Follow-the-Leader Deployment of Steerable Needles Using a Magnetic Resonance-Compatible Robot With Stepper Actuators1

Abstract

Epilepsy is a debilitating, potentially fatal, seizure-causing neurological disorder that will affect approximately 1% of people worldwide in their lifetimes [1]. Medication-based treatment is ineffective for an estimated 40% of epilepsy patients [1]. As an alternative to medication, surgical removal of the hippocampus (commonly, the origin of epileptic seizures) successfully cures epileptic seizures in about 70% of cases [2]; however, 50–90% of eligible patients forgo surgery due to risks associated with highly invasive brain surgery [2,3].Magnetic resonance image-guided (MRI-guided) laser ablation of the hippocampus is a promising avenue for minimally invasive surgical treatment of epilepsy. Recent clinical trials using various needle-based, MRI-guided laser ablation systems to treat epilepsy have reported positive results; however, seizure outcomes were worse than those of epilepsy surgery [4]. These ablation systems exhibit one major limitation: linear needle trajectories are unable to traverse the entire curved structure of the hippocampus.Steerable needles—comprising concentric tubes of pre-curved superelastic nitinol—address this limitation by enabling curvilinear needle trajectories in soft tissue. The potential benefits of curvilinear trajectories are twofold: (1) they enable therapy delivery to a larger region of the hippocampus and (2) they enable accurate needle placement while avoiding sensitive, untreated tissue that might otherwise obstruct a typical linear trajectory [5]. To achieve curvilinear trajectories without shearing tissue, however, steerable needles must be deployed in a “follow-the-leader” (FTL) fashion, whereby the needle backbone follows the path created by the needle tip [5]. Precise coordination of needle insertion and rotation required for FTL deployment necessitates robotic actuation.Research on MRI-compatible robotic needle-actuation systems has focused primarily on straight needle placement (see, e.g., Ref. [6]; for a more general review of MRI-compatible robotics, see Ref. [7]). To enable use of steerable needles for MRI-guided epilepsy surgery, we previously developed a compact, pneumatically actuated, additively manufactured, fail-safe, MR-compatible robotic needle-driving system [8]. This paper presents a joint-level trajectory coordinator for FTL deployment of a steerable needle using our MRI-compatible robot. FTL deployment is validated experimentally.Our MRI-compatible robot system, shown in Fig. 1, uses a two degrees-of-freedom pneumatic stepper actuator to drive a helical steerable needle. The actuator is an additively manufactured monolithic structure comprising both a linear and a rotary flexible fluidic actuator (FFA). Actuation is achieved by inflation of the FFAs, causing translational or rotational deformation, respectively. Flat diaphragm grippers are inflated around clamshell inserts to grasp a transmission tube at the needle base. Detailed design, operation, and low-level nonlinear position control are presented in Ref. [8].During operation, the superelastic needle deploys from a fixed, straight outer cannula (not shown in Fig. 1), and the distal end of the needle returns to its helical shape as it exits the cannula. Since a portion of the needle remains straightened inside the cannula, the displaced arc length and rotation of the deployed helix, respectively, equal the translation, x, and rotation, θ, of the needle base at the actuator. During FTL deployment, x and θ must be coordinated such that they follow the geometric relationship between helix arc length and rotation:(1)x=θr2+p2where r and p are the helix radius and pitch, respectively.To achieve FTL deployment using the stepper actuator, a joint-level trajectory coordinator determines the desired translation and rotation of the actuator (denoted by subscript des) during each actuation step. The trajectory coordinator accepts final desired displacements as inputs. For each actuation step (denoted by superscript k), the