Tomographic cerebral blood flow measurement using xenon inhalation and serial CT scanning: normal values and its validity

Potential use of quantitative bedside CBF monitoring (Xe-CT) for decision making in neurosurgical intensive care

During a 3-year period, mobile xenon-computerized tomography (Xe-CT) for bedside quantitative assessment of cerebral blood flow was used as an integrated tool for decision making during the care of complicated patients in our neurosurgical intensive care units (NSICU), in an attempt to make a preliminary evaluation regarding the usefulness of this method in routine work in the neurosurgical intensive care. With approximately 200 studies involving 75 patients, we identified six different categories where the use of bedside Xe-CT significantly influenced (or, with more experience, could have influenced) the decision making, or facilitated the handling of patients. These categories included identification of problems not apparent from other types of monitoring, avoidance of adverse effects from treatment, titration of standard treatments, evaluation of the vascular resistance reserve, assessment of adequate perfusion pressure and better utilization of resources from access to the bedside cerebral blood flow (CBF) technology. We conclude that quantitative bedside measurements of CBF could be an important addition to the diagnostic and monitoring arsenal of NSICU-tools.

Bedside monitoring of CBF with xenon-CT and a mobile scanner: a novel method in neurointensive care

Combining previously independently established techniques our objective was to develop and evaluate a method for bedside qualitative assessment of cerebral blood flow in neurointensive care (NICU) patients. The CT-protocol was optimized using phantoms and comparing a mobile CT-scanner (Tomoscan-M, Philips) with two stationary CT scanners. Thirty-two per cent xenon was delivered with standard equipment (Enhancer 3000). Mean cortical flow in volunteers was 48 ml/min/100 g, with the mean vascular territorial flow varying between 45 and 66 ml/min/100 g. The potential clinical usefulness was illustrated in three patients with vasospasm following subarachnoid haemorrhage. Our conclusion is that quantitative bedside measurements of CBF can be repeatedly performed in an easy and safe way in a standard NICU-setting, using xenon-inhalation and a mobile CT-scanner. The method is useful for the decision-making, and is a good example of how the quality of multi-modality monitoring in the NICU can be developed and further diversified.

Brain stem blood flow, pupillary response, and outcome in patients with severe head injuries

OBJECTIVE

Acute pupillary dilation in a head-injured patient is a neurological emergency. Pupil dilation is thought to be the result of uncal herniation causing mechanical compression of the IIIrd cranial nerve and subsequent brain stem compromise. However, not all patients with herniation have fixed and dilated pupils, and not all patients with nonreactive, enlarged pupils have uncal herniation. Therefore, we have tested an alternative hypothesis that a decrease in brain stem blood flow (BBF) is a more frequent cause of mydriasis and brain stem symptomatology after severe head injury. We determined the relation of BBF to outcome and pupillary response in patients with severe head injuries.

METHODS

One hundred sixty-two patients with a Glasgow Coma Scale score of 8 or less underwent stable xenon computed tomographic blood flow determination at the level of the superior colliculus, and this blood flow was correlated with pupillary features, intracranial pressure, computed tomographic scan pathology, and outcome.

RESULTS

A BBF of less than 40 ml/100 g/min was significantly associated with poor outcome (P < 0.009). In patients with bilaterally nonreactive pupils, the BBF was 30.5+/-16.8 ml/100 g/min, and in those with normally reactive pupils, the BBF was 43.8+/-18.7 ml/100 g/min (P < 0.001). Intracranial pressure and the presence of a brain stem lesion observed on the computed tomographic scan did not correlate with BBF, pupillary size, or reactivity. Unfavorable outcome at 12 months was directly related to age (P = 0.062) and inversely related to pupillary responsiveness (P = 0.0006), pupil size (P = 0.005), and BBF of less than 40 ml/100 g/min (P = 0.009).

CONCLUSION

These findings suggest that pupillary dilation is associated with decreased BBF and that ischemia, rather than mechanical compression of the IIIrd cranial nerve, is an important causal factor. More important, pupil dilation may be an indicator of ischemia of the brain stem. If cerebral blood flow and cerebral perfusion pressure can be rapidly restored in the patient with severe head injury who has dilated pupils, the prognosis may be good.

Regional cerebral blood flow and CO2 reactivity in fulminant hepatic failure

Alterations in cerebral hemodynamics are postulated to contribute to brain herniation, a major cause of death in patients with severe hepatic encephalopathy due to fulminant hepatic failure (FHF). In an effort to identify these changes in cerebral hemodynamics, regional and global cerebral blood flow (CBF) and CO2 reactivity were measured using stable xenon-enhanced computed tomography (Xe/CT) in 24 patients within 72 h of onset of severe hepatic encephalopathy. Regional variations in CBF, most notably, a relative decrease in CBF in the anterior circulation and an increase in CBF in the posterior circulation were found. CBF was significantly lower in FHF patients compared with controls, however, these values are well out of the established ischemic range. FHF patients also showed significant impairment in CBF response to hypoventilation, while the CBF response to hyperventilation remained intact. This study suggests that FHF patients demonstrate early changes in both CBF patterns and CO2 reactivity. The relatively "normal" CBF values obtained in FHF patients in severe hepatic encephalopathy coupled with the lack of vasodilatation to hypoventilation suggest a state of uncoupled CBF and metabolism or "luxury perfusion" that could theoretically contribute to vasogenic edema, brain swelling, and cerebral herniation.

