The C15O2 build-up technique to measure regional cerebral blood flow and volume of distribution of water

[15O]H2O PET: Potential or Essential for Molecular Imaging?

Imaging water pathways in the human body provides an excellent way of measuring accurately the blood flow directed to different organs. This makes it a powerful diagnostic tool for a wide range of diseases that are related to perfusion and oxygenation. Although water PET has a long history, its true potential has not made it into regular clinical practice. The article highlights the potential of water PET in molecular imaging and suggests its prospective role in becoming an essential tool for the 21st century precision medicine in different domains ranging from preclinical to clinical research and practice. The recent technical advances in high-sensitivity PET imaging can play a key accelerating role in empowering this technique, though there are still several challenges to overcome.

Day-to-day test-retest variability of CBF, CMRO2, and OEF measurements using dynamic 15O PET studies

Purpose

We assessed test–retest variability of cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO₂), and oxygen extraction fraction (OEF) measurements derived from dynamic ¹⁵O positron emission tomography (PET) scans.

Procedures

In seven healthy volunteers, complete test–retest ¹⁵O PET studies were obtained; test–retest variability and left-to-right ratios of CBF, CBV, OEF, and CMRO₂ in arterial flow territories were calculated.

Results

Whole-brain test–retest coefficients of variation for CBF, CBV, CMRO₂, and OEF were 8.8%, 13.8%, 5.3%, and 9.3%, respectively. Test–retest variability of CBV left-to-right ratios was <7.4% across all territories. Corresponding values for CBF, CMRO₂, and OEF were better, i.e., <4.5%, <4.0%, and <1.4%, respectively.

Conclusions

The test–retest variability of CMRO₂ measurements derived from dynamic ¹⁵O PET scans is comparable to within-session test–retest variability derived from steady-state ¹⁵O PET scans. Excellent regional test–retest variability was observed for CBF, CMRO₂, and OEF. Variability of absolute CBF and OEF measurements is probably affected by physiological day-to-day variability of CBF.

Alcohol consumption and cerebral blood flow among older adults

A substantial epidemiological literature now supports the existence of a J or U-shaped association between alcohol consumption and a broad range cardiovascular health outcomes including stroke. Although it is well documented that alcoholics exhibit both global and regional cerebral hypoperfusion in the sober state, little is known regarding the effects of a broader range of alcohol consumption on cerebral blood flow (CBF). The present study employed positron emission tomography with H(2)(15)O to assess quantitative global and regional CBF in 86 participants (51 men and 35 women; mean age 60.1) as a function of self-reported weekly alcohol consumption (none, <1, 1 to <7, 7 to <15, and >15 drinks per week). Analyses controlling for age, gender, and vascular health (carotid intima-media thickness) revealed that, relative to the weighted population mean, global CBF was greater in the lightest alcohol consumption group (<1 per week) and lower in the heaviest (>15 per week). Effects did not vary across regions of interest. This report is the first to describe an inverted J-shaped relationship between alcohol consumption and CBF in the absence of stroke.

Evaluation of Basis Function and Linear Least Squares Methods for Generating Parametric Blood Flow Images Using 15O-Water and Positron Emission Tomography

PURPOSE

Parametric analysis of (15)O-water positron emission tomography (PET) studies allows determination of blood flow (BF), perfusable tissue fraction (PTF), and volume of distribution (V (d)) with high spatial resolution. In this paper the performance of basis function and linear least squares methods for generating parametric flow data were evaluated.

PROCEDURES

Monte Carlo simulations were performed using typical perfusion values for brain, tumor, and heart. Clinical evaluation was performed using seven cerebral and 10 myocardial (15)O-water PET studies. Basis function (BFM), linear least squares (LLS), and generalized linear least squares (GLLS) methods were used to calculate BF, PTF, or V(d).

RESULTS

Monte Carlo simulations and human studies showed that, for low BF values (<1 ml/min(-1)ml(-1), BF, PTF, and V(d) were calculated with accuracies better than 5% for all methods tested. For high BF (>2 ml/min(-1)ml(-1)), use of BFM provided more accurate V(d) compared with (G)LLS.

