Absorption Of NMR Signals At High Frequencies In Physiological Phantoms

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Abstract
In magnetic resonance (MR) imaging, improved signal to noise ratio (S/N) can be achieved with higher magnetic field strength. Such improvement, however, will be limited eventually by the absorption of the radiofrequency (rf) within the patient. Radiofrequency propagation into an infinite cylinder of physiological dielectric properties has been calculated at frequencies from 1 to 100 MHz for a concentric single loop antenna. The amplitude of the rf wave at the center of the phantom can be either enhanced or attenuated depending on the dielectric properties of the phantom. Measurements performed at 25 MHz in a 28-cm diameter cylindrical phantom of 154 mM saline show an attenuation of 16% and a phase shift of 57° which agree well with the calculations for this medium.

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