Advances In Skull Base Imaging

Otolaryngol Clin N Am 40 (2007) 439–454

Advances in Skull Base Imaging Colin L.W. Driscoll, MDa,*, John I. Lane, MDb a

Department of Otorhinolaryngology, Mayo Clinic College of Medicine, 200 First Street SW, Rochester, MN 55905, USA b Department of Radiology, Mayo Clinic College of Medicine, 200 First Street S.W., Rochester, MN 55905, USA

Despite being introduced in the 1970s and 1980s, CT and MRI continue to be the primary imaging modalities for the temporal bone and skull base. Although the general concepts and physics remain the same, the images obtained currently are far superior. Augmenting these traditional modalities are nuclear medicine imaging techniques, functional imaging, and the fusion of different techniques [1]. Advances in these areas are expanding our options and illuminating pathology in unique ways. As basic physiologic processes are better understood, new opportunities for novel imaging techniques should arise. This article focuses on the advantages of the latest MRI, CT, and nuclear medicine technology and the impact it has on clinical practice and research. Not only is the image acquisition technology changing but also our methods for viewing the images are evolving. It is becoming increasingly uncommon to have actual printed films, and images are routinely viewed in an electronic format. Although this transition to electronic viewing can be uncomfortable because it is a change, there are some distinct advantages. To a large degree, the success of the transition to an electronic system depends on the specific image viewing software and hardware. An intuitive, easy-to-use program and computer monitors of sufficient size and quality ease the transition. Tools that allow the clinician to quickly identify image series of interest (eg, axial postcontrast), rapidly scroll through images, make measurements, and compare scans from different times are all critical for a successful experience. The ability to scroll up and down through a series of images makes it easier to visualize the three-dimensional image of the pathology in comparison to looking at traditional films. This ability is particularly important because of the large number of images obtained for most examinations. It is not uncommon to have 400 to 1000 images for a single * Corresponding author. E-mail address: [email protected] (C.L.W. Driscoll). 0030-6665/07/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.otc.2007.03.001

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CT scan. Reviewing a stack of printed MRI or CT scans requires identifying the different series and much organizing, which in the electronic environment is unnecessary. The clinician can simply choose the series of interest (eg, T1 postcontrast) for review. Improved labeling by radiologists and user-friendly software are making review an efficient task. CT CT scanning became widely used in the 1970s. Because of the exquisite bony detail it can provide, it is beneficial for assessing temporal bone and skull base lesions. Although the fundamental technology has remained the same, every facet of the process has been improved. Higher quality images are obtained in less time with less radiation exposure. The most recent technologic advance is the advent of multidetector (multislice) scanners [2–4]. The current generation of 64-slice scanners (Fig. 1) allows for rapid volumetric image acquisition with resolution in the range of 0.3 to 0.6 mm. Because the data are acquired for an entire volume in a single pass, there is no need for performing scans in multiple planes (axial, direct coronal, and sagittal). The volumetric data can be reformatted rapidly in any desired plane and without loss of resolution, which is a tremendous improvement that allows visualization of the anatomy in novel ways [5]. Another advantage is that current scanning protocols require approximately 5 to 10 seconds for image acquisition. The incredibly rapid acquisition time greatly reduces motion artifact and often the need for sedation in infants or young children, which reduces not only the risk of the procedure but also the cost to the health care system. Enhanced resolution has improved diagnostic accuracy. Superior semicircular canal dehiscence syndrome (Figs. 2 and 3), otosclerosis (Fig. 4),

Fig. 1. SOMATOM Sensation 64 scanner (Siemens, Munich, Germany) equipped with 32 detectors and two sources. Images through the skull base can be obtained in less than 10 seconds. The volumetric acquisition allows for reconstruction of the data in any plane without loss of resolution.

