From the March 2011 issue of HealthCare Business News magazine
This report originally appeared in the March 2011 issue of DOTmed Business News
By Joel Kellogg
MRI systems can be negatively impacted by electromagnetic interference generated by a variety of sources — including moving metal, subways, electric buses, transformers, and electric lines. Typically, EMI sources will impact gradient echo sequences, functional MRI (fMRI), and spectroscopy. While OEMs and end-users will try to avoid situating in areas impacted by EMI, it can be very difficult to find “quiet” sites in densely populated urban areas.
The challenge: electromagnetic interference
Areas such as New York City represent an extreme challenge for OEMs and users attempting to site EMI sensitive equipment. Failing to identify a solution for a site with large environmental EMI disturbances decreases the usefulness of an MRI system. Sensitive sequences like fMRI will be impacted and the images produced will suffer. Additionally, gradient echo imaging sequences can be impacted resulting in degraded images that show signs of image ghosting.
Solutions: evaluating cost, performance, and convenience
Essentially, there are five possible solutions for a site experiencing EMI disturbances. The first three are generally not options. They would consist of identifying a new site free of disturbance, operating a hobbled MRI or abandoning the project altogether.
Shielding solution
Of the more reasonable options, the first would be to implement a passive shielding design around the MRI in order to reduce the EMI levels inside the MRI suite. A passive shielding solution involves using magnetic shielding material — typically, some sort of steel — to reduce the EMI. Unfortunately, the shielding properties of these materials are very poor when attempting to address EMI disturbances generated by quasi-DC sources such as subways and moving metal. Often, these solutions come with a price tag in the hundreds of thousands of dollars. Further, it is difficult to guarantee the performance of these shields as there are additional technical challenges. For example, the shielding can be rendered completely ineffective if it becomes saturated due to elevated DC magnetic fields. Inevitably, either a portion or all of the passive shielding around an MRI will become saturated rendering the magnetic shielding ineffective for reducing quasi-DC EMI. In order to create an effective passive shield, the shield would need to be designed in layers with the layers closest to the MRI designed to contain the Gauss fields and the layers furthest from the MRI designed to reduce the EMI disturbances. This would require that the layers closest to the MRI to provide much higher Gauss containment than is typically required. In most installations, only the five Gauss must be contained from public areas per FDA recommendations, but in this case, the initial shield for Gauss containment would most likely need to provide one Gauss protection or better to avoid saturation of the outer. Further, the outer layer would need to be comprised of a very high permeability material such as “mu-metal.” These materials tend to be very expensive.
