What limitation is associated with increasing NEX in MRI?

Increasing the number of excitations (NEX) in MRI is associated with a direct increase in scan time. This is because NEX refers to the number of times the data is acquired for each slice or voxel, which means that for each increase in NEX, the time it takes to complete the scan effectively doubles. For example, if an MRI sequence with an NEX of 1 takes 5 minutes, changing it to an NEX of 2 would increase the scan time to 10 minutes, as the sequence is repeated to improve the signal-to-noise ratio (SNR).

This aspect of increasing NEX is crucial to consider during MRI acquisition, as longer scan times can impact patient comfort, the likelihood of motion artifacts, and the overall efficiency of the imaging process in busy clinical settings. While increasing NEX does improve SNR to some extent, the trade-off is that the prolonged scan times can lead to patient issues and scheduling constraints for the radiology department.