Researchers at IIT-Madras have announced a breakthrough in ultrasound imaging through the use of ‘metamaterials’. This development has the potential to both revolutionise medical imaging and improve non-destructive testing processes in industry.

X-ray produces sharp resolution; MRI scans even sharper. (These days, doctors tend to rely more on MRI than X-ray because the latter is blind to some defects.) However, X-ray is not quite safe as it is based on ionising radiation; MRI scans are expensive and not suitable for people with metal implants.

Ultrasound imaging, which uses ‘sound’ rather than ‘light’ for imaging, is done with non-ionising radiation but its resolution is not so good — X-ray’s resolution is 1,000 times sharper. X-ray can achieve resolutions as fine as 0.5 microns in high-end applications, compared with ultrasound’s 0.5 mm.

Ultrasound’s resolution can be increased but then the imaging will be superficial, not deep.

In a recent paper published in Nature, Prof Prabhu Rajagopal of the Centre for Non-destructive Evaluation, IIT-Madras, describes the problem thus: “Electromagnetic methods such as radiographic (X-ray) testing can achieve high resolution but with reduced penetration in solids; they typically involve ionising radiation while also being expensive, limiting wider field application. Ultrasound can be an effective alternative with better penetration of thicker samples while being cost-effective and non-ionising, thus allowing for the possibility of rapid and largescale online/in-situ material diagnostics. However, conventional (linear, bulk) ultrasound has limited applicability for imaging microscopic defect features due to the longer wavelengths involved. Techniques such as scanning acoustic microscopy (SAM) can offer better resolution for ultrasonics at elevated frequencies on the order of 100 MHz but are restricted to the sample surface. Thus, techniques for achieving very high-resolution imaging using low-frequency bulk linear ultrasonics could offer an elusive breakthrough for material diagnostics and imaging deeper inside solids.”

Special material

Now, Rajagopal and his team have come up with a new technique that improves the resolution of ultrasound by using metamaterials, which are specially engineered materials. The one Rajagopal uses for high-resolution ultrasound imaging is a meta lens — a tiny silicon block with hundreds of square channels drilled into it. This was done using a well-known technique called ‘deep reactive ion etching’, used in micro-fabrication mainly for micro electromechanical systems (MEMS). As ultrasound waves pass through these channels, they get amplified and are picked up by a laser-based receiver.

So, the architecture is simple — an ultrasonic transmitter, the sample, the meta lens and the laser-based receiver. The sample has to be kept in a water bath because ultrasound is of very small wavelength compared to audible sound, so it will be scattered by air particles and cannot propagate through air. (In medical applications, a gel is used in place of water.)

From the transmitter, the ultrasonic wave propagates through the water medium, passes through the object being imaged and emerges through the water-filled meta lens. It is picked up by the laser Doppler vibrometer.

At the heart of the setup is the meta lens, which is quite difficult to make.

Drilling perfectly square channels through the silicon material is itself daunting. Further, since the channels are too narrow, the capillary effect will distort the water level. To hold an equal level of water, Rajagopal’s team made the insides of the tubes hydrophilic (water attracting) through oxidation.

“It has been my quest for many years to bring ultrasound to the same range as X-ray using metamaterials,” Rajagopal, who was awarded the prestigious Shanti Swarup Bhatnagar award last year, told Quantum.

“This research by Dr Prabhu Rajagopal’s team showcases a breakthrough in ultrasonic inspection, achieving an unprecedented 50-micron resolution using commercially available low-frequency probes. Their innovative approach, which combines micro-fabricated metamaterial lenses with advanced signal processing, offers a powerful and cost-effective alternative to traditional radiation-based imaging techniques, with transformative potential across various industries,” says Dr David Fan, associate professor at the School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.