Development of Biocompatible Antennas for Medical Applications: Ensuring MRI safety, Implantation, and Catheterization

Abstract

Intravascular catheters are evolving to be more adaptable, enabling a broad spectrum of diagnostic and therapeutic capabilities such as tracking, imaging, sensing, ablation, and drug delivery. Several techniques can be employed to ensure accurate catheter placement, including the use of anatomical landmarks, aspiration, or imaging modalities such as X-ray, compute tomography scan, ultrasound, or magnetic resonance imaging (MRI). Among these options, MRI stands out as a highly advantageous method for image-guided interventions due to its radiation free procedures and superior visualization of soft tissues compared to other imaging modalities. Furthermore, MRI offers real-time 3D anatomical data similar to ultrasound and X-rays, making it well-suited for minimally invasive surgeries. The design of interventional MRI (iMRI) devices faces significant challenges arising from the strong magnetic fields and RF transmission within the MRI environment. The visualization of interventional devices during MRI necessitates their interaction with the main magnetic field (B0) and proton spins. Various iMRI coils have recently been proposed to overcome these challenges. Passive iMRI coils introduce metallic susceptibility artifacts, producing positive and negative contrast in the image, as well as using T1 shorting agents. The visualization of soft tissues and lesions in MRI scans often involves the passive utilization of metallic needles that are commercially accessible. However, passive devices have limitations such as longer scan times, lower image quality, and limited applications. A semi-tracking method for iMRI devices has been utilized, employing an inductively coupled RF coil to stimulate the surrounding tissues at an amplified angle, resulting in a contrasting image. However, these devices are associated with drawbacks including cost-effectiveness, bulkiness, electromagnetic interference, and complex setup. The integration of electronic components within the receiver chain of MRI hardware in active iMRI devices results in improved visibility during MR imaging. Various coils, such as loopless antennas, asymmetric arm coils, regular loop coils, multimodal imaging coils, and solenoid coils, have been actively integrated with MRI to improve image quality, orientation, and catheter tracking. However, the aforementioned active coils have several limitations, such as integration of the active coil with an interventional devices, larger physical size, and limited capabilities. These limitations emphasize the need for a miniaturized MR antenna that is small, flexible, versatile, and has a high-quality factor. By overcoming these limitations, an optimized MR antenna can greatly improve intravascular high-resolution MRI sensitivity, precision, and effectiveness.

Additionally, most studies on exterior coil arrays and multi-channel coil arrays cover the entire face for imaging. Despite its advantages, it also has limitations when imaging small areas with sub-millimeter dimensions and in dental imaging. A conventional surface coil often has limited sensitivity, which can result in insufficient high signal-to-noise ratio (SNR) for imaging sub-millimeter root canals. In addition, the extraoral surface coils are located approximately 30-50 mm from the teeth, allowing optimum sensitivity, but also generating a strong signal from the buccal fat and cheeks. Therefore, a fully flexible, lightweight, and compact coaxial intraoral antenna is required to be placed between the teeth and cheek to overcome the aforementioned limitations, and thus increase the SNR and sensitivity in the region of interest. Furthermore, medical devices such as intraoral antenna, intravascular antenna, and tattoos may have limitations when undergoing MRI because of the potential of the radiofrequency (RF) induced heating around the devices. The electromagnetic field distribution may change around the tattoos and other medical devices, and the amount of heating depending on various factors including resonance frequency, patient’s posture, and anatomy. While extensive research has been conducted to understand the RF-induced heating of medical implants, there is currently a lack of studies examining the interaction between tattoos and the RF field of an MRI. Therefore, it is necessary to investigate the effects of tattoos, considering different shapes, size, thickness, pigments, and ink at various field strength of MRI. This thesis focus on a miniature antenna for an intravascular device in the MRI system that offers a high-quality factor, SNR, precise visibility, orientation

with respect to the B0 field, low specific absorption rate, and the smallest volume is presented. The optimization of the antenna was carried out using both the finite element method and the finite difference time domain. Additionally, a fabricated prototype was integrated into an electrophysiological catheter model. The performance of the fabricated prototype was evaluated in a saline solution and heart to measure the reflection coefficient both in bent and flat conditions. The MR antenna exhibit satisfactory performance with a quality factor of 28 and SNR of 58, indicating optimum sensitivity and high-quality imaging. Furthermore, the effect of the metallic and non-metallic surfaces of the catheter on the proposed antenna is analyzed. The catheter-integrated MR antenna creates a homogeneous magnetic field and maintains persistent visibility of the catheter during MRI. The sum of B1 field strengths (ΣB1) and average B1 field in the region of interest was improved by approximately 61% and 12%, respectively. Finally, safety considerations were taken into account when analyzing the performance of the MR antenna. Next, a fully flexible coaxial dipole antenna is designed for intraoral applications at 3 T. The dipole design is characterized by its open distal end, which allows for a lack of restriction on the movement of the tongue, and also offers an increased depth of sensitivity that allows for dental roots. The finite element method and finite-difference time-domain simulations were used to optimize the antenna performance and identify the optimum gaps in the shield that ensure a uniform current distribution at the desired frequency band. Finally, a fabricated prototype was evaluated in minced pork to measure the reflection coefficient in open- and closed-mouth scenarios. The optimal configurations were determined with gaps on

both sides of the fully flexible dipole antenna at small and large curvatures. Furthermore, the implementation of the dipole antenna has led to improvement in the B+ 1 field homogeneity, resulting in higher transmit and specific absorption rate efficiency. Finally, safety considerations were considered when analyzing the performance of the intraoral antenna under MRI. A novel intraoral flexible antenna and its advantages are described to overcome the challenges associated with intraoral RF coils. Finally, the RF-induced heating of single and multiple tattoos was evaluated during magnetic resonance imaging (MRI) at 1.5 T and 3 T. Various tattoos of different shapes, positions, pigment, length, diameter, and gap between the tattoos was investigated. Finite-difference time-domain based electromagnetic and thermal simulations were performed to study the specific absorption rate (SAR) and temperature rise, respectively. The results indicated that tattoos influenced the induced electric field distribution and maximum magnitude of the SAR on the surface of the skin. A notable enhancement in the SAR were observed around the sharp edges, long strips, and circular loops of tattoos. Interestingly, the maximum local SAR and increase in tissue temperature strongly depend on the shape of the tattoo. Furthermore, the relative position and size of the tattoos affected RF-induced heating. The RF-induced heating of multiple tattoos were investigated considering the worst case scenarios. Our results confirm that RF-induced heating of multiple tattoos is quite different from that of single tattoo and does not follow a simple superposition of the results from a single tattoos. Moreover, the procedures presented in the simulation environment are used to facilitate RF-induced heating for patients with tattoos undergoing clinical MRI.