Basilar and middle cerebral artery reserve: a comparative study using transcranial Doppler and breath-holding techniques

BACKGROUND AND PURPOSE

A 1997 report suggests that the posterior circulation of the normal brain has diminished vasoreactivity compared with the anterior circulation. To further study this, we quantified and compared the vasodilatory capacities of the middle cerebral (MCA) and basilar artery (BA) territories in response to changes in PaCO(2), as indices of respective cerebrovascular reserve (CVR). If posterior circulation CVR is indeed physiologically lower than that of the MCA, it might indicate a greater risk of low-flow ischemia distal to basilar obstructive cerebrovascular lesions and provide a rationale for earlier treatment of such lesions with interventional techniques. We also wished to establish normal baseline CVR values for the posterior circulation.

METHODS

Twelve patients with signs and/or symptoms suggestive of posterior circulation disease but without flow-limiting obstructive changes and 11 normal controls were entered into the study. With the use of transcranial Doppler techniques, alterations in blood flow velocity in response to sequential breath-holding trials of varying duration were simultaneously monitored in both MCAs and the BA. CVR was measured as the percent velocity increase (during breath-holding) from resting baseline values.

RESULTS

No significant differences were found in CVR between the MCA and BA territories in or between patients and controls.

CONCLUSIONS

Our study suggests that the anterior and posterior circulations have similar reserve capacities in individuals without flow-limiting cerebrovascular obstructive lesions and that the BA territory, relative to the MCA territory, is not at increased risk for low-flow stroke on the basis of limited reserve potential.

Contrast-enhanced transcranial color-coded duplexsonography in stroke patients with limited bone windows

BACKGROUND AND PURPOSE

Thickening of the temporal bone in stroke-age patients may obviate sonographic evaluation of the circle of Willis in 20% to 30% of patients. We assessed the diagnostic efficacy of contrast-enhanced transcranial color-coded duplexsonography (TCCD) for noninvasive evaluation of the circle of Willis in stroke patients with limited bone windows.

METHODS

Of 171 consecutive patients who presented with ischemic symptoms in the middle cerebral artery (MCA) territory, 49 patients (32 female, 17 male; age range, 70.5+/-10.6 years) had no detectable colorflow signals from the circle of Willis by TCCD because of limited acoustic windows. These 49 patients received an IV injection of a sonographic contrast-enhancing agent, Levovist (Schering; Berlin, Germany), and were re-examined. Correlative imaging studies of the circle of Willis were obtained in 42 of 49 of these patients.

RESULTS

In 38 of 49 patients, contrast-enhanced TCCD enabled full visualization of the circle of Willis bilaterally; in an additional five patients, contrast-enhanced TCCD revealed only the portion of the circle of Willis ipsilateral to the probe through one temporal bone. In six of these 43 patients, contrast-enhanced TCCD showed MCA stenosis and MCA occlusion in three; three of the six cases of MCA stenosis and all three cases of the MCA occlusion were found on the symptomatic side. In six of 49 patients, no colorflow signals were obtained after contrast enhancement. All contrast-enhanced TCCD findings were confirmed by CT angiography, transfemoral digital subtraction angiography, MR angiography, or a combination of all three correlative studies. Levovist produced no serious adverse events.

CONCLUSION

In stroke-age patients with limited acoustic windows, contrast-enhancement with Levovist can markedly increase the sensitivity of TCCD and increase the detection of clinically relevant intracranial arterial disease.

Effects of tissue type plasminogen activator in embolic versus mechanical models of focal cerebral ischemia in rats

