*Excerpted with permission from, Regarding the Value Reported for the Term “Spatial Gradient Magnetic Field” and How This Information Is Applied to Labeling of Medical Implants and Devices*, Shellock FG, Kanal E, Gilk, TB. American Journal of Roentgenology, 2011;196:142-145. To obtain the original article including the illustrations, please visit www.IMRSER.org.
Frank G. Shellock, Ph.D., University of Southern California
Emanuel Kanal, M.D., University of Pittsburgh Medical Center
Tobias B. Gilk, M.Arch, Mednovus
Differences in interpretations of testing and reporting criteria, methodologies, and perhaps even concerns about manufacturer legal liability have created a contemporary environment in which questions and confusion abound in the magnetic resonance imaging (MRI) industry. Of particular concern regarding the management of patients with implants and devices in the MRI environment is the disparities in the way that the spatial gradient magnetic field information is presented.
The intensity of the static magnetic field around an MR system varies with respect to the distance from the scanner. This so-called "fringe-field" of the MR system creates a “spatial gradient magnetic field”. By definition, the spatial gradient magnetic field is a magnetic field that varies in intensity over distance. The spatial gradient magnetic field should not be confused with the time-varying, gradient magnetic fields produced by the gradient coils that are used during the imaging process for spatial encoding of the MRI signals.
The spatial gradient of the magnetic field produces an attractive displacement (or translational) force on ferromagnetic objects placed into the static magnetic field of the MR system (1-4). Importantly, the MRI-specific labeling that is used for medical implants and devices typically provides information for the “highest spatial gradient magnetic field” at which the medical device was tested and that is allowed for the object to be exposed to in order to ensure the safety of the patient relative to the device’s translational attraction (1-6). The assessment of translational attraction (or displacement force) is just one aspect of implant testing that is performed when evaluating a medical device (1-5).
In most cases, the Food and Drug Administration (FDA) accepts the determination of translational attraction for a medical device that is conducted according to the procedure described by the American Society for Testing and Materials (ASTM) International Designation. F 2052, Standard test method for measurement of magnetically induced displacement force on passive implants in the magnetic resonance environment. Using this technique, a test apparatus with a protractor (also called the “fixture’) is placed in an MR system with a horizontal magnetic field at the point of the highest “accessible” spatial gradient magnetic field (6). In general, the highest spatial gradient magnetic field used to assess translational attraction for a medical device is located off-axis, at a side-wall and near the opening of the bore of the scanner (1, 3).
Alternatively, the medical device is assessed for translational attraction at the point where the highest deflection angle occurs in association with the particular MR system used for the assessment. The angular “deflection” of the device from the vertical is measured and the translational attraction is calculated (6).
Notably the placement of the test fixture (apparatus with the protractor) in the MR system is at a position where it can be utilized properly (i.e., securely positioned) for the test procedure. This is almost always a worst-case position of greatest attractive force that the patient with the device will “pass through” when entering the bore of the MR system for an examination.
With regard to the translational attraction measurement for a medical device, if the deflection angle is less than 45-degrees, and the magnetic force is in the horizontal direction, the deflection force is less than the gravitational force associated with the device’s weight and it is assumed that any risk imposed by the application of the magnetically-induced deflection force is no greater than any risk imposed by normal daily activity in the Earth’s gravitational field (4, 6). However, even if a device exceeds 45-degrees of deflection, it may still be acceptable for a patient undergoing an MRI examination if sufficient counter-forces are present (e.g., from sutures, scarring, tissue in-growth, etc.) that prevent it from being moved or dislodged (3).
There is confusion about the term “spatial gradient magnetic field” (or “highest spatial gradient” or “magnetic spatial gradient”) and how this parameter is reported for a given MR system relative to this term’s use in the labeling of a medical device. Note: The term “spatial gradient magnetic field” may also be misunderstood because some MRI professionals see the term “gradient” and presume that it refers to the “time-varying or gradient magnetic fields” (dB/dt) used for spatial location in association with MR imaging (2). The term “spatial gradient magnetic field” refers to the rate at which the static magnetic field strength changes over space or distance per unit length. This parameter is indicated as dB/dx, using the units of Tesla/meter or gauss/centimeter.
