The mechanism of burst suppression, a spontaneous neural activity arising pathologically or from anesthesia, was studied by determining the cerebral rate of metabolic oxygen during EEG bursts with near-infrared and diffuse correlation spectroscopy. OCIS codes: (170.5380) Physiology; (170.5280) Photon migration Background and MotivationBurst suppression is a state operationally defined by a spontaneous, global EEG activity consisting of quasi-periodic intervals of activity and quiescence [1,2]. This state can arise naturally in pathological conditions such as hypothermia [3] and coma [4] or can be induced by deep levels of general anesthesia [5]. The potential harms or benefits from entering or remaining in a state of burst suppression are not understood and the basic mechanisms which initiate and sustain burst suppression have not been established. While EEG patterns of burst suppression are well known, the underlying cerebral metabolism during burst suppression has been less investigated but has been implicated in the mechanism driving neuronal activity [6]. Here, we make simultaneous, real-time measurements of both EEG and cortical hemodynamics in rats to investigate the coupling between cerebral oxygen metabolism and neuronal activity in the burst suppressed state. MethodologyUsing near infrared spectroscopy (NIRS) and optical diffusion correlation spectroscopy (DCS), we are able to measure, in a non-invasive manner suitable for both humans and animals, changes in cortical oxy-and deoxyhemoglobin (HbO and HbR, respectively) and cerebral blood flow (CBF) synchronously with EEG activity as bursting occurs. Burst-related hemodynamic changes were measured in rats as a range of isoflurane concentrations were used to modulate cortical electrical activity from continuous activity under low anesthesia to fully isoelectric EEG under deep anesthesia, with various levels of burst suppression occurring in between. The measured changes in hemoglobin concentrations were used to estimate the oxygen extraction fraction (OEF), which, together with the CBF measurement, was used to calculate cerebral metabolic rate of oxygen (CMRO 2 ).Changes in cortical hemoglobin concentrations were measured with a CW6 system (TechEn Inc., Milford, MA) with laser sources at 690 and 830 nm and calculated by the modified Beer-Lambert law [7]. Cortical blood flow was measured with a home-built diffuse correlation spectroscopy (DCS) system employing a solid-state long coherence length laser at 785 nm for illumination and four photon-counting avalanche photodiodes for detection, as previously described [8]. The intensity auto-correlation function of each channel is computed by a digital correlator and the cerebral blood flow index (CBF i ) is determined by fitting the measured electric field autocorrelation functions to a model of dynamic light scattering in deep tissues.The cerebral metabolic rate of oxygen index (CMRO 2i ) was estimated as previously described [8]. For these experiments, an assumption of baseline total hemoglobin concentration of...

Magnetic resonance-guided radiotherapy (MRgRT) is a new and evolving treatment modality that allows unprecedented visualization of the tumor and surrounding anatomy. MRgRT includes daily 3D magnetic resonance imaging (MRI) for setup and rapidly repeated near real-time MRI scans during treatment for target tracking. One of the more exciting potential benefits of MRgRT is the ability to analyze serial MRIs to monitor treatment response or predict outcomes. A typical radiation treatment (RT) over the span of 10-15 minutes on the MRIdian system (ViewRay, Cleveland, OH) yields thousands of “cine” images, each acquired in 250 ms. This unique data allows for a glimpse in image intensity changes during RT delivery. In this report, we analyze cine images from a single fraction RT of a glioblastoma patient on the ViewRay platform in order to characterize the dynamic signal changes occurring during RT therapy. The individual frames in the cines were saved into DICOM format and read into an MIM image analysis platform (MIM Software, Cleveland, OH) as a time series. The three possible states of the three Cobalt-60 radiation sources—OFF, READY, and ON—were also recorded. An in-house Java plugin for MIM was created in order to perform principal component analysis (PCA) on each of the datasets. The analysis resulted in first PC, related to monotonous signal increase over the course of the treatment fraction. We found several distortion patterns in the data that we postulate result from the perturbation of the magnetic field due to the moving metal parts in the platform while treatment was being administered. The largest variations were detected when all Cobalt-60 sources were OFF. During this phase of the treatment, the gantry and multi-leaf collimators (MLCs) are moving. Conversely, when all Cobalt-60 sources were in the ON position, the image signal fluctuations were minimal, relating to very little mechanical motion. At this phase, the gantry, the MLCs, and sources are fixed in their positions. These findings were confirmed in a study with the daily quality assurance (QA) phantom. While the identified variations were not related to physiological processes, our findings confirm the sensitivity of the developed approach to identify very small fluctuations. Relating these variations to the physical changes that occur during treatment shows the methodical ability of the technique to uncover their underlying sources.