Electrophysiological Cardiovascular Magnetic Resonance (EP-CMR)-Guided Interventional Procedures: Challenges and Opportunities.

PURPOSE OF REVIEW: Cardiovascular magnetic resonance (CMR) imaging excels in providing detailed three-dimensional anatomical information together with excellent soft tissue contrast and has already become a valuable tool for diagnostic evaluation, electrophysiological procedure (EP) planning, and therapeutical stratification of atrial or ventricular rhythm disorders. CMR-based identification of ablation targets may significantly impact existing concepts of interventional electrophysiology. In order to exploit the inherent advantages of CMR imaging to the fullest, CMR-guided ablation procedures (EP-CMR) are justly considered the ultimate goal. RECENT FINDINGS: Electrophysiological cardiovascular magnetic resonance (EP-CMR) interventional procedures have more recently been introduced to the CMR armamentarium: in a single-center series of 30 patients, an EP-CMR guided ablation success of 93% has been reported, which is comparable to conventional ablation outcomes for typical atrial flutter and procedure and ablation time were also reported to be comparable. However, moving on from already established workflows for the ablation of typical atrial flutter in the interventional CMR environment to treatment of more complex ventricular arrhythmias calls for technical advances regarding development of catheters, sheaths and CMR-compatible defibrillator equipment. CMR imaging has already become an important diagnostic tool in the standard clinical assessment of cardiac arrhythmias. Previous studies have demonstrated the feasibility and safety of performing electrophysiological interventional procedures within the CMR environment and fully CMR-guided ablation of typical atrial flutter can be implemented as a routine procedure in experienced centers. Building upon established workflows, the market release of new, CMR-compatible interventional devices may finally enable targeting ventricular arrhythmias.

Access to MRI in Patients With Cardiac Implantable Electronic Devices is Variable and an Issue in Australia.

AIMS: This study aimed to characterise the level of access to magnetic resonance imaging (MRI) in Australian hospitals for patients with MR-conditional and non-MR-conditional cardiac implantable electronic devices (CIED), and to identify any barriers impeding this access. METHODS: All Australian Tertiary Referral Public Hospitals (n=38) were surveyed with a mixed qualitative and quantitative questionnaire. Provision of MRI to patients with MR-conditional and non-MR-conditional CIEDs; patient monitoring strategies during scan and personnel in attendance; barriers impeding MRI access. RESULTS: Of the 35 (92%) hospitals that completed the survey, a majority (85.7%) scan MR-conditional CIEDs, while a minority (8.6%) scan non-MR-conditional CIEDs. MR-conditional device scanning is often limited to non-pacing dependent patients, excluding implantable cardioverter-defibrillators. In total, 21% of sites exclude thoracic MR scans for CIED patients. Although most centres scan on 1.5 Tesla (T) machines (59%), 10% scan at 3T and 31% scan at both strengths. Sites vary in patient monitoring strategies and personnel in attendance; 80% require staff with Advanced Cardiac Life Support to be present. Barriers to service expansion include an absence of national guidelines, formal training, and logistical device support. CONCLUSIONS: Most surveyed Australian hospitals offer MRI for patients with MR-conditional CIEDs, however many still have exclusions for particular patient groups or scan requests. Only three surveyed sites offer MRI for patients with non-MR-conditional CIEDs in Australia. A national effort is needed to address the identified barriers including the development of national guidelines, formal training, and logistical support.

Development of a novel MR-conditional microwave needle for MR-guided interventional microwave ablation at 1.5T.

