How is scan time calculated for 2D images?

The calculation of scan time for 2D images is indeed based on the parameters TR (repetition time), NPE (number of phase encoding steps), and NSA (number of signal averages).

In this context, TR represents the time between successive pulse sequences applied to the same slice, while NPE refers to the number of phase encoding steps taken to cover the field of view in the phase encoding direction. NSA is the number of times each slice is sampled, which helps improve the signal-to-noise ratio in the final image.

When you multiply TR by NPE and then by NSA, you effectively account for the total time necessary to complete the imaging sequence across all the phase encoding steps while also considering the multiple acquisitions for the same slice. This multiplication gives you the total time taken to gather all the data needed for the 2D image, as each phase encoding step contributes additional time based on the repetition time and signal averages.

This formula accurately reflects the factors that contribute to the overall scan time for creating 2D images, which is essential for optimizing imaging protocols and ensuring efficient use of MRI resources.