MR safety assessment of active implantable medical devices.

BACKGROUND: Increasing numbers of patients with active implantable medical devices (AIMDs) require magnetic resonance (MR) examinations. The manufacturers are continuing to improve the MR compatibility of their AIMDs. To this end, a variety of measurement methods and numerical simulations are used to evaluate the risks associated with magnetic resonance imaging (MRI). OBJECTIVE: In this article, test methods used to investigate interactions between AIMDs with radio frequency fields and time-varying magnetic gradient fields are reviewed. MATERIALS AND METHODS: A literature review of known test methods for radio frequency and gradient field exposure of AIMDs with leads, in particular for neurostimulators, cochlear implants, and implanted infusion pumps, is presented. The state of the art and promising methods are discussed. RESULTS: ISO/TS 10974 describes the design of high frequency and gradient injection setups to test conductive materials. A large number of sensor designs have been published to measure the induced voltages and currents through radio frequency and gradient fields and for monitoring AIMDs during MR examinations in in vitro tests. CONCLUSION: The test methods should be planned to be as conservative as possible to cover the worst case scenario. However, in vitro measurements and computer simulation are far from being able to cover all possible configurations in their complexity and uniqueness. For safer MR examinations, current research recommends in vivo testing prior to MR, parallel radiofrequency transmission techniques, and new sequences with reduced energy input in the presence of AIMDs.

[MR safety assessment of active implanted medical devices. German version]. / MR-Sicherheitsbewertung von aktiven implantierbaren medizinischen Geräten.

BACKGROUND: Increasing numbers of patients with active implantable medical devices (AIMDs) require magnetic resonance (MR) examinations. The manufacturers are continuing to improve the MR compatibility of their AIMDs. To this end, a variety of measurement methods and numerical simulations are used to evaluate the risks associated with magnetic resonance imaging (MRI). OBJECTIVE: In this article, test methods used to investigate interactions between AIMDs with radio frequency fields and time-varying magnetic gradient fields are reviewed. MATERIALS AND METHODS: A literature review of known test methods for radio frequency and gradient field exposure of AIMDs with leads, in particular for neurostimulators, cochlear implants, and implanted infusion pumps, is presented. The state of the art and promising methods are discussed. RESULTS: ISO/TS 10974 describes the design of high frequency and gradient injection setups to test conductive materials. A large number of sensor designs have been published to measure the induced voltages and currents through radio frequency and gradient fields and for monitoring AIMDs during MR examinations in in vitro tests. CONCLUSION: The test methods should be planned to be as conservative as possible to cover the worst case scenario. However, in vitro measurements and computer simulation are far from being able to cover all possible configurations in their complexity and uniqueness. For safer MR examinations, current research recommends in vivo testing prior to MR, parallel radiofrequency transmission techniques, and new sequences with reduced energy input in the presence of AIMDs.

A novel MR-compatible sensor to assess active medical device safety: stimulation monitoring, rectified radio frequency pulses, and gradient-induced voltage measurements.

PURPOSE: To evaluate the function of an active implantable medical device (AIMD) during magnetic resonance imaging (MRI) scans. The induced voltages caused by the switching of magnetic field gradients and rectified radio frequency (RF) pulse were measured, along with the AIMD stimulations. MATERIALS AND METHODS: An MRI-compatible voltage probe with a bandwidth of 0-40 kHz was designed. Measurements were carried out both on the bench with an overvoltage protection circuit commonly used for AIMD and with a pacemaker during MRI scans on a 1.5 T (64 MHz) MR scanner. RESULTS: The sensor exhibits a measurement range of ± 15 V with an amplitude resolution of 7 mV and a temporal resolution of 10 µs. Rectification was measured on the bench with the overvoltage protection circuit. Linear proportionality was confirmed between the induced voltage and the magnetic field gradient slew rate. The pacemaker pacing was recorded successfully during MRI scans. CONCLUSION: The characteristics of this low-frequency voltage probe allow its use with extreme RF transmission power and magnetic field gradient positioning for MR safety test of AIMD during MRI scans.

Experimental setup for transfer function measurement to assess RF heating of medical leads in MRI: Validation in the case of a single wire.

