Guiding surgical needles

Doctors frequently use thin, hollow needles to apply anesthetic to individual nerves, to extract samples of amniotic fluid, and to perform other minimally invasive procedures. Knowing the precise needle-tip location is crucially important—and challenging. Ultrasound can help, but with the two-dimensional scans that are typically used, it can be unclear what part of the needle is in view, and the tip is often out of sight. And if the needle’s angle of insertion is steep, it will not be seen with ultrasound imaging. To address those limitations, Wenfeng Xia of University College London and his colleagues have installed a tiny fiber-optic ultrasound sensor with a Fabry–Pérot cavity into a surgical needle. The reflectance of the cavity is altered by impinging ultrasound waves, and it is measured continuously by a wavelength-tunable laser via the optical fiber in the needle’s cannula. During an ultrasound-guided procedure, transducer elements in the ultrasound probe generate pulses used both for imaging tissue and for sensing the tip. Because of the reversibility of the wave equation, the tip’s position can be inferred in real time from sensor data, knowledge of the positions of the transducers, and the time it takes their pulses to reach the tip. Last year Xia and his colleagues demonstrated the feasibility of their device on a live sheep fetus. In their newest paper, they report using pulse compression techniques borrowed from radar ranging to boost the accuracy of needle location by almost an order of magnitude. (W. Xia et al., Med. Phys. 43, 4065, 2016, doi: 10.1118/1.4953205 .)