Radiofrequency safety modelling of parallel transmit magnetic resonance imaging

Radio-frequency (RF) safety plays an important role in Magnetic Resonance Imaging (MRI) with the goal of assuring patient safety and preventing excessive RF heating. Historically, RF safety in MRI has been monitored using pre-calculated specific absorption rate (SAR) limits which indicate the amount of energy that can be delivered safely to the subject. Electromagnetic (EM) simulations have usually been performed to assess the electric field and RF energy levels that are experienced by the subject. In order to validate these EM simulations, the distribution of radiofrequency (B1) field can be measured experimentally, and compared with simulation results. Also, the measurement of temperature in phantom and animal models has been used to assess RF heating directly and validate the EM simulations. Well-characterized phantom models are needed to assess RF heating experimentally. Meat phantoms can be used for RF heating tests, but these are difficult to store and challenging for their shape to be well described. Agar-gel phantoms can also be used for RF heating tests, in which one can control their dielectric properties with use of carefully controlled ingredients, such as NaCl and polyethylene powder to achieve specific conductivity and permittivity values. Once a phantom is made, the dielectric properties can be verified experimentally using an open-ended coaxial cable. A number of EM simulation methods have been proposed, which help one understand EM field phenomena in high-field MRI. But there are still many questions remaining which need to be further studied, including better characterization of thermal behaviours. In Chapter 3, three simulation methods are compared against experimental thermal elevation measures. A framework of validating EM simulations using Proton Resonance Frequency Shift (PRF) based MR thermometry in the context of parallel transmit (pTx) MRI is described in Chapter 4. Also described in this chapter is a 3D gradient-echo sequence used to monitor temperature-induced phase shifts, as well as the required image reconstruction techniques and field-stabilisation methods. The need for personalised SAR models has arisen quickly, especially in ultra-highfield pTx studies. Use of a safety margin of 1.5 (150%) to account for morphometric differences across the population has been reported previously. To reduce the need for overestimation, the development of a personalized SAR models using non-linear registration is described in Chapter 5 and assessed for robustness using three 'standard' models. Finally, the practical advantages of using the personalised model in pTx MRI are discussed in Chapter 6. With the aid of high computing capacity the personalised model approach could help MRI users reduce safety margins, so that the scanners can operate closer to the true SAR limits representing the subject in the scanner.</p

Radiofrequency safety modelling of parallel transmit magnetic resonance imaging

Radio-frequency (RF) safety plays an important role in Magnetic Resonance Imaging (MRI) with the goal of assuring patient safety and preventing excessive RF heating. Historically, RF safety in MRI has been monitored using pre-calculated specific absorption rate (SAR) limits which indicate the amount of energy that can be delivered safely to the subject. Electromagnetic (EM) simulations have usually been performed to assess the electric field and RF energy levels that are experienced by the subject. In order to validate these EM simulations, the distribution of radiofrequency (B1) field can be measured experimentally, and compared with simulation results. Also, the measurement of temperature in phantom and animal models has been used to assess RF heating directly and validate the EM simulations. Well-characterized phantom models are needed to assess RF heating experimentally. Meat phantoms can be used for RF heating tests, but these are difficult to store and challenging for their shape to be well described. Agar-gel phantoms can also be used for RF heating tests, in which one can control their dielectric properties with use of carefully controlled ingredients, such as NaCl and polyethylene powder to achieve specific conductivity and permittivity values. Once a phantom is made, the dielectric properties can be verified experimentally using an open-ended coaxial cable. A number of EM simulation methods have been proposed, which help one understand EM field phenomena in high-field MRI. But there are still many questions remaining which need to be further studied, including better characterization of thermal behaviours. In Chapter 3, three simulation methods are compared against experimental thermal elevation measures. A framework of validating EM simulations using Proton Resonance Frequency Shift (PRF) based MR thermometry in the context of parallel transmit (pTx) MRI is described in Chapter 4. Also described in this chapter is a 3D gradient-echo sequence used to monitor temperature-induced phase shifts, as well as the required image reconstruction techniques and field-stabilisation methods. The need for personalised SAR models has arisen quickly, especially in ultra-highfield pTx studies. Use of a safety margin of 1.5 (150%) to account for morphometric differences across the population has been reported previously. To reduce the need for overestimation, the development of a personalized SAR models using non-linear registration is described in Chapter 5 and assessed for robustness using three 'standard' models. Finally, the practical advantages of using the personalised model in pTx MRI are discussed in Chapter 6. With the aid of high computing capacity the personalised model approach could help MRI users reduce safety margins, so that the scanners can operate closer to the true SAR limits representing the subject in the scanner.</p

