Comparative validation of speckle contrast optical spectroscopy against diffuse correlation spectroscopy for monitoring human cerebral blood flow

Significance: Within diffuse optics, speckle contrast optical spectroscopy (SCOS) has emerged as a promising alternative to the state-of-the-art technique of diffuse correlation spectroscopy (DCS) for continuous, noninvasive bedside monitoring of cerebral blood flow (CBF). Although the two methods theoretically yield equivalent relative indices of CBF (rCBFi), in practice, SCOS-based measurements require experimental calibration to obtain unbiased rCBFi values. To date, there are limited validation studies comparing SCOS and DCS in human subjects, particularly at long source-detector separations (SDS) relevant to adult brain monitoring. Aim: We aim to compare rCBFi from SCOS and DCS during concurrent, colocalized measurements on tissue-mimicking phantoms and humans at a large SDS of 3 cm. Approach: We conducted concurrent SCOS and DCS measurements on temperature-ramped, two-layer, and flow phantoms, respectively. We also conducted concurrent CBF measurements on 10 healthy volunteers undergoing various physiological challenges (breath-holding, hyperventilation, pressure modulation, and squatting) designed to elicit measurable changes in blood flow. SCOS and DCS operated from a shared pulsed laser source, which enabled pulsation-resolved human CBF measurements at 3 cm SDS (DCS necessitated the use of cardiac-gated averaging). Results: measurements. Phantom experiments showed a small depth-sensitivity disadvantage for SCOS at the same SDS; however, this was far outweighed by its superior noise performance, yielding an order-of-magnitude improvement in SCOS contrast-to-noise ratio. Conclusions: The results demonstrate the equivalence of properly calibrated SCOS and DCS rCBFi measurements at long SDS in adults, establishing SCOS as a viable alternative to DCS for monitoring CBF. Further validation across larger cohorts and clinical populations is warranted.