PURPOSE Patients with active implants such as deep brain stimulation (DBS) devices are often denied access to MRI due to safety concerns associated with the radiofrequency (RF) heating of their electrodes. The majority of studies on RF heating of conductive implants have been performed in horizontal close-bore MRI scanners. Vertical MRI scanners which have a 90° rotated transmit coil generate fundamentally different electric and magnetic field distributions, yet very little is known about RF heating of implants in this class of scanners. We performed numerical simulations as well as phantom experiments to compare RF heating of DBS implants in a 1.2T vertical scanner (OASIS, Hitachi) compared to a 1.5T horizontal scanner (Aera, Siemens). METHODS Simulations were performed on 90 lead models created from post-operative CT images of patients with DBS implants. Experiments were performed with wires and commercial DBS devices implanted in an anthropomorphic phantom. RESULTS We found significant reduction of 0.1 g-averaged specific absorption rate (30-fold, P < 1 × 10-5 ) and RF heating (9-fold, P < .026) in the 1.2T vertical scanner compared to the 1.5T conventional scanner. CONCLUSION Vertical MRI scanners appear to generate lower RF heating around DBS leads, providing potentially heightened safety or the flexibility to use sequences with higher power levels than on conventional systems.

Vertical open-bore MRI scanners generate significantly less radiofrequency heating around implanted leads: A study of deep brain stimulation implants in 1.2T OASIS scanners versus 1.5T horizontal systems.

Ultra-high field MRI at 7 T can produce much better visualization of sub-cortical structures compared to lower field, which can greatly help target verification as well as overall treatment monitoring for patients with deep brain stimulation (DBS) implants. However, use of 7 T MRI for such patients is currently contra-indicated by guidelines from the device manufacturers due to the safety issues. The aim of this study was to provide an assessment of safety and image quality of ultra-high field magnetic resonance imaging at 7 T in patients with deep brain stimulation implants. We performed experiments with both lead-only and complete DBS systems implanted in anthropomorphic phantoms. RF heating was measured for 43 unique patient-derived device configurations. Magnetic force measurements were performed according to ASTM F2052 test method, and device integrity was assessed before and after experiments. Finally, we assessed electrode artifact in a cadaveric brain implanted with an isolated DBS lead. RF heating remained below 2°C, similar to a fever, with the 95% confidence interval between 0.38°C-0.52°C. Magnetic forces were well below forces imposed by gravity, and thus not a source of concern. No device malfunctioning was observed due to interference from MRI fields. Electrode artifact was most noticeable on MPRAGE and T2*GRE sequences, while it was minimized on T2-TSE images. Our work provides the safety assessment of ultra-high field MRI at 7 T in patients with DBS implants. Our results suggest that 7 T MRI may be performed safely in patients with DBS implants for specific implant models and MRI hardware.

/pdf/safety-and-image-quality-at-7t-mri-for-deep-brain-3abiwwczv1.pdf

Safety and image quality at 7T MRI for deep brain stimulation systems: Ex vivo study with lead-only and full-systems.

Interaction of an active electronic implant such as a deep brain stimulation (DBS) system and magnetic resonance imaging (MRI) radiofrequency (RF) fields can induce excessive tissue heating, limiting MRI accessibility. Efforts to quantify RF heating mostly rely on electromagnetic (EM) simulations to assess individualized specific absorption rate (SAR), but such simulations require extensive computational resources. Here, we investigate if a predictive model using machine learning (ML) can predict the local SAR in the tissue around tips of implanted leads from the distribution of the tangential component of the MRI incident electric field, E tan. A dataset of 260 unique patient-derived and artificial DBS lead trajectories was constructed, and the 1 g-averaged SAR, 1 g SARmax, at the lead-tip during 1.5 T MRI was determined by EM simulations. E tan values along each lead's trajectory and the simulated SAR values were used to train and test the ML algorithm. The resulting predictions of the ML algorithm indicated that the distribution of E tan could effectively predict 1 g SARmax at the DBS lead-tip ( R = 0.82). Our results indicate that ML has the potential to provide a fast method for predicting MR-induced power absorption in the tissue around tips of implanted leads such as those in active electronic medical devices.

Machine Learning-Based Prediction of MRI-Induced Power Absorption in the Tissue in Patients With Simplified Deep Brain Stimulation Lead Models

One of the major issues in the surgical treatment of gliomas is the concern about maximizing the extent of resection while minimizing neurological impairment. Thus, surgical planning by carefully observing the relationship between the glioma infiltration area and eloquent area of the connecting fibers is crucial. Neurosurgeons usually detect an eloquent area by functional MRI and identify a connecting fiber by diffusion tensor imaging. However, during surgery, the accuracy of neuronavigation can be decreased due to brain shift, but the positional information may be updated by intraoperative MRI and the next steps can be planned accordingly. In addition, various intraoperative modalities may be used to guide surgery, including neurophysiological monitoring that provides real-time information (e.g., awake surgery, motor-evoked potentials, and sensory evoked potential); photodynamic diagnosis, which can identify high-grade glioma cells; and other imaging techniques that provide anatomical information during the surgery. In this review, we present the historical and current context of the intraoperative MRI and some related approaches for an audience active in the technical, clinical, and research areas of radiology, as well as mention important aspects regarding safety and types of devices. 

