Static Magnetic Fields: The interaction of static magnetic fields on paramagnetic and ferromagnetic materials is dependent not only upon the static magnetic field strength itself but also the magnetic spatial gradient, or the rate at which the magnetic field changes across space or across a device or i implant of foreign body. The relative spatial distribution of these forces - where they are greatest and where they are weakest - will be reviewed and the clinical significance of these forces and spatial distributions will be clarified. Emphasis will be placed on translational forces; comparisons with rotational/torque related forces will also be highlighted. Lenz’s forces can be exerted on any electrically conductive material moving through changing magnetic fields. A discussion of Lenz’s forces, what causes them, and how to safely manage them, will be presented.
Radiofrequency Magnetic Fields:
Diffuse RF Power Deposition: Each RF pulse (used in the MR imaging process) transmitted by the MR scanner deposits energy into the patient so exposed. The relationship between these RF pulses and patient core temperature is discussed. Mechanisms of human thermal auto-regulation are also reviewed and their importance in MR imaging environments is emphasized.
Focal RF Power Deposition: While diffuse power deposition from RF pulses is exceedingly unlikely to cause any harm in MR environments, focal RF power deposition, or burns, are the most common non-pharmaceutical based adverse event reported associated with MR imaging units. In this section I introduce the idea that not all MR-related RF thermal injuries are the same. We will discuss 4 different mechanisms or potential etiologies for burns in the MR imaging environment, and highlight not only different causes for these various types of burns but also different steps that one would have to undertake in order to mitigate the thermal (i.e., burn) risks to our patients and/or research subjects. Multiple clinical examples of various types of RF related focal thermal injuries/burns will be presented and analyzed. We will discuss how one can differentiate the type of RF thermal injury that transpired by the provided history as well as the shape and distribution of the alleged RF thermal injury.
Gradient Magnetic Fields: While transmitted RF energies, tuned to megahertz frequencies, can be efficiently absorbed by the human body as heat, the frequency of modulating time varying imaging gradient magnetic fields used in the MR imaging process are typically only a few thousand times per second. Such frequencies are relatively poor at converting into heat. However, gradients are switched at slew rates, or rise and fall times, that are able to induce direct neuromuscular discharge. In other words, the changing gradient magnetic fields used in the MR imaging process can stimulate a nerve to discharge an action potential that would be propagated along that neuron’s axon and stimulate whatever is at the other end of that nerve. Thus, the MR imaging gradients used in the MR imaging process can induce sufficient voltage in a perpendicularly oriented nerve to stimulate a voluntary/skeletal muscle. The factors that affect the likelihood of stimulating such a discharge of a peripheral nerve will be discussed. Additionally, the switched gradients produce a vibration in electrically conductor devices or implants or foreign bodies if appropriately oriented. These, in turn, vibrate the surrounding air at those same very low frequencies. These sounds and noises can be quite loud, can readily exceed published auditory safety thresholds, and have been found to be associated with temporary or even permanent hearing loss.
Cryogen Safety Considerations: The wires in the magnet coil remain in a superconductive state as a result of being bathed in super-cooled liquid helium. The safety of the liquid helium as well as the environment in the event of a quench, or loss of superconductivity, is the focus of this presentation.
Implant Safety Clinical Cases: “Would you scan this patient?” A novel process of analyzing the safety issues associated with MR scanning of various devices, implants, and foreign bodies will be presented. Based on the methodology used by Dr. Kanal in his clinical practice, the safety issues associated with exposure to powerful static magnetic fields and strong stationery/static magnetic field gradients, time varying imaging gradients, and time varying RF magnetic energy fields on various implants is discussed. This session is strongly coupled with active audience participation, and teaches a formal, organized, standardized approach to assessing safety of implants, devices, and foreign bodies in the MR imaging environment.
MR Safety Implant Risk Assessment App; MR Safety Checklist Review: The aviation industry has taken dramatic steps to ensure that it possesses one of the most enviably safe professional environments. Two of the most powerful tools used in aviation to accomplish this level of safety are a) the checklist and b) the extensive use of prospective training and environment/challenge simulation. This presentation introduces the attendee to the first of two of iOS apps, Kanal’s MR Safety Implant Risk Assessment app. It is designed to function essentially as a formalized prospective MR safety checklist to help assess the safety of MR scanning of patients in whom there is an implanted device, implant, and/or foreign body.
MagnetVision™ App: In these presentations the attendees will be introduced to MagnetVision™, the second iOS platform MR safety app developed by Dr. Kanal. Its design and objectives is to serve as an MR safety simulator/model in which the user can model the patient, the implant(s), the MR scanner hardware, patient positioning, and patient centering. All of these collectively interact in real time to determine whether any labeled MR safety thresholds were approached or exceeded in the process of positioning the patient being modeled in whom there is a specific implant/device being modeled for an MR examination in a specified MR scanner and positioned for a specified MR imaging examination.
