"Concerns for UHF MRI:
Prior to the construction of the 8 tesla system, there were significant technological hurdles to overcome. Clearly, the construction of a whole body magnet that could operate at such high magnetic fields was the foremost technological concern. However, even if such a device could be constructed, the ability to obtain a human image in the ultra high frequency (UHF) range remained uncertain. Indeed, at 340 MHz, scientists are approaching the microwave region of the electromagnetic spectrum. As such, there was the possibility that the radiofrequency (RF) energy required to excite the spin would not penetrate to the center of the human head. In addition, even if the RF could penetrate, there were concerns that dielectric resonances (much like standing waves) would be produced inside the head which would deteriorate the quality of the resulting images. Most importantly, there were numerous human safety concerns. Foremost amongst these were the theoretical predictions that a tremendous amount of RF power would be required to obtain the image. There was the possibility that RF burns could be produced in the subject. The high RF power requirements at 340 MHz could also act to raise the internal temperature, much like heating in a microwave oven. As such, anticipated high RF power requirements became the key concern against a move to the UHF region of the spectrum. In addition, humans had yet to be exposed to such high static magnetic field strengths. Previous experience at 4 tesla indicated that exposure to high magnetic fields could produce transient dizziness in the subject following rapid head motion in the scanner. This presumably occurs due to the interaction of the magnetic field with ions contained within the inner ear. It is assumed that eddy currents are set up during rapid motion within these canals. In turn, these currents cause a local osmotic pressure disruption, much like what is experienced when consuming high quantities of alcohol. Naturally, there was a fear of the unknown. Yet, a thorough review of the scientific literature pointed to the relative safety of high magnetic fields. Nonetheless, it would be naïve to assert that everything is known about the interaction of static magnetic fields with a system which in itself remains filled with mystery, namely the human body."
P.-M. L. Robitaille, 2000 Yearbook of Science and Technology.