CONCLUSIONS

In general, BFM provided the most accurate estimates of BF, PTF, and V(d).

Positron emission tomographic imaging of angiogenesis and vascular function

Surrogate markers of clinical outcome are important in anticancer drug research, since clinical criteria of response develop only slowly and may be confounded by other processes than drug effect. The need for surrogate outcome markers is especially great with newer agents that may act by tumour stabilization as opposed to shrinkage. Neoplastic angiogenesis is associated with a number of detectable changes at molecular and microcirculatory levels. Therefore, direct study of angiogenic molecular biology and tumour circulation before during and after treatment may offer useful surrogate markers for vascular-targeted therapies. The main advantage of radiotracer imaging with positron emission tomography (PET) is its functional specificity. This article will review two main areas: (a) the methodology behind PET imaging of tumour blood supply with 15O-oxygen labelled compounds; and (b) newer tracers in development as markers of angiogenetic biology.

Principles of tracer kinetic analysis

This article described basic ideas and concepts that are related to the performance and analysis of dynamic tracer kinetic PET studies of brain. There are many aspects of these studies that require careful consideration, because there is always a compromise between accuracy and precision [101]. Important issues that were not discussed in this article include the appropriate use of anatomic information for the interpretation and analysis of the functional PET imaging data, ROI sampling, or parametric image generation; statistical analyses of ROI and parametric data; as well as steps involved in the evaluation of novel radiotracers and the identification of an analysis-of-choice or issues related to methodologic optimization. Fig. 5 is a summary diagram that links these ideas and provides a more complete picture of the multiple components that are involved in tracer kinetic PET imaging research.

H(2)(15)O PET validation of steady-state arterial spin tagging cerebral blood flow measurements in humans

Steady-state arterial spin tagging approaches can provide quantitative images of CBF, but have not been validated in humans. The work presented here compared CBF values measured using steady-state arterial spin tagging with CBF values measured in the same group of human subjects using the H(2)(15)O IV bolus PET method. Blood flow values determined by H(2)(15)O PET were corrected for the known effects of incomplete extraction of water across the blood brain barrier. For a cortical strip ROI, blood flow values determined using arterial spin tagging (64+/-12 cc/100 g/min) were not statistically different from corrected blood flow values determined using H(2)(15)O PET (67+/-13 cc/100 g/min). However, for a central white matter ROI, blood flow values determined using arterial spin tagging were significantly underestimated compared to corrected blood flow values determined using H(2)(15)O PET. This underestimation could be caused by an underestimation of the arterial transit time for white matter regions.

Differences in cerebral activation during smooth pursuit and saccadic eye movements using positron-emission tomography

BACKGROUND

Abnormalities of smooth pursuit eye movements occur commonly in schizophrenia, but the pathophysiological significance of these abnormalities is unknown. To address this, the authors conducted a pilot study in which we examined differences in regional cerebral activation using positron-emission tomography (PET) in normal volunteers as they performed two types of eye movements.

METHODS

Cerebral activation in 10 normal volunteers was studied using C15O2 PET while subjects tracked a visual target using smooth pursuit and saccadic eye movements. A left-hand movement comparison task provided a physiologic landmark for verification of the location of the frontal eye fields (FEFs).

RESULTS

Subjects exhibited FEF activation during both smooth pursuit and saccadic eye movements, which was greater in the latter. During smooth pursuit, subjects also exhibited increased cerebral activation in the left temporal-occipital border and left superior frontal cortex and decreased activation in medial superior parietal and insular regions relative to saccades. Other cortical visual and eye-movement brain regions also demonstrated differences in activation between the two visual tasks.

CONCLUSIONS

Significant fEF activation appears to underlie both smooth pursuit and saccadic eye movements but may be more critical in the former. Dysfunction of the frontal lobe, and possibly of other areas in the pursuit pathway such as the temporo-occipital motion area, may contribute to observed eye-movement abnormalities in patients with schizophrenia.

