All-Optical Ultrasound Transducer Could Transform Keyhole Surgeries

An optical ultrasound needle has been developed that allows heart tissue to be imaged in real-time during keyhole procedures. The technology was successfully used for minimally invasive heart surgery on pigs, giving a high-resolution view of soft tissues up to 2.5 cm in front of the instrument, inside the body.

The all-optical ultrasound transducer, developed by a research team from University College London (UCL) and Queen Mary University of London (QMUL), uses light guided by miniature optical fibers to generate and receive ultrasound. The optical fiber is encased within a customized clinical needle to deliver a brief pulse of light which generates ultrasonic pulses. Reflections of these ultrasonic pulses from tissue are detected by a sensor on a second optical fiber, providing real-time ultrasound imaging to guide surgeons.

The sharp inner needle (schematic and inset photo) used to puncture the cardiac septum to gain access to the left atrium can be safely recessed within a blunt outer needle cannula. After puncturing, the dilator sheath is advanced over the needle into the left atrium. The probe includes two optical fibers positioned within the inner needle for pulse-echo ultrasound imaging: one for transmission with the delivery of pulsed excitation light to an optically absorbing coating and one for reception with the delivery of continuous-wave light to a Fabry-Pérot cavity. Acoustic isolation between the transmission and reception fibers is provided by a thin metal septum. Scale bar = 500 µm. Courtesy of Finlay et al.

Broad-bandwidth ultrasound generation was achieved photoacoustically through light pulses delivered to an optically absorbing composite coating on the distal end of one of the optical fibers. Ultrasound waves reflected from tissue were detected through a Fabry–Pérot cavity positioned on the distal end of the other optical fiber. Depth scans were concatenated across time and displayed in real-time as M-mode ultrasound images.

The use of this needle within the beating heart of a pig provided real-time views (50 Hz scan rate) of cardiac tissue (depth: 2.5 cm; axial resolution: 64 μm) and revealed the critical anatomical structures required to safely perform a transseptal crossing — i.e., the right and left atrial walls, the right atrial appendage, and the limbus fossae ovalis.

Researchers believe that this new technology could allow ultrasound imaging to be integrated into a broad range of minimally invasive devices in different clinical contexts to generate ultrasound images of areas of the body for which they were previously unavailable.

“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 Dr. Malcolm Finlay, co-lead of the study and consultant cardiologist at QMUL.

Since its inception, ultrasound imaging has been achieved using electronic transducers. All-optical ultrasound transducers, which perform ultrasonic generation using pulsed light and optical reception of ultrasonic reflections from tissues, could serve as viable alternatives.

Two-dimensional all-optical ultrasound imaging (B-Mode) acquired during the manual translation of the needle tip across a distance of 4 cm. As the needle tip progressed from the high right atrium to the inferior vena cava, the thin foramen ovale manifested as a hypoechoic region between the thick limbus fossae ovalis and the tendon of Todaro (with a diagonal artifact from the ICE catheter and sheath). X-ray fluoroscopic imaging was acquired concurrently (inset). Courtesy of Finlay et al.

“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. The technology has been designed to be completely compatible with MRI and other current methods, so it could also be used during brain or fetal surgery, or with guiding epidural needles,” said Finlay.

The team developed the technology for use in a clinical setting, making sure it was sensitive enough to image centimeter-scale depths of tissues when moving; that it fit into the existing clinical workflow; and worked inside the body.

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

The team is now working toward translating the technology for clinical use in patients.

The research was published in Light: Science & Applications (doi:10.1038/lsa.2017.103).