Chapter 3:

Neuro Imaging of the Posterior Fossa

Thomas J. Vogl, Marc Harth

Contents:

1. Introduction

2. Topographical overview

3. Imaging techniques

4. Pathological findings

Department of Radiology

University of Frankfurt

Theodor-Stern-Kai 7

60590 Frankfurt am Main, Germany

E-mail:

Thomas J. Vogl, MD, PhD: T.Vogl@em.uni-frankfurt.de

Marc Harth, MD: marc.harth@gmx.de

Business phone:+ 49-69-6301-7291

Business fax: + 49-69-6301-5252

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1. Introduction

Neuro imaging of the posterior fossa means a diagnostic challenge for neuroradiology,

radiology, neurology and neurosurgery. Due to the dramatical technical progress of imaging

modalities increasing knowledge of the clinicians about the technique and indications of

these examinations is necessary. In this chapter we provide an overview of the present use

and interpretation of imaging modalities for the posterior fossa. It would go beyond the scope

of this chapter to include the different physical and technical basic principles of the presented

imaging modalities. Anatomical details of the posterior fossa, some historical background

information, a very short summary of different imaging techniques, indications and the major

advantage of focused techniques will be summarized. Radiological cases will demonstrate

subtle pathological findings. In addition, interlinking different modalities of neuro imaging

can confirm the diagnosis.

2. Topographical anatomical details of the posterior fossa

The posterior cranial fossa is the deepest and most capacious of the three cranial fossae. It

contains the cerebellum, pons, and medulla oblongata. The great foramen is centrally located

in the posterior fossa. The posterior fossa is surrounded by deep grooves containing the

transverse and sigmoid sinuses.

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Figure 3-1: 3D Volume-rendering (VR) scan images of skull base processed with 3D volume

data of a 16-row multidetector CT (MDCT).

The indications for neuroimaging is to reach this detailed neuroanatomy for evaluating the

posterior fossa.

The brain stem is formed by the midbrain, pons and medulla oblongata, and partially

obscured by the cerebral hemispheres and cerebellum. Small nuclei of motor or sensory

nuclei of the cranial nerves are scattered in the gray matter in the brain stem. The midbrain is

located between the pons and the cerebral hemispheres. The dorsal segment of the midbrain

is called the tectum and the more central and ventral part is called the tegmentum. The pons

lies anterior to the cerebellum and superior to the medulla, from which it is separated by a

groove through which the abducens and the facial and acoustic nerves emerge. The medulla

oblongata is the pyramid-shaped segment of the brain stem between the spinal cord and the

pons. The lower half contains the remnants of the central canal.

The posterior portion of the superior half forms the floor of the body of the fourth ventricle.

The cerebellum is located in the posterior fossa of the skull, behind the pons and the medulla.

It is separated from the overlying cerebrum by an extension of dura mater and the tentorium

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of cerebellum.

The tentorium of cerebellum is oval in form, with its widest diameter along the transverse

axis. It is composed of a small, unpaired central portion—the vermis—and two large lateral

masses—the cerebellar hemispheres.

Figure 3-2: Sagittal view of a T2-w MRI scan of the head of an 8-year-old boy. The

indication of MRI examination was continuous headache after the boy fell from his bicycle.

Chirari I malformation: herniation of the cerebellar tonsils through the great foramen into the

cervical spinal canal.

3. Different imaging modalities for the diagnosis of the posterior fossa

The different techniques can be divided into noninvasive and invasive imaging.

Ultrasound:

Since its introduction in the late 1950s ultrasonography has become a very useful prenatal

diagnostic tool in obstetrics and gynecology. Ultrasound scan is currently considered to be a

safe, noninvasive, accurate and cost-effective examination in the fetus. Many structural

abnormalities in the fetus can be reliably diagnosed by an ultrasound scan, and these can

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usually be performed before the 20th gestational week. Common examples of neurological

malformations include hydrocephalus, anencephaly and myelomeningocele. Ultrasound scan

will be supplemented with Doppler ultrasound, and the 3-D and 4-D ultrasound techniques.

Plain x-ray skull:

The oldest modality of imaging is plain x-ray skull. This technique can show abnormalities

and fracture of the skull, signs of chronic intracranial hypertension, and calcification. In case

of a dermoid cyst, a bone defect with sclerotic margins may be detected. But this technique is

limited to direct or indirect bone lesions.

Figure 3-3:

A 25-year-old drunk and unconscious man. Can you see the injury in the conventional

frontal plain film of the head? At the 3D volume-rendering (VR) scan image of the skull

base it should be no problem to recognize a fracture of the occipital bone on the right side.

