Answer 1: C The basiocciput
- The bones of skull base are formed by endochondral ossification.
- The bones of calvaria and face are formed by intramembranous ossification.
- The occipital bone has interparietal,
supraoccipital and exoccipital ossification centers.
The interparietal and the supraoccipital parts of the occipital bone are separated by the mendosal suture,
that disappears during the first year of the life.
The most upper part of the occipital bone (interparietal) shows intramembranous ossification whereas the rest of the occipital bone shows endochondral ossification.
Answer 2: B Petrous ridges limit the central and posterior skull base.
Divisions of the skull base are (Figure 1):
– Anterior limit is the posterior wall of the frontal sinuses
– Posterior margin is the posterior margin of the lesser sphenoid wings,
– Anterior limit is the posterior margin of the anterior skull base
– Posterior margin is the petrous ridge and anterior margin clivus
– Anterior margin is posterior margin of the middle cranial fossa
Fig. 1: Divisions of the skull base.
Answer 3: D Posterior ethmoidal foramen
Origins of the major foramina and associated structures:
- Anterior skull base has cribiform palate (CNI- olfactor nerve,
anterior and posterior ethmoidal foramen (anterior ve posterior ethmoidal artery)
- Central skull base has optic canal (CNII- optic nerve),
superior opthalmic fissure (CNIII-oculamotor nerve,
CNIV- trochlear nerve and CNV,V1-opthalmic nerve,
superior ophthalmic vein),
foramen rotundum (CNV,V2- maxillary nerve),
foramen ovale (CN,V3- mandibular nerve) and foramen spinosum (middle meningeal artery)
- Posterior skull base has internal acoustic canal (CNVII- facial nerve,
jugular foramen (CNIX- glossopharyngeal nerve,
CNX- vagus nerve,
CNXI- accessory nerve,
inferior petrosal vein,
jugular vein) and hypoglossal canal (XII- hypoglossal nerve).
Answer 4: C Pneumosinus dilatans (in conjunction with meningioma)
• Pneumosinus dilatans is abnormal dilatation of one or more paranasal s inuses without bone destruction,
or mucosal thickening.
• Frontal sinus,
followed by the sphenoid,
and maxillary sinuses are most commonly affected.
• It may be associated with meningioma,
nerve sheath tumors,
Dyke- Davidoff-Masson syndrome,
Answer 5: D High-resolution heavily T2-weighted 3D imaging
- A skull base cerebrospinal (CSF) leak is an abnormal communication of the subarachnoid space with the sinonasal or tympanomastoid cavities,
and presents clinically with clear rhinorrhea or otorrhea.
- Imaging is essential in determining the site of a CSF leak but it can also aid in determining the underlying cause.
- High-resolution CT of the paranasal sinuses and mastoids should be the first line of imaging.
It shows the site of bony defect in the majority of cases.
- High-resolution heavily T2-weighted three-dimensional (3D) imaging techniques (T2 spc/ciss/DRIVE/FIESTA/B-FFE) can characterize the contents of tissue herniating through an osseous defect and differentiate them from inflammatory mucosal changes (Figure 7).
Answer 6: C Asymmetric pneumatization of the anterior clinoid process
- Asymmetric pneumatization of the anterior clinoid process may mimic aneurysm on MRI.
- Recognizing the fat and air signal on all images and a precise knowledge of the anatomy is important in diagnosis.
- It is easy to pick up pneumatized cells on CT (Figure 9).
- Pterygoid process of the sphenoid bone and petrous apices may also show asymmetric pneumatization.
Non-pneumatized areas may mimic enhancing lesion on contrast enhanced images (Figure 10).
Fig. 9: Bilateral pneumatization of the anterior clinoid process.
Answer 7: E Fluid entrapment; CT
- Pterygoid process of the sphenoid bone and petrous apices may show asymmetric pneumatization.
- Fluid entrapment may be seen in the pneumatized petrous apices and pterygoid process of the sphenoid bone.
