Ultra High Field MRI Functional MRI, Any Field Strength Limits?
Sou-Tien Wang, WangNMR, California, USA and AST, Taipei, Taiwan
In 1995, WangNMR Inc. has successfully deigned, constructed and delivered a 4.7 T 110 cm bore functional MRI System to New York Neurological research Institute, Medical School at Columbia University, New York, New York. The human functional MRI has been successfully operated and generated many sodium and phophorus image/spectroscopy for both brain and breast cancer research. This year, we have applied for 4 T MRI FDA 510K approval in preparation for 4 T clinical use.
WangNMR and AST have started design and construction of a 500 MHz (11.74 T) 210 mm bore functional MRI for whole body animal research. This system will have strong grandient (>20 G/cm), fast pulse (80 – 100 ms) actively shielded gradient coil with two RF transmitters and four RF receivers allowing multiple functional imaging and spectrosopic research.
These high field functional MRI provides exceptional signal-to-noise (S/N) ratio and excellant spectral resolution. They are needed to do carbon, phosphorus, or sodium imaging/spectroscopy study.
For human, the highest functional MRI is the 7 T MRI at University of Minnesota and the highest MRI imager at 8 T at Ohio State University. There are 9.4 T adn 11.74 T MRI proposal underway. For animal, the highest functional MRI is the 11.74 T system right now.
From safety consideration, heating damage to the tissue set the limit, although there is discomfort level already at 3 – 4 T. The technical difficult in RF field penetration and thus generating nonuniform RF field homogeneity. On the other hand, the demands for low noise gradient amplifier and fast pulse rate pose great technical challenge as well. Final limitation will be the field limit of superconductor and the cost of MRI machine. All of these issues are presented and discussed in the paper.