The SOLUS instrument: Optical characterization of the first hand-held probe for multimodal imaging (ultrasound and multi-wavelength time-resolved diffuse optical tomography)

SOLUS is a multimodal imaging system comprising the first miniaturized handheld device to perform time domain Diffuse Optical Tomography at 8 visible and near infrared wavelengths. The hand-held probe also includes B-mode ultrasounds, Shear Wave Elastography and Color Doppler sonography, being its first goal the multiparametric non-invasive diagnosis of breast cancer. This work aims at presenting the system and its main capabilities, focusing on the optical characterization carried out to assess the overall performance of the developed photonics technologies (picosecond pulsed lasers, high-sensitive time-gated sensors and integrated electronics) and of the software for tomographic reconstructions (perturbative model based on Born approximation). Systematic measurements performed on tissue-mimicking phantoms, reproducing a perturbation (e.g., a lesion) in a homogenous background, helped understand the system efficiency range. Variations in absorption are tracked with acceptable quality, which is key to estimate tissue composition, up to 0.25 cm−1 for the bulk (relative error on average of 16 %) and 0.16 cm−1 for sufficiently big perturbations (relative error on average of 26 % for 6 cm3 inhomogeneities). Instead, the system showed low sensitivity to a localized perturbation in scattering and a relative error on average of 17 % for the scattering bulk assessment. An example case of clinical measurement is also discussed.