Final answer:
The gradient active during echo sampling in MRI is the Readout Gradient, which encodes spatial information along the frequency axis and works together with other types of gradients to produce an image.
Step-by-step explanation:
The gradient that is on during the sampling or production of an echo in magnetic resonance imaging (MRI) is called the Readout Gradient. This gradient is applied along the frequency encoding axis and its function is to spatially encode the precessing spins within the slice which allows for the creation of an image. The Readout Gradient works in conjunction with both the Slice Selection Gradient, which selects a particular slice of the body to be imaged by applying a gradient along one axis and using a corresponding RF pulse, and the Phase Encoding Gradient, which is applied perpendicular to the slice selection and readout gradients to encode spatial information in the phase of the signal received.