Journal of Medical Imaging and Radiation Oncology 58 (2014) 458–463 bs_bs_banner
RADIOLO GY —P I CTO R I A L E SSAY
‘Do not touch’ lesions of the skull base Mircea C Dobre and Nancy Fischbein Department of Radiology, Stanford Hospitals and Clinics, Stanford, California, USA
MC Dobre MS, MD; N Fischbein MD. Correspondence Dr Mircea C Dobre, Stanford Radiology, 300 Pasteur Drive, S047, Stanford, CA 94305, USA. Email: [email protected] Conflict of interest: Authors stated no financial relationship to disclose. No grant funding was utilised for this project. Submitted 29 January 2014; accepted 22 May 2014.
Summary Imaging of the skull base presents many challenges due to its anatomical complexity, numerous normal variants and lack of familiarity to many radiologists. As the skull base is a region which is not amenable to physical examination and as lesions of the skull base are generally difficult to biopsy and even more difficult to operate on, the radiologist plays a major role in directing patient management via accurate image interpretation. Knowledge of the skull base should not be limited to neuroradiologists and head and neck radiologists, however, as the central skull base is routinely included in the field of view when imaging the brain, cervical spine, or head and neck with computed tomography or magnetic resonance imaging, and hence, its nuances should be familiar to general radiologists as well. We herein review the imaging findings of a subcategory of lesions of the central skull base, the ‘do not touch’ lesions.
doi:10.1111/1754-9485.12195
Key words: head neuroradiology.
Introduction The central skull base is anatomically complex and consists of an osseous foundation that is pierced by critical vessels and nerves that bridge the intra-cranial and extra-cranial compartments. As this region is difficult to assess clinically, cross-sectional imaging with CT and MRI plays a major role in evaluating patients with symptoms potentially referable to the central skull base, and therefore, the interpreting radiologist plays a vital role in directing patient care. Fludeoxyglucose positron emission tomography (FDG PET) may be extremely helpful in instances where there is concern for skull base malignancies; however, it may offer little benefit and even be misleading in patients with certain benign conditions, for example, fibrous dysplasia (FD). In this article, we review a select group of conditions that occur in and around the skull base, focusing on lesions that should be recognised by imaging characteristics and therefore generally not biopsied, the ‘do not touch’ lesions (Table 1).
FD FD is a mesenchymal disorder that can affect any bone in the body. The skull and facial bones are the affected sites in 10–25% of patients with monostotic FD and in 50% of patients with polyostotic FD. CT findings include three varieties: the ground-glass pattern (56%), the homoge458
and
neck
imaging;
magnetic
resonance
imaging;
neously dense pattern (23%) and the cystic pattern (21%).1 The most common appearance of FD on CT is expanded bone with a ground-glass appearance, and this appearance is easily recognised and characteristic (Fig. 1). Localised FD on MR imaging, however, may mimic a tumour because fibrous tissue has low to intermediate T1 and heterogeneous T2 signal and enhances after the administration of contrast material.2,3 When MR is performed as the primary imaging modality in a patient with headache, for example, FD of the skull base is often diagnosed as an aggressive lesion, and the patient is referred to a tertiary care center for biopsy of what is often interpreted as a clival chordoma or metastatic focus. Recognising the benign expansion of bone with preservation of normal contours and the characteristically dark T2 signal of the fibrous tissue and suggesting a confirmatory CT of the skull base can help the patient to avoid an unnecessary biopsy (Fig. 2). The risk of unnecessary biopsy can be compounded if the patient undergoes an FDG PET CT for further evaluation of this ‘aggressive’ skull base mass since FD is typically FDG avid and presents as a hypermetabolic focus (Fig. 3).
