinical indices and are used for assessing ischemic degree in cerebrovascular disorders. These quantitative images can be measured by PET using (15)O-labelled tracers such as C(15)O, C(15)O(2) and (15)O(2). To reduce the time of scan, one possibility is to omit the use of CBV data. The present study investigated the influence of fixing the CBV to OEF and CMRO(2) values on subjects with and without cerebrovascular disorders.

METHODS

The study consisted of three groups, namely, GROUP-0 (n = 10), GROUP-1 (n = 9), and GROUP-2 (n = 10), corresponding to--without significant disorder, with elevated CBV, and with reduced CBF and elevated OEF, respectively. All subjects received PET examination and using the PET data OEF and CMRO(2) images were computed by fixing CBV and with CBV data. The computed OEF and CMRO(2) values were compared between the methods.

RESULTS

The OEF and CMRO(2) values obtained by fixing the CBV were around 10% underestimation against that with CBV data. The regression analysis showed that these values were comparable (r = 0.93-0.98, P < 0.001). The simulation showed that fixing of the CBV would not derive significant error in either OEF or CMRO(2) values, when changed from 0 to 0.08 ml/g.

CONCLUSION

This study shows the feasibility of fixing the CBV value for computing OEF and CMRO(2) values in the PET examination, suggesting the CO scan could be eliminated.

A physiologic model for recirculation water correction in CMRO2 assessment with 15O2 inhalation PET

Cerebral metabolic rate of oxygen (CMRO(2)) can be assessed quantitatively using (15)O(2) and positron emission tomography. Determining the arterial input function is considered critical with regards to the separation of the metabolic product of (15)O(2) (RW) from a measured whole blood. A mathematical formula based on physiologic model has been proposed to predict RW. This study was intended to verify the adequacy of that model and a simplified procedure applying that model for wide range of species and physiologic conditions. The formula consists of four parameters, including of a production rate of RW (k) corresponding to the total body oxidative metabolism (BMRO(2)). Experiments were performed on 6 monkeys, 3 pigs, 12 rats, and 231 clinical patients, among which the monkeys were studied at varied physiologic conditions. The formula reproduced the observed RW. Greater k values were observed in smaller animals, whereas other parameters did not differ amongst species. The simulation showed CMRO(2) sensitive only to k, but not to others, suggesting that validity of determination of only k from a single blood sample. Also, k was correlated with BMRO(2), suggesting that k can be determined from BMRO(2). The present model and simplified procedure can be used to assess CMRO(2) for a wide range of conditions and species.

Estimation of the regional cerebral metabolic rate of oxygen consumption with proton detected 17O MRI during precision 17O2 inhalation in swine

Despite the importance of metabolic disturbances in many diseases, there are currently no clinically used methods for the detection of oxidative metabolism in vivo. To address this deficiency, (17)O MRI techniques are scaled from small animals to swine as a large animal model of human inhalation and circulation. The hemispheric cerebral metabolic rate of oxygen consumption (CMRO(2)) is estimated in swine by detection of metabolically produced H(2)(17)O by rapid T(1rho)-weighted proton magnetic resonance imaging on a 1.5T clinical scanner. The (17)O is delivered as oxygen gas by a custom, minimal-loss, precision delivery breathing circuit and converted to H(2)(17)O by oxidative metabolism. A model for gas arterial input is presented for the deeply breathing large animal. The arterial input function for recirculation of metabolic water is measured by arterial blood sampling and high field (17)O spectroscopy. It is found that minimal metabolic water "wash-in" occurs before 60s. A high temporal resolution pulse sequence is employed to measure CMRO(2) during those 60s after delivery begins. Only about one tidal volume of (17)O enriched oxygen gas is used per measurement. Proton measurements of signal change due to metabolically produced water are correlated with (17)O in vivo spectroscopy. Using these techniques, the hemispheric CMRO(2) in swine is estimated to be 1.23+/-.26 micromol/g/min, consistent with existing literature values. All of the technology used to perform these CMRO(2) estimates can easily be adapted to clinical MR scanners, and it is hoped that this work will lead to future studies of human disease.

