MRI is the diagnostic procedure of choice because of its ability to demonstrate the exact location, extent, and relationship of the arachnoid cyst to adjacent brain or spinal cord. Myelography and CT myelography remain of diagnostic value, especially for cases that are not definitive on MRI.

Plain radiographic findings are nonspecific and have little to offer in the diagnosis of arachnoid cysts, although changes in skull contour may be detected on skull radiographs performed for other indications, such as trauma.

Cranial ultrasonography is an important diagnostic tool during the first year of life, and although symptomatic arachnoid cysts are comparatively rare in infants, ultrasound provides a noninvasive imaging technique with high yield in the detection and characterization of cystic masses.

Although angiography can show associated anomalies of venous drainage and the relationship of the lesion to normal vasculature if needed for surgical planning, in practice, angiography is rarely performed, because CT angiography or magnetic resonance angiography may provide the same information noninvasively.