Lateral sphenoid meningocele The current nomenclature regarding the contents of osteodural defects is confusing and includes terms such © 2014 The Royal Australian and New Zealand College of Radiologists
‘Do not touch’ lesions of the skull base
Table 1. ‘Do not touch’ lesions Category Anatomical variant Benign proliferative or acquired Dysplatic
Condition Arrested pneumatisation Arachnoid (Pacchionian) granulations Fibrous dysplasia Aneurysm Petrous apex cephalocele Lateral sphenoid meningocele
as meningocele, meningoencephalocele, encephalocele, meningeal or arachnoid hernia, and arachnoid diverticulum, among others. Cephalocele is commonly utilised as an all-inclusive term, and if it contains brain tissue, then
it is termed an encephalocele; however, if it contains merely meninges, it becomes a meningocele. Furthermore, imaging may not always display the full range of tissues contained within a lesion. Lateral sphenoid meningoceles (LSM) are rare lesions and should be considered when a defect lateral to the foramen rotundum within the anterior portion of the inferolateral recess is identified on CT or MRI. Depending on the contents of the lesion, MRI may show a signal identical to cerebrospinal fluid (CSF) or to brain as the inferior temporal gyrus may herniate into the lesion or appear ‘tented’ or ‘tethered’ towards it (Fig. 4). When a meningocele extends into the sphenoid sinus, it may mimic a far more commonly occurring primary sinus lesion, such as a retention cyst, polyp or mucocele, and it is therefore at
Fig. 1. Fibrous dysplasia. (a) Gadoliniumenhanced fat-suppressed T1-weighted MR image shows expansion of the clivus and replacement of part of the normal marrow by enhancing soft tissue. The lesion also involves the greater wing of the sphenoid bone bilaterally. (b) Coronal CT image shows how the lesion expands the clivus as well as the greater wings of the sphenoid bone. The expansion plus the internal ‘ground-glass’ matrix of this lesion clinches the diagnosis of fibrous dysplasia.
Fig. 2. Cystic fibrous dysplasia. (a) Axial CT image shows a lesion replacing the normal bone marrow of the left aspect of the clivus (black arrow). (b) Coronal CT image utilising soft tissue windows shows this lesion to represent soft tissue. (c) Axial fast spin echo T2-weighted MR image shows this lesion has low T2 signal, and (d) axial gadoliniumenhanced T1-weighted MR image demonstrates the lesion is avidly enhancing. Due to the concerning appearance of the lesion, a biopsy was performed rendering the diagnosis of cystic fibrous dysplasia.
a
b
a
b
d
c
© 2014 The Royal Australian and New Zealand College of Radiologists
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MC Dobre and N Fischbein
Fig. 3. Fibrous dysplasia. Axial-fused positron emission tomography (PET)/CT image demonstrating fludeoxyglucose (FDG) hypermetabolism in the left aspect of the clivus in a patient with biopsy-proven skull base fibrous dysplasia.
risk of being biopsied or removed by an unsuspecting endoscopic sinus surgeon. When a fluid containing structure is therefore seen in the sphenoid sinus, the diagnosis of LSM should be considered, and the adjacent sinus walls should always be carefully examined.4 LSMs are often asymptomatic and incidentally discovered on imaging studies; however, patients may present with spontaneous CSF rhinorrhea and/or prior history of meningitis and in these cases may require surgical repair.5
Petrous apex cephalocele Petrous apex cephalocele (PAC) is a lesion representing protrusion of arachnoid or dura mater, usually from
Meckel’s cave, into the petrous apex. PACs are thought to be caused by chroni¬cally increased intracranial pressure that is trans¬mitted into Meckel’s cave through a patent porus trigeminus. PACs are associated with empty sella and idiopathic intracranial hypertension. Typical PACs are bilateral and occur more often in women than in men. These lesions are rare and usually found incidentally but may be symptomatic, and in these cases, patients may present with symptoms of headache, cranial nerve palsy, including cranial nerves 3 through 6, and sensorineural hearing loss (SNHL).6 It remains unclear why SNHL has been reported seen in these patients; however, some authors suggest a possible association with dural dysplasia affecting the inner ear, for example, as seen in Usher syndrome.7,8 On CT imaging, PACs cause unilateral or bilateral smooth erosion and remodelling of the petrous apex. MRI shows a lesion that is isointense to CSF on T1- and T2-weighted images and that does not enhance after contrast administration (Fig. 5) or demonstrate reduced diffusion; deviation from CSF signal on any MRI sequence should raise the possibility of another diagnosis. These lesions can typically be identified as ‘budding off’ from Meckel’s cave on coronal T2-weighted images. Asymptomatic lesions may be followed on serial imaging to ensure that they do not enlarge over time, while the rare lesion that produces symptoms can be surgically obliterated.3,6
Arrested pneumatisation Arrested pneumatisation of the skull base is a benign anatomical variant that occurs when pneumatisation is regionally interrupted or never commences.9 The most common location of arrested pneumatisation is the basisphenoid, though other commonly affected regions include the pterygoid process and petrous apex. Multiple bones may be involved in the same patient, and the process may be bilateral but asymmetric.10,11 On imaging, the non-aerated bone shows high signal intensity on T1-weighted MR images and no evidence of enhancement beyond the mild enhancement that can be seen in normal marrow on post-gadolinium, fatsuppressed images (Fig. 6) – the T1 bright material is fat Fig. 4. Lateral sphenoid meningocele. (a) Axial CT image shows a lobulated soft tissue density mass filling the right lateral aspect of the sphenoid sinus, with attenuation and focal discontinuity of the bony margin of the sinus (arrow). (b) Axial fast spin echo T2-weighted MR image shows brain tissue tenting towards and herniating through the defect (arrow) in the sphenoid bone, accompanied by a meningocele.