According to test reports and resulting labeling for many implants tested at 3-Tesla, the highest “accessible” spatial gradient magnetic field of 720-gauss/cm was determined using highly involved methodology for the 3-Tesla MR system used for the measurements of translational attraction. (Researchers and MRI professionals, however, must typically rely on the magnitude and location of the spatial gradient magnetic field as it is provided by the MR system manufacturer.) To test the device, the apparatus with the protractor was placed at that position (720-gauss/cm) to measure the deflection angle. This information (i.e., the value for the highest spatial gradient magnetic field) is reported in the labeling for the device. Thus, the MRI-specific labeling would then state that the acceptable static magnetic field information a given implant would be, as follows (2):
“3-Tesla or less
Highest spatial gradient magnetic field of 720-gauss/cm or less”
Notably, to date, the Food and Drug Administration has approved MRI-specific labeling for more than three thousand medical devices according to the above-described process, especially with regard to how the displacement force is determined i.e., by following the procedure described in the ASTM document (6) and with respect to how the findings are presented in the labeling (i.e., reporting the value for the highest spatial gradient magnetic field that was used for the determination of the deflection angle for the medical device, which was based on where the apparatus with the protractor was positioned). Furthermore, the ASTM document that addresses the measurement of translational attraction for a medical device clearly states to report “10.1.7….the magnitude of the spatial gradient of the magnetic field, at the test location.” That is why this particular spatial gradient magnetic field value has been used in the FDA-approved MRI labeling for thousands of medical devices. Of note is that the reported results describe the MR system variables under which the implant or device was tested and do not necessarily represent the safety threshold for translational forces.
Recently, MR system manufacturers have provided data to MRI professionals pertaining to the “spatial gradient magnetic field” values for a given scanner, including information for the highest spatial gradient magnetic field. Presumably, this was done in an effort to help address the case when a patient presents with a medical device that has labeling stating the “highest spatial gradient magnetic field” value permitted for that implant and that parameter is unknown for a specific MR system. The format, values, locations for measurement and presentation of these reported MR system values have varied, and even conflicted between and within the MR system vendors that have provided these data.
Considerable confusion has inadvertently arisen as a result of these recently reported spatial gradient magnetic field values. The reason for this misunderstanding is simple: The location, and, therefore, the values, of the highest spatial gradient magnetic field measured and reported by the MR system manufacturers is sometimes not the same as that which is utilized for the location at which the implant/device displacement force testing was performed. To elaborate, the location of measurement of the highest spatial gradient magnetic field that is used when evaluating the displacement force for a medical implant or device (i.e., the place in the MR system where the apparatus with the protractor is positioned; which is generally off-axis, near the opening of the bore of the scanner) is not necessarily the same as the location where the MR system manufacturer measures the scanner’s highest spatial gradient magnetic field.
The MR system manufacturer may have performed the measurement with the covers or shroud removed from the MR system, and/or without the patient table present. This permits access to stronger static magnetic fields and spatial gradient magnetic fields and, thus, can result in a greater measured value for the “highest spatial gradient magnetic field” as it is reported by the manufacturer. This region and, therefore, the measured value, however, is not one that can be reached by a patient with an implant and, thus, it does not represent a reasonable assessment of risk exposure for that situation.
We recognize that the values reported for the regions not normally accessible to patients may have significance or relevance for MR system manufacturers and their manufacturing and/or service employees. Nevertheless, from the point of view of clinical patient care, these “patient inaccessible regions” and the spatial gradient magnetic field values reported for them are of no particular benefit. Indeed, reporting these values seems to have resulted in confusion.
Furthermore, making decisions regarding the patients with implants or devices based on these higher spatial gradient magnetic field values measured in patient inaccessible regions might inadvertently lead to canceling requested and needed clinical MRI examinations for patients with devices tested and cleared to the patient accessible region’s translational forces – but not to those higher values reported at patient inaccessible regions.
