Blood flow to white matter during maturation of the brain

Cerebral Blood Flow Monitoring in High-Risk Fetal and Neonatal Populations

Cerebrovascular pressure autoregulation promotes stable cerebral blood flow (CBF) across a range of arterial blood pressures. Cerebral autoregulation (CA) is a developmental process that reaches maturity around term gestation and can be monitored prenatally with both Doppler ultrasound and magnetic resonance imaging (MRI) techniques. Postnatally, there are key advantages and limitations to assessing CA with Doppler ultrasound, MRI, and near-infrared spectroscopy. Here we review these CBF monitoring techniques as well as their application to both fetal and neonatal populations at risk of perturbations in CBF. Specifically, we discuss CBF monitoring in fetuses with intrauterine growth restriction, anemia, congenital heart disease, neonates born preterm and those with hypoxic-ischemic encephalopathy. We conclude the review with insights into the future directions in this field with an emphasis on collaborative science and precision medicine approaches.

Is There a "Right" Amount of Oxygen for Preterm Infant Stabilization at Birth?

The amount of oxygen given to preterm infants within the first few minutes of birth is one of the most contentious issues in modern neonatology. Just two decades ago, pure oxygen (FiO2 1.0) was standard of care and oximetry monitoring was not routine. Due to concerns about oxidative stress and injury, clinicians rapidly adopted the practice of using less oxygen for the respiratory support of all infants, regardless of gestational maturity and pulmonary function. There is now evidence that initial starting fractional inspired oxygen may not be the only factor involved in providing optimum oxygenation and that the amount of oxygen given to babies within the first 10 min of life is a crucial factor in determining outcomes, including death and neurodevelopmental injury. In addition, evolving practice, such as non-invasive respiratory support and delayed cord clamping, need to be taken into consideration when considering oxygen delivery to preterm infants. This review will discuss evidence to date and address the major knowledge gaps that need to be answered in this pivotal aspect of neonatal practice.

Fetal Cerebrovascular Maturation: Effects of Hypoxia

The human cerebral vasculature originates in the fourth week of gestation and continues to expand and diversify well into the first few years of postnatal life. A key feature of this growth is smooth muscle differentiation, whereby smooth muscle cells within cerebral arteries transform from migratory to proliferative to synthetic and finally to contractile phenotypes. These phenotypic transformations can be reversed by pathophysiological perturbations such as hypoxia, which causes loss of contractile capacity in immature cerebral arteries. In turn, loss of contractility affects all whole-brain cerebrovascular responses, including those involved in flow-metabolism coupling, vasodilatory responses to acute hypoxia and hypercapnia, cerebral autoregulation, and reactivity to activation of perivascular nerves. Future strategies to minimize cerebral injury following hypoxia-ischemic insults in the immature brain might benefit by targeting treatments to preserve and promote contractile differentiation in the fetal cerebrovasculature. This could potentially be achieved through inhibition of receptor tyrosine kinase-mediated growth factors, such as vascular endothelial growth factor and platelet-derived growth factor, which are mobilized by hypoxic and ischemic injury and which facilitate contractile dedifferentiation. Interruption of the effects of other vascular mitogens, such as endothelin and angiotensin-II, and even some miRNA species, also could be beneficial. Future experimental work that addresses these possibilities offers promise to improve current clinical management of neonates who have suffered and survived hypoxic, ischemic, asphyxic, or inflammatory cerebrovascular insults.

Combined diffusion tensor imaging and transverse relaxometry in early-onset bipolar disorder

OBJECTIVE

Transverse relaxation time (T2) imaging provides the opportunity to examine membrane fluidity, which can affect a number of cellular functions. The objective of the present work was to examine T2 abnormalities in children with unmodified DSM-IV-TR bipolar disorder (BD) in bilateral cingulate-paracingulate (CPC) white matter.

METHOD

A total of 21 children and adolescents with BD and 16 healthy control subjects underwent magnetic resonance imaging at 1.5 Tesla and were compared using a region-of-interest analysis. A post hoc diffusion tensor imaging (DTI) analysis was also performed on selected subjects.

RESULTS

The T2 values were significantly decreased on the right-side of the subjects with BD compared with that of the control subjects. Hemispheric difference was also observed in the BD group, with decreased T2 on the right side compared with the left side. No significant difference was observed between left and right CPC T2 in control subjects. For participants who had both T2 and DTI measurements, significant DTI differences were observed: On the left side, fractional anisotropy was reduced and trace and radial diffusivity were increased, whereas on the right side, trace was increased and T2 was decreased in subjects with BD compared with control subjects.

