Needle insertion and radioactive seed implantation in human tissues: simulation and sensitivity analysis

− To facilitate training and planning for medical procedures such as prostate brachytherapy, we are developing an interactive simulation of needle insertion and radioactive seed implantation in soft tissues. We describe a new 2D dynamic FEM model based on a reduced set of scalar parameters such as needle friction, sharpness, and velocity, where the mesh is updated to maintain element boundaries along the needle shaft and the effects of needle tip and frictional forces are simulated. The computational complexity of our model grows linearly with the number of elements in the mesh and achieves 24 frames per second for 1250 triangular elements on a 750MHz PC. We use the simulator to characterize the sensitivity of seed placement error to physician-controlled and biological parameters. Results indicate that seed placement error is highly sensitive to physician-controlled parameters such as needle position, sharpness, and friction, and less sensitive to patient-specific parameters such as tissue stiffness and compressibility. (a) Human prostate with target implant location (b) Needle insertion (c) Needle reaches target (d) Seed implanted at target (e) Needle extraction (f) Seed placement error Fig. 1: Simulation of needle insertion based on an ultrasound image of a human prostate. Frame (a) outlines the prostate and displays the target implant location (white cross), which is fixed in the world frame. The simulated needle is inserted and places a radioactive seed (small square) at the target (d). After needle retraction, the seed placement error, the distance between the target and resulting seed location shown in (f), is 20% of the width of the prostate. Needle plans that compensate for tissue deformation can reduce placement errors like this that damage healthy tissue and fail to kill cancerous cells.