Sensorless motion planning for medical needle insertion in deformable tissues

Abstract

Minimally invasive medical procedures such as biopsies, anesthesia drug injections, and brachytherapy cancer treatments require inserting a needle to a specific target inside soft tissues. This is difficult because needle insertion displaces and deforms the surrounding soft tissues causing the target to move during the procedure. To facilitate physician training and preoperative planning for these procedures, we develop a needle insertion motion planning system based on an interactive simulation of needle insertion in deformable tissues and numerical optimization to reduce placement error. We describe a 2-D physically based, dynamic simulation of needle insertion that uses a finite-element model of deformable soft tissues and models needle cutting and frictional forces along the needle shaft. The simulation offers guarantees on simulation stability for mesh modifications and achieves interactive, real-time performance on a standard PC. Using texture mapping, the simulation provides visualization comparable to ultrasound images that the physician would see during the procedure. We use the simulation as a component of a sensorless planning algorithm that uses numerical optimization to compute needle insertion offsets that compensate for tissue deformations. We apply the method to radioactive seed implantation during permanent seed prostate brachytherapy to minimize seed placement error.

Fig. 1

Four vertical frames illustrate brachytherapy needle insertion based on deforming ultrasound images of the human prostate using simulation. The left column shows results without planning, producing substantial placement error. The right column shows results with the sensorless plan, with minimal placement error. The target implant location is indicated in all frames with a cross fixed in the world frame. Frame (a) outlines the undeformed prostate. In Frame (b), the needle is inserted and the radioactive seed (small square) is released at the needle tip. In Frame (c), the needle is retracted. Frame (d) indicates the resulting placement error, the distance between the target and resulting actual seed location. Without planning, placement error is substantial: 26% of the prostate diameter, resulting in damage to healthy tissue and failure to kill cancerous cells. With sensorless planning, placement error is negligible.

Fig. 2

During permanent seed prostate brachytherapy (a), needles carrying radioactive seeds are inserted transperineally into the patient, who is lying on his back [3]. Intra-operative transrectal ultrasound can be used for imaging (b), but these images do not provide sufficient quality signal to track tissue deformations.

Fig. 3

The needle is in the interior of the mesh with needle tip node c = i at point p′ moving in direction r′ in the world frame (a). Mesh modification is performed in the reference mesh (b) to represent the path cut by the needle through the tissue. Vector r′ is transformed to r in the reference mesh and the tip node is moved along r a cut distance b as described in Sec. IV-D.

Fig. 4

The simulation user interface, which is based on an ultrasound image, is intended to mimic the experience of a physician performing brachytherapy. The physician interactively guides the needle using a mouse and implants seeds (small squares). Tissue deformations and seed locations are predicted and displayed. The implantation error is the distance between the seed and its target (cross) after needle retraction.

Fig. 5

When the needle pushes against the lower half of the prostate from the right, the prostate rotates clockwise because it is stiffer than the surrounding tissue. This rotation can lead to significant changes in the optimal needle insertion height.

Fig. 6

Twelve sample points were selected as targets marked “+” inside the prostate. Actual seed placements using simulation are marked “•”. Lack of planning results in major placement errors averaging 17% of the prostate diameter (a), which will lead to a poor radioactive dose distribution. Seed placement error was negligible using the planner (b).

Fig. 7

(a) Needles should generally be inserted deeper than the target depth to compensate for tissue deformations and minimize placement error. The bold portion of the line denotes feasible seed placements inside the prostate. (b) The motion planner computes the optimal insertion height y_r and corresponding optimal depth $z_r^{*}$ to minimize placement error ∊. Placement error is minimized for (y_r, z_r) = (1.59cm; 3.80cm).