Number of shots

The Number of shots selection determines the number of Time to Repeat or Repetition Time (TR) periods to be repeated for an EPI acquisition. This also determines the speed and image quality of an EPI sequence.

• As the number of shots increase, the effective echo spacing decreases and blurring in the image decreases.
• As the number of shots decrease, scan time decreases.

Formula to calculate the scan time for an EPI sequence: Scan Time = TR × Number of Shots × NEX

A single-shot EPI sequence is one in which all the required phase encoding steps are collected within one TR period. One shot (single-shot) is the only selection for Diffusion Weighted EPI.

EPI protocols that use more than one shot to complete the image acquisition are referred to as multi-shot EPI. Multi-shot EPI fills k-space lines in an interleaved manner. The interleaved k-space yields a reduced effective echo spacing.