Ultrasonically marked instruments for ultrasound-guided interventions

In order to enhance needle and catheter visibility under ultrasound and enable tool tracking, it has been proposed to embed ultrasound receivers near the tip of interventional tools. Using these sensors to receive and analyze signals coming from the imaging probe as its beams sweep the field of view, the position of the sensor can be estimated with <;0.5mm accuracy even when the tool tip is invisible in the pulse-echo image. One crucial design parameter for these ultrasonically marked instruments is the sensitivity and directivity pattern of the sensor: the ideal sensor has to maintain adequate SNR at a wide range of depths and insonification angles depending on the clinical intervention. This paper presents the acoustical characterization of small (~1mm) high frequency (>10MHz) PZT sensors in different form factors for acoustical marking of needles and catheters. PZT-equipped tools including guidewires, steerable sheaths, and biopsy needles are also presented and the tracking capability is demonstrated ex vivo. Maximum SNRs of >50dB were found with all sensors in the water tank at an imaging depth of 8cm and insonification MI of 1.1. Complex directivity patterns were observed varying with sensor size and shape, likely owing to mode conversion inside the sensor and excitation of the materials' transverse piezoelectric modes. Small disks (≤1mm diameter) were found to be most appropriate with flattest directivity. Ex vivo experiments demonstrated reliable tracking of an ultrasound-equipped needle at depths >10cm. In conclusion, it is feasible to manufacture PZT-equipped instruments exhibiting good tracking accuracy and robustness.