Chapter 9: Recording in the Magnetic Resonance Imager

MRI Overview
fMRI and Blood Oxygen Level Dependent Contrast Imaging
MRI Measurement Processes
Magnetic Field Susceptibility and Material Type
Larmor Frequency and Precession
Practical Considerations when Recording Physiological Data in the MRI
Signal Artifact Associated with MRI
Electrode Leads
Patch Panel Filtering
Instrumentation Amplifier Behavior, High-frequency Inputs, Rectification
Rate Limiting, Non-linear Filtering
Comb Bandstop Filtering
Electrical Stimulation in the MRI
MRI Phantoms
Pneumatic Devices – Pressure Pad, Air Flow
Common Elements and Compounds Compatibility with MRI
MRI-related classifications

MRI Overview

Magnetic Resonance Imaging (MRI) is an imaging method used to produce pictures of anatomical structures inside humans and other animals. MRI is based on the phenomena of Nuclear Magnetic Resonance (NMR) which is subject to the principles of atomic absorption and emission of radio frequency (RF) energy. Protons inside atomic nuclei have a property known as Spin. Spin behaves like a small magnetic field and the collective Spin field causes the nucleus to produce an NMR signal.

In the MRI bore, a large magnetic field is applied to the protons in the tissue. This applied field causes the protons to precess (rotate) at a specific frequency. The magnetic moment of protons is 42.58 MHz per Tesla. When radiated with pulsed radio energy, at this frequency, the protons become excited. As the protons return to their unexcited state, they emit RF energy. These radio signals are recorded and analyzed. By changing the type and sequence of the RF energy pulses, and the strength of the magnetic field, different types of images can be created.

fMRI and Blood Oxygen Level Dependent Contrast Imaging

Functional Magnetic Resonance Imaging (fMRI) encompasses the discovery that MRI can be used to map changes in brain hemodynamics that correspond to mental activity. This discovery has resulted in the fMRI technique of Blood Oxygen Level Dependent (BOLD) contrast imaging. Significant advantages of this technique, for imaging brain activity, are short scan times and that subject injections of contrast enhancing agents or radioactive isotopes are not required. fMRI has become a significant research technique used in cognitive neuroscience.

The most commonly used fMRI method measures and contrasts BOLD changes associated with metabolic functions that are presumed to support neuronal activity in the brain. An important contributory principle to BOLD imaging behavior is that hemoglobin is paramagnetic when it carries no oxygen and is diamagnetic when full of oxygen. Accordingly, the magnetic susceptibility of red blood cells is a function of how much oxygen the cell transports. Variations in local magnetic susceptibility cause changes in the T2 transverse relaxation time in nearby water protons, thus contributing to the BOLD imaging process.

MRI Measurement Processes

Magnetic Resonance Imaging (MRI) operation relies on the fact that body tissue contains water (H2O). Each hydrogen atom contains one isolated proton which will align along a specific orientation in a strong magnetic field, like a miniature compass needle. When body tissue is inside the magnetic field of the MRI, the many hydrogen nuclei (protons) becomes aligned with the direction of the field. Specifically, the magnetic vector of the proton, in the hydrogen atom, aligns with this field. In this aligned condition, when submitted to a stimulation pulse of resonant radio waves, the energy content of the hydrogen nuclei increases. After the stimulus pulse, a response resonance wave is emitted when the nuclei return to their previous aligned state. The frequency of this wave is called the Larmor frequency and is 42.7*Tesla MHz for water protons.

The MRI’s primary magnetic field is established via a super conducting magnet. During imaging, this magnetic field changes tilt (also called gradient or slope) in a time-sequential fashion. The gradient of the magnetic field establishes a physical circumstance whereby the field will vary as a function of position. Through a specific volume of the subject’s body, the magnetic field will vary over that volume. Depending upon the gradient, the field may vary more or less over that same volume.

The location of the origin of the response resonance wave, in body tissue, can be determined as a consequence of knowing the established gradient of the magnetic field through the body tissue volume. The field gradient (slope) is generated by passing electric currents through gradient coils which are present in the primary magnet. These coils can vary the magnetic field strength, in a precise and controllable manner, over the body tissue volume scanned.

