Practical methods for improving B1+ homogeneity in 3 Tesla breast imaging.

IN RECENT YEARS, there is increasing interest to move breast MRI toward higher static field strengths. The motivation for higher field strengths lies in the promise of higher signal-to-noise ratio (SNR) (1), however, higher field (e.g., 7 Tesla [T]) human MRI remains challenging due to several difficulties including the inhomogeneity of the transmitted radio frequency ( B1+) field, which leads to nonuniform image signal and contrast over the region of interest (2). Even at moderately high field strengths such as 3T, B1+ inhomogeneity effects are observed and are severe enough in certain applications to warrant detailed understanding and correction—this is especially important at 3T given the clinical importance of this field strength (3). In the case of 3T breast MR, this B1+ nonuniformity results in a distinct left–right asymmetry, such that the B1+ field magnitude in the left breast is increased by a factor of approximately 1.3 with regard to the right breast for various patient orientations and types of body coils (2–5). An example of the B1+ distribution is shown in Figure 1a, with another example in Figure 1b showcasing the MR image shading that results from such an inhomogeneous B1+ distribution. Figure 1 Example for the B1+ distribution (flip angle in degrees) (a) and the resulting image shading in the breast at 3T (b), obtained with a body coil. c: Computational model of a 3T body coil with body model (Virtual Family, Ella, 26 years, IT’IS foundation). ... Recent research efforts have led to several techniques aimed at reducing B1+ nonuniformity for various anatomies; the most recent and theoretically most versatile methods being Transmit SENSE (6,7) and multi-channel RF shimming (8). These methods currently receive considerable attention and are eventually expected to provide ultimate control over transmit field uniformity and safety for future high-field imaging; however, they are also complex methods that require costly additional hardware and considerable computational effort to calculate the optimized excitation parameters. The work presented here instead focuses on two simple methods without the need for subject-specific optimization, which are moreover readily implementable on today’s clinical systems without significant additional hardware or training effort. The first method is a cost-efficient passive shimming approach, whereby dielectric materials are inserted in the vicinity of the breast to perturb the field lines toward a uniform distribution within the region of interest (ROI). The second method presented is two-channel RF shimming, nowadays readily implemented on several scanners that provide a range of amplitude and phase adjustment possibilities. The ability to alter the RF field distribution by means of dielectric coil loading was first observed by Foo et al (9); this concept has subsequently been studied by several authors (10–17) in the form of dielectric shimming, with the most extensive analyses by Yang et al (10–12) and Webb et al (13–15). The investigated anatomies were mostly limited to the brain, however, with a small number of publications focused on abdominal, cardiac, and pelvic imaging. Moreover, the available literature is limited to empirical tests of dielectric materials in the vicinity of the body and/or phantoms; while there are only limited modeling and parametric studies available. Therefore, this study aims at the modeling of the effect for the case of 3T breast MR in particular—especially in view of the fact that the presented passive shimming method is simple and low-cost, permits moderate field correction, and is directly usable in clinical settings. In addition, dielectric pads increase the B1+ signal locally and, therefore, ultimately provide an increase of SNR and in turn image contrast (11). Moreover, we study the combination of two-channel RF shimming and dielectric shimming to understand the performance benefits of combining these two most widely available and practical shimming methods. The study also studies the safety aspects of our proposed shimming methods to ensure that SAR is maintained within regulatory limits.