Zonally-magnified oblique multislice (ZOOM) EPI.

Introduction Echo-Planar Imaging (EPI) is an ultrafast technique which has the drawback of sensitivity to off-resonance effects caused by the presence of local gradients. This gives rise to geometric distortions and signal drop-outs in the EP image. The magnitude of these distortions depends on the gradient performance, field of view (FOV), and resolution of the image and as such provides an additional constraint to the resolution obtainable. Snap-shot EPI is the technique of choice for functional and diffusion studies; multi-shot navigated EPI has been used succesfully in Diffusion Tensor Imaging (l), but some images have to be discarded, and the distortions can still constrain image resolution. If only a small section of a larger sample is required then zonally-magnified EPI can reduce gradient requirements by only sampling a limited FOV. Resolution is enhanced but the shorter echo-train length gives less distortion than the equivalent full FOV acquisition. Such inner-volume excitation techniques only work in single-slice mode (Z-3), as in multislice mode successive inversions by all the orthogonal 180-degree pulses lead to a drastic signal reduction. Another approach uses 2dimensional slice selective pulses with very rectangular slice profiles to select an inner volume (4). Here we present a modified version of the zonally-magnified EPI technique which is not so dependent on the 180 pulse profile and can be used in a mutli-slice mode without loss of signal to noise.