Present MRI methods require that the magnetic field be both stable and predictable during a imaging scanning pass. A consecutive time-series of magnetic fields, with specific gradients, are established through a volume of body tissue and the volume is stimulated with radio frequency pulses. During each scanning pass, the established gradient-shifted, stable magnetic field causes the frequency of the response resonance wave to be dependent on its origin in a pre-defined manner. Accordingly, the distribution of protons in the body tissue can be mathematically determined from the signal by the use of spatial transformations.

A sequence of transmitted resonant radio wave pulses can be used to emphasize particular tissues or abnormalities over others. This emphasis occurs because different tissues have different rates of decay for their response resonance waves. The time taken for protons to relax, in the tissue, is measured in two ways. The first, T1 relaxation, is the time period for the magnetic vector (longitudinal magnetization) to return to its resting state. The second, T2 relaxation, is the time period for the axial spin (transverse magnetization) to return to its resting state. Both of these time period measurements, T1 and T2, occur simultaneously, with T2 always being equal to or less than T1. In general, relaxation results in recovery of magnetization in the longitudinal plane and decay of magnetization in the transverse plane.

There are two basic types of MRI pulse sequences, namely spin echo and gradient echo. All other MRI pulse sequences are variations of these. Spin echo was the first type of pulse sequence used for MRI [1]. A spin echo sequence has good signal to noise ratio, but can be relatively slow. Gradient echo is a pulse sequence which allows for faster imaging at the expense of reduced signal to noise ratio.

The following types of imaging methods are commonly used:

1. Diffusion imaging allows the mapping of the diffusion process of molecules, typically water, in the biological volume scanned.

2. Echo-planar imaging (EPI) is a fast, gradient echo based, scanning technique capable of acquiring an entire MR image in only a fraction of a second.

3. Inversion recovery imaging allows for mapping, while nulling the impact of fluids, in the biological volume scanned.

4. Spiral imaging is similar to EPI, but slightly faster in operation, because spiral gradient waveforms are easier to produce than those for EPI.

Magnetic Field Susceptibility and Material Type

All materials’ magnetic behavior fall into one of five categories, based on their bulk magnetic susceptibility. Most materials can be classified as diamagnetic, ferromagnetic and paramagnetic. The two most common types of magnetism are diamagnetism and paramagnetism. Two uncommon types of magnetism are ferrimagnetism and anti-ferromagnetism.

Diamagnetic materials have a small and negative susceptibility to magnetic fields. Diamagnetic materials have only paired electrons, so their atoms have no net magnetic moment. Diamagnetic materials are slightly repelled by a magnetic field and the materials do not retain magnetic properties when the external field is removed. Diamagnetic properties arise from changes to the normal electron paths, when under the influence of an external magnetic field. Diamagnetic elements include carbon, copper and gold.

Ferromagnetic materials have a large and positive susceptibility to magnetic fields. Ferromagnetic materials have unpaired electrons, so their atoms have a net magnetic moment. Ferromagnetic material are strongly attracted to magnetic fields and the materials are able to retain magnetic properties when the external field is removed. Ferromagnetic material get their magnetic properties from magnetic domains. In a domain, large numbers of atom’s moments are similarly aligned, so the magnetic force in the domain is strong. When a ferromagnetic material is in an unmagnetized state, domains are randomly organized, so the net magnetic field for the material is zero. When a magnetizing force is applied, domains become aligned to produce a strong magnetic field within the material. Ferromagnetic elements include cobalt, iron and nickel.

Paramagnetic materials have a small and positive susceptibility to magnetic fields. Paramagnetic materials have unpaired electrons, so their atoms have a net magnetic moment. Paramagnetic materials are slightly attracted by a magnetic field and the materials do not retain magnetic properties when the external field is removed. Paramagnetic properties arise from changes to the normal electron paths, when under the influence of an external magnetic field. Paramagnetic elements include aluminum, magnesium and oxygen.

