When running fMRI experiments it's not uncommon for the scanner to prohibit what you'd like to do because of a gradient stimulation limit. You may even hit the limit "out of the blue," e.g. when attempting an oblique slice prescription for a scan protocol that has run just fine for you in the past. I'd covered the anisotropy of the gradient stimulation limit as a footnote in an old post on coronal and sagittal fMRI, but it's an issue that causes untold stress and confusion when it happens so I decided to make a dedicated post.
Some of the following is take from Siemens manuals but the principles apply for all scanners. There may be vendor-specific differences in the way the safety checking is computed, however. Check your scanner manuals for details on the particular implementation of stimulus monitoring on your scanner.
According to Siemens, then:
The scanner monitors the physiological effects of the gradients and prohibits initiating scans that exceed some predefined thresholds. On a Siemens scanner the limits are established according to two models, used simultaneously:
The scanner computes the expected stimulation that will arise from the gradient waveforms in the sequence you are attempting to run. If one or both models suggests that a limit will be exceeded, you get an error message. I'll note here that the scanner also monitors in real time the actual gradients being played out in case some sort of fault occurs with the gradient control.
