Pulse Sequence used in the Dynamic MR angiogram

Spoiled T1-weighted GRE sequences (3D fast field echo, 3D turbo field echo) are more suitable for dynamic subtraction MR angiography than a nonspoiled GRE sequence. With a spoiled GRE sequence, the signal intensity increases as the blood level of gadopentetate dimeglumine increases. However, with a nonspoiled GRE sequence, the signal intensity decreases at a blood level of gadopentetate dimeglumine higher than 2 mmol/L due to reduction of T2 relaxation (actually T2* decay) time 

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Among spoiled GRE sequences, the signal-to-noise ratio and vascular contrast are higher in 3D fast field echo images than in 3D turbo field echo images, whereas the artifact from cardiac motion is much less severe in 3D turbo field echo images than in 3D fast field echo images. Therefore, we use the 3D fast field echo sequence for imaging any anatomic areas other than the chest and the 3D turbo field echo sequence for imaging the chest .

Sequence parameters that influence the signal-to-noise ratio of vessels at 3D fast field echo or 3D turbo field echo imaging are the repetition time and flip angle . The short repetition time reduces the acquisition time, thus permitting breath-hold data acquisition and enabling imaging of arteries before contrast material reaches the veins. However, as the repetition time is decreased, the signal-to-noise ratio of enhanced vessels decreases in proportion to the square root of the repetition time. Thus, high-quality vascular imaging can be achieved even with a moderately short repetition time when the timing of data acquisition is appropriate.

The flip angle should be optimized according to the repetition time to achieve a high signal-to-noise ratio in vessels. For contrast material–induced T1 values of vessels enhanced with bolus injection, the optimal flip angle for a repetition time of 10 msec is 25°–40° .

Intravoxel dephasing can be reduced by using short echo times and velocity-compensating gradients.