During which process is the readout gradient typically turned on along with the sampling or readout of the peak echo?

The process during which the readout gradient is activated alongside the sampling or readout of the peak echo is frequency encoding.

In MRI, frequency encoding is essential for spatially resolving the signals received from the imaging area. Once a radiofrequency pulse has been applied and the resulting echo is generated, the readout gradient is applied in the frequency encoding direction. This gradient causes resonating protons in that direction to precess at different frequencies based on their position within the imaging slice. When the echo is sampled, the frequency differences are used to determine the spatial location of the signals within that slice.

This process directly contrasts with other options like slice orientation selection, slice selection, and phase encoding, where different gradients and processes come into play. In slice selection, for instance, a specific frequency is targeted using a gradient to excite protons only within a particular slice. In phase encoding, a gradient is applied before the readout to create varying phases in the precessing spins, enabling differentiation in the resulting image.

Thus, frequency encoding is specifically linked to the readout of the echo, allowing for accurate spatial representation of the MRI data.