Department of Biomedical Engineering, Stony Brook University, Stony Brook, New York, United States of America.
PLoS One. 2019 May 16;14(5):e0216952. doi: 10.1371/journal.pone.0216952. eCollection 2019.
Terahertz (THz) imaging is a widely used technique in the study and detection of many chemicals and biomolecules in polycrystalline form because the spectral absorption signatures of these target materials often lie in the THz frequencies. When the size of dielectric grain boundaries are comparable to the THz wavelengths, spectral features can be obscured due to electromagnetic scattering. In this study, we first investigate this granular scattering effect in identification of chemicals with THz spectral absorption features. We then will propose a signal processing technique in the so-called "quefrency" domain to improve the ability to resolve these spectral features in the diffuse scattered THz images. We created a pellet with α-lactose monohydrate and riboflavin, two biologically significant materials with well-known vibrational spectral resonances, and buried the pellet in a highly scattering medium. THz transmission measurements were taken at all angles covering the half focal plane. We show that, while spectral features of lactose and riboflavin cannot be distinguished in the scattered image, application of cepstrum filtering can mitigate these scattering effects. By employing our quefrency-domain signal processing technique, we were able to unambiguously detect the dielectric resonance of lactose in the diffused scattering geometries. Finally we will discuss the limitation of the new proposed technique in spectral identification of chemicals.
太赫兹（THz）成像是一种广泛应用于研究和检测多晶态许多化学物质和生物分子的技术，因为这些目标材料的光谱吸收特征通常位于太赫兹频率范围内。当介电晶粒尺寸与太赫兹波长相当时，由于电磁散射，光谱特征可能会被掩盖。在这项研究中，我们首先研究了在具有太赫兹光谱吸收特征的化学物质识别中这种颗粒散射效应。然后，我们将在所谓的“准频率”域中提出一种信号处理技术，以提高在漫散射太赫兹图像中解析这些光谱特征的能力。我们创建了一个包含α-乳糖一水合物和核黄素的颗粒，这两种都是具有已知振动光谱共振的生物上重要的物质，并将颗粒埋在高散射介质中。在覆盖半焦平面的所有角度进行了太赫兹透射测量。我们表明，虽然在散射图像中无法区分乳糖和核黄素的光谱特征，但倒谱滤波的应用可以减轻这些散射效应。通过应用我们的准频率域信号处理技术，我们能够在漫散射几何中明确地检测到乳糖的介电共振。最后，我们将讨论新提出的技术在化学物质光谱识别中的局限性。
