Exploiting Phase Encoding Capabilities of Parallel Excitation for Improved Spatial Se- Lectivity in Inner-volume Imaging

Introduction: During the last years, Spatially Selective Excitation (SSE) has become practicable in combination with parallel RF transmission. SSE applications like inner-volume imaging (IVI) [1] or targeted spectroscopy [2] utilize the possibility to restrict the generation of transverse magnetization MT to specified target volumes by spatial amplitude modulation according to a given target pattern. However, the potential of SSE to additionally control the spatial phase distribution of MT has been barely exploited so far, although spatial phase modulation offers new promising applications [3]. This work introduces Inner Volume Imaging by Phase Encoding (IVIP) and demonstrates that phase modulation during excitation can be used instead of amplitude modulation to realize spatial selectivity for IVI. It is shown that with this approach spatial selectivity can be enhanced by mitigating certain limitations of SSE pulses which otherwise significantly deteriorate the amplitude modulation accuracy. In order to select an inner-volume by amplitude modulation, RF pulses are typically calculated to excite MT with a finite flipangle (FA) inside a target volume, while outside this volume FA should be zero. However because of physical limitations (e.g. Gibbs ringing due to limited resolution) and experimental imperfections (off-resonances, gradient imperfections, etc.), artifacts excited by small, but finite flip-angles (typically less than 10% of the target FA) may be generated outside the target volume. When exciting with small target flip-angles the signal amplitude related to such artifacts is typically smaller by more than one order of magnitude than the amplitude from the desired signal (Fig. 1a) as long signal amplitude depends linearly on FA. However, for short repetition times (TR) or large flip-angles, saturation effects can no longer be neglected and the dependency of signal amplitude on FA in the steady state is highly non-linear as described by the equation and the graph in Fig. 2 for the example of gradient echo signals. This extremely non-uniform weighting of desired and undesired signal results in highly pronounced artifacts as shown in Fig. 1b,c. This limits the application of SSE, e.g. for 3D scans with reasonable flipangles and short TRs. The IVIP approach overcomes this limitation by exciting the whole object with homogenous FA amplitude close to the Ernst angle (FAe) and realizing the volume selection by spatial phase encoding of the generated MT.