[The spinal canal: from imaging anatomy to diagnosis].

The spinal canal is divided into epidural, subdural and subarachnoid spaces. Intraspinal processes should be correctly placed into their space of origin. MRI is the best imaging modality to achieve this task. Accurate determination of the space of origin routinely requires the acquisition of two different pulse sequences, typically T1W and T2W images, in two orthogonal planes, usually axial and sagittal. Simple imaging features can assist in determining the site of origin: changes to the epidural fat, compression or widening of subarachnoid spaces. The epidural space, bordered medially by dura, contains fat and vascular structures. The subdural space is a virtual space in between the dura and arachnoid membrane. The subarachnoid space is home to the CSF, spinal cord and nerve rootlets. An epidural process replaces the epidural fat, displaces the dura and narrows the subarachnoid space. A subarachnoid process widens the subarachnoid space and spares the epidural fat. Epidural processes usually are infectious or tumoral, either primary or secondary to spinal involvement. Subarachnoid processes include primary tumors, leptomeningeal metastases, arachnoiditis and hemorrhage. Nerve sheath tumors and meningiomas are the most frequent intradural extramedullary tumors.