This method can help answer key clinical questions about ultrasound needle tracking, even when the needle is outside the imaging plane. The main advantage of this technique is it helps to eliminate complications and injuries that can arise from poorly tracked needle punches. Demonstrating the procedure with me will be Sunish Mathews, a research associate at UCL.
Begin by selecting a fiber optic ultrasound hydrophone made up of a single-mode optical fiber with a Fabry-Perot Optical Cavity at the distal end. Using a scalpel, partially remove the 900 micrometer jacket along the length of the fiber optic hydrophone close to its distal end to expose the buffer layer, until the hydrophone can fit within the needle cannula. Next, affix a medical needle horizontally to a manual horizontal translation stage, and use a stereo microscope to visualize the needle tip with the optical access of the microscope aligned horizontally and perpendicular to the needle, rotating the needle about its axis as necessary, until the bevel can be observed.
With the distal end of the needle in view, insert the fiber optic ultrasound receiver through the cannula of a Tuohy-Borst Sidearm adapter and subsequently through the lure connector of the needle until the sensing region of the hydrophone is just proximal to the bevel surface of the needle. Affix the hydrophone to the translation stage with polyimide tape to avoid movement of the device within the needle. Vertically affix a 20-microliter pipette to the vertical translation stage with the tip facing downward, and use both the horizontal and vertical translation stages to position the micropipette tip until it is adjacent to the fiber optic hydrophone and about 0.5 millimeters proximal to the sensing region at the distal end.
Place a drop of optical adhesive at the proximal end of the micropipette, and adjust the needle to allow a direct path from the micropipette tip to the fiber optic ultrasound receiver. Then use a 10-milliliter syringe to slowly inject the adhesive from the distal end of the micropipette into the fiber optic ultrasound receiver, taking care to avoid applying adhesive to the sensing region, or occluding the cannula, and illuminate the needle tip with ultraviolet light, until the optical adhesive is cured. To integrate the hydrophone into the imaging system, connect the hydrophone to the optical console, and a custom ultrasound imaging probe to the ultrasound console.
To confirm that the hydrophone is ready for clinical use, select the operation mode on the ultrasound imaging probe, and add ultrasound gel to the probe. Next, add water to a fetal ultrasound phantom to mimic amniotic fluid, and select B-mode imaging on the probe. Identify the amniotic fluid in the phantom as the insertion target.
Then, insert the needle toward the insertion target using the switch on the custom probe to continuously alternate between the imaging and tracking operation modes. In this representative experiment, a needle was inserted into the uterine cavity and the tip was tracked along a trajectory that attained an out-of-plane distance of 15 millimeters, and a depth of 38 millimeters. Golay coding improved the signal-to-noise ratio, with a 7.5-fold increase relative to the conventional bipolar excitation.
The 3D-tracked needle tip positions were then overlayed onto the 2D ultrasound image, using crosses with widths indicative of the out-of-plane distances, and colors indicative of the imaging plane. Ultrasound tracking is a technique that allows practical nurse in many clinical fields to accurately locate medical devices within the body, relative to ultrasound imaging plan. After watching this video, you should have a good understanding of the key steps involved in integrating of fiberoptic hydrophone into a clinical needle to allow ultrasonic tracking.
A compact ultrasound probe with transducer elements adjacent to the main imaging array, and an intuitive user interface are useful to maintain compatibility with clinical workflow.
