MRI is the study of choice for the diagnostic evaluation of arachnoiditis. T1-weighted MRIs may reveal an indistinct or absent cord outline due to the increase in the signal intensity of the surrounding CSF. T2-weighted MRIs may demonstrate CSF loculation and obliteration of the subarachnoid space or irregularly thickened, clumped nerve roots, which occasionally may be misinterpreted as a tethered cord or a thickened filum terminale. With more severe arachnoiditis, progression of nerve root clumping and leptomeningeal adhesions may lead to angular defects in the dural sac. Peripheral adherence of the nerve roots to the walls of the thecal sac produces the so-called featureless, or empty, sac.

Arachnoiditis is inflammation of some part of the arachnoid membrane that covers the spinal cord. It can produce severe chronic pain. Arachnoiditis may occur as a result of infection, but most commonly is seen after surgical procedures and use of contrast material to enhance visualization of structures with X-rays during myelography. Some people are more sensitive than others to contrast material. An MRI scan has about a 90% chance of showing this abnormality if it is present. A negative MRI scan for arachnoiditis is a strong argument that it is not present.

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