Time-resolved rapid MR imaging blood flow velocity measurement in the carotid arteries: A comparison with duplex sonography

Quantification of blood flow velocity by MR imaging is usually time-consuming. With the technique used, flow velocity is measured simultaneously for 13 different points of time within the heart cycle and an equal spacing of 30 msec with only one multiecho pulse sequence (16 echoes). By changing the trigger delay, the time interval of 390 msec (13 x 30 msec) can be adjusted within the heart cycle. The time required is about 8.5 min, the slice thickness is 25 mm. The technique is based on the introduction of a flow term to the Bloch equations, which results in a flow-dependent relaxation time, 1/T2F = 1/T2 + V/S, where T2 is given for stationary blood, V is the mean velocity, and S is the section thickness. To avoid errors caused by odd/even echo rephasing a moving average of four adjacent echoes is used. In 16 patients with stenosis of the carotid arteries, the time-resolved absolute flow velocity was determined using a 1.0-T Siemens whole-body imager. Results were compared with the results of duplex US. There was a close relationship between MR imaging and US regarding the time velocity curves and the absolute flow velocity. As sonography measures peak velocity and MR imaging measure mean velocity, MR imaging values were about half the US values, which is in good agreement with theory, assuming laminar flow