Overview of techniques to obtain vascular contrast
Endogenous vascular contrast can be obtained by 3 different techniques:
- either by using modifications linked to blood volume displacement, which will not be subjected to all the radiofrequency pulses, unlike the stationary tissue: time-of-flight phenomenon
- or by exploiting the transverse magnetization dephasing of moving spins subjected to a gradient: phase contrast
- or by optimizing spatial encoding and the duration of an ultrafast spin echo sequence, synchronized to ECG: FBI
In all three techniques, the signal and resulting image are produced by physiological flow. Depending on the underlying physical phenomenon, these methods will be more or less sensitive to certain flow velocities or types of flow. They will often come into difficulty, namely when faced with complex or turbulent flows.
In the case of contrast-enhanced angiography, it is more a question of morphological imagery: the distribution of the contrast agent in the vascular sector is imaged. The main criterion is thus to acquire data at the right moment, during the passage of the contrast agent bolus.
Except for FBI, these MR angiography techniques use sequences derived from T1-weighted echo gradient sequences.
Vessel signals in spin echo sequences
Vessels generally appear as hyposignals in spin echo sequences due to the outflow effect. The spins in the blood are excited during the slice selection pulse (a.). At time TE/2 some of these spins move out of the slice and so are not subjected to the 180° pulse: therefore there is a reduction in the signal from their initial region (b.). This signal is even lower when blood velocity is high. There will be a complete vascular signal void if all the spins have moved out of the slice at time TE/2.