Tissue type plasminogen activator (tPA) can be effective therapy for embolic stroke by restoring cerebral perfusion. However, a recent experimental study showed that tPA increased infarct size in a mouse model of transient focal ischemia, suggesting a possible adverse effect of tPA on ischemic tissue per se. In this report, the effects of tPA in two rat models of cerebral ischemia were compared. In experiment 1, rats were subjected to focal ischemia via injection of autologous clots into the middle cerebral artery territory. Two hours after clot injection, rats were treated with 10 mg/kg tPA or normal saline. Perfusion-sensitive computed tomography scanning showed that tPA restored cerebral perfusion in this thromboembolic model. Treatment with tPA significantly reduced ischemic lesion volumes measured at 24 hours by >60%. In experiment 2, three groups of rats were subjected to focal ischemia via a mechanical approach in which a silicon-coated filament was used intraluminally to occlude the origin of the middle cerebral artery. In two groups, the filament was withdrawn after 2 hours to allow for reperfusion, and then rats were randomly treated with 10 mg/kg tPA or normal saline. In the third group, rats were not treated and the filament was not withdrawn so that permanent focal ischemia was present. In this experiment, tPA did not significantly alter lesion volumes after 2 hours of transient focal ischemia. In contrast, permanent ischemia significantly increased lesion volumes by 55% compared with transient ischemia. These results indicate that in these rat models of focal cerebral ischemia, tPA did not have detectable negative effects. Other potentially negative effects of tPA may be dependent on choice of animal species and model systems.

Dodecafluoropentane ultrasonic contrast enhancement in carotid diagnosis: preliminary results

To assess the efficacy in carotid diagnosis of an investigational dodecafluoropentane ultrasonic contrast enhancing agent, we compared B-mode, color flow, and duplex Doppler findings in 16 patients with common carotid artery bifurcation disease after dodecafluoropentane and saline injections. Dodecafluoropentane produced enhanced backscatter in all patients for 4 to 20 min (mean, 8.4+/-4.74 min) after intravenous injection. In six patients this enhancement improved the color flow and pulsed Doppler signal detection in areas of sonographic shadowing. The enhanced color flow information changed the diagnostic impression in one case. Dodecafluoropentane produced enhanced backscatter in the carotid artery in all patients, and for a mean duration longer than that reported for other agents. It has the potential to improve the efficacy of carotid ultrasonic evaluation.

Gadolinium-enhanced MR angiography of the carotid arteries

To assess the efficacy of gadolinium (Gd)-enhanced magnetic resonance angiography (gdMRA) in overcoming signal dropout artifacts on conventional MRA (cMRA), the authors examined 13 patients with suspected neurovascular stenotic/occlusive lesions on MRA pre- and post-gadolinium enhancement. The sample consisted of 18 internal carotid artery (ICA) les ons (16 extracranial, 2 cranial). In 13 of 16 stenotic vessels, gdMRA better characterized the pathoanatomy of moderate to severe stenotic lesions, changing MRA diagnosis in 3 patients. In zero of three vessels with no flow enhancement by cMRA, the lumen remained nonvisualized on gdMRA. For an ICA lesion at the skull base, enhancement of surrounding normal structures limited the usefulness of the technique. The authors conclude that Gd MRA is a fast, accurate, and convenient noninvasive technique for documenting the morphology and severity of carotid stenotic disease, especially extracranially.

[Ultrasound contrast media for neurovascular applications]

Ultrasound is widely used in the assessment of neurovascular diseases. In spite of its effectiveness there are considerable limitations such as low flow detection in carotid disease or limited bony windows in transcranial Doppler. One approach to overcome these limitations is the use of ultrasound contrast enhancing agents. The usefulness of ultrasound contrast enhancing agents Levovist, EchoGen and BY 963 in neurovascular applications has been evaluated. Contrast enhanced colourflow Doppler for the diagnosis of carotid disease has been investigated in three small trials and might be effective for improving the diagnostic yield in severe disease. Contrast enhanced transcranial colourflow Doppler has been relatively more widely explored also with promising results. Based on the combined findings out of these preliminary investigational trials, it appears to be reasonable to undertake larger trials for assessment of usefulness of ultrasound contrast agents for a variety of neurovascular applications.

