Toward noninvasive assessment of flap viability with time-resolved diffuse optical tomography: a preclinical test on rats.

The noninvasive assessment of flap viability in autologous reconstruction surgery is still an unmet clinical need. To cope with this problem, we developed a proof-of-principle fully automatized setup for fast time-gated diffuse optical tomography exploiting Mellin-Laplace transform to obtain three-dimensional tomographic reconstructions of oxy- and deoxy-hemoglobin concentrations. We applied this method to perform preclinical tests on rats inducing total venous occlusion in the cutaneous abdominal flaps. Notwithstanding the use of just four source-detector couples, we could detect a spatially localized increase of deoxyhemoglobin following the occlusion (up to 550 µM in 54 min). Such capability to image spatio-temporal evolution of blood perfusion is a key issue for the noninvasive monitoring of flap viability.

Spatial resolution in depth for time-resolved diffuse optical tomography using short source-detector separations.

Diffuse optical tomography for medical applications can require probes with small dimensions involving short source-detector separations. Even though this configuration is seen at first as a constraint due to the challenge of depth sensitivity, we show here that it can potentially be an asset for spatial resolution in depth. By comparing two fiber optic probes on a test object, we first show with simulations that short source-detector separations improve the spatial resolution down to a limit depth. We then confirm these results in an experimental study with a state-of-the-art setup involving a fast-gated single-photon avalanche diode allowing maximum depth sensitivity. We conclude that short source-detector separations are an option to consider for the design of probes so as to improve image quality for diffuse optical tomography in reflectance.

Time-resolved diffuse optical tomography using fast-gated single-photon avalanche diodes.

We present the first experimental results of reflectance Diffuse Optical Tomography (DOT) performed with a fast-gated single-photon avalanche diode (SPAD) coupled to a time-correlated single-photon counting system. The Mellin-Laplace transform was employed to process time-resolved data. We compare the performances of the SPAD operated in the gated mode vs. the non-gated mode for the detection and localization of an absorbing inclusion deeply embedded in a turbid medium for 5 and 15 mm interfiber distances. We demonstrate that, for a given acquisition time, the gated mode enables the detection and better localization of deeper absorbing inclusions than the non-gated mode. These results obtained on phantoms demonstrate the efficacy of time-resolved DOT at small interfiber distances. By achieving depth sensitivity with limited acquisition times, the gated mode increases the relevance of reflectance DOT at small interfiber distance for clinical applications.

Time-domain reflectance diffuse optical tomography with Mellin-Laplace transform for experimental detection and depth localization of a single absorbing inclusion.

We show how to apply the Mellin-Laplace transform to process time-resolved reflectance measurements for diffuse optical tomography. We illustrate this method on simulated signals incorporating the main sources of experimental noise and suggest how to fine-tune the method in order to detect the deepest absorbing inclusions and optimize their localization in depth, depending on the dynamic range of the measurement. To finish, we apply this method to measurements acquired with a setup including a femtosecond laser, photomultipliers and a time-correlated single photon counting board. Simulations and experiments are illustrated for a probe featuring the interfiber distance of 1.5 cm and show the potential of time-resolved techniques for imaging absorption contrast in depth with this geometry.

Time-domain diffuse optical tomography processing by using the Mellin-Laplace transform.

We investigate the use of the Mellin-Laplace transform for reconstructing optical parameters from time-resolved optical tomography in diffusive media. We present here its definition, its mathematical properties, and its sensitivity to variations of optical properties. The method was validated on two-dimensional reconstructions from simulation in the reflection geometry. We conclude that reconstructions based on the Mellin-Laplace transform are more robust to noise than the methods using first moments.