Due to high absorption in water, terahertz waves don’t travel deeply in tissue. So, in the clinic, terahertz imaging has often focused on surface water content and structural details — ideally suited for dermatology. Those who enjoyed the related theme last issue will learn more in this edition about what the terahertz regime may offer in skin cancer diagnostics and ulcer identification. There is significant disagreement about what terahertz waves can accomplish in biology and medicine; most commercial applications apply this wavelength range (between 30 µm and 3 mm on the spectrum) for manufacturing purposes such as materials analysis. But in a study published last year, “Terahertz in vivo imaging of human skin: Toward detection of abnormal skin pathologies,” a collaboration between the University of Queensland in Australia and the University of Leeds in England used a coherent terahertz system built around quantum cascade lasers. The system used laser feedback interferometry, in which the laser is used as a light source, with the beam then reflected back into the laser cavity with the aid of scanning mirrors, enabling the measurement of surface properties. The fiber-based system used a broad scanning field (50 × 40 mm) and was able to differentiate lesions associated with angioma, a benign tumor made of dilated blood vessels, and basal cell carcinoma from tattoos and other forms of scarring. It established the necessary contrast that showed how far lesions had spread. This success dovetails well with conversations I had for my own feature on terahertz imaging on page 40. Research led by M. Hassan Arbab at Stony Brook University has revolved around the use of terahertz imaging to reveal data about the severity of burn wounds, which can expand into additional tissue layers. This information helps determine when excision and graft are necessary, which is why their work has been funded in part by the U.S. Army Medical Research. Elsewhere, a group led by Enrique Castro Camus at the Centro de Investigaciones en Óptica in Mexico, translated their analysis of plant hydration into human study, using terahertz waves to examine diabetic ulcers. Their method was called moisture mapping by terahertz. With the use of femtosecond laser pulses divided between a transmitter and receiver, they learned that while patients lose hydration as they get older, it happens more quickly in diabetic patients. Commercial viability of biomedical terahertz imaging has proved to be elusive. But according to Future Market Insights, the medical terahertz market is expected to grow from $135.3 million in 2022 — with a compound annual growth rate of 17.1% — to more than $768 million in 2032. This technology is already employed in the pharmaceutical industry; it may soon find a clinical niche. It will be a fascinating evolution to watch. Enjoy the issue! Douglas J. Farmer