Gradient echo MRI. Do the gradients have to be the same duration? I currently got hold of the book "Handbook of MRI Pulse Sequences" and am looking into computing the duration, gradient strength and ramp times for the phase encoding gradient, readout prehasing and the slice prephasing gradients. 

Now, every GE pulse sequence diagram that I look at, shows these gradients to be of the same duration i.e. they start and end at the same time during the sequence. Now, going through some of the calculations in the book for a scanner with some maximum gradient strength, slew rate, receiver BW, field of view and the k-space matrix size, I can compute the optimal gradient strengths and the durations for these gradients. 

However, the optimal gradients are not the same duration. Does this mean I have made a mistake in my calculations or do they overlap these gradients in the pulse sequence diagrams for convenience?
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