Electromagnetic Simulation
Cuts Cost of Protective MRI Enclosures

by Ben Turner, Lindgren RF Enclosures, Inc.

Simulation of electromagnetic fields substantially reduced the cost of the protective shields surrounding magnetic resonance imaging (MRI) suites at Chicago Northwestern Memorial Hospital. Also, by using simulation as part of the design process, engineers were able to ensure that the shields would perform perfectly prior to installation, eliminating the need for costly modifications.

Software enabled engineers at Lindgren RF Enclosures, Inc., Glendale Heights, IL, to quickly evaluate different shield materials and material thicknesses to achieve the desired level of protection at the lowest price. The company is a manufacturer and supplier of shielding systems for electromagnetic and radio frequency interference (EMI/RFI), installing more than 25,000 enclosures within a variety of industrial, governmental, and medical environments in more than 75 countries. Its services include architectural assistance, site evaluation, design, engineering, fabrication, on-site installation, and certification testing to support all application needs.

Strong field

An MRI system generates an image of the body through the use of a very strong magnetic field and RF signals. The magnetic field causes the alignment of every molecule in the body along the magnetic lines, while RF signals cause the molecules to vibrate. Different tissues vibrate in response to different frequencies, so as the MRI system pulses the body with varying RF signals, pickup coils detect the vibrations and generate an image of the different tissues within the body. The resulting scan resembles an X-ray but it is better for viewing soft tissues such as the liver and heart.

A basic MRI system's magnetic field is so strong that if it were not shielded, it would extend out from the facility and potentially harm other equipment -- such as monitors, computers and other test apparatus -- or even people outside the MRI room who have pacemakers or metal implants. Shielding an MRI suite with certain materials, which are also magnetic, contains much of the field and can prevent these problems. Shielding is also done to protect the MRI system. Large masses of metal, such as a truck passing by outside, could disrupt the imaging process by altering the MRI's uniform magnetic field.

The challenge for Lindgren is that each project the company undertakes is unique, as an enclosure must accommodate both the MRI system as well as the facility in which it will reside. There are many different MRI systems, all with differing magnetic fields. The sites can be fairly simple, such as those with flat, simple rectangular walls and ceilings, or they can be complicated by the need to position the shield around structural columns or beams.

An MRI system generates an image of the body through the use of RF signals and a very strong magnetic field.

In the past, companies would need to perform calculations by hand to predict how well a particular enclosure design would contain the magnetic field of a given MRI system, but they couldn't know for certain how well the enclosure would perform until the facility was actually built. Magnetic field simulation software can decrease design time with increased accuracy. Simulation allows a company to know whether an enclosure was effective before it was built, and gives engineers the facility to quickly try out different designs to achieve effective containment while minimizing costs.

Lindgren engineers use OPERA software from Vector Fields, Aurora, IL, because its graphical user interface made it easy to model the numerous MRI systems, shield materials, and configurations. In addition, its powerful scripting language makes it possible to automate the analysis in order to rapidly evaluate a large number of proposed designs.

Better process

The company's work with Chicago's Northwestern Memorial Hospital is a good example of how Lindgren uses up-front simulation to deliver an optimized enclosure. In this case, Lindgren was asked to design shields for several MRI systems at the multi-story facility. Initially, the hospital was not sure which type of MRI system it would purchase. Lindgren engineers helped them research the technology, showing how much shielding the different types of machines would require, and how shielding would affect construction costs and schedules. This information helped the hospital narrow the possibilities to three MRI systems.

Once the hospital selected an MRI vendor, Lindgren engineers began designing the enclosures in detail, starting with the architect-provided floor plan of the MRI suites. Starting with the MRI's system's free field (its magnetic field without any shielding), and borrowing from past experience, a preliminary shield design was developed. The next step was entering this information into OPERA to create the analysis model. Engineers graphically generated a model of the enclosure dimensions in 2D plan and extruded into the third dimension. Then shielding material was added, based upon heat-treating and chemistry specifications in the model. The materials and B-H properties of the shield components were a part of the library of material data contained in the Vector Field system. The program then divided the model into finite elements.

Electromagnetic simulation is especially useful in MRI shielding applications because each project is unique.

With this information, the TOSCA electromagnetic analysis solver, also from Vector Fields, predicted how well the shielding material would contain the magnetic field in this problem. The TOSCA package provides three-dimensional analysis of magnetostatic, electrostatic and synchronous fields through state-of-the-art numeric methods. The analysis determined where the shield material saturated and presented the information by means of graphs, histograms, and contour plots showing magnetic field values superimposed on the facility model. From this output, engineers could clearly see where the shielding material was inadequate and where it was over-applied. Engineers then went back to the original design and, through an iterative design-analysis process, trimmed the shield to the lightest weight possible while still achieving effective containment of the magnetic field. Because they could evaluate many different combinations of materials and material thicknesses, the shielded enclosures for this facility were the most cost-effective solutions possible.

Unique projects

Another Lindgren project, involving an MRI facility in Texas, demonstrates dealing with unusual design parameters. The site where this particular MRI system was going to be installed had very poor soil. The architects were aware of this and had designed the facility so that there was one slab under the MRI machine and a separate slab under the rest of the MRI suite. These two slabs were also independent from the rest of the facility. The architects knew that the weight of MRI machine would cause the slab underneath it to settle, but the exact amount of settling was unknown. Likewise, the MRI room slab would also settle. In addition, they knew that the two slabs would settle at different rates. Lindgren engineers had to design a shielded enclosure that would not be compromised by the settling, while still providing RF and magnetic shielding. Solving this problem without extensive experience and the use of simulation software would have been very difficult and only time would have told if the design were adequate. With electromagnetic simulation software, engineers were able to evaluate many different settling scenarios and design a shield that performed well in all of them.

Typically, the use of simulation software saves the customer between $6,000 and $8,000 per facility. Electromagnetic simulation has also given Lindgren an important competitive advantage over companies that rely on trial and error, and qualified guesswork.

For more information:

Circle 420 - Lindgren RF Enclosures, Inc., or connect directly to their website via the Online Reader Service Program at http://www.1rs.com/012-df420

Circle 421 - Vector Fields or connect directly to their website at http://www.1rs.com/012-df421