STANDING WAVES IN UNIFORM WATER PHANTOMS

To measure and correct nonuniformity in magnetic resonance images, large cylindrical water phantoms have traditionally been used. Above about 0.5 T, the large dielectric constant of water causes standing wave effects in water phantoms, giving a nonuniform RF field inside the phantom, even when the external field is uniform. The held and signal inside a nonconducting long cylinder are calculated for coils with circular and linear polarization, as a function of phantom diameter and field strength. At 1.5 T, the signal drops by 10% at the edge of a 9.5 cm diameter water cylinder. Oil phantoms are preferred, since the dielectric constant is lower (80 for water, about 5 for oil). At 1.5 T, the signal drops 10% at the edge of a 38 cm oil cylinder. A previous expression (G. H. Glover et al., J. Magn. Reson. 64, 255, 1985) for the internal field and signal for a coil with circular polarization is in error. (C) 1994 Academic Press, Inc.