VIBRATION ISOLATION OF MAGNETIC RESONANCE IMAGING MACHINERY (MRI)

Magnetic resonance imaging is an analysis that uses a magnetic field and radio waves to obtain detailed images of organs and structures of the body. The magnetic resonance equipment is made up of a large ring-shaped magnet, inside which is placed the stretcher on which the patient will later be placed.

During the examination, radio waves manipulate the magnetic position of the body's atoms, which is detected by an antenna and sent to the computer in the neighbouring room. This computer modifies the required information to obtain images from it. These images can even be converted into three-dimensional images.

During this process, the disturbances in this room are required to be as small as possible, since this would affect the quality of the obtained images. Taking into account that there are many elements in hospitals that create disturbances in the way of vibrations of different frequencies, for example air conditioning units, X-ray machine, tomographic machines, ... it is necessary to correctly insulate both the MRI room and the machine itself, so it will be necessary to use the anti vibration mounts. These anti vibration mounts will be in charge of attenuating these excitations transmitted through the structure of the building before they are transmitted to the base of the MRI machine.

MRI rooms are based on the Faraday cage principle. Faraday cages are a fundamental and essential part of an MRI equipment. The Magnet is in an enclosure specially constructed with electrically conductive materials to prevent radio waves and strong magnetic fields generated by it from interfering with other hospital equipment, both the resonance images treating equipment and the other existing equipment.

Its mission is to eliminate radio frequency interference from the outside so that it does not affect the reception and generation of images, as well as to avoid the emission of strong magnetic fields generated by it to the outside of the enclosure. The Radio Frequency enclosure is based on a metal box that surrounds the Magnet and prevents electromagnetic waves from entering or leaving it, maintaining a null electric field.

VIBRATION ISOLATION OF MAGNETIC RESONANCE MACHINERY

As it has been seen, it is essential to keep a complete enclosure of the room to maintain the principle of the Faraday Cage. Therefore, it is necessary to follow some guidelines when installing the acoustic insulation system.

The first is that the anchors, both for the room and for the elements inside, must not be in contact with any metallic element external to the room, that is, they cannot be in contact with any reinforcement of the lower concrete slab or other structural metal of the building.

The anchors must be electrically connected to the Faraday cage, so they must be composed of electrically conductive materials but must not be magnetic conductors or cause galvanic corrosion with the Faraday cage.

There are different solutions for the acoustic insulation of the MRI machines, being able to combine the different options at the same time.

1. Under a concrete slab

The isolation of floating floors, walls and ceilings is known as “Box in Box” vibration and noise insulation principle. The room is insulated by means of anti vibration mounts from the structure of the building on its entire surface to insulate the floor, walls and ceiling of the room. This way we avoid the possible transmission of noise (soundproofing of the room) and vibrations that could be transmitted through the structure of the building, as can be seen in the following picture:

To get this, using the details of the previously exposed image, different solutions are used, with different types of anti vibration mounts, in each of the areas to be insulated. The roof is insulated by means of a “suspended ceiling”, thus joining a composite block with insulating materials (both thermal and acoustic) to the upper slab using an acoustic hanger that allows the vibrations transmitted by the structure of the building to be insulated, achieving the rooms soundproofing:

A. Roof detail

The interior walls are also joined to the structural walls of the building by wall acoustic isolators, designed to achieve high acoustic insulation from possible excitations transmitted structurally in the building.

B. Wall detail

Finally, the acoustic insulation of the floor of the room is done by placing floating floor mounts under a concrete slab, either a continuous element like the one in the image or by placing elastomeric acoustic hangers (with non-magnetic steel) distributed throughout the base of the slab. This concrete slab serves to achieve a more even distribution of loads over the entire surface.

C. Ground detail

For this, materials of different densities can be combined according to the load that they will be forced to support in the different areas of the room. The following image shows an example of the acoustic insulation for one of these rooms.

