Morphological cardiac imaging
In morphological cardiac MRI, the main priority is to clearly delineate the heart chambers and vascular lumen, using a “dark blood” contrast (PRESTO / dark-blood / black-blood) . This imaging is based on spin echo sequences, in their fast and ultrafast variants, with prospective gating and diastole acquisition. « Dark blood » contrast results from the outflow effect of the moving spins in the blood, and suppression of the blood signal prior to acquisition.
Fast spin echo sequences, combined with the multiplane technique, can be carried out in breath-hold (5 to 15 seconds depending on the machine). Ultrafast spin echo sequences can acquire a slice in a single R-R interval (a heartbeat). Breath-hold is not therefore indispensable.
Suppression of the blood signal (Dark blood / PRESTO)
The blood signal is suppressed to obtain a dark blood contrast, using a double inversion preparation scheme (figure 11.5):
- The first 180° inversion pulse is non selective, and applied to the entire volume explored
- This is immediately followed by a second 180° inversion pulse selecting the slice to scan: restoring the longitudinal magnetization of the spins of the slice of interest. However, the longitudinal magnetization of the spins outside the slice plane remains inverted, and regains equilibrium depending on the T1 of the blood.
- The acquisition sequence is launched after an inversion time TI corresponding to the cancellation of the blood signal: the signal of the blood flowing into the slice is suppressed.
Nevertheless, a blood signal will persist when it is in the slice during the two inversions and during the imaging sequence (case of slow-flowing blood moving through the slice plane).
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