CONCLUSIONS

Our findings suggest that the observed T2 difference is a reflection of cerebral blood flow rather than an alteration of the fluidity of cell membranes. It is possible that myelin damage occurs on the left side in early-onset BD, in addition to changes in the blood flow. Prospective studies with larger numbers of subjects are warranted to further explore the relevance of the presented results.

Magnetic resonance imaging of the neonatal brain

Neonatal magnetic resonance (MR) imaging is rapidly becoming the preferred modality for the evaluation of central nervous system disorders in the newborn. Recent literature supports the value of this imaging technique in diagnosing ischemic, hemorrhagic and infectious disease processes in the premature and full-term neonatal brain. Recent data in premature newborns with neurological injury also suggest a role for MR imaging in determining long-term neurodevelopmental outcomes. This review article provides a framework and overview on neonatal MR imaging techniques and examines the literature or radiological disease patterns and prognostic implications in common neurological disorders.

A model of the development of frontal lobe functioning: findings from a meta-analysis

Although past research has provided an initial examination of maturational trends of frontal lobe functioning, it has not yielded a unifying developmental model. The purpose of this study was to generate a model representing the maturation of frontal lobe function as determined principally through neuropsychological tests. A meta-analytic review of the literature on the development of frontal lobe functioning was conducted. Journal articles were identified through an initial search of PsycInfo, Medline, and ERIC for the years 1984-2004 using key words executive function*, frontal function*, development*, and age. Analyses of effect size differences across age groups assisted in determining the developmental patterns for commonly used measures of frontal functioning by providing a common metric of growth. Age-related increases across the different frontal functions were averaged providing overall age-related increases in performance. A plot was made of the development of frontal lobe functioning using the mean effect size of change in performance across age groups. The model of the development of frontal lobe functioning suggests a staging of development that begins in early childhood with the maturation of frontal functioning and continues, although at a decreased rate, into adolescence and early adulthood.

Accelerated myelination associated with venous congestion

Magnetic resonance imaging is currently the gold standard in the assessment of brain myelination. The normal pattern of brain myelination conforms to a fixed chronological sequence. Focal accelerated myelination is a usual pathological state and previously has only been associated with Sturge-Weber syndrome. The purpose of our study is to describe alternate causes for accelerated myelination. We retrospectively reviewed serial MR scans, MR angiography, conventional angiography and the clinical progress of three children with accelerated myelination. Two patients with accelerated myelination had an underlying cerebral sinovenous thrombosis. The third patient had Sturge-Weber syndrome. Our study strongly suggests that cerebral venous thrombosis with the consequent restriction of venous outflow could be a key factor in the induction of accelerated myelination. We recommend that in patients with accelerated myelination, the search for an underlying etiology should include careful evaluation of the intracranial vascular pathology, especially cerebral venous thrombosis.

Brain perfusion in children: evolution with age assessed by quantitative perfusion computed tomography

OBJECTIVE

The objective of this study was to assess the age-related variations of brain perfusion through quantitative cerebral perfusion computed tomography (CT) results in children without brain abnormality.

METHODS

Brain perfusion CT examinations were performed in 77 children, aged 7 days to 18 years. These patients were admitted at our institution for both noncontrast and contrast-enhanced cerebral CT. Only children whose conventional cerebral CT and clinical/radiologic follow-up, including additional investigations, were normal were taken into account for this study (53 of 77).

RESULTS

The average regional rCBF amounts to 40 (mL/100 g per minute) for the first 6 months of life, peaks at approximately 130 (mL/100 g per minute) at approximately 2 to 4 years of age, and finally stabilizes at approximately 50 (mL/100 g per minute) at approximately 7 to 8 years of age, with a small increase of rCBF values at approximately 12 years of age. The rCBF in the gray matter averages 3 times that in the white matter, except for the first 6 months of life. The global CBF represents 10% to 20% of the global cardiac output for the first 6 months of life, peaks at approximately 55% by 2 to 4 years of age, and finally stabilizes at approximately 15% by 7 to 8 years of age. Specific age-related evolution patterns were identified in the different anatomic areas of the cerebral parenchyma, which could be related to the development of neuroanatomic structures and to the emergence of corresponding cognitive functions.

CONCLUSIONS

Quantitative perfusion CT characterization of brain perfusion shows specific age variations. Brain perfusion of each cortical area evolves according to a specific time course, in close correlation with the psychomotor development.