a 460
b © 2014 The Royal Australian and New Zealand College of Radiologists
‘Do not touch’ lesions of the skull base
Fig. 5. Petrous apex cephalocele. (a) Axial CT image shows a focal lytic lesion involving the left petrous apex and lateral clivus (arrow). The borders are smoothly corticated, suggesting the long-standing nature of this lesion. (b) Coronal T2-weighted MR image of a different patient shows a similar lesion which is bright and clearly shows the communication with Meckel’s cave (arrow). It was isointense to cerebrospinal fluid on all other imaging sequences, including DWI and post-contrast (not shown).
a
b
and will suppress on a pre-gadolinium fat-suppressed image. CT reliably shows the normal trabeculated bone with similar or lower attenuation compared with the remaining non-aerated bones of the skull base and typically helps clinch the diagnosis, especially when other features, such as thin sclerotic margins, lack of mass effect and normal appearance to the margins of the adjacent neural foramina, are present.3
Aneurysm Most aneurysms arise from the Circle of Willis, but sometimes, a large aneurysm is detected in the skull base and, especially if thrombosed, can mimic a mass lesion.12 The radiologist must thus be alert in order to prevent potentially catastrophic complications associated with biopsy or surgery. Intact internal carotid artery (ICA) aneurysms at the skull base are often found incidentally, but depending on the direction of the aneurysm, the patient may present with headache and/or cranial nerve palsies. On non-contrast CT, there may be thinning or dehiscence of the bony walls adjacent to the ICA, and if the aneurysm is located along the petrous segment of
a
b
the ICA, then there may be an expansion of the carotid canal.13 Regular, thin peripheral calcification on CT can also be a helpful clue for the diagnosis of aneurysm. Non-contrast MRI may show a range of signal characteristic depending on the proportion of intraluminal thrombus to patent lumen and the presence or absence of turbulent flow (Figs 7,8). Contrast-enhanced CT, timeof-flight MRA or contrast enhanced MRA, as well as digital subtraction angiography, can be used to confirm the presence of an aneurysm, especially if it is not completely thrombosed. Although aneurysms are ‘do not touch’ lesions in the meaning intended by this article (i.e. biopsy), invasive treatment options may be warranted.
Arachnoid (pacchionian) granulations Arachnoid granulations are CSF-filled, pia-arachnoidlined protrusions that extend through openings in the dura into the venous sinuses or venous lacunae (Fig. 9). They are particularly common in the region of the transverse sinuses but may also be found associated with the cavernous and other major venous sinuses. When they
c
Fig. 6. Arrested pneumatisation. (a) Coronal T1-weighted MR image shows the asymmetry of the sphenoid sinuses. There is a right-sided ‘lesion’ in contiguity with the pterygoid process that has fatty signal intensity (arrow). (b) Coronal gadolinium-enhanced fat-suppressed T1-weighted MR image shows the ‘lesion’ is dark after fat suppression and does not enhance (arrow). (c) Coronal CT image shows that this ‘lesion’ is clearly in continuity with the sphenoid bone and is marginated by cortical bone (arrow). Its Hounsfield unit density was negative. © 2014 The Royal Australian and New Zealand College of Radiologists
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MC Dobre and N Fischbein
a
b
c
Fig. 7. Aneurysm. (a) Coronal CT shows ill-defined, mass-like enhancement in the right cavernous sinus and right sella. (b) Coronal gadolinium-enhanced fatsuppressed T1-weighted MR image shows a brightly enhancing lesion in the right sella immediately adjacent to the cavernous sinus and intimately associated with the right internal carotid artery (ICA) (arrow). Subtle pulsation artefact was present in the phase encoding axis (left to right, not shown). (c) Right ICA angiogram shows an aneurysm arising from the proximal cavernous segment of the ICA (black arrow).
