Black Blood Versus Bright Blood Imaging

Abstract

Spin echo pulse sequences exhibit intrinsic black-blood contrast, caused by the movement of blood out of the image slice between the 90° and 180° pulses, known as spin washout. In the presence of slow blood flow, the intrinsic black blood contrast of spin echo pulse sequences becomes inconsistent. Black Blood Contrast can be improved by using a black blood preparation scheme, in combination with a spin echo pulse sequence. The black blood preparation scheme consists of two 180° inversion pulses, one non-selective and one slice selective, to invert the blood magnetisation outside the image slice. As the magnetisation recovers towards zero, the inverted blood moves into the slice. The spin echo pulse sequence is applied as the inverted blood magnetisation reaches zero leading to no signal from blood. Gradient Echo pulse sequences with short repetition times commonly exhibit an intrinsic bright blood appearance from flowing blood. The short TR causes saturation of the magnetisation and a reduction in signal of stationary tissue within the slice. Flowing blood entering the slice is fully magnetised and is able to yield a higher signal than the surrounding tissue, resulting in a bright blood appearance. This effect is known as inflow enhancement and it is the main contrast mechanism employed in time-of-flight TOF MR angiography, in addition to bright blood imaging of the heart.