Oscillating Dual Equilibrium Steady State Angiography ODESSA

A novel technique of generating non-contrast angiograms was proposed [20]. This method utilized a modified steady-state free precession (SSFP) pulse sequence (see Fig. 3.32). The SSFP sequence was modified such that flowing material reaches a steady state that oscillates between two equilibrium values, while stationary material attains a single, non-oscillatory steady state. Subtraction of adjacent echoes results in large, uniform signal from all flowing spins and zero signal from stationary spins. Venous signal can be suppressed based on its reduced T2. ODESSA arterial signal was more than three times larger than that of traditional phase-contrast angiography (PCA) in the same scan time, and also compares favorably with other techniques of MR angiography (MRA). Pulse sequences are implemented in 2D, 3D, and volumetric-projection modes. Angiograms of the lower leg, generated in as few as 5 seconds, showed high arterial signal-to-noise ratio (SNR) and full suppression of other tissues.

Figure 3.32: (a) Two-dimensional and (b) 3D ODESSA pulse sequences. Two TR intervals are shown. Each axis has zero net gradient area over the TR interval. During odd TRs (at left), a bipolar flow-encoding pulse follows readout on any axis. A triphasic pulse after even readouts, though not necessary, is included to mitigate imaging system nonidealities. The numbered locations correspond to spin states.

Figure 3.32: (a) Two-dimensional and (b) 3D ODESSA pulse sequences. Two TR intervals are shown. Each axis has zero net gradient area over the TR interval. During odd TRs (at left), a bipolar flow-encoding pulse follows readout on any axis. A triphasic pulse after even readouts, though not necessary, is included to mitigate imaging system nonidealities. The numbered locations correspond to spin states.

3.4.21 Fat-Suppressed 3D MRA

Appropriate rate of fat-suppression pulses (using spec IR spectral selective inversion recovery) were determined for fat-suppressed 3D magnetic resonance angiography (MRA) with an elliptical centric view order [21]. In abdominal 3D fast spoiled gradient echo (fast SPGR) wit an elliptical centric view order, the spec IR pulse rate was changed from zero to one every 15 repetitions (in nine steps) in eight volunteers. In the equilibrium phase, abdominal contrast-enhanced 3D MRA was obtained by 3D fast SPGR using an elliptical centric order without fat-suppression and with two spec IR, and by fat-suppressed 3D fast SPGR with a sequential-centric view order (3D-EFGRE). Fat and vascular signals were estimated. Although 3D fast SPGR using an elliptical centric order with two spec IR placed every 15 TR and 3D-EFGRE effectively decreased fat signals, these sequences lengthened the breath-hold by 4-6 seconds compared with non-fat suppressed sequence. 3D fat SPGR using an elliptical centric order and two spec IR reduced the fat signal by 30%. And provided good 3D MR angiography without substantial prolongation of breath-hold. Two spec IR can be used for generation of partially fat-suppressed abdominal 3D MRA without prolongation of the breath-hold when performing 3D fast SPGR using an elliptical centric view order.

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