Neuroradinlngt]

Neuroradiology 19, 81-88 (1980)

© by Springer-Verlag t980

CT Sagittal Reconstruction of Posterior Fossa Tumors J. Delavelle and M. Megret Department of Neurology,GenevaCanton Hospital, Geneva, Switzerland

Summary. A method of CT sagittal reconstruction, by which the exact topography and extent of tumors in the posterior fossa can be demonstrated, is described.

Key words: CT - Tumors, posterior fossa - Sagittal reconstruction

Computed tomography (CT) has become the screening examination for all patients suspected of expanding processes in the posterior fossa. The topographical diagnosis of these tumors should be improved with the latest equipment which makes their sagittal and coronal reconstructions possible. After a short review of the normal radiological anatomy in the sagittal plane, the different tumors of the posterior fossa will be studied according to their topography, and the value of and the indications for sagittal reconstructions will be discussed.

Method When a mass in the posterior fossa is suggested by the clinical symptoms and radiological features, CT is systematically performed in two steps. Firstly, the precontrast axial investigation is performed with an angulation of 20 ° between the scanning plane and the orbitomeatal line, and with a slice 5 mm thick. There is general agreement that this angulation allows a better study of the posterior fossa seen in its whole diameter. But our CT scanner (Ohio Nuclear Delta 25 head scanner, matrix 256 × 256) produces unsatisfactory sagittal reconstructions with such an angulation of the gantry. They have a "step-

ped" aspect because of the inability of the computer to make the necessary angular correction. This phenomenon is not observed on coronal reconstructions and may soon be corrected. Secondly, the patient receives a bolus injection of 100 ml meglumine amidotrizoate (Urovist) and the postcontrast scan is performed in the orbitomeatal position; the X-ray beam is parallel to the orbitomeatal plane. Overlapping technique and normal speed are applied to produce a slice 5 mm thick. At least 24 sections are necessary, and often 34, to explore the entire posterior fossa. The picture interpretation is made with filtration and magnification technique. During this second part of the investigation, it is necessary to infuse 50 to 100 ml of contrast medium slowly to opacify the basilar artery and veins. The computed data may be applied later for coronal reconstructions.

Radioanatomy The ventricular structures comprise the 4th ventricle and the aqueduct. The 4th ventricle is seen as median on the axial view (Fig. l a). The small hyperdense area just behind the 4th ventricle on the postcontrast investigation is consistent with the choroid plexus. The triangular shape of the 4th ventricle is easily recognized in the lateral view (Fig. I b) and it may be localized with the help of Twinning's line (drawn from the tuberculum sellae to the internal occipital protuberance). The midpoint of this line should project in the center or near the floor of the 4th ventricle (Fig. 1 c). The aqueduct of Sylvius and the 3rd ventricle are normally not seen on sagittal reconstructions, but they are easily seen when they are dilated.

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J. Delavelle and M. Megret: CT Sagittal Reconstruction of Posterior Fossa Tumors

Fig. 1 a--c. Normal anatomy of the posterior fossa, a Normal axial CT showing median 4th ventricle and the cerebellopontine angle cistern. b Normal sagittal reconstruction: 4th ventricle (4); premedullary cistern (m); prepontine cistern (p); interpeduncular cistern (/); galenic cistern (g); vermian cistern (v); basilar artery (b); vein of Galen (gv); straight sinus (s). e Twinning's line on a normal sagittal reconstruction

Fig. 2a, b. Glioma of brainstem, a Four serial slices of postcontrast axial scan demonstrate displacement of 4th ventricle to the right and an increased density tumor invading pons, mesencephalon and thalamus, b Sagittal reconstruction of same patient shows posterior displacement of 4th ventricle behind the midpoint of Twinning's line; basilar artery is close to clivus; cranio-caudal extension to surrounding tissues is easier demonstrated

In the anterior compartment of the posterior fossa, the brainstem and the subarachnoid cisterns are well defined on the midsagittal reconstructions. The brainstem, composed of the medulla oblongata, pons and mesencephalon, are separated from the clivus by the medullary, pontine and interpeduncular cisterns (Fig. l b). The pontomedullary notch is often well delineated.

The peripeduncular and cerebellopontine cisterns are not demonstrated on sagittal reconstructions but are well seen on the axial projection.

The posterior compartment of the posterior fossa includes the vermis of the cerebellum and the cerebeUar hemispheres. On the midsagittal reconstructions the cisterna magna separates the inferior vermis

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Fig. 3a-d. Glioma of brainsten (decreased density tumor), a 4th ventricle displaced to left on axial scan. b Sagittal reconstruction of same case demonstrates posterior displacement of 4th ventricle behind midpoint of Twinning's line. c Four serial slices of same patient show obstruction of peripeduncular cistern, displacement of basilar artery to the left and decreased density of the brainstem, d Sagittal reconstuction demonstrates obstruction of premedullary, prepontine and partly interpeduncular cisterns; basilar artery (b) is not displaced; tumor extends from superior part of pons to mesencephalon

from the occipital bone; the superior vermis is well outlined by the vermian cistern. The normal quadrigeminal cistern (or cistern of the great vein of Galen) is perfectly delineated (Fig. 1 b). With the help of the continuous infusion of contrast medium the major vascular structures of the posterior fossa are easily identified on the sagittal reconstructions. The entire length of the basilar artery is seen in the subarachnoid cisterns in front of the brainstem (Fig. 1 b). The straight sinus and great vein of Galen form the superior boundary of the posterior fossa (Fig. lb). So the free edge of the tentorium is clear

on the sagittal reconstructions. The median and lower margins, however, are well shown only on the coronal reconstructions. Moreover, the free edge enclosing the upper part of the brainstem is generally seen in the axial study after contrast enhancement of the veins and an coronal reconstructions.

