Human soft tissue phantom for terahertz imaging and spectroscopy

Over the past decades, terahertz radiation has found many biomedical applications, such as marker-free diagnosis of malignant tumors, monitoring wound healing, studying brain pathologies, monitoring graft viability, etc. Most of these applications assume that soft tissues are optically homogeneous in the terahertz wavelength range, and the marker of the pathological process is the differences in the values of the complex dielectric permittivity obtained within the framework of the formalism of the effective medium theory. Meanwhile, recent advances in terahertz imaging with subwavelength spatial resolution have made it possible to detect spatial heterogeneities in the distribution of complex dielectric permittivity with dimensions comparable to the terahertz wavelength in neural, fibrous, muscle and other types of tissue. The presence of such contrasting inclusions can lead to the effects of scattering of terahertz waves at their boundaries. This raises the problem of studying the phenomena of absorption and scattering of terahertz waves in soft tissues. To solve it, it is necessary to use a phantom with previously known parameters. At the moment, there are no phantoms with scattering properties for the terahertz range. In the interests of this task, a tissue-simulating phantom was proposed in this work, which has the shape of a gelatin plate and is a highly absorbent hydration matrix into which silicon dioxide (SiO$_2$) microspheres are embedded with a lower refractive index and absorption coefficient, as well as subwavelength or mesoscale diameters. The terahertz images of this phantom are similar to those of a number of soft tissues, which allows its use in studies of new methods of terahertz imaging and spectroscopy.