Magnetic resonance signal loss in turbulent shear flow

NMR signal loss due to turbulent shear flow is discussed, and a general expression for the phase fluctuation is derived. In the presence of flow shear, the velocity fluctuation perpendicular to the direction of magnetic gradient and the Reynolds stress can cause loss of MR signal. Most of signal loss results from the boundary layer, where the flow shear is strong in turbulent pipe flow. Half the signal loss within the mixing layer distal to a moderate stenosis is caused by the velocity fluctuation in the direction of magnetic gradient, while the remaining results from the velocity fluctuation perpendicular to the magnetic gradient. The use of eddy diffusivity for the description of signal dephasing in a spin echo sequence is also addressed; A positive, constant eddy diffusivity can not describe the temporal change of phase fluctuation correctly.