It should be differentiated from tumors,
meningoceles and cholesteatomas.
- On CT,
fluid entrapment presents as a opacification without expansion and preservation of the bony septations and the cortex (Figure 12).
it has T2 high signal with low or mildly elevated T1 signal and shows no enhancement.
- Entrapped fluid in the petrous apex air cells is more fruquent than other pathologies (Figure 13).
Fig. 12: Fluid entrapment in the pneumatized pterygoid process of the sphenoid bone (CT images of the patient in Question-7)
Answer 8: D Posterior condylar emissary vein canal
- Foramen rotundum (CNV2- maxillary nerve),
foramen ovale (CNV3- mandibular nerve) and foramen spinosum (middle meningeal artery) are originated from the central skull base.
- Foramen vesalius is a tiny,
variably present foramen in the greater wing of the sphenoid bone,
located anterior and medial to the foramen ovale.
It transmits a sphenoidal emissary vein linking the pterygoid venous plexus in the infratemporal fossa to the cavernous sinus.
- Posterior condylar emissary vein canal is originated from the posterior skull base.
It is a variably present emissary vein that courses between the superior bulb of the internal jugular vein and deep cervical vein (Figure 15).
Fig. 15: Posterior condylar emissary vein canal (arrows).
Answer 9: C Juvenile nasal angiofibroma
- Juvenile nasal angiofibroma is a benign,
fibrovascular tumor that is locally invasive.
- Clasically occurs in adolescent males and usually presents with epistaxis and nasal obstruction.
- Juvenile nasal angiofibroma is located in the lateral aspect of the nasal cavity,
adjacent to the sphenopalatine foramen.
It expands the sphenopalatine foramen (Figure 17)
- Larger tumors are lobulated and may bulge into the nasopharynx,
sphenoid sinus and pterygopalatine fossa (Figure 18)
- Juvenile nasal angiofibroma is a extremely vascular neoplasm and usually needs preoperative embolisation.
Fig. 17: Juvenile angiofibroma (The same patient in Question-9)
Answer 10: E Pseudolesion
- Sphenoid pseudolesion or arrested pneumatization of the sphenoid bone is an incidentally identified benign lesion.
- On CT,
it is characterized by a non-expansile or mildly expansile,
hypodense lesion with well-defined sclerotic margins.
It may contain calcification.
It has internal fat density (<-5 HU).
Cortical erosion and prominent bone expansion should exclude the diagnosis (Figure 20).
- On MRI,
it has macroscopic fat signal on all sequences and shows fat suppression.
Answer 11: C Chondrosarcoma
- This patient is a 52-year-old patient with lung cancer that represented with a solitary clival bone metastases.
- Clival lesions may arise from the skull base itself,
from the intracranial compartment or from below the base of skull.
They may present with cranial neuropathies.
invasive pituitary macroadenoma,
central skull base osteomyelitis,
nasopharynegeal carcinoma are the main differential diagnosis.
- A pituitary lesion (invasive macroadenoma,
craniopharyngioma) may extend into the sphenoid sinus and central skull base.
- Nasopharyngeal carcinoma may invade skull base.
The clue is the origin of the lesion from nasopharynx (Figure 22).
- Skull base osteomyelitis are initiated by ear infections in older diabetic patients (Figure 23),
but central or atypical skull base osteomyelitis occurs much less frequently and does not begin with otitis externa (Figure 24).
Diffuse skull base infiltration without an obvious mass and high ADC values may help diagnosis.
- Plasmacytoma may be solitary or part of multiple myeloma.
It originates from bone marrow but may have soft tissue component.
- Chordrosarcoma originates from petroclival synchondrosis.
It usually has very high T2 signal intensity and high ADC values (Figure 25).
Answer 12: E Persistent trigeminal artery
- The persistent trigeminal artery is not a primitive notochord remnant lesion.
It is the most common and most cephalic of the persistent carotid-vertebrobasilar anastomoses.