occur in unusual locations and do not appear to communicate with any major sinuses, they may be a cause of spontaneous CSF leakage secondary to cortical bone erosion.13 On imaging, they are seen in as many as 24% of contrast-enhanced CT scans and 13% of MR studies, and they normally display signal intensities similar to CSF on all sequences as well as lack of enhancement after contrast administration.14 When these are greater than 1 cm, they are termed ‘giant’ granulations, and it
has been reported that they are no longer isointense to CSF on all sequences; in fact, up to 80% of these ‘giant’ granulations have been reported to show CSFincongruent signal on at least one MR sequence, most commonly FLAIR.15 It is imperative for the interpreting radiologist to distinguish these benign normal variants from pathology in order to avoid risking potentially dangerous complications such as meningitis if biopsy or surgical excision is pursued.
Fig. 8. Aneurysm. (a) Axial CT images shows an expansile lesion in the right clivus with remodelling of the adjacent bone (white arrow). The lesion appears in contiguity with the carotid canal. (b) Axial fast spin echo T2-weighted MR image shows a heterogeneous appearance to the mass which is inseparable from the petrous segment of the internal carotid artery (white arrow). Diagnostic cerebral angiogram (not shown) later demonstrated this lesion to be a partially thrombosed aneurysm.
a
b
Fig. 9. Arachnoid granulations. (a) Axial CT image show a lytic lesion of the lateral aspect of the greater wing of the sphenoid. The outer cortex is intact, and no additional lesions were seen. (b) Axial fast spin echo T2-weighted MR image shows a benign-appearing, slightly lobulated structure of cerebrospinal fluid intensity at this location.
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b © 2014 The Royal Australian and New Zealand College of Radiologists
‘Do not touch’ lesions of the skull base
Conclusion The skull base is anatomically complex and is best evaluated radiologically by cross-sectional imaging. CT and MRI are complementary, and both are often necessary to determine whether a lesion belongs to the ‘do not touch’ category. The radiologist should be familiar with the skull base conditions described in this article in order to optimally guide patient management and spare the patient unnecessary invasive procedures.
References 1. Brown EW, Megerian CA, McKenna MJ et al. Fibrous dysplasia of the temporal bone. AJR Am J Roentgenol 1995; 164: 679–82. 2. Chong V, Khoo J, Fan Y-F. Fibrous dysplasia involving the base of the skull. AJR Am J Roentgenol 2002; 178: 717–20. 3. Razek A, Huang B. Lesions of the petrous apex: classification and findings at CT and MR imaging. Radiographics 2012; 32: 151–73. 4. Settecase F, Harnsberger HR, Michel MA et al. Spontaneous lateral sphenoid cephaloceles: anatomic factors contributing to pathogenesis and proposed classification. AJNR Am J Neuroradiol 2014; 35: 784–9. 5. Schuknecht B, Simmen D, Briner H. Nontraumatic skull-base defects with spontaneous CSF rhinorrhea and arachnoid herniation: imaging findings and correlation with endoscopic sinus surgery in 27 patients. AJNR Am J Neuroradiol 2008; 29: 542–9. 6. Moore KR, Fischbein NJ, Harnsberger HR et al. Petrous apex cephaloceles. AJNR Am J Neuroradiol 2001; 22: 1867–71.