/pdf/intraoperative-mr-imaging-during-glioma-resection-3fxhw2x5.pdf

Intraoperative MR Imaging during Glioma Resection

The majority of studies that assess magnetic resonance imaging (MRI) induced radiofrequency (RF) heating of the tissue when active electronic implants are present have been performed in horizontal, closed-bore MRI systems. Vertical, open-bore MRI systems have a 90° rotated magnet and a fundamentally different RF coil geometry, thus generating a substantially different RF field distribution inside the body. Little is known about the RF heating of elongated implants such as deep brain stimulation (DBS) devices in this class of scanners. Here, we conducted the first large-scale experimental study investigating whether RF heating was significantly different in a 1.2 T vertical field MRI scanner (Oasis, Fujifilm Healthcare) compared to a 1.5 T horizontal field MRI scanner (Aera, Siemens Healthineers). A commercial DBS device mimicking 30 realistic patient-derived lead trajectories extracted from postoperative computed tomography images of patients who underwent DBS surgery at our institution was implanted in a multi-material, anthropomorphic phantom. RF heating around the DBS lead was measured during four minutes of high-SAR RF exposure. Additionally, we performed electromagnetic simulations with leads of various internal structures to examine this effect on RF heating. When controlling for RMS B1+, the temperature increase around the DBS lead-tip was significantly lower in the vertical scanner compared to the horizontal scanner (0.33 ± 0.24°C vs. 4.19 ± 2.29°C). Electromagnetic simulations demonstrated up to a 17-fold reduction in the maximum of 0.1g-averaged SAR in the tissue surrounding the lead-tip in the vertical scanner compared to the horizontal scanner. Results were consistent across leads with straight and helical internal wires. Radiofrequency heating and power deposition around the DBS lead-tip were substantially lower in the 1.2 T vertical scanner compared to the 1.5 T horizontal scanner. Simulations with different lead structures suggest that the results may extend to leads from other manufacturers.

A comparative study of RF heating of deep brain stimulation devices in vertical vs. horizontal MRI systems

Patients with deep brain stimulation (DBS) implants are often denied access to magnetic resonance imaging (MRI) due to safety concerns associated with RF heating of implants. Although MR-conditional DBS devices are available, complying with manufacturer guidelines has proved to be difficult as pulse sequences that optimally visualize DBS target structures tend to have much higher specific absorption rate (SAR) of radiofrequency energy than current guidelines allow. The MR-labeling of DBS devices, as well as the majority of studies on RF heating of conductive implants have been limited to horizontal close-bore MRI scanners. Vertical MRI scanners, originally introduced as open low-field MRI systems, are now available at 1.2 T field strength, capable of high-resolution structural and functional imaging. No literature exists on DBS SAR in this class of scanners which have a 90° rotated transmit coil and thus, generate a fundamentally different electric and magnetic field distributions. Here we present a simulation study of RF heating in a cohort of forty patient-derived DBS lead models during MRI in a commercially available vertical openbore MRI system (1.2 T OASIS, Hitachi) and a standard horizontal 1.5 T birdcage coil. Simulations were performed at two major imaging landmarks representing head and chest imaging. We calculated the maximum of 0.1g-averaged SAR (0.1g-SAR Max ) around DBS lead tips when a B 1 + = 4 µT was generated on an axial plane passing through patients body. For head landmark, 0.1g-SAR Max reached 220±188 W/kg in the 1.5 T birdcage coil, but only 14±11 W/kg in the OASIS coil. For chest landmark, 0.1g-SAR Max was 24±17 W/kg in the 1.5 T birdcage coil and 3±2 W/kg in the OASIS coil. A paired two-tail t-test revealed a significant reduction in SAR with a large effect-size during head MRI (p < 1.5×10-8, Cohen’s d = 1.5) as well as chest MRI (p < 6.5×10-10, Cohen’s d = 1.7) in 1.2 T Hitachi OASIS coil compared to a standard 1.5 T birdcage transmitter. Our findings suggest that open-bore vertical scanners may offer an untapped opportunity for MRI of patients with DBS implants.

/pdf/rf-heating-of-deep-brain-stimulation-implants-during-mri-in-3m5fd96lrl.pdf

RF heating of deep brain stimulation implants during MRI in 1.2 T vertical scanners versus 1.5 T horizontal systems: A simulation study with realistic lead configurations

Purpose: To evaluate safety of MRI in patients with fragmented retained leads (FRLs) through numerical simulation and phantom experiments. 
Methods: Electromagnetic and thermal simulations were performed to determine the worst-case RF heating of 10 patient-derived FRL models during MRI at 1.5 T and 3 T and at imaging landmarks corresponding to head, chest and abdomen. RF heating measurements were performed in phantoms implanted with reconstructed FRL models that produced highest heating in numerical simulations. The potential for unintended tissue stimulation was assessed through a conservative estimation of the electric field induced in the tissue due to gradient-induced voltages developed along the length of FRLs. 
Results: In simulations under conservative approach, RF exposure at B1+<2 uT generated CEM43<40 at all imaging landmarks at both 1.5 T and 3 T indicating no thermal damage for acquisition times (TA) <10 minutes. In experiments, the maximum temperature rise when FRLs were positioned at the location of maximum electric field exposure was measured to be 2.4 degrees Celsius at 3 T and 2.1 degrees Celsius at 1.5 T. Electric fields induced in the tissue due to gradient-induced voltages remained below the threshold for cardiac tissue stimulation in all cases. 
Conclusions: Simulation and experimental results indicate that patients with FRLs can be scanned safely at both 1.5 T and 3 T with majority of clinical pulse sequences.

Safety of magnetic resonance imaging in patients with retained cardiac leads.

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Vertical open-bore MRI scanners generate significantly less radiofrequency heating around implanted leads: A study of deep brain stimulation implants in 1.2T OASIS scanners versus 1.5T horizontal systems.

RF heating of deep brain stimulation implants during MRI in 1.2 T vertical scanners versus 1.5 T horizontal systems: A simulation study with realistic lead configurations

Safety of magnetic resonance imaging in patients with retained cardiac leads.