3T MRI made its name creating high-quality brain studies in research institutes. But today's 3T MR scanners are whole body imagers used at both academic medical centers and community-based imaging facilities, performing clinical brain, orthopedic and musculoskeletal imaging, and showing promise in areas such as breast, cardiac and molecular imaging. 3T is considered the wave of the future - but exactly when that wave will hit is still the question.
When it comes to the routine clinical utilization of 3T MRI, the imaging community is on the fence. Users of 3T are believers that their practices have much to gain from increasing field strength; not only the ability to perform imaging techniques not possible at 1.5T, but to offer patients a possible diagnosis using higher-quality digital images. They also recognize that owning and operating a 3T scanner differentiates them in the marketplace.
On the other hand, hospitals and imaging centers that do bread-and-butter MR on high-end 1.5T systems are not yet convinced the time is right for 3T; there is not enough clinical literature and immature coil and antenna technology currently limits how many clinical whole body applications can be performed at 3T - although vendors say this has improved tremendously. The convincing argument to spend the extra million on 3T is harder won.
Despite the clinical squabble, most everyone agrees the overall benefit of 3T is the doubled magnetic field strength. Higher field strength means a higher signal to noise ratio (SNR), which might be used to increase the imaging matrix, to choose thinner slices, to reduce the number of signal averages, or in some cases, to decrease the amount of contrast agent administered. Simply stated, 3T scanners produce higher resolution images and, in some cases, scan faster than their 1.5T cousins. They also perform applications better, more clearly, less invasively, with more detail or in less time than 1.5T.
Experienced, routine clinical applications at 3T include brain, musculoskeletal and orthopedic imaging. High-end brain applications include functional imaging, spectroscopy, and diffusion tensor imaging. For musculoskeletal applications, increased SNR improves image quality of bone structures, tendons, ligaments and cartilage. MR angiography (MRA) is typically better due to the signal advantages of T1 in the 3T magnet. Other applications that are SNR-starved and are sure to see benefit are breast and cardiac MR - once more surface coils become available.
"3T is terrific for the brain, spine, musculoskeletal, abdominal and pelvic imaging, vascular imaging, especially for contrast-enhanced MR angiography of carotid, renal and pulmonary arteries," says Paul Finn, MD, professor of radiology and director of cardiovascular imaging at University of Los Angeles Medical Center. "It is also excellent for the intra-cranial arteries. Still, one of its more limited applications is functional imaging of the heart. If you were going to take a 1.5T system and a 3T system and do the same patient for this specific technique, you will probably get more reliable and quality images with a 1.5T system."
"We use 3T for almost every application for which 1.5T is used including MR angiography of the renal arteries and peripheral arterial system, in addition to routine imaging of the liver and pancreas," says Bruce Distell, MD, director of body imaging at Cape Fear Valley Health Systems of Fayetteville, N.C. "However, being that our 3T is sited at an outpatient center ... and there are indications for which 3T is particularly useful, we steer certain studies to this magnet. For example, we use it for many outpatient brain studies and high resolution orthopedic work."
For almost a year, clinicians have been using GE Healthcare's Signa Excite 3T at the imaging center. The facility is the first in the region to make clinical use of a 3T magnet. Distell says both the desire to perform state-of-the-art imaging and to move into the realm of molecular imaging motivated them to adopt 3T.
"Since there is increased signal from the 3T magnet, there is the opportunity to routinely perform very high resolution imaging," says Distell. "The superb quality of the images increases the accuracy rate and the confidence level for making subtle diagnoses. The referring physicians have been very impressed with the images. For example, the orthopedic surgeons find it clinically useful when I can give them a more accurate assessment of the state of the articular cartilage. This is achieved by obtaining smaller voxels while maintaining appropriate signal."
Neuroskeletal Imaging Institutes in Florida operates a 3T scanner at each of its four imaging centers in the Orlando area. They are currently the only 3T MRI owners in Brevard and Orange counties.
"Brain imaging, musculoskeletal, MRAs in the brain and in the body, and spine imaging make up a bulk of our MRI services," says Marc Shapiro, MD, chief of neuroradiology at Neuroskeletal Imaging. "For brain imaging, 3T is superb. Without question, our scans are much more detailed than on any 1.5T system. We do between 1mm and 3mm slice thickness on all of our brain pulse sequences. On 1.5T, the standard is 5mm. We have picked up several lesions [tumors, small aneurysms] with the 3T because we used such thin slices."
Referrals are increasing. "At least 40 to 50 percent of requisitions from neurosurgeons, orthopedists and neurologists now ask for the 3T system," he adds.
The ultra high field scanner is attracting members of the academic community, too. Neuroskeletal Imaging has been selected by the University of Miami School of Medicine to teach board-certified radiologists as part of an annual MRI fellowship. "The musculoskeletal fellows from the Mayo Clinic Jacksonville will rotate through our MRI centers in July," says Shapiro. "An MRI fellowship-trained cardiologist does cardiac MRI on our 3T system in Winter Park, where we have a 3T cardiac coil."
Age matures 3T
Both field strengths - 1.5 and 3T - are producing great clinical results. The numbers indicate that 1.5T is currently the gold standard, but physicians using 3T see it as the wave of the future.
"Most clinical imaging can be done at 1.5T, but don't forget, before 1.5T became a standard, people thought you did not need 1.5T," opines Distell. "It then became the industry standard to use 1.5T. I anticipate over time that the 3T magnet will become the new industry standard because you can image with higher resolution in the same amount of time, or you can image at equal imaging parameters in lesser time because you have more signal to use."