© 2014 The Royal Australian and New Zealand College of Radiologists
7. Isacsson B, Coker NJ, Vrabec JT et al. Invasive cerebrospinal fluid cyst and cephaloceles of the petrous apex. Otol Neurotol 2006; 27: 1131–41. 8. Stark TA, McKinney AM, Palmer CS et al. Dilation of the subarachnoid spaces surrounding the cranial nerves with petrous apex cephaloceles in usher syndrome. AJNR Am J Neuroradiol 2009; 30: 434–6. 9. Spaeth J, Krugelstein U, Schlondorff G. The para-nasal sinuses in CT-imaging: development from birth to age 25. Int J Pediatr Otorhinolaryngol 1997; 39: 25–40. 10. Welker K, DeLone D, Lane J et al. Arrested pneumatization of the skull-base: imaging characteristics. AJR Am J Roentgenol 2008; 190: 1691–6. 11. Singh A, Smoker WRK, Policeni B. Imaging algorithm of petrous apex lesions. Neurographics 2012; 2: 126–38. 12. Hacein-Bey L, Provenzale J. Current imaging assessment and treatment of intracranial Aneurysms. AJR Am J Roentgenol 2011; 196: 32–44. 13. Lee MH, Kim HJ, Lee IH et al. Prevalence and appearance of the posterior wall defects of the temporal bone caused by presumed arachnoid granulations and their clinical significance: CT findings. AJNR Am J Neuroradiol 2008; 29: 1704–7. 14. Leach J, Jones B, Tomsick T et al. Normal appearance of arachnoid granulations on contrast-enhanced CT and MR of the brain: differentiation from dural sinus disease. AJNR Am J Neuroradiol 1996; 17: 1523–32. 15. Trimble CR, Harnsberger HR, Castillo M et al. ‘Giant’ arachnoid granulations just like CSF?: not!! AJNR Am J Neuroradiol 2010; 31: 1724–8.
463
RADIOLO GY —P I CTO R I A L E SSAY
‘Do not touch’ lesions of the skull base Mircea C Dobre and Nancy Fischbein Department of Radiology, Stanford Hospitals and Clinics, Stanford, California, USA
MC Dobre MS, MD; N Fischbein MD. Correspondence Dr Mircea C Dobre, Stanford Radiology, 300 Pasteur Drive, S047, Stanford, CA 94305, USA. Email: [email protected] Conflict of interest: Authors stated no financial relationship to disclose. No grant funding was utilised for this project. Submitted 29 January 2014; accepted 22 May 2014.
Summary Imaging of the skull base presents many challenges due to its anatomical complexity, numerous normal variants and lack of familiarity to many radiologists. As the skull base is a region which is not amenable to physical examination and as lesions of the skull base are generally difficult to biopsy and even more difficult to operate on, the radiologist plays a major role in directing patient management via accurate image interpretation. Knowledge of the skull base should not be limited to neuroradiologists and head and neck radiologists, however, as the central skull base is routinely included in the field of view when imaging the brain, cervical spine, or head and neck with computed tomography or magnetic resonance imaging, and hence, its nuances should be familiar to general radiologists as well. We herein review the imaging findings of a subcategory of lesions of the central skull base, the ‘do not touch’ lesions.
doi:10.1111/1754-9485.12195
Key words: head neuroradiology.
Introduction The central skull base is anatomically complex and consists of an osseous foundation that is pierced by critical vessels and nerves that bridge the intra-cranial and extra-cranial compartments. As this region is difficult to assess clinically, cross-sectional imaging with CT and MRI plays a major role in evaluating patients with symptoms potentially referable to the central skull base, and therefore, the interpreting radiologist plays a vital role in directing patient care. Fludeoxyglucose positron emission tomography (FDG PET) may be extremely helpful in instances where there is concern for skull base malignancies; however, it may offer little benefit and even be misleading in patients with certain benign conditions, for example, fibrous dysplasia (FD). In this article, we review a select group of conditions that occur in and around the skull base, focusing on lesions that should be recognised by imaging characteristics and therefore generally not biopsied, the ‘do not touch’ lesions (Table 1).
