Changes In Cerebral Blood Oxygenation Research Articles

Magnetic resonance imaging of regional cerebral blood oxygenation changes under acetazolamide in carotid occlusive disease.

Gradient-echo magnetic resonance imaging can demonstrate changes in cerebral blood oxygenation with high spatiotemporal resolution. We have previously shown that this technique allows monitoring of autoregulatory responses under vasodilatory stress in the healthy human brain. Here the approach has been extended to assess impairment of the autoregulatory reserve capacity in patients with carotid occlusive disease. We studied four patients with unilateral occlusion of the internal carotid artery on a 2.0-T clinical high-field magnetic resonance system. Oxygenation-sensitive imaging was based on long-echo-time, gradient-echo sequences (repetition time, 62.5 milliseconds; echo time, 30 milliseconds) with low flip angles (10 degrees) to emphasize changes in blood oxygenation rather than flow velocity. Dynamic recording monitored signal intensities before and after injecting 1 g of acetazolamide. In sections covering the hand area of the primary sensorimotor cortex, acetazolamide-induced magnetic resonance signal increases were attenuated in the vascular territories of occluded arteries. Lateralization of responses in the left and right hemispheric parts of the section corresponded to decreased hemodynamic reserve capacity as measured globally by transcranial Doppler ultrasonography. The present findings indicate that magnetic resonance imaging can demonstrate exhaustion of the autoregulatory reserve capacity when monitoring cerebral blood oxygenation changes during vasodilatory stress. We suggest that this method can help to evaluate regional cerebral hemodynamics in patients with carotid occlusive disease.

High‐resolution functional magnetic resonance imaging of cortical activation during tactile exploration

AbstractCortical activation during tactile exploration of macrogeometric objects was investigated in six healthy individuals with the use of magnetic resonance imaging (MRI) sensitized to changes in cerebral blood oxygenation. Dynamic measurement of task‐related signal alterations were performed at 2.0 T using a rapid gradient‐echo MRI sequence (TR/TE=63/30 ms, flip angle 10°, measuring time 6 s) at high spatial resolution (0.8 × 1.6 mm2). Four contiguous sections (thickness 4 mm) parallel to the bicommissural plane covered the hand area of the primary sensorimotor cortex (M1, S1), the supplementary motor area (SMA), premotor areas (PMA), and superior parts of the parietal cortex (PC). Task‐related activation was determined by correlating signal intensity time courses with the stimulus protocol on a pixel‐by‐pixel basis. In contrast to predominantly contralateral M1 activation, effects in the hand area of S1 were not restricted to the contralateral side but were equally present in the posterior section of ispilateral S1. Furthermore, bilateral responses were encountered in SMA and PC, while observations within PMA remained inconsistent. These findings in single subjects readily demonstrate a highly resolved and interindividually reproducible pattern of cortical activation in relation to exploratory finger movements and associated intergration of somatosensory information. © 1995 Wiley‐Liss, Inc.

Functional magnetic resonance imaging (FMRI) of the human brain

Functional magnetic resonance imaging (FMRI) can provide detailed images of human brain that reflect localized changes in cerebral blood flow and oxygenation induced by sensory, motor, or cognitive tasks. This review presents methods for gradient-recalled echo-planar functional magnetic resonance imaging (FMRI). Also included is a discussion of the hypothesized basis of FMRI, imaging hardware, a unique visual stimulation apparatus, image post-processing and statistical analysis. Retinotopic mapping of striate and extrastriate visual cortex is discussed as an example application. The described echo-planar technique permitted acquisition of an image in 40 ms with a repetition rate of up to 2 per second. However, FMRI responses are slow compared to changes in neural activity. Onset of a visual checkerboard test pattern evoked a response that was delayed by 1–2 s and reached 90% of peak in 5 s. Return to baseline following stimulation was slightly slower. Alternating control (blank) and test (checkerboard) patterns every 20 s induced a cyclic response that was detected in the presence of noise using a cross-correlation technique that was verified by parametric statistics. FMRI revealed retinotopically organized patterns of visually evoked activity in response to annular stimuli that increased in visual field eccentricity. Retinotopy was also observed with test patterns rotated around the fixation point (center of gaze). Results from repeated tests 1 week apart were highly similar. Compared to passive viewing, an active visual discrimination task enhanced responses from extrastriate association cortex.

