See the remove even lines page of the K-Space Tool.
SENSE factor 2.
SENSitivity Encoding is a parallel imaging technique which uses knowledge of the sensitivities of the elements of a phased-array coil to "undo" the foldover demonstrated in the image on the remove even lines page of the K-Space Tool. An aliased image is produced from the k-space from each coil element, and linear algebra is used to create a new, unaliased image. The way k-space is filled in this example is a Cartesian or rectilinear sampling scheme (in horizontal lines). In this case, the reduction of the number of phase encoding steps with respect to full Fourier encoding (filling all the lines) is described by the SENSE reduction factor, R. Here, we've increased the spacing between the acquired lines by 2, so R=2.
R needn't be an integer: the spacing between the lines can be altered (you just have to set the right phase encoding gradients, remember) to suit non-integer reduction factors. R can be between 1 and the number of coil elements used.
Further reading on this topic:
Books: MRI From Picture to Proton p340-341