2.5 Hz sample rate time-domain near-infrared optical tomography based on SPAD-camera image tissue hemodynamics

Abstract

Time-domain near-infrared optical tomography (TD NIROT) techniques based on diffuse light were gaining performance over the last years. They are capable of imaging tissue at several centimeters depth and reveal clinically relevant information, such as tissue oxygen saturation. In this work, we present the very first in vivo results of our SPAD camera-based TD NIROT reflectance system with a temporal resolution of ∼116 ps. It provides 2800 time of flight source-detector pairs in a compact probe of only 6 cm in diameter. Additionally, we describe a 3-step reconstruction procedure that enables accurate recovery of structural information and of the optical properties. We demonstrate the system's performance firstly in reconstructing the 3D-structure of a heterogeneous tissue phantom with tissue-like scattering and absorption properties within a volume of 9 cm diameter and 5 cm thickness. Furthermore, we performed in vivo tomography of an index finger located within a homogeneous scattering medium. We employed a fast sampling rate of 2.5 Hz to detect changes in tissue oxygenation. Tomographic reconstructions were performed in true 3D, and without prior structural information, demonstrating the powerful capabilities of the system. This shows its potential for clinical applications.

Fig. 1.

Diagram of the Pioneer system (a) and optical probe (b). Photo of the Pioneer system (c).

Fig. 2.

Schematic of the mesh used for modelling. Black contour line represents the geometry and relative position of the actual phantom.

Fig. 3.

Illustration of the finger measurement with Pioneer (a). The Pioneer probe was placed on the top surface of the phantom. The view of the surface shows the locations of sources and the FoV of the detectors. A conventional meter measured the pressure in the cuff on the arm. A similar measurement was performed with the oximeter OxyPrem 1.4 (b). OxyPrem 1.4 features two detectors and two sources with several wavelengths.

Fig. 4.

ToF histogram representing the distribution of photons as function of time at the central pixel of the Piccolo array. It was measured on a homogeneous silicone phantom and is compared to the distribution expected from simulations (main plot a). Instrumental response function (inset b), normalized intensity (c) and phase delay (d) over the field of view, measured on the same phantom. Each pixel corresponds to an area of 1.06 × 1.06 mm² on the surface of the phantom.

Fig. 5.

Distribution of reconstructed ${\mu }_a$ (a), with insets showing the ground truth (blue) and reconstructed (red) values along the dashed black lines drawn through the center of the inclusion. The dotted black frames show projections of the inclusion (a). Results of the region-based reconstruction for both ${{\mu }_s}^{\mathrm{\prime }}$ and ${\mu }_a$ for the two regions (bulk and inclusion) at three different wavelengths (b): comparison of the initial guess, ground truth and reconstructed values.

Fig. 6.

Reconstructed finger region (white cloud) and the ground-truth finger shape (blue surface) (a) with the inset showing the top view (b). Note that the modeled volume is smaller than the real phantom. Hemodynamic responses during the arterial occlusion (c) and venous occlusion tests (d). The values were set to zero at the first data point. Shaded areas indicate an inflated cuff.