Separation of input function for rapid measurement of quantitative CMRO2and CBF in a single PET scan with a dual tracer administration method

Cerebral metabolic rate of oxygen (CMRO(2)), oxygen extraction fraction (OEF) and cerebral blood flow (CBF) images can be quantified using positron emission tomography (PET) by administrating (15)O-labelled water (H(15)(2)O) and oxygen ((15)O(2)). Conventionally, those images are measured with separate scans for three tracers C(15)O for CBV, H(15)(2)O for CBF and (15)O(2) for CMRO(2), and there are additional waiting times between the scans in order to minimize the influence of the radioactivity from the previous tracers, which results in a relatively long study period. We have proposed a dual tracer autoradiographic (DARG) approach (Kudomi et al 2005), which enabled us to measure CBF, OEF and CMRO(2) rapidly by sequentially administrating H(15)(2)O and (15)O(2) within a short time. Because quantitative CBF and CMRO(2) values are sensitive to arterial input function, it is necessary to obtain accurate input function and a drawback of this approach is to require separation of the measured arterial blood time-activity curve (TAC) into pure water and oxygen input functions under the existence of residual radioactivity from the first injected tracer. For this separation, frequent manual sampling was required. The present paper describes two calculation methods: namely a linear and a model-based method, to separate the measured arterial TAC into its water and oxygen components. In order to validate these methods, we first generated a blood TAC for the DARG approach by combining the water and oxygen input functions obtained in a series of PET studies on normal human subjects. The combined data were then separated into water and oxygen components by the present methods. CBF and CMRO(2) were calculated using those separated input functions and tissue TAC. The quantitative accuracy in the CBF and CMRO(2) values by the DARG approach did not exceed the acceptable range, i.e., errors in those values were within 5%, when the area under the curve in the input function of the second tracer was larger than half of the first one. Bias and deviation in those values were also compatible to that of the conventional method, when noise was imposed on the arterial TAC. We concluded that the present calculation based methods could be of use for quantitatively calculating CBF and CMRO(2) with the DARG approach.

Rapid quantitative measurement of CMRO(2) and CBF by dual administration of (15)O-labeled oxygen and water during a single PET scan-a validation study and error analysis in anesthetized monkeys

Cerebral blood flow (CBF) and rate of oxygen metabolism (CMRO(2)) may be quantified using positron emission tomography (PET) with (15)O-tracers, but the conventional three-step technique requires a relatively long study period, attributed to the need for separate acquisition for each of (15)O(2), H(2)(15)O, and C(15)O tracers, which makes the multiple measurements at different physiologic conditions difficult. In this study, we present a novel, faster technique that provides a pixel-by-pixel calculation of CBF and CMRO(2) from a single PET acquisition with a sequential administration of (15)O(2) and H(2)(15)O. Experiments were performed on six anesthetized monkeys to validate this technique. The global CBF, oxygen extraction fraction (OEF), and CMRO(2) obtained by the present technique at rest were not significantly different from those obtained with three-step method. The global OEF (gOEF) also agreed with that determined by simultaneous arterio-sinus blood sampling (gOEF(A-V)) for a physiologically wide range when changing the arterial PaCO(2) (gOEF=1.03gOEF(A-V)+0.01, P<0.001). The regional values, as well as the image quality were identical between the present technique and three-step method for CBF, OEF, and CMRO(2). In addition, a simulation study showed that error sensitivity of the present technique to delay or dispersion of the input function, and the error in the partition coefficient was equivalent to that observed for three-step method. Error sensitivity to cerebral blood volume (CBV) was also identical to that in the three-step and reasonably small, suggesting that a single CBV assessment is sufficient for repeated measures of CBF/CMRO(2). These results show that this fast technique has an ability for accurate assessment of CBF/CMRO(2) and also allows multiple assessment at different physiologic conditions.