PURPOSE: To develop an MR-conditional microwave needle that generates a spherical ablation zone and clear MRI visibility for MR-guided microwave ablation. METHODS: An MR-conditional microwave needle consisting of zirconia tip and TA18 titanium alloy tube was investigated. The numerical model was created to optimize the needle's geometry and analyze its performance. A geometrically optimized needle was produced using non-magnetic materials based on the electromagnetics simulation results. The needle's mechanical properties were tested per the Chinese pharmaceutical industry standard YY0899-2013. The MRI visibility performance and ablation characteristics of the needle was tested both in vitro (phantom) and in vivo (rabbit) at 1.5T. The RF-induced heating was evaluated in ex vivo porcine liver. RESULTS: The needle's mechanical properties met the specified requirements. The needle susceptibility artifact was clearly visible both in vitro and in vivo. The needle artifact diameter (A) was small in in vivo (Ashaft: 4.96 ± 0.18 mm for T1W-FLASH, 3.13 ± 0.05 mm for T2-weighted fast spin-echo (T2W-FSE); Atip: 2.31 ± 0.09 mm for T1W-FLASH, 2.29 ± 0.08 mm for T2W-FSE; tip location error [TLE]: -0.94 ± 0.07 mm for T1W-FLASH, -1.10 ± 0.09 mm for T2W-FSE). Ablation zones generated by the needle were nearly spherical with an elliptical aspect ratio ranging from 0.79 to 0.90 at 30 W, 50 W for 3, 5, 10 min duration ex vivo ablations and 0.86 at 30 W for 10 min duration in vivo ablations. CONCLUSION: The designed MR-conditional microwave needle offers excellent mechanical properties, reliable MRI visibility, insignificant RF-induced heating, and a sufficiently spherical ablation zone. Further clinical development of MR-guided microwave ablation appears warranted.

Evaluation of a Workflow to Define Low Specific Absorption Rate MRI Protocols for Patients With Active Implantable Medical Devices.

BACKGROUND: MRI exams for patients with MR-conditional active implantable medical devices (AIMDs) are contraindicated unless specific conditions are met. This limits the maximum specific absorption rate (SAR, W/kg). Currently, there is no general framework to guide meeting a lower SAR limit. PURPOSE: To design and evaluate a workflow for modifying MRI protocols to whole-body SAR (WB-SAR ≤0.1 W/kg) and local-head SAR (LH-SAR ≤0.3 W/kg) limits while mitigating the impact on image quality and exam time. STUDY TYPE: Prospective. POPULATION: Twenty healthy volunteers on head (n = 5), C-spine (n = 5), T-spine (n = 5), and L-spine (n = 5) with IRB consent. ASSESSMENT: Vendor-provided head, C-spine, T-spine, and L-spine protocols (SARRT ) were modified to meet both low SAR targets (SARLOW ) using the proposed workflow. in vitro SNR and CNR were evaluated with a T1 -T2 phantom. in vivo image quality and clinical acceptability were scored using a 5-point Likert scale for two blinded readers. FIELD STRENGTH/SEQUENCES: 1.5T/spin-echoes, gradient-echoes. STATISTICAL ANALYSIS: In vitro SNR and CNR values were evaluated with a repeated measures general linear model. in vivo image quality and clinical acceptability were evaluated using a generalized estimating equation analysis (GEE). The two reader's level of agreement was analyzed using Cohen's kappa statistical analysis. RESULTS: Using the workflow, SAR limits were met. LH-SAR: 0.12 ± 0.02 W/kg, median (SD) values for LH-SAR were 0.12 (0.02) W/kg and WB-SAR: 0.09 (0.01) W/kg. Examination time did not increase ≤2x the initial time. SARRT SNR values were higher and significantly different than SARLOW (P < 0.05). However, no significant difference was observed between the CNR values (value = 0.21). Median (IQR) CNR values were 14.2 (25.0) vs. 15.1 (9.2) for head, 12.1 (16.9) vs. 25.3 (14.2) for C-spine, 81.6 (70.1) vs. 71.0 (26.6) for T-spine, and 51.4 (52.6) vs. 37.7 (27.3) for L-spine. Image quality scores were not significantly different between SARRT and SARLOW (median [SD] scores were 4.0 [0.01] vs. 4.3 [0.2], P > 0.05). DATA CONCLUSION: The proposed workflow provides guidance for modifying routine MRI exams to achieve low SAR limits. This can benefit patients referred for an MRI exam with low SAR MR-conditional AIMDs. LEVEL OF EVIDENCE: 1 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2020;52:91-102.

