Ankylosing Spondylitis is a disease characterized by abnormal bone structures (syndesmophytes) growing at intervertebral disk spaces (IDS). The growth of syndesmophytes is typically monitored by visual inspection of radiographs. The limitations inherent to the modality (2D projection of a 3D object) and rater (qualitative human judgment) entail a possibly important loss in sensitivity. We previously presented a method designed to overcome both limitations: a computer algorithm that quantitatively measures syndesmophytes in the 3D space of a high-resolution computed tomography scan. To establish the method's usefulness for longitudinal studies, it is necessary to assess its precision (repeatability) which can be affected by the limitations of both the algorithm itself and the imaging modality. To this end, an anthropomorphic vertebral phantom with syndesmophytes in 4 IDSs was manufactured. It was scanned 22 times with varying positions and resolutions. The syndesmophyte volumes extracted by our algorithm have an average coefficient of variation of 1.6% per IDS and 0.85% for the total.