Computed Tomography of the Brain Stem with Intrathecal Metrizamide. Part I: The Normal Brain Stem Michel E. Mawad 1 Detailed anatomy of the brain stem and cervicomedullary junction can be accurately A. John Silver demonstrated with metrizamide computed tomographic cisternography. Specifically. Sadek K. Hilal surface anatomy is unusually well outlined. Nine distinct and easily recognizable levels S. Ramaiah Ganti of section are described: four levels in the medulla, three in the pons, and two in the mesencephalon. Surface features of the brain stem, fine details in the floor of the fourth ventricle, cranial nerves, and vascular structures are shown and discussed. Reliably accurate imaging of the brain stem and cervicomedullary junction has now become available using high-resolution computed tomographic (CT) scan­ ning following intrathecal admini stration of metrizamide [1 -6]. The demonstration of surface features of the brain stem such as the ventral fissure, ventrolateral su lcus, pyramids, and olivary protuberance has become commonplace; suc h details have not been routinely demonstrable in the past. Many authors [1, 2] have already emphasized the value of metrizamide CT cisternography and its superiority to both angiography and pneumoencephalog­ raphy. These latter procedures rely on subtle displacement of vessels or distor­ tion of the air-filled fourth ventricle and posterior fossa cisterns. Compared with air, metrizamide spreads much more readily in th e entire subarachnoid space without the problem of meniscus formation or " air lock. " CT permits the sepa­ ration of the various collections of contrast agent and avoids th e superimposition of features encountered in nontomographic contrast studies. Improved visualization of the details of the brain stem by metrizamide CT has allowed the detection of subtle morphologic changes in the brain stem and subarachnoid space not previously appreciated. Di stortion of th e brain stem by tortuous vessels, focal atrophic changes, and early changes of mass lesions are good examples of the value of this method. A detailed famili arity with the normal anatomy of the brain stem, cerebellum , and fourth ventricle on CT cisternography is essential in order to fully utilize th e diagnostic capability of metrizamide CT. Materials and Methods From those patients who had a metrizamide CT c isternogram at the New York Neurolog­ This article appears in the January / February ical Institute in an 18 month period for a variety of indications, we analyze the morphology 1983 issue of AJNR and the March 1983 issue of the brain stem in 30 who had no c linical signs or symptoms of brain stem involvement. of AJR. Metrizamide was introduced in the subarachnoid space through a lumbar puncture or Received March 2, 1982; accepted after revi­ more often through a C1-C2 puncture, wh ich allowed us to use a lesser amount of contrast sion July 23, 1982. medium. The average amount used was 8 ml of contrast material at 170 mg / ml, except in 'All aulhors: Department of Radiology, Neuro­ children. The use of metrizamide in children is not recommended by the manufacturer, logical In stilule, Columbia-Presbyteri an Medical however, in our experience, the diagnosti c va lue of this method may outweigh the potential Center, 7 10 W. 168th St., New York, NY 10032. Address reprint requests to M . E. Mawad . ri sks of the contrast agent in difficult cases. Scanning was performed in the supine position in the hour after injection. All patients were scanned on an 0450 Pfizer high-reSOlution AJNR 4:1-11 , January/ February 1983 0195-6108/ 83/ 0401-0001 $00.00 scanner using primary reconstru c ti on of an area of interest measuring about 10 cm in © Ameri can Roentgen Ray Society diameter on a 512 x 5 12 matri x. Th e usual sli ce thickness was 5 mm , bu t we often used 2 MAWAD ET AL. AJNR:4. Jan./Feb. 1983 Optic Chiasma and Tracts Infundibulum of III Ventricle Cerebral Peduncle Mamillary Bodies 9 Interpeduncular Fossa Oculomotor Nerve 8 Trochlear Nerve 7 Pons Trigeminal Nerve 6 Middle Cerebellar Peduncle Abducens Nerve 5 Facial and Vestibulocochlear Nerves Glossopharyngeal Nerve Cerebellar Flocculus Lateral Recess of IV Ventricle 3 Vagus Nerve - - - / Spinal Accessory Nerve 2 Hypoglossal Nerve ------./ Olivary Eminence Pyramid of the Medulla Pyramidal Decussation Ventral Median Fissure Antero Lateral Sulcus Lateral Funiculus A Fi g. 1.- Anteri or (A) and posterior (6) views of brain stem show th e nine tevets of section. The plane of section is usually higher anteriorly than posteriorl y. thinner sli ces of 2- 4 mm ; the average increment between two 2. transition from closed to open me­ consecutive slices was 5 mm . Th e angle of secti on was th at used dulla (obex; foramen of Ma­ for the standard examinati on of the brain , about 10°-15° inclinati on gendie) above the canthomeatalline. The anatomic description in this work 3. open medulla (olivary nu cleus; floor refers to characteristic secti ons most commonly encountered. Of­ of fourth ventricle) ten, however, because of difference in skull configurati on or degree 4. medullary pontine junction of angulati on, the plane of section is not precisely reproduced , and one may obtain an image combining the anteri or aspect of one Pons: 5. lower pons (seventh and eighth standard level of th e brain stem with the posterior aspect of an nerves) adjacent one. The description of coronal sections will be the subject 6 . mid pons (fifth nerve) of a future report. 