Pathology Masses in the posterior fossa can be divided into four groups according to their topography. 1. Lesions of the anterior compartment of the pos-

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J. Delavelle and M. Megret: CT Sagittal Reconstruction of Posterior Fossa Tumors

Fig,4a-fl, Chordoma of clivus: correlation between CT, pneumoencephalography and angiography, a Sagittal reconstruction shows 4th ventricle (4) and aqueduct (a) displaced posteriorly. b Pneumoencephalography shows the same. c Sagittal reconstruction: upper part of basilar artery (b) displaced posteriorly by decreased density mass arising from superior clivus, d Vertebral arteriography with subtraction shows same posterior bend of basilar artery

terior fossa include brainstem tumors and extra-axial masses lying ventral to the brainstem. a) Brainstem tumors, seen mostly in children, generally involve the pons, but also the medulla oblongata and mesencephalon. Most are infiltrative tumors like glioma. Pontine tumors displace the floor Of the 4th ventricle posteriorly; this displacement can be appreciated on the sagittal reconstructions with Twinning's line (Figs. 2 b, 3 b). Eccentric brainstem tumors will cause lateral displacement of the ventricular structure, seen on the axial study (Figs. 2 a, 3 a). Mesencephalic tumors, on sagittal reconstructions, show the same pattern but the aqueduct is generally obstructed and the interpeduncular and qua-

drigeminal cisterns are compressed (Fig. 3d). The axial study shows narrowing of the ipsilateral peripeduncular cistern (left side, Fig. 2 a; right side, Fig. 3 c). The basilar artery appears close to the clivus on the sagittal reconstructions, so, the diagnosis of an intra-axial tumor is possible (Figs. 2 b, 3 d). The sagittal reconstruction provides complementary data for the evaluation of the size and relations of the tumor. b) Extrinsic brainstem tumors are polymorphic and are seen mostly in adults. They are meningiomas or tumors arising from the clivus (chordoma, cholesteatoma, osteoma, metastasis). Posterior displacement of the 4th ventricle and stretching of the aqueduct are perfectly identified on the sagittal

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Fig. 5 a, b. Superior vermian medulloblastoma, a Four serial slicesof postcontrast axial investigation showing median 4th ventricle and small increased density tumor which appears median and close to tentorium, b Sagittal reconstruction of same patient demonstrates 4th ventricle slightly displaced downward and compression of quadrigeminal cistern (g). The diagnosis of extra-axial tumor of superior vermis is possible. The size is better evaluated in this view

Fig. 6a, b. Right cerebellar metastasis, a Axial scan (four selected slices) shows obstruction of 4th ventricle and increased density tumor high in posterior fossa close to tentorium, b Tumor appears intraparenchymatous and deep on lateral sagittal reconstruction

reconstructions (Fig. 4a). The basilar artery is displaced backward in its u p p e r part (Fig. 4 c) (the axial investigation shows only a small lateral displacement of the artery). A mass of decreased density is found between the clivus, which is eroded (erosion better seen with window for b o n e densities), and the basilar artery, and its whole extension from pons to posterior clinoids appreciated (Fig. 4c). This case strongly indicates the need for a continuous infusion of contrast in order to show the basilar artery perfectly.

2. Lesions of the posterior compartment of the posterior fossa include tumors of the vermis and of the cerebellar fossa.

a) Vermian tumors are responsible for the rapid obstruction of the 4th ventricle with obstructive hydrocephalus. When the 4th ventricle is not obstructed (Fig. 5) the distorsion and displacement on the sagittal reconstructions are similar to those described by Corrales and Greitz [1] after p n e u m o e n c e p h a l o g r a phy. The 4th ventricle is slightly displaced anteriorly

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Fig. 7. Tentorial meningioma, a Two axial sections demonstrate displacement to right of 4th ventricle by increased density tumor close to left margin of tentorium and expanding into left cerebellopontine angle cistern with "retrodilatation" of the left peripeduncular cistern. b Coronal reconstruction of same patient shows 4th ventricle displaced downward and expanding process (t) arising from left part of tentorium

