The new design of image slicer developed at Durham University has many advantages that make it particularly well suited for an integral field spectrograph on NGST. Integral field spectroscopy is a method to obtain the spectra of all the positions in a 2D image at the same time. This is usually done by cutting the field into many sub- fields that are re-imaged at the entrance focal plane of a spectrograph. Among these methods, a slicer is different in the way that it cut the field: in one direction only into 'slices' instead of in the 2 directions of space into 'pixels'. This implies that a slicer will give the highest product of the number of spectra by the number of spectral elements of resolution, maximizing the size of the field of view, the spatial resolution and the spectral length. Also, the huge focal ration degradation present when the spatial sampling is smaller than the diffraction PSF is significant in one direction only, so the optics of a slicer must be longer but no larger as must the other methods. Other advantages are the high transmission, the very large bandwidth since it is all reflective, and the ability to easily be cooled to cryogenic temperatures. For other methods to given the same performances, the spectrograph would have to be massively oversized. The new design is also much smaller than previous slicer designs. We will show that the multiplex advantage of a slicer on NGST is one to 2 order of magnitude larger when compared to the other methods. A few possible designs with up to 64000 spatial elements are described.