Reducing RF-induced Heating near Implanted Leads through High-Dielectric Capacitive Bleeding of Current (CBLOC).

Patients with implanted medical devices such as deep brain stimulation or spinal cord stimulation are often unable to receive magnetic resonance imaging (MRI). This is because once the device is within the radiofrequency (RF) field of the MRI scanner, electrically conductive leads act as antenna, amplifying the RF energy deposition in the tissue and causing possible excessive tissue heating. Here we propose a novel concept in lead design in which 40cm lead wires are coated with a ~1.2mm layer of high dielectric constant material (155 < ε r < 250) embedded in a weakly conductive insulation (σ = 20S/m). The technique called High-Dielectric Capacitive Bleeding of Current, or CBLOC, works by forming a distributed capacitance along the lengths of the lead, efficiently dissipating RF energy before it reaches the exposed tip. Measurements during RF exposure at 64 MHz and 123 MHz demonstrated that CBLOC leads generated 20-fold less heating at 1.5 T, and 40-fold less heating at 3 T compared to control leads. Numerical simulations of RF exposure at 297 MHz (7T) predicted a 15-fold reduction in specific absorption rate (SAR) of RF energy around the tip of CBLOC leads compared to control leads.

[Pacemaker and MRI in clinical practice]. / Herzschrittmacher und MRT im klinischen Alltag.

The number of cardiac pacemaker wearers is continuously increasing in Germany as well as worldwide. The probability of indications for a magnetic resonance imaging (MRI) examination during the lifetime is approximately 50-75% for every person. An MRI examination is nowadays possible for pacemaker wearers under certain conditions. Due to the technical developments during the last 10 years certain MRI-conditional pacemakers are available. The recommendations of the German and American medical specialist societies currently allow an MRI examination in patients with conventional pacemakers beyond the approval conditions (off-label use) under prespecified conditions, based on the study data. This article summarizes the information on conditions of use and reprogramming strategies as well as on the study situation for the clinical routine.

State-of-the-art approach towards magnetic resonance imaging of the nervous system structures in patients with cardiac implantable electronic devices.

INTRODUCTION: MRI generated forces are the source of potential complications in patients with cardiac implantable electronic devices (CIED). The technological progress, and growing clinical evidence concerning the operation of the contemporary MR non-conditional CIEDs during MRI, have started to significantly change our every-day clinical practice. Nevertheless, a lot of patients who could have an MRI performed safely, still have been refused the examination. STATE-OF-THE-ART: In many clinical situations, an MRI examination in a patient with a CIED is reasonable, and is linked to a negligible risk of complications if performed under strict precautions. The MagnaSave Registry that evaluated the influence of nonthoracic MRI on the function of MR non-conditional CIEDs, and numerous studies involving thoracic and non-thoracic MRIs in patients with legacy CIEDs, have confirmed the feasibility and safety of such examinations. In this article, practical tips aimed towards improving the safety of MRI in MR conditional and non-conditional CIED patients are largely based on the very recently released (2017) HRS expert consensus statement. CLINICAL IMPLICATIONS: Clinical data emphasize the necessity of making the MRI more accessible to CIED patients, also in the case of MR non-conditional systems or when the thorax MR imaging is clinically reasonable. This goal should be achieved by increasing the number of centers complying with respective recommendations and applying protocols that would guarantee the highest safety level. FUTURE DIRECTIONS: Further studies are warranted to assess safety issues related to the main current contraindication to MRI, i.e., the presence of abandoned leads.

Provision of magnetic resonance imaging for patients with 'MR-conditional' cardiac implantable electronic devices: an unmet clinical need.

