Cerebral Blood Flow-Based Resting State Functional Connectivity of the Human Brain using Optical Diffuse Correlation Spectroscopy

This protocol utilizes variable optical probes to evaluate brain blood flow during the resting state. An important advantage of this technique is its portability. Making it well-suited for bedside monitoring.

The wearable probe allows resting state functional connectivity measurements to be obtained in natural environments of the subjects for both diagnostic and therapeutic applications. This protocol utilizes wearable optical probes to evaluate brain blood flow during the resting state functional connectivity measurements. Demonstrating the procedure with Chien Poon will be Ben Rinehart, a grad student from my laboratory.

At least 10 minutes before beginning the analysis, power up the FD-fNIRS and DCS with a light modulation and detector voltage. Use a tape measure to measure the distance between the nasion and the inion on the subject’s head. Using the nasion as the starting point, use an ink marker to designate the location that is 10%of the distance to the inion.

Place an EEG 10/20 cap onto the subject’s head such that the marked point is between Fp1 and Fp2. Mark the point between Fp1 and F7 on the left cortex and the point between Fp2 and F8 on the right cortex to make the boundaries between the superior prefrontal cortex and the dorsolateral prefrontal cortex and between the dorsolateral prefrontal cortex and the inferior prefrontal cortex for the left and right hemispheres, respectively. Using a 3D-printed probe, connect the multi-mode fibers to the 785 nanometer laser light source.

Then place another single-mode fiber one centimeter below the multi-mode fibers at the Ds location on both sides of the cortex and connect each of the single-mode fibers to individual single-photon counting machines. Place the single-mode fibers 2.75 centimeters away from the multi-mode fibers and place one fiber on the left and right dorsal lateral prefrontal cortices and one fiber on the inferior prefrontal cortex. And place the multi-mode fibers on the newly marked points.

To prepare the FD-fNIRS system for calibration, turn off any lights and open the graphic user interface data acquisition software. Click the auto-bias button to adjust the detector gain to achieve an optimal signal with the sensor attached and secured to a calibration phantom. If the overvoltage warning flashes, lower the gain.

After the maximum signal has been obtained, disconnect one of the source fibers so that the background light leakage can be measured by the detector. And verify that the direct current is less than 20 counts per measurement period for the corresponding source fiber. Next, verify the proper signal level readout on every source and detector and click calibrate.

The system will take measurements and apply calibration factors to correctly measure the optical properties of the known phantom. Then log the calibration data, which will provide a record of the system performance on a standard phantom. To set up the DCS, warm up the system laser light sources and the single-photon counting machines for at least 10 minutes.

In the graphic user interface system acquisition software, verify the contact of each fiber by checking the graphical user interface so as to obtain at least 5, 000 counts per second and below 1, 000, 000 counts per second. To verify the sufficient photon count levels are being obtained from each detector, check the photon count level and the near real-time autocorrelation curves. To verify a sufficient fiber contact without any ambient light leakage, check the y-intercept of the autocorrelation curve.

The optimal value is approximately 1.5 without the use of polarizers. To verify that the probe and measurements are not prone to motion artifacts tighten the elastic band so that it is tight enough to resist motion but loose enough to prevent any discomfort to the subject. Then check the autocorrelation curves such that the autocorrelation curve decays to one for longer correlation times.

Then confirm that the subject is seated in a comfortable position with their eyes closed. Place the FD-fNIRS system optical probe on the forehead adjacent to the DCS probe and click Acquire in the FD-fNIRS acquisition software. This data will provide static optical properties, absorption parameters, and scattering parameters that will be used for quantification of the dynamic optical parameter.

When all of the equipment is ready, instruct the subject to minimize any movements during the measurement and turn off the lights. After completion of the FD-fNIRS measurements, click Run in the DCS data acquisition interface and collect data for a total of eight minutes with a maximum two second integration time. In this representative analysis, The resting state functional connectivity in the prefrontal cortices was measured in nine subjects.

a higher correlation was observed in the intra-regional region of the left and right cortices than was measured the inter-regional region of the left and right cortices. Further, T-test analysis comparing the inter and inter-regional resting state functional connectivities of both cortices revealed a significant difference between these values. It is important to verify that the DCS parameters are within the acceptable ranges as failing to adequately perform these steps may result in the acquisition of unusable data.

Diffuse correlation spectroscopy can provide a non-invasive measurement of the blood flow. Making it a useful tool for studying the functional connectivity of the resting or active brain on the various stimulations. Non-invasive measurements of blood flow are useful for the evaluation of treatments and therapies for neurological diseases.

When performing this procedure, be sure to always follow the appropriate laser safety guidelines.