High-field MRI looms larger on clinical horizon

Since the advent of 3T MRI 10 years ago, high-field systems have remained largely a tool for research in Europe. Now, developments in scanner technology are increasing the likelihood that 3T MRI will play a significant role in clinical radiology.

Although 1.5T systems currently dominate clinical MRI in European hospitals, switching to a higher field scanner would scale up performance advantages considerably, according to Prof. Dr. Peter Boesiger, head of MRI research at the Institute for Biomedical Engineering, University of Zurich and ETHZ, Switzerland.

"MRI at 3T is not a completely new MRI, but a very important extension of what is already in daily use. The physical fundamentals are the same," Boesiger said .

Superior signal-to-noise ratio is a key benefit of high-field MRI. Doubling the field strength from 1.5T to 3T correspondingly doubles the SNR, enhancing spatial resolution and reducing partial volume effects. The increased field strength also alters MR tissue contrast parameters, making it easier to distinguish vessels from surrounding tissue. This capability may be especially useful for MR angiography.

The increased sensitivity to blood oxygenation of 3T MRI is likely to benefit functional MRI studies, bolus perfusion studies, and hemorrhage detection. And the higher SNR at 3T, coupled with an increased chemical shift resolution, brings examination times for MR spectroscopy in line with the fast speed of other MRI techniques.

"The new generation of 3T MRI scanners has proven to be an excellent tool for diagnosis. I expect that 3T MRI will very rapidly gain acceptance, first mainly in the area of neuroradiology and later in other areas," Boesiger said.

The small-bore 3T MRI systems on the market are suitable for installation and operation in a clinical environment, he said. Previous models built around a longer bore were neither designed nor approved for routine clinical use. But limited availability of the user-friendly, small-bore scanners and the dedicated coils needed for different applications is keeping 3T MRI out of many European hospitals, according to Boesiger.

The lack of high-field MRI equipment is particularly acute for whole-body imaging, said Prof. Robert Lenkinski, director of experimental radiology and the 3T MRI/MRS program at Beth Israel Deaconess Medical Center in Boston. Sufficient high-quality MR body multicoils and optimization of scanning parameters for 3T MRI are critical to widespread adoption of the technology, he said. Once those barriers have been addressed, education of radiologists will be the next limiting factor.

Delegates will have the chance to view the clinical potential of whole-body 3T MRI for themselves. Lenkinski will present numerous imaging studies performed on a 3T scanner at Beth Israel Deaconess, which has been fitted with a prototype whole-body RF coil. Possible applications for such a system include visualization of prostate and breast cancer and measurement of vulnerable carotid artery plaques. High-field imaging using phosphorus-31 may also be used to identify viable ischemic and necrotic tissue in patients suffering from lower extremity ischemic disease.

But why stop at 3T? Wouldn't raising the field strength further produce even better imaging results? It's not quite as simple as that, according to Lenkinski. The specific absorption rate (SAR), which is related to RF power deposition, places limitations on MRI scanning. Although SNR varies linearly with the static field, the SAR rises as the square of the field.

"Three-T is a good compromise between SNR and SAR; 4T would lead to moderate gains in SNR with larger restrictions on SAR. These restrictions on SAR would lead to fewer slices and longer repetition times," he said.

Boesiger is similarly lukewarm about the merits of moving from 3T to 4T. Another doubling in field strength, to 6T or even 7T, would be necessary to achieve the same level of progress as moving from 1.5T to 3T, he said. While some sites are already discussing such a move, these ultrahigh field strengths are unlikely to find use outside research facilities, he added.

Most experts agree that 3T MRI is safe for clinical work, provided precautions are taken. Lenkinski recommends using good-quality, accurate power monitors during 3T scans, which will have a higher SAR than standard 1.5T examinations. Patient screening is also important because hazards associated with metal objects will also be higher, he said.