Currently, doctors rely on external ultrasound probes and pre-operative imaging to visualize soft tissue and organs during keyhole procedures. In this study, the team of surgeons, engineers, physicists and material chemists designed and built an all-optical ultrasound transducer that fits into existing single-use medical devices, such as a needle.

"The optical ultrasound needle is perfect for procedures where there is a small tissue target that is hard to see during keyhole surgery using current methods and missing it could have disastrous consequences," said study co-lead Malcolm Finlay, consultant cardiologist at Queen Mary and Barts Heart Centre. "We now have real-time imaging that allows us to differentiate between tissues at a remarkable depth, helping to guide the highest risk moments of these procedures. This will reduce the chances of complications occurring during routine but skilled procedures such as ablation procedures in the heart."

Fibre formation

The all-optical ultrasound transducer comprises two 300 µm optical fibres encased within a customized clinical needle (diameter 1.08 mm; length 78 cm) and separated by a thin metal septum. The researchers created a multiwalled carbon nanotube-polydimethylsiloxane composite and used this to coat the distal end of one fibre. Pulsed laser light guided through the fibre is absorbed by the carbon nanotubes, leading to generation of an ultrasound wave via the photoacoustic effect.

Reflections of these ultrasonic pulses from tissue are then detected by a sensor on the second optical fibre. The researchers developed highly sensitive fibre sensors based on a Fabry–Pérot cavity positioned on the distal end of the fibre.

To demonstrate the clinical utility of real-time in vivo all-optical ultrasound imaging, the team used the device during a cardiac transseptal puncturing in a pig. This procedure – in which a needle is inserted into the right atrium of the heart via the femoral vein, to access the left atrium for therapeutic intervention - is usually guided by X-ray fluoroscopy, which does not provide soft-tissue contrast.

Here, the use of the optical ultrasound needle within the animal's beating heart provided real-time (50 Hz scan rate) imaging of cardiac tissue up to 2.5 cm in front of the instrument, with a resolution of 64 µm. The images revealed the critical anatomical structures required to safely perform the transseptal crossing.

"This is the first demonstration of all-optical ultrasound imaging in a clinically realistic environment," explained study co-lead Adrien Desjardins from UCL. "Using inexpensive optical fibres, we have been able to achieve high-resolution imaging using needle tips under 1 mm. We now hope to replicate this success across a number of other clinical applications where minimally invasive surgical techniques are being used."