What forms the basis of the echo train length in a fast/turbo spin echo sequence?

In a fast or turbo spin echo sequence, the echo train length is determined by the number of 180-degree radiofrequency (RF) pulses applied after the initial 90-degree RF pulse. When the initial 90-degree pulse is applied, it tips the magnetization into the transverse plane, creating a signal that starts to decay due to T2 relaxation. By following up with a series of 180-degree RF pulses, the spins are "refocused" at regular intervals, generating a series of echoes.

The duration and number of these 180-degree refocusing pulses directly influence the echo train length, as each refocusing pulse results in an additional echo being produced. Therefore, the correct answer is based on the fundamental principle that the series of 180-degree RF pulses allows for the sampling of multiple echoes within a single imaging sequence, thus enhancing efficiency and signal-to-noise ratio while providing a more comprehensive image in less time compared to traditional spin echo sequences.

The other options do not appropriately describe the mechanism behind echo train length. For example, a single 90-degree RF pulse does initiate the process but does not create a train of echoes on its own. Similarly, a series of gradient pulses is related to different types of imaging sequences but does