Production, automatic delivery and bolus injection of [15O]water for positron emission tomography studies

An automatic system allowing repetitive bolus injection of oxygen-15-labeled water for PET studies is described in this report. The production of this radiopharmaceutical by the 16O(p,pn)15O nuclear reaction on H2(16O), its purification and delivery nearby the PET camera, the injection system, and the quality controls are presented.

Noninvasive quantification of rCBF using positron emission tomography

This study proposes a new method for the pixel-by-pixel quantification of regional CBF (rCBF) with positron emission tomography and H(15)(2)O by using a reference tissue region. No arterial blood is required. Simulation studies revealed that the calculation of rCBF was fairly stable provided that the frame time was relatively short compared with total scan time. In practice, calculated CBF images correlated significantly with those obtained with the dynamic/integral method. Because the method accurately detects changes in CBF, it is particularly suitable for brain activation studies.

Validation of noninvasive quantification of rCBF compared with dynamic/integral method by using positron emission tomography and oxygen-15 labeled water

This study proposes a new solution for the quantification of rCBF pixel-by-pixel using PET and 15O-H2O. The method represents an application of weighted integration that used PET image only, requiring no input function of arterial blood. It generates the rCBF image quickly and automatically. Simulation studies revealed that the calculation of rCBF was fairly stable as long as a relatively shorter scan frame time and longer scan time were selected. Calculated images of actual PET study by this method correlated significantly with those identified by the dynamic/integral method. Because this procedure could detect whole brain CBF change between different studies as accurately as by the dynamic/integral method, this procedure may be the most suitable for brain activation studies.

Is positron emission tomography a useful tool for studying migraine?

Despite the high prevalence of migraine in the population, the pathophysiology of this condition remains poorly understood. Vascular changes have been postulated. With positron emission tomography and various radiotracers, it is possible to have a non-invasive access to a number of parameters of interest in migraine research. These are presented and discussed in this article. PET has great potential for answering some basic questions concerning the physiological or biochemical changes that can occur during and between migraine attacks. Few studies have been published, however, probably because of (i) limitations inherent to the technique and (ii) problems in study design.

Brain stem activation in spontaneous human migraine attacks

Evidence from animal experiments shows that the brain stem is involved in the pathophysiology of migraine. To investigate human migraine, we used positron emission tomography to examine the changes in regional cerebral blood flow as an index of neuronal activity in the human brain during spontaneous migraine attacks. During the attacks, increased blood flow was found in the cerebral hemispheres in cingulate, auditory and visual association cortices and in the brain stem. However, only the brain stem activation persisted after the injection of sumatriptan had induced complete relief from headache and phono- and photophobia. These findings support the idea that the pathogenesis of migraine is related to an imbalance in activity between brain stem nuclei regulating antinociception and vascular control.

Imaging the head: functional imaging

Images

Functional neuroanatomy of antisaccade eye movements investigated with positron emission tomography

Increasing interest in the role of the frontal lobe in relation to psychiatric and neurologic disorders has popularized tests of frontal function. One of these is the antisaccade task, in which both frontal lobe patients and schizophrenics are impaired despite normal performance on (pro)saccadic tasks. We used position emission tomography to examine the cerebral blood flow changes associated with the performance of antisaccades in normal individuals. We found that the areas of the brain that were more active during antisaccades than saccades were highly consistent with the oculomotor circuit, including frontal eye fields (FEFs), supplementary motor area, thalamus, and putamen. Superior parietal lobe and primary visual cortex were also significantly more active. In contrast, prefrontal areas 46 and 9 were not more active during antisaccades than during saccades. Performance of some frontal patients on the antisaccade task has been likened to a bradykinesia, or the inability to initiate a willed movement. It is the necessity to will the movement and inhibit competing responses that intuitively linked this task to the dorsolateral prefrontal cortex in frontal patients. Our data suggest that it is the FEFs in prefrontal cortex that differentiate between conditions in which the required oculomotor response changes while the stimulus remains the same, rather than areas 46 and 9, which, in human studies, have been linked to the performance of complex cognitive tasks. Such a conclusion is consistent with single-unit studies of nonhuman primates that have found that the FEFs, the executive portion of the oculomotor circuit, can trigger, inhibit, and set the target of saccades.

