and 6 is selectively excited by such a pulse the flip angle varies along the direction of its thickness that is the z direction which represents the direction of blood flow as shown in fig1 the flip angle is very small and nearly 0 u00b0 on the upstream side on the left hand side of the excitation slab 4 increasing gradually as one moves to the downstream side to the right hand side such a flip angle profile can be obtained by means of a pulse sequence shown in fig2 in other words the magnitude of the slice selective gradient magnetic field expressed as g z the slice selection being in the z direction is gradually changed and the carrier frequency of the excitation rf pulse is changed at the same time as shown in fig2 alternatively use may be made of a differential rf coil 1 as shown in fig3 such a differential rf coil i may comprise four coil segments 21 22 31 and 32 arranged on a cylinder such that segments 21 and 22 and segments 31 and 32 and opposite to each other with respect to the cylinder and that segments 21 and 31 and segments 22 and 32 are disposed next to each other along the longitudinal direction if currents are caused to flow in.