Clinical personnel responsible for assessing the labeling that states the “MR conditional” aspects of an implant or device, rated as acceptable to a value of 720 gauss/cm, when their MR system by the specifications provided by the MR system manufacturer is identified as having a maximum value 200 gauss/cm higher (e.g., 900 gauss/cm), may refuse to scan the patient with the implant on that MR system which, ironically, is equivalent to the scanner used to demonstrate safety relative to translational attraction for the implant!
While the value for the highest spatial gradient magnetic field that the MR system manufacturer reports for a given scanner does not have direct relevance to a patient with a medical device, if the MR system manufacturer provides measurements that are obtained in radial increments from that position, these values may be useful for managing patients with medical implants and devices, if the position used to perform the deflection angle test on the device was known.
In consideration of the discrepancy between the MR system manufacturer’s reported value for the highest spatial gradient magnetic field for a given scanner and the highest “accessible “spatial magnetic gradient value used for medical device testing that has led to confusion and frustration when MRI professionals must consider how to manage patients presented with MRI labeling information, we strongly recommend that standardization is necessary. Notably, it would be helpful and appropriate for all MR system manufacturers to report the greatest static magnetic field strength as well as the highest spatial gradient magnetic field values in regions that are readily accessible by patients and/or healthcare professionals working around an “intact” (i.e., with the covers or shroud not removed) MR system.
In order to accomplish such a task, the FDA needs to understand the nature of the present problems related to the MRI specific labeling for implants and devices, develop an appropriate strategy to address this matter, and implement a solution in a timely manner.
Clinical Implications
For today, a recognition that the spatial gradient magnetic field values provided by the MR system manufacturers may differ (i.e., typically the values are higher) from those at which the implant might have undergone testing for translational attraction is the first step towards understanding the important issues presented in this Editorial Commentary. Hopefully, this information will enable an appropriate determination of the risk-benefit for the management of patients with implants.
If the deflection angle information for the implant can be obtained from the device manufacturer, and the deflection angle value was somewhat trivial (e.g., 15-degrees or lower) for the specific MR system to be used for the examination, it would be reasonable to expect that a minor variation in the highest spatial gradient magnetic field value reported by the MR system manufacturer (i.e., compared to that reported in the labeling for the device) would not produce a clinically significant difference in the safety outcome. Of course, an entirely different conclusion might be reached if the translational attraction testing documented a significant deflection angle for the implant.
Conclusions
There are two different positions where the highest spatial gradient magnetic field may be measured and reported. One, the highest “patient accessible” spatial gradient magnetic field, which pertains to that used for the deflection angle test for a medical implant or device and through which a patient with an implant or device may pass. The other, the scanner’s highest spatial gradient magnetic field, as reported in the specifications for the magnet of a given scanner, that reports the region of greatest dB/dx regardless of its accessibility to patients or healthcare providers. The values measured by the MR system manufacturers are inherently higher than those reported and used in the labeling of medical devices are of dubious clinical value, and should be replaced, or at the very least supplemented, with the greatest dB/dx values reachable or accessible by patients and healthcare professionals.
Excerpted with permission from the American Journal of Roentgenology. To obtain the original article including the figures/illustrations, please visit www.IMRSER.org.
References
(1) American Society for Testing and Materials (ASTM) International Designation: F 2503–05. Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment. ASTM International. West Conshohocken, PA.
(2) Shellock FG, Woods TO, Crues JV. MRI Labeling Information for Implants and Devices: Explanation of Terminology. Radiology 2009;253:26-30.
(3) Shellock FG. Reference Manual for Magnetic Resonance Safety Implants and Devices: 2014 Edition. Biomedical Research Publishing Group, Los Angeles, CA, 2014.
(4) Woods TO. Standards for medical devices in MRI: present and future. Journal of Magnetic Resonance Imaging 2007;26:1186-1189.
(5) www.MRIsafety.com
(6) American Society for Testing and Materials (ASTM) International Designation. F 2052. Standard test method for measurement of magnetically induced displacement force on passive implants in the magnetic resonance environment. ASTM International. West Conshohocken, PA.
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