Roger Y. Shifrin, MD, chief of MRI and CT for Radiology Associates (Daytona Beach, Fla), makes an eye-opening comparison when describing the power of 3-Tesla (3T) MRI technology: “The magnetic strength is about 60,000 times as strong as Earth’s magnetic field.”
It staggers the imagination. To bring it into perspective-and to make it easier to intellectually grasp-3T MRI boasts twice the strength and speed of 1.5T MRI. Currently used for research and clinical applications, 3T MRI provides more information (about both structure and function), twice as fast.
Until recently, 3T MRI usage had been restricted to neurological studies in the research realm, where the improved resolution revealed brain structure in ways never before possible. It particularly benefited scientists working with functional MRI, spectroscopy, and perfusion and diffusion imaging.
Now, prompted by recent developments in magnets and surface coils, usage is beginning to move from the large research institutions into large and midsize hospitals and even into imaging centers-for applications beyond neurology. “We’re using it for everything from head to toe,” says Shifrin, who uses the whole-body 3T Excite model from GE Healthcare (Waukesha, Wis) at the Twin Lakes Imaging Center (Daytona Beach, Fla), a facility that operates under the umbrella of Radiology Associates.
Still, encroachment into the smaller facilities is not overwhelming. Right now, interest is higher than actual usage, comments J. Paul Finn, MD, professor of radiology and director of cardiovascular imaging at University of California, Los Angeles (UCLA). “There aren’t a lot of machines installed,” he says. “The trend is not yet significant.” But the interest sure is.
Medical professionals at smaller facilities are intrigued by the enhanced image quality and faster scan times of 3T MRI. Thus, clinical usage is expected to increase. In addition, vendors-Philips Medical Systems (Bothell, Wash), Siemens Medical Solutions (Malvern, Pa), and GE Healthcare, the leading developers of 3T technology-are developing systems that enable both brain and whole-body imaging and that surmount some of the complexities involved with higher field systems.
3T “Currency”
Two other compelling advantages of 3T MRI are higher contrast and better visualization of microstructures. Improved contrast can, in some cases, eliminate the need for gadolinium and enable earlier disease detection. Better visualization of smaller structures enables the identification of the more subtle anomalies.
Many of the advantages that attract interest toward 3T MRI-particularly the better images, higher resolution, and reduced acquisition time-result from the increased signal-to-noise ratio (SNR). “With MRI, we’re always concerned with signal to noise,” Shifrin explains. “SNR roughly scales linearly with field strength, so if you increase your field strength, you increase your SNR.”
One of the more interesting advantages of 3T MRI is reduced acquisition time, which not only reduces artifacts, but also results in faster scans and increased patient throughput-a very attractive element to the more bottom-line oriented freestanding imaging clinics. However, some users might not want to take advantage of the reduced acquisition time. By employing the same acquisition time they use at 1.5T, users will acquire images of even higher resolution. This approach would be more desirable for many users, particularly those performing fMRI and MR spectroscopy studies.
This “currency” element, often spoken about in connection with 3T MRI, concludes that a large part of how a system is used depends on how a user wants to “spend” the currency. “SNR is the currency of MRI, and that’s the key to 3T technology,” Shifrin says. “If you have more SNR, then you have more currency to buy things.” He notes that the things users would want to buy are greater spatial resolution (which enables visualization of smaller structures) and greater temporal resolution (which speeds up procedure, increases patient throughput, and enables the user to perform studies that they couldn’t at lower field strength).
Increasing Applications
Facilities best served by 3T MRI would be those performing neurological, musculoskeletal, and cardiac imaging. In neurological applications, clinical 3T MRI is expected to improve diagnosis for schizophrenia, stroke, Alzheimer’s disease, and multiple sclerosis. It also will provide more information about how brain tumors respond to treatment.
Proving especially useful in musculoskeletal applications, 3T provides high-quality images of bone structure, tendons, ligaments, cartilage, and median and ulnar nerves. Further, it has demonstrated great promise in pediatric applications. In July 2003, the Cincinnati Children’s Hospital Medical Center (CCHMC) became the first US pediatric hospital to use a whole body 3T MR scanner for clinical imaging.
Bernard Dardzinsky, MD, division chief of research and radiology at CCHMC, reports that the hospital uses Siemens’ Magnetom Trio 3T scanner for brain, spine, heart, and joint exams. “We have found that it enables more accurate diagnosis of injury and disease,” he says.
When it comes to imaging pediatric patients, Dardzinsky finds that a big advantage of 3T MRI is its acquisition speed. “We’ll use the higher resolution on children who are more compliant, but the faster scan times work better for children who find it hard to stay still for more than a few minutes at a time,” he says.
