5 Skull Base Imaging

• Surgeons should treat every request they make for imaging in the same way as making a request for a consultation from a clinical specialist. Requests and responses should be tailored to the patient’s symptoms and signs, and should provide the radiologist with clinical information that will aid in diagnosis. This is critical for optimum patient care (Fig. 5.1).

Role of Neuroimaging in Skull Base Surgery

Diagnosis

Final diagnosis comes from the pathologist. However, a radiologist’s opinion regarding diagnosis is extremely helpful to the team making management decisions. A purely descriptive report that skirts or avoids a working diagnosis is of much less value.

Location

Information regarding the location of the tumor should also specify the origin and extent of the tumor and relationships to critical structures.

• Determining the origin can provide a clue to pathology (Fig. 5.2). The origin can be ascertained in many ways: by the epicenter of the lesion, by focal hyperostosis or erosion in certain lesions, and by the way the surrounding structures are displaced.

Fig. 5.1a,b (a) A tuberculum sellae meningioma (arrow) elevating the chiasm is readily apparent on the appropriate magnetic resonance imaging (MRI) sequences: sagittal postgadolinium T1 (left), coronal T2 (right). Note an incidental microadenoma within the pituitary gland (arrow). (b) A small tuberculum sellae meningioma, however, may be difficult to diagnose on axial computed tomography (CT) unless the appropriate technique is used (thin axial sections and a sagittal reformat in this example). The radiologist cannot plan the appropriate CT technique or MRI protocol without adequate clinical information on the requisition.

• Extent: Information about patterns of growth is important in understanding pathology (Figs. 5.3 and 5.4). Extension into foramina and fissures, across bone and dura, in proximity to and with encasement of vessels and nerves can alter management and help determine the surgical plan.¹

• Understanding the extent also helps to determine the stage and volume of the tumor. The determination of volume and any change in volume, signifying growth, is critical in deciding on the management of benign tumors such as meningiomas.

Fig. 5.2a–c A cerebellopontine angle meningioma on axial T2 (a), postgadolinium T1 axial (b), and coronal (c) views. The eccentric relationship to the internal auditory canal (IAC), the broad base on the dura, an obtuse angle of contact, and the enhancing dural tail are obvious clues to the diagnosis. In addition, a converging leash of vessels (a, arrow) and focal hyperostosis (if present) can provide additional clues to the exact site of origin.

• Relationships: Three-dimensional (3D) rendering can help in understanding in detail the relationships among bone, soft tissue, and vessels. Because 3D images can be rotated, sectioned, and viewed at any desired angle to simulate the surgical approach, they can help guide management (Fig. 5.5).

Fig. 5.3 Facial schwannoma presenting as an enhancing extra-axial mass in the middle fossa. Such a lesion can be mistaken for a meningioma; enhancement of the facial nerve is the clue to the diagnosis (arrow). Careful inspection of all possible pathways of spread is an essential step in skull base imaging. Note fat suppression used on these postgadolinium T1 images.

Fig. 5.4a,b Facial schwannoma on a heavily T2-weighted sequence (T2 DRIVE) (a) and corresponding postgadolinium T1 section (b). The enhancing “ice-cream cone”–like cisternal and IAC component of the mass is similar to a vestibular schwannoma. The distinguishing feature is the involvement of the labyrinthine segment of the facial nerve (thin arrow) and the geniculate ganglion (thick arrow).

Fig. 5.5 A volume-rendered 3D image can be rotated and sectioned to simulate the surgical approach.

Resectability and Risk Assessment

By understanding the location and extent of the tumor, an understanding of resectability and the risks of resection are possible. Thus, with a proper and careful diagnosis, it is useful to determine which treatment approach—observation, surgery, radiation, or palliation—can achieve the goals desired for the patient. Proper diagnosis can help to plan the surgical approach or the radiation plan, determine the prognosis, and decide on a follow-up schedule.2

Surgical Navigation

Good-quality imaging data transferred to the operating room are essential and routinely used for safe surgical navigation.³ The surgeon must understand not only the location of the tumor and the pathology but also where normal structures are expected to be found. For example, does the meningioma originate medial to or lateral to the entrance of cranial nerve (CN) V into Meckel’s cave? If medial, CN V will be on the superficial side of the tumor when accessed by a retrosigmoid or petrosal approach, meaning that the surgeon will encounter the trigeminal nerve at the beginning of the tumor resection. If the origin is lateral, then the surgeon accessing the tumor from any posterolateral approach will encounter the nerve at the end of the tumor resection, preferably deep to the arachnoid of the trigeminal cistern. The same principles can be enunciated for the arteries and veins around the tumor and along the pathway of access. Understanding these relationships is important for optimal surgical outcomes. Understanding these navigational issues requires close communication between radiologists and surgeons.

