Resolving fine defects, together with magnifying the image is of much interest in industrial and biomedical diagnostics applications. One of the ways of doing this is by using superlenses, that use metamaterials (engineered materials that can manipulate waves) to go beyond the diffraction limit (the resolution performance of any classical imaging technique).
The resolution of ultrasonic imaging systems is also constrained by the diffraction limit, just like their acoustic and electromagnetic counterparts. This is because ‘evanescent’ or high frequency components in the scattered wavefield, that carry information about fine features, die out within the near-field. Imaging within the near-field is challenging due to poor signal-to-noise ratios and moreover, the post-processing required to achieve this is also typically complicated. Thus harnessing the information carried by evanescent waves by successfully transferring them to the far-field has been of research interest. However, most metamaterial concepts require the acquisition of wavefields transmitted through their geometric features, which are of subwavelength order, to be useful in imaging applications. Sophisticated equipment is also required to achieve this.
Hyperlenses are a type of metamaterial that produce magnified images of objects smaller than the wavelength used. Hyperlenses convert evanescent waves into propagating ones, thus carrying information to the far-field. Although there have been demonstrations of hyperlenses in the electromagnetic and acoustic domains, they are rarely studied for ultrasonics. This is perhaps because hyperlens parameters and other details are not widely known for ultrasonics, and reception of waves is also challenging in this domain (as compared to acoustics for e.g.).
In this study, the authors Dr. M. Subair S.A. Ali and Prof. Prabhu Rajagopal from the Centre for Nondestructive Evaluation, Department of Mechanical Engineering, Indian Institute of Technology (IIT) Madras, Chennai, India, have developed a hyperlens and a novel waveguide based reception technique that can magnify subwavelength features up to 5 X times, and achieve super-resolution ultrasonic imaging in the far-field.
This is one of the world’s first experimental demonstrations of magnification and super-resolution using a hyperlens in the ultrasonic domain. The results of this study have important implications for higher resolution ultrasonic imaging.
Prof. Bruce Drinkwater from the Department of Mechanical Engineering, University of Bristol, UK, acknowledged the importance of this study by giving the following comments:
“Ultrasonic imaging is equally critical in medical diagnostics and in detecting cracking in engineering structures. Our health depends on this technology, as does the safety of aircraft, trains and power stations. In 1879 Lord Rayleigh showed that the resolution of such imaging is limited by diffraction effects. This recent work uses an ultrasonic hyperlens to beat this limit. The hyperlens is made from a metamaterial that extracts the near-field scattering information, expanding it in space such that it can be more easily measured with standard equipment. The paper shows that concept is quite versatile and hyperlenses can be realized as curved or flat structures and this will undoubtedly lead to a broad range of future applications. Hence, the work is an important step forward in making a practical imaging systems with better resolution.”
Article by Akshay Anantharaman
Here is the original link to the paper:
https://www.nature.com/articles/s41598-022-23046-7
