3T scanners may dethrone 1.5T as high-field imaging benchmark

Increased signal fuels expanded clinical and research use

Vendors are redefining the meaning of high-field MRI, positioning 3T scanners as the heir apparent to advanced clinical and research applications. They expect to deliver between three and four dozen 3T scanners by the end of 2002 in the U.S. alone. Within the next two to three years, 3T scanners could account for a quarter or more of the total MR market, according to industry executives. In the process, these scanners will assume much of the role now being played by 1.5T scanners.

"Three-T is rapidly proving to be clinically advantageous in many mainstream applications as well as being relatively cost-effective," said David Weber, Ph.D., manager of high-field MR business for GE Medical Systems. "For these reasons, the clinical community is recognizing that 3T will become the workhorse of clinical MRI in coming years."

Three major players‹first GE, then Siemens and Philips‹have implemented plans to build on their knowledge of very high field systems to create production line 3T whole-body products: GE¹s Sigma VH/i, Philips¹ Integra 3.0T, and Siemens¹ Magnetom Trio, which complements the company¹s dedicated 3T head scanner Allegro. The jump to these systems will result from the need to fill a void in certain clinical applications created by the shortcomings of 1.5T systems, according to Yuri Warmed, Ph.D., manager of ultrahigh field MRI at Siemens. He cited a need for better detail in anatomic imaging, improved perfusion-diffusion and angiographies, and improved functional and structural imaging of the brain.

"I think 3T is being introduced for the interest of exploring a new field," Warmed said. "As far as clinical MRI is concerned, no one has determined that 3T is going to be the standard, but 3T is still in a leading-edge category. A lot of people are convinced this is going to be the next wave, as with 1.5T."

Some pundits believe 3T will displace 1.5T systems as the benchmark for high-field imaging, raising the bar for clinical imaging with the improved signal to noise possible with these more powerful machines. These two product types together will compose the high-field imaging segment, according to Heinrich Kolem, president of the Siemens MR division.

"Most of the routine and fast routine scanning will be done at 1.5T; the research segment will focus on 3T," he said. "So, high-field imaging, which three or five years ago was 1.5T and 1T, will become 3T and 1.5T."

Dr. Alistair Howseman, senior clinical scientist for Philips Medical Systems, said neuropathology has already begun to recognize the clinical advantages of 3T. These advantages will provide the basis for further adoption, even for relatively routine applications.

"Where signal-to-noise ratio is a limitation--in diffusion-weighted imaging, for example--the extra signal can be used to boost the SNR," he said. "In applications where SNR is not a limitation but extra resolution would be valuable, then 3T provides an alternative way in which the additional signal can be used."

In this context, Howseman said, signal is a commodity and 3T provides more of it. Improved SNR translates into more opportunity and greater flexibility.

SNR is the reason many radiologists today favor 1.5T over lower field strengths, and it is the reason 3T will overshadow 1.5T technology, according to David G. Norris, Ph.D., an MR physicist at the F.C. Donders Center for Cognitive Neuroimaging in Nijmegen, the Netherlands.

"If you are looking at anatomical imaging, you have a clear signal-to-noise advantage by going to a higher field strength," Norris said.

The foundation for the widening acceptance of 3T will come from physicians who have maximized the capabilities of 1.5T systems and not achieved the results they want. J. Thomas Vaughan, Ph.D., an associate professor of Roentgen diagnosis at the University of Minnesota, believes the introduction of 3T systems into clinical practice, the prospect of further expansion into clinical 4T, and the widening adoption of 7T and 8T megascanners for research will force the MR community to change its definition of high-field MRI. With 90 3T scanners operating in the field, according to Vaughn, the transformation to a higher field strength is already happening. Clinical applications are leading this transformation.

Examples in which 3T can make a difference, according to Howseman, are noncontrast perfusion, functional MRI, and particularly spectroscopy.

"MR spectroscopy, which has been around since the early 1980s, will benefit from the extra SNR of high-field MR," he said. "MRS, therefore, is likely to be proportionately more important at 3T than 1.5T."

Performed at 3T, hydrogen-1 MR spectroscopy can improve the surgical outcomes of patients with temporal lobe epilepsy because the clinician gets a better look at the extent of lateralization, metabolic severity, and multifocal nature of the disease, according to Dr. Jullie W. Pan, an associate professor of neuroscience at Albert Einstein College of Medicine in New York City. MRS performed at 3T may soon track the effectiveness of breast cancer chemotherapy. Future applications may also extend to the use of phosporus-31 MRS to gauge myocardial function, an application explored by researchers at the University of Alabama, Birmingham. The improved SNR of 3T imaging may also be harnessed for physiological studies of diabetes.

Another improvement in clinical applications with 3T may affect angiography, where higher resolution data can be acquired, making possible the imaging of very small vessels, according to Howseman. Higher resolution is not only achievable, but better background suppression may be possible at 3T due to longer T1s. Thanks to the additional signal, acquisition times may be shortened as well.

"The importance of body imaging applications is less proven at this time although the prognosis looks very encouraging," he said. "Excellent images of breast, prostate, joints, coronary arteries, and runoff angiography show the way to using higher resolution imaging to aid in diagnosis."

Other clinical applications that may reap the rewards of higher field MRI, according to Weber, could be orthopedic and whole-body functional imaging techniques.

This potential is being achieved on the heels of technical advances including the development of a magnet with a field-of-view and homogeneity required for whole-body imaging with 3T.

"For whole-body imaging, you need at least a 40-cm isotropic field-of-view, and you need homogeneity over that field-of-view to support all applications including fat saturation and spectroscopy," Weber said. "Another technical challenge is designing a radio-frequency subsystem that can properly manage the increased RF power that is required by 3T. This involves the design of optimized pulse sequences and algorithms that monitor SAR (specific absorption rate, which indicates energy deposition in the subject) that allow mainstream clinical sequences such as fast spin-echo to be performed routinely."

Some disadvantages remain, such as increased acoustic noise, increased susceptibility to artifacts, and restricted use in some cases due to SAR. The main challenge to widespread acceptance of whole-body 3T systems, however, may be their cost. The new breed of 3T scanners cost at least $1 million more than 1.5T products, and vendors are limited in how much they can reduce this differential. The price of a 3T scanner will exceed that of an equivalent 1.5T partly because of the cost of the superconducting wire used in the magnet.

"The advantages, however, are numerous," Howseman said. "And they will certainly lead to 3T becoming the gold standard for clinical MRI in the coming years."