• MR Safe Рan item that poses no known hazards in all MRI environments. Using the terminology, “MR Safe” items are non-conducting, non-metallic, and non-magnetic items such as a plastic Petri dish. An item may be determined to be MR Safe by providing a scientifically based rationale rather than test data.

• MR Conditional Рan item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use. “Field” conditions that define the MR environment include static magnetic field strength, spatial gradient magnetic field, dB/dt (time rate of change of the magnetic field), radio frequency (RF) fields, and specific absorption rate (SAR). Additional conditions, including specific configurations of the item (e.g., the routing of leads used for a neurostimulation system), may be required.

Larmor Frequency and Precession

In the context of magnetic resonance imaging (MRI), the Larmor frequency refers to the frequency of the radio wave that will resonate with the protons in the nucleus of any specific element.

Hydrogen protons have a gyromagnetic ratio of 42.58 MHz/Tesla
Water protons have a gyromagnetic ratio of 42.7 MHz/Tesla

The Larmor equation describes the resonant precessional frequency of a nuclear magnetic moment in an applied static magnetic field. When placed in a magnetic field (B), the atomic nucleus of an element will precess (resonate) at a frequency which is determined by the gyromagnetic ratio of the element and the strength of the magnetic field, in accordance to the following formula:

w = g * B

Where:

w = precessional frequency (resonant frequency)
g = gyromagnetic ratio (MHz/Tesla)
B = magnetic field (Tesla)

Proton resonance frequency is affected by the surrounding medium. This is why hydrogen protons and water protons resonate at slightly different frequencies. For protons in water, the resulting g values are called “shielded” values, referring to the shielding by the electrons in the water molecule. The dependence of the proton resonance frequency upon the surrounding environment is called a “chemical shift” and this phenomena is used to explore the elemental makeup surrounding protons in magnetic resonance imaging.

Practical Considerations when Recording Physiological Data in the MRI

When recording physiological signals from subjects in MRI systems, two issues are very important.

1. The MRI system incorporates a VERY strong magnetic field. It is extremely important that no significant magnetically susceptible objects are taken into the room that houses the MRI System. All materials have some degree of magnetic susceptibility. However, materials with mass magnetic susceptibility values, similar to or have magnitude less than that of water (-0.91E-8 m^3/kg), are typically permitted to be used in the vicinity of MRI Systems. No electrodes, electrode leads, transducers or cabling, that contain significant magnetically susceptible components, should be used when recording data from a subject in the MRI.

2. The MRI System both emits and records radio frequency (RF) energy. This energy is typically in the range of 5 to 500 MHz. For a 3T MRI, the characteristic proton precession frequency will be 3T * 42.7 MHz or 128.1 MHz. MRI generated RF energy will, at least, partially reflect from encountered conductors and induce current flow in those conductors. Signal reflections and induced currents can disrupt sensitive imaging operations by creating signal artifacts. Also, RF induced currents, if not properly controlled, can cause local heating.

Care must be taken to limit the flow of RF energy both in and out of the room that houses the MRI. RF energy from the MRI superimposes with the very small biopotentials and transducer signals in the vicinity of the subject. Alternatively, RF energy from recording or other equipment in the MRI Control Room (e.g. digital clocks and power supply switching noise) can leak into the MRI chamber, via subject cabling, and cause degradation of the image created by the MRI. It can be complicated to record biopotentials, from a subject’s body, during an MRI scan. These signals are very tiny and the RF energy, and associated magnetic field shifting, generated by the MRI corrupts the recording. Accordingly, successful biopotential recording in the MRI involves the use of specialized amplifiers, optimized cabling, patch-panel filtering, advanced signal processing and/or synchronization with MRI scanning processes to perform clean measurements.

When RF energy is pulsed into the subject, this energy spreads throughout the subject because the subject is a volume conductor. As the MRI magnetic gradient shifts in synchrony with RF energy pulsing, the gradient shift induces a current flow in conductors that intersect with the field. From a practical standpoint, RF energy pulsing and gradient-induced current flows often manifest themselves as repetitive artifact signals directly in the band of the physiological generated biopotentials. This is because RF energy pulsing and magnetic field gradient switching are performed synchronously for imaging and the repetition rates for image slice generation usually fall within the range of 1-100 Hz. Depending upon the biopotential frequency range (from lowest to highest: EGG, EOG, ECG, EEG and EMG), different amplification and signal processing techniques can be used to isolate the signal of interest. Furthermore, it’s often helpful to control imaging processes so that the MRI artifact signal spectrum does not unduly impact the measured physiological signal spectrum.

