1. A method for three-dimensional parallel magnetic resonance imaging (MRI) using an MRI system, the steps of the method comprising: a) selecting a criterion; b) estimating a first acceleration factor along a first phase-encoding direction and a second acceleration factor along a second phase-encoding direction such that the product of the first and second acceleration factors is substantially equal to a desired net acceleration factor and such that the selected criterion is optimized; c) generating a k-space sampling pattern using the estimated acceleration factors; d) directing the MRI system to acquire k-space data while sampling k-space in accordance with the generated k-space sampling pattern; and e) reconstructing an image from the acquired k-space data.