In-plane vascular imaging: pulse sequence design and strategy.
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Magnetic resonance (MR) angiography is a noninvasive method of obtaining images without contrast agents. Recent developments in sequence design have allowed images of moving spins to be obtained without a loss of signal by rephasing the spins with three or four gradient pulses to compensate for constant velocity or acceleration, respectively. At longer echo times (TE), this approach allowed for low readout gradients and high signal-to-noise ratios. Angiograms with a resolution of 300 micron were obtained. With additional sequences that allow some dephasing but minimal signal loss, separate images of arteries and veins were obtained. Phase information was used to estimate flow velocity. Application of the rephasing scheme to gradient-echo sequences allowed for ungated, fast MR angiograms. Acceleration correction was important for long TE sequences, but velocity-corrected, gradient-echo sequences with a very short TE were comparable to velocity- and acceleration-corrected, gradient-echo sequences with slightly longer TEs. With ungated three-dimensional, gradient-echo sequences, susceptibility artifacts were minimized and excellent contrast-to-noise ratios were obtained.
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