Additional data for "An investigation into the minimum number of tissue groups required for 7T in-silico parallel transmit electromagnetic safety simulations in the human head"

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Assessment of radio-frequency heating of a parallel transmit coil in a phantom using multi-echo proton resonance frequency shift thermometry

We propose a workflow for validating parallel transmission (pTx) radio-frequency (RF) magnetic field heating patterns using Proton-Resonance Frequency shift (PRF)-based MR thermometry. Electromagnetic (EM) and thermal simulations of a 7 T 8-channel dipole coil were done using commercially available software (Sim4Life) to assess RF heating. The fabrication method for a phantom with electrical properties matched to human tissue is also described, along with methods for its electrical and thermal characterisation. Energy was deposited to specific transmit channels, whilst acquiring 3D PRF data using a pair of interleaved RF shim transmit modes. A multi-echo readout and pre-scan stabilisation protocol were used for increased sensitivity and to correct for measurement-to-measurement instabilities. The electrical properties of the phantom were found to be within 10% of the intended values. Adoption of a 14-min stabilisation scan gave sufficient suppression of any evolving background spatial variation in the B0 field to achieve <0.001 °C/mm thermometry drift over 10 min of subsequent scanning. Using two RF shim transmit modes enabled full phantom coverage and combining multiple echo times enabled a 13-54% improvement in the RMSE sensitivity to temperature changes. Combining multiple echoes reduced the peak RMSE by 45% and visually reduced measurement-to-measurement instabilities. A reference fibre optic probe showed temperature deviations from the PRF-estimated temperature to be smaller than 0.5 °C. Given the importance of RF safety in pTx applications, this workflow enables accurate validation of RF heating simulations with minimal additional hardware requirements

DataSheet1_Short-pulsed micro-magnetic stimulation of the vagus nerve.pdf

Vagus nerve stimulation (VNS) is commonly used to treat drug-resistant epilepsy and depression. The therapeutic effect of VNS depends on stimulating the afferent vagal fibers. However, the vagus is a mixed nerve containing afferent and efferent fibers, and the stimulation of cardiac efferent fibers during VNS may produce a rare but severe risk of bradyarrhythmia. This side effect is challenging to mitigate since VNS, via electrical stimulation technology used in clinical practice, requires unique electrode design and pulse optimization for selective stimulation of only the afferent fibers. Here we describe a method of VNS using micro-magnetic stimulation (µMS), which may be an alternative technique to induce a focal stimulation, enabling a selective fiber stimulation. Micro-coils were implanted into the cervical vagus nerve in adult male Wistar rats. For comparison, the physiological responses were recorded continuously before, during, and after stimulation with arterial blood pressure (ABP), respiration rate (RR), and heart rate (HR). The electrical VNS caused a decrease in ABP, RR, and HR, whereas µM-VNS only caused a transient reduction in RR. The absence of an HR modulation indicated that µM-VNS might provide an alternative technology to VNS with fewer heart-related side effects, such as bradyarrhythmia. Numerical electromagnetic simulations helped estimate the optimal coil orientation with respect to the nerve to provide information on the electric field’s spatial distribution and strength. Furthermore, a transmission emission microscope provided very high-resolution images of the cervical vagus nerve in rats, which identified two different populations of nerve fibers categorized as large and small myelinated fibers.</p

The MotoNet: A 3 Tesla MRI-Conditional EEG Net with Embedded Motion Sensors

We introduce a new electroencephalogram (EEG) net, which will allow clinicians to monitor EEG while tracking head motion. Motion during MRI limits patient scans, especially of children with epilepsy. EEG is also severely affected by motion-induced noise, predominantly ballistocardiogram (BCG) noise due to the heartbeat. Methods: The MotoNet was built using polymer thick film (PTF) EEG leads and motion sensors on opposite sides in the same flex circuit. EEG/motion measurements were made with a standard commercial EEG acquisition system in a 3 Tesla (T) MRI. A Kalman filtering-based BCG correction tool was used to clean the EEG in healthy volunteers. Results: MRI safety studies in 3 T confirmed the maximum heating below 1 °C. Using an MRI sequence with spatial localization gradients only, the position of the head was linearly correlated with the average motion sensor output. Kalman filtering was shown to reduce the BCG noise and recover artifact-clean EEG. Conclusions: The MotoNet is an innovative EEG net design that co-locates 32 EEG electrodes with 32 motion sensors to improve both EEG and MRI signal quality. In combination with custom gradients, the position of the net can, in principle, be determined. In addition, the motion sensors can help reduce BCG noise

Aluminum Thin Film Nanostructure Traces in Pediatric EEG Net for MRI and CT Artifact Reduction