Nevertheless, at the plain x-ray skull this fracture can be seen in projection at the right frontal

sinus.

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Computerized tomography (CT) scanning:

Since 1976 examinations of the head have been performed with computerized tomography.

As CT scan times have got faster, more anatomy can be scanned in less time. With an older

CT scanner the examination of the posterior fossa was limited because of the artifact

produced from the surrounding thick bone. However, with the new technique of multidetector

(4-row, 16-row) CT scanners high quality images can be reconstructed in multiple planes

from a single volume data set.

Figure 3-4: Thrombosis in the right transversal sinus. This image is reconstructed with the

same data set as the images in Figure 3-1.

MDCT is rapidly becoming the new standard in radiological imaging.

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Spiral mode MAs KV Section Collimation

(mm)

Rotation

time (s)

Scantime (s) Slice

(mm)

Volume of Flow contrast

(ml) rate

Delay

(s)

Head 330 120 16x0.75 1 3,76 3 100 2 50/60 Head-Angio 200 120 16x0.75 0.5 10.45 4 100/120 3 4

Table 1: 16-row MDCT scan parameters

The most important primary indication for CT imaging including CT angiography and CT

venography in neuroradiology are acute head trauma, suspicion of acute intracranial

hemorrhage, immediate postoperative evaluation for surgical treatment, shunted

hydrocephalus, brain herniation, suspected mass or tumor and acute cerebral infarction.

Figure 3-5: CT of brain stem hemorrhage (bleeding into the pons)

Usually, CT is the imaging method of choice performed in patients with posterior fossa

masses who often present with nausea, vomiting, ataxia, and other signs of increased

intracranial pressure. CT is a quick, available, and relatively inexpensive method to assess

neurological emergencies including hydrocephalus, hemorrhage, and herniation syndromes.

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Magnetic resonance imaging (MRI) scanning:

In the early 1980s, MRI caught the attention of clinicians by its ability to visualize

abnormalities in the posterior fossa of the brain and in the upper cervical spine. Since its

clinical development the application of magnetic resonance imaging (MRI) sequences has

rapidly evolved. New techniques and sequences are constantly being updated. In order to

select the right technique, an understanding of the relationships between the various imaging

parameters is necessary. With all these techniques it is evident that MR is not a modality

where pathology will show itself. The parameters mentioned should be changed to bring the

optimum contrast for that particular pathology under investigation.

Sequence TR

[ms]

TE

[ms]

FA

[°]

SN

[n]

Orientation ST

[mm]

FOV

[mm]

DF

[%]

BR Bandwidth

[Hz/Px]

Head

T2 se 3590 13 150 25 Tran 5 240 10 256 65

T2 tse 5900 122 150 25 Tran 5 240 10 256 65

T1 nativ 522 14 90 25 Tran 5 240 10 256 89

T1 + KM 522 14 90 25 tran+cor 5 240 10 256 89

Head-Angio FISP 3D 40 4.97 25 40 Tran 0.83 81.3 -37.5 512 65

FLASH 3D 3.3 1.07 25 60 Cor 1.3 360 20 512 390 Abbreviations:

BR base resolution ST slice thickness

cor coronar se spin echo

DF distance factor SN slice number

FISP fast imaging with steady precession TE time of echo

FLASH fast low angle shot TR time of repetition

FOV field of view tran transversal

FA flip angle tse turbo spin echo

Table 2: Sequence and parameters for MRI of the head and head angio (Sonata, 1.5 Tesla,

Siemens)

Furthermore intrauterine magnetic resonance imaging are leading to a subtle prenatal

neurological diagnostic. The most important primary indications for MRI imaging in

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neuroradiology are congenital disorders, posterior fossa lesions, early infarction,

demyelinating and other white matter diseases, sensorineural deafness and inflammatory

lesions. The technical parameters of MRI examination are specifically different due to the

questions of the clinicians.

Diagnostic cerebral angiography:

Searching for an intracerebral aneurysm, the second diagnostic procedure to be performed

immediately after emergency CT is selective cerebral angiography. Both carotid and vertebral

arteries must be injected. External carotid arteries should also be visualized, particularly if

intracranial angiography is negative, as subarachnoid haemorrhage may sometimes be due to

a rupture not of an aneurysm but of dural arteriovenous malformations. Angiography should

aim at recognizing the aneurysm, its precise location, size, size of the neck, relationship with

the parent vessel and multiplicity. To achieve this goal the ideal procedure is rotational

angiography with three-dimensional reconstruction.

