Fresh Blood Imaging (FBI) MRA
The magnetic resonance angiography technique of the FBI type (“Fresh blood imaging”) is based on:
- 3D ultrafast spin echo acquisition in half Fourier plane
- prospective ECG synchronization
- no contrast agent
- magnetization preparation by inversion pulse to suppress the fat signal (STIR)
The spin echo type sequence with half Fourier plane is chosen for its speed (less than one second per slice) in fixing movements. The effective TE in these sequences is short, to fill the center of k-space before the flow-induced dephasings reduce the vascular signal. The volume reference plane is coronal, parallel to the axis of the vessels.
The phase encoding direction is chosen parallel to the axis of the vessels in order to:
- preferentially orientate the flow along the axis of the vessels due to the short relaxation time T2 of the blood
- stop the readout gradient from weakening the vascular signal, as it is of a higher amplitude than the phase gradient for the central lines of k-space.
Prospective ECG synchronization triggers acquisitions at the same moment in the cardiac cycle. The optimal interval between R wave and acquisition, for the vessel of interest, is determined by a prior calibration sequence. This sequence consists in producing different 2D images incrementing the time interval each time in relation to the R wave.
The signal of slow flowing vessels of the venous type is globally the same, regardless of the time interval in relation to the R wave. A complementary 3D acquisition with only the slow flows can be used as a reference and be subtracted from the arterial acquisition to conserve only the fast flows.
STIR preparation reduces the signal from the surrounding fat tissues.
For faster flows, ultrafast spin echo acquisition can be faulty, with the appearance of hyposignals or signal voids inside vessels. One way to get around this problem is to choose the diastolic period to take advantage of the slowing of the blood flow. However subtraction with venous flows will not perform as well.