Novel Methods & Models for B1 Mapping

Introduction Roughly 30 peer reviewed papers have been published on B1+ mapping in the last 2 years from 2010 to 2011. This shows the vitality of a research area that has started back in the 80’s [1, 2]. The RF field behavior in biological tissue and its spatial variations was assessed very early [3]. The transmit RF field can be described by a time independent, spatially varying B1+ map times the time dependent transmit voltage. By definition the complex B1+ map is the RF field distribution per transmit voltage unit. In the 90’s techniques have been developed, which were more practical for in-vivo measurements and some of them became gold standard for more than a decade [4-6]. The main interest for doing B1+ mapping was to quantify MRI and MRS results e.g. to correct for B1+ non-uniformities in T1 maps. In the first years after 2000, the interest in measuring the B1+ field distributions was increased due to the emerging high field strengths and the more pronounced B1+ field variations [7-8]. Recently the upcoming parallel transmission (pTX) technology defined new requirements for B1+ mapping and boosted the development of new techniques: complex B1+ field distributions (in amplitude and phase) of multiple transmit coils with large dynamic ranges need to be acquired for pTX pulse design within a limited measurement time frame [38-43]. Electrical property mapping, a new contrast for MR which has recently been proposed, requires highest accuracy for B1+ mapping [9-11]. The number of new B1+ mapping techniques recently published reflects the changing demands on these methods but might also indicate that there is no gold-standard yet fitting all application needs. In comparison with other MR imaging techniques B1+ mapping in general suffers from rather long acquisition times. The “B1 contrast” to be encoded is special in the sense that all manifold contrast mechanisms (luckily) existing in MR need to be eliminated from signal generation to crystallize the pure effect of RF excitation on the MR signal. The course focuses on presenting  concepts of encoding the B1+ field  characteristics to describe performance and the area of validity of B1+ mapping concepts  special techniques for mapping multi transmit coil arrays  an overview of the most recent and popular methods The goal is to show different concepts, which might be used to tailor B1+ mapping methods or to assess existing methods for specific applications.