Fig. 8. Meningioma right cerebellopontine angle, a Four serial slices of postcontrast axial CT showing the obstruction of the right cerebellopontine angle cistern and retrodilatation of right peripeduncular cistern (~--) by meningioma of cerebellopontine angle; 4th ventricle slightly deviated to left. b Coronal reconstruction of same case. This type of reconstruction may help to differentiate between a tumor arising in cerebellopontine angle and one expanding to cerebellopontine angle

and downward by a superior vermian tumor; the quadrigeminal cistern is compressed forward (Fig. 5 b). The sagittal reconstructions are important in those tumors because they show the exact situation of the mass in the vermis, permit the diagnosis of an extraparenchymal tumor as in this case (the contiguity with the straight sinus is obvious) and also give a more accurate idea of the volume of the mass. The axial investigation undervalues the size of the tumor (Fig. 5 a). b) Tumors of the cerebellar fossa include tumors of the cerebellar hemispheres (astrocytomas) and those arising from the tentorium (meningiomas). The hemispheric tumors, when parasagittal, have the same CT pattern as the vermian already seen. Figure 6 shows the value of the sagittal reconstructions of such tumors. On the axial examination the

increased density mass appears quite high and close to the tentorium (Fig. 6 a). The sagittal reconstruction (Fig. 6b) shows that the tumor is intraparenchymatous and deeply situated, topographical data fundamental for the surgeon. We have no case of a lateral hemispheric cerebellar process. Those tumors set the problem of their topography (intra- or extraparenchymatous); thus, the main problem seems to consist in their differentiation from tentorial processes which have the same clinical signs. In the tentorial processes, as in Figure 7, the axial examination shows lateral displacement of the 4th ventricle, compression of the ipsilateral peripeduncular cistern and the quadrigeminal cistern. The cerebellopontine cistern may be narrowed or obstructed indicating extension of the tumor into the cerebellopontine angle (Fig. 7 a).

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Fig. 9a and b. Chordoma of foramen magnum: correlation between CT and pneumoencephalogram, a Sagittal reconstruction showing obstruction of premedullary cistern by increased density tumor with calcification arising from anterior border of foramen magnum and inferior clivus. Note good visualization of medulla oblongata and cervical subarachnoid space. b Pneumoencephalography is supplemental to sagittal reconstruction

As we have already seen in the normal radioanatomy, only the coronal reconstructions can show the tentorium in its whole lateral extension; thus, when a meningioma is suspected, such reconstructions would be necessary. Their value is evident since they show the extraparenchymatous topography of the process (contiguity with the tentorium), the implantation area, and an estimation of the extension of the tumor above and below the tentorium (Fig. 7 b). 3. Lesions arising in the cerebellopontine angle are generally extraparenchymatous (neurinomas of the VIII or V cranial nerves). Numerous authors have described the CT pattern of these tumors as seen on axial scans. The sagittal reconstructions, impeded by artefacts due to the petrous pyramids, are not useful for the topographical diagnosis. The coronal reconstructions may be useful for more delicate differential diagnosis (Fig. 8b). Thus, the search for the direct sign (tumor visualization) and indirect signs (lateral displacement of the 4th ventricle, obstruction of the cerebellopontine cistern with or without dilatation of the peripeduncular cistern) must be made from the axial scan (Fig. 8 a). We would just like to emphasize the importance of the overlapping technique with half speed (low noise scan) to reduce the artefacts due to the petrous bones and make detection of tumors, of less than 1 cm diameter possible. 4. Lesions of the cervico-occipital area may be divided into intramedullary (medulloblastoma,

glioma, ependymoma) and extramedullary expanding processes (chordoma, meningioma of the foramen magnum). Sagittal reconstructions facilitate this topographic diagnosis. Thus, in Figure 9, the narrowing of the premedullary cistern caused by a tumor arising from the anterior border of the foramen magnum and the inferior clivus (the bony structures are eroded) is demonstrated as well by sagittal reconstruction as by pneumoencephalography. The prepontine cistern and the cisterna magna appear obstructed (Fig. 9a). It should be emphazised that the sagittal reconstruction demonstrates the upper cervical cord and the anterior and posterior cervical subarachnoid spaces.

Discussion and Conclusions We would like to emphasize the value of reconstructions in the topographic diagnosis of these tumors. It does not seem necessary and even advisable to proceed to a three dimensionnal study for every posterior fossa tumor. The radiologist may choose between sagittal and coronal reconstructions, according to the clinical features and data from the axial examination, providing that the method of axial sections already described had been used. Sagittal reconstructions would be indicated for anterior compartment masses (differentiation between intra- and extra-axial lesions), vermian or

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parasagittal hemispheric cerebellar tumors (more accurate anatomic localization) and for processes in the cervico-occipital area. On the other hand, coronal reconstructions would be indicated to show the tentorial implantation of a meningioma and to differentiate this tumor from one in a cerebellar hemisphere. For the investigation of cerebellopontine expansive lesions the axial study remains the method of choice, provided technical improvements are realized permitting the detection of exclusively intracanalicular lesions.

References 1. Corrales, M., Greitz, T.: Fourth ventricle. II. Tumours of the cerebellum. Acta Radiol~. [Diagn.] (Stockh.) 12, 241-270 (1972) Received: 18 September 1979 Dr. M. Megret Service de Neuroradiologie H6pital Cantonal Geneva, Switzerland