Doctors currently rely on external ultrasound probes, combined with pre-operative imaging scans, to visualize soft tissue and organs during minimally invasive procedures, as the miniature surgical instruments used do not support internal ultrasound imaging.
A team designed and built optical ultrasound technology to fit into existing single-use medical devices such as needles. The technology has been used successfully for minimally invasive heart surgery in pigs, giving a high-resolution view of soft tissues up to 2.5 cm in front of the instrument inside the body.
Real-time imaging allows surgeons to differentiate between tissues at a remarkable depth, helping to guide the highest-risk moments of these procedures. This could 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.
The all-optical ultrasound imaging technology was developed for use in a clinical setting over four years, and is sensitive enough to image centimeter-scale depths of tissues when moving; it fit into the existing clinical workflow, and worked inside the body.
Using inexpensive optical fibers, high-resolution imaging using needle tips less than 1 mm was achieved. The technology uses a miniature optical fiber 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 realtime ultrasound imaging to guide surgery (see figure).
One of the key innovations was the development of a flexible material that included a mesh of carbon nanotubes enclosed within clinical-grade silicone precisely applied to an optical fiber. The carbon nanotubes absorb pulsed laser light, and this absorption leads to an ultrasound wave via the photoacoustic effect. A second innovation was the development of highly sensitive optical fiber sensors based on polymer optical microresonators for detecting the ultrasound waves.
The process occurs extremely quickly, giving a 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; its sensitivity allows soft tissues to be readily differentiated.