During the readout phase of a spin echo pulse sequence, which gradient is primarily used?

During the readout phase of a spin echo pulse sequence, the frequency encoding gradient is primarily used to collect the signal emitted by the excited spins in the area of interest. This gradient is responsible for differentiating the signals based on their frequency, allowing for spatial encoding of the data in the frequency domain.

As the readout occurs, the frequency encoding gradient is applied along the direction of signal acquisition, helping to ensure that different locations within the imaging slice contribute distinct frequencies to the received signal. This directionality is crucial for reconstructing the final image accurately since it allows delineation of anatomical structures based on their position and the frequencies of the signals they emit.

In contrast, the slice encoding gradient is used to select the specific slice of tissue that will be imaged before any signals are acquired, and the phase encoding gradient is utilized to add spatial information in the orthogonal direction during the imaging process. The refocusing gradient, which is a part of the spin echo sequence, serves to refocus the spins but is not the primary gradient during the signal collection phase. Thus, the frequency encoding gradient is key to successfully capturing the data needed for image formation in the readout phase.