OPTXE is plotted in logarithmic scale for the central sagittal plane of the “Duke” head model and the spherical sample with radius a = 10 cm, for B0 = 1.5T, 7T, 10.5T, 21T. OPTXE profiles along the x, y, and z axes are in agreement between the two objects, especially for the case of the y-profile, for which the dimension of the head matches the radius of the sphere. OPTXE decreased with increasing field strength for both the heterogeneous head model and the uniform sphere.