Adjunctive Treatment

Preoperative embolization is the most common adjunctive treatment. Intra-arterial chemotherapy and procedures to open the blood–brain barrier are more recent additions to the role of interventional neuroradiologists (see Chapter 12).⁴

Imaging Choices

The complementary role of computed tomography (CT) and magnetic resonance imaging (MRI) is long known and well established, but cannot be overemphasized.⁵ Each technique has advantages, but most skull base pathology requires both modalities for a complete evaluation.

Computed Tomography

• It is superior in detecting subtle erosions of cortical margins and foramina, and in demonstrating reactive changes in bone (e.g., hyperostosis and pneumosinus dilatans around meningiomas) or tiny areas of calcification within lesions.5,6

• A thin-section axial helical CT data set is initially acquired usually with contrast; 64-slice scanners are standard in most skull base centers, but some centers have access to 256-slice or higher scanners with better computed tomography angiography (CTA) and 3D reconstructions, although there is no clear clinical advantage in the skull base.⁷

• Axial images of desired thickness are then generated, and high-quality coronal and sagittal images are reconstructed, using soft tissue and bone algorithms.

• It is important to use the correct “bone algorithm” images, which are generated from the same data set with edge enhancement to evaluate bone in sharp detail. Looking at the routine soft tissue images using a bone window setting is suboptimal.

• Intravenous contrast is necessary to look for intracranial or extracranial disease, to evaluate the vessels and cavernous sinus, and to help characterize the lesion. Contrast provides an initial assessment of tumor vascularity and may determine the need for dedicated vascular imaging [CTA/magnetic resonance angiography (MRA) or digital subtraction angiography (DSA)]. CT and MR perfusion studies have been used for grading the tumor, determining the prognosis, assessing the treatment response, and differentiating treatment/radiation effects and nonneoplastic lesions from neoplasms.⁸

• Contrast may be omitted on follow-up CT of osseous tumors with no soft tissue components, of sinonasal lesions clearly without intracranial extension, or if there is a contraindication.

• An unenhanced CT should be performed first if intracranial hemorrhage (or calcification) is a concern; contrast images can follow.

• Patients known to be at higher risk for contrast allergy may be premedicated using the following regimen (all three medications) to reduce the frequency or severity of reactions, after obtaining informed consent⁹:

Prednisone 50 mg PO at 13 hours, 7 hours, and 1 hour before

Diphenhydramine PO 50 mg at 13 hours, 7 hours, and 1 hour before

Ranitidine PO at 1 hour before contrast administration

Computed Tomography Angiography

• Computed tomography angiography (CTA) is invaluable in preoperative planning, particularly when intracranial disease is in close proximity to arteries, the cavernous sinus, or the dural sinuses. It is also important to recognize variant vascular anatomy such as a persistent trigeminal artery or unexpected vascular lesions such as aneurysms.10

• Three-dimensional images can be postprocessed from the CTA source data set (Fig. 5.5).

• A “less is more” approach is advocated when ordering CTA studies. A focused region of interest (ROI) should be clearly defined in the radiology request.

• Coverage beyond the ROI results in unnecessary radiation, redundant images, and lower quality images at the region of interest. One study quotes an effective dose of 0.67 millisievert (mSv) for CTA of the cerebral vessels and 4.85 mSv for the cervicocerebral vessels.¹¹ No radiation risk is acceptable, however small, without benefit to the patient.¹² As an example, a CTA covering from C2 to the circle of Willis will provide high-quality vascular imaging at the skull base. This advantage is lost on a CTA covering the aortic arch to the vertex.

• Computed tomography angiography will show vascular variants of surgical importance^10,13:

The internal carotid artery (ICA) medially positioned, bulging into sphenoid sinus. The bony wall may be thin or deficient, putting the ICA at risk during transsphenoidal surgery. Manipulation of a septum attached to this thin wall may also jeopardize the ICA.

A persistent trigeminal artery may also bulge into the sphenoid sinus.

A persistent hypoglossal artery traverses the hypoglossal canal (which may be enlarged). This rare variant may be the major or sole contributor to the basilar system. Another rare variant is the proatlantal artery, an ICA or external carotid artery (ECA) branch that supplies the vertebral artery.

A high-riding jugular bulb may be at risk during translabyrinthine surgery; often visible on coronal CTA images; best seen on computed tomography venography (CTV).

Venous varix at the basiocciput can occasionally be mistaken for a bony lesion.

