Sensory and motor stimulation thresholds of the ulnar nerve from electric and magnetic field stimuli: implications to gradient coil operation

What factors affect a patient's subjective perception of MRI examination

MRI is becoming increasingly available and more common. However, it is a long examination, within a limited space, and making strong demands on the patient for proper co-operation. Using survey data collected by prospective questionnaire, this work examines the influence of patient preparation and type of MRI device on patients' subjective perception of the examination. The work analysed 800 patient questionnaires from 7 radiology centres, 12 MRI machines from 3 manufacturers. It was shown that 20% of patients were not informed at all or only insufficiently about the MRI examination by the referring physician, and this had a statistically significant effect on subjective perception as to the length of the examination. In claustrophobic patients, there was no significant difference in the perception of MRI examination between machine types (open vs. closed) or between bore size. This work demonstrated the influence of technical parameters of MRI devices on some other evaluated aspects in terms of patients' perception of MRI examinations (such as noise perception or peripheral nerves irritation) and that the preparation prior to the examination itself plays also an important role. Sufficient explanation from the referring physician, good workplace time management, and sufficient communication with the patient influence the subjective perception of the examination and thus indirectly its diagnostic benefit.

Exploring sensory, motor, and pain responses as potential side or therapeutic effects of sub-2 mA, 400 Hz transcranial pulsed current stimulation

BACKGROUND

Various brain stimulation devices capable of generating high-frequency currents are readily available. However, our comprehension of the potential side or therapeutic effects associated with high-frequency transcranial pulsed current stimulation (tPCS), particularly concerning the new 400 Hz tPCS device, AscenZ-IV Stimulator, developed by AscenZion Neuromodulation Co. Pte. Ltd. in Singapore, remains incomplete.

OBJECTIVE

This study examines preliminary parameters for the safe and comfortable application of 400 Hz tPCS at intensities below 2 mA.

METHODS

In a cross-sectional study, 45 healthy participants underwent sub-2 mA 400 Hz tPCS to assess sensory, motor, and pain thresholds on the dominant side. Study 1 (N = 15) targeted the primary motor cortex of the right-hand area, while study 2 (N = 30) focused on the back of the right forearm.

RESULTS

Study one showed that increasing the current intensity gradually resulted in no responses at sub-0.3 mA levels, but higher intensities (p < 0.001) induced sensory perception and pain responses. Study two replicated these findings and additionally induced motor responses along with the sensory and pain responses.

CONCLUSION

Despite the theoretical classification of tPCS as a subsensory level of stimulation, and the expectation that individuals receiving this type of current should not typically feel its application on the body, this high-frequency tPCS device generates different levels of stimulation due to the physiological phenomenon known as temporal summation. These novel levels of stimulation could be viewed as either potential "side-effects" of high frequency tPCS or as additional "therapeutic benefits". This dual capacity may position the device as one that generates both neuromodulatory and neurostimulatory currents. Comprehensive comprehension of this is vital for the development of therapeutic protocols that incorporate high-frequency tPCS.

Electric field calculation and peripheral nerve stimulation prediction for head and body gradient coils

PURPOSE

To demonstrate and validate electric field (E-field) calculation and peripheral nerve stimulation (PNS) prediction methods that are accurate, computationally efficient, and that could be used to inform regulatory standards.

METHODS

We describe a simplified method for calculating the spatial distribution of induced E-field over the volume of a body model given a gradient coil vector potential field. The method is easily programmed without finite element or finite difference software, allowing for straightforward and computationally efficient E-field evaluation. Using these E-field calculations and a range of body models, population-weighted PNS thresholds are determined using established methods and compared against published experimental PNS data for two head gradient coils and one body gradient coil.

RESULTS

A head-gradient-appropriate chronaxie value of 669 µs was determined by meta-analysis. Prediction errors between our calculated PNS parameters and the corresponding experimentally measured values were ~5% for the body gradient and ~20% for the symmetric head gradient. Our calculated PNS parameters matched experimental measurements to within experimental uncertainty for 73% of ∆G_min estimates and 80% of SR_min estimates. Computation time is seconds for initial E-field maps and milliseconds for E-field updates for different gradient designs, allowing for highly efficient iterative optimization of gradient designs and enabling new dimensions in PNS-optimal gradient design.

CONCLUSIONS

We have developed accurate and computationally efficient methods for prospectively determining PNS limits, with specific application to head gradient coils.

Reducing PNS with minimal performance penalties via simple pulse sequence modifications on a high-performance compact 3T scanner

One of the major concerns associated with high-performance gradients is peripheral nerve stimulation (PNS) of the subject during MRI exams. Since the installation, more than 680 volunteer subjects (patients and controls) have been scanned on a compact 3 T MRI system with high-performance gradients, capable of 80 mT m^-1 gradient amplitude and 700 T m^-1 s^-1 slew rate simultaneously. Despite PNS concerns associated with the high-performance gradients, due to the smaller physical dimensions of the gradient coils, minimal or no PNS sensation was reported with most pulse sequences. The exception was PNS reported by only five of 252 subjects (about 2%) scanned with a specific 3D fast spin echo pulse sequence (3DFLAIR). Rather than derating the entire system performance across all pulse sequences and all gradient lobes, we addressed reported PNS effect with a simple and specific modification to the targeted lobes of the problematic pulse sequence. in addition, the PNS convolutional model was adapted to predict sequence-specific PNS threshold level and its reduction after derating. The effectiveness of the targeted pulse sequence modification was demonstrated by successfully re-scanning four of the subjects who previously reported PNS sensations without further reported PNS. The pulse sequence modification did not result in noticeable degradation of image quality or substantial increase in scan time. The results demonstrated that PNS was rarely reported on the compact 3 T, and when it was, utilizing a specific modification of the gradient waveform causing PNS was an effective strategy, rather than derating the performance of the entire gradient system.