Resistance to exercise-induced increase in glucose uptake during hyperinsulinemia in insulin-resistant skeletal muscle of patients with type 1 diabetes

Insulin and exercise have been shown to activate glucose transport at least in part via different signaling pathways. However, it is unknown whether insulin resistance is associated with a defect in the ability of an acute bout of exercise to enhance muscle glucose uptake in vivo. We compared the abilities of insulin and isometric exercise to stimulate muscle blood flow and glucose uptake in 12 men with type 1 diabetes (age 24 +/- 1 years, BMI 23.0 +/- 0.4 kg/m(2)) and in 11 age- and weight-matched nondiabetic men (age 25 +/- 1 years, BMI 22.3 +/- 0.6 kg/m(2)) during euglycemic hyperinsulinemia (1 mU. kg(-1). min(-1) insulin infusion for 150 min). One-legged exercise was performed at an intensity of 10% of maximal isometric force for 105 min (range 45-150). Rates of muscle blood flow, oxygen consumption, and glucose uptake were quantitated simultaneously in both legs using [(15)O]water, [(15)O]oxygen, [(18)F]-2-fluoro-2-deoxy-D-glucose, and positron emission tomography. Resting rates of oxygen consumption were similar during hyperinsulinemia between the groups (2.4 +/- 0.3 vs. 2.0 +/- 0.5 ml. kg(-1) muscle. min(-1); normal subjects versus patients with type 1 diabetes, NS), and exercise increased oxygen consumption similarly in both groups (25.3 +/- 4.3 vs. 20.1 +/- 3.0 ml. kg(-1) muscle. min(-1), respectively, NS). Rates of insulin-stimulated muscle blood flow and the increments in muscle blood flow induced by exercise were also similar in normal subjects (129 +/- 14 ml. kg(-1). min(-1)) and in patients with type 1 diabetes (115 +/- 12 ml. kg(-1). min(-1)). The patients with type 1 diabetes exhibited resistance to both insulin stimulation of glucose uptake (34 +/- 6 vs. 76 +/- 9 micromol. kg(-1) muscle. min(-1), P < 0.001) and also to the exercise-induced increment in glucose uptake (82 +/- 15 vs. 162 +/- 29 micromol. kg(-1) muscle. min(-1), P < 0.05). We conclude that the ability of exercise to increase insulin-stimulated glucose uptake in vivo is blunted in patients with insulin-resistant type 1 diabetes compared with normal subjects. This could be caused by either separate or common defects in exercise- and insulin-stimulated pathways.

Brief vibrotactile stimulation does not increase cortical oxygen consumption when measured by single inhalation of positron emitting oxygen

Vibrotactile stimulation of the hand elicits no increase in oxygen consumption commensurate with the increase in blood flow measured in human sensory cortex. To test the hypothesis that previous failures to detect a proportionate increase in oxygen consumption could be an artefact of the sequential bolus, or three-step, method used to measure this parameter in the human brain in vivo, the authors compared the measurements with the results of a novel single bolus, or one-step, method of measuring oxygen consumption. The time of completion of the three-step method was 40 to 50 minutes, whereas the one-step method lasted only 3 minutes. The baseline whole-brain oxygen consumption averaged 185 +/- 32 micromol hg(-1) min(-1) by the three-step method and 153 +/- 15 micromol hg(-1) min(-1) by the one-step method. Vibrotactile stimulation did not elicit a significant increase in oxygen consumption measured by either method. This finding rejects the hypothesis that failure to detect an increase of oxygen consumption could be an artefact caused by limitations of the method used previously. Conversely, it also rejects the hypothesis that observations of an increase of oxygen consumption by the new method are artefacts caused by limitations of the one-step method.

Rapid algorithms for the construction of cerebral blood flow and oxygen utilization images with oxygen-15 and dynamic positron emission tomography

Two rapid estimation algorithms for construction of cerebral blood flow (CBF) and oxygen utilization (CMRO) images with dynamic positron emission tomography (PET) are presented. These algorithms are based on the linear least squares (LLS) and generalized linear least squares (GLLS) methodologies. Using the conventional two-compartmental model and multiple tracer studies, we derived a linear relationship for brain tissue activity to arterial blood activity, time-integrated arterial blood activity and time-integrated brain tissue activity. The LLS technique is computationally efficient as no regression analysis is required, while GLLS is used to refine the estimates obtained from LLS. A comparative study using non-linear least squares regression (NLS) revealed excellent correlation between the new algorithms for various noise levels expected in clinical applications. A sensitivity analysis was performed to examine reliability and identifiability of the parameter estimates. In view of the results, LLS and GLLS provide rapid and reliable estimates of CBF and CMRO when applied to dynamic PET data. These algorithms are particularly suitable for pixel-by-pixel construction of high resolution and highly accurate PET functional images.

