Calculation of SAR and B/sub 1/-field within human head excited by MRI birdcage coils

Abstract

Birdcage coils have been widely used to generate radiofrequency (RF) pulses for magnetic resonance imaging (MRI) because of their capability to produce a highly homogeneous B/sub 1/-field over a large volume within the coil. This is true even if the coil is loaded with a human body or a human head provided that the frequency of the B/sub 1/-field is low (less than 50 MHz). However, when birdcage coils are employed for high frequency MRI, the interaction between the electromagnetic (EM) field and the biological object to be imaged deteriorates the B/sub 1/-field homogeneity. Such a deterioration reduces the quality of the MR images and causes difficulties for advanced MRI studies such as functional MRI and MR spectroscopy. Moreover, the EM interaction at high frequencies also leads to an increased electric field, thus, an increased specific energy absorption rate (SAR) in the biological object, causing concern about the possible adverse effects of MRI on human health. Jin et al. (see Phys. Med. Biol., vol.41, p.2719-38, 1996) proposed a method that was suitable for modelling open MRI coils; however, it has difficulties in modelling coils with an RF shield, which is often used to reduce the unwanted interactions between the EM fields and the exterior structures. We employ the finite-difference time-domain (FDTD) method to study this problem. It is found that the FDTD method can effectively model the complex anatomical structure of the human head, the currents in the MRI coil, and the RF shield.