FD FD is a mesenchymal disorder that can affect any bone in the body. The skull and facial bones are the affected sites in 10–25% of patients with monostotic FD and in 50% of patients with polyostotic FD. CT findings include three varieties: the ground-glass pattern (56%), the homoge458
and
neck
imaging;
magnetic
resonance
imaging;
neously dense pattern (23%) and the cystic pattern (21%).1 The most common appearance of FD on CT is expanded bone with a ground-glass appearance, and this appearance is easily recognised and characteristic (Fig. 1). Localised FD on MR imaging, however, may mimic a tumour because fibrous tissue has low to intermediate T1 and heterogeneous T2 signal and enhances after the administration of contrast material.2,3 When MR is performed as the primary imaging modality in a patient with headache, for example, FD of the skull base is often diagnosed as an aggressive lesion, and the patient is referred to a tertiary care center for biopsy of what is often interpreted as a clival chordoma or metastatic focus. Recognising the benign expansion of bone with preservation of normal contours and the characteristically dark T2 signal of the fibrous tissue and suggesting a confirmatory CT of the skull base can help the patient to avoid an unnecessary biopsy (Fig. 2). The risk of unnecessary biopsy can be compounded if the patient undergoes an FDG PET CT for further evaluation of this ‘aggressive’ skull base mass since FD is typically FDG avid and presents as a hypermetabolic focus (Fig. 3).
Lateral sphenoid meningocele The current nomenclature regarding the contents of osteodural defects is confusing and includes terms such © 2014 The Royal Australian and New Zealand College of Radiologists
‘Do not touch’ lesions of the skull base
Table 1. ‘Do not touch’ lesions Category Anatomical variant Benign proliferative or acquired Dysplatic
Condition Arrested pneumatisation Arachnoid (Pacchionian) granulations Fibrous dysplasia Aneurysm Petrous apex cephalocele Lateral sphenoid meningocele
as meningocele, meningoencephalocele, encephalocele, meningeal or arachnoid hernia, and arachnoid diverticulum, among others. Cephalocele is commonly utilised as an all-inclusive term, and if it contains brain tissue, then
it is termed an encephalocele; however, if it contains merely meninges, it becomes a meningocele. Furthermore, imaging may not always display the full range of tissues contained within a lesion. Lateral sphenoid meningoceles (LSM) are rare lesions and should be considered when a defect lateral to the foramen rotundum within the anterior portion of the inferolateral recess is identified on CT or MRI. Depending on the contents of the lesion, MRI may show a signal identical to cerebrospinal fluid (CSF) or to brain as the inferior temporal gyrus may herniate into the lesion or appear ‘tented’ or ‘tethered’ towards it (Fig. 4). When a meningocele extends into the sphenoid sinus, it may mimic a far more commonly occurring primary sinus lesion, such as a retention cyst, polyp or mucocele, and it is therefore at
Fig. 1. Fibrous dysplasia. (a) Gadoliniumenhanced fat-suppressed T1-weighted MR image shows expansion of the clivus and replacement of part of the normal marrow by enhancing soft tissue. The lesion also involves the greater wing of the sphenoid bone bilaterally. (b) Coronal CT image shows how the lesion expands the clivus as well as the greater wings of the sphenoid bone. The expansion plus the internal ‘ground-glass’ matrix of this lesion clinches the diagnosis of fibrous dysplasia.
Fig. 2. Cystic fibrous dysplasia. (a) Axial CT image shows a lesion replacing the normal bone marrow of the left aspect of the clivus (black arrow). (b) Coronal CT image utilising soft tissue windows shows this lesion to represent soft tissue. (c) Axial fast spin echo T2-weighted MR image shows this lesion has low T2 signal, and (d) axial gadoliniumenhanced T1-weighted MR image demonstrates the lesion is avidly enhancing. Due to the concerning appearance of the lesion, a biopsy was performed rendering the diagnosis of cystic fibrous dysplasia.
a
b
a
b
d
c
© 2014 The Royal Australian and New Zealand College of Radiologists
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MC Dobre and N Fischbein
Fig. 3. Fibrous dysplasia. Axial-fused positron emission tomography (PET)/CT image demonstrating fludeoxyglucose (FDG) hypermetabolism in the left aspect of the clivus in a patient with biopsy-proven skull base fibrous dysplasia.
