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This Standard Reference Material is for determining the length scales within medical computed tomography (CT). This reference device consists of solid polytetrafluoride ethylene balls separated by precision plastic spacers. The central positions of the distances between the balls are certified with an uncertainty of 0.15 mm. The distances are traceable to the International System of Units (SI). The entire package may be held in the palm of the hand and contains only common, inert materials. All of the materials are selected to be within the range of radiodensity (measured in Hounsfield units in medical CT) that is found in the human body. Yet there is high contrast between the parts, so a simple threshold procedure may be used to determine the extent of each ball in a CT reconstruction. Moreover, each ball is held symmetrically, so the fixture will have little influence on the position of the centers of the balls as measured in the CT. A typical experiment may search for distortions within the CT field. For example, the ratio of the length scales in the transverse to longitudinal directions may be found. Differences in the length scale from the isocenter of the CT to the edge of the field may also be investigated. The identification of any commercial product or trade name does not imply endorsement or recommendation by the National Institute o