This artery originates from the internal carotid artery immediately after its exit from the carotid canal and anastomoses with the midbasilar artery.
- Notochordal lesions are located in midline of the craniospinal axis,
reaching from the dorsum sellae to the sacrococcygeal region.
- The notochordal remnant lesions are:
- Ecchordosis Physalipohora
- Tornwaldt cyst
- Fossa navicularis magna
- Persistan canalis bazalis medianus
- Persistent craniofaryngeal canal
Answer 13: D Ecchordosis Physalipohora
- Ecchordosis physaliphora is a small,
gelatinous tissue that is considered an ectopic notochordal remnant.
- It is typically found intradurally in the prepontine cistern,
where it is attached to the dorsal wall of the clivus via a small pedicle.
- They are usually asymptomatic and found incidentally.
it is hypointense on T1-weighted images and hyperintense on T2-weighted images,
and shows no contrast enhancement.
Answer 14: B Thumb sign (chordoma)
- Ecchordosis physaliphora is the least aggressive whereas chordoma is the most aggressive notochordal retroclival lesions.
- Chordomas are expansile lesions with high T2 signal intensity,
heterogeneous enhancement and bone destruction.
- Typically the lesion projects in the midline posteriorly indenting the pons (thumb sign).
Answer 15: C Petrous apex cephalocele
- Petrous apex cephaloceles are located just above the anterior petrous apex and are continuous with the Meckel cave.
- They are well-marginated and have the same signal intensity characteristics as CSF with all MR imaging sequences.
- CT may show extensive nonaggressive erosion of the petrous apex with a well-defined or scalloped border.
- Cholesterol granulomas are usually hyperintense on both T1- and T2-weighted images and show no enhancement.
prominent hyperintensity on nonenhanced T1 -weighted MR image is a characteristic of a cholesterol granuloma (Figure 30).
Answer 16: E The most appropriate next step is MRI.
Answer 17: C Rhabdomyosarcoma
- Rhabdomyosarcoma is the most common soft-tissue malignancy in children and the most common primary malignancy of the temporal bone.
It may clinically simulate chronic otitis media.
- On CT,
it shows an aggressive osseous destruction.
- On MR imaging,
it has intermediate signal intensity on T1-weighted images,
variable signal intensity on T2-weighted images,
and variable enhancement.
- Differential diagnosis of destructive enhancing temporal bone lesions in a child include metastatic neuroblastoma and langerhans cell histiocytosis (LCH).
- Rhabdomyosarcoma usually involves the anterior portion of the temporal bone (petrous apex and middle ear) whereas LCH usually involves the mastoid.
This difference in primary location may be helpful in predicting the pathology of these lesions on the basis of imaging.
- LCH presents with bilateral aggressive masses without cranial neuropathies. A well-defined destructive lesion with nonsclerotic margins and marked enhancement favors the diagnosis of LCH in pediatric patients (Figure 33).
Answer 18: C Glomus jugulare
- Glomus tumors (paragangliomas,
glomangiomas) are neuroendocrine neoplasms in the head and neck region originating from paraganglia.
- Glomus jugulare arises from glomus bodies in the jugular foramen.
- They tend to show superolateral extention to the middle ear (glomus jugulotympanicum)
- On MR,
there are T1 hyperintense foci representing hemorrhage or slow ﬂow (“salt”),
and T2 hypointense foci due to high-velocity arterial ﬂow voids and calciﬁcation (“pepper”).
- On CT,
they show permeative and destructive changes in the surrounding bone (Figure 35).
- Jugular foramen schwannoma is the main differential diagnosis.
Well-defined enlargement of the jugular foramen with sclerotic borders,
craniocaudal extension of the lesion to the carotid space (dumbbell shape),
and no salt and pepper (flow voids) appearance are typical features of the jugular foramen schwannoma.
Larger tumors may show cystic areas (Figure 36)
Fig. 35: Glomus jugulare. The permeative and destructive changes in the surrounding bone (arrow)
Fig. 36: Jugular foramen schwannoma.