@MISC{Wiesinger_limitationsand, author = {Florian Wiesinger}, title = {Limitations and High Field Potential of Parallel MRI}, year = {} }

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Abstract

This contribution aims at explaining the limitations and potential inherent to parallel imaging (PI), which have recently been studied based on fundamental physical considerations (1-3). Using the principle of reciprocity (4), PI performance is shown to be determined by the electrodynamic fields of the radiofrequency (RF) receiver coil array. This translates into the following important conclusions: (1) PI performance is inherently limited. (2) The limitation softens for high field strength (B0) and/or large object size (L). Results are presented using terms of wave optics, in particular the RF wave length λ and the relative object size L λ. The latter is shown to be a key parameter for characterizing the signal-to-noise ratio (SNR) yield that can be achieved in PI. Principally, the electrodynamic fields of an array of RF receiver coils are governed by the Maxwell equations. For the following, a homogeneous and isotropic object is considered, where coils are exclusively arranged outside the sample. In this case, the Maxwell equations can equivalently be stated in the adimensional Helmholtz representation as (3,5): ( ) ( ) ( ) ( ) () ∆ + κ = ∇ = = ∇×γ µ2 01L 0,. L 0, L Li B LE r E r H r E r [1] with ()κ = µγ εγ + σ0 0B B i L the adimensional wave number, E and H the electric and magnetic field, respectively, r the spatial position vector, γ the gyromagnetic ratio, ε the permittivity, σ the conductivity and µ the permeability. Hereby, the characteristic length L (i.e. the diameter of the object) severs as a means to normalize the spatial position vector r and to describe the electromagnetic fields on an object size independent, unified length scale. Note that the adimensional wave number in Eq. [1] combines B0, γ, ε, σ, µ and L to form one complex number, which solely describes the curvature, and therefore the spatial variation, of E and H. Alternatively, κ can also be expressed in terms