The measurement of blood flow in humans using radioactive tracers

Since the first description of the movement of blood around the body by William Harvey, the accurate measurement of blood velocity has provided a major challenge for medical science. This review looks at the contribution made by techniques using radioactive tracers. Initially consideration is given to the fundamental problem of how to measure the amount of radiotracer in an organ with sufficient accuracy, using both single-photon and positron-emitting tracers. The various models used to link tracer behaviour with blood flow are then discussed and the article closes with a detailed review of the clinical applications of blood flow measurements.

Automatic control device for the continuous administration of (15)O labeled gaseous tracers for PET measurements

An automatic control device for the administration of (15)O labelled gaseous PET tracers with constant dose per time (continuous inhalation) is presented. The system controls the flow through the target, the flow to the patient and the dose to the patient simultaneously. The desired values for the dose and the flow to the patient are variable. The dose to the patient is kept constant with +/- 4% even if the beam current raises or if the beam is off for some seconds.

Rapid measurement of regional cerebral blood flow in the baboon using 15O-labelled water and dynamic positron emission tomography

The sensitivity and reproducibility of rapid measurements of regional cerebral flow (rCBF) using a bolus injection of H2 15O and dynamic positron emission tomography (PET) were investigated in anaesthetised baboons. The cerebrovascular reactivity to changes in arterial pCO2 was used as an experimental support. PET data were acquired over 4 min following a single bolus intravenous injection of H2 15O, while arterial blood was withdrawn for continuous activity counting. Images were reconstructed with a dynamic sequence of 45 x 2s + 15 x 10s, including a correction for decay. Regional values of CBF were derived from non-linear least-squares fits of the time activity curves using a four-parameter two-compartment model. The results obtained with a four-parameter fitting method were compared with those obtained with two other rapid estimation methods, first fitting two parameters only, CBF and partition coefficient (p), and secondly autoradiography (with p fixed at 0.95 ml brain ml blood-1). Twelve regions of interest were analysed. The values for the basal CBF obtained from 13 measurements in two baboons were close to published values obtained with other techniques. Reproducibility checks showed a mean variation of 9.7 per cent. The CBF measurements performed in hypercapnic conditions gave results similar to published data in other animal species, showing a 4.5 +/- 0.9 per cent increase in CBF per mm Hg paCO2. The results obtained with the three estimation techniques were closely correlated. The dynamic bolus H2 15O method appeared to be suitable for high blood flow measurements.

Measurement of regional cerebral blood flow with positron emission tomography: a comparison of [15O]water to [11C]butanol with distributed-parameter and compartmental models

To further our understanding of the best way to measure regional CBF with positron emission tomography (PET), we directly compared two candidate tracers ([15O]water and [11C]butanol, administered intravenously) and two popular implementations of the one-compartment (1C) model: the autoradiographic implementation representing a single PET measurement of tissue radioactivity over 1 min and a dynamic implementation representing a sequence of measurements of tissue radioactivity over 200 s. We also examined the feasibility of implementing a more realistic, and thus more complex, distributed-parameter (DP) model by assigning fixed values for all of its parameters other than CBF and tracer volume of distribution (Vd), a requirement imposed by the low temporal resolution and statistical quality of PET data. The studies were performed in three normal adult human subjects during paired rest and visual stimulation. In each subject seven regions of interest (ROIs) were selected, one of which was the primary visual cortex. The corresponding ROI were anatomically equivalent in the three subjects. Regional CBF, Vd, tracer arrival delay, and dispersion were estimated for the dynamic data curves. A total of 252 parameter sets were estimated. With [11C]butanol both implementations of the 1C model provided similar results (r = 0.97). Flows estimated using the 1C models were lower (p < 0.01) with [15O]water than with [11C]butanol. In comparison with the 1C model, the constrained version of the DP used in these studies performed inadequately, overestimating high flow and underestimating low flow with both tracers, possibly as the result of the necessity of assigning fixed values for all of its parameters other than CBF and Vd.

