Simple measurement of scanner stability for functional NMR imaging of activation in the brain.

Functional MR imaging (fMRI) of activation in the brain is one of the more demanding applications required of an NMR imaging instrument. Since the signal changes in activation imaging are quite small, any instrumental variations can easily compromise the fMRI contrast. The authors describe a simple method for measuring these scanner instabilities, which is implementable on all scanners, to determine whether instability is degrading the fMRI contrast-to-noise. In this method, a long time series of images is acquired with identical imaging parameters to the chosen fMRI scans, the fluctuations are measured as a function of region-of-interest (ROI) size, and compared with the single image SNR. By plotting these fluctuations versus ROI size, and comparing them with the theoretically approachable value, the impact of these fluctuations can be assessed. We also present a simplified version of the test, which can be implemented with minimal calculations.