Acoustic Reflector-Integrated Encapsulation for Needle-Aligned Ultrasound-Guided PCNL Access

Percutaneous Nephrolithotomy (PCNL) is a procedure to remove kidney stones by needle insertion into the kidney followed by dilation of the needle path for surgical tools to enter and stones to be removed. Current techniques for needle insertion either do not visualize the needle entry point into tissue, or require careful hand-hand coordination to visualize the needle. This research aims to develop a stable system to visualize the entire needle insertion path during ultrasound-guided PCNL. A novel method, designed by Tang et al. (2022), implemented two parallel acoustic reflectors to reflect ultrasound into the patient, and a needle inserted through a gap between the reflectors. Results showed the ability to visualize the needle path at any needle pitch angle, and, most importantly, the ability to visualize the needle from its insertion point to the target. In this current research, a modular design was created that allowed any probe to fit into the system by altering just three components. Several structural modifications were made to make the system waterproof, allowing the fluid material that housed the reflectors to be replaced with a fluid with better sound transmission. An algorithm was created to allow better needle tip and needle path visualization by analyzing raw image data. Custom acoustic impedance-matched materials were used to allow better sound transmission and better image quality. Through multiple tests and a mathematical model, the team developed an optimal combination of materials that produced the best image quality, determined through Contrast to Noise Ratio (CNR), Signal to Noise Ratio (SNR), and Full-Width Half-Maximum (FWHM).

I. Introduction

PCNL is a procedure for the removal of kidney stones that are branched, partially branched, or larger than 20mm in diameter. The procedure involves needle insertion into the target calyx of the kidney, and subsequent insertion of a dilator over the needle to make a tract large enough for tools to enter and kidney stones to be removed [1]. Ultrasound images consist of a single plane image, with a depth and one axial dimension, where objects outside the plane are not visible. To keep the needle in-plane, it can be inserted next to the long edge of the probe at an angle, as shown in Fig 1. This technique, called longitudinal insertion, may make it hard to visualize the entry point of the needle in the skin. Insertion of the needle at an oblique angle may also make subsequent dilation more challenging. It is possible to continuously move the probe to allow for better visualization of the needle throughout the insertion process, but this requires careful hand-hand coordination. Another needle visualization technique is transverse insertion, in which the needle is inserted closer to the probe center, but is not in-plane so the probe must be rocked back and forth to see the entire needle in segments [2]. This is shown in Fig 1.