MXR Imaging acquired Oxford Instrument Healthcare in May 2020
Why Use an Ultra High-Field MRI? New Insights That the Technology Offers
Ultra High Field MRI Systems Are Offering Amazing Medical Insights
The quality of the images captured by an MRI depends on the strength of the magnetic field: the stronger the magnetic field the MRI system can produce, the higher quality the images will usually be. While typical MRI systems produce field strengths between 1.5T and 3T, there are ultra high-field MRI systems available that produce magnetic fields at 7T. However, these ultra high-field MRI systems are still somewhat rare in hospital settings since the first 7T MRI was only recently approved for clinical imaging last year.
What is Magnetic Field Strength?
MRI stands for Magnetic Resonance Imaging. It's the only diagnostic imaging system that uses magnets instead of X-rays to capture images of a patient's internal organs and structures. It works by using its powerful magnets to produce a magnetic field that causes the protons in the body's water molecules to become aligned.
The MRI also produces a radio frequency current. The protons absorb this energy and flip their spins. Once the magnetic field is turned off, the spin of the protons returns to normal. It's this return to their normal spins that are measured by the system's receivers and converted into an image. The stronger the strength of the magnetic field, the less close the patient has to be to the magnet and the faster the process will be completed.
The clarity and resolution of the images captured by the MRI also depend on the magnetic field strength. There are MRI systems that produce different magnetic field strengths. As a result, they are typically used for different purposes. The following is a brief rundown of the four main types of magnetic field strengths:
- Low-field - Low-field MRIs produce magnetic fields below 0.3T. Because the speed of the scan is largely dependent on the strength of the magnetic field, low-field MRI scans take longer to perform. Extremity MRI systems are typically low-field because they don't need to produce a large magnetic field to capture images of hands, arms, legs, or feet--and the magnets are often directly adjacent to the extremities.
- Medium-field - Medium-field MRIs produce magnetic fields between 0.3T and 1T. They are usually open MRI systems, meaning the patient isn't completely enclosed. They are used to capture images of the torso and head, something that isn't possible with an extremity MRI.
- High-field - High-field MRIs produce magnetic fields between 1T and 3T. High-field MRI’s are closed systems, which means the patient is completely surrounded by the magnet except for where they entered on their back. High-field MRIs provide higher resolution images that make it easier to diagnose certain conditions than medium-field MRIs.
- Ultra high-field - Ultra high-field MRIs produce magnetic fields at 7T or stronger. These closed MRI systems are the most powerful MRIs available for clinical applications. They are more effective than high-field MRIs for diagnosing or tracking neurological conditions.
What Makes a 7T High-Field Scanner so Powerful?
The power of a 7T ultra high-field MRI allows for clinical applications that were not previously possible with lower strength MRIs. For example, both X-nuclei imaging and spectroscopy are only feasible at ultra-high magnetic fields. A 7T magnetic field also improves the resolution and clarity of the scans, which makes the system capable of more accurate brain and joint imaging. It's one of the reasons why 7T MRIs have been so heavily relied on for research purposes over the past few years. In fact, some of the newer 7T MRI systems approved for clinical use actually offer dual-mode functionality for both clinical and research settings.
Although you might assume that with such a huge increase in magnetic field strength the MRI system must be much larger and more difficult to integrate into your facility, but this isn't actually true. Technology has advanced to the point where not only is a 7T magnetic field easily produced, but the magnets that produce it are 50 percent lighter than the magnets that were being used in previous MRI systems. Nor will you have to deal with the added cost of having to refill the system's liquid helium more often due to the use of zero helium boil-off technology.
As advanced as the new 7T MRI systems may seem, there are actually 10.5T MRI systems currently on the market. Manufacturers are even working to surpass the power of those MRIs as they are currently in the midst of developing MRI systems capable of producing 11.7T magnetic fields.
What Kind Of Insights Can We Gain From Ultra High Field MRI?
The 7T MRI can capture anatomical images and functional images that are more than twice the resolution of those generated by a typical 1.5T MRI system. This makes it possible for doctors to make much more accurate diagnoses in patients who have complex neurological and musculoskeletal conditions. For example, an ultra high-field MRI can be used to diagnose and track Alzheimer's disease, epilepsy, Parkinson's disease, brain tumors, traumatic brain injuries, cerebrovascular diseases, and more.
Many conditions (such as those previously listed) are difficult to identify using lesser powered MRI systems. Multiple sclerosis is a condition that tends to be particularly challenging to assess. However, a 7T MRI system basically allows doctors to perform a virtual biopsy by visualizing cortical lesions as well as central veins in white matter lesions, which are notable features of multiple sclerosis. Such capabilities could allow doctors to better understand and treat progressive forms of multiple sclerosis, which currently has very few forms of treatment. Additionally, the ability to identify lesions (which 3T MRI systems typically cannot), significantly improves the chance of a better surgical result for the patient.
Certain early stage structural pathologies cannot be identified by lesser strength MRI systems as well. An ultra high-field MRI has that ability and can be used by orthopedic surgeons to capture non-invasive, detailed pictures of the patient. For example, using a 7T MRI, an orthopedic surgeon can get to the source of the knee pain a patient might be experiencing by capturing detailed images of the trabecular bone, the tendon and soft-tissue, the collagen ultrastructure and make-up in osteoarthritis, and the microstructure in trauma and osteoarthritis.
The good news is an ultra high-field MRI allows doctors to capture images in a resolution and clarity that were previously impossible, making it easier to diagnose numerous conditions at earlier stages. Such capabilities should also help promote the development of alternative treatments for many of those conditions.
Are There Any Disadvantages?
Even though both the clinical and research benefits of an ultra high-field MRI are almost countless, there are a few disadvantages that you should be aware of. The biggest disadvantage is arguably the cost of a 7T MRI. It might be twice as powerful as a high-field, 3T MRI system, but it's also more than twice as expensive--often even more so. Don't be surprised to invest anywhere from $7 million to $10 million in a 7T MRI system.
Besides the cost, one potential concern you might have is the specific absorption rate (SAR). The SAR is the rate at which energy is absorbed by the patient when exposed to a radio frequency electromagnetic field. The SAR is much higher due to the more powerful magnetic field produced by a 7T MRI. It's one of the reasons why ultra high-field MRIs were not cleared for clinical use for so long.
However, the FDA set SAR limits to ensure the safety of patients undergoing high-powered MRI scans before they approved any ultra high-field MRI system for clinical use. As a result, any 7T MRI system being used for clinical use is equipped with monitoring systems to ensure the SAR limits aren't surpassed. On top of that, lower-flip angle sequences have been designed to reduce RF energy deposition. Without these advancements, the 7T MRI would not have been cleared for clinical use by the FDA. Meeting these SAR limits means it's now safe for use.
The other drawback is that due to the strength of the magnet, ferrous metal objects are even more susceptible to the magnetic field produced. Fortunately, as long as the proper safety precautions are adhered to, this should not pose a risk.
Do All Hospitals Need an Ultra High Field MRI?
We acknowledge that this system won’t be necessary or in the budgets for all hospitals, but the possibilities of this new technology certainly have raised a lot of excitement among medical professionals.
An ultra high-field MRI system is a major investment to make, and only consider making it if you have plenty of room in your budget. Even if you have the budget for a 7T MRI system, consider who your patients are. If you see a lot of patients with neurological diseases or musculoskeletal conditions, then a 7T MRI system can be incredibly helpful. However, if you do not treat many such patients, then a 1.5T or 3T MRI system is more than capable of providing the high-quality diagnostic imaging that you need.