Noninvasive optical measures of CBV, StO(2), CBF index, and rCMRO(2) in human premature neonates' brains in the first six weeks of life

Effects of acetazolamide on the micro- and macro-vascular cerebral hemodynamics: a diffuse optical and transcranial doppler ultrasound study

Acetazolamide (ACZ) was used to stimulate the cerebral vasculature on ten healthy volunteers to assess the cerebral vasomotor reactivity (CVR). We have combined near infrared spectroscopy (NIRS), diffuse correlation spectroscopy (DCS) and transcranial Doppler (TCD) technologies to non-invasively assess CVR in real-time by measuring oxy- and deoxy-hemoglobin concentrations, using NIRS, local cerebral blood flow (CBF), using DCS, and blood flow velocity (CBFV) in the middle cerebral artery, using TCD. Robust and persistent increases in oxy-hemoglobin concentration, CBF and CBFV were observed. A significant agreement was found between macro-vascular (TCD) and micro-vascular (DCS) hemodynamics, between the NIRS and TCD data, and also within NIRS and DCS results. The relative cerebral metabolic rate of oxygen, rCMRO(2), was also determined, and no significant change was observed. Our results showed that the combined diffuse optics-ultrasound technique is viable to follow (CVR) and rCMRO(2) changes in adults, continuously, at the bed-side and in real time.

Validation of diffuse correlation spectroscopy measurements of rodent cerebral blood flow with simultaneous arterial spin labeling MRI; towards MRI-optical continuous cerebral metabolic monitoring

Cerebral blood flow (CBF) during stepped hypercapnia was measured simultaneously in the rat brain using near-infrared diffuse correlation spectroscopy (DCS) and arterial spin labeling MRI (ASL). DCS and ASL CBF values agree very well, with high correlation (R=0.86, p< 10(-9)), even when physiological instability perturbed the vascular response. A partial volume effect was evident in the smaller magnitude of the optical CBF response compared to the MRI values (averaged over the cortical area), primarily due to the inclusion of white matter in the optically sampled volume. The 8.2 and 11.7 mm mid-separation channels of the multi-distance optical probe had the lowest partial volume impact, reflecting ~75 % of the MR signal change. Using a multiplicative correction factor, the ASL CBF could be predicted with no more than 10% relative error, affording an opportunity for real-time relative cerebral metabolism monitoring in conjunction with MR measurement of cerebral blood volume using super paramagnetic contrast agents.

Effects of muscle fiber motion on diffuse correlation spectroscopy blood flow measurements during exercise

The influence of muscle fiber motion during exercise on diffuse correlation spectroscopy (DCS) measurements of skeletal muscle blood flow is explored. Isotonic (with muscle fiber motion) and isometric (without muscle fiber motion) plantar flexion exercises were performed at 30% of maximal force on a dynamometer, and muscle blood flow was continuously monitored on the medial gastrocnemius (calf) muscle of a healthy volunteer using DCS. During exercise, dynamometer recordings including footplate position, footplate angular velocity, and plantar flexion torque were obtained. Muscle fiber motions introduced artifacts into the DCS signals, causing an overestimation of blood flow changes. We show how proper co-registration of dynamometer recordings and DCS measurements enables separation of the true blood flow responses during exercise from those affected by the motion artifacts.

Effects of congenital heart disease on brain development

Brain and heart development occurs simultaneously in the fetus with congenital heart disease. Early morphogenetic programs in each organ share common genetic pathways. Brain development occurs across a more protracted time-course with striking brain growth and activity-dependent formation and refinement of connections in the third trimester. This development is associated with increased metabolic activity and the brain is dependent upon the heart for oxygen and nutrient delivery. Congenital heart disease leads to derangements of fetal blood flow that result in impaired brain growth and development that can be measured with advanced magnetic resonance imaging. Delayed development results in a unique vulnerability to cerebral white matter injury in newborns with congenital heart disease. Delayed brain development and acquired white matter injury may underlay mild but pervasive neurodevelopmental impairment commonly observed in children following neonatal congenital heart surgery.

