What characterizes a single shot or snapshot fast/turbo spin echo sequence?

A single shot or snapshot fast/turbo spin echo sequence is characterized by the technique of acquiring multiple lines of k-space in a single excitation. In this sequence, the effective echo train length (ETL) corresponds to the number of phase encoding steps. This means that for each excitation, multiple echo signals are collected, allowing for faster imaging while maintaining the benefits of spin echo sequences, such as improved contrast and reduced artifacts compared to gradient echo sequences.

The process takes advantage of the rapid acquisition of echoes that follow a single radiofrequency pulse, where the number of phase encoding steps is closely tied to the ETL. This mechanism allows for efficient and rapid imaging, making it particularly useful in clinical settings where time is a critical factor.

The other options do not accurately reflect the principles of a snapshot fast/turbo spin echo sequence. For instance, the equality of TR and TE, or TR equating to ETL, is not inherent to the definition of this imaging sequence. Instead, the relationship between ETL and phase encoding steps is the most defining characteristic.