Correcting for rf inhomogeneities in multiecho pulse sequence MRI dosimetry

Abstract

The growing interest in using MRI to measure distributions of dose in specialized materials has prompted the need to improve existing methods for measuring NMR relaxation rates throughout large volume phantoms. The change in transverse relaxation rate (R2) has proven to be a more sensitive measure of dose than longitudinal relaxation rate (R1) in some materials; however, the accuracy of R2 measurements is limited by the presence of artifacts resulting from imperfect MRI tip angles throughout large phantoms. A novel echo quotient technique allowing accurate measurement of changes in R2 (delta R2) in regions of imperfect tip angle is presented. Application of this technique to the measurement of a dynamically wedged photon beam dose distribution demonstrates the improved accuracy of the echo quotient technique compared with other methods. Agreement of the MRI data with film and ion chamber data is within 2.5% throughout the volume of interest, indicating the potential for delta R2 MRI dosimetry to become a clinically useful tool.