Neuroimaging looks to the future

Images

Arteriovenous malformation hemodynamics: a transcranial Doppler study

Congenital arteriovenous malformation (AVM) of the brain represents a defect in capillary development resulting in a high flow fistula between arterial and venous systems. In this study, AVM hemodynamics were related with clinical findings. Volume flow was calculated based on transcranial Doppler (TCD) and angiographic data. Forty patients admitted to the Massachusetts General Hospital for proton beam therapy (33 +/- 10 yr old; mean +/- SD) were studied. Four symptoms were considered: intracranial bleeding, progressive neurological deficit, seizures, and headache. Fourteen control subjects aged 30 +/- 7 years (mean +/- SD) were normal volunteers. Angiography with calibrated markers permitting magnification correction was available for all patients. Lateral and medial depth limits of the intracranial basal arteries in relation to the TCD temporal window were determined by TCD and angiogram with excellent correlation. Selected depth for data acquisition was determined independently in the angiogram and by TCD. The difference between the two techniques was less than 4 mm. Mean flow velocity, pulsatility index, and vessel diameter were studied. Flow volume was calculated from these data. Mean flow velocity, pulsatility index, vessel diameter, and flow volume were significantly different among AVM feeders, non-feeders, and control arteries. The non-feeding middle cerebral artery, anterior cerebral artery, and posterior cerebral artery flows were 254 +/- 13, 136 +/- 14, and 79 +/- 8 ml/min, respectively. Accordingly, the estimated cerebral flow volume was 938 ml/min. The feeding middle cerebral artery, anterior cerebral artery, and posterior cerebral artery flows were 552 +/- 47, 369 +/- 70, and 484 +/- 67 ml/min, respectively (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Dipole-tracing of abnormal slow brain potentials after cerebral stroke--EEG, PET, MRI correlations

A patient with major neurological deficits 5 years after a left cerebral infarction underwent correlative EEG, MRI and PET studies of cerebral blood flow and oxygen metabolism. The EEG showed abnormal slow electroencephalographic activity in the frontopolar region. The intracranial location of the slow electrical activity was estimated, as an equivalent current dipole, by using a newly developed dipole tracing (DT) method. The DT analysis showed that the dipole equivalent of the slow wave is approximately located at the frontal part of the left cingulate gyrus, away from the margins of the infarction and enlarged left lateral ventricle demonstrated by MRI, and in a region with intact oxygen consumption rate. The genesis of the slow wave is discussed.

Evaluation of the 11CO2 positron emission tomographic method for measuring brain pH. II. Quantitative pH mapping in patients with ischemic cerebrovascular diseases

A practical method has been developed that, using 11CO2 and positron emission tomography (PET), computes and maps (a) "effective pH" (pHt), a weighted average of intra- and extracellular pH, and (b) "clearance" (K1), product of blood flow and 11CO2 extraction. This method, together with measurements of cerebral blood flow (CBF) and oxygen extraction fraction (OEF), was applied to 12 patients with cerebral ischemia or stroke. The regional K1 was positively correlated with CBF (n = +0.78). The k1/CBF ratio, representing the extraction fraction ratio of 11CO2 to H2 15O, was negatively correlated with CBF (r = -0.54), suggesting that 11CO2 extraction decreases as flow increases. In five acute stroke patients within 2 days of onset, the injured cortex had lower CBF (20.6 ml/min/100 g), higher OEF (78.1%), and lower pHt (6.96) than the contralateral cortex (CBF = 41.4 ml/min/100 g, OEF = 53.3%, pHt = 7.00), suggesting intracellular acidosis with intact cell membranes. In three stroke patients 5-8 days after onset, the injured cortex had higher CBF (60.9 ml/min/100 g), lower OEF (32.0%), and higher pHt (7.12) than the contralateral cortex (CBF = 45.3 ml/min/100 g, OEF = 58.0%, pHt = 7.06), which suggested an increase in extracellular volume compartment reflecting loss of cell membrane integrity. This method provides information on the regional tissue acid-base status and cell membrane integrity, which may be prognostic of tissue viability.

The 15O steady-state method: correction for variation in arterial concentration

One of the factors limiting the accuracy of the 15O steady-state method for the measurement of regional cerebral blood flow and oxygen metabolism is the requirement that a constant arterial blood concentration be maintained over long periods. A new method has been developed to correct for the variation of the arterial concentration in the C15O2 and 15O2 steady-state inhalation technique. The time course of the arterial activity is obtained by multiple sampling over the study period. The same 15O model as is used in the steady-state method is employed but is solved without assuming equilibrium. Look-up tables are generated to relate flow and oxygen extraction fraction to tissue activity, and from them the regional parameters are estimated. Theory and simulation studies suggest that substantial improvement in accuracy can be obtained with no increase in statistical error. The validity of the method was checked experimentally by making repeated measurements in the same subject after perturbing the gas delivery. The conventional steady-state method showed significantly larger deviations in repeat measurement than did the new method. Thus, it is concluded that the proposed method is superior.

