Diffuse Correlation Spectroscopy to Measure Cerebral Blood Flow in a Human Subject

Begin with a diffuse correlation spectroscopy system prepared for measuring cerebral blood flow.

Switch off the laser and detectors.

The system includes a probe composed of source and detector fibers.

Sanitize the probe. Apply double-sided tape, avoiding the optical fibers.

Sanitize the participant’s forehead and place laser safety goggles on the participant.

Secure the probe over the region-of-interest on the participant’s forehead.

Turn on the laser and detectors, then begin cerebral blood flow measurement.

Long-coherent, near-infrared light from the laser penetrates deep into the brain, illuminating the cerebral tissue via the source fiber.

Scattered light from the tissue and moving blood cells in the cerebral microvasculature interfere constructively and destructively.

This interference pattern is recorded by the detector.

Light scattered by moving blood cells causes fluctuations in the interference pattern, which are analyzed to calculate the cerebral blood flow index—a measure of blood flow in the probed cerebral tissue.

Before moving the system into the ICU, connect all the fibers to the relevant lasers and detectors and make sure they are properly attached to the optical probe. Then obtain consent from either the patient or a legal representative and bring the cart to the patient's room. Upon arrival, turn off the DCS laser and in the File menu, click Load Settings File to load the appropriate probe settings file.

Place the probe on the curved side of the calibration phantom, ensuring good contact with the surface, and click Optimize All Detectors to optimize the PMT bias voltage. In the Calibrate menu, click Calculate Waveform Calibration Values for Optical Props and Multiple Distances to run the calibration for multiple source detector separations. In the Text-Mon menu, open the user-defined option to check that the measured optical properties match the prespecified values and that the fitting R^2 is close to 1.

Then probe the phantom again to measure the optical properties of the phantom marked as check to ensure that the calibration was adequate. The measured optical properties should be within 10% of the values specified in the phantoms. To prepare the patient for the analysis, first use sanitizing wipes to clean both the probe and the patient's forehead. Place double-sided tape on the probe, taking care that the tape is not in direct contact with the optical fiber tips, and place laser safety goggles onto the patient. Place the probes over the region of interest and wrap the elastic straps around the patient's head. Then turn on the DCS laser.

Before beginning the data acquisition, enter the DCS source detector separations in the Settings tab of the graphic user interface. To begin the acquisition, press Start in the system software. Click Optimize All Detectors to optimize the PMT bias voltage for the patient, and check the optical properties in R^2 of the DOS fitting as demonstrated. Turn on the DCS detectors and check that each DCS detector is measuring an adequate light intensity. Typically, more than 10 kilohertz are required.

If the measured intensity exceeds 800 kilohertz, use a neutral density filter to reduce the photon counts to avoid damaging the detectors. Check the autocorrelation curves in the Correlation Curves tab of the graphic user interface to ensure a good skin coupling and reposition the optical probe if necessary. When a good contact between the probe and the skin is achieved, click Stop to stop the data collection and set the experiment and patient identifiers in the folder text box and the region of interest name in the file name text box. Then press Start to start the data acquisition.

After starting the measurement, enter the relevant patient information in the Experiment Info tab and click Mark to ensure that any relevant event that occurs during the monitoring period is marked. After each mark, enter the event description in the Experiment Info tab. At the end of the assessment, press Stop in the graphic user interface to stop the data collection. Turn off the DCS detector switch and turn the DCS laser key to the off position. Click All Detectors Off to turn off the PMTs and remove the probe from the patient's head and the double-sided tape from the probe. Then clean the probe and the system and accessories with sanitizing wipes.