In a spin echo pulse sequence, which gradient encodes the slice?

In a spin echo pulse sequence, the slice selection occurs during the application of a specific gradient known as the slice encoding gradient. This gradient is applied at the same time as a radiofrequency (RF) pulse and is responsible for delineating the thickness and position of the slice being imaged.

When the slice encoding gradient is applied, it creates a variation in the magnetic field strength across the imaging volume. This causes the spins within the selected slice to experience a different frequency than those outside of it, allowing only the desired slice to be excited and subsequently imaged.

The other gradients mentioned play different roles in the imaging process. The frequency encoding gradient is applied after the RF pulse to determine the position of signals along one axis of the image, while the phase encoding gradient helps to encode spatial information along a perpendicular axis. The readout gradient is specifically used to read out the signal during the acquisition of data but does not play a role in slice selection.

Thus, the slice encoding gradient is essential for precisely defining and isolating the area of interest in MRI imaging.