Furthermore, cerebral angiography is useful to assess the vascular supply of the tumor. With

the wide availability of MRI, cerebral angiography is no longer used as the first option in

brain tumor assessment.

The risks of cerebral angiography are bleeding at the site of the catheter insertion, allergic

reaction to x-ray contrast (approximately 1/50,000 to 1/150,000 people), arterial embolism

and stroke.

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4. Examples of pathological findings in the posterior fossa:

Child Adult Extraaxial (outside brain)

Dandy-Walker Malformations

Epidermoid Cyst

Arachnoid Cyst

Rhabdoid Cyst

Schwannoma

Meningioma

Epidermoid Cyst

Aneurysm

Metastasis

Arachnoid Cyst

Petrous Lesions

Intraaxial (inside brain) PNET (Medulloblastoma)

Pilocytic Astrocytoma

Ependymoma

Pontine Astrocytoma

Infarction

Metastasis

Hemangioblastoma

Multiple Sclerosis

Abscess

Osmotic Myelinolysis

Lhermitte-Duclos

Table 3: Common lesions of the posterior fossa (by James G. Smirniotopoulos)

Figure 3-6: Postcontrast axial T1-weighted MRI: At the right cerebellopontine angle a

contrast-enhanced acoustic neurinoma is located.

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Figure 3-7: Multiple sclerosis with lesions in the posterior fossa at the right cerebellopontine

angle.

Figure 3-8: Native CT shows bilateral symmetric calcification of the cerebellar hemispheres

(reasons: idiopathic, familial cerebrovascular ferrocalcinosis, postinflammatory, congenital,

post-anoxic/toxic)

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Figure 3-9: Edema of the cerebrum and cerebellum with compression of the mesencephalon

by the tentorium of cerebellum.

Figure 3-10: Dandy-Walker-Malformation with key features: large posterior fossa cyst,

elevated confluence of the sinus (torcular Herophili), hypoplastic vermis and hypoplastic

cerebellar hemispheres.

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Figure 3-11 : Thrombosis of basal artery with an infarction area developing in 12 hours.

Figure 3-12: Ruptured aneurysma of the basal artery.

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Figure 3-13: CT scan shows a subtle hypodens area in the pons at the left side, whereas

diffusion MRI scan detects ischemia in this region with high signal intensity.

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References:

1. List for ACR Practice Guidelines and Technical Standards:

http://www.acr.org/departments/stand_accred/standards/dl_list.html

2. RCR guidelines Head imaging:

http://www.rlbuht.nhs.uk/content/pol.asp?web=72&sub=124&page=331

3. Provenzale JM. CT and MR Imaging of Nontraumatic Neurologic Emergencies. AJR.

2000; 174:289-299.

4. Luna AL, Goldstein RB. Sonographic Visualization of Neonatal Posterior Fossa

Abnormalities Through the Posterolateral Fontanelle. AJR. 2000; 174:561-567.

5. The WHO 2000 CLASSIFICATION of Brain (CNS) TUMORS,

http://rad.usuhs.mil/rad/who/who-index.html. James G. Smirniotopoulos

6. The new WHO Classification of Tumors affecting the Central Nervous System,

http://neurosurgery.mgh.harvard.edu/newwhobt.htm. Stephen B. Tatter

7. Albright L. Posterior Fossa tumors. Neurosurg Clin N Am. 1992; Oct;3(4):881-91.

8. Fukui MB, Hogg JP, Martinez AJ. Extraaxial ependymoma of the posterior fossa. AJNR.

1997; 18(6):1179-81.

9. Eslick GD, Hammond SR. Cystic lesion of the posterior fossa. Lancet. 2002;

2;359(9304):396.

10. Maeta M, Saito R, Nameki H. False-positive magnetic resonance image in the diagnosis

of small acoustic neuroma. J Laryngol Otol. 2001; 115(10):842-4 .

11. Mangels KJ, Tulipan N, Tsao LY, Alarcon J, Bruner JP. Fetal MRI in the evaluation of

intrauterine myelomeningocele. Pediatr Neurosurg. 2000; 32(3):124-31.

12. Loevner LA. Imaging features of posterior fossa neoplasms in children and adults. Semin

Roentgenol. 1999;34(2):84-101.

13. Tan TY, Teh HS. Contrast-enhanced magnetic resonance imaging of the internal auditory

canals and posterior fossa. Ann Acad Med Singapore. 1998;27(2):168-72.