Numerically simulated exposure of children and adults to pulsed gradient fields in MRI

PurposeTo determine exposure to gradient switching fields of adults and children in a magnetic resonance imaging (MRI) scanner by evaluating internal electric fields within realistic models of adult male, adult female, and child inside transverse and longitudinal gradient coils, and to compare these results with compliance guidelines. Materials and MethodsPatients inside x-, y-, and z-gradient coils were simulated using anatomically realistic models of adult male, adult female, and child. The induced electric fields were computed for 1 kHz sinusoidal current with a magnitude of 1 A in the gradient coils. Rheobase electric fields were then calculated and compared to the International Commission on Non-Ionizing Radiation Protection (ICNIRP) 2004 and International Electrotechnical Commission (IEC) 2010 guidelines. The effect of the human body, coil type, and skin conductivity on the induced electric field was also investigated. ResultsThe internal electric fields are within the first level controlled operating mode of the guidelines and range from 2.7V m(-1) to 4.5V m(-1), except for the adult male inside the y-gradient coil (induced field reaches 5.4V m(-1)).The induced electric field is sensitive to the coil type (electric field in the skin of adult male: 4V m(-1), 4.6V m(-1), and 3.8V m(-1) for x-, y-, and z-gradient coils, respectively), the human body model (electric field in the skin inside y-gradient coil: 4.6V m(-1), 4.2V m(-1), and 3V m(-1) for adult male, adult female, and child, respectively), and the skin conductivity (electric field 2.35-4.29% higher for 0.1S m(-1) skin conductivity compared to 0.2S m(-1)). ConclusionThe y-gradient coil induced the largest fields in the patients. The highest levels of internal electric fields occurred for the adult male model. J. Magn. Reson. Imaging 2016;44:1360-1367

A durable gain in motor and non-motor symptoms of Parkinson’s Disease following repeated caloric vestibular stimulation: A single-case study

Objective: To gain ‘first-in-man’ evidence that repeated caloric vestibular stimulation (CVS), a non-invasive form of neuro-modulation, can induce a lasting and clinically-relevant reduction in Parkinson’s Disease (PD) symptoms. Methods: A 70yr old male, diagnosed with PD 7 years prior to study enrolment, self-administered CVS at home 2x20 minutes per day for three months using a solid-state portable device. Standardised neuropsychological assessments of motor, cognitive, affective and independent function were carried out prior to stimulation, at the start and end of the sham (month 1) and active (months 2-3) phases, and 5 months post-stimulation. Results: Relative to the pre-stimulation baseline, behavioural improvements that exceeded the minimal detectable change were observed on the EQ5D, Unified Parkinson’s Disease Rating Scale, Schwab and England scale, 2 minute walk, Timed up and go, Non-motor symptom assessment scale for PD, Montreal cognitive assessment, Hospital depression scale and Epworth sleepiness scale. The level of change exceeded the threshold for a minimal clinically important difference on all scales for which a threshold has been published. By contrast, little improvement was seen during the sham (i.e. placebo) phase. Conclusion: Caloric vestibular stimulation may offer a novel, home-based method of relieving everyday symptoms of PD, and merits further evaluative study

Simulated design strategies for SPECT collimators to reduce the eddy currents induced by MRI gradient fields

Combining single photon emission computed tomography (SPECT) with magnetic resonance imaging (MRI) requires the insertion of highly conductive SPECT collimators inside the MRI scanner, resulting in an induced eddy current disturbing the combined system. We reduced the eddy currents due to the insert of a novel tungsten collimator inside transverse and longitudinal gradient coils. The collimator was produced with metal additive manufacturing, that is part of a microSPECT insert for a preclinical SPECT/MRI scanner. We characterized the induced magnetic field due to the gradient field and adapted the collimators to reduce the induced eddy currents. We modeled the x-, y-, and z-gradient coil and the different collimator designs and simulated them with FEKO, a three-dimensional method of moments / finite element methods (MoM/FEM) full-wave simulation tool. We used a time analysis approach to generate the pulsed magnetic field gradient. Simulation results show that the maximum induced field can be reduced by 50.82% in the final design bringing the maximum induced magnetic field to less than 2% of the applied gradient for all the gradient coils. The numerical model was validated with measurements and was proposed as a tool for studying the effect of a SPECT collimator within the MRI gradient coils

