20.4 3D Surgical Alignment with 100µm Resolution Using Magnetic-Field Gradient-Based Localization

Substantial advances in the field of surgery have taken place in recent years, which aim at decreasing patient morbidity through innovations in endoscopy, optical imaging, laparoscopic and robotic technologies. However, real-time imaging and navigation during high precision surgery necessitates the use of X-Ray fluoroscopy with most existing technologies to achieve precise localization. Intramedullary (IM) nailing is a common example of such high precision orthopedic surgery, which requires insertion of a nail into the medullary canal of a fractured bone followed by locking screws [1]. Proximal screw locking is performed using a mechanical guide, which is not possible for distal locking owing to the deformation (≈15mm) caused during insertion [2]. Freehand technique is typically used to localize distal holes, in which the surgical drill is aligned with the hole axis through fluoroscopic imaging. This process is time-consuming and exposes the patient and surgical team to high ionizing radiation. Various other methods, which reduce or eliminate irradiation during distal locking, are not widely used. This is attributed to their lack of compensation for significant deformation of the nail, added requirements such as computing systems, extra robotic arms, CT images, sophisticated hardware and software that require training for the surgeon and staff. This paper presents a navigation system for 3D localization using magnetic field gradients, which is inspired by Magnetic Resonance Imaging (MRI) but does not require the strong magnetic field used in MRI [3], and can replace fluoroscopy during IM nailing. The goal is to generate monotonically varying magnetic fields in the desired Field-of-View (FOV) such that each spatial point corresponds to a unique field value. The field is sensed by miniaturized devices in the FOV and communicated wirelessly to a receiver, which maps the field to spatial coordinates and displays the device location in real-time. Monotonically varying magnetic fields are generated using low-cost planar electromagnet coils, placed beneath the patient's leg. A battery-less wireless device capable of measuring and transmitting the value of its local magnetic field, is attached to the distal end of the implanted nail. This device consists of a 3D magnetic sensor (AK09970N), a CMOS chip to interface with the sensor, an off-chip coil for wireless power transfer (WPT) and data communication, and off-chip capacitors for energy storage. Another similar device is installed in the surgical drill. In the measurement phase, both devices measure magnetic fields in X, Y and Z at their respective locations. A computing system decodes the spatial position of both the devices from the received magnetic field data and displays their relative locations on a monitor, which helps the surgeon to maneuver to the hole location in real-time. The resolution of magnetic field measured by the devices (AB) and strength of the applied magnetic field gradient (G), together set the localization resolution (LR) of the system as LR = AB/G.