Phase enhancement for time-of-flight and flow-sensitive black-blood MR angiography.

A magnitude-based MR angiography method of standard time-of-flight (TOF) employing a three-dimensional gradient-echo sequence with flow rephasing is widely used. A recently proposed flow-sensitive black-blood (FSBB) method combining three-dimensional gradient-echo sequence with a flow-dephasing gradient and a hybrid technique, called hybrid of opposite-contrast, allow depiction of smaller blood vessels than does standard TOF. To further enhance imaging of smaller vessels, a new enhancement technique combining phase with magnitude is proposed. Both TOF and FSBB pulse sequences were used with only 0th-order gradient moment nulling, and suitable dephasing gradients were added to increase the phase shift introduced mainly by flow. Magnitude-based vessel-to-background contrast-to-noise ratios in TOF and FSBB were further enhanced to increase the dynamic range between positive and negative signals through the use of cosine-function-based filters for white- and black-blood imaging. The proposed phase-enhancement processing both improved visualization of slow-flow vessels in the brains of volunteer subjects with shorter echo time in TOF, FSBB, and hybrid of opposite-contrast and reduced wraparound artifacts with smaller b values without sacrificing vessel-to-background contrast in FSBB. This method of enhancement processing has excellent potential to become clinically useful.