risk of being biopsied or removed by an unsuspecting endoscopic sinus surgeon. When a fluid containing structure is therefore seen in the sphenoid sinus, the diagnosis of LSM should be considered, and the adjacent sinus walls should always be carefully examined.4 LSMs are often asymptomatic and incidentally discovered on imaging studies; however, patients may present with spontaneous CSF rhinorrhea and/or prior history of meningitis and in these cases may require surgical repair.5
Petrous apex cephalocele Petrous apex cephalocele (PAC) is a lesion representing protrusion of arachnoid or dura mater, usually from
Meckel’s cave, into the petrous apex. PACs are thought to be caused by chroni¬cally increased intracranial pressure that is trans¬mitted into Meckel’s cave through a patent porus trigeminus. PACs are associated with empty sella and idiopathic intracranial hypertension. Typical PACs are bilateral and occur more often in women than in men. These lesions are rare and usually found incidentally but may be symptomatic, and in these cases, patients may present with symptoms of headache, cranial nerve palsy, including cranial nerves 3 through 6, and sensorineural hearing loss (SNHL).6 It remains unclear why SNHL has been reported seen in these patients; however, some authors suggest a possible association with dural dysplasia affecting the inner ear, for example, as seen in Usher syndrome.7,8 On CT imaging, PACs cause unilateral or bilateral smooth erosion and remodelling of the petrous apex. MRI shows a lesion that is isointense to CSF on T1- and T2-weighted images and that does not enhance after contrast administration (Fig. 5) or demonstrate reduced diffusion; deviation from CSF signal on any MRI sequence should raise the possibility of another diagnosis. These lesions can typically be identified as ‘budding off’ from Meckel’s cave on coronal T2-weighted images. Asymptomatic lesions may be followed on serial imaging to ensure that they do not enlarge over time, while the rare lesion that produces symptoms can be surgically obliterated.3,6
Arrested pneumatisation Arrested pneumatisation of the skull base is a benign anatomical variant that occurs when pneumatisation is regionally interrupted or never commences.9 The most common location of arrested pneumatisation is the basisphenoid, though other commonly affected regions include the pterygoid process and petrous apex. Multiple bones may be involved in the same patient, and the process may be bilateral but asymmetric.10,11 On imaging, the non-aerated bone shows high signal intensity on T1-weighted MR images and no evidence of enhancement beyond the mild enhancement that can be seen in normal marrow on post-gadolinium, fatsuppressed images (Fig. 6) – the T1 bright material is fat Fig. 4. Lateral sphenoid meningocele. (a) Axial CT image shows a lobulated soft tissue density mass filling the right lateral aspect of the sphenoid sinus, with attenuation and focal discontinuity of the bony margin of the sinus (arrow). (b) Axial fast spin echo T2-weighted MR image shows brain tissue tenting towards and herniating through the defect (arrow) in the sphenoid bone, accompanied by a meningocele.
a 460
b © 2014 The Royal Australian and New Zealand College of Radiologists
‘Do not touch’ lesions of the skull base
Fig. 5. Petrous apex cephalocele. (a) Axial CT image shows a focal lytic lesion involving the left petrous apex and lateral clivus (arrow). The borders are smoothly corticated, suggesting the long-standing nature of this lesion. (b) Coronal T2-weighted MR image of a different patient shows a similar lesion which is bright and clearly shows the communication with Meckel’s cave (arrow). It was isointense to cerebrospinal fluid on all other imaging sequences, including DWI and post-contrast (not shown).
a
b
and will suppress on a pre-gadolinium fat-suppressed image. CT reliably shows the normal trabeculated bone with similar or lower attenuation compared with the remaining non-aerated bones of the skull base and typically helps clinch the diagnosis, especially when other features, such as thin sclerotic margins, lack of mass effect and normal appearance to the margins of the adjacent neural foramina, are present.3
Aneurysm Most aneurysms arise from the Circle of Willis, but sometimes, a large aneurysm is detected in the skull base and, especially if thrombosed, can mimic a mass lesion.12 The radiologist must thus be alert in order to prevent potentially catastrophic complications associated with biopsy or surgery. Intact internal carotid artery (ICA) aneurysms at the skull base are often found incidentally, but depending on the direction of the aneurysm, the patient may present with headache and/or cranial nerve palsies. On non-contrast CT, there may be thinning or dehiscence of the bony walls adjacent to the ICA, and if the aneurysm is located along the petrous segment of
a
b
the ICA, then there may be an expansion of the carotid canal.13 Regular, thin peripheral calcification on CT can also be a helpful clue for the diagnosis of aneurysm. Non-contrast MRI may show a range of signal characteristic depending on the proportion of intraluminal thrombus to patent lumen and the presence or absence of turbulent flow (Figs 7,8). Contrast-enhanced CT, timeof-flight MRA or contrast enhanced MRA, as well as digital subtraction angiography, can be used to confirm the presence of an aneurysm, especially if it is not completely thrombosed. Although aneurysms are ‘do not touch’ lesions in the meaning intended by this article (i.e. biopsy), invasive treatment options may be warranted.
