Abstract: A quality control phantom for a computed tomographic imaging instrument, such as a nuclear magnetic resonance scanner, is provided for the on-site calibration and standardization of image slice offset and angulation so as to maintain and insure proper operation. The phantom includes a cylindrical slab-like cast acrylic casing or body having a frustoconical groove formed therein. The groove is filled with a known material to effect a signal from the instrument. A scan taken by the NMR instrument across the frustoconical groove at an angle perpendicular to the central axis of the groove yields a circular image. The method for determining the image slice offset includes the step of relating the diameter of two such circular images so as to verify the distance one such image is from the other along the central axis of the groove. A scan taken by the NMR instrument across the frustoconical groove at an angle other than perpendicular to the central axis of the groove yields an elliptical image.