Perfusion-weighted magnetic resonance imaging of the brain: techniques and application in children

Perfusion-weighted magnetic resonance imaging (PWI) has been proposed as an attractive non-invasive tool for evaluating cerebral haemodynamics. Quantitative maps of cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), time to peak (TTP) and various other haemodynamic parameters can be obtained. Recent advances in hard- and software made PWI available for clinical routine. Although PWI became common in adult neuroradiology, it remains challenging in pediatric neuroradiology. In this article, the different PWI techniques that render haemodynamic maps of the brain are presented and discussed. The normal developmental changes of the cerebral haemodynamics in children as measured by PWI are presented as well as the application of PWI in cerebral ischaemia, primary and secondary cerebral vasculopathies and in cerebral tumours.

Magnetic resonance imaging in pediatric stroke

Pediatric stroke is a term that can be used to encompass everything from hypoxic-ischemic injury to the fetal central nervous system, and especially the premature neonate, to bland versus hemorrhagic infarction from arterial or venous causes in the infant and older child. Pediatric stroke is a chronically underrecognized and therefore underdiagnosed problem that may have significant economic implications. The risk factors for stroke in children are numerous and differ from those in adults. However, with adequate workup, the etiology can be identified in about 75% of cases. Cardiac disorders and hemoglobinopathy are the most common causes of ischemic infarction in children, whereas various congenital anomalies of the blood vessels or defects in coagulation or platelet function often are found in children with parenchymal hemorrhage. Magnetic resonance imaging provides a noninvasive method of investigating childhood stroke, aiding in both better diagnosis and management of this problem.

Anaerobic glycolysis preceding white-matter destruction in experimental neonatal hydrocephalus

The metabolic changes in neonatal hydrocephalus that lead to permanent brain injury are not clearly defined, nor is the extent to which these changes can be prevented by a cerebrospinal fluid shunt. To clarify these processes, cerebral glucose utilization was examined using [14C]2-deoxyglucose autoradiography in 1-month-old kittens, kaolin-induced hydrocephalic littermates, and hydrocephalic kittens in which a ventriculoperitoneal shunt had been inserted 10 days after kaolin injection. The hydrocephalic kittens showed thinning of the cerebral mantle and an anterior-to-posterior gradient of enlargement of the ventricular system, with a ventricle:brain ratio of 24% for the frontal and 35% for the occipital horns compared with control (< 0.5%) and shunted (< 5%) animals. White matter in hydrocephalic animals was edematous. Myelination was delayed in the periventricular region and in the cores of the cerebral gyri. Glucose utilization in hydrocephalic and shunted animals was unchanged from control animals in all gray-matter regions examined. However, in hydrocephalic animals, the frontal white matter exhibited a significant increase in glucose utilization (25 mumol.100 gm-1.min-1) in the cores of gyri compared with normal surrounding white-matter values (14.8 mumol.100 gm-1.min-1). Very low values (mean 4 mumol.100 gm-1.min-1) were found in areas corresponding to severe white-matter edema, and these areas were surrounded by a halo of increased activity (24 mumol.100 gm-1.min-1). In contrast, cytochrome oxidase activity in white matter was homogeneous. Shunting resulted in restoration of the cerebral mantle thickness, a return to normal levels of glucose utilization in the white matter, and an improvement in myelination. It is suggested that the areas of increased glucose utilization seen in the white matter represent anaerobic glycolysis which, if untreated, progresses to infarction. The pattern of this increased glucose utilization matches that of expected myelination and, during this period of high energy demand, white matter may be susceptible to the hypoperfusion associated with hydrocephalus.

Anatomical Features of the Developing Brain Implicated in Pathogenesis of Hypoxic-Ischemic Injury

The developing nervous systems is subject to damage from lack of vital substances necessary for normal maturation and function as well as from trauma or a variety of toxins and infectious agents. By far, the most important of these is inadequate oxygen delivery to the fetus in utero, and/or during the intrapartum and/or early neonatal period. Many types of lesions have been described under the rubric of hypoxic-ischemic encephalopathy, a major proportion of which are found only in the immature nervous system and essentially are never seen later in life. Moreover, a large number are primarily hemorrhagic rather than ischemic in character. The unique character and distribution of these lesions results from a collision of the changing anatomy of the developing nervous system and pathophysiological factors afflicting the immature organism. Whereas the majority of hypoxic-ischemic lesions in the fetus/neonate fall into this group, abnormalities characteristically found in the mature nervous system are also seen. Recognition of the anatomic and physiological features peculiar to the developing nervous system will assist in diagnosis of hypoxic-ischemic damage peculiar to the fetus and neonate.

