Abstract
Terahertz pulsed imaging (TPI) was evaluated for nondestructively characterizing the 3-D internal structures of pharmaceutical tablets. The structural information of a pharmaceutical tablet, such as coating thickness and interface uniformity, was obtained directly from the analysis of the time-domain terahertz waveform. The chemical map of a sample was obtained by using frequency-domain terahertz spectra, together with spectral matching techniques such as cosine correlation mapping. The axial spatial resolution achieved was 30 mum, limited by the confinement of terahertz pulses in time (pulse width); and the lateral spatial resolution was determined to be 150 mum at 90 cm-1 (2.7 THz), limited by the confinement of terahertz pulses in space (focus size, which is diffraction limited and thus frequency dependent). In addition, the buried structure within a tablet was mapped using the TPI, and its chemical composition was successfully identified through spectral-time analysis of the recorded terahertz waveform. We also present a rigorous electromagnetic theory for simulating the terahertz propagation in a multilayered sample, to facilitate terahertz data analysis and interpretation. In conclusion, the TPI is a powerful tool for assaying the tablet coating layer thickness and interface uniformity, and for identifying polymorphs.
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