Rapidly changing magnetic fields, such as those produced by a switched magnetic field gradient coil can activate nerve fibers in ways that can be perceived as sensation or even induce involuntary muscle contraction or twitching. Gradient induced peripheral nerve stimulation (PNS) will be discussed as well as the limits it imposes on the operation of gradient system.
Following this session, attendees should be able to:
(1) Understand the criteria affecting PNS
(2) Identify configurations in which these criteria are maximized
(3) Identify practices which may avoid or minimize PNS
In the human body, electrical impulses are used to communicate signals, such as muscle and sensory stimulation. When an external electric field is applied, peripheral nerve stimulation may be experienced as twitching or sensations such tingling, pinpricks and pressure [1].
Gradient magnetic fields are used to encode spatial information into the received signal [2]. Image acquisition strategies, such as echo planar [3] and spiral [4] imaging techniques, as well as simultaneous multi-slice excitation techniques such as PINS [5] rely on rapidly switched magnetic fields. These rapidly changing magnetic fields induce electric fields which can cause peripheral nerve stimulation (PNS), and medical devices can affect the threshold of nerve stimulation. Safety agencies have issued guidelines regarding exposure to time varying magnetic field [6], and the experience of painful PNS is a key factor determining operational limits for gradient coils systems. Despite its impact on operational limits, PNS considerations are not always included in the design process. The purpose of this talk is the improve understanding and awareness of peripheral nerve stimulation and its effect on MRI operation.
Since the magnitude of the electric field is proportional to the rate of change of the magnetic field, both gradient coil and pulse sequence can be optimized to attempt to minimize PNS. However, for some applications PNS can be difficult to avoid.
One approach to limiting PNS is a modification of the gradient coil design. Approaches including novel gradient pulse design, as well as localized, asymmetrical, non-linear, and other non-conventional gradient coil designs have shown promise [8-12]. However, predicting PNS thresholds for new gradient coil designs can be challenging. Although simple simulations can provide an initial estimate of gradient induced electric fields, the human body is irregular and inhomogeneous, making it a challenge to accurately calculate the induced electric field and predict PNS . Frameworks are being developed which integrate detailed simulations of the human body to assist in gradient coil design [13-16].
[1] Plonsey R; Barr RC. Electric field stimulation of excitable tissue. IEEE Transactions in Biomedical Engineering, 42(4):329-336, 1995
[2] Nishimura DG. Principles of Magnetic Resonance Imaging. Stanford University, 1996
[3] Mansfield P et al. Human whole body imaging and detection of breast tumors by n.m.r. Phil. Trans. R.Soc. Lond. B 289, 503-510, 1980
[4] Yudilevich E, Stark H. Spiral Sampling in Magnetic Resonance Imaging – The effect of Inhomogeneities. IEEE Transactions on Medical Imaging 6(4)”337-345, 1987
[5] Norris DG, Kiipmans PJ, Boyacioglu R, Barth M. Power Independent of Number of Slices (PINS) Radiofrequency Pulses for Low-Power Simultaneous Multislice Excitation. Magnetic Resonance in Medicine 66:1234-1240, 2011.
[6] International Commission on non-Ionizing Radiation Protection ‘Guidelines for Limiting Exposure to Time-Varying Electric Magnetic, and Electromagnetic Fields. 11 July 2018
[7] Reilly JP. Applied Bioelectricity. Springer-Verlag, New-York, 1998
[8] Feldman RE et al. Experimental Determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient system. Magnetic Resonance in Medicine 62(3):763-770, 2009
[9] Lee SK et al. Peripheral Nerve Stimulation Characteristics of an Asymmetric Head-Only Gradient Coil Compatible with a High-Channel-Count Receiver Array. Magnetic Resonance in Medicine 76:1939-1950, 2016
[10] Smith E, Freschi F, Repetto M, Crozier S. The Coil Array Method for Creating a Dynamic Imaging Volume. Magnetic Resonance in Medicine 78:784-793, 2017
[11] Winkler et al. Gradient and shim technologies for ultra high field MRI. Neuroimage 168: 59-70, 2018
[12] Weiger M et al. A High-Performance Gradient Insert for Rapid and Short-T2 Imaging at Full Duty Cycle. Magnetic Resonance in Medicine 79:3256-3266, 2018
[13] Chronik BA, Ramachandran M. Simple anatomical measurements do not correlate significantly to individual peripheral nerve stimulation thresholds as measured in MRI gradient coils. Journal of Magnetic Resonance Imaging. 17(6): 716-721, 2003.
[14] Feldman RE, Odegaard J, Handler WB, Chronik BA. Simulation of Head-Gradient-Coil Induced Electric Fields in a Human Model. Magnetic Resonance in Medicine 68L1973-1982, 2012
[15] Samoudi AM et al. Numerically Simulated Exposure of Children and Adults to Pulsed Gradient Fields in MRI. Journal of Magnetic Resonance Imaging 44:1360-1367 (2016)
[16] Davids M et al. Predicting Magnetostimulation Thresholds in the Peripheral Nervous System using Realistic Body Models. Scientific Reports 7:5316, 2017