Measurement of end-capillary PO2 with positron emission tomography

The analysis of positron emission tomography measurements of oxygen metabolism has been extended to provide a quantitative estimate of end-capillary PO2. The principle of this extension rests on the idea that the oxygen extraction fraction can be used to calculate the end-capillary oxygen saturation of the blood. The relation between oxygen saturation and PO2 is obtained through the oxygen dissociation curve. Our studies show that in addition to the local oxygen extraction fraction, arterial PO2 and pH values are needed in the calculation, whereas fairly large variations in factors such as PCO2, hematocrit, hemoglobin, and plasma protein levels have little or no effect. Rough estimates of end-capillary PO2 can be made using standard O2 dissociation nomograms. Blood gas and acid-base properties of blood have been known for decades, making it possible to account accurately for individual differences that may be encountered when studying patients. Measurements in nine normal subjects yielded a mean end-capillary PO2 value of 31.2 mm Hg. The ability to make a quantitative visualization of altered patterns of end-capillary PO2 provides an additional dimension to the investigation of stroke disease and tumor metabolism.

Evaluation of the 11CO2 positron emission tomographic method for measuring brain pH. I. pH changes measured in states of altered PCO2

The 11CO2 method for measuring local brain pH with positron emission tomography (PET) has been experimentally evaluated, testing the adequacy of the kinetic model and the ability of the method to measure changes in brain pH. Plasma and tissue time/activity curves measured during and following continuous inhalation of 11CO2 were fit with a kinetic model that includes effects of tissue pH, blood flow, and fixation of CO2 into compounds other than dissolved gas and bicarbonate ions. For each of ten dogs, brain pH was measured with PET at two values of PaCO2 (range 21-67 mm Hg). The kinetic model fit the data well during both inhalation and washout of the label, with residual root mean square (RMS) deviations of the model from the measurements consistent with the statistical quality of the PET data. Brain pH calculated from the PET data shows a linear variation with log(PaCO2). These results were in good agreement with previously reported measurements of brain pH, both in absolute value and in variation with PCO2. The interpretation of these pH values in normal and pathological states is discussed.

N-[11C-Methyl]chlorphentermine and N,N-[11C-dimethyl]chlorphentermine as brain blood-flow agents for positron emission tomography

N-[11C-methyl]chlorphentermine ([11C]NMCP) and N,N-[11C-dimethyl]chlorphentermine ([11C]NDMCP) were prepared from chlorphentermine and 11CH3I in DMF and evaluated in rats as brain blood-flow agents for positron emission tomography (PET). Tissue distribution of [11C]NMCP showed that brain uptake was 2.70 +/- 0.40% of injected dose per organ at 5 min with no change in radioactivity concentration up to 30 min after i.v. injection. Approximately 80% of the initial brain uptake remained at 60 min. On the other hand, initial brain uptake of [11C] NDMCP (3.66 +/- 0.31 and 3.63 +/- 0.88% injected dose per organ at 5 and 15 min, respectively) was greater than that of [11C]NMCP. The brain activity however, rapidly decreased to 2.38 +/- 0.17 and 1.82 +/- 0.32% at 30 and 60 min, respectively. Because of its longer retention in the brain compared with [11C]NDMCP, [11C]NMCP would be a potential brain blood-flow agent for quantitative PET studies.

Analysis of some errors in the measurement of oxygen extraction and oxygen consumption by the equilibrium inhalation method

Some sources of error in the equilibrium inhalation method for the measurement of oxygen extraction fraction and CMRO2 by positron emission computed tomography scanning have been evaluated by computer simulation. Emphasis has been placed on errors that have not been thoroughly studied in past work. These include effects of random statistical errors, systematic errors in arterial blood radioactivity concentrations, and errors due to perturbations of the equilibrium state, to tissue inhomogeneity, and to subject motion.