Evaluation of regional cerebral blood flow in acute head injury by stable xenon-enhanced computerized tomography

Measurement of regional cerebral blood flow (rCBF) in head-injured patients is considered useful for understanding the cerebral hemodynamics of brain trauma and for determining the optimal therapy. Most data thus far obtained with 133Xe clearance techniques have made only relative contribution, due to limitations of the 133Xe method. More recently, is has become possible to measure rCBF by xenon-enhanced computerized tomography (Xe-CT), which obviates most problems inherent to the 133Xe method. On the other hand, computational errors and concerns regarding the safety of xenon inhalation have thwarted the clinical use of Xe-CT. Recent advances in CT technology, however, have largely eliminated these problems. Xe-CT CBF measurements in severe head injury demonstrate a good correlation between CBF values obtained with 133Xe and Xe-CT. By consistently applying these studies in conjunction with conventional CT, information on very early flow derangements (within 1 to 2 hours after injury) can be obtained, in relation to anatomical lesions. Preliminary data reveal higher incidences of global and focal ischaemia than found previously. Local ischaemia tends to evolve to hyperemia in the ensuing days.

CBF measured by Xe-CT: approach to analysis and normal values

Normal reference values and a practical approach to CBF analysis are needed for routine clinical analysis and interpretation of xenon-enhanced computed tomography (CT) CBF studies. We measured CBF in 67 normal individuals with the GE 9800 CT scanner adapted for CBF imaging with stable Xe. CBF values for vascular territories were systematically analyzed using the clustering of contiguous 2-cm circular regions of interest (ROIs) placed within the cortical mantle and basal ganglia. Mixed cortical flows averaged 51 +/- 10ml.100g-1.min-1. High and low flow compartments, sampled by placing 5-mm circular ROIs in regions containing the highest and lowest flow values in each hemisphere, averaged 84 +/- 14 and 20 +/- 5 ml.100 g-1.min-1, respectively. Mixed cortical flow values as well as values within the high flow compartment demonstrated significant decline with age; however, there were no significant age-related changes in the low flow compartment. The clustering of systematically placed cortical and subcortical ROIs has provided a normative data base for Xe-CT CBF and a flexible and uncomplicated method for the analysis of CBF maps generated by Xe-enhanced CT.

Stable xenon-enhanced CT measurement of cerebral blood flow in reversible focal ischemia in baboons

When the lateral striate arteries of the baboon are temporarily occluded for either 20 or 60 minutes, a near-cessation of blood flow is followed by a dramatic, transient local increase in blood flow values. These findings are evident from serial xenon (Xe)-computerized tomography (CT) measurement of cerebral blood flow (CBF). In this study, 20 minutes of vessel occlusion resulted in brief (less than 1 hour) hyperemia, with no subsequent CT alteration and minimal random neuronal injury. Sixty minutes of occlusion resulted in a more prolonged hyperemia, a low-density area on CT images within 3 hours of reperfusion, and infarction of all cellular elements within the anterior lentiform nucleus. The Xe-CT method provides a sensitive, noninvasive technique for examining sequential alterations of CBF in small regions deep within the brain. This method of recording CBF also permits correlative studies of cerebral infarction, both clinically and experimentally, and allows reasonable inference about the probabilities of neuronal tissue damage with or without reperfusion.

Cerebral blood flow determination within the first 8 hours of cerebral infarction using stable xenon-enhanced computed tomography

Cerebral blood flow mapping with stable xenon-enhanced computed tomography (Xe/CT) was performed in conjunction with conventional computed tomography (CT) within the first 8 hours after the onset of symptoms in seven patients with cerebral infarction. Six patients had hemispheric infarctions, and one had a progressive brainstem infarction. Three patients with very low (less than 10 ml/100 g/min) blood flow in an anatomic area appropriate for the neurologic deficit had no clinical improvement by the time of discharge from the hospital; follow-up CT scans of these three patients confirmed infarction in the area of very low blood flow. Three patients with moderate blood flow reductions (15-45 ml/100 g/min) in the appropriate anatomic area had significant clinical improvement from their initial deficits and had normal follow-up CT scans. One patient studied 8 hours after stroke had increased blood flow (hyperemia) in the appropriate anatomic area and made no clinical recovery.

LCBF values decline while L lambda values increase during normal human aging measured by stable xenon-enhanced computed tomography

Results of measurements of LCBF and L lambda values utilizing optimal CT-CBF methods under resting conditions are reported among thirty-two neurologically normal volunteers aged between 20 and 88 years. Measurements were made during inhalation of 26-30% stable xenon gas for 8 min and serial scanning utilizing a state-of the-art CT scanner with both eyes closed and ears unplugged. LCBF values for cortical gray matter were lowest in occipital cortex and highest in frontal cortex. Gray matter flow values were also high in subcortical structures with highest values measured in the thalamus. For white matter, highest flow values were measured in the internal capsule. Changes in LCBF and L lambda values were analyzed with respect to advancing age. Significant age-related declines in LCBF values were observed in occipital cortex and frontal white matter. Significant age-related increases in L lambda values were measured in frontal and temporal cortex, caudate nucleus and thalamus. Possible explanations are offered for these age-related increases in L lambda values for gray matter, such as accumulation of lipofuscin in neurons and relative compacting of gray matter with advancing age. The latter increases the numbers of nerve cells sampled per volume of gray matter measured.

Stable xenon enhanced computed tomography in the study of clinical and pathologic correlates of focal ischemia in baboons

When the lateral striate arteries of baboons are occluded, an immediate cessation of blood flow followed by a transient, minimal restitution of flow occurs in that vascular distribution. These findings are evident from serial xenon/computed tomography cerebral blood flow imaging. In our study, infarction consistently accompanied arterial occlusion for 6 hours or more. The xenon/computed tomography method provides a sensitive, noninvasive technique for examining sequential alterations of cerebral blood flow in small regions deep within the brain. This methodology for recording cerebral blood flow permits correlative studies of cerebral infarction, clinically and experimentally, and allows reasonable inferences about the probabilities of neural tissue damage.