Frontiers in optical imaging of cerebral blood flow and metabolism

Abstract

In vivo optical imaging of cerebral blood flow (CBF) and metabolism did not exist 50 years ago. While point optical fluorescence and absorption measurements of cellular metabolism and hemoglobin concentrations had already been introduced by then, point blood flow measurements appeared only 40 years ago. The advent of digital cameras has significantly advanced two-dimensional optical imaging of neuronal, metabolic, vascular, and hemodynamic signals. More recently, advanced laser sources have enabled a variety of novel three-dimensional high-spatial-resolution imaging approaches. Combined, as we discuss here, these methods are permitting a multifaceted investigation of the local regulation of CBF and metabolism with unprecedented spatial and temporal resolution. Through multimodal combination of these optical techniques with genetic methods of encoding optical reporter and actuator proteins, the future is bright for solving the mysteries of neurometabolic and neurovascular coupling and translating them to clinical utility.

Figure 1

Comparison of spatial resolution, temporal resolution, and penetration depth of neurovascular and neurometabolic optical imaging techniques. Plot of the spatial and temporal resolutions of different optical techniques, with color-coded penetration depth. These are guidelines, intended to relate the currently reported capabilities of different optical methods. Technological advances continue to improve the resolution and penetration depth of each technique. Consequently, this figure does not constitute a definitive comparison of these techniques.

Figure 2

Measurements of hemoglobin concentration and oxygenation. (A) Optical intrinsic signal imaging (OISI) of 2-second forepaw stimulation in rodent somatosensory cortex results in a transient increase in oxy-hemoglobin (HbO) (red) and total hemoglobin concentration (HbT=HbO+Hb) (green) and a decrease in Hb. (B) Photoacoustic tomography (PAT) can provide high-resolution angiograms of microvessels and estimate the hemoglobin oxygen saturation (SO₂) (Tsytsarev et al, 2011). (C) Near-infrared spectroscopy (NIRS) can be used to image hemodynamic changes from visual stimulation noninvasively in humans (Gregg et al, 2010).

Figure 3

Different measurements of velocity and flow. (A) Laser speckle contrast imaging (LSCI) measures changes in blood flow as exemplified here for a middle cerebral artery occlusion in a rodent (Ayata et al, 2004). (B) Two-photon microscopy (TPM) is used to obtain a depth-resolved angiogram of the microvessels and red blood cell (RBC) velocity within individual vessels (Schaffer et al, 2006). (C) Doppler optical coherence tomography (OCT) can obtain angiograms of the microvessels as well as quantify volumetric flow within individual vessels, as illustrated by flow conservation at branching vessels (Srinivasan et al, 2011). (D) Diffuse correlation spectroscopy (DCS) can obtain noninvasively an index of flow changes in humans that correlates with Xenon computed tomography (Kim et al, 2010).

Figure 4

Optical imaging of oxygen availability and metabolism. (A) Two-photon partial pressure of oxygen (pO₂) imaging in cerebral tissue. Each plot shows baseline pO₂ as a function of the radial distance from the center of the blood vessel—diving arteriole (left) or surfacing venule (right)—for a specific cortical depth range. Data from multiple vessels are overlaid on each plot (Devor et al, 2011). (B) Two-photon measurement of pO₂ in cortical microvasculature. Measured pO₂ values are overlaid on a vasculature graph (grayscale) (Sakadzic et al, 2010). Scale bar: 200 μm. (C) Spatial variation in baseline nicotinamide adenine dinucleotide (NADH) fluorescence, imaged with two-photon microscopy (TPM), can be explained by distance from the vasculature. NADH fluorescence is superimposed with pseudocolored arterioles (red) and venules (blue). The yellow sketches highlight circular borders enclosing perpendicularly oriented vessels; the orange sketches highlight linear borders parallel to horizontally oriented vessels (Kasischke et al, 2010). Scale bar: 200 μm.

Figure 5

Optical manipulation of neuronal activity. (A) Intracellular electrophysiological recordings from an inhibitory cortical neuron that was genetically engineered to express a light-sensitive protein, ChETA (Gunaydin et al, 2010). Bars under the electrophysiological traces indicate light stimulation (472-nm, 2-milliseconds light pulse widths). (B) Photoactivation of a pyramidal cortical neuron by photolysis of a caged-glutamate compound (Fino et al, 2009). The upper panel shows the neuron loaded with a fluorescent dye through the recording pipette. Uncaging laser targets are indicated by dots within the soma. Scale bar: 20 μm. The lower panel shows electrophysiological recordings of neuronal spikes triggered by uncaging. The duration and relative intensity of the uncaging light are indicated by square-pulse traces below the electrophysiological recordings.