MRI simulator with object-specific field map calculations

Bloch simulations with intra-voxel spin dephasing

“…The signal from the voxel affected by IVD can thus be calculated by integration of the phase over the whole voxel volume [8][9] [13]: …”

“…The Bloch simulator used this field map to interpolate the inhomogeneity induced by the gradients through the voxel [8]. The following parameters were used in both simulations and experiments: gradient echo sequence with FOV = 120 mm, 128x128-matrix size, TE = 10, 15 and 25 ms, TR = 1000 ms, 90 Hz bandwidth per pixel.…”

“…However, this approach requires special treatment of the time scale in order to generate proper spin-echo signal formation by 180 0 refocusing pulses. Another approach assumes a linear phase change across the voxel which determines the dephasing slope for the phase of the neighboring isochromats, and finally, integration of transverse magnetization is used to calculate the signal generated by each element [8]. An alternative approach is to calculate intra-voxel dephasing iteratively using an analytical formula for partial derivatives of the magnetization vectors [9].…”

Geometric distortion correction for echo planar images using nonrigid registration with spatially varying scale

“…The obtained binary image is further processed by morphological 'dilate' and 'erode' operations to remove small gaps or speckles. A water/air susceptibility map is created based on this binary image and then convolved with a kernel to generate a simulated field map using the method described in [21]. A distortion field at each location (x, y, z) is computed from this field map and used as the y component of D 0 (x, y, z), which is 0 in the x and the z components, as shown in (1).…”

“…Note that accurate simulation of the field map is a complicated task in itself [21], [22], [27], and it requires precise head models. To initiate our scale map calculation, however, we need only a rough approximation.…”