Terahertz tomography is a class of tomography where sectional imaging is done by terahertz radiation. Terahertz radiation is electromagnetic radiation with a frequency between 0.1 and 10 THz; it falls between radio waves and light waves on the spectrum; it encompasses portions of the millimeter waves and infrared wavelengths. Because of its high frequency and short wavelength, terahertz wave has a high signal-to-noise ratio in the time domain spectrum. Tomography using terahertz radiation can image samples that are opaque in the visible and near-infrared regions of the spectrum. Terahertz wave three-dimensional (3D) imaging technology has developed rapidly since its first successful application in 1997, and a series of new 3D imaging technologies have been proposed successively.
Terahertz imaging Terahertz imaging has advantages over the more expensive and shorter range X-ray scanners. A variety of materials are transparent to terahertz radiation, which allows it to measure the thickness, density, and structural properties of materials that are difficult to detect. Since terahertz is not ionizing radiation, the use of terahertz does not cause damage to living tissue, making terahertz a safe, non-invasive biomedical imaging technique. Moreover, because many materials have a unique spectral signature in the terahertz range, terahertz radiation can be used to identify materials. Terahertz imaging is widely used in the study of semiconductor material properties, biomedical cell imaging, and chemical and biological examination. Terahertz time domain systems (THz-tds) have made significant advances in 2D imaging. THz-tds is able to determine the sample complex dielectric constant, usually 0.1–4 THz, and provides information about the static characteristics of the sample over dozens of frequencies. However, this technology has some limitations. For example, due to the lower power of the beam, the sensor must be more sensitive. Low image acquisition speeds may force a tradeoff between time and resolution.
Applications Terahertz imaging can be useful for luggage and postal mail screening because it can identify substances on the basis of their characteristic spectra in this frequency band, such as explosives and illicit drugs; for example, several liquid explosives can be distinguished by the change in dielectric response in the terahertz range as a function of alcohol percentage. Although dangerous metal objects, such as knives, can be recognized by their shapes through certain pattern recognition algorithms, it is impossible to see through metallic packages with terahertz waves. Thus, terahertz spectrometers cannot replace X-ray scanners, even though they provide more information than X-ray scanners for low-density materials and chemical separation. Terahertz systems are used for production control in the paper and polymer industries. They can detect thickness and moisture content in paper and conductive properties, moisture level, fiber orientation and glass-transition temperature in polymers. Terahertz systems facilitate the detection of metallic and nonmetallic contamination in food. For example, terahertz waves made it possible to detect metallic and nonmetallic foreign matter in chocolate bars, since food with low water contents, such as chocolates, are almost transparent in the terahertz band. Terahertz tomography is also useful in the wine and spirits industries for quantifying moisture and analysing cork non-destructively. Terahertz imaging can detect different isomers have different spectral fingerprints in the terahertz range, which enables terahertz spectroscopy to distinguish between stereoisomers—a crucial distinction in pharmacy, where one isomer may be the active compound and its enantiomer may be inactive or even dangerous. Terahertz systems are also used for gauging tablet coating quality. Terahertz imaging enables non-destructive analysis of valuable artworks and can be conducted onsite. It can reveal hidden layers and via the transmittance of various pigments. It is also being investigated as a tool for 3D visualization.
Skin Cancer Imaging Terahertz tomography typically relies on pulsed THz time-domain, where short bursts of terahertz radiation are emitted and detected to capture both the amplitude and phase of the transmitted or reflected signal. As the sample rotates (in transmission geometry) or the beam is scanned across the surface (in reflection geometry), a sinogram is built, encoding spatial and spectral information essential for tomographic reconstruction. In the case of skin cancer imaging, the use of reflection geometry allows for the encoding of skin cancer lesions over the range of the image obtaining multiple data points. Experimental acquisition of skin cancer data often employs quantum cascade lasers (QCLs) and laser feedback interferometry, where the laser functions as both the source and the detector. This approach offers high sensitivity, coherent detection, self-alignment, and high frame rates, while also mitigating the limitations associated with traditional detectors. Once the sinogram is acquired, the data is reconstructed into volumetric images. For weakly absorbing samples, conventional filtered back-projection (FBP) suffices. However, for strongly scattering tissues or limited-view problems, more advanced methods are employed:
… excerpt ends here. Continue reading the full article.
