Practical implementation of robust MR-thermometry during clinical MR-guided microwave ablations in the liver at 1.5 T

Abstract

Practical non-invasive equipment modifications and effective acquisition methods to achieve robust and reliable real-time MR thermometry for monitoring of clinical hepatic microwave ablations were implemented. These included selection of the microwave generator location (inside versus outside the MR scan room), the number of radiofrequency chokes added to the microwave generator's coaxial lines, and the use of copper wool to maximize their electrical grounding. Signal-to-noise ratio (SNR) of MR thermometry images of a small fluid-filled phantom acquired during activation of microwave antenna were used to evaluate image quality as a function of each modification. SNR measurements corresponding to both locations of the microwave generator were comparable and so it was located outside the MR scan room. For this location, addition of one RF choke on the power and four chokes on the sensor coaxial lines was found to be optimal, corresponding to a 68% increase in SNR. Furthermore, image quality strongly depended on the proper electrical grounding of the power and sensor lines. SNR ratio (relative to SNR of baseline images) during activation of microwave generator was found to be 0.49 ± 0.28 without adequate grounding, and 0.88 ± 0.08 with adequate grounding (p = 0.002, Student's ttest). These SNR measurements were sufficiently sensitive to detect issues related to equipment performance and hence formed part of the quality assurance testing performed prior to each clinical treatment. Incorporating these non-invasive approaches resulted in significant improvements to image quality and, importantly while maintaining the clinical integrity of the microwave system which is of paramount importance in a highly regulated healthcare environment.

A review of antenna designs for percutaneous microwave ablation

“…Microwave ablation (MWA) is one of the energy-based minimally invasive modalities in the treatment of various tumors, such as liver tumors [1], lung tumors [2], renal tumors [3], and bone tumors [4]. In general, tumor ablation in MWA is achieved with one or more MW antennas inserted into the targeted tissue percutaneously under the guidance of an image-guided device [5][6][7][8][9][10]. High-frequency electromagnetic energy (commercially available at 915 MHz or 2.45 GHz) generated by a microwave power generator is transferred through a coaxial cable as an antenna into surrounding tissues to cause a rise in temperature to 60 • C or above, which leads to protein denaturation and coagulative necrosis [11][12][13][14][15][16][17].…”

Contactless Thermometry by MRI and MRS: Advanced Methods for Thermotherapy and Biomaterials

“…Robust thermometry of the liver is particularly challenging, in view of the target location deep inside the body. The practical implementation of some required modifications to equipment and workflow, taking into account the significant clinical constraints in the context of clinical MR-guided MW ablation, was presented recently (Gorny et al, 2019); see also Table S1. The most important modifications made concern the addition of RF filters (chokes) to the MW generator's coaxial lines and electrical grounding of the MW generator through copper wool.…”

Continuous cardiac thermometry via simultaneous catheter tracking and undersampled radial golden angle acquisition for radiofrequency ablation monitoring

“…In clinical applications, the gradient echo (GRE) sequence is widely used for MRTI due to its robustness to magnetic field inhomogeneity artifacts and its ease of use. On static organs, this sequence usually produces temperature maps with an in-plane spatial resolution around 1 mm 2 and a temporal resolution around 3 s 20 , 32 .…”