Brain oxidative metabolism of the newborn dog: correlation between 31P NMR spectroscopy and pyridine nucleotide redox state

The effects of both anoxia and short- and long-term hypoxia on brain oxidative metabolism were studied in newborn dogs. Oxidative metabolism was evaluated by two independent measures: in vivo continuous monitoring of mitochondrial NADH redox state and energy stores as calculated from the phosphocreatine (PCr)/Pi levels measured by 31P nuclear magnetic resonance (NMR) spectroscopy. The hemodynamic response to low oxygen supply was further evaluated by measuring the changes in the reflected light intensity at 366 nm (the excitation wavelength for NADH). The animal underwent surgery and was prepared for monitoring of the two signals (NADH and PCr/Pi). It was then placed inside a Phosphoenergetics 260-80 NMR spectrometer magnet with a 31-cm bore. Each animal (1-21 days old) was exposed to short-term anoxia or hypoxia as well as to long-term hypoxia (1-2 h). The results can be summarized as follow: (a) In the normoxic brain, the ratio between PCr and Pi was greater than 1 (1.2-1.4), while under hypoxia or asphyxia a significant decrease that was correlated to the FiO2 levels was recorded. (b) A clear correlation was found between the decrease in PCr/Pi values and the increased NADH redox state developed under decreased O2 supply to the brain. (c) Exposing the animal to moderately long-term hypoxia led to a stabilized low-energy state of the brain with a good recovery after rebreathing normal air. (d) Under long-term and severe hypoxia, the microcirculatory autoregulatory mechanism was damaged and massive vasoconstriction was optically recorded simultaneously with a significant decrease in PCr/Pi values.(ABSTRACT TRUNCATED AT 250 WORDS)

Positron emission tomography study of human brain functional development

From over 100 children studied with 2-deoxy-2[18F]fluoro-D-glucose and positron emission tomography we selected 29 children (aged 5 days to 15.1 years) who had suffered transient neurological events not significantly affecting normal neurodevelopment. These 29 children were reasonably representative of normal children and provided an otherwise unobtainable population in which to study developmental changes in local cerebral metabolic rates for glucose (lCMRGlc). In infants less than 5 weeks old lCMRGlc was highest in sensorimotor cortex, thalamus, brainstem, and cerebellar vermis. By 3 months, lCMRGlc had increased in parietal, temporal, and occipital cortices; basal ganglia; and cerebellar cortex. Frontal and dorsolateral occipital cortical regions displayed a maturational rise in lCMRGlc by approximately 6 to 8 months. Absolute values of lCMRGlc for various grey matter regions were low at birth (13 to 25 mumol/min/100 gm), and rapidly rose to reach adult values (19 to 33 mumol/min/100 gm) by 2 years. lCMRGlc continued to rise until, by 3 to 4 years, it reached values of 49 to 65 mumol/min/100 gm in most regions. These high rates were maintained until approximately 9 years, when they began to decline, and reached adult rates again by the latter part of the second decade. The highest increases of lCMRGlc over adult values occurred in cerebral cortical structures; lesser increases were seen in subcortical structures and in the cerebellum. This time course of lCMRGlc changes matches that describing the process of initial overproduction and subsequent elimination of excessive neurons, synapses, and dendritic spines known to occur in the developing brain. The determination of changing metabolic patterns accompanying normal brain development is a necessary prelude to the study of abnormal brain development with positron emission tomography.

The ultrastructure of the nervous tissue in a benign teratoma

The ultrastructure of the nervous tissue in a benign ovarian teratoma is described. This tissue was organized into areas having both "meningeal" and "ependymal" surfaces, between which were found astrocytes, ependymal cells, neurones with synapses and microglia. These cells all had ultrastructural similarities to their normal counterparts in the nervous system. In addition, some signs of degenerative change--due possibly to the abnormal location of the nervous tissue--were observed. Oligodendrocytes and myelin were absent, possibly because of vascular insufficiency.