Moreover, because 3T MRI provides exquisite detail of smaller anatomical structures, it naturally lends itself well to imaging small children, especially their musculoskeletal structures, like the wrist, hands, ankles, and elbows. “I had been using 3T for about 10 years, mostly on research systems, and the Siemens Trio has brought it into the clinical setting where we can do really good musculoskeletal imaging,” Dardzinsky adds.
Besides the obvious impact that 3T adoption has had on patient care, the technology has generated a great deal of new referrals for the hospital. “Most of the referrals come from orthopedic specialists who recognize the quality in the images as compared to the quality of images from lower field systems or the so-called patient-friendly open systems,” Dardzinsky explains. “They just can’t produce the image quality that we get with this high-field magnet.”
Meanwhile, some facilities continue to use 3T MRI strictly as a research tool. Others use it for its increasing number of clinical applications. Breast and abdominal imaging are other areas where 3T MRI shows great potential. During abdominal imaging, the faster imaging could eliminate the need for patient breath holds.
The Oregon Health and Science University (OHSU of Portland) was one of the first facilities to use 3T in a clinical setting. Gary M. Nesbit, MD, chief of neuroradiology and an associate professor of diagnostic and interventional neuroradiology, reports that OHSU uses Philips’ 3T magnets for neurological, body, and cardiac applications. “We first used it exclusively for neurological applications and then increased our applications by using body, spine, and cardiac coils,” he says. “As these surface coils come online, we are certainly adopting 3T in other areas. Cardiac is an emerging application.”
Nesbit says the main reason that OHSU adopted 3T was for the SNR, so that they could use the extra signal to cut down on slice thickness to obtain increased resolution. “In some cases, we’ll use it for the speed,” he adds. “But, because we are a tertiary imaging center, we tend to use this [element] to improve imaging rather then to cut down on time.”
As at CCHMC, 3T MRI has impacted referrals at OHSU. “We tend to have patients referred specifically for 3T because physicians recognize the increased quality,” Nesbit notes. “Typically, these referrals come from neurology specialists.”
Still, other facilities use it as both a research and a clinical tool. Such is the case, Finn reports, at UCLA, where the Siemens’ Magnetom Trio 3T system is being used for cardiovascular applications, abdominal imaging, and neurological studies. “Our machine is technically a research system, but we also use it for some clinical work,” Finn says. “We use it mainly for cardiovascular research, but we also use it for clinical cardiovascular imaging. We do a certain amount of neurological research with it as well.”
Mastering 3T MRI
Adopting and using 3T MRI is not as straightforward as one might expect. Protocols performed at 1.5T cannot simply be transferred to a 3T system, Finn explains. “It just doesn’t work like that,” he says. “Some things just don’t run properly at 3T, while other things run better at 3T than at 1.5.”
Therefore, he points out, it’s important to educate a staff on the reality of a 3T MRI system: what it can and cannot do, its challenges, and the limitations that come with the advantages. “To some degree, 3T MRI is application specific, and people need to be aware of this [circumstance] up front,” he says.
Shifrin emphasizes that many intricate physics come into play at the 3T level. “You’ll need to have a radiologist in your group who dives deep into MRI to really understand how to best use 3T, because some things must be done differently,” he says. “Some significant technological hurdles must be overcome. That’s where some extra education comes in.”
One of the elements this education involves is the reading of images. Essentially, clinicians must learn how to read the images. As it turns out, the images are so good that they need to know exactly what they’re seeing.
New users also need to know when it is most appropriate to use the 3T system. As Nesbit points out, when referring physicians and patients learn more and more about new technology, they want to go with the “latest and greatest.” However, as far as MRI is concerned, some questions are still better answered with a 1.5T scanner. “Currently, we’ve taken that approach to head and neck imaging, only because there are some issues with susceptibility problems. So certain scans we don’t necessarily triage toward [the 3T] direction,” Nesbit says.
Susceptibility problems are one of the prices that come with 3T. Higher magnetic field strengths tend to exacerbate differences in magnetic susceptibility in different tissues. These problems can be addressed by increasing the spatial resolution or acquiring thinner sections.
Other complexities that come with higher field strength include:
- Specific absorption rate (SAR)-The SAR is a measure of the amount of energy that enters the body. Limits have been set on that amount. “The SAR is increased in 3T,” Shifrin says. “If we were to take the standard pulse sequences that we use with 1.5T and translate them over to 3T, we would wind up banging up against that SAR ceiling frequently.”
- Relaxation times-With the increased magnetic field strength, the T1 relaxation times are increased. Users need to adjust for this increase.
- Contrast agents-Users need to modify contrast dose to prevent misinterpretation of data.
For some facilities considering 3T adoption, cost is a notion far more formidable than those aforementioned physics complexities. The price tag can run from $2.5 to $3 million. This number might be feasible within the budget at a large research institution, but hospitals and imaging centers would most likely be content to stick with their 1.5T systems. What could sway an institution toward 3T is intended use. A facility that does a high volume of neurology or cardiovascular studies would be much more inclined to purchase a 3T system.
