Choosing the right MRI imaging equipment for your hospital or clinic can be a complex decision.
As advances in technology have continued in the MRI field, you have more choices available for machines of higher field strength. Typically, these machines can cost at least twice as much as the commonly used 1.5T machines, so clinics find themselves weighing up the cost-benefit.
One of the big questions people ask is, how strong does that MRI magnet really need to be? Does bigger mean better? Let’s take a closer look at MRI and the relative merits of field strength:
A primer on MRI field strength
“Field strength” refers to the magnetic field strength of the magnet used in the MRI machine. This correlates with signal-to-noise ratio — the stronger the field the stronger the signal. Magnetic field strength is measured in teslas (T) and higher field strength can also equate to faster throughput.
The measurement in tesla is proportional. Therefore, an MRI machine at 3.0T is twice as strong as a machine at 1.5T.
Commercially available MRI scanners for routine clinical use exist from 0.2T to 3.0T, while research facilities currently perform human imaging in fields up to 11.7T. The vast majority of machines in use in a clinical setting are 1.5T (including our mobile fleet at DMS Health). The diagram below shows examples of field strengths and applications that they are used for:
Channels and coils
One of the most important elements of an MRI to consider is the coils. The coils correlate with how many channels your MRI machine is able to offer. This is through the number of coil elements, so four elements means four channels. A channel refers to the receiver pathway of the MRI system.
More channels mean better image quality and improved speed of acquisition. Many MRI machines in use have 4, 8, 16, even 32 or more channels. On-coil digitization is a technology that is allowing more channels off a lower number of coils, too. This technology can help with the overall price as more elements mean a higher price.
If you’re looking at field strength of an MRI magnet, it’s important to keep in mind that channels play a big role. For example, a unit with a comparatively weaker field strength may have more channels available, which may be more beneficial than one with a higher strength field and less channels.
How strong should your MRI magnet be?
There is no correct answer to this question. The MRI magnet needs to be as strong as is needed to get a result for the specific test being run. For example, some things show up better on 3.0T or higher, while other things don’t require nearly as much field strength.
For the purchaser of this equipment, the key question is, how much strength do I need for the tests that we need to perform? This is usually asked with the consideration that the stronger the equipment is, the more you will pay for it.
In a clinical setting, 1.5T is generally considered to be more than enough for most routine scans. Many state-of-the-art facilities use 1.5T MRI machines and consider them to provide excellent imaging.
There are pros and cons to each field strength in terms of what they can and can’t do. It’s important to remember that stronger doesn’t always mean “better.” For example, higher field strength may not be able to be used with certain implants or devices and may not be advised for pregnant patients. On the other hand, higher field strength can allow you to see things at a better resolution and potentially faster than a lower field strength. On applications where you need higher resolutions or more signal, such as on smaller body parts, breast tissue or spectroscopy, the higher field strength can be an advantage.
This does not mean that you won’t be getting diagnostic value from a lower field strength. For example, if we were to compare 1.5T and 3.0T, there would be some applications for which the higher field strength has little advantage.
1.5T vs. 3.0T MRI machines
In comparing 1.5T against 3.0T MRI machines, an understanding of signal-to-noise ratio is important. We explained earlier that the stronger field strength produces a stronger signal — a clearer image may be produced because the stronger signal overcomes background noise. This noise shows up as graininess in images, which will be reduced in 3T as compared to 1.5T machines. It’s like turning up the volume on your music to drown out background noise.
Higher field strength also correlates with higher temporal resolution and higher spatial resolution. Higher spatial resolution is helpful in gaining clearer images of small, complex structures, such as detecting nerve root, spinal cord or neuroforaminal pathologies when imaging the spine.
Higher temporal resolution makes for a more efficient MRI scanner. Comparing 3.0T and 1.5T machines, it means that the 3.0T scanner may be able to throughput more patients in the same amount of time with similar image quality to the 1.5T scanner. Of course, in facilities with relatively low-volume scanning needs, throughput may be a negligible factor.
An artifact is a visual anomaly on an MRI image. Sometimes these artifacts can have an impact on diagnostic quality, although sometimes they don’t interfere with diagnosis. Artifacts may be classified as signal-processing dependent, patient-related or machine-related.
Out of the two strengths of machines we are comparing, the 3.0T can have a higher likelihood of artifacts appearing on images, such as blood or fluid.
Every imaging facility is concerned with the cost-benefit ratio on their equipment. It’s typically a key driver in purchase (or interim) decisions. Right out of the gate, we can tell you that a 3.0T machine is considerably more expensive than a 1.5T (even refurbished, second-hand 3.0T machines can easily cost you double that of a 1.5T machine), but what of cost-benefit?
Here you might weigh up the cheaper 1.5T machine with the throughput efficiency of the 3.0T. This throughput may be a significant factor in a high-volume facility but it is important to know that throughput can only be increased to a certain point before it impacts the patient and image quality. Think of it this way; even if you have a car that can go 140 mph, it can’t run that fast all the time and it’s important to make sure that you don’t push it so hard it overheats. There are pros and cons to increased throughput and it is important to keep patient care and safety in mind.
It’s also worth noting that parts are more readily available for the 1.5T machines should they be required. Maintenance and repair cost a lot more on 3.0T machines.
In terms of reimbursements, it’s important to note that Medicare, Medicaid and private insurance payors reimburse the same amount for an MRI scan. It doesn’t matter if the scan was conducted on a more expensive machine. This can definitely play a role in whether patients are willing to pay for scans from the 3.0T machine when their co-pay may be less elsewhere.
On the other hand, patients with needs for very detailed imaging may be better-served with the higher-strength machine and willing to pay for it. Advanced equipment may be a source of marketing differentiation in the eyes of the patient. You will probably want to assess how much need your facility typically has for fine detail imaging work.
How strong does your MRI magnet really need to be? The answer lies with your individual facility in terms of your needs for highly detailed images and the volumes of patients you need to have scanned. A 1.5T field strength is the most commonly used and is considered appropriate for most clinical needs.