Maturational changes in cerebral function in infants determined by 18FDG positron emission tomography

2-Deoxy-2[18F]fluoro-D-glucose positron emission tomography performed in human infants during development revealed progressive changes in local cerebral glucose utilization. In infants 5 weeks of age and younger, glucose utilization was highest in the sensorimotor cortex, thalamus, midbrain-brainstem, and cerebellar vermis. By 3 months, glucose metabolic activity had increased in the parietal, temporal, and occipital cortices and the basal ganglia, with subsequent increases in frontal and various association regions occurring by 8 months. These functional changes measured with positron emission tomography are in agreement with behavioral, neurophysiological, and anatomical alterations known to occur during infant development.

The effect of stimulation of the reticulo-hypothalamic-hippocampal systems on the cerebral blood flow and neocortical and hippocampal electrical activity in cats

The effect of stimulation of the medial and lateral reticulo-hypothalamic-hippocampal (RHH) systems on cerebral blood flow (CBF) and electrical activity of the hippocampus and neocortex was examined in 19 encéphale isolé cats. ECoG was recorded from posterior sigmoid gyri and marginal gyri and hippocampal activity from dorsal hippocampus. Changes in hippocampal activity were evoked by electrical stimulation of RHH systems. CBF was measured by external monitoring of the clearance of 133Xe given as a single bolus in the carotid artery. Stimulation of the lateral system resulted in desynchronisation of ECoG and hippocampal activity without changes in CBF. Stimulation of the medial system elicited desynchronisation in ECoG modulated by theta-like synchrony, theta activity in the hippocampus and a 45% CBF increase. After atropine administration, low frequency, high voltage waves appeared in both ECoG and hippocampal activity, but no change in CBF was observed. During stimulation of the medial system there were no changes in the type of electrical activity but the CBF response was still preserved (increase by 50%). Stimulation of the lateral system did not change either the type of electrical activity or the CBF. The results indicate that the two systems of neuronal pathways, which mediate two different patterns of electrical response in the dorsal hippocampus but similar ECoG activity in the neocortex, elicit different CBF responses. It is argued that the alterations of electrical activity of the neocortex and hippocampus mediated by these two pathways depend on the cholinergic system, whereas the CBF changes depend on a different mechanism.

Cerebral blood flow and blood-brain influx of some neutral amino acids in control and hypothyroid 16-day-old rats

Rats were made hypothyroid by a daily subcutaneous injection of propylthiouracil beginning the first day after birth. CBF, brain plasma volume, blood-brain extraction, and influx of some neutral amino acids were studied in 16-day-old animals. In hypothyroid rats, the brain plasma volume was decreased by approximately 30%. CBF was decreased by greater than 50%. This decrease was the highest in cerebellum. Blood-brain extraction of small neutral amino acids (alanine, serine, cysteine) was greatly enhanced. This greater extraction compensated for the decreased supply of alanine brought about by its decreased plasma concentration and the lower CBF. In contrast, the extraction of the large amino acids tested (leucine, phenylalanine) was hardly increased, and the influx of phenylalanine was slightly decreased. These results suggest an alteration in the maturation of the brain capillary bed and capillary transport for neutral amino acids in hypothyroidism. The differential effect of hypothyroidism on some small and large amino acids is an additional argument for the existence of two systems of transport for neutral amino acids at the luminal membrane of brain capillary endothelial cells of immature rats.

Beagle puppy model of intraventricular hemorrhage. Effect of superoxide dismutase on cerebral blood flow and prostaglandins

Intraventricular hemorrhage (IVH) represents a major problem for preterm neonates and is thought to occur secondary to alterations in cerebral blood flow (CBF) to damaged germinal matrix tissues. Many investigators believe that both local CBF and changes in capillary morphology and permeability may be partially controlled by prostaglandins. To evaluate this hypothesis, the authors have studied the effects of superoxide dismutase (SOD), a known free-radical scavenger, on newborn beagle pups that were randomly assigned by computer to four groups consisting of either SOD- or saline-pretreated animals that underwent either insult by hemorrhagic hypotension/volume reexpansion or no insult. Prostaglandin levels were determined prior to and 60 minutes following the administration of the solutions, and carbon-14 iodoantipyrine autoradiography was performed for determination of CBF. It was demonstrated that, although SOD significantly decreased the incidence of IVH in this model (p less than 0.05), it caused no alterations in baseline CBF or prostaglandin levels. In addition, SOD did not prevent either the systemic blood pressure changes or the alterations in CBF found in response to a hemorrhagic hypotensive insult. The authors propose that neonatal IVH results from a combination of factors, one of which is prostaglandin-mediated alterations in CBF to a damaged capillary matrix.

