A study on multiple array method of birdcage coils to improve the signal intensity and homogeneity in small-animal whole-body magnetic resonance imaging at 7 T

Abstract

Birdcage (BC) coils generate a homogeneous magnetic field ( divide B-1 divide ), whose strength depends on the length of the coils, the number of legs, the resonance frequency, the excitation method, and the length-to-diameter ratio. BC coils with a shorter distance between the end rings produce a more intense divide B-1 divide field with a higher level of homogeneity. Here, we propose a combination of overlapping and nonoverlapping radiofrequency volumetric BC coils for small-animal magnetic resonance imaging (MRI) working at 7 T with a frequency of 300 MHz to improve the divide B-1 divide -field homogeneity and intensity along the main magnetic field ( divide B-0 divide ) direction. The first combination consisted of two overlapping BC (o-BC) coils, in which each coil is half the length of the imaging object. A combination of two nonoverlapping BC coils separated by a small gap (nog-BC) was also developed. In order to improve the performance of the nog-BC coil, we attached a decoupling capacitor. In computational calculations, the proposed o-BC coil configuration showed an increased homogeneity in the divide B-1 divide field along the divide B-0 divide direction compared with the other coils. The experimental bench tests of the o-BC coil configuration showed a high decoupling coefficient between the BC coils, which allowed each of them to be used individually for transmission and reception. The o-BC coil was able to maintain a high transmission efficiency without signal interference while exhibiting an increased divide B-1 divide -field homogeneity compared with other coils. In this study, the advantages of the o-BC coils are demonstrated using electromagnetic simulations and MRI on a homogenous phantom and an in vivo mouse body.