A Switched-Mode Breast Coil for 7 T MRI Using Forced-Current Excitation

Abstract

In high-field magnetic resonance imaging, the radio frequency wavelength within the human body is comparable to anatomical dimensions, resulting in B1 inhomogeneity and nonuniform sensitivity patterns. Thus, this relatively short wavelength presents engineering challenges for RF coil design. In this study, a bilateral breast coil for 1H imaging at 7 T was designed and constructed using forced-current excitation. By forcing equal current through the coil elements, we reduce the effects of coupling between the elements to simplify tuning and to ensure a uniform field across both breasts. To combine the benefits of the higher power efficiency of a unilateral coil with the bilateral coverage of a bilateral coil, a switching circuit was implemented to allow the coil to be reconfigured for imaging the left, right, or both breasts.

Homogeneous B1+ for bilateral breast imaging at 7 T using a five dipole transmit array merged with a high density receive loop array

“…A homogeneous B 1 + field is particularly important for MR sequences such as turbo spin echo (TSE), which can still not compete with the clinical standard at 3 T . One approach to overcome B 1 inhomogeneity is forced current excitation as demonstrated in previous studies . Another approach is the use of a multi‐transmit setup where RF shimming can be used to improve transmit uniformity for every subject …”

“…still not compete with the clinical standard at 3 T. 4 One approach to overcome B 1 inhomogeneity is forced current excitation as demonstrated in previous studies. [5][6][7] Another approach is the use of a multi-transmit setup where RF shimming can be used to improve transmit uniformity for every subject. 1,8 A wide variety of breast MRI studies at 7 T have been presented with various RF coil setups.…”

A 32‐channel receive array coil for bilateral breast imaging and spectroscopy at 7T

“…The left and right quadrature coils were connected to produce B + 1 fields with the same polarity, consistent with the previously reported version. 24 Concentric 3.5-mm-wide coplanar shields surrounded each saddle and Helmholtz loop to improve B + 1 transmit efficiency at high fields. 25 All four saddle elements were connected in parallel at a common voltage point located at the center of the bilateral volume coil along the x-axis via λ/4 twin-axial transmission lines (RG108; Pasternack Enterprises, Irvine, CA) to ensure equal currents through all elements, irrespective of loading.…”

“…22 The FCE approach is also beneficial for bilateral configurations because of its ability to provide homogeneous excitation throughout the region of interest without the need for multiple transmit channels, its ability to mitigate effects of the mutual coupling between the left and right volume excitation coils, and the potential for a switchable, unilateral single breast excitation implementation. 23,24 Here we describe the practical aspects of the design, construction, and evaluation of a bilateral 32-channel receive array for imaging at 7T, together with a bilateral FCE transmit coil modified to accommodate the array. Because of the the lack of commercially available high-channel count 1 H breast coils for 7T, the performance of the array coil was evaluated by comparison with the bilateral FCE volume coil in transmit/receive (T/R) mode, as well as to the performance previously reported on the 16-channel unilateral version of this coil.…”

Experimental and numerical analysis of B 1 + field and SAR with a new transmit array design for 7 T breast MRI

“…Additionally while many high field breast MRI studies published promising results [1, 10–13, 28–33]; to our knowledge, so far, only one study has evaluated B 1 + field and peak/average SAR variations in different breast models and compared to breast phantoms [34]. In our study, numerical analysis was performed using FDTD method in a homogeneous spherical model and in four different breast models.…”