Detection of thirty-second cognitive activations in single subjects with positron emission tomography: a new low-dose H2(15)O regional cerebral blood flow three-dimensional imaging technique

Positron emission tomography regional CBF (rCBF) studies of cognitive processes have traditionally required 30-60 mCi of H2(15)O per scan and intersubject averaging to achieve statistical significance. However, intersubject anatomical, functional, and disease variability can make such an approach problematic. A new method that produces significant results in single subjects is presented. It is based upon high-sensitivity three-dimensional imaging and a "slow" bolus administration of < 15 mCi of H2(15)O per scan. The method is validated in four normal volunteers using control and auditory-language activation tasks with four scans per condition and statistical parametric mapping analysis. It is demonstrated that the rCBF distribution associated with the cognitive state is detected during the arrival of radiotracer in the brain. This occurs over 30 s and constitutes a critical temporal window during which stimulation should be performed. A 90-s acquisition time is found to produce results of greater significance than a 60-s acquisition time. The implications of the results and the functional neuroanatomical findings are discussed. This method is suitable for the study of individual functional neuroanatomy in many neuropsychological, pharmacologic, and symptom states in normal subjects and in patients with psychiatric and neurologic disorders.

Individual patterns of functional reorganization in the human cerebral cortex after capsular infarction

We have previously shown bilateral activation of motor pathways and the recruitment of additional motor areas in studies of groups of patients with recovery from motor stroke. We have now developed a new positron emission tomographic technique to measure the changes in regional cerebral blood flow elicited during a motor task in individual patients, relative to the cerebral activation found in normal subjects. The patterns of cerebral activation in each of 8 individual patients with capsular lesions of the pyramidal tract and complete recovery from hemiplegia are described by comparison with the pattern found in a representative sample of 10 normal subjects. We found a large ventral extension of the hand field of the contralateral (sensori)motor cortex in all patients with lesions of the posterior limb of the internal capsule. Greater activation than in normal subjects was found in variable combinations of the supplementary motor areas, the insula, the frontal operculum, and the parietal cortex. Structures belonging to motor pathways ipsilateral to the recovered limb were also more activated in the patients than in normal subjects. However, additional activation of the ipsilateral (sensori)motor cortex was only found in the 4 patients who exhibited associated movements of the unaffected hand when the recovered hand performed the motor task. We conclude that recovery from motor stroke due to striatocapsular damage is associated with individually different patterns of functional reorganization of the brain. These patterns are dependent on the site of the subcortical lesion and the somatotopic organization of the pyramidal tract, both of which may determine the precise potential for recovery of limb function following this type of brain injury.

A study on statistically reliable and computationally efficient algorithms for generating local cerebral blood flow parametric images with positron emission tomography

With the advent of positron emission tomography (PET), a variety of techniques have been developed to measure local cerebral blood flow (LCBF) noninvasively in humans. A potential class of techniques, which includes linear least squares (LS), linear weighted least squares (WLS), linear generalized least squares (GLS), and linear generalized weighted least squares (GWLS), is proposed. The statistical characteristics of these methods are examined by computer simulation. The authors present a comparison of these four methods with two other rapid estimation techniques developed by Huang et al. (1982) and Alpert (1984), and two classical methods, the unweighted and weighted nonlinear least squares regression. The results show that these methods can take full advantage of the contribution from the fine temporal sampling data of modern tomographs, and thus provide statistically reliable estimates that are comparable to those obtained from nonlinear LS regression. These methods also have high computational efficiency, and the parameters can be estimated directly from operational equations in one single step. Therefore, they can potentially be used in image-wide estimation of local cerebral blood flow and distribution volume with PET.

