Which characteristic defines gradient echo sequences?

Gradient echo sequences are well-defined by their use of low flip angles during excitation. This characteristic allows for quicker imaging times and is particularly useful in dynamic studies where speed is crucial. Low flip angles generally create less RF power deposition and can enhance signal-to-noise ratio under certain conditions, particularly in conjunction with gradient-based imaging techniques.

Using low flip angles also contributes to a reduced T1 weighting effect, as the signal recovery time is not as lengthy as that required for high flip angle sequences. This results in images that are often more efficient for specific applications, such as functional MRI or in cases where quick imaging is necessary.

High flip angles, which are typically associated with spin echo sequences, and the use of 180° refocusing pulses, define a different imaging strategy that emphasizes T2 contrast. Similarly, requiring longer TR intervals does not align with the rapid acquisition nature of gradient echo sequences, which typically aim for shorter TR times to facilitate faster imaging.