In a spin echo pulse sequence, what is the first gradient used?

In a spin echo pulse sequence, the first gradient used is the slice encoding gradient. This gradient is essential as it helps define the spatial location of the tissue being imaged. The slice encoding gradient (often referred to as the slice selection gradient) is applied during the radiofrequency (RF) excitation pulse and is responsible for selecting a specific slice of the body to be imaged.

When this gradient is applied, it alters the magnetic field across the slice, enabling the RF pulse to excite only the spins within that particular slice, while spins in the adjacent slices remain unaffected. This is crucial for obtaining high-quality images, as it ensures that the signal received is from a well-defined area, permitting accurate spatial localization.

Other gradients, such as phase encoding and frequency encoding gradients, play significant roles in different stages of the imaging process, but they come into play after the slice selection has been made. The phase encoding gradient is utilized to encode spatial information in one direction, while the frequency encoding gradient is used during data acquisition to distinguish signals based on their frequency, which correlates with their position along another axis. Understanding the sequence and purpose of these gradients is vital for successful MRI imaging.