Measurement of flow velocity by magnetic resonance imaging using 2D phase contrast technique. Estimation of oblique flow

This study analyzed the effects of the obliqueness of flow on the accuracy of measuring flow velocity by means of the 2D phase contrast MR technique. A constant flow phantom consisting of a pump, a polyethylene tube and a flow meter was assembled. A commercial 1.5 Tesla MR imager was used to perform flow velocity measurements. The phase contrast technique was used to estimate the flow velocity of saline through the phantom. The effects of changes in obliqueness of flow to the plane of imaging were studied. The obliqueness had a significant effect that was more pronounced with increasing section thickness. An increase in the obliqueness angle caused underestimation of the average and maximum velocities. The obliqueness was found to be an important parameter affecting the estimation with the 2D phase contrast MR technique. The application of MR to flow velocity measurement was first described by Singer. Various pulse sequences have been proposed to evaluate the flow velocity by MR. The two main classes of techniques are the time-of-flight method and the phase contrast method. The time-of-flight technique derives its contrast from the flow-related enhancement of inflowing blood. The phase contrast method, on the other hand, is based on applying balanced gradient pulses, and derives its contrast by detecting spin phase differences as blood moves across a magnetic field gradient. The phase contrast technique is not only a simple subtraction of phase images on a pixel by pixel basis but a complex difference of phase obtained with a known different flow encoding gradient. Phase images are collected using balanced gradient pulses to produce a phase shift for moving spins. The difference in phase is directly proportional to the flow velocity of the spins within the corresponding voxels. The phase contrast technique is generally better than the time-of-flight technique for quantitative measurement of flow velocity in cardiovascular an cerebrospinal abnormalities. Advantages of the phase contrast method include bi-directional flow sensitivity, ability to obtain both a conventional magnitude image an a velocity image from the same data set and the relative simplicity in interpretation, processing, and display. Cardiac gating can be used in conjunction with phase contrast to provide flow velocity encoded phase images at equally spaced temporal intervals during the cardiac cycle. Interpretation of flow velocity values during the cycle can provide instantaneous average and maximum flow velocity information. The purpose of this study is to evaluate the effects of obliqueness of the flow on the accuracy of the MR flow velocity measurement. (author)