This project investigates how Terahertz frequencies could be used in future medical applications.
Compared to visible light, Terahertz waves are affected by different material properties. This means that something which is difficult to visualize with light could become much clearer in the Terahertz area. This could be used, for instance, for the identification of cancerous cells in a cell test, in so called histopathology. Since THz radiation is non-ionizing, it could also be useful as a safer alternative to X-rays, for instance at dentist examinations. There are also a whole range of non-medical applications that are of interest.
This project benefits from the closely related microwave research, since similar algorithm strategies can be applied to THz. Thanks to the high frequency of THz waves, researchers hope to be able to generate images of even higher resolution compared to the imaging systems based on microwaves.
The system consists of two THz antennas facing each other which can be moved independently of each other. A thin sample is mounted in between the antennas to enable transmission and reflection measurements. Since human tissues absorb a lot of the THz radiation, the applications are limited to thin structures, surfaces or material with a low content of water.
Figure: Images of a leaf. The measured amplitude (unprocessed data) is shown in (a) and the real and imaginary parts of the contrast in relative permittivity are shown in (b) and (c).
Terahertz (THz) radiation refers to electromagnetic waves at frequencies in the terahertz range, which is located in between infrared radiation and microwaves. Terahertz radiation is non-ionizing which means it is safer than radiation from, for instance, X-rays. Like microwaves, THz waves are capable of penetrating a wide variety of non-conducting materials, including clothing, paper, wood, plastic and ceramics. It has however difficulties in penetrating fog and clouds, and cannot penetrate metal or water.
The most common applications for THz used today are related to safety, such as hidden weapon detection at airports. Plans also exist to use THz for various communication applications with high transmission capacity. A lot of research also goes into using THz for identification of explosive materials from a distance.
The simplest method for creating THz images is by using a single transmitter and detector configuration, i.e. line-of-sight detection. The image is obtained on a point-by-point basis by scanning the transmitter/detector pair over the sample under test and recording amplitude and phase at each point. In this way THz images of macroscopic objects can be obtained without the need for any particular image reconstruction algorithm.
In a practical system operating at 1 THz, the spatial resolution could approach about 0.5 mm and is limited by the diffraction of the THz radiation. Thus it is not possible to resolve smaller objects and study them in detail. This works in the same way as for optical frequencies where the smallest resolvable features in an image are not generally smaller than the wavelength.
The aim of the research is to stretch the resolution beyond what can be obtained with a simple point-to-point imaging scheme. The strategy to achieve this for THz imaging is similar to that of microwave imaging. The sample is mounted in the holder and the two THz antennas are used to collect measurement data. The transmitting antenna is kept in a fixed position whereas the receiving antenna is scanned in order to collect the scattering pattern from the sample. The transmitting antenna is then moved to a new position where a new set of measurements is made. The procedure is then repeated until the target sample has been fully covered.
As a next step, the image reconstruction is performed. The basis for this is a numerical computer model of the system which is used and compared to the measurement data in an image reconstruction algorithm to generate an image of the sample target. Chalmers research has shown that resolution can be improved over traditional point to point imaging strategies.
The Chalmers team has now built a working imaging system. The plan for the future is closer investigation of various applications and assessment of their potential.
While it is early days for this technology as a medical imaging modality, it does have the potential for a wide range of clinical applications where it can improve and aid the detection and diagnosis of disease. Some potential applications of interest to Chalmers researchers include histopathology and dental applications. There are also a whole range of non-medical applications of interest.