Magnetic Resonance Imaging is based on the idea that a sequence of transmitted resonant radio wave pulses can be used to emphasize particular tissues or abnormalities over others. This emphasis occurs because different tissues have different rates of decay for their response resonance waves. The time taken for protons to relax, in the tissue, is measured in two ways. The first, T1 relaxation, is the time period for the magnetic vector (longitudinal magnetization) to return to its resting state. The second, T2 relaxation, is the time period for the axial spin (transverse magnetization) to return to its resting state. Both of these time period measurements, T1 and T2, occur simultaneously, with T2 always being equal to or less than T1. In general, relaxation results in recovery of magnetization in the longitudinal plane and decay of magnetization in the transverse plane.

In the case of Blood Oxygen Level Dependent (BOLD) imaging, T1 is unaffected by blood oxygen levels. However, T2 is greatly affected by blood oxygen levels. Changes in T2 relaxation are caused by changes in the amount of oxygenated hemoglobin in the venous circulation of the brain. As hemoglobin becomes more saturated with oxygen, the associated T2 relaxation time increases because oxygenated hemoglobin has smaller magnetic susceptibility than deoxygenated hemoglobin. When brain neuronal activity increases there is a momentary decrease in local blood oxygenation. This action is followed by a period where the blood flow increases and overcompensates for the increased oxygen demand. The result is that blood oxygenation increases after neural activation. Accordingly, the magnetic susceptibility of blood decreases after neural activation, thus increasing T2 relaxation time.

Types of Blood Oxygen Level Dependent Effect (BOLD) Imaging

Echo Planar Imaging (EPI)

EPI is a magnetic resonance imaging (MRI) method valuable to neuroscience. EPI is employed for nearly all functional MRI (fMRI) and diffusion imaging of neural fiber connections in the brain. EPI is a high speed MRI technique where one saturation radio frequency (RF) pulse is used to generate nuclear magnetic resonance data to create a two-dimensional planar image. EPI uses rapidly varying magnetic field gradients, in the transverse plane, to cause multiple refocusing gradient echoes from a single RF pulse. EPI generates a single two dimensional image in a fraction of a second. However, EPI requires 2 to 3 seconds to acquire multiple slices for whole brain imaging during fMRI. EPI-based diffusion imaging takes even more time.

Multi-band (MB) Excitation

MB excitation, during EPI, allows for the simultaneous acquisition of multiple slices during imaging. MB factor 6, means that six slices are acquired in the same time that one slice would be acquired during regular EPI. This technique is helpful for fMRI studies to improve the statistical definition of neural fiber connections and for diffusion-based imaging neural fiber tractography, to visualize structural connections in the brain.

Multiplexed-EPI (M-EPI) Sequencing

M-EPI sequencing combines two forms of image acquisition multiplexing: time-based multiplexing (T) and space-based multiplexing (S) with MulTi-band RF pulses to acquire (T x S) images in an EPI sequence instead of just one image. M-EPI provides a substantial increase in time resolution and higher statistical power for fMRI, resulting in improved image resolution.

Slice Definitions for fMRI

Axial Slice:

The plane that chops off the top of the head, with x-axis coordinates going from left-to-right (ear-to-ear) and y-axis coordinates from front-to-back (face to back of head).

Coronal Slice:

The plane parallel to the face, with x-axis coordinates going from left-to right (ear to ear) and y-axis coordinates from top-to-bottom (head to foot).

Sagittal Slice:

The plane parallel to the side of the head, with x-axis coordinates going from left to right (face to back of head) and y-axis coordinates from top-to-bottom (head to foot).