The effect of different anesthetics on neurovascular coupling

To date, the majority of neurovascular coupling studies focused on the thalamic afferents' activity in layer IV and the corresponding large spiking activity as responsible for functional hyperemia. This paper highlights the role of the secondary and late cortico-cortical transmission in neurovascular coupling. Simultaneous scalp electroencephalography (EEG) and diffuse optical imaging (DOI) measurements were obtained during multiple conditions of event-related electrical forepaw stimulation in 33 male Sprague-Dawley rats divided into 6 groups depending on the maintaining anesthetic - alpha-chloralose, pentobarbital, ketamine-xylazine, fentanyl-droperidol, isoflurane, or propofol. The somatosensory evoked potentials (SEP) were decomposed into four components and the question of which best predicts the hemodynamic responses was investigated. Results of the linear regression analysis show that the hemodynamic response is best correlated with the secondary and late cortico-cortical transmissions and not with the initial thalamic input activity in layer IV. Baseline cerebral blood flow (CBF) interacts with neural activity and influences the evoked hemodynamic responses. Finally, neurovascular coupling appears to be the same across all anesthetics used.

Diffuse Optics for Tissue Monitoring and Tomography

This review describes the diffusion model for light transport in tissues and the medical applications of diffuse light. Diffuse optics is particularly useful for measurement of tissue hemodynamics, wherein quantitative assessment of oxy- and deoxy-hemoglobin concentrations and blood flow are desired. The theoretical basis for near-infrared or diffuse optical spectroscopy (NIRS or DOS, respectively) is developed, and the basic elements of diffuse optical tomography (DOT) are outlined. We also discuss diffuse correlation spectroscopy (DCS), a technique whereby temporal correlation functions of diffusing light are transported through tissue and are used to measure blood flow. Essential instrumentation is described, and representative brain and breast functional imaging and monitoring results illustrate the workings of these new tissue diagnostics.

Interpreting oxygenation-based neuroimaging signals: the importance and the challenge of understanding brain oxygen metabolism

Functional magnetic resonance imaging is widely used to map patterns of brain activation based on blood oxygenation level dependent (BOLD) signal changes associated with changes in neural activity. However, because oxygenation changes depend on the relative changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO(2)), a quantitative interpretation of BOLD signals, and also other functional neuroimaging signals related to blood or tissue oxygenation, is fundamentally limited until we better understand brain oxygen metabolism and how it is related to blood flow. However, the positive side of the complexity of oxygenation signals is that when combined with dynamic CBF measurements they potentially provide the best tool currently available for investigating the dynamics of CMRO(2). This review focuses on the problem of interpreting oxygenation-based signals, the challenges involved in measuring CMRO(2) in general, and what is needed to put oxygenation-based estimates of CMRO(2) on a firm foundation. The importance of developing a solid theoretical framework is emphasized, both as an essential tool for analyzing oxygenation-based multimodal measurements, and also potentially as a way to better understand the physiological phenomena themselves. The existing data, integrated within a simple theoretical framework of O(2) transport, suggests the hypothesis that an important functional role of the mismatch of CBF and CMRO(2) changes with neural activation is to prevent a fall of tissue pO(2). Future directions for better understanding brain oxygen metabolism are discussed.

Optical measurement of cerebral hemodynamics and oxygen metabolism in neonates with congenital heart defects

We employ a hybrid diffuse correlation spectroscopy (DCS) and near-infrared spectroscopy (NIRS) monitor for neonates with congenital heart disease (n=33). The NIRS-DCS device measured changes during hypercapnia of oxyhemoglobin, deoxyhemoglobin, and total hemoglobin concentrations; cerebral blood flow (rCBF(DCS)); and oxygen metabolism (rCMRO(2)). Concurrent measurements with arterial spin-labeled magnetic resonance imaging (rCBF(ASL-MRI), n=12) cross-validate rCBF(DCS) against rCBF(ASL-MRI), showing good agreement (R=0.7, p=0.01). The study demonstrates use of NIRS-DCS on a critically ill neonatal population, and the results indicate that the optical technology is a promising clinical method for monitoring this population.