Fast silicon photomultiplier improves signal harvesting and reduces complexity in time-domain diffuse optics
Diffuse optical measurements of tissue inform clinicians of deep tissue hemodynamics continuously, without ionizing radiation or invasive proves. The most ubiquitous of these systems is the pulse-oximeter, which measures arterial oxygen saturation by tracking changes in absorption over the cardiac cycle. Cerebral oximeters instead measure blood oxygenation in the brain, primarily in the capillary bed. However, current commercial devices rely on simple light sources and detectors, temporally modulated at under 10 kHz. These systems do not provide quantitative monitoring tools, as they are unable to measure photon path lengths in tissue. Over the last several decades, a series of quantitative research systems have been developed, utilizing both time- and frequency-modulated light to measure photon path lengths, permitting extraction of both tissue scattering and absorption coefficients and therefore blood hemoglobin oxygenation and concentration. However, these systems tend to be bulky (small refrigerator size), utilize expensive detectors and laser sources (>$100k) mounted in an opto-electronics rack, and are not amenable to large-scale commercialization. A major factor in the difficulty of translating these quantitative devices into commercial and widespread clinical use is the reliance on optical fibers to couple light into and out of the tissue. These fragile connections impose strong limits on light delivery and collection while preventing exploitation of the many developments in micro-electronics over the past few decades to miniaturize these systems. Together, solving these issues will significantly advance clinical adaptation of quantitative diffuse optics.
Mora et al. have recently demonstrated an extremely compact proof-of-concept system which moves the light sources and detectors into direct tissue contact requiring only electrical connections with the instrument. While this advance may seem trivial, the engineering to place picosecond pulsed light sources and detectors with high temporal resolution directly on tissue has eluded researchers up until now and opens up significant clinical possibilities. This system brings together several advances which together permit the device to rival the much more bulky and expensive systems reported in the literature. Specifically, the required source power is reduced by eliminating fiber coupling losses and light collection is increased by utilizing the full numerical aperture of the detector. Additionally, by using silica-based detectors in place of bulky, expensive, and fragile photomultiplier tubes, the detector quantum efficiency is increased and both the size and cost of the detector system are substantially reduced.
By removing the necessity of fiber optics connecting the opto-electronics and the subject, time-domain systems developed from this work can utilize lightweight, flexible, robust wires to connect subjects with the instrument. Further, integration of a wireless data connection and battery would allow quantitative continuous measurement of freely moving subjects, rather than the trend monitoring now available from battery-powered devices. The potential for lightweight wearable modules and robust connections is of especial importance to one of the critical areas of application for diffuse optics – pediatric intensive care, where the additional information provided by these quantitative devices may profoundly impact clinical practice.