We demonstrated three possible applications of a real-time THz color scanner in the biomedical field.
In the first application, imaging of pharmaceutical tablets, 116 consecutive THz-TDS images of four tablets with an image size of 20 mm in height by 60 mm in width (232 pixels by 600 pixels) were acquired within only 60 s and were used for material characterization of unknown tablets based on their THz spectral fingerprints. Although the sample speed of 1 mm/s in this application is much fast than that available in conventional point-scanning THz-TDS imaging systems, further speed-up will be required to expand the application scope of the THz color scanner more widely to manufacturing processes. For example, the maximum moving speed of a conveyor belt can reach 1,000 mm/s. If the sample speed is increased to 10 mm/s while keeping the same image acquisition rate ( = 10 line/s), the number of horizontal pixels in the spectral image is reduced to 10%, namely 60 pixels, although the DR and SNR are maintained at the same level. When the sample moving at a speed of 1,000 mm/s is measured at the maximum image acquisition rate in the present system ( = 500 line/s), THz-TDS line-images can be captured at intervals of 2 mm. To achieve this application at the maximum image acquisition rate, DR and SNR have to be further enhanced by highly efficient generation of THz pulse, discussed later.
In the second application, imaging of a sliced human tooth, the THz transmittance images significantly changed depending on the sample position and THz frequency. We consider that such local and spectral dependence of the THz transmittance is mainly related to the crystal structure of the dental tissues. Therefore, the THz color scanner provides a potential indicator of the crystallinity of mineralized hard tissues and is expected to become a powerful tool to clarify the growth mechanism of the initial stages of caries or osteoporosis.
In the third application, sensitive measurement of the water content of human hair was performed by exploiting the strong THz absorption caused by liquid water. The drying process of wet hairs was clearly visualized by the temporal evolution of the THz transmittance image. The THz color scanner is also promising in the field of hair cosmetics.
While these demonstrations showed the high potential of the THz color scanner for practical applications in the biomedical field, they revealed technical problems that need to be solved. The THz transmittance images obtained at higher frequencies were less clear than those obtained at lower frequencies. One reason for this is the insufficient DR at higher frequencies, as shown in . Also, although many THz spectral fingerprints are located within the 3 THz band [1
], the presented system cannot fully cover them. Furthermore, when the THz color scanner is applied to faster moving objects or more absorbent materials, higher DR will be required. On the other hand, the SNR should be further improved to make quantitative analysis more precise. Recently, highly efficient generation of THz pulses has been proposed based on Cherenkov radiation with tilted pulse-front excitation [22
] and laser induced plasma in air [23
]. Work is in progress to increase DR, SNR, and/or the spectral bandwidth in a real-time THz color scanner using these methods.