In vivo estimation of cerebral blood flow, oxygen consumption and glucose metabolism in the pig by [15O]water injection, [15O]oxygen inhalation and dual injections of [18F]fluorodeoxyglucose

There is a need for suitable non-primate laboratory animals for studies of brain function by positron emission tomography (PET). To provide a comparative index of the circulatory physiology of the pig, we have applied novel PET tracer methodology to seven anaesthetized pigs, and measured cerebral regional oxygen consumption (CMR[O2]), cerebral blood flow (CBF), and cerebral glucose metabolism (CMR[glc]). Blood flow and flow-metabolism couple were estimated for selected cerebral regions of interest. We found an average hemispheric CMR(O2) of 171 +/- 18 micromol/100 cm3/min. Individual hemispheric CBF measurements varied between 33 and 41 ml/100 cm3/min, with an average of 37 +/- 3 ml/100 cm3/min at an average PaCO2 of 4.3 +/- 0.9 kPa. The blood flow dependency on arterial PCO2 was calculated from the results of the carbon dioxide response in two pigs in which the CBF measurements obeyed the equation CBF (ml/100 cm3/min) = 8.9 PaCO2 (kPa). In each pig, CMR(glc) was studied twice with a double-injection FDG method. In the first session, the values of CMR(glc) averaged 27 +/- 3 and 23 +/- 4 micromol/100 cm3/min, estimated by multilinear and linear regression analysis, respectively. In the second session, the corresponding averages were 27 +/- 3 and 24 +/- 3 micromol/100 cm3/min, respectively. The average oxygen extraction fraction was 0.46 +/- 0.09 and the oxygen-glucose ratio was 6.1 +/- 0.8. The findings indicate that the pig is suitable for PET studies of cerebral blood flow, cerebral oxygen consumption and glucose metabolism.

Cerebral [15O]water clearance in humans determined by PET: I. Theory and normal values

When used to measure blood flow in the brain, water leaves a residue in the vascular bed that influences the estimation of blood flow by current methods. To assess the magnitude of this influence, we developed a two-compartment model of blood flow with separate parameters for transport and vascular distribution of brain water. Maps of the water clearance, K1 into brain tissue, separated from the circulation by a measurably resistant blood-brain barrier (BBB), were generated by time-weighted integration. Depending on the validity of the assumptions underlying the two-compartment model presented here, the maps revealed a significant overestimation of the clearance of water when the vascular residue was ignored. Maps of Vo, the estimate of the apparent vascular distribution volume of tracer H2(15)O, clearly revealed major cerebral arteries. Thus, we claim that the accumulation of radioactive water in brain tissue also reflects the volume of the arterial vascular bed of the brain.

Noninvasive quantification of regional myocardial metabolic rate for oxygen by use of 15O2 inhalation and positron emission tomography. Theory, error analysis, and application in humans

BACKGROUND

A method has been developed to measure the regional myocardial metabolic rate of oxygen consumption (rMMRO2) and oxygen extraction fraction (rOEF) quantitatively and noninvasively in humans by use of 15O2 inhalation and positron emission tomography. This article describes the theory, an error analysis of the technique, and procedures of the method used in a human feasibility study.