1.5T wears the ribbon as today's clinical workhorse, but research suggests 3T is gaining stride. In 2003, Frost & Sullivan estimated that 3T systems accounted for slightly more than $100 million in revenues, representing 8.5 percent of total revenues garnered by 1.5 and 3T combined. In the future, the firm estimates the presence of 3T systems will grow to represent approximately 20 percent of total high-field system sales as adoption moves from academic medical centers out into the base of community hospitals and free-standing imaging centers.
According to UCLA's Finn, if a hospital just has one MR scanner today, chances are it's going to be 1.5T. "3T is still an evolving technology," he says. "For certain applications, the future looks very bright. But it is a maturing technology. Some of the ways we do things at 3T are going to be a little less sufficient than they are at 1.5T because the peripherals are not as highly evolved, as well as the coils, the antenna and the techniques."
The right coils impact the performance and ease of use of the system. "The real big advantage that people are starting to see in the 3T market, in particular in it becoming a clinical system as opposed to research only, is the development of the radiofrequency technology designed for 3T," says Kyle Salem, Siemens Medical Solutions manager of MR application and research and development. "In the past, vendors were in such a rush to get 3T onto the market, they tried to take 1.5T RF technology and modify them to work at 3T. They quickly found out that that was certainly less than optimal. The same thing occurred with sequences.
"What is happening now is that vendors are coming out with coils specifically designed to work 3T," he continues. "It's not the material, the electrical design is a little bit different. Typically at 3T you are going to push for more coil elements and you also are going to look at designs that fit your strategy. What really has improved across the board are the number of channels that you are getting in a coil and the fidelity in which the 3T signal is received."
High field strength presents a number of obstacles. First there's cost: 3T systems range in price between $2.5 and $3 million, which is significantly higher than the $1.5 to $1.8 million price tag range of a high-end 1.5T system. Other challenges include limited coil technology, increased specific absorption rate (SAR) and prolonged T1 relaxation times.
SAR, the energy deposited in the body by the RF coils, increases by a factor of four when going from 1.5T to 3T. This means that SAR limits that are set for patient safety are more rapidly reached, needing alternative developments to ensure patient safety. Siemens, GE and Philips Medical Systems - the three developers of 3T - have come out with different technologies to address this.
"There is increased SAR, but the vendors have introduced multiple techniques into the system that make those levels safe," says Distell. "We have not run into any problems with SAR in clinical use."
However, Finn poses that increased SAR can limit the end-user in some ways. "For instance, at 1.5T you may be able to use a very, very short repetition time for MRA or cardiac cine imaging," says Finn. "And you would like to use a fairly high clip angle and that will amount to relatively high SAR even at 1.5T. But once you go to 3T, if you use exactly the same parameters, it quadruples the dose. What that means is that you have to back off on some of those parameters. So in fact, it may take longer to do the scan if for no other reason than for SAR considerations."
Vendors of 3T systems are designing second- and third-generation short-bore, whole-body imagers that are similar in look and feel to their 1.5T cousins. However, customers will need to take additional measures to address potential fringe field complications. To be properly installed, users must consider both the structural strength of the floor and how far the magnetic field stretches out around the magnet.
When it comes to informatics and image management, vendors say that concerns are not field-strength related. Both 1.5T and 3T systems produce images similar in size. The main issue is dealing with the volumetric nature of MR datasets. IT will want to evaluate how physicians will handle three-dimensional data sets and determine the direction of post processing. For example, should the physician do this or the technologist?
Most clinicians using 3T say there is a learning curve involved. Shapiro says that sophisticated users of 3T will love the system more than 1.5T. Users today are typically large hospitals that tend to have sub-specialized radiologists and outpatient imaging centers that have radiologists that want to be on the cutting edge, says Shapiro.
Cape Fear Valley's Distell says that you can't just shift over the protocols from 1.5T to 3T and expect them to work the same or better because there are different nuances with 3T. "That requires a small learning curve. If you are already familiar with 1.5T and you're up-to-date with MR technology, it still takes a little getting used to."
While the MR community remains divided, there is no doubt that the high-field scanners are finding clinical homes and gaining valuable clinical experience. If you are looking to go 3T, Neuroskeletal Imaging's Shapiro suggests - in addition to testing out each and every system - that end-users closely examine how well the system handles SAR issues, the quality of spine images, computer processing power and coil development. Widespread adoption will be influenced by the amount of literature that comes out discussing the clinical advantages of 3T over 1.5T. Molecular imaging techniques, also SNR-starved, may be better enabled by the use of a 3T system. "I think that the future of imaging is to detect disease at an earlier stage so there is a higher potential for effective therapy and or cure," says Distell. "I believe that 3T, along with PET/CT, will be the standards for state-of-the-art general and molecular imaging."
3T SEES RF IMPROVEMENTS
GE Healthcare expanded its MR surface coil innovation and production when it acquired USA Instruments in December 2002. GE has a full coil portfolio for 1.5T and 3T field strengths, except a breast MR coil at 3T. The company says 3T currently represents more than 25 percent of all MR purchases. GE sees the rate of 3T adoption accelerating over the next five years, with a bit swifter growth in the overall MR market. GE's current 3T installed base is more than 150 systems.
Siemens Medical Solutions offers two platforms of surface coils for 1.5T - Tim and non-Tim - and the company recently filed its 510(k) submission for Tim technology at 3T. Currently, the Magnetom Trio 3T whole-body scanner can be equipped with up to 12 RF imaging coils. Siemens has installed 80 Magnetom Trios worldwide and installed 48 nationally.