7. isthmus of the pons Mesencephalon : 8. inferior colliculus 9 . superior colliculus (third nerve) Results: Normal Brain Stem and Surrounding Spaces We subdivide the brain stem into th e following nine distinct 1. Closed Medulla (fig . 2) and recognizabl e levels (fig. 1): This extends from the first cervi cal roots, which are usu­ Medulla: 1. closed medulla (C1 roots to obex) ally slightly below th e foramen magnum, to the obex. In AJ NR:4 . Jan./Feb. 1983 CT OF NORMAL BRAIN STEM 3 ~~~----\---- Pineal Gland ~-r--+--ri-- Pu Ivi nar 9 JII',"""",-,!'" i~=~~~~ Superior Colliculus 8 Inferior Colliculus ~---- Trochlear Nerve 7 /::::':::!I:lF-<--~"""------ Superior Medullary Velum 6 ."..--4Ir----- - Superior Cerebellar Peduncle ~;----- Middle Cerebellar Peduncle 5 ./""- -\----- Inferior Cerebellar Peduncle '><...,.:::...l,--- i---- Median Sulcus l Floor of IV 4 ~k-----'r--+---- Medial Emminencel Ventricle .r"":r~""'''''''----- Lateral Recess of IV Ventricle ~F-"f--:,----'~!I---- Inferior Medullary Velum Cuneate Nucleus Foramen of Magendie '-------- Obex '--------Gracile Nucleus .......f.--j '-fl--------- Dorsal Median Fissure ftH-.---------Gracile and Cuneate Fasciculi ~+-------- Postero Lateral Sulcus ,..,.---- ---- Lateral Funiculus B cross section, the closed medulla has a pear-shaped ap­ a normal variant as it does not correlate with any underlying pearance with a narrow ventral surface featuring a short, pathology. often oblique, median fissure and a broader, dorsal surface with a less pronounced dorsal median fissure. Anteriorly, on 2. Transitional Medulla (fig. 3) either side of the ventral fissure are two eminences that correspond to the pyramids of the medulla (largely formed Starting at the obex, the posteri or aspect of the med ulla by the corticospinal tracts). The ventrolateral sulcus is a is cleft by a narrow midline groove th at corresponds to the very shallow groove posterior and lateral to the pyramidal most caudal part of the fourth ventricle and gives to th e eminence. The vertebral arteries usually lie close to this medulla the posteriorly open appearance on cross section . ventrolateral sulcus. The lateral and posterior medullary The obex is the point of transition from the closed medulla segments of the posterior inferior cerebellar artery are often to what we choose to call th e open medulla. Th e lower, seen lateral and posterior to the medulla within the medullary open med ulla is a short segment, only a few millimeters cistern . The hypoglossal and accessory nerves arise re­ thick, extending from the obex to the upper margin of the spectively from the ventrolateral sulcus and the lateral sur­ foramen Magendie. The latter foramen is sometimes seen in face of the medulla, but they are very rarely seen on metri­ the midline bounded on both sides by a thin membrane that zamide CT scanning. comprises the ependyma and the choroid. Lateral to th is On several occasions, we have found an axial rotation of membrane are the em in ences of the gracile and cuneate the medulla in a well positioned scan; we consider this to be nuclei. Anteri orly, the cross section at this level features th e 4 MAWAD ET AL. AJNR:4 , Jan./ Feb. 1983 LATERAL FUNICULUS OF THE MEOULLA DORSAL COLUMNS DORSAL MEDIAN FISSUR E A B Fig. 2. -Level 1. Closed medulla at level of foramen magnum is pear-shaped and usually somewhat rotated. olivary emin ence, which is lateral to the pyramid and cor­ ventral fissure, the open medulla has two: the medullary responds to a large underlying cellular structure called the pyramid medially and the olivary eminence laterally. The inferior olivary nucleus. The anterior surface characteristics preolivary sulcus separates the olivary eminence from the of the medulla are better discussed on the following higher pyramid. Lateral to the olive is the shallow postolivary sul­ section where the olivary nucleus is somewhat more prom­ cus, which gives to the open medulla its laterally biconcave inent. symmetry. The dorsal aspect of the medulla is now formed by the floor of the fourth ventricle where one can sometimes recognize the eminences of the nuclei of the twelfth, tenth, 3 . Open Medulla (fig. 4) and eighth nerves located on either side of the median Above th e foramen of Magendie, the medulla (which is sulcus in this order from medial to lateral. The roof of the now open posteriorly) has, on cross section, a characteristic fourth ventricle is formed by the inferior medullary velum, laterally biconcave appearance due to the shallow postoli­ which can be identified on CT.