The Effect of Acetazolamide on Regional Cerebral Blood Oxygenation at Rest and under Stimulation as Assessed by MRI

The sensitivity of gradient echo magnetic resonance imaging (MRI) to changes in cerebral blood oxygenation (CBO) has been introduced for mapping functional brain activation. Here, we report that this approach allows monitoring autoregulation in the human brain under vasodilatory stress. Following the administration of acetazolamide, signal intensities of deoxyhemoglobin-sensitive images increased in cortical and subcortical gray matter and to a lesser extent in white matter. This result reflects a venous hyperoxygenation stemming from an increase in cerebral perfusion with oxygen consumption remaining constant. In addition, pharmacologic induction of vasodilation attenuated activity-related MRI signal changes in the visual cortex under photic stimulation. Although intersubject variability was high, this finding indicates individually persisting autoregulatory responsiveness to functional challenge despite an "exhausted" reserve capacity. It is suggested that recording CBO by MRI will foster our understanding of modulation of vasomotor tone and cerebral perfusion. Furthermore, this technique may prove valuable for assessing the cerebrovascular reserve capacity in patients with carotid artery occlusive disease.

Effect of behavioral states on changes of cerebral blood volume and oxygenation during endotracheal suctioning: Near infrared spectroscopy study in ventilated preterm neonates

Effect of behavioral states on changes of cerebral blood volume and oxygenation during endotracheal suctioning: Near infrared spectroscopy study in ventilated preterm neonates

Functional cooperativity of human cortical motor areas during self-paced simple finger movements. A high-resolution MRI study.

Magnetic resonance imaging of changes in cerebral blood oxygenation (CBO) delineated areas of neural activation during self-paced unilateral middle finger tapping in five normal volunteers. Four contiguous imaging sections parallel to the bicommissural plane covered the hand area of the primary sensori-motor cortex bilaterally. All measurements were performed at 2.0 T using rapid gradient-echo sequences (TR/TE = 63/30 ms) with high spatial resolution (0.8 x 1.6 x 4 mm) and both strong (40 degrees flip angle) and weak (10 degrees) radiofrequency excitation pulses. This allows differentiation of flow and CBO contributions to the observed signal alterations. Functional cooperativity was analysed by a pixel-by-pixel correlation of signal intensity time courses with the stimulus protocol. Areas of activation included the contralateral primary motor cortex, the homologue part of the primary sensory cortex, the supplementary motor area (SMA) and the lateral premotor areas in all volunteers. Task-related activation of ipsilateral primary motor cortex above a threshold correlation coefficient of 0.5 was seen in two out of five volunteers (at 40 degrees) and one out of five (at 10 degrees) when performing the right-hand task. The present MRI findings readily demonstrate in single subjects that the SMA is involved in self-paced finger tapping. Only sparse activation in the ipsilateral primary motor cortex is consistent with the motor paradigm used.

Advances in clinical neuroimaging: functional MR imaging techniques.

U INTRODUCTION Initial efforts at developing useful magnetic resonance (MR) neuroimaging techniques were hampered by the limitations of conventional methods ofMR imaging. Ti and T2, the image contrast parameters used in conventional imaging, are not always sensitive or specific in characterizing tissue or distinguishing healthy from pathologic states. To address these limitations, developments were made that allow water diffusion (1-5), temperature (6-8), and motion (9) to be measured. These functional imaging techniques have significandy advanced neuroradiology by extending the anatomic capabilities of conventional MR imaging methods to the acquisition of tissue function maps of high spatial and temporal resolution. Hemodynamic imaging is perhaps the most clinically significant of these functional MR imaging techniques. Progress in imaging tissue blood flow, blood volume, and blood oxygenation has been spurred by two parallel developments. First, high-speed gradientecho (10) and echo-planar (1 1) imaging have now been implemented on commercially available equipment, resulting in a greatly increased speed of MR data collection. Second, a greater understanding now exists concerning high-magnetic-susceptibility exogenous and endogenous MR imaging contrast agents. Because of the intimate link between tissue function, metabolism, and blood supply (12), functional MR imaging oftissue hemodynamics can supply specific and sensitive information on healthy and pathologic states. By combining the use of magnetic-susceptibility contrast material and high-speed imaging, we can measure cerebral blood volume (CBV) in tumors and stroke, as well as estimate changes in cerebral blood flow (CBF) and blood oxygenation to visualize human brain activity in real time.

Magnetic resonance imaging mapping of brain function. Human visual cortex.