Coronal scans occur in the same plane as the distribution of electrodes over the subject’s body. Because gradient shifts occur in this plane, induced scanning artifacts are substantially reduced. In the case of Axial scans, gradient shifts occur in the orthogonal plane to the distribution of body-attached subject electrodes. In this case, scanning artifacts are maximized, because magnetic gradient-induced signals are optimally situated to be picked up by the recording electrodes.

A disadvantage of Coronal fMRI scans is that they take longer to generate a complete scan of the brain, due to differences in the brain’s dimensions in the Axial, Coronal and Sagittal planes. However, physiological data is substantially less corrupted by induced MRI artifact if the recording plane of the electrodes matches the plane of fMRI scanning.

Signal Artifact Associated with MRI

When performing MRI and simultaneously recording time-series physiological data, there are two basic types of artifact associated with MRI processes. The first type of artifact is associated with the presence of the large constant MRI magnetic field. This type of artifact is typically called “magnetohydrodynamic effect” (MHD). MHD artifact results from movement of conductive fluid (blood) and from the movement of charge inside the body, when exposed to a magnetic field. A motional electromotive force (Vemf) is generated when a conductor moves through a magnetic field. Also, a charge moving in a magnetic field may have its travel path affected by the field. This two phenomenon can distort and superimpose with an existing biopotential signal. The most prominent MHD effects occur in the vicinity of the heart. The aortic arch is aligned perpendicularly to the MRI magnetic field and this geometry is conducive to creating large magnitude MHD artifacts in the ECG signal. Aortic blood flow is highest during systole, which corresponds to the ST segment in the ECG signal, so the largest MHD artifacts occur during this period. MHD effects are easily seen when measuring ECG inside the MRI, even when scanning is off. In this situation, the ECG signal will usually distort to the point of being non-diagnostic, such as lengthening QRS time and increasing T-wave amplitude.

The second type of artifact is associated with the time-based scanning processes occurring within the MRI. During scanning, two sources generate artifacts during biopotential measurements in the MRI. The first source is associated with magnetic field gradient switching during EPI or other scanning sequence. As the magnetic field changes orientation with respect to the biopotential electrode lead-subject-amplifier loop, a current is induced in the loop. The magnitude of induced current is proportionally linked to the loop area and number of turns in the loop. The second source is associated with radio-frequency (RF) pulsing coincident with gradient switching during the scanning sequence. The RF is at the Larmor frequency and couples to the biopotential loop conductors, which include electrodes, electrode leads and subject. RF energy can circulate in this loop because the biopotential signal conductors will travel through the cable harness to the patch panel connectors separating chamber room from control room. RF energy will couple throughout the cabling harness due to distributed capacitance in the cable, thus establishing an RF conducting loop that includes subject, electrodes, electrode leads and distributed capacitance in cable harness.

Electrode Leads

To help reduce these two scanner-generated current artifacts, it’s optimal to employ high resistance carbon composition electrode leads. These types of electrode leads have a specific amount of resistance per unit length. Because resistance is distributed along the length of the lead, they are more effective at blocking RF currents than lumped resistive elements. To reduce gradient field switch artifact, the electrode leads should be held to as short a distance as practical and leads should be uniformly axially twisted, from subject electrode connections to cable harness. This twisting will reduce the effective cross sectional area of the conducting loop, thus helping to minimize gradient switching artifact. Axially twisting the electrode leads will also cause the RF transmitted signal to couple to both electrode leads similarly, thus helping to reduce differentially-generated RF currents due to unequal coupling. By twisting the electrode leads, coupled RF energy is induced as common mode.

Patch Panel Filtering

The MRI produces a great deal of radio frequency (RF) energy at the frequency of 42.7*T MHz, where T is the field strength of the MRI in units of Tesla. This is because water is the most common element in the body. This RF energy will be present on all conductors which may be attached to a subject in the MRI. The MRI incorporates sensitive detectors to measure the re-radiated RF, from the body tissue, after it has been initially stimulated. This combination of high power RF generation, coupled with the need for sensitive RF detection, requires that the MRI chamber room be electromagnetically shielded. The room shielding contains the high power RF energy and also keeps outside RF energy from corrupting sensitive RF measurement.

