Over the past five to ten years, magnetic resonance imaging (MRI) has become an important tool for medical diagnostic applications. Newer techniques are leading to the use of higher static magnetic fields; more rapidly switched gradient fields and higher radiofrequency (RF) magnetic fields. Although use of the increasingly stronger electromagnetic fields is causing concern about patient safety, to date, only simplified homogeneous spherical, cylindrical and disc models have been used to obtain induced current density distributions and rates of energy absorption (specific absorption rates or SARs). Since this knowledge is vitally important, particularly for the critical regions of the body, we will adapt the anatomically based modeling techniques that we have successfully used for electromagnetic dosimetry. We propose to use the previously tested highly efficient impedance method and its new generalization for higher frequencies to calculate internal induced current densities produced by gradient magnetic fields, and SARs for RF magnetic fields, of realistic polarizations and variations typical of present and planned MRI systems. We also plan to use the newly developed modified finite-difference time-domain method, which has previously been used successfully for a wide variety of RF electromagnetic exposure conditions. For the various calculations we will use the new high- resolution model of a human volunteer based on MRI scans (with resolutions of 1.875-3 mm) and its scaled versions where different scaling factors alphax, alphay, and alphaz on the order of 0.6-1.0 will be taken to obtain models of different heights and weights representative of human adults and children. This latter step is needed since induced current densities and SARs are strongly dependent on the size and shape of the body. The current densities and SARs thus calculated will be compared with the safety guidelines suggested by USFDA and NRPB (U.K.).
| Gandhi, O P; Chen, X B (1999) Specific absorption rates and induced current densities for an anatomy-based model of the human for exposure to time-varying magnetic fields of MRI. Magn Reson Med 41:816-23 |