Magnetic resonance imaging (MRI) studies of human brain activity are described. Task-induced changes in brain cognitive state were measured using high-speed MRI techniques sensitive to changes in cerebral blood volume (CBV), blood flow (CBF), and blood oxygenation. These techniques were used to generate the first functional MRI maps of human task activation, by using a visual stimulus paradigm. The methodology of MRI brain mapping and results from the investigation of the functional organization and frequency response of human primary visual cortex (V1) are presented.

Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation.

Neuronal activity causes local changes in cerebral blood flow, blood volume, and blood oxygenation. Magnetic resonance imaging (MRI) techniques sensitive to changes in cerebral blood flow and blood oxygenation were developed by high-speed echo planar imaging. These techniques were used to obtain completely noninvasive tomographic maps of human brain activity, by using visual and motor stimulus paradigms. Changes in blood oxygenation were detected by using a gradient echo (GE) imaging sequence sensitive to the paramagnetic state of deoxygenated hemoglobin. Blood flow changes were evaluated by a spin-echo inversion recovery (IR), tissue relaxation parameter T1-sensitive pulse sequence. A series of images were acquired continuously with the same imaging pulse sequence (either GE or IR) during task activation. Cine display of subtraction images (activated minus baseline) directly demonstrates activity-induced changes in brain MR signal observed at a temporal resolution of seconds. During 8-Hz patterned-flash photic stimulation, a significant increase in signal intensity (paired t test; P less than 0.001) of 1.8% +/- 0.8% (GE) and 1.8% +/- 0.9% (IR) was observed in the primary visual cortex (V1) of seven normal volunteers. The mean rise-time constant of the signal change was 4.4 +/- 2.2 s for the GE images and 8.9 +/- 2.8 s for the IR images. The stimulation frequency dependence of visual activation agrees with previous positron emission tomography observations, with the largest MR signal response occurring at 8 Hz. Similar signal changes were observed within the human primary motor cortex (M1) during a hand squeezing task and in animal models of increased blood flow by hypercapnia. By using intrinsic blood-tissue contrast, functional MRI opens a spatial-temporal window onto individual brain physiology.

Effects of phenylephrine and dopamine on cerebral blood flow, blood volume, and oxygenation in young rabbits

The effects of phenylephrine and dopamine on cerebral blood flow, blood volume, and oxygenation were studied in young rabbits using a hydrogen gas clearance method and near-infrared spectrometry. Phenylephrine infusion led to an increase in blood pressure and cerebral blood flow without changes in cerebral blood volume and oxygenation. This result suggests that phenylephrine increases arterial blood flow and vascular resistance in vascular beds which may lead to the rupture of weak vessels in premature infants. Cytochrome a, a3, however, was not affected by the drug. Conversely, a low-dose dopamine infusion had no effect on cerebral hemodynamics, but high-dose decreased blood pressure and cerebral oxyhemoglobin, and increased deoxyhemoglobin without appreciable changes in cerebral blood flow, which suggests venous congestion and arterial blood reduction in the brain.

The EEG in Hypoxic-Ischemic Encephalopathy

ABSTRACT.The EEG is sensitive to changes in cerebral blood flow and oxygenation and a useful test in the neurological evaluation of patients after hypoxic-ischemic insults. This paper describes the spectrum of EEG changes that typically occur in patients following cardiac arrest.

Computed point spread functions for light in tissue using a measured volume scattering function.

Optical techniques are increasingly being used in the field of medicine in areas as diverse as surgery (for cutting and coagulation), cancer treatment (through photoradiation therapy) and blood flow monitoring (by laser doppler measurements). At University College, we are using the technique of near infrared spectroscopy (nirs) to monitor changes in cerebral blood and tissue oxygenation in newborn infants (1), and are investigating methods of optical imaging across the head (2). In all these applications, a detailed knowledge of light transport in tissue is required. For spectroscopy studies, in order to quantitate data, one needs to know the effective photon pathlength through the tissue, and a knowledge of the path also allows one to calculate the volume of tissue from which results are being obtained. In the case of imaging through tissue, data is required on the point spread function (PSF) for the tissue, both for the prediction of the image quality that could be obtained using various imaging schemes, and for use in image enhancement and reconstruction computations.

Methods of quantitating cerebral near infrared spectroscopy data.

Non invasive infrared spectroscopy is a well established technique for monitoring changes in the oxygenation status of tissues (1). The technique has in particular been successfully employed to monitor changes in cerebral blood and tissue oxygenation by observing the absorption of haemoglobin and cytochrome aa3 respectively. Because of the highly light scattering nature of the tissues studied, it has normally not been possible to quantitate the observed changes.