Prediction of peripheral nerve stimulation thresholds of MRI gradient coils using coupled electromagnetic and neurodynamic simulations

PURPOSE

As gradient performance increases, peripheral nerve stimulation (PNS) is becoming a significant constraint for fast MRI. Despite its impact, PNS is not directly included in the coil design process. Instead, the PNS characteristics of a gradient are assessed on healthy subjects after prototype construction. We attempt to develop a tool to inform coil design by predicting the PNS thresholds and activation locations in the human body using electromagnetic field simulations coupled to a neurodynamic model. We validate the approach by comparing simulated and experimentally determined thresholds for 3 gradient coils.

METHODS

We first compute the electric field induced by the switching fields within a detailed electromagnetic body model, which includes a detailed atlas of peripheral nerves. We then calculate potential changes along the nerves and evaluate their response using a neurodynamic model. Both a male and female body model are used to study 2 body gradients and 1 head gradient.

RESULTS

There was good agreement between the average simulated thresholds of the male and female models with the experimental average (normalized root-mean-square error: <10% and <5% in most cases). The simulation could also interrogate thresholds above those accessible by the experimental setup and allowed identification of the site of stimulation.

CONCLUSIONS

Our simulation framework allows accurate prediction of gradient coil PNS thresholds and provides detailed information on location and "next nerve" thresholds that are not available experimentally. As such, we hope that PNS simulations can have a potential role in the design phase of high performance MRI gradient coils.

Predicting Magnetostimulation Thresholds in the Peripheral Nervous System using Realistic Body Models

Rapid switching of applied magnetic fields in the kilohertz frequency range in the human body induces electric fields powerful enough to cause Peripheral Nerve Stimulation (PNS). PNS has become one of the main constraints on the use of high gradient fields for fast imaging with the latest MRI gradient technology. In recent MRI gradients, the applied fields are powerful enough that PNS limits their application in fast imaging sequences like echo-planar imaging. Application of Magnetic Particle Imaging (MPI) to humans is similarly PNS constrained. Despite its role as a major constraint, PNS considerations are only indirectly incorporated in the coil design process, mainly through using the size of the linear region as a proxy for PNS thresholds or by conducting human experiments after constructing coil prototypes. We present for the first time, a framework to simulate PNS thresholds for realistic coil geometries to directly address PNS in the design process. Our PNS model consists of an accurate body model for electromagnetic field simulations, an atlas of peripheral nerves, and a neurodynamic model to predict the nerve responses to imposed electric fields. With this model, we were able to reproduce measured PNS thresholds of two leg/arm solenoid coils with good agreement.

Functionalized anatomical models for EM-neuron Interaction modeling

The understanding of interactions between electromagnetic (EM) fields and nerves are crucial in contexts ranging from therapeutic neurostimulation to low frequency EM exposure safety. To properly consider the impact of in vivo induced field inhomogeneity on non-linear neuronal dynamics, coupled EM-neuronal dynamics modeling is required. For that purpose, novel functionalized computable human phantoms have been developed. Their implementation and the systematic verification of the integrated anisotropic quasi-static EM solver and neuronal dynamics modeling functionality, based on the method of manufactured solutions and numerical reference data, is described. Electric and magnetic stimulation of the ulnar and sciatic nerve were modeled to help understanding a range of controversial issues related to the magnitude and optimal determination of strength-duration (SD) time constants. The results indicate the importance of considering the stimulation-specific inhomogeneous field distributions (especially at tissue interfaces), realistic models of non-linear neuronal dynamics, very short pulses, and suitable SD extrapolation models. These results and the functionalized computable phantom will influence and support the development of safe and effective neuroprosthetic devices and novel electroceuticals. Furthermore they will assist the evaluation of existing low frequency exposure standards for the entire population under all exposure conditions.

Magnetic resonance imaging: fundamental safety issues

Medical practitioners have a variety of imaging modalities at their disposal. The exquisite soft tissue delineation available with magnetic resonance imaging (MRI) has resulted in the rising utilization of this particular modality. Increasingly, physical therapists around the world are actively involved in not only referring patients with musculoskeletal conditions for MRI but also in the acquisition of MRI data in both the clinical and research arenas. The MRI process involves the use of a very strong static magnetic field, time-varying (gradient) fields, and radiofrequency energy. To ensure the well-being of patients, staff, and visitors, an understanding of the primary hazards of this environment and the rigorous safety procedures that must be followed is imperative to the clinician. This paper describes the basic components of an MRI system, discusses various MRI safety issues, and presents the screening procedure necessary prior to using MRI. Primary hazards associated with the imaging process are also reviewed. J Orthop Sports Phys Ther 2011;41(11):820-828. doi:10.2519/jospt.2011.3906.