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.