Computed Tomography Venography

• Computed tomography venography is done by acquiring a similar axial data set following intravenous contrast, but later in the venous phase.

• Performing both CTA and CTV involves twice the amount of radiation.¹¹ The second acquisition may be difficult to achieve, resulting in suboptimal CTV images, particularly in patients with cardiac dysfunction.

Cisternography

• Cisternography is occasionally indicated for detection or confirmation of the site of a CSF leak along the skull base. It is usually performed if unenhanced high-resolution CT of the skull base and/or MR cisternography fail to show the site of CSF leakage.14,15

• Ideally, to have maximum sensitivity, the patient should be actively leaking at the time of the test.

• The procedure involves intrathecal administration of nonionic iodinated myelographic contrast medium under fluoroscopy, usually by lumbar puncture, occasionally by lateral C1-C2 puncture. The usual adult doses are 10 to 15 mL of nonionic contrast at concentrations not exceeding 300 mg/mL, and up to a maximum dose of 3 g. Strict adherence to safe practice guidelines is mandatory, as inadvertent use of ionic contrast can cause seizures and death.¹⁶

• The contrast column is gravitated cranially by tilting the fluoroscopy table, followed by high-resolution CT of the skull base with coronal reconstructions. Direct coronal CT with the patient in the prone position may be attempted in CSF rhinorrhea.

• Also see Magnetic Resonance Cisternography, below.

Magnetic Resonance Imaging

• Magnetic resonance imaging is clearly superior in evaluating the soft tissues, particularly neoplastic intracranial extension, dural integrity, leptomeningeal spread, and brain invasion.^17,18

• The same applies to perineural spread, orbital extension, cavernous sinus involvement, and extension into fissures and foramina.¹⁹

• Routine brain sequences are insufficient; dedicated skull base protocols are required. These commonly include T1, fat-suppressed T2, or short tau inversion recovery (STIR) in appropriate planes, focused on the ROI and not the whole brain (Tables 5.1 and 5.2).

• Specifying anterior, middle, or posterior skull base, and specifying regions of interest such as orbital apex, cavernous sinus, cerebellopontine angle (CPA), sella, clivus, foramen magnum, etc., will prompt a protocol dedicated to that region by the radiologist. Without this information, studies cannot be tailored to the ROI (Fig. 5.1).

• Gadolinium is required for the detection of intracranial disease, cavernous sinus involvement, and meningeal and perineural spread, usually with fat suppression. Documenting enhanced scans in the three primary orthogonal planes is very helpful in tumor surgery planning.

• Bright signal from fat and marrow may mask a lesion or make enhancement unappreciable. Fat-suppression techniques (Figs. 5.3 and 5.6) are therefore invaluable, but can be a double-edged sword, as they are prone to susceptibility artifacts. Artifacts from dental hardware, clips, and implants are markedly exaggerated on fat-suppressed sequences.

Table 5.1 Common Magnetic Resonance Imaging (MRI) Sequences Used in the Skull Base

Table 5.2 Magnetic Resonance Imaging Appearance of Intracranial Hematoma

Fig. 5.6 Sinonasal mass (esthesioneuroblastoma) with intracranial extension that is best identified on the fat-suppressed postgadolinium coronal T1 image on right (arrow). Fat-suppressed sequences are invaluable in the skull base and orbit.

• Patients tolerate shorter scans better. Long studies are often degraded by patient motion.

This is performed using highly T2-weighted 3D thin-section “myelographic” sequences. These can be reconstructed in multiple planes. Intrathecal injection of gadolinium is also an option, but is not widely used as of yet.20

Cerebrospinal Fluid Flow Studies

Cerebrospinal fluid flow studies traditionally use the phase contrast technique; newer techniques, which use variations of arterial spin labeling, are now available.²¹ These techniques can demonstrate the presence or absence of CSF flow across the foramen magnum in a Chiari malformation, across the aqueduct of Sylvius, within an obstructed ventricular system (such as posterior fossa tumor), across a third-ventriculocisternostomy, and across a fenestrated arachnoid cyst. A series of images are acquired over short periods of time, which are then viewed as a cine sequence.

Magnetic Resonance Tractography

Advanced diffusion tensor imaging methods on 3-tesla or higher systems can map the facial nerve preoperatively in patients with large CPA tumors.²² Similarly, preoperative mapping of CNs II, III, V, and VIII has been achieved and its clinical utility is being examined.²³

Magnetic Resonance Angiography

• Magnetic resonance angiography is usually performed using gadolinium. If vascular imaging is not a high priority and gadolinium is not otherwise indicated, or the patient has an allergy to gadolinium, time of flight (TOF) MRA may be considered.