Temporary neurological deterioration after extracranial-intracranial bypass

Five patients who experienced temporary neurological deterioration after extracranial to intracranial bypass procedures are reported in detail. These patients suffered transient ischemic attacks or more prolonged deficits usually of a different nature than the preoperative symptoms. All patients had a good outcome and the spells ceased; the neurological deficits improved within a maximum of 2 weeks. Obvious causes of deterioration such as intra- or extracerebral hematomas, occlusion of a previously stenotic vessel, or graft occlusion were ruled out by computed tomography and angiography in each case. Intraoperative causes of neurological deterioration such as anesthetic effect, hypotension, and temporary occlusion of the cortical vessel or sacrifice of its small branches were not likely to be the cause of the deficits because in each case, the patient awoke satisfactorily and deterioration occurred hours to days later. In each case, postoperative angiography showed good perfusion of at least one major division of the middle cerebral territory. Anticoagulation with heparin in three patients did not change the clinical course. In one patient who was not anticoagulated, embolism could have been responsible for a single prolonged ischemic event, but in the other patients thromboembolism does not seem likely to have been responsible for the deficits. The cause of the deterioration in these patients remains unexplained. We speculate that hyperperfusion of chronically ischemic brain tissue and shifts in the watershed region resulting from the new flow pattern after bypass grafting are two mechanisms that may have been of importance in the etiology of these deficits.

Measurement of brain pH using 11CO2 and positron emission tomography

We have examined the feasibility of measuring local brain pH in vivo with 11CO2 and positron emission tomography. In particular, we have addressed two objections that have been raised against this method: the assumed need to estimate local tissue PCO2 and the rapid fixation of 11C in tissue. From a reexamination of the basic theory, we argue that after administration of 11CO2 the time-dependent distribution of 11C between tissue and blood is independent of the distribution of CO2 already in the body, making it unnecessary to estimate local tissue PCO2. Assuming that the blood--brain barrier is impermeable to bicarbonate ions, there will be equal partial pressures of 11CO2 in blood and tissue at equilibrium. To overcome the problem of fixation in the tissue we have developed a kinetic model of the time-dependent distribution of 11C that accounts for regional variations in blood flow, CO2 extraction, pH, and rate of fixation. The values of the model parameters can be estimated from sequential measurements of tissue activity concentration during administration of 11CO2. Tissue pH can then be calculated from one of the parameter values, a measurement of arterial pH, and known constants. Numerical calculations based on the kinetic model with assumed values of the parameters were used to optimize the experimental design. The calculations show that problems with fixation are much less severe with continuous infusion of activity than with bolus administration. During infusion the tissue curve depends strongly on tissue pH but only weakly on the rate of fixation.(ABSTRACT TRUNCATED AT 250 WORDS)

Strategy for the measurement of regional cerebral blood flow using short-lived tracers and emission tomography

This report describes a strategy for measurement of regional CBF that rigorously accounts for differing tracer partition coefficients and recirculation, and is convenient for use with positron emission tomography. Based on the Kety model, the measured tissue concentration can be expressed in terms of the arterial concentration, the rate constant K, and the blood flow f. The local partition coefficient may be computed as p = f/K. In our approach, maps of K and f are computed from two transverse section reconstructions. The reconstructions are based on weighted sums of projection data measured frequently during the observation period. Theoretical studies of noise propagation in the estimates of K and f were carried out as a function of tomographic count rate, total measurement time, and tracer half-life for varying input functions. These calculations predict that statistical errors in f of between 5 and 10% at a resolution of 1 cm full width at half maximum can be obtained with existing tomographs following i.v. injection. To compare theory and experiment, a series of flow studies were carried out in phantoms using a positron tomograph. These measurements demonstrate close agreement between computed flow and noise estimates and those measured in a controlled situation. This close agreement between theory and experiment as well as the low statistical errors observed suggest that this approach may be a useful tool in clinical investigation.