METHODS AND RESULTS

Inhaled 15O2 is transported to peripheral tissues, where it is converted to 15O-labeled water of metabolism, which exchanges with the relatively large extravascular tissue space. Quantification of this buildup of radioactivity allows the calculation of rMMRO2 and rOEF. However, a correction for the spillover of the pulmonary gas radioactivity signal into myocardial regions is required and has been made by use of a gas volume distribution estimated from the transmission scan. This was validated by comparative measurements using the inert gas [11C]CH4 in four greyhounds. Spillover of the cardiac chamber radioactivity has been corrected for with an inhaled [13O]CO (blood volume) scan. The underestimation of myocardial radioactivity due to wall motion and thickness has been corrected for by use of values of tissue fraction obtained from the flow measurement [15OKCO2 scan). Values of rOEF were similar (within 4%) whether obtained from gas volume measurements determined from the transmission or [11C]CH4 scan data. 15O2 scan information from six healthy volunteers showed a clear distribution of myocardial radioactivity after the vascular and pulmonary gas 15O background was subtracted. Subsequent compartmental analysis resulted in values for rOEF and rMMRO2 of 0.60 +/- 0.11 and 0.10 +/- 0.03 mL.min-1.g-1 in the human myocardium at rest.

CONCLUSIONS

The results of this study are in good agreement with established values. This is the first known approach to allow the direct quantitative determination of rOEF and oxygen metabolism to be made noninvasively on a regional basis.

Modeling [15O]oxygen tracer data for estimating oxygen consumption

The most direct measure of oxidative tissue metabolism is the conversion rate of oxygen to water via mitochondrial respiration. To calculate oxygen consumption from the analysis of tissue residue curves or outflow dilution curves after injection of labeled oxygen one needs realistic mathematical models that account for convection, diffusion, and transformation in the tissue. A linear, three-region, axially distributed model accounts for intravascular convection, penetration of capillary and parenchymal cell barriers (with the use of appropriate binding spaces to account for oxygen binding to hemoglobin and myoglobin), the metabolism to [15O]water in parenchymal cells, and [15O]water transport into the venous effluent. Model solutions fit residue and outflow dilution data obtained in an isolated, red blood cell-perfused rabbit heart preparation and give estimates of the rate of oxygen consumption similar to those obtained experimentally from the flow times the arteriovenous differences in oxygen contents. The proposed application is for the assessment of regional oxidative metabolism in vivo from tissue 15O-residue curves obtained by positron emission tomography.

Oxygen consumption of the living human brain measured after a single inhalation of positron emitting oxygen

We measured the rate of washout of 15O-labeled water generated from labeled oxygen accumulated in brain after bolus [15O]O2 inhalation, and compared the washout with that of labeled water measured with H215O. Contrary to the original expectation, the radioactive water generated from labeled oxygen failed to leave the brain tissue at the rate predicted by exogenous water. Therefore, the use of a separately measured value for exogenous water clearance led to an error in the calculation of oxygen consumption. A new method presented in this paper eliminated the error by yielding oxygen consumption in a single oxygen study. We used time-weighted integration to estimate three parameters, including the unidirectional clearance from blood to brain (KO2(1)), the fractional clearance of the distribution volume in brain (kO2(2)), and the vascular volume correction (VO). We showed that the clearance of oxygen from blood to brain can be estimated with acceptable precision by this new approach, and that the new method yields a reliable measure of oxygen consumption.

Modelling approach for separating blood time-activity curves in positron emission tomographic studies

A modelling approach is developed to generate the full time course of an injected radiotracer and its labelled metabolites in plasma/blood, based on measurements of the total radioactivities in withdrawn plasma/blood samples. A compartmental model is used to describe the conversion of an injected tracer to its metabolites in the body. The model equation is formulated with the total radioactivity concentration curve as the input function. The utility and characteristics of the approach in quantitative positron emission tomographic (PET) studies are shown with two examples. In the first example, using the tracer 6-[18F]fluoro-L-dopa (FDOPA), the approach is shown to derive the full time course of plasma FDOPA and its metabolites. In the second example of dynamic 15O oxygen PET, the approach is used to solve a deconvolution problem to give separated time-activity curves of 15O oxygen and 15O water in blood. The modelling approach improves the separation of blood/plasma time-activity curves and leads to better quantitative interpretation of PET results.

