In a Fast Spin Echo sequence, the number of shots is calculated by:

In a Fast Spin Echo (FSE) sequence, the number of shots is calculated by taking the number of phase encodings and dividing it by the Echo Train Length (ETL). This is because the ETL defines how many echoes (or data points) can be collected in a single shot of the sequence. Each shot results in multiple phase encodings being sampled at once due to the use of multiple refocusing pulses within the echo train.

By dividing the total number of phase encodings required by the ETL, you can determine the total number of shots needed to complete the acquisition of the image. This relationship allows for efficient data collection and is an essential concept in optimizing the FSE sequence for various imaging needs.

Other calculations in the options relate to different aspects of MRI imaging sequences, such as overall imaging time or pulse sequences but do not accurately reflect how to derive the number of shots specifically in a Fast Spin Echo context.