Comparison of 2- and 3-18F-fluoro-deoxy-D-glucose for studies of tissue metabolism

2- and 3-18F-fluoro-deoxy-D-glucose were proposed as sugar analogs to study glucose metabolism in brain and heart tissues. To evaluate their usefulness, the in vivo behavior of 2- 28FDG and 3-18FDG was investigated in mice and rats and for 3-18FDG in dogs at various times post-injection. Positron emission tomographic (PET) imaging was performed for heart and brain of anesthetized dogs with both radiopharmaceuticals. In all species studied, a higher uptake in brain, heart and kidney was observed for 2-18FDG compared with 3-18FDG. Radioactivity also cleared blood and liver more rapidly with 2-18FDG than with 3-18FDG. Estimates of brain kinetic model parameters revealed the metabolic trapping of 2-18FDG, making this agent favorable for studies of tissue metabolism, and the relative lack of phosphorylation of 3-18FDG, which makes it a potential agent for studies of glucose transport.

Positron imaging in ischemic stroke disease

Positron emission tomography (PET) is well suited to the study of ischemic stroke disease. It has the potential to help elucidate pathophysiological mechanisms, differentiate viable from nonviable tissue, and provide a more rational basis for developing specific therapies for ischemic lesions. The different tracer strategies that may be applied to the study of ischemic disease, however, all have relative limitations, which may be related to the physical or biological determinants of the tracer distributions, to the tracer half-lives, or to the methods required for quantitation of the data. Determination of blood flow and oxygen metabolism are useful for characterizing stroke lesions, but other parameters, such as the oxygen extraction fraction, blood volume, and glucose metabolism, can provide important interpretative information. Correlation of the physiological PET data with the clinical presentation and course is a primary requisite for the development of the full potential of PET.

Quantitation of regional cerebral glucose metabolism

Kinetic analysis of 18F-labeled 2-fluoro-2-deoxy-D-glucose (2FDG) has been carried out in 28 studies on 25 subjects. The object of the analysis was to determine the practical problems of quantitation of glucose metabolic rate (GMR) using the Sokoloff model with 2FDG. We found that arterial and venous plasma concentration of 2FDG yielded equivalent values of the integrated plasma concentration (IPC*) and that one arterial or venous plasma sample at 30 min serves to predict IPC* to within +/- 7%. These observations suggest that quantitation is indeed possible in such subjects without using complex arterial or venous sampling procedures. The average values of K1, K2, and K3 are observed to be 0.14 +/- 0.08, 0.20 +/- 0.10, and 0.030 +/- 0.012 min-1. The data are consistent with a value of lumped constant of 0.4 and a considerable spread in global values of GMR (30%) in an unselected group of subjects.

Cerebral blood flow and cerebrovascular CO2 reactivity in stroke-age normal controls

We used the noninvasive 133-xenon inhalation technique to determine cerebral hemodynamics in 55 normal volunteers aged 18 to 88. Values for cerebral blood flow and cerebrovascular CO2 reactivity in fast-clearing tissue (flow gray) and slow-clearing tissue (flow white) were examined as functions of age and in relation to hematocrit, blood pressure, and evidence of extracranial vascular disease. Flow gray declined linearly with age, but no corresponding change was found in flow white or in CO2 reactivity. The data suggest that the progressive fall in flow gray is due to a physiologic aging process.

Noninvasive diagnosis of carotid disease in the era of digital subtraction angiography

The author discusses the noninvasive tests and their roles relative to intravenous digital subtraction angiography in the assessment of patients with carotid disease.

Complementary roles of a noninvasive test battery and DSA in evaluating carotid artery disease

Initial comparisons of the results of digital subtraction angiography (DSA) and a battery of tests for noninvasive diagnosis of carotid artery disease indicate the two techniques are complementary, rather than competitive. DSA provides important morphologic information at the carotid bifurcation and siphon, but the images are sometimes difficult to interpret with precision. Noninvasive testing gives discrete physiologic information related to hemodynamics, but the findings are less specific for the level of the lesion in the carotid/ophthalmic system. Noninvasive tests may be more useful than DSA for determining if a lesion is hemodynamically significant and whether advanced disease shows evidence of progression on sequential studies. The noninvasive tests are not definitive procedures, but are useful in selecting patients for a contrast study. They are the initial procedure of choice for the patient with asymptomatic bruit. Depending on the clinical situation and quality of the study, DSA can sometimes be a definitive procedure, but in some situations correlative noninvasive test results are necessary to assess whether the patient is a candidate for arteriography and/or surgery.

