Sparse-deconvolution terahertz near-field microprobe tomography enabling non-destructive inspection of small solid dosage forms

Terahertz (THz) pulsed imaging is a powerful tool for investigating solid dosage forms. However, traditional far-field systems struggle with physically small samples and strongly bent surfaces due to inherently limited lateral resolution. The present study introduces a novel approach using photo-conductive near-field microprobes (PC-NFMs) with a THz time-domain spectroscopy module to overcome the limitations of far-field setups concerning their achievable lateral resolution. In addition, a modified sparse deconvolution algorithm for advanced THz signal processing is presented, enabling the reconstruction of fainting interfaces in scattering media. This approach is particularly valuable for specialized dosage forms with extremely thick coatings, such as the investigated controlled-release dosage form, aiding as a worst-case scenario. While most pharmaceutical coatings are <100 µm thick, our method's ability to analyze thicker coatings up to nearly 400 µm at an extraordinarily high spatial resolution of 87 µm in the x-direction and 175 µm in the y-direction sets it apart from approaches presented so far. This versatility makes the approach relevant for standard pharmaceutical products and niche, specialized dosage forms, including mini tablets.