However, when recording physiologically-generated signals during MRI, electrical conductors may be required to carry biopotentials and excitation/sense electrical currents between the Chamber room and Control room. The point of junction, between Chamber room and Control room, is typically an aluminum sheet metal panel. This panel, known as a patch panel, usually is populated with a variety of connectors to permit electrical conductors to pass though the chamber’s shielding. Unless these conductors are properly filtered, as they pass through the patch panel, they will carry RF energy both directions. Lack of filtering on these conductors will permit RF energy, generated by the MRI, to leak into the adjoining room. Similarly, RF energy already present in the adjoining room, can leak into the Chamber room and corrupt MRI imaging ability. Suitable patch panel filters will greatly reduce the RF energy which can pass through the panel. Filters should be designed so they greatly attenuate frequencies in the vicinity of 42.7*T MHz. Typically, this filtering is constructed in the form of lowpass or band-reject (bandstop) filters. These filters should be designed to satisfy medical IEC60601 standards to minimize leakage currents to ground, in the event of a fault condition, when recording human biopotentials.

Instrumentation Amplifier Behavior, High-frequency Inputs, Rectification

Instrumentation amplifier front ends, used to record biopotentials and low-level electrical signals from transducers, will typically be non-linearly sensitive to RF energy present on the amplifier inputs. This sensitivity manifests as a DC offset on the input stages of the amplifier. If the RF energy is pulsed and large enough, then the amplifier may typically show a corresponding shift in output level, coincident with the pulsing. This behavior happens because large levels of RF energy will be rectified by the input transistor stages of the instrumentation amplifier. Adequate patch panel filtering will help to reduce this potential problem.

Rate Limiting, Non-linear Filtering

Non-linear filtering methods are useful for handling MRI-generated, time-varying, artifact. All MRI imaging processes, including echo planar imaging (EPI) sequences, can induce large transient artifacts on recorded biopotentials and transducer signals. Predictable, time-series, artifacts are generated from two sources; shifting magnetic field gradients and RF pulsing. These transient artifacts, because of distinctly different character, can often be readily identified and largely removed from physiologically-sourced data. As example, considering the ECG, the maximum rate of signal change is about 280mV per second*. If observed components in the ECG recording exceed 0.28V per second, then those signals can be deemed artifact. A non-linear filter, such as a slew rate limiter, can be employed to remove such artifact.

Comb Bandstop Filtering

During MRI imaging processes, such as echo planar imaging (EPI), two linked processes are initiated. One process involves shifting of the X, Y or Z direction of the magnetic field gradient (slope). The other process introduces pulses of RF energy, which are synchronized with the gradient shifts. Both of these processes will contribute to periodic electrical signal artifact in simultaneously recorded physiological data.

Using the example of EPI, these linked processes occur at a fixed repetition rate (Tr). During scan intervals, the gradient may typically be shifted a number (X) times. The formula (X/Tr) determines the gradient shift rate. As an example, for Tr = 2.0 seconds and number of gradients (slices)= 37, then the gradient shift rate is 37/2 or 18.5Hz. A comb bandstop (CBS) filter set to 18.5Hz, plus all harmonics (37Hz, 55.5Hz, etc.) will effectively remove this periodic artifact. CBS filtering methods can be useful when periodic MRI artifact results in signals that fall into the physiological signal bandwidth of interest.

It can be problematic to employ CBS filtering on physiological signals that have critical frequency components in the vicinity of the CBS bandstop frequencies. An example is the QRS wave in the ECG. This wave has frequency components peaking around 17Hz. A CBS filter (plus harmonics) that fall on, or close, to 17Hz will impact the shape of the QRS wave passing through the CBS filter. However, if the CBS fundamental frequency and harmonics are shifted away from 17Hz, then the QRS wave shape can be relatively unaffected.

Electrical Stimulation in the MRI

Electrical signal artifact pulses, during EPI or other scanning sequence, can be generated along conductors by magnetic field gradient switching occurring within the MRI. As the magnetic field shifts, it will attempt to induce a current in any conductor that crosses the moving field lines. The induced current is allowed to flow if a complete electrically-conductive loop is formed with the conductor and its associated attachments.