Computational modeling of a single-element transcranial focused ultrasound transducer for subthalamic nucleus stimulation

Objective. While transcranial focused ultrasound is a very promising neuromodulation technique for its non-invasiveness and high spatial resolution, its application to the human deep brain regions such as the subthalamic nucleus (STN) is relatively new. The objective of this study is to design a simple ultrasound transducer and study the transcranial wave propagation through a highly realistic human head model. The effects of skull morphology and skull and brain tissue properties on the focusing performance and energy deposition must therefore be known. Approach. A full-wave finite-difference time-domain simulation platform was used to design and simulate ultrasound radiation from a single-element focused transducer (SEFT) to the STN. Simulations were performed using the state-of-the-art Multimodal Imaging-based and highly Detailed Anatomical (MIDA) head model. In addition, the impact of changes in sound speed, density, and tissue attenuation coefficients were assessed through a sensitivity analysis. Main results. A SEFT model was designed to deliver an intensity of around 100 W m(-2) to the STN region; 20% of the STN volume was sonicated with at least half of the maximum of the peak intensity and it was predicted that 61.5% of the volume of the beam (above half of the peak intensity) falls inside the STN region. The sensitivity analysis showed that the skull's sound speed is the most influential acoustic parameter, which must be known with less than 1.2% error to obtain an acceptable accuracy in intracranial fields and focusing (for less than 5% error). Significance. Ultrasound intensity delivery at the STN by a simple single element transducer is possible and could be a promising alternative to complex multi-element phased arrays, or more general, to invasive or less focused (non-acoustic) neuromodulation techniques. Accurate acoustic skull and brain parameters, including detailed skull geometry, are needed to ensure proper targeting in the deep brain region

Deep transcranial magnetic stimulation : improved coil design and assessment of the induced fields using MIDA model

Stimulation of deep brain structures by transcranial magnetic stimulation (TMS) is a method for activating deep neurons in the brain and can be beneficial for the treatment of psychiatric and neurological disorders. To numerically investigate the possibility for deeper brain stimulation (electric fields reaching the hippocampus, the nucleus accumbens, and the cerebellum), combined TMS coils using the double-cone coil with the Halo coil (HDA) were modeled and investigated. Numerical simulations were performed using MIDA: a new multimodal imaging-based detailed anatomical model of the human head and neck. The 3D distributions of magnetic flux density and electric field were calculated. The percentage of volume of each tissue that is exposed to electric field amplitude equal or greater than 50% of the maximumamplitude of E in the cortex for each coil was calculated to quantify the electric field spread (V50). Results show that only the HDA coil can spread electric fields to the hippocampus, the nucleus accumbens, and the cerebellum with V50 equal to 0.04%, 1.21%, and 6.2%, respectively

Analysis of Eddy currents induced by transverse and longitudinal gradient coils in different tungsten collimators geometries for SPECT/MRI integration

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Assessment of nerve cathodal block for the percutaneous auricular vagus nerve stimulation

Nerve cathodal block mechanism for the percutaneous auricular vagus nerve stimulation is investigated. The response of individual axons to stimulation will be assessed in terms of excitation, blocking and propagation of action potentials in order to optimize stimulation patterns. It was seen that the response obeyed the activating function remarkably well. The found sensitivity indices of the blocking threshold for variations in diameter and temperature (61 % and 15 % respectively) are significantly higher than for the excitation threshold. Finally, the threshold needed for cathodal block (around -5 V) is far from the amplitudes used to stimulate the nerves (around -1 V). More investigations by performing an uncertainty analysis varying axonal trajectories and electrode placement can lead to the conclusion that cathodal block is less likely to occur when stimulating with clinically used amplitudes in pVNS