Minimisation of parameter estimation errors in dynamic PET: choice of scanning schedules

We addressed the general problem of finding an optimal scan schedule in positron emission tomography (PET) dynamic studies which minimises the errors in estimating the transfer constants between a set of compartments. As an example, the influence of scan intervals in PET on the accuracy of estimation of the rate constants and vascular component in the deoxyglucose method was examined using an empirical noise model. The simulated noisy curves used in the analysis were compared with patient data to validate the noise model. A series of scan schedules were compared for accuracy of fit by evaluating the determinant of the variance-covariance matrix of the fitted parameters as an index of parameter accuracy. For realistic noise levels there is a monotonic improvement in the index of parameter accuracy with increasing sampling frequency, particularly over the initial minutes after the tracer injection. However, since faster schedules are more susceptible to errors introduced by time mismatches between plasma and tissue curves and impose greater computational and memory overhead, an initial scan duration of 30 s provide a practical trade-off for dynamic PET 18F-fluoro-deoxyglucose studies.

Direct comparison of single-scan autoradiographic with multiple-scan least-squares fitting approaches to PET CMRO2 estimation

The time course of local cerebral radioactivity concentration after bolus inhalation of oxygen gas labeled with O-15 was measured in a rapid dynamic sequence of positron tomographic images. Four normal subjects were studied at rest. In each study, 15 multiple-slice image sets were acquired over a 3-min period in a Scanditronix model 384 tomograph. The radioactivity concentration in arterial blood was measured at 1-s intervals by means of a pump-fed flow-through detector. Pump effluent was directed to discrete samples that were separated into plasma and cell fractions to estimate the accumulation of labeled, recirculating water arising from systemic metabolism. Stereotactically matched scans of local cerebral blood flow and volume were acquired in the same imaging session, and the derived values were used as fixed parameters in the model fits of the time courses of pixel radioactivity in the oxygen study. Rapid nonlinear least-squares parameter optimization was used to estimate simultaneously the local CMRO2 and the brain/blood relative distribution volume for water in each image pixel. The same scan data were combined into effective single frames of various starting times and durations for analysis using the single-scan ("autoradiographic") approach to CMRO2 estimation, which requires a presumed value for relative distribution volume. Oxygen use values derived using this approach were observed to be strongly dependent on the relative distribution volume value chosen, particularly for long study durations. However, for each gray matter region of interest studied, a uniform value for the relative distribution volume existed such that the estimated CMRO2 values were independent of the starting time and duration of the single scan used, and were furthermore the same as that yielded by the multiple-scan least-squares fitting of the total time course in the same region. We conclude that the properties of the single-scan and multiple-scan approaches are very similar at the same total study duration, provided that the value selected for the water relative distribution volume brings the measured and computed tissue time courses into correspondence.

An evaluation of Bayesian regression for estimating cerebral oxygen utilization with oxygen-15 and dynamic PET

The use of Bayesian regression for improving the estimation of cerebral oxygen utilization rate (CMRO) was investigated. Based on plasma time-activity curves of (15)O oxygen and (15)O water that were obtained previously in a normal subject, realistic tissue time-activity curves of (15)O radioactivity in gray and white matter as obtained in dynamic PET (positron emission tomography) (15)O oxygen studies were computer-simulated for various noise conditions and blood flow variations. Bayesian regression and regular least-square regressions were applied to estimate CMRO from the kinetics. The accuracy and precision of the estimates were evaluated. The improvement in precision was as high as 50% for gray matter of 1 cm (3) size in a study of 0.5 million counts/slice over 3 min. Similar improvement was achieved for white matter in 1-million-count studies. The amount of improvement is larger for higher noise cases.

Influx of a choline analog to dog brain measured by positron emission tomography

The influx of the 11C-labeled choline analog pyrrolidinocholine into tissue was measured in the brain of three dogs by positron emission tomography (PET). During the first 90 s after the intravenous bolus injection of the tracer, transfer of tracer from plasma to tissue was unidirectional. The influx constant for pyrrolidinocholine into intracranial tissue, Kin, was 0.017 ml/g/min (0.008 SD), and the initial volume of distribution, V0, was 0.08 ml/g (0.03 SD). The influx constant was at least five times larger than the value expected if simple diffusion were to account for tissue uptake. The method presented in this paper can be used to investigate the availability of plasma choline and its analogs to the living human brain and other tissue in degenerative diseases affecting the cholinergic system, and to provide in vivo information on a choline transport system.