In the case of an electrical stimulation setup, a conductive loop is formed that includes the subject, skin surface electrodes, electrode lead wires, cabling and the electrical stimulator output. Any scanning-induced artifact currents can flow into the Control Room, through the electrical stimulator output and then back out to the Chamber Room and to the subject (or phantom) in the magnet.

In order to suppress this artifact, the use of high resistance, carbon composition, electrode leads is recommended. These leads help reduce artifact in two ways. High resistance leads reduce the Q of the resonant frequency of any conductive loop and so reduces energy coupling from scanner-induced RF pulsing. Also, the distributed resistance of the leads reduces the potential for capacitive bypass coupling, as compared to the case of lumped resistances in the leads. Artifact can also be reduced by axially-twisting the electrode leads to minimize the cross-sectional area of the conductive loop exposed to the magnetic field gradient shifts. In addition, axially-twisting the electrode leads will encourage pulsed RF scanner signals to establish themselves as a common-mode signal on the leads to reduce the possibility for differential RF current flow.

MRI Phantoms

Phantom testing is highly recommended before performing any biopotential measurements or electrical stimulation protocols in the MRI, to determine the magnitude and extent (if any) of artifact stimulation pulses evident during any specific imaging sequence. A salt-water phantom, in a plastic cup, can be used to emulate the subject stimulation site. The phantom can be crafted with a 0.9% salt (NaCl) solution of similar volume to the stimulated tissue volume. Normal saline (0.9% NaCl) is isotonic to the human body. Electrodes normally attached to the subject’s body can be immersed in a similar volume of saline, as referenced to the distance between normally body-applied skin surface electrodes.

Pneumatic Devices – Pressure Pad, Air Flow

Measurement devices which are based upon pneumatic principles are very useful in the MRI. Pneumatic-based transducers can be developed from materials which are only slightly diamagnetic or paramagnetic (essentially non-magnetic) and non-conductive. These qualities are very helpful for use in the MRI, because such transducers are unaffected (low magnetic susceptibility) by very strong magnetic fields and, because of their non-conductive nature, do not affect the radio waves employed during magnetic resonance imaging. Furthermore, they do not manifest localized heating because electrical direct currents do not flow through non-conductive materials.

Common Elements and Compounds Compatibility with MRI

MRI Compatible Compounds

Examples of common compounds, elements and plastic materials, with low mass magnetic susceptibility (X), measured in m3/kg * 10-8 (SI units), as follows:

Common Compounds:

Carbon Dioxide (CO2) = -0.59
Quartz (SiO2) = -0.6
Salt (NaCl) = -0.64
Water (H2O) = -0.91

Elements:

Aluminum = +0.78
Carbon = -0.62
Copper = -0.11
Gallium = -0.30
Gold = -0.18
Indium = -0.14
Lead = -0.15
Mercury = -0.21
Nitrogen = -0.54
Oxygen = +133.60
Platinum = +1.22
Silicon = -0.16
Silver = -0.23
Tin = -0.31
Titanium = +4.01
Tungsten = +0.39
Zinc = -0.22

Plastic Materials:

Plexiglass (Perspex) = -0.50
Polyvinyl Chloride (PVC) = -0.75
Polyethylene = +0.20

MRI Incompatible Compounds

Cobalt
Ferromagnetic Compounds (ferrites)
Iron
Nickel
Stainless Steel (martensitic alloys)
Steel

MRI-related Classifications

For safety and performance purposes, it’s important to classify the transducers, electrodes and leads (parts) that may be introduced to the MRI environment.

MRI Safe: The parts are non-magnetic, non-electrically conductive and non-radio frequency reactive. These parts are not influenced by MRI processes, nor do they affect MRI imaging.

MRI Conditional: The parts may contain trace magnetic, electrically conductive or radio frequency reactive elements that remain safe for operations within the confines of the MRI, as long as they are used at (or under) the specified magnetic field strength (e.g. up to 3 Tesla) and under any other specified conditions.

MR Unsafe: Parts that can’t be classified as MRI Safe or Conditional.