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Zusammenfassung:
Purpose/Introduction: Extended field-of-view (FoV) imaging is relevant for many applications, e.g. whole-body MR/PET attenuation correction [1], radiotherapy [2], and biopsy. Recently, a method to extend the axial FoV using an optimal readout (RO) gradient to compensate gradient nonlinearities
and B0 inhomogeneities has been shown [3]. However, the optimal RO gradient depends on the position of the region of interest and therefore differs
for optimization of the left and the right patient’s arm. In this work we developed a 2D SE-based sequence using a dual-echo acquisition with optimized
RO gradients for distortion-reduced imaging of both the left and the right patient’s arm.
Subjects and Methods: A dual-echo SE sequence was modified to handle RO gradients differing in polarity and amplitude (FIG. 1A). An additional prephase moment ensures the spin echo condition for the second echo. The readout gradients have been adapted to acquire a distortion-reduced axial slice of the right patient’s arm in the first echo and of the left patient’s arm in the second echo. The optimal space-depending RO gradients were calculated automatically by the sequence according to
GRO (x,y,z) = - δB0(x,y,z) / c(x,y,z), where δB0 is the B0 inhomogeneity and c is the relative error in the gradient field. The main magnetic field inhomogeneities
and the gradient field nonlinearities were measured using a magnetic probe array as described in [3]. The field coefficients were stored in spherical harmonics as input files for the sequence calculation.
A volunteer experiment was performed on a 3T whole-body system, and a transversal slice at z=0 was acquired. The field-of-view was set to 600mm with 1.18*1.18mm in-plane resolution and 5mm slice thickness.
