Radiologic anatomy of the sacral canal.

The extradural space is currently investigated through fluoroscopy and ultrasound for surgical approach, whereas magnetic resonance imaging has been used to provide detailed information. The aim of the present paper is to describe the radiologic anatomy of the sacral canal through a review of its appearance in the different radiologic techniques. CT is able to visualise also the sacrum and the content of the sacral canal, triangular in shape in the transverse images, being able to establish the measurement of the transverse area of the dural sac and of the canal diameter. On the sagittal CT scans, the sacrococcygeal membrane appears as a hypodense structure, between the posterior end of the sacral vertebra and the posterior tip of the coccyx. In magnetic resonance imaging, on T2-sagittal plane images, the sacral canal appears hyperintense, due to the presence of the liquor. The dural sac appears as a hypointense band and its termination as hypointense cul de sac in the context of the hyperintensity of the sacral canal. The sacrococcygeal membrane appears as a hypointense band between the posterior end of the sacral vertebra and the posterior tip of the coccyx. On ultrasound imaging, in the transverse sonographic view, two hyperechoic reversed U-shaped structures correspond to the two bony prominences of sacral cornua, between which there were two hyperechoic band-like structures. The band-like structure on top is the sacrococcygeal ligament. The band-like structure at the bottom is the dorsal surface of the sacrum. The sacral hiatus corresponds to the hypoechoic region observed between the two hyperechoic band-like structures.