Arachnoid (pacchionian) granulations Arachnoid granulations are CSF-filled, pia-arachnoidlined protrusions that extend through openings in the dura into the venous sinuses or venous lacunae (Fig. 9). They are particularly common in the region of the transverse sinuses but may also be found associated with the cavernous and other major venous sinuses. When they
c
Fig. 6. Arrested pneumatisation. (a) Coronal T1-weighted MR image shows the asymmetry of the sphenoid sinuses. There is a right-sided ‘lesion’ in contiguity with the pterygoid process that has fatty signal intensity (arrow). (b) Coronal gadolinium-enhanced fat-suppressed T1-weighted MR image shows the ‘lesion’ is dark after fat suppression and does not enhance (arrow). (c) Coronal CT image shows that this ‘lesion’ is clearly in continuity with the sphenoid bone and is marginated by cortical bone (arrow). Its Hounsfield unit density was negative. © 2014 The Royal Australian and New Zealand College of Radiologists
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MC Dobre and N Fischbein
a
b
c
Fig. 7. Aneurysm. (a) Coronal CT shows ill-defined, mass-like enhancement in the right cavernous sinus and right sella. (b) Coronal gadolinium-enhanced fatsuppressed T1-weighted MR image shows a brightly enhancing lesion in the right sella immediately adjacent to the cavernous sinus and intimately associated with the right internal carotid artery (ICA) (arrow). Subtle pulsation artefact was present in the phase encoding axis (left to right, not shown). (c) Right ICA angiogram shows an aneurysm arising from the proximal cavernous segment of the ICA (black arrow).
occur in unusual locations and do not appear to communicate with any major sinuses, they may be a cause of spontaneous CSF leakage secondary to cortical bone erosion.13 On imaging, they are seen in as many as 24% of contrast-enhanced CT scans and 13% of MR studies, and they normally display signal intensities similar to CSF on all sequences as well as lack of enhancement after contrast administration.14 When these are greater than 1 cm, they are termed ‘giant’ granulations, and it
has been reported that they are no longer isointense to CSF on all sequences; in fact, up to 80% of these ‘giant’ granulations have been reported to show CSFincongruent signal on at least one MR sequence, most commonly FLAIR.15 It is imperative for the interpreting radiologist to distinguish these benign normal variants from pathology in order to avoid risking potentially dangerous complications such as meningitis if biopsy or surgical excision is pursued.
Fig. 8. Aneurysm. (a) Axial CT images shows an expansile lesion in the right clivus with remodelling of the adjacent bone (white arrow). The lesion appears in contiguity with the carotid canal. (b) Axial fast spin echo T2-weighted MR image shows a heterogeneous appearance to the mass which is inseparable from the petrous segment of the internal carotid artery (white arrow). Diagnostic cerebral angiogram (not shown) later demonstrated this lesion to be a partially thrombosed aneurysm.
a
b
Fig. 9. Arachnoid granulations. (a) Axial CT image show a lytic lesion of the lateral aspect of the greater wing of the sphenoid. The outer cortex is intact, and no additional lesions were seen. (b) Axial fast spin echo T2-weighted MR image shows a benign-appearing, slightly lobulated structure of cerebrospinal fluid intensity at this location.
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b © 2014 The Royal Australian and New Zealand College of Radiologists
‘Do not touch’ lesions of the skull base
Conclusion The skull base is anatomically complex and is best evaluated radiologically by cross-sectional imaging. CT and MRI are complementary, and both are often necessary to determine whether a lesion belongs to the ‘do not touch’ category. The radiologist should be familiar with the skull base conditions described in this article in order to optimally guide patient management and spare the patient unnecessary invasive procedures.
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