In the picture, different areas of different colours (which would indicate the density of the floating floor mounts´ insulating material) can be differentiated, in which the turquoise area would indicate the location of the MRI machine, therefore, the load will be higher in this zone requiring a material with a higher density. The areas on the edges of the room, on which the metal structure rests to close the Faraday cage or the windows of the room are placed, also require a higher density isolator, represented in the colours orange and red, which in the rest of the room, where the lowest density isolator is used, represented by the yellow colour.

For all this, AMC-MECANOCAUCHO has a variety of anti vibration mounts for each of the areas to be isolated.

Floor acoustic insulation:

Depending on the excitation frequency that must be isolated and the level of isolation required, AMC-MECANOCAUCHO has different types of anti vibration mounts.

To achieve lower natural frequencies and, therefore, a higher level of isolation, the ideal solution would be to use Vibrabsorber + Sylomer® spring type mounts, which allow to obtain natural frequencies below 5Hz. These mounts are placed under the concrete slab. The springs provide high acoustic insulation level against low frequency excitations while the Sylomer® strips provide acoustic insulation against high frequency waves that could be transmitted through the spring coils.

Another possible solution would be to use acoustic insulation mounts based on Sylomer®, with which a natural frequency close to 10Hz would be obtained, being the indicated solution when soundproofing of the room is needed, as they offer great acoustic isolation level against high frequency noises. AMC-MECANOCAUCHO has a wide range of acoustic insulation mounts based on this solution. Some, like the FZH, are installed embedded in the concrete slab, making them a good solution when the height of the room is limited. An additional insulating material can be added between the floating floor mounts to improve its acoustic and thermal performance.

Others, such as plates Sylomer® (example previously described) or Akustik Floor Blocks are placed under the slab. In this case, it is also recommended to cover the excess space using insulating materials to optimize de level of soundproofing and the thermal isolation of the room.

A third option is the use of rubber pads under the concrete slab. This solution is indicated when the excitation frequency to be isolated is higher, since natural frequencies below 15Hz are achieved. The rubber blocks are placed under the concrete slab and the excess space is filled with mineral wool or another absorbent material to obtain a better level of thermal and acoustic insulation.

Ceiling acoustic Insulation:

In the same way that Floating floor mounts contribute to its acoustic insulation, acoustic hangers are the effective solution to reduce structural noise through the ceiling. The efficiency of the acoustic hangers depends mainly on their natural frequency to the load they are supporting.

In the graph below, the complete range of AMC-MECANOCAUCHO acoustic hangers and mounts can be seen, indicating the natural frequency obtained by each acoustic hanger at its optimal workload.

Wall acoustic insulation:

Acoustic wall acoustic isolators are often used for high acoustic requirements. The use of these isolators optimizes the acoustic insulation of the suspended ceiling, preventing disturbances from being transmitted to the room through the partitions. For this purpose, the rigidity of the acoustic isolator and the impedance of the elastic material play a key role, so its installation must be carried out correctly to obtain optimal acoustic insulation results.

2. Under the benches:

In this case, the surface on which the placement of the anti vibration mounts (insulating material) is limited to the bench on which the system rests. For this, the floor of the room is placed leaving the space corresponding to the bench of the resonance machine.

Once the floor of the room has been placed, the plates are placed, made up of anti vibration supports (the insulating material of the corresponding density to be able to support the weight of the machine) and a metal plate to distribute the weight of the supports on a greater surface.

Once the plates are installed, with the corresponding holes prepared to join the bedplate as it has been shown in the previous picture, the bedplate of the machine is screwed to them, always using elements that are not magnetic conductors.

Finally, the resulting necessary elements would be placed to carry out the medical tests and the machine enclosures.

3. Under the magnet supports:

In the latter case, the anti vibration mounts (the insulating material) would just be placed under the magnet supports, so it would be a solution similar to the previous one, but using a less amount of insulating material, although this should be of higher density because the surface on which the load is distributed is less, so the level of acoustic insulation would also be lower.