Brain uptake and organ distribution of 11C from 11C-labeled glucose

The time course of the distribution of carbon-11 (11C, t1/2 = 20.4 min) in brain after the i.v. administration of 11C-labeled glucose [( 11C]glucose) was studied in an effort to understand and explore its behavior in relation to the known factors concerning the catabolic fate of glucose carbon in the brain. The biodistribution of 11C from [11C]glucose was studied in rats using organ dissection. Human radiation doses were estimated from rat biodistribution data. All the rat organs except the brain cleared with a half time of 30-60 min. The brain showed delayed uptake that plateaued from 20 to 60 min. The 11C distribution in normal, non-ischemic, brain 30 min after intravenously administered [11C]glucose is due to labeled carbon incorporation into amino acids associated with tricarboxylic acid cycle intermediates. External imaging with the Massachusetts General Hospital positron camera, PC I, was performed in dogs and humans and the time course of 11C incorporation was similar to the rat brain results. Regional uptake paralleled known metabolic differences between grey and white matter in normal human volunteers. A patient with progressive dementia had less uptake in an area of decreased perfusion as demonstrated angiographically, suggesting that the image obtained 20 min after tracer administration could be used to detect abnormalities in cerebral metabolism due to pathology.

Positron brain imaging--normal patterns and asymmetries

Regional brain physiology was investigated in 11 normal resting right-handed subjects using positron emission tomography. Cerebral blood flow was studied in all subjects. Cerebral oxygen metabolism was studied in six subjects, and cerebral glucose metabolism was also studied in one subject. In five subjects, physiological activity was higher in left frontotemporal regions than right. These findings may be related to structural cerebral asymmetries or to activation of brain language centers.

Estimation of the local statistical noise in emission computed tomography

A simple modification of the filtered backprojection algorithm is presented for the computation of the local statistical noise in emission computed tomography. The technique is general in that any distribution of radioactivity may be accommodated. When applied to positron emission tomography, it is shown that the effects of photon absorption, random coincidences, radioactive decay, and detector nonuniformity may be included. Calculations have shown the effects of resolution, object size, and photon absorption on the statistical noise of disk-shaped emitters. Comparison of calculation and experiment show close agreement both in magnitude and spatial variation. Measurements of the noise level in tomograms of the brain obtained during continuous inhalation of 150-CO2 demonstrate that estimates of radioactivity concentration with a precision of a few percent are readily attainable.

Positron imaging in ischemic stroke disease using compounds labeled with oxygen 15. Initial results of clinicophysiologic correlations

Initial results in over 50 patients with stroke suggest that positron images made during continuous inhalation of carbon dioxide labeled with oxygen 15 and molecular oxygen labeled with oxygen 15 provide data on tissue function that may be relevant to acute stroke management. Five cases illustrate the following findings: 15O-activity patterns observed in areas of ischemic injury or infarction are what one would expect if the 15O distributions represented physiologic functions, such as cerebral blood flow and metabolism. Areas of abnormal 15O activity correlate with the clinical or computed tomographic (CT) localization of the deficit. In studies performed acutely, changes in 15O distributions anticipate alterations in CT scans and may be predictive of outcome. Data related to oxygen metabolism correlate better with tissue viability than do those reflecting cerebral blood flow.

F-18-labeled 3-deoxy-3-fluoro-D-glucose for the study of regional metabolism in the brain and heart

Glucose is the major physiological substrate of the brain and an important physiological substrate for the myocardium. [19F]fluoro-3-deoxy-glucose [3-FDG(F-18)] was studied to determine whether it is a suitable tracer for evaluating the metabolic function of the brain and myocardium. 3-FDG(F-18) was rapidly accumulated in the mouse myocardium (10-12% injected dose/g) and remained constant up to 120 min. Blood, liver, and lung activities exhibited a rapid accumulation of activity (4% injected dose/g) at 1 min, followed by elimination of activity up to 30 min (2% injected dose/g), and then remaining unchanged for a period of 120 min. The arterial blood curve in the dog was fit best by three exponential components (T 1/2 = 0.52 min, 2.75 min, and 142.8 min). Transverse-section images were obtained of the dog's brain and myocardium. From sequential two-dimensional images, a clearance half-time of 26.88 min was determined for the canine brain. Radiation doses for man were calculated from tissue distribution data for mice.

