Interferometric speckle visibility spectroscopy (ISVS) for human cerebral blood flow monitoring

Abstract

Infrared light scattering methods have been developed and employed to non-invasively monitor human cerebral blood flow (CBF). However, the number of reflected photons that interact with the brain is low when detecting blood flow in deep tissue. To tackle this photon-starved problem, we present and demonstrate the idea of interferometric speckle visibility spectroscopy (ISVS). In ISVS, an interferometric detection scheme is used to boost the weak signal light. The blood flow dynamics are inferred from the speckle statistics of a single frame speckle pattern. We experimentally demonstrated the improvement in the measurement of fidelity by introducing interferometric detection when the signal photon number is low. We apply the ISVS system to monitor the human CBF in situations where the light intensity is ∼100-fold less than that in common diffuse correlation spectroscopy (DCS) implementations. Due to the large number of pixels (∼2 × 105) used to capture light in the ISVS system, we are able to collect a similar number of photons within one exposure time as in normal DCS implementations. Our system operates at a sampling rate of 100 Hz. At the exposure time of 2 ms, the average signal photoelectron number is ∼0.95 count/pixel, yielding a single pixel interferometric measurement signal-to-noise ratio (SNR) of ∼0.97. The total ∼2 × 105 pixels provide an expected overall SNR of 436. We successfully demonstrate that the ISVS system is able to monitor the human brain pulsatile blood flow, as well as the blood flow change when a human subject is doing a breath-holding task.

Optical imaging and spectroscopy for the study of the human brain: status report

“… 76 For instance, pulsatile iDWS BFI measurements at 3.5-4.0 cm source-detector separation were recently demonstrated with a 333 kHz line-scan CMOS camera, where interferometric channels were achieved. 76 Further improvements of the SNR-to-cost ratio will be possible with iDWS methods that utilize low frame rate two-dimensional sensors 84 , 85 with many megapixels. 80 , 85 Keeping in mind that the field autocorrelation decays at half the rate of the intensity autocorrelation, detectors that provide access to time scales are needed to maximize brain specificity.…”

“…SVS can be further improved by adding a reference beam to enable interferometric measurements—a method termed interferometric speckle visibility spectroscopy (iSVS). 84 The iSVS scheme is illustrated in Fig. 1 .…”

Performance optimisation of a holographic Fourier domain diffuse correlation spectroscopy instrument

“…Thus, FD-DCS, which is inherently an interferometric technique, also lends itself well to multispeckle detection. Additionally, the interferometric measurement interrogates the electric field directly, rather than intensity, and therefore the Siegert relation, and the assumptions therein, do not constrain FD-DCS [ 18 ].…”

Interferometric diffuse optics: recent advances and future outlook

“… Timeline of representative advances in NIRS 2 (red, top), DCS/DWS (blue, bottom), and related methods (iNIRS shares characteristics with NIRS and DCS/DWS, appearing in both colors). fNIRS: functional NIRS; SFDI: spatial frequency domain imaging; 7 fDCS: functional DCS; 8 iNIRS: interferometric NIRS; 9 TD-DCS: time domain DCS; 10 SCOS: speckle contrast optical spectroscopy (for human brain 11 ); iDWS: interferometric DWS; 12 FD-DCS: Fourier domain DCS with heterodyne holographic detection; 13 iSVS: interferometric speckle visibility spectroscopy; 14 and MiDWS: multiexposure iDWS. 15 A growing consensus is emerging on the insertion of the letter “i” before a technique to denote “interferometric.” …”

“…Over the past 5 years, motivated in part by the successes of DCS, several groups have independently investigated interferometric detection for diffusely scattered light in living biological tissue. 12 – 15 , 36 , 39 – 43 This area of investigation remains active. Already, several clear and tangible advantages of the interferometric approach have emerged.…”