Gradient echo MRI. Do the gradients have to be the same duration?
The question is what optimal is. It is correct that very often certain gradients do have the same duration in sequence diagrams - and actual imaging sequences (e.g. slice rewinder, read-out prewinder, and phase-encoding gradient). Normally, you would compute the longest duration for these gradients and recalculate the others to be of the same length by stretching them in time and reducing their amplitude. This may have benefits, e.g. the peripheral nerve stimulation may be reduced, and also the noise that the MRI creates. Also it reduces the stress for the gradient coils, i.e. heating and vibration. Certainly it is not always the best idea to stretch all gradients: When the fall below a certain amplitude, there may be inaccuracies due to system imperfections. Also the parasitic diffusion weighting of the gradients may be an issue in certain applications.

So the main reasons for certain design choices are:


reduction of patient discomfort
reduction of stress for the MR-system
reduction of image artifacts