MRI Shielding and Magnetic Field Management

The invention of the MRI (Magnetic Resonance Imaging) Scanner in the late 1970's heralded a major advance in diagnostic medicine.

Today the MRI allows the entire human body to be scanned, and presented in sections for review by Radiographers and Consultants. MRI's differ from X-Rays and CT Scanners in that they don't use ionizing radiation, and are therefore inherently safer for patients, they are also far superior in providing diagnostic imaging of soft tissue such as the brain, spinal cord, muscles and ligaments - hence their frequent use in sports injury clinics.

How does an MRI work?
When outside of a strong magnetic field, the protons in the nuclei (the central cluster of particles) of the atoms which make up every chemical element, and in turn make up every part of our bodies, are randomly aligned so that they point in any direction. These are rather like tiny needles on a compass without a Magnetic North to align them. An MRI scanner is effectively a very powerful magnet (or more accurately a number of powerful magnets) and when the atoms in our bodies encounter a strong magnetic field, the protons align themselves with it like flags in a strong breeze.

Once a scan commences, RF (Radio Frequency) energy or Radio Waves at specific frequencies are pulsed through the patient, this additional energy excites the protons and causes them to change their orbit slightly, shifting their alignment as they spin around their rotational axis (these two energy states are often termed 'Spin-up' and 'Spin-down'). Between pulses, the protons release this additional energy and re-align with the magnetic field - the rate at which they re-align and the quantity of energy released is captured by sensors in the MRI allowing the Imaging Software to define precisely which chemical compounds are present. With the aid of gradient magnets which travel around the bore of the MRI magnet changing the magnetic field strength in specific locations, it is possible to pin point the exact location in space where these chemicals are present and from there to identify precisely the nature and composition of the tissue in that area.

The images provided by an MRI Scanner can be set to take minute slices through the entire body, or just a particular part of the body to provide an extraordinary level of detail to the Radiographer. This level of detail is absolutely key to the effective use of the MRI as a diagnostic tool. There are two key elements which are essential to the safe use and correct function of the MRI Unit and to achieving reliable high resolution diagnostic imaging, these are RF (Radio Frequency) Energy and Magnetism and each requires careful management.

How is RF Energy managed and why is this important?

Copper RF Shielding for MRI Applications

Because the way that an MRI Scanner works relies on the use of RF Energy, it is important that the only energy that reaches the patient is that which is produced and can be quantified by the MRI itself. This means that stray RF Energy from outside the MRI Examination Room must be excluded or it will affect the energy state of the protons in the patients body. This stray energy would result in 'artifacts' on the scan where the information received by the sensors and software doesn't correlate or cannot be understood by the software. This would be effectively like looking at the scan results through a dirty window.

The exclusion of unwanted energy is achieved with the introduction of RF (Radio Frequency) Shielding and often employs copper shielding panels and flooring and special interface components, doors and windows.

The design, manufacture, installation, testing and maintenance of RF Shielding is a highly specialised field of Engineering and requires carefully coordinated, collaborative design and implementation. Shielding Specialists like Response MRI work closely with MRI Magnet manufacturers, and with the Facilities Managers along with the Fit-out and Construction companies charged with delivering and managing the facility. As the performance of MRI Room Shielded Doors and of the Shielding itself is exceptionally important to scan performance during the lifetime of the MRI facility, regular maintenance and servicing by a qualified professional is an absolute neccessity.

Why is it important to manage the magnetic field of an MRI Scanner, and how is this achieved?

There are two main reasons that the management of the Magnetic Field produced by an MRI Scanner is important, these are scan resolution and clarity, and safety.

Magnetic Field Management for Scan Resolution and Clarity
In order to accurately define the location of a particular compound within the patient, the MRI magnet must be able to precisely understand the magnetic field strength in the location of interest, and to be able to make predictable changes to this field to assist in defining this location. This means that external influences to this field must be eliminated or effectively mitigated, for instance if there is a car park next to the MRI Examination Room, the ferrous mass of a car can 'attract' the magnetic field, if not managed, the magnetic field could be pulled outward if the car began to move - clearly this would be undesirable during a scan and can significantly impact the quality of the imaging that the MRI can provide. Whilst the magnet can be 'shimmed' (a process where blocks of ferrous material are attached to the magnet bore to manipulate the field shape) there are practical limits to how much shimming can be employed before the MRI performance is degraded. Many MRI Manufacturers prefer that external measures are employed to reduce the need to shim, particularly as shimming cannot adapt to changing conditions, like a moving vehicle.

Magnetic Field Management for Safety
As we can see from the magnetic field plot to the right, the magnetic field around an MRI propagates like a bubble, and expands further from the Magnet at either end of the bore (the hole through the centre). These field strengths are very high close the MRI, and fall away the further away they are. The magnetic field strength of the Earth is 0.25 to 0.65 Gauss, whereas an MRI can yield field strengths of 200 Gauss and higher - particularly for specialist diagnostic MRI units.

A typical magnetic field plot for a 1.5 Tesla MRI Scanner

Research has concluded that field strengths of 5 Gauss and above can adversely affect medical equipment and appliances, of particular concern are the Pacemakers used in some Cardiac Patients. For this reason, 5 Gauss and higher fields must be either contained in the MRI Room or the area into which they project must only be accessible by trained and authorised personnel. In the image above, we see that without effective management, the 5 Gauss field would project outside the MRI Room. In this particular case, it would have been present in a public corridor.

How is Magnetic Field Management achieved?
To manage the field for the purposes of scan quality, there are two main approaches, each of which has its advantages:
Active Magnetic Field Compensation
In this approach, cable loops are run around the perimeter of the MRI Exam room in all three Axes. These are linked to a control unit which uses a 3 Axis Magnetometer to rapidly sample changes in the magnetic field around the MRI Unit and to provide a precisely opposing field to cancel out the incoming change.

Passive Magnetic Shielding
For applications which exceed the capabilities of an Active Cancellation System, and for containment of the 5+ Gauss fields for safety purposes, passive shielding is often employed. The idea behind this is to capture and flatten the magnetic bubble by placing ferrous material in its path to effectively provide an easier route for it to follow when compared to air we can see this in the image to the left.

The materials used vary by field strength, proximity and the final requirement. Because this approach works in both directions, it can also be highly effective against external field disturbances like the moving vehicle discussed above. The calculations required to define the materials, thicknesses and extent of magnetic shielding are highly complex and are typicaly undertaken by the Magnet Manufacturer themselves or Specialist Consultants.

It should also be noted that these two solutions can also be employed in combination which can be an exceptionally powerful, flexible and effective approach especially for Electron-Microscopy and Research Facilities.