Number of EXcitations (NEX)

The Number of EXcitations (NEX) is the number of times the phase encoding steps are repeated in a given acquisition. NEX is a factor in calculating scan time: (TR) × (Phase Steps) × (NEX) = basic 2D scan time.

Select a NEX value that produces sufficient Signal-to-Noise Ratio (SNR). Adjust NEX to change SNR and/or scan time.

• An increase in the NEX value increases SNR and scan time. The SNR increases by the square root of the ratio of NEX. For example, increasing the NEX from 2 to 4 results in a 40% increase in SNR and a doubling of the scan time.
• Typically, as resolution increases, the NEX value needs to be increased. An exception to this is 3D imaging. Select the NEX value that produces adequate SNR to make the diagnosis. Too much SNR wastes time, too little SNR produces un-diagnostic images.