A fast MR-thermometry method for quantitative assessment of temperature increase near an implanted wire

Technical advances in motion-robust MR thermometry

Proton resonance frequency shift (PRFS) MR thermometry is the most common method used in clinical thermal treatments because of its fast acquisition and high sensitivity to temperature. However, motion is the biggest obstacle in PRFS MR thermometry for monitoring thermal treatment in moving organs. This challenge arises because of the introduction of phase errors into the PRFS calculation through multiple methods, such as image misregistration, susceptibility changes in the magnetic field, and intraframe motion during MRI acquisition. Various approaches for motion correction have been developed for real-time, motion-robust, and volumetric MR thermometry. However, current technologies have inherent trade-offs among volume coverage, processing time, and temperature accuracy. These tradeoffs should be considered and chosen according to the thermal treatment application. In hyperthermia treatment, precise temperature measurements are of increased importance rather than the requirement for exceedingly high temporal resolution. In contrast, ablation procedures require robust temporal resolution to accurately capture a rapid temperature rise. This paper presents a comprehensive review of current cutting-edge MRI techniques for motion-robust MR thermometry, and recommends which techniques are better suited for each thermal treatment. We expect that this study will help discern the selection of motion-robust MR thermometry strategies and inspire the development of motion-robust volumetric MR thermometry for practical use in clinics.

Remotely detuned receiver coil for high-resolution interventional cardiac magnetic resonance imaging

Introduction

Interventional cardiac MRI in the context of the treatment of cardiac arrhythmia requires submillimeter image resolution to precisely characterize the cardiac substrate and guide the catheter-based ablation procedure in real-time. Conventional MRI receiver coils positioned on the thorax provide insufficient signal-to-noise ratio (SNR) and spatial selectivity to satisfy these constraints.

Methods

A small circular MRI receiver coil was developed and evaluated under different experimental conditions, including high-resolution MRI anatomical and thermometric imaging at 1.5 T. From the perspective of developing a therapeutic MR-compatible catheter equipped with a receiver coil, we also propose alternative remote active detuning techniques of the receiver coil using one or two cables. Theoretical details are presented, as well as simulations and experimental validation.

Results

Anatomical images of the left ventricle at 170 µm in-plane resolution are provided on ex vivo beating heart from swine using a 2 cm circular receiver coil. Taking advantage of the increase of SNR at its vicinity (up to 35 fold compared to conventional receiver coils), real-time MR-temperature imaging can reach an uncertainty below 0.1°C at the submillimetric spatial resolution. Remote active detuning using two cables has similar decoupling efficiency to conventional on-site decoupling, at the cost of an acceptable decrease in the resulting SNR.

Discussion

This study shows the potential of small dimension surface coils for minimally invasive therapy of cardiac arrhythmia intraoperatively guided by MRI. The proposed remote decoupling approaches may simplify the construction process and reduce the cost of such single-use devices.

New Insights into MR Safety for Implantable Medical Devices

Over the last two decades, the status of MR safety has dramatically changed. In particular, ever since the MR-conditional cardiac device was approved by the Food and Drug Administration (FDA) in 2008 and by the Pharmaceuticals and Medical Devices Agency (PMDA) in 2012, the safety of patients with an implantable medical device (IMD) has been one of the most important issues in terms of MR use. In conjunction with the regulatory approvals for various IMDs, standards, technical specifications, and guidelines have also been rapidly created and developed. Many invaluable papers investigating and reviewing the history and status of MR use in the presence of IMDs already exist. As such, this review paper seeks to bridge the gap between clinical practice and the information that is obtained by standard-based tests and provided by an IMD's package insert or instructions for use. Interpretation of the gradient of the magnetic flux density intensity of the static magnetic field with respect to the magnetic displacement force is discussed, along with the physical background of RF field. The relationship between specific absorption rate (SAR) and B1+RMS, and their effects on image quality are described. In addition, insofar as providing new directions for future research and practice, the feasibility of safety test methods for RF-induced heating of IMDs using MR thermometry, evaluation of tissue heat damage, and challenges in cardiac IMDs will be discussed.