PURPOSE: The measurement of the transfer function is a good tool to evaluate the radiofrequency heating of complex conductive wires, such as pacemaker leads. The aim is to describe precisely the design of a transfer function bench and compare the measurements to simulations. METHODS: The transfer function was measured by mean of an excitation probe and a receiving probe, both connected to a two-port vector network analyzer. The experimental results were compared with the simulated results, reproducing the excitation scheme. This procedure was applied to two different cables with different geometrical and insulation properties to test the robustness of the setup. RESULTS: It is possible to touch the cable electrode with the excitation probe without inducing an error in the measured transfer function, which solves the direct coupling problem. There is a good agreement between the measured and simulated transfer function for both tested cables. CONCLUSIONS: A valid transfer function measurement bench is described. Magn Reson Med 79:1766-1772, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Toward nitrogen-14 nuclear quadrupole resonance imaging by nutation experiments performed with a radio-frequency field gradient.

Until now, NQR imaging has been considered mainly in the case of Chlorine-35. This is a spin 3/2 resonating at relatively high frequency (around 30MHz) thus affording a favorable sensitivity. Conversely, Nitrogen-14 (spin 1) NQR is much less sensitive because its resonances frequencies are below 6MHz. In contrast to already existing methodologies for object localization by 14N NQR, we present here a new, more straightforward, approach and the principles of space dependent 14N Quadrupole Resonance are laid down. The method is based on nutation curves obtained with an inhomogeneous radio-frequency field produced by a dedicated transmit-receive coil. The gradient created that way does not need to be uniform although more accurate results would be obtained with a uniform gradient. Nutation curves have been simulated by a specially designed algorithm which takes into account the 14N Quadrupole Resonance particularities including the so-called powder average. Preliminary experiments were carried out with the highest resonance frequency (4.64MHz) of sodium nitrite (NaNO2). A cylindrical sample of powder sodium nitrite containing a spacer was used. Simulations of the corresponding nutation curves not only demonstrate the existence of this spacer but, in addition, provide its position and its thickness. This clearly ascertains the feasibility of 14N Quadrupole Resonance imaging.

¹4N Quadrupole Resonance line broadening due to the earth magnetic field, occuring only in the case of an axially symmetric electric field gradient tensor.

As demonstrated before, the application of a weak static B0 magnetic field (less than 10 G) may produce definite effects on the ¹4N Quadrupole Resonance line when the electric field gradient tensor at the nitrogen nucleus level is of axial symmetry. Here, we address more precisely the problem of the relative orientation of the two magnetic fields (the static field and the radio-frequency field of the pure NQR experiment). For a field of 6G, the evolution of the signal intensity, as a function of this relative orientation, is in very good agreement with the theoretical predictions. There is in particular an intensity loss by a factor of three when going from the parallel configuration to the perpendicular configuration. By contrast, when dealing with a very weak magnetic field (as the earth field, around 0.5 G), this effect drops to ca. 1.5 in the case Hexamethylenetetramine (HMT).This is explained by the fact that the Zeeman shift (due to the very weak magnetic field) becomes comparable to the natural line-width. The latter can therefore be determined by accounting for this competition. Still in the case of HMT, the estimated natural line-width is half the observed line-width. The extra broadening is thus attributed to earth magnetic field. The latter constitutes therefore the main cause of the difference between the natural transverse relaxation time (T2) and the transverse relaxation time derived from the observed line-width (T2(⁎)).

Effect of a weak static magnetic field on nitrogen-14 quadrupole resonance in the case of an axially symmetric electric field gradient tensor.

The application of a weak static B0 magnetic field (less than 1 mT) may produce a well-defined splitting of the (14)N Quadrupole Resonance line when the electric field gradient tensor at the nitrogen nucleus level is of axial symmetry. It is theoretically shown and experimentally confirmed that the actual splitting (when it exists) as well as the line-shape and the signal intensity depends on three factors: (i) the amplitude of B0, (ii) the amplitude and pulse duration of the radio-frequency field, B1, used for detecting the NQR signal, and (iii) the relative orientation of B0 and B1. For instance, when B0 is parallel to B1 and regardless of the B0 value, the signal intensity is three times larger than when B0 is perpendicular to B1. This point is of some importance in practice since NQR measurements are almost always performed in the earth field. Moreover, in the course of this study, it has been recognized that important pieces of information regarding line-shape are contained in data points at the beginning of the free induction decay (fid) which, in practice, are eliminated for avoiding spurious signals due to probe ringing. It has been found that these data points can generally be retrieved by linear prediction (LP) procedures. As a further LP benefit, the signal intensity loss (by about a factor of three) is regained.