Radiofrequency Ablation, MR Thermometry, and High-Spatial-Resolution MR Parametric Imaging with a Single, Minimally Invasive Device

Abstract

Purpose To develop and demonstrate in vitro and in vivo, a single interventional MR-active device that integrates the functions of precise identification of a tissue site with the delivery of RF energy for ablation, high-resolution thermal mapping to monitor thermal dose, and with quantitative MRI relaxometry to document ablation-induced tissue changes for characterizing ablated tissue. Materials and Methods All animal studies were approved by our Institutional Animal Care and Use Committee. A loopless MRI antenna comprised of a tuned micro-cable either 0.8 or 2.2mm in diameter with an extended central conductor, was switched between a 3T MRI scanner and an RF power source, to monitor and perform RF ablation in bovine muscle and human artery samples in vitro, and in rabbits in vivo. High-resolution (250–300μm) proton resonance frequency shift MR thermometry was interleaved with ablations. Quantitative spin-lattice (T1) and spin-spin (T2) relaxation time MRI mapping was performed pre- and post-ablation, and compared with gross tissue examination of the region of ablated tissue, post-MRI. Results High-resolution MRI afforded temperature mapping in under 8s for monitoring ablation temperatures exceeding 85°C delivered by the same device, producing irreversible thermal injury and necrosis. Quantitative MRI relaxation time maps revealed up to a two-fold variation in mean regional T1 and T2 post-vs. pre-ablation. Conclusion A simple, integrated, minimally-invasive interventional probe that provides image-guided therapy delivery, thermal mapping of dose and the detection of ablation-associated MRI parametric changes was developed and demonstrated. This single-device approach avoided coupling-related safety concerns associated with multiple conductor approaches.

High‐resolution intravascular MRI‐guided perivascular ultrasound ablation

“…All MRI studies were performed on a Philips Achieva (Koninklijke Philips N.V., Amsterdam, Netherlands) 3T scanner with an in-room image monitor for interventional studies, used previously. [20][21][22][23][24] MRI-HIFU ablation catheters were comprised of a loopless IV-MRI antenna formed from an 0.8-mm outer diameter (OD), 0.88-m-long, biocompatible nitinol coaxial cable with a 42-mm whip tuned to resonate at 128 MHz in a lossy medium (0.35% saline). The antenna was deployed as a receiver coil which was turned-off during excitation by means of a positive-intrinsic-negative (PIN) diode in its matching circuit.…”

“…Postablation, thermal dose maps, measured in cumulative equivalent minutes at 43°C ("CEM 43 "), were derived by temporally integrating the dynamic real-time temperature maps 32,33 after de-noising and co-registering them with anatomical images. A CEM 43 of 340 has been assumed as a threshold for producing necrosis, 24,34,35 so the MRI-based lesion-dose-threshold CEM 43 = 340 contours were calculated, smoothed to generate continuous loops, and overlaid on co-registered photographs of lesions. The areas of the continuous CEM 43 = 340 loops were measured as a potential IV-MRI proxy for lesion size.…”

“…They potentially offer a precise, minimally invasive, diagnostic, and interventional imaging modality, at least within a range of a few centimeters from the probe. RFA using a single IV‐MRI probe for image guidance, energy delivery and thermometry for monitoring dose, has been demonstrated in tissue specimens and in vivo in a clinical 3T MRI scanner . However, the RFA lesions produced this way are limited primarily to the point of contact of the MRI‐sensitive portion of the catheter with the tissue, causing injury to the vessel wall in IV applications.…”

Radiofrequency applicator concepts for simultaneous MR imaging and hyperthermia treatment of glioblastoma multiforme

“…The invasiveness of this approach is a major road blocker for wider clinical application, thus the need for non-invasive thermal therapy solutions is obvious. Simultaneous RFheating and MR imaging at ultrahigh fields (B 0 =7.0T, f=298MHz) [3][4][5][6][7] has the potential of delivering and managing sufficiently large thermal dosage for hyperthermia of relatively large tumor areas. En route to a potential RFinduced hyperthermia treatment of glioblastoma in the human brain, this work focuses on electromagnetic field (EMF) simulations and provides realistic RF applicator designs tailored for simultaneous RF-heating and MRI.…”

Potentials, Limitations and Future Directions of MR Contrast Media in Ablation Therapies

“…The advantages of MR imaging arise from: the superior soft tissue contrast, noninvasiveness, lack of radiation, high spatial and temporal resolution, inherent 3D data acquisition with unlimited orientation, intrinsic contrast exploitable for tissue characterization, sensitivity to blood flow, temperature measurement, and the different ways in which MR signal produced and processed [10][11][12][13][14]. Recently investigators demonstrated that a single loopless antenna could be configured to acquire high-spatial-resolution MR images at 3T, locally deliver radiofrequency energy, monitor that delivery with MR thermometry, and assess the outcome of the ablation by means of relaxometry [15]. T 1 weighted imaging is useful for assessing structures that are high in fat or structures that are near water filled structures, such as joints, which otherwise would be hard to be seen on T 2 weighted imaging.…”