Scientists from University of Warwick and Chinese University of Hong Kong have refined a new type of camera to make it a hundred times faster than the previous state of the art. The technique utilizes terahertz radiation, or Trays, that sit in-between infrared and Wi-Fi on the electromagnetic spectrum. Trays can ‘see’ through common materials such as plastics and clothing but are non-ionizing — making them ideal for non-invasive screening.
The researchers have reached a crucial milestone towards developing single-pixel terahertz imaging technology for use in biomedical and industrial applications. Their single-pixel terahertz camera reached 100 times faster acquisition than the previous state-of-the-art without adding any significant costs to the entire system or sacrificing the sub-picosecond temporal resolution needed for the most sought-after applications.
T-rays have different properties from other electromagnetic waves, most notably they can see through many common materials such as plastics, ceramics and clothes, making them potentially useful in non-invasive inspections. Another quality is that the low-energy photons of T-rays are non-ionizing, making them very safe in biological settings including security and medical screening. They are also highly sensitive to water and can observe minute changes to the hydration state of biological matter. This means that diseases perturbing the water content of biological matter, such as skin cancer, can potentially be detected using T- rays in vivo without any histological markers.
Efficient detection and generation of T-rays has been possible in laboratory settings for the last 25 years. However, THz technology is still not widely used in commercial settings as the cost, robustness and/or ease of use is still lagging behind for commercial adoption in industrial settings.
Very few clinical trials have been performed for biomedical applications, most notably due to the equipment not being user-friendly and imaging being too slow due to the need for measuring multiple terahertz frequencies for accurate diagnosis. Finally, equipment and running costs need to be within hospital budgets. As a result, a lot of research into terahertz technology is currently focused on developing the equipment to improve imaging speed, without reducing diagnosis accuracy or incurring large costs. As a result, the researchers are exploring alternative imaging techniques from those currently used in modern day smart phones.
The researchers use what is called ‘a single-pixel camera’ to obtain the images. They spatially modulate the THz beam and shine this light onto an object. Then, using a single-element detector, they record the light that is transmitted (or reflected) through the object they want to image. They keep doing this for many different spatial patterns until they can mathematically reconstruct an image of their object.
They have to keep changing the shape of the THz beam many times, which means this method is usually slower compared to multi-pixel detector arrays. However, multi-pixel arrays for the terahertz regime usually lack sub-picosecond temporal resolution, require cryogenic temperatures to operate or incur large equipment costs (more than $350,000). The setup developed by the Warwick team, which is based on a single-element detector, is more reasonably priced ($20,000), robust, has sub-picosecond temporal resolution (needed for accurate diagnosis), and operates at room temperature.
Their work improves upon the acquisition rate of single-pixel terahertz cameras by a factor of 100 from the previous state-of-the-art, acquiring a 32x32 video at 6 frames per second. They do that by first determining the optimal modulation geometry, second by modeling the temporal response of the imaging system for improvement in signal-to-noise, and third by reducing the total number of measurements with compressed sensing techniques. In fact, part of their work shows that they can reach a five-times faster acquisition rate if they have sufficient signal-to-noise ratio.
The researchers have previously developed several THz devices including THz modulators that make use of the total internal reflection geometry to achieve high MDs across a broadband frequency range and a new approach for amplitude and phase modulation exploiting the Brewster angle. They are also working to improve the resolution of single pixel THz imaging through signal processing approaches. Future work will focus on improving the signal-to-noise and optimizing the software needed for accurate medical diagnosis, with the ultimate goal being to use single pixel THz imaging for in vivo cancer diagnosis.
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