3T or 1.5T-A New Standard?
Currently, 3T is considered the standard for neuroimaging, but opinions differ on whether 3T MRI will completely replace 1.5T as the clinical standard in other areas of the body. According to Shifrin, “3T is the standard in terms of providing the best images, but I don’t think it’s going to be the standard in terms of being the most represented magnet that’s out there.”
Finn thinks that rather than 3T totally replacing 1.5T, the two scanners will complement each other inside an imaging facility. Nesbit agrees, and he describes the kind of approach that he thinks will become the standard for potential purchasers: “The practices that have multiple MRI scanners would want to consider purchasing a 3T system when they’re thinking about replacing a scanner or adding another one.”
Clinical demand, he believes, will dictate such an approach. “As more people become aware of what it can do, they’re going to want that latest-and-greatest technology.” That goes for the institution’s clinicians as well as the referring physicians and the patients.
Of course, all of that could change in the next year or two, as the capabilities of 3T MRI become more fully realized. It could indeed become the overwhelming standard. “They way I see it now, we’re only at the tip of the iceberg of what 3T can do,” Shifrin admits. “I only expect it to improve as clinicians and vendors learn together.”
As Finn points out, and which has been amply demonstrated in the past, you never say never when it comes to anything having to do with MRI.
The Color of MoneyA 3T MRI scanner comes with a hefty price tag. It has been suggested-a bit wryly, perhaps-that it costs about $1 million per Tesla. It’s not quite that high, notes Roger Y. Shifrin, MD, of Radiology Associates. But with double the strength of a 1.5T system, purchasers will be hit with almost double the price. “It works out to be just shy of $3 million. If I was to purchase a good 1.5 Tesla, it would cost about $1.6 or $1.7 million,” he indicates. Faced with such an expense, some people look for any economic benefits that might help pay off the new purchase. The most immediate benefit a facility might realize would come from increased patient throughput. Because of its increased signal-to-noise ratio (SNR), a 3T MRI system offers the advantage of reduced acquisition time, which results in a faster scan time. Some folks see this speed as a way to pay for the system. “You can get an economic benefit, depending on how you use it,” says Gary M. Nesbit, MD, of Oregon Health and Science University. “I’ve heard that some people take advantage of the extra signal by cutting down on scan time so that patient throughput is higher.” However, faster scan times-and the increased patient throughput-is not the main reason why many facilities adopt 3T. The faster scan times work best for routine cases, Nesbit indicates. “That way, it’s used more for an economic standpoint because they’re dealing with the rule-out-a-disease kind of patients,” he explains. But other times, a clinician could take advantage of the extra time available to do a more thorough and specific examination. “If there is a specific question about the patient, they would use the time to make better images,” Nesbit says. And that is the more significant advantage of 3T. J. Paul Finn, MD, of UCLA agrees: “For most of the applications we’ve looked at, in certain areas, you can get faster scans, but that’s not what stands out most as being a strong advantage.” For one thing, he reports that scan times with 3T are faster than 1.5T, but not all that much faster. “The scan times really take a similar length of time,” he comments, depending on the technique. In some cases, he adds, 3T scans might even be longer, because of the specific absorption rate (SAR) issues. “We can’t really use the same sort of radio frequency that we use at 1.5, because we’d give too high a dose of SAR,” Finn points out. “We have to modify our protocols somewhat and maybe use a longer repetition time for sequences if we want to get studies comparable to what we get at 1.5. So I don’t think a community hospital is going to look at this [information] and think that with all the extra SNR, they’ll be scanning much faster and have a significantly higher throughput. That’s not really the way it works.” Another economic factor that facilities need to be aware of is that payors do not reimburse more for 3T than for 1.5T, despite the fact that the former is so much more expensive. “To the payors, an MR is an MR is an MR,” Nesbit says. So, increased patient throughput might or might not help pay off the 3T, but the more significant economic advantages could possibly come from increased patient referrals and from the marketing perspective. A facility equipped with 3T MRI has the “bragging rights” to cutting-edge technology. “With 3T, particularly for musculoskeletal and brain imaging, you can get very exquisite pictures with high resolution,” Finn says. “For diagnostic purposes and for potentially competitive marketing purposes, the community hospital and private-practice hospitals could find that advantageous. However, I don’t think they should count on a very big increase in throughput just by going to 3T.” Shifrin adds, “For anyone planning to buy 3T, they should see it as a way of positioning themselves as the leader in their area in terms of technology. They’re not necessarily going to make more per unit time on the magnet, but they will increase their capabilities. Then, perhaps, they will become the preferred imaging center because of the technological advantages they offer. But per unit time will not necessarily make [a facility] more money.” -DH |
Dan Harvey is a contributing writer for Medical Imaging.