3T Defines A Role in Clinical Imaging

The marketplace for high-field MRI, specifically 3-tesla, is transitioning from research institutes to a more clinically centered customer base at the community and specialty facility level. Originally dedicated to head imaging, developments in magnet and surface coil technology have morphed these systems into whole-body imagers - covering applications in neuro imaging, spectroscopy, body imaging and cardiovascular imaging. Coupled with its increased signal-to-noise ratio and ability to scan faster or with distinctly higher resolution, 3T magnets are clearly gaining attraction.

For years, 3T MRI has been used as a research tool for brain imaging. In 2002, 3T systems were welcomed into the spectrum of clinical MR when the U.S. Food and Drug Administration approved some 3T scanners for brain and whole body imaging. Expanded applications and the availability of more radiofrequency (RF) surface coils paved the way for 3T's inception in the clinical arena. While academic medical centers are the dominant users, 3T vendors forecast a future change in end-user demographics to large to medium-size hospitals.

Philips Medical Systems, Siemens Medical Solutions and GE Healthcare are the leaders in 3T technology. Similar in size and user interface to their 1.5T cousins, what differentiates the two is price - approximately $1 million extra a tesla - and increased signal to noise ratio (SNR). Users practically double their SNR when they go to 3T. "When you go to 3T, it gives you extra SNR and how you choose SNR is really up to you as a physician," says Gregory Sorensen, MD, a neuroradiologist at Massachusetts General Hospital. "You can use it to go faster or you can use it for higher resolution."

Buying SNR means buying flexibility on how it's spent. Increased signal to noise can be used for images that require higher spatial resolution or at times when a faster scanning time is needed, which is beneficial for patients who do not like to be scanned or are frail.

Physicians at MGH use both Siemens' Magnetom Trio 3T whole body scanner and Siemens' Magnetom Allegra 3T scanner, which is dedicated to advanced neuro studies. Siemens holds the position as the only major vendor that has two 3T scanners in its product portfolio. MGH clinicians utilize high-field in the investigation of conditions such as dementia, drug addiction, migraines, and other disorders. Sorensen says they are also studying Alzheimer's and schizophrenia patients on the 3T system and "the tools we are developing to diagnose these illnesses are clearly going to work better at 3T than 1.5T."

From the beginning, it's been clear that 3T benefits the neurosciences, particularly functional MR (fMRI), diffusion imaging, perfusion imaging and spectroscopy. "We routinely use spectroscopy for evaluating patients with brain tumors and multiple sclerosis," says Michael Lipton, MD, medical director of MRI services at Montefiore Medical Center and Albert Einstein College of Medicine in New York. Lipton adds that both MR angiography and perfusion imaging, especially arterial spin labeling techniques (ASLT), also benefit from higher field strength.

Philips' Intera 3T (the forerunner to the company's newest system released at RSNA 2003, the Intera Achieva 3T) has been used at Montefiore for more than two months. The department is learning more about its clinical applications as more RF coils are released. "The main thing that you need to get used to is that there are many more possibilities that you can do at 3T than at 1.5T," poses Lipton.


MORE, BETTER, FASTER

Using an older generation, long-bore 3T MRI system, physicians at Michigan State University (MSU) College of Human Medicine have performed more than 12,000 clinical exams since July 2002. About 60 percent of exams are neuro and 40 percent are musculoskeletal, says Mark Delano, MD, physician director of MRI and director of Radiology Research at the facility.

Delano believes there is an amazing capacity to improve image quality in musculoskeletal imaging with 3T MR. "What we are seeing is that there is no compromise as far as contrast features in the musculoskeletal system," he says. The facility routinely performs soft-tissue neck, abdomen, pelvis, hip, long-bone extremity, knees, ankles, feet and shoulder exams at 3T.

Makers of 3T systems have invested quite a bit into creating a comprehensive imaging platform that permitted both brain and whole body scanning. While fMRI and spectroscopy are clearly advantageous at 3T, not every medical center performs these sophisticated exams on a daily basis. Being able to visualize and detect the pathology of tendons and ligaments, bone structure and cartilage due to enhanced image quality makes 3T much more attractive for smaller-sized hospitals.

High-field scanners are showing promise in orthopedics, abdominal, cardiovascular and breast imaging. Faster imaging allows images of the abdomen to be produced without patient breath-holds. Cardiology is buying the benefits of improved SNR in myocardial perfusion, development of cardiac blood oxygen level-dependent (BOLD) imaging, MR-guided catheter placement and cardiac spectroscopy, to name a few.

"I am doing bread-and-butter MRI at high quality at faster scan speeds than in the past," opines Delano. "But it's taken some effort to understand 3T. It's not simply a matter of transferring your protocols from the 1.5T scanner and making them faster. There are various aspects that require getting used to."

