In the past years there have been many advances in imaging technologies, paving the way for the highest resolution and image quality to date. At one time, medical professionals thought that 1.5 T, and after that 3 T would be the pinnacle of MRI field strength. When 7 T was developed, nobody ever thought it could be used for anything other than research purposes. Now, several companies have taken interest in developing 7 T clinical MRI systems for use in large hospitals.
3 T can be compared to a sports car as it has plenty of horsepower, torque and is featurepacked. However problems occur when the 3 T sports car mutates into a 7 T Formula 1 racing car, which makes daily use even more challenging. The major question is under what circumstances are the increased costs of 7 T and the inherent limitations in coverage and safety outweighed by diagnostic benefits.
Enter Dr. Siegfried Trattnig, professor, founder and chairman of the MR Center of Excellence (MRCE), in Vienna, Austria, who believes that 7 T is the future, due to its exceptional signaltonoise (SNR) ratio, spectral and spatial resolution compared with 1.5 T and 3 T systems.
"I believe 7 tesla is the top of it, and a further investment into 10 or 11 tesla does not make sense, because the costs are so high," he said. "In my mind, 7 Tesla is the best field strength for this clinically oriented research."
But, Trattnig admits there are several serious drawbacks to 7 T, including susceptibility artifacts, absorption rate problems, hot spots in images and compatibility and safety issues. With improvement of 7 T technology, these problems are being gradually solved by new coil designs (multiarray, receive and transmit), pulse programming, 3D turbospin echo and parallel reconstruction techniques.
"After many years, we have now studied more than 100 patients in our 7 T and so far we haven't had any severe side effects," he said. "When you consider the same safety issues and regulations that you have at 3 T, things also work nicely at 7 T. The only difference we have seen is that some physiologic effects can be worse, like dizziness."
The strong suits of 7 T include neuroimaging (multiple sclerosis, peripheral nerves, hippocampus, cerebrovascular and degenerative brain diseases), musculoskeletal (cartilage, triangular fibrocartilage, osteoarthritis) and breast imaging.
"Very recent work on sodium imaging for breast cancer offers exciting possibilities, and sodium can provide information that we cannot get from any other technique noninvasively," he said. "Even more important is the followup after chemotherapy, because until now it takes two to three weeks to differentiate between the responder and the nonresponder."
MRI systems worldwide
It is important to remember that although there are relatively few studies directly comparing 1.5 T, 3 T and 7 T, it is generally considered that higher field strength does not necessarily mean higher diagnostic accuracy for all diseases and abnormalities. This means that 7 T MRI will fill a specific niche in the market for certain sequences and imaging methods. They include: 31P MR spectroscopy (31PMRS), which can be used as a noninvasive tool for measuring the relative intracellular concentrations of several phosphorus metabolites in different organs.
This information combined with magnetization transfer techniques, enable measurement of metabolic activity at rest in vivo. This can prove to have tremendous diagnostic capabilities as various pathological conditions are characterized by different metabolic patterns
"Hepatic energy metabolism is of high interest as its alterations are indicative for inflammatory and neoplastic liver diseases as demonstrated in type 2 diabetes patients at 3 T," Trattnig said, adding that invasive liver biopsy is the only method currently used to distinguish between nonalcoholic fatty liver disease and steatohepatitis.
As more companies start developing 7 T, and it eventually makes its way into the clinical setting, researchers and radiologists will have a chance to peer further into the how and why of illnesses.
Wattjes, M. P., & Barkhof, F. (2012). Diagnostic relevance of high field MRI in clinical neuroradiology: the advantages and challenges of driving a sports car. European Radiology, 22(11), 2304–2306. doi:10.1007/s0033001225529