Towards Tracking of Deep Brain Stimulation Electrodes Using an Integrated Magnetometer

Abstract

This paper presents a tracking system using magnetometers, possibly integrable in a deep brain stimulation (DBS) electrode. DBS is a treatment for movement disorders where the position of the implant is of prime importance. Positioning challenges during the surgery could be addressed thanks to a magnetic tracking. The system proposed in this paper, complementary to existing procedures, has been designed to bridge preoperative clinical imaging with DBS surgery, allowing the surgeon to increase his/her control on the implantation trajectory. Here the magnetic source required for tracking consists of three coils, and is experimentally mapped. This mapping has been performed with an in-house three-dimensional magnetic camera. The system demonstrates how magnetometers integrated directly at the tip of a DBS electrode, might improve treatment by monitoring the position during and after the surgery. The three-dimensional operation without line of sight has been demonstrated using a reference obtained with magnetic resonance imaging (MRI) of a simplified brain model. We observed experimentally a mean absolute error of 1.35 mm and an Euclidean error of 3.07 mm. Several areas of improvement to target errors below 1 mm are also discussed.

Keywords: deep brain stimulation; image guided intervention; magnetic field mapping; magnetic tracking system; three-dimensional magnetometer.

Figure 1

Scheme of the magnetic tracking system in the DBS context. Patient lying on the operating bed with the magnetic source located underneath the head (left). Top view illustrating the position of the Leksell’s stereotactic guiding apparatus (right).

Figure 2

Sequences of the current flowing through the coils A, B and C, and the corresponding measured magnetic field intensity at a location point P (x = 1.4 cm, y = 5 cm, z = 2.66 cm).

Figure 3

Magnetic field camera developed to experimentally map the magnetic field generated by the source. An arrangement of 4 × 4 × 4 = 64 magnetometer chips, separated by 48 mm in the X, Y and Z directions, provides the 64 magnetic field intensity values which are then interpolated to obtain a fine map of the field.

Figure 4

Experimental setup: the DBS electrode model is inserted along an engraved trajectory. A visual assessment of the position is then performed at each tested location.

Figure 5

(a) Experimental setup: the DBS electrode model is inserted in a watermelon. The watermelon is then placed on a Lego^® holder on top of the tracking magnetic source. (b) 100 mT MRI scanner used to image the watermelon with the inserted electrode.

Figure 6

(a) Magnetic field intensity map obtained by the tri-cubic interpolation of experimental mapping data when all coils are turned off. (b–d) Magnetic field intensity maps obtained by the tri-cubic interpolation of mapping experimental data when the coils A, B and C are respectively turned on. (e) Superimposition of (b–d) maps to illustrate the trilateration principle.

Figure 7

DBS electrode model with the integrated three-dimensional Hall magnetometer. The carbon tube has a diameter of 5 mm. This version has been used for the experimental accuracy estimation of the tracking system.

Figure 8

Miniaturized version of the DBS electrode model with the integrated three-dimensional Hall magnetometer, mounted on a Leskell system. The tip of the electrode has a diameter of 2 mm.

Figure 9

Experimental setup: the DBS electrode model is inserted along the engraved trajectory. The plots represent the reference linear trajectory in red vs. the trajectory estimated by the magnetic tracking system in blue for a Z-level of 2.66 cm.

Figure 10

MR images of the watermelon with the DBS electrode model inserted. Position of the tip of the electrode measured on MR images (in red) vs. measured position obtained with the proposed tracking system (in blue).