From the March 2011 issue of HealthCare Business News magazine

Magnetic active cancellation system solution
The second option would be a magnetic active cancellation system. A magnetic active cancellation system typically includes a magnetometer, controller, amplifier and compensation coils. The magnetometer is utilized to monitor the environmental EMI. The controller interprets the data provided by the magnetometer and determines the level of magnetic field cancellation required. An amplifier then drives current through a set of compensation coils, which provide an opposing cancelling field. A magnetic active cancellation system, unlike passive shielding, can provide high levels of attenuation at frequencies less than 100 Hz.

Determining the best solution for an MRI facility

There are pros and cons to passive and active shielding solutions. Passive shielding requires significant construction and can be extremely expensive when utilized for quasi-DC EMI issues. Due to the construction methods required for passive solutions, an owner may have to accept that the volume of the room may decrease substantially due to the passive shield requirements. Further, passive shielding systems are designed and installed to meet a specified attenuation level. As a result, the passive shielding may become insufficient if the environment changes for the worse. Passive shielding, however, does not require maintenance nor rely on electronics that could potentially fail and result in MRI downtime or limited functionality of the MRI.

Magnetic active cancellation, on the other hand, does not require significant amounts of construction. Typically, the construction is limited to the installation of cabling for the system coils. These coils can be installed at the time of the RF shielding installation and concealed behind the interior finish walls. Additionally, a magnetic active cancellation system can be retrofitted into a room. Finally, magnetic active cancellation typically costs less than passive shielding and provides much better performance than passive shielding at frequencies less than 100 Hz.

Implementing magnetic active cancellation
Magnetic active cancellation is a unique solution to EMI issues as it does not employ traditional shielding materials such as steel, copper or aluminum. The system utilizes Helmholtz coils to generate a canceling field to an EMI disturbance. In order to effectively install a cancellation system, it is important to understand the EMI issue for a particular site and what the impact will be on the MRI. Provided that the RF shielding meets the magnet vendor’s requirements, RF should not pose an issue. EMI on the other hand, can create some interesting issues that can be difficult to identify. When a site is within the magnet vendor’s EMI specifications, there should not be an EMI issue, but a site that is out of specification will pose some unique challenges. For example, it is not always enough to simply reduce the magnitude of the EMI at the site. A proper assessment of the EMI will determine if there are significant gradients (the rate that the disturbance changes over distance) generated by the EMI. The gradients can sometimes be more problematic than the magnitude of the EMI. Therefore, when employing any solution for EMI it is important to first perform a proper evaluation of the site to determine the magnitude of the EMI in the B0 direction (main direction of the MRI’s magnetic field) and the gradient across the bore of the MRI. While a magnetic active cancellation system may work well in many applications, there are applications where it is necessary to customize the coil design in order to effectively reduce gradients associated with an EMI issue.

March 2011 Magazine

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