Exploring the Capabilities of Terahertz Imaging

Originally Published MDDI February 2002

R & D DIGEST

Although terahertz (THz or T-ray) wave radiation occupies a large part of the electromagnetic spectrum between infrared and microwave bands, it has been the focus of limited research to date. It has continued, however, to be generally viewed as offering great potential for biomedical applications.

According to Xi-Cheng Zhang, PhD, J. Erik Jonsson '22 Distinguished Professor of Science at Rensselaer Polytechnic Institute (Troy, NY), the T-ray portion of the spectrum "has not been particularly useful because there were neither suitable emitters available to send out controlled T-ray signals, nor were there efficient sensors that could collect these signals and record the information."

Recently, however, researchers at Rensselaer became the first group to image biological tissue using single pulses of terahertz radiation. The group believes that, in medical applications, this single-pulse approach could reduce diagnostic time from hours or days to minutes or even seconds. The technique could eventually provide the basis for computerized medical diagnoses performed in the physician's office, they add.

Says Zhang, "Three key properties of T-ray radiation triggered research in developing this frequency band for medical applications. T-rays have low photon energies (4 meV at 1 THz) and will not cause harmful photoionization in biological tissues. At terahertz frequencies, numerous organic molecules exhibit strong absorption and dispersion due to dipole-allowed rotational and vibrational transitions; these transitions are specific to the molecule and enable T-ray fingerprinting. [Finally,] coherent T-ray signals can be detected in the time domain by mapping the transient of the electric field in amplitude and phase. This gives access to absorption and dispersion spectroscopy."

The researchers combined T-rays with a technique capable of delivering single picosecond-long blasts or "chirped pulses" of light. Using a brief single pulse of radiation provides significant benefits when imaging biological tissue, according to the researchers. The unique properties of terahertz radiation allow it to see farther, and in more detail, than imaging methods such as x-rays, ultrasound, and radar. For example, T-rays have been demonstrated to effectively image skin burn severity, tooth cavities, and skin and breast cancer.

"Basically, T-ray imaging potentially provides spectroscopic information in the terahertz range, while x-ray and ultrasound imaging do not," says Zhang. "Several groups are working in this field to get the fingerprint or signature of cancer tissues, for comparison with those of normal tissues."

The researchers suggest that as an alternative method of mammography, for example, T-rays can detect breast cancer and see underground toxins better than other technologies, such as conventional x-rays. Mapping of DNA and RNA could also be enhanced with the technology, they add. "Our idea is to fully automate analysis of these images," says Zhang. "One day it could lead to diagnostic tools based on the terahertz response."

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