The cisternal tap may be used in myelography when the upper margin of a spinal block needs to be defined; however, magnetic resonance imaging (MRI) has become the procedure of choice for defining the upper and lower limits of spinal cord or spinal cord-compressing lesions. It is necessary at times in the intrathecal administration of irritating medications, such as amphotericin B. Medications are diluted more rapidly in the larger and more rapidly circulating volume of cisterna magna than in the smaller lumbar sac.