There still exist limitations to the technology, such as susceptibility artifacts, RF energy deposition and changes in T2 and T2 relaxation times. Power deposition is an issue because electrical energy deposited in patients increases as the user goes up in field strength. Protocol changes can be made to control the SAR (specific absorption rate), but manufacturers have also made additional strides toward improvement. For example, GE's Signa Excite 3T MRI helps users not exceed SAR guidelines with Perform, a feature that monitors power deposition in patients.

"One of the differences of higher field imaging is that the image contrast is not the same," explains Montefiore's Lipton. "Therefore, we are using different types of images for certain applications. For example, in seizure imaging, given the way some of the behavior tissue changes at 3T, we are now moving to use gradient echo techniques."


PEDIATRIC IMAGING

High-field MRI also improves the visibility of small structural details - depicting smaller anatomy better - but physicians have only recently begun to use 3T systems on smaller patients. Cincinnati Children's Hospital Medical Center is the first U.S. pediatric hospital to install a whole body 3T MR scanner for clinical imaging. Its adoption improves advanced imaging techniques at the institution, brings research into the clinical arena and continues the hospital's leading edge in pediatric care, says Blaise Jones, MD, chief of neuroradiology at the hospital.

The institute uses Siemens' Magnetom Trio 3T scanner for brain, spine, heart and joint exams. Patients are usually scheduled equally across the magnets, but exceptions include functional MRI and cardiac studies.

As with any new diagnostic imaging tool, users must assess its contribution to the specific patient population on which it will be used. "The process of obtaining a 3T scanner was a fairly careful and meticulous one," explains Jones. "It started over three years ago when a proposal was brought to administration. It was something that was approached with great due process and careful assessment. All the places that had 3T scanners in place were not doing pediatric work. So we had to assess them and see how it would apply to our patient population."

At the same time, MRI systems with stronger imaging power are not intended to be used in place of lower field strength systems. "A 3T system should not be considered as a one-to-one replacement for a 1.5T system, rather it should be seen as a complementary modality that can allow you to do better and additional things that you were doing with 1.5T," acknowledges Jones.


STATE OF THE INDUSTRY

Despite its clinical triumphs, some clinicians are pessimistic that 3T will replace 1.5T any time soon. "With 3T you can get finer detail and superior image quality," expresses John Anastos, MD, chairman of the department of radiology at Advocate Lutheran General Hospital in Park Ridge, Ill. "The question is, since you are paying significantly for that superior image quality, are you getting information that is really clinically useful? There are applications, especially in neuro, that are enhanced by 3T. But for the vast majority of MR users, 1.5T is sufficient." On the other hand, proponents argue that the same was said about 1.5T when 0.6T was the dominant magnet.

Anastos believes that academic medical centers have the money to spend on higher field technology, but clinicians in private enterprises are less willing to pay the extra million for 3T. Market numbers agree, but not for long. Research firm Frost & Sullivan reports that within the high-field market segment, 1.5T systems are the dominant sub-segment in terms of units sold and revenues achieved. However, 3T systems are experiencing growth rates and doubled sales in recent years.

While 3T systems accounted for slightly more than $100 million in revenues in 2003, representing 8.5 percent of total revenues garnered by 1.5T and 3T combined, Frost & Sullivan predicts the presence of 3T systems will eventually represent approximately 20 percent of the high-field market segment. How quickly that happens depends on when the technology moves from university settings to community hospitals and free-standing imaging centers. Stephen Mitchell, director of product marketing for Philips' MR, predicts that hospitals will buy more SNR because 3T is the system of choice for neuro imaging and soon will become the system of choice for cardiovascular studies, particularly coronary artery imaging. The high-field technology also helps attract referrals and provides better access to grant funding. Hospitals on the fence about purchasing a 3T system want to know that it is going to significantly expand their referral base.


REDUCING SCAN TIMES

Installing GE's Signa Excite 3T system has made Edison Imaging, a free-standing, for-profit imaging center affiliated with JFK Medical Center in Edison, N.J., and New Jersey Neuroscience Institute, more competitive. Neurology referrals started insisting on 3T after seeing the first few cases, along with orthopedic surgeons and oncologists.

"The real key thing is that this is the wave of the future," opines Lawrence Tanenbaum, MD, FACR, section chief of MR and CT at the facility. "The quality of the exam in terms of resolution, signal and speed that 3T will deliver should shift the high-field market away from 1.5T to 3T in the next five years."

While the clinical benefits and relevance of 3T systems raise more eyebrows, and perhaps more believers, going to higher field strength requires site visits and talks with people using the technology. "The argument for a 3T system is that in some cases you need the clinical benefits," explains MGH's Sorensen. "In the rest of the cases you can shorten your imaging time. The way to pay for a 3T is to move to 20-30 minute imaging slots instead of the 45-60 imaging slots. On occasion you will spend an extra 10 to 15 minutes getting super quality images when you don't know what's going on. The rest of the time you are going to be able to image people at faster throughput."