Regional response differences within the human auditory cortex when listening to words

The relationship between activity within the human auditory cortices and the presentation rate of heard words was investigated by measuring changes in regional cerebral blood flow with positron emission tomography. We demonstrate that in the primary auditory cortices and middle regions of the superior temporal gyri there is a linear relationship between the rate of presentation of heard words and blood flow response. In contrast, the blood flow response in an area of the left posterior superior temporal gyrus (Wernicke's area) is primarily dependent on the occurrence of words irrespective of their rate of presentation. The primary auditory cortices are associated with the early processing of complex acoustic signals whereas Wernicke's area is associated with the comprehension of heard words. This study demonstrates for the first time that time dependent sensory signals (heard words) detected in the primary auditory cortices are transformed into a time invariant output which is channelled to a functionally specialised region--Wernicke's area. Wernicke's area is therefore distinguished from other areas of the auditory cortex by direct observation of signal transformation rather than by association with a specific behavioural task.

Functional reorganization of the brain in recovery from striatocapsular infarction in man

We used positron emission tomography (PET) to study organizational changes in the functional anatomy of the brain in 10 patients following recovery from striatocapsular motor strokes. Comparisons of regional cerebral blood flow maps at rest between the patients and 10 normal subjects revealed significantly lower regional cerebral blood flow in the basal ganglia, thalamus, sensorimotor, insular, and dorsolateral prefrontal cortices, in the brainstem, and in the ipsilateral cerebellum in patients, contralateral to the side of the recovered hand. These deficits reflect the distribution of dysfunction caused by the ischemic lesion. Regional cerebral blood flow was significantly increased in the contralateral posterior cingulate and premotor cortices, and in the caudate nucleus ipsilateral to the recovered hand. During the performance of a motor task by the recovered hand, patients activated the contralateral cortical motor areas and ipsilateral cerebellum to the same extent as did normal subjects. However, activation was greater than in normal subjects in both insulae; in the inferior parietal (area 40), prefrontal and anterior cingulate cortices; in the ipsilateral premotor cortex and basal ganglia; and in the contralateral cerebellum. The pattern of cortical activation was also abnormal when the unaffected hand, contralateral to the hemiplegia, performed the task. We showed that bilateral activation of motor pathways and the recruitment of additional sensorimotor areas and of other specific cortical areas are associated with recovery from motor stroke due to striatocapsular infarction. Activation of anterior and posterior cingulate and prefrontal cortices suggests that selective attentional and intentional mechanisms may be important in the recovery process. Our findings suggest that there is considerable scope for functional plasticity in the adult human cerebral cortex.

In vivo measurement of the volume of distribution of water in cerebral grey matter: effects on the calculation of regional cerebral blood flow

The present study was undertaken to determine the apparent value for the volume of distribution of water to be used in the dynamic/integral technique for generating functional CBF images. A value of 0.86 resulted in only a minor loss of accuracy compared to the more accurate (but time-consuming) dynamic only analysis, which incorporated the regionally fitted estimates of the volume of distribution of water. In contrast to the traditionally used in vitro value of 0.95, the value of 0.86 allows for the inclusion of a significant part of the washout phase in the integral analysis, thereby producing statistically improved CBF images.

A new approach of weighted integration technique based on accumulated images using dynamic PET and H2(15)O

We developed a new technique of weighted integration for the measurement of local cerebral blood flow (LCBF) and the blood-tissue partition coefficient (p) using dynamic positron emission tomography (PET) and H2(15)O. The weighted integration in the new technique is carried out on the equation of the first time integration of the Kety-Schmidt differential equation. Practically, serially accumulated images with sequentially prolonged accumulation times are weighted by two arbitrary functions. The weighting functions do not have to be differentiated because of the exclusion of the differential term in the starting equation. Consequently, the method does not require data at the end of the scan. The technique was applied to H2(15)O dynamic PET performed on four normal subjects, and was verified to provide a better signal-to-noise ratio than the previously developed integrated projection (IP) technique. Computer simulations were carried out to investigate the effects of statistical noise, tissue heterogeneity, and time delay and dispersion in arterial input function. The simulation showed that the new technique provided about a 1.4 times lower statistical error in both LCBF and p at 50 ml 100 g-1 min-1 compared to the IP technique, and it should be noted that the new technique was less sensitive to the shape of the weighting functions. The new technique provides a new strategy with respect to the statistical error for estimation of LCBF and p.