Magnetic resonance imaging (MRI) and continuous electroencephalogram (EEG) monitoring are essential in the clinical management of neonatal seizures. EEG electrodes, however, can significantly degrade the image quality of both MRI and CT due to substantial metallic artifacts and distortions. Thus, we developed a novel thin film trace EEG net (“NeoNet”) for improved MRI and CT image quality without compromising the EEG signal quality. The aluminum thin film traces were fabricated with an ultra-high-aspect ratio (up to 17,000:1, with dimensions 30 nm × 50.8 cm × 100 µm), resulting in a low density for reducing CT artifacts and a low conductivity for reducing MRI artifacts. We also used numerical simulation to investigate the effects of EEG nets on the B1 transmit field distortion in 3 T MRI. Specifically, the simulations predicted a 65% and 138% B1 transmit field distortion higher for the commercially available copper-based EEG net (“CuNet”, with and without current limiting resistors, respectively) than with NeoNet. Additionally, two board-certified neuroradiologists, blinded to the presence or absence of NeoNet, compared the image quality of MRI images obtained in an adult and two children with and without the NeoNet device and found no significant difference in the degree of artifact or image distortion. Additionally, the use of NeoNet did not cause either: (i) CT scan artifacts or (ii) impact the quality of EEG recording. Finally, MRI safety testing confirmed a maximum temperature rise associated with the NeoNet device in a child head-phantom to be 0.84 °C after 30 min of high-power scanning, which is within the acceptance criteria for the temperature for 1 h of normal operating mode scanning as per the FDA guidelines. Therefore, the proposed NeoNet device has the potential to allow for concurrent EEG acquisition and MRI or CT scanning without significant image artifacts, facilitating clinical care and EEG/fMRI pediatric research

Development, validation, and pilot MRI safety study of a high-resolution, open source, whole body pediatric numerical simulation model.

Numerical body models of children are used for designing medical devices, including but not limited to optical imaging, ultrasound, CT, EEG/MEG, and MRI. These models are used in many clinical and neuroscience research applications, such as radiation safety dosimetric studies and source localization. Although several such adult models have been reported, there are few reports of full-body pediatric models, and those described have several limitations. Some, for example, are either morphed from older children or do not have detailed segmentations. Here, we introduce a 29-month-old male whole-body native numerical model, "MARTIN", that includes 28 head and 86 body tissue compartments, segmented directly from the high spatial resolution MRI and CT images. An advanced auto-segmentation tool was used for the deep-brain structures, whereas 3D Slicer was used to segment the non-brain structures and to refine the segmentation for all of the tissue compartments. Our MARTIN model was developed and validated using three separate approaches, through an iterative process, as follows. First, the calculated volumes, weights, and dimensions of selected structures were adjusted and confirmed to be within 6% of the literature values for the 2-3-year-old age-range. Second, all structural segmentations were adjusted and confirmed by two experienced, sub-specialty certified neuro-radiologists, also through an interactive process. Third, an additional validation was performed with a Bloch simulator to create synthetic MR image from our MARTIN model and compare the image contrast of the resulting synthetic image with that of the original MRI data; this resulted in a "structural resemblance" index of 0.97. Finally, we used our model to perform pilot MRI safety simulations of an Active Implantable Medical Device (AIMD) using a commercially available software platform (Sim4Life), incorporating the latest International Standards Organization guidelines. This model will be made available on the Athinoula A. Martinos Center for Biomedical Imaging website

High-Frequency Pulsed Electric Field Ablation in Beagle Model for Treatment of Prostate Cancer

Conventional irreversible electroporation (IRE) with low-frequency pulsed electric field (LF-PEF) is used to induce cell death; however, it has several disadvantages including a long procedure time and severe muscle contraction due to high-voltage electric field. This study investigates a novel IRE protocol with high-frequency pulsed electric field (HF-PEF) of 500 Hz repetition to ablate the prostate tissue in beagles for treatment of prostate cancer. A finite element analysis was performed to validate optimal electrical field strength for the procedure. In total, 12 beagles received HF-PEF of 500 Hz and were sacrificed at 4 h, 4 days, and 28 days (3 each). The remaining three beagles underwent sham procedure. The outcomes of HF-PEF were assessed by histological responses. HF-PEF successfully decellularized the prostate tissues 4 h after the treatment. The prostate glands, duct, and urethra were well preserved after IRE with HF-PEF. The ablated prostatic tissues were gradually regenerated and appeared similar to the original tissues 28 d after IRE with HF-PEF. Moreover, electrocardiography and hematology demonstrated that IRE with HF-PEF did not seriously affect the cardiac tissue. HF-PEF was effective and safe in the beagle prostate and effectively induced the ablation and gradually recovered with cellular regeneration