One needs to be careful with the naming of the quantities. Notice that B1(x,y,z,f0) refers to a field with two components, namely (B1+, B1-) !!!! I personally find the name a bit unfortunate but it was directly taken from MRI jargon where they denote B1 to the RF B-field to distinguish it from the static magnetic field, denoted as B0.
Do NOT confuse with B(x,y,z,f0) that is a C^3 quantity with (Bx, By, Bz) components!!
Check in the manual B1(x,y,z,f0) for more details.
