Axial magnetic resonance images of the thoracic spinal cord of 50 patients are analyzed in order to measure the length of the dorsal arc span between the dorsal root entry zones for the purpose of optimizing the mechanical design dimensions of the Human Spinal Cord Modulation System (HSCMS), which delivers electrical stimuli directly to the spinal cord. Two mathematical approaches are used to assess the data, with validation of the results conducted via a direct physical measurement using a magnified image. Results show that the nominal value of the arc length is 6.7 士 1.0 mm (1 a), with high and low values of 8.8 and 5.1 mm, respectively. The mean radius of the spinal cord was found to be 3.6 士 0.5 mm. Taking into account previously reported measurements, it is suggested that values at the high end of this range be used for further morphometric studies of the cord in the thoracic region. The implications of these findings on the design of the HSCMS are discussed.