The use of cerebral activation procedures with single photon emission tomography

Single photon emission tomography allows the imaging of dynamic brain functioning. The use of cerebral activating procedures within the scan protocol enables investigation of the mechanisms involved in specific brain functions in health and disease. Activation studies involve the comparison of at least two data sets describing brain activity generated in conditions that differ for the specific function in question. When designing an activation study, decisions regarding methodology include: the nature of the activation regime, the tracer-ligand utilized, the SPET instrument and the manner of subsequent data analysis. These issues are discussed in this review, both theoretically and with reference to published studies. Means of activating particular cerebral structures and functions are reviewed, as are the limitations of the techniques with respect to temporal and spatial resolution and the potentially confounding nature of preconceived ideas regarding the mechanisms of brain function.

Combination of dynamic and integral methods for generating reproducible functional CBF images

A new method to measure regional CBF is presented, applying both dynamic and integral analyses to a dynamic sequence of positron emission tomographic scans collected during and following the administration of H2(15)O (inhalation of C15O2). The dynamic analysis is used to correct continuously monitored arterial whole-blood activity for delay and dispersion relative to tissue scans. An integral analysis including corrections for this delay and dispersion is then used to calculate CBF on a pixel-by-pixel basis. Normal values and reproducibility over a 2-h period are presented, together with the results of validation and simulation studies. The results indicate that the single-tissue compartment model adequately describes the distribution of H2(15)O in the brain, without recourse to postulating a nonexchanging water pool.

The relationship between global and local changes in PET scans

In order to localize cerebral cognitive or sensorimotor function, activation paradigms are being used in conjunction with PET measures of cerebral activity (e.g., rCBF). The changes in local cerebral activity have two components: a global, region independent change and a local or regional change. As the first step in localizing the regional effects of an activation, global variance must be removed by a normalization procedure. A simple normalization procedure is division of regional values by the whole brain mean. This requires the dependence of local activity on global activity to be one of simple proportionality. This is shown not to be the case. Furthermore, a systematic deviation from a proportional relationship across brain regions is demonstrated. Consequently, any normalization must be approached on a pixel-by-pixel basis by measuring the change in local activity and change in global activity. The changes associated with an activation can be partitioned into global and local effects according to two models: one assumes that the increase in local activity depends on global values and the other assumes independence. It is shown that the increase in activity due to a cognitive activation is independent of global activity. This independence of the (activation) condition effect and the confounding linear effect of global activity on observed local activity meet the requirements for an analysis of covariance, with the "nuisance" variable as global activity and the activation condition as the categorical independent variable. These conclusions are based on analysis of data from 24 scans: six conditions over four normal subjects using a verbal fluency paradigm.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of cerebral blood flow with a bolus of oxygen-15-labelled water: comparison of dynamic and integral methods

A method is presented for the measurement of cerebral blood flow (CBF) with a bolus of water labelled with oxygen 15. The method, which has been evaluated in normal volunteers, is based on Kety's model, with two additional parameters to account for the difference in the time of tracer arrival in the radial and carotid arteries ("delay") and for dispersion of the tracer in the body and/or blood counting systems. It combines the advantages of: (i) dynamic data collection for estimation of delay and dispersion; (ii) robustness and linearity of CBF estimates with an integral method; and (iii) simplicity of continuous external monitoring of arterial blood radioactivity, particularly with repeated measurements. An optimized protocol is proposed for routine applications in neurological and neurophysiological studies.