AIMS: Increasing need for magnetic resonance imaging (MRI) has driven the development of MR-conditional cardiac implantable electronic devices (CIEDs; pacemakers and defibrillators); however, patients still report difficulties obtaining scans. We sought to establish current provision for MRI scanning of patients with CIEDs in England. METHODS AND RESULTS: A survey was distributed to all hospitals in England with MRI, to assess current practice. Information requested included whether hospitals currently offer MRI to this patient group, the number and type of scans acquired, local safety considerations, complications experienced and perceived obstacles to service provision in those departments not currently offering it. Responses were received from 195 of 227 (86%) of hospitals surveyed. Although 98% of departments were aware of MR-conditional devices, only 46% (n = 89) currently offer MRI scans to patients with CIED's; of these, 85% of departments perform ≤10 scans per year. No major complications were reported from MRI scanning in patients with MR-conditional devices. Current barriers to service expansion include perceived concerns regarding potential risk, lack of training, logistical difficulties, and lack of cardiology support. CONCLUSION: Provision of MRI for patients with CIEDs is currently poor, despite increasing numbers of patients with MR-conditional devices and extremely low reported complication rates.

Clinical safety and performance of a MRI conditional pacing system in patients undergoing cardiac MRI.

BACKGROUND: Utilization of cardiac magnetic resonance imaging (cMRI) as an imaging modality in clinical practice is rapidly increasing. More evidence from randomized studies establishing clinical safety and performance of pacing systems in patients undergoing a cMRI scan is needed. OBJECTIVES: The purpose of this prospective, multicenter, randomized study was to demonstrate safety and efficacy of the Accent MRI™ conditional pacing systems (St. Jude Medical, St. Paul, MN, USA) in patients undergoing cMRI scan. METHODS: Patients (n  =  283) indicated for dual-chamber pacemaker implant were randomized to either the MRI Group (MG) (n  =  140) or the Control Group (CG) (n  =  143) after successful device implantation of the Accent MRI™ system. Clinical evaluation and device interrogation were performed at pre- and post-MRI scan, and 1 month post-MRI for all patients. At 9-12 weeks postimplant, patients in MG underwent a nondiagnostic cMRI scan at 1.5 Tesla (T), while patients in CG underwent device interrogation and clinical evaluation twice with a 45-minute waiting period in between. The safety endpoint was freedom from MRI scan-related complications and that for efficacy was significant changes in right atrial/ventricular capture threshold and sensing amplitude between right before MRI, immediately after MRI, and 1 month post-MRI. RESULTS: Results showed 100% freedom from MRI scan-related complications. There were no significant changes in device performance between pre-MRI scan and 1 month post-MRI scan time points in both study groups. CONCLUSIONS: cMRI scanning with 1.5 T scanners is safe in patients implanted with the Accent MRI™ conditional pacing system and has no significant effect on the electrical parameters of the device and leads.

Characterization and Control of a Pneumatic Motor for MR-conditional Robotic Applications.

Magnetic Resonance (MR) guided interventional robots have recently been developed for a variety of surgeries, such as biopsy, ablation, and brachytherapy. The actuators and encoders that power and track such robots must be MR-conditional. In this paper, we propose an MR-conditional pneumatic motor with an integrated and custom-built fiber-optical encoder that provides powerful and accurate actuation. The motor is coupled with a modular plastic gearbox that provides a variety of gear ratio options so that the motor can be adapted to application requirements. With a 100:1 gear reduction at 0.55 MPa, the motor achieves 460 mNm stall torque and 370 rpm no-load speed, which leads to the peak output power of 6W. The motor has the bandwidth of approximately 1.1 Hz and 3.5 Hz when connected to 8 m and 0.2 m air hoses, respectively. The motor was tested in a 3T MRI scanner. No image artifact was observed and maximum signal to noise ratio (SNR) variation was less than 5%. Different from most of the existing MR-conditional pneumatic actuators, the proposed motor shape is more like the traditional electric motors, which offers more flexibility in the MR-conditional robot design.