• If a CTA has already been performed, MRA is usually unnecessary. Exceptions include suspected dissection at the skull base with a negative CTA, or the presence of metal artifact on the CTA from coils.

Magnetic Resonance Venography

• Magnetic resonance venography (MRV) is best achieved using gadolinium, as TOF MRV is prone to artifacts. A common indication is to look for encroachment of a dural sinus by a skull base meningioma.

Digital Subtraction Angiography

• Time-resolved MRA techniques [such as Time-Resolved Imaging of Contrast KineticS (TRICKS), 4D Time-Resolved Angiography using Keyhole (4D TRAK), and Time-resolved angiography WIth Stochastic Trajectories (TWIST)] are a promising tool that can occasionally obviate the need for a catheter angiogram, but they cannot match the spatial or the temporal resolution of conventional angiography as of yet.24,25

• Neurologic complications occur in 1.3% of catheter angiographies, of which some are transient or reversible, but 0.5% of deficits are permanent.²⁶

• Additional complications include hematoma around the puncture site (4.2%), nausea/vomiting and transient hypotension (1.2%), and headache, chest pain, arrhythmia, urticaria, anaphylaxis, circulatory failure, and acute renal failure (each below 1%). Risk of death from all complications is 0.06%, mostly related to neurologic complications.²⁷

• Relative contraindications to catheter cerebral angiography include hypotension, coagulopathy, clinically significant sensitivity to contrast material, renal insufficiency, and congestive heart failure. The patient can be optimized prior to angiography (for example, with premedication for contrast allergy), provided that the benefits outweigh the risks. Planning and prior discussion with the radiologist is essential.28

Nuclear Medicine

• Thallium-201 and technetium-99m (Tc-99m) glucoheptonate in conventional nuclear imaging, as well as sugar analogues such as fluorodeoxyglucose (FDG), radiolabeled amino acids, and cell membrane analogues in positron emission tomography (PET) are available to distinguish viable tumor from edema and necrosis in the skull base.^29,30

• Osteomyelitis imaging in the skull base is best performed with tagged white blood cells, which is commonly achieved with either a Tc-99m or indium-111 (In-111) label rather than gallium-67, as the latter often fails to accumulate in marrow-rich bone such as skull.^31–33

• Shunt studies are frequently performed via injection of 0.1 to 0.5 cc of pyrogen-free Tc-99m diethylenetriamine pentaacetic acid (DTPA) into the reservoir using sterile technique.³⁴ Delay in tracer migration beyond an hour with ambulation and shunt pumping is highly suspicious for obstruction.

• Tumors of the skull base including meningiomas and pituitary adenomas can be assessed by both somatostatin analogues such as In-111 octreotide and norepinephrine analogues such as iodinated meta-iodo-benzyl-guanosine.^35–38

• Phosphorus-32 is an electron emitter with a 14.3-day half-life, usually prepared as a colloid, that has gained acceptance for the control of craniopharyngioma usually via direct transnasal injection, with some authors reporting a 70% tumor control rate at 10 years.^39–42

Plain X-Rays

• Plain radiography has a very limited role in modern skull base imaging, except in the initial evaluation of the craniovertebral junction anomalies, basilar invagination, and ligamentous instability.⁴³

• Lines and measurements can be used in the assessment of basilar invagination (digastric line and bimastoid line on the anteroposterior radiograph, and McRae, Chamberlain, and McGregor lines on the lateral radiograph).⁴⁴ CT or MRI can provide the same information and also demonstrate any mass effect on neural structures.⁴⁵

• Useful information may still be obtained from plain X-rays in a low-resource environment. Lytic or sclerotic bone involvement, focal hyperostosis, enlargement of skull base foramina, vascular grooves (especially in the presence of occluded venous sinuses), opacification, enlargement or destruction of a paranasal sinus, displacement of soft tissue planes, and soft tissue calcifications can provide evidence of an underlying lesion.46

Radiology of Selected Skull Base Pathologies

See boundaries of the skull base fossae in Chapter 2, Fig. 2.3, and in Fig. 5.7. Tables 5.3 and 5.4 summarize the imaging choice criteria and main radiological findings of skull base pathologies. Table 5.5 summarizes anterior skull base (ASB) pathologies.

Meningioma

Related posts:

Hi-Tech Tools in Skull Base Surgery Pediatric Skull Base Functional Anatomy of the Cranial Nerves Skull Base Infections Skull Base Reconstruction Techniques Transcranial Approaches to the Skull Base

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