Beagle puppy model of intraventricular hemorrhage: ethamsylate studies

Intraventricular hemorrhage (IVH) remains a major problem of preterm neonates, and ethamsylate, an inhibitor of specific prostaglandin-synthetic enzymes has been demonstrated to prevent IVH in these patients. We have examined the effects of ethamsylate on newborn beagle pups who were, by randomized computerized design, assigned to four cells consisting of (a) either ethamsylate or saline pretreatment and (b) either insulted or not insulted with hemorrhagic hypovolemia/volume re-expansion. Prostaglandin levels were obtained prior to and thirty minutes following administration of the solutions and 14C iodoantipyrine autoradiography was performed for cerebral blood flow (CBF) determinations. Ethamsylate produced a significant decrease in the incidence of IVH in this model (p less than 0.05). Following drug administration, ethamsylate-pretreated pups had significant declines in thromboxane B2 and 6-keto PGF1 alpha levels, the major breakdown products of thromboxane A2 and prostacyclin. Although ethamsylate significantly lowered baseline CBF in all brain regions examined in insulted and non-insulted pups (p less than 0.05), in the drug-treated group it did not prevent the changes seen in CBF to the germinal matrix region which were detected in the saline-pretreated pups. Nor did it significantly blunt the blood pressure changes in response to the hemorrhagic hypovolemia/volume re-expansion insult found in the latter group of animals. In addition, only ethamsylate pretreated pups had marked hypotensive responses to the reperfusion phase of the insult. Although the diminution of baseline CBF may contribute to the prevention of neonatal IVH which this drug has been demonstrated to exhibit, ethamsylate may also act as a capillary stabilizing agent.

Noninvasive method of estimating human newborn regional cerebral blood flow

A noninvasive method of estimating regional cerebral blood flow (rCBF) in premature and full-term babies has been developed. Based on a modification of the xenon-133 inhalation rCBF technique, this method uses eight extracranial NaI scintillation detectors and an i.v. bolus injection of xenon-133 (approximately 0.5 mCi/kg). Arterial xenon concentration was estimated with an external chest detector. Cerebral blood flow was measured in 15 healthy, neurologically normal premature infants. Using Obrist's method of two-compartment analysis, normal values were calculated for flow in both compartments, relative weight and fractional flow in the first compartment (gray matter), initial slope of gray matter blood flow, mean cerebral blood flow, and initial slope index of mean cerebral blood flow. The application of this technique to newborns, its relative advantages, and its potential uses are discussed.

Local cerebral glucose utilization in the beagle puppy model of intraventricular hemorrhage

Local cerebral glucose utilization has been measured by means of carbon-14(14C)-autoradiography with 2-deoxyglucose in the newborn beagle puppy model of intraventricular hemorrhage. Our studies demonstrate gray matter/white matter differentiation of uptake of 14C-2-deoxyglucose in the control pups, as would be expected from adult animal studies. However, there is a marked homogeneity of 14C-2-deoxyglucose uptake in all brain regions in the puppies with intraventricular hemorrhage, possibly indicating a loss of the known coupling between cerebral blood flow and metabolism in this neuropathological condition.

Beagle puppy model of intraventricular hemorrhage

Intraventricular hemorrhage (IVH) is a major neurological problem of the preterm infant and originates in the germinal matrix tissues of the developing brain. The newborn beagle pup has been demonstrated to provide an excellent model for this neonatal neuropathological condition. By the production of hemorrhagic hypotension followed by relative volume reexpansion, such hemorrhages can be produced in this animal. Carbon-14 (14C)-iodoantipyrine was used for measuring the cerebral blood flow in this experimental model and demonstrated that, although the germinal matrix is a relatively low-flow area in the newborn beagle pup, the selective flow to this region increases significantly in animals with IVH.

Validity of cerebral blood flow measurements obtained with quantitative tracer techniques

A great number of results for the cerebral blood flow obtained in the animal with quantitative tracer techniques have been collected from the literature. They are exposed in order to compare both normal flow values in different laboratory species, and the characteristics, accuracy and sensitivity of each technique. A dramatic overall dispersion of flow values is observed, allowing neither the flow level particular to each species to be estimated, nor the average value provided by a given technique to be found. The physiological and technological causes of such a dispersion are discussed. Several techniques seem to have limitations which even alter the interpretation of their results, and especially the origin of the local or regional blood flow results. Other techniques may be criticized from the quantitative standpoint, but give more reliable results.