Minimally-invasive surgery — usually known as keyhole surgery — is becoming more prevalent, because small wounds heal faster and require less care, cutting hospital stays and reducing costs for hard-pressed health services. But working in the reduced spaces accessed by keyhole incisions can be difficult for surgeons. Although operations are planned in advance using scans, and external ultrasound is often used during the operation itself, but the precise site of the surgery can be hard to access and visualise.
The UCL team, working with colleagues from Queen Mary University of London (QMUL), has developed an optical ultrasound probe that can be mounted in a single-use surgical needle. This can provide images in real time, and in tests (on pigs) has been used to provide high-resolution views of the heart, 2.5cm in front of the tip of the instrument
Optical ultrasound, also known as photoacoustic imaging, uses pulses of light to generate ultrasound waves. In this project, the team developed a black flexible material that included a mesh of carbon nanotubes enclosed within clinical grade silicone precisely applied to an optical fibre. The carbon nanotubes absorb pulsed laser light, and this absorption leads to an ultrasound wave via the photoacoustic effect.
"The whole process happens extremely quickly, giving an unprecedented real-time view of soft tissue. It provides doctors with a live image with a resolution of 64 microns, which is the equivalent of only nine red blood cells, and its fantastic sensitivity allows us to readily differentiate soft tissues," said UCL medical physicist Dr Richard Colchester.
The researchers, who included materials scientists, electrical engineers, surgeons and engineers specialising in surgical instruments, built a fibre-optic optical ultrasound generator into a customised surgical needle with a point less than 1mm across. The needle also houses a highly sensitive optical fibre sensor based on polymer optical microresonators for detecting the ultrasound waves, relaying this data back to equipment that interprets them to produce the high-res images.
"This is the first demonstration of all-optical ultrasound imaging in a clinically realistic environment,” commented Adrien Desjardins, study co-lead at UCL’s Wellcome EPSRC Centre for Interventional and Surgical Sciences and a principal investigator on optical ultrasound. His fellow co-lead, Dr Malcolm Finlay, consultant cardiologist at QMUL and Barts Heart Centre, added: "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.”
The results of the four-year study are published in the Nature journal Light: Science & Applications. The sensor was also developed at UCL, by James Guggenheim, and reported recently in Nature Photonics. The team believes the technique will be particularly useful for operations such as cardiac ablation, where tissues that cause abnormal heat rhythms are destroyed by heat or freezing, and is now working to replicate its results in other minimally-invasive operations, such as foetal surgery or guidance of epidural needles.
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