Preliminary imaging results with 18F-2-fluoro-2-deoxy-D-glucose

Transverse section imaging with 18F-2-fluoro-2-deoxy-D-glucose (2FDG) is of considerable interest because quantitative values of regional glucose metabolism may be obtained. We present preliminary results using 2FDG produced by the Brookhaven National Laboratory and imaged with the PCII at the Massachusetts General Hospital. Three studies are reported showing the distribution in dog, monkey, and in brain of an essentially normal patient.

Positron imaging of cerebral blood flow during continuous inhalation of C15O2

This investigation tests the hypothesis that the normal cerebral image obtained non-invasively during continuous inhalation of C15O2 is related to cerebral blood flow. Trace amounts of CO2 labeled with the positron-emitting radionuclide 15O were administered to 4 normal subjects at normo- and hypocapnia and to 2 of these subjects at hypercapnia. Hypocapnia typically caused a marked decrease in cerebral 15O activity, and hypercapnia a small increase in activity. The relative difference in the change in count rate in response to hypo- and hypercapnia is what one would expect if the activity represented bloow flow, according to a mathematical model which assumes the 15O label enters the brain as water of perfusion. The findings in this study suggest that the normal cerebral image obtained during continuous inhalation of C15O2 is related to cerebral blood flow, but in a non-linear fashion, and that the technique would be more sensitive to ischemic events than to hyperemic phenomena.

Mechanisms of acute carotid stroke

In a three-year study, the clinical course and results of intracranial angiography were compared in patients having an acute stroke in the carotid artery territory combined with angiographic abnormalities indicating severe extracranial carotid stenosis or occlusion. Two major mechanisms of stroke were delineated. In one group, the angiographic intracranial abnormalities strongly suggested the presence of embolism in the cerebral vessels supplied by the stenotic or occluded carotid artery; many of these patients had no obvious transient ischemic attacks prior to their stroke and experienced a moderate to severe clinical deficit. In the other group, evidence of embolism was absent; many showed a widespread delay in cerebral arterial perfusion, experienced a greater frequency of transient ischemic attacks before their stroke, and had a milder stroke than did those with embolism.

Symptomatic middle cerebral artery stenosis

The clinical course of 16 consecutive patients with stenosis of the middle cerebral artery angiographically diagnosed between 1970 and 1977 was reviewed. All were managed nonsurgically with medical treatment including anticoagulation. Prior to therapy, transient ischemic attacks had occurred in 15 and cerebral infarction in 11. Initially, none exhibited more than a minor neurological deficit. Follow-up from one month to six years showed a benign course in 14 patients: 13 experienced no subsequent transient attacks or new stroke; 1 had repeated transient attacks for two years but not in the following four years. Two of the 16 developed a severe stroke early in the course, before medical therapy was started. No distinctive clinical or radiographic features were identified that permitted prediction of the outcome. This small series supports the need for a randomized study of bypass efficacy in these patients.

Computed tomography of cerebral infarction: hemorrhagic, contrast enhancement, and time of appearance

Large confluent petechial hemorrhages and/or hemorrhage within infarct may be seen on CT scan. Small petechial hemorrhages are not resolved by current equipment and techniques. Elevation of absorption values of an infarct following contrast media primarily occurs in the first month after onset, and may be occasionally confused with a tumor. Sequential CT changes in infarcts correlate well with established pathologic changes. Cerebral infarction may be seen on CT scan in some cases as early as 24--48 hr after its onset.

CSF enhancement for computerized tomography

Metrizamide cisternography, combined with hypocycloidal tomography, has been performed in 12 patients with possible posterior fossa or parasellar mass lesions with finely detailed images of the basal cisterns resulting. Computed tomography was performed in nine of these patients and produced exceptional images of the cisternal anatomy. The technique of cerebrospinal fluid enhancement may be efficacious in identifying small basal masses not shown with conventional computed tomography, and thus may form an important complement to enhancement by intravenous injection of contrast medium.