Which parameter is crucial for coherent gradient echo sequences to maintain coherence?

Coherent gradient echo sequences rely on specific parameters to maintain signal coherence and optimize image quality. The re-winder gradient plays a critical role in preserving the phase relationship among the spins in the examined tissue. By appropriately re-phasing the spins during the sequence, this gradient helps to ensure that the signals from different parts of the tissue can remain in sync, which is essential for generating a clear and coherent image.

In coherent sequences, the timing and application of the re-winder gradient are pivotal because they allow for the maintenance of the longitudinal magnetization and the effective use of the transverse magnetization, thus maximizing the signal intensity in the final images. This gradient works in conjunction with the frequency encoding and phase encoding processes to help achieve the desired image resolution and contrast.

Other parameters mentioned, while important in different contexts of MRI, do not directly impact the coherence of gradient echo sequences to the same extent as the re-winder gradient. For instance, a long repetition time (TR) primarily affects tissue contrast and relaxation effects rather than coherence. Short echo time (TE) can enhance image quality but is also not the primary factor for maintaining coherence, while high flip angles mainly influence signal amplitude rather than coherence in the context of gradient echo sequences. Therefore, focusing on the