OPTOACOUSTICS/OXIMETRY: Real-time photoacoustics beats pulse oximetry by measuring oxygenation in single cells

Red blood cells ferry oxygen to a body's cells and tissues by way of arteries, veins, and capillaries. The tool used today to quantify blood oxygenation, the pulse oximeter, does not provide a full picture of oxygen metabolism because it measures oxygen only in arteries. By contrast, a new photoacoustic approach measures oxygen in individual blood cells in real time.1

Called photoacoustic flowoxigraphy (FOG), the approach allows the viewing of red blood cells as they flow through capillaries. According to scientists at Washington University in St. Louis (WUSTL; St. Louis, MO), the method has applications for further biological research as well as in clinical settings. Lihong Wang, the Gene K. Beare Distinguished Professor of Biomedical Engineering, says the technology could answer a number of biomedical questions—for instance, how cancer or diabetes impacts oxygen metabolism, or how chemotherapy and other cancer treatments affect oxygen level. "We'd like to see if we could use this technique to monitor or predict therapeutic efficacy," he notes.

Because oxy- and deoxy-hemoglobin absorb photons differently (a), oxygen content can be detected through the ultrasound waves each cell emits when exposed to two sequential pulses of different color laser light. False coloring (b) shows the oxygen content of red blood cells: Red represents high and green represents low oxygen levels.

Using the method involves using two different-color laser pulses, offset by just 20 μs, to hit a single red blood cell at nearly the same location. This produces signals at both colors, helping researchers figure out the color of the blood cells at any given moment. By watching the color change, they can determine how much oxygen is delivered from each red blood cell per unit of time or distance. From there, they calculate the average oxygen delivery per unit length of capillary segment.

The research team was able to watch red blood cells choose which direction to travel when they encountered a "fork" in the capillary. The cells travel in bunches to where oxygen is most needed in the body at that time and although they move quickly, the researchers could watch in real time, thanks to the speed of the FOG device: 200 Hz, or 20 3D frames/s.

Wang and colleagues would like to license the technique to a company that would move it forward to make it available to biologists and physicians for applications.

1. L. Wang, K. Maslov, and L. V. Wang, Proc. Nat. Acad. Sci., doi:10.1073/pnas.1215578110 (2013).

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