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Fig. 2. The diagnosis of superior semicircular canal dehiscence syndrome remains difficult and controversial. The clinical history and vestibular testing can suggest the diagnosis. The CT scan can exclude the disease when there is clearly intact bone over the canal, but inadequate resolution may not allow thin bone to be appreciated. These images obtained on the Somoton 64 slice scanner demonstrate the effects of resolution. The coronal images were initially reconstructed from the processed axial data set rather than the raw data, and image quality was not optimal (top). The image looks more typical of a coronal reconstruction from an older CT scanner. The superior canal was considered dehiscent. The raw data were reformatted in the sagittal oblique orientation, however, and there is clearly bone over the entire canal (bottom). Surgical repair is clearly not indicated in this patient.

ossicular chain abnormalities (Fig. 5), labyrinthine ossification, cochlear implant location, and inner ear malformations (Fig. 6) are just a few areas that have benefited from improved resolution [6,7].

CT angiography CT angiography can be performed on multidetector CT scanners as a noninvasive means to obtain information concerning the skull base arterial anatomy [8]. The technique can be useful for identifying an aberrant carotid artery or assessing vascular malformations or the interface between tumors and arterial vasculature. Contrast is given during image acquisition and the image viewing software is used to subtract out bone and other structures to allow isolated views of the arteries. The high quality of these images often obviates the need for a formal angiogram.

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Fig. 3. Superior canal dehiscence syndrome: Sagittal oblique reformatted images demonstrate a normal right canal (top) and a left superior canal dehiscence. The ability to reformat in the plane of the canal reduces the likelihood of an errant diagnosis.

Fig. 4. Axial multidetector CT images obtained on a 64-slice scanner clearly demonstrate an otosclerotic focus classic for fenestral otosclerosis. Portions of the stapes superstructure can be seen. Improved CT resolution allows for preoperative diagnosis of otosclerosis in most patients with this disease when hearing loss has progressed to the point at which they are surgical candidates.

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Fig. 5. Sagittal oblique reformat of a right temporal bone demonstraties a normal endolymphatic duct (black arrow). Typically the diagnosis of enlarged vestibular aqueduct syndrome is made based on axial images. The sagittal oblique image orientation provides a more complete view of the course of the duct compared with standard axial images, particularly the entrance of the duct into the vestibule. An abnormal stapes and incus configuration is appreciated (white arrow), and intraoperatively the patient was noted to have a malformation of the long process of the incus and stapes superstructure.

Office-based CT scanners are becoming increasingly popular and offer the advantage of convenience for the patient, perhaps obviating the need for a return visit. Image quality is not as good as the latest generation standard scanners but is certainly adequate for some clinical situations.

Rotational tomography One limitation of conventional CT is the artifact caused by metallic implants, such as a cochlear implant, stapes prosthesis, or other ossicular prosthesis. Rotational tomography is a technique that reduces the metallic artifact and allows for more precise identification of prosthesis location [9]. It uses equipment typically used for angiography and is widely available. The DynaCT capable angiography unit has a rotating C-arm that acquires images (Fig. 7). Slice thickness and voxel size are 0.4 mm, and images are acquired in approximately 5 to 20 seconds, depending on the number of projections. Because data are volumetric, they can be reformatted and processed in the same manner as data acquired with a multislice CT scanner. Another potential advantage of this technique is a reduced radiation exposured10 to 15 mGy compared with 20 to 60 mGy for conventional CT [9]. Although the latest CT technology demonstrates less metallic artifact, rotational tomography may result in more sharply defined prosthesis location. This clinical technique is being used in some centers after cochlear implantation, but more experience is needed to define its role compared with conventional CT. Both technologies are moving closer together and are not dissimilar beyond the physical structure of the device. Cadaver and temporal bone studies suggest that it may be useful in the evaluation of ossicular prosthesis, but more clinical experience is needed.

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Fig. 6. Multidetector CT demonstrates an enlarged vestibular aqueduct malformation in the sagittal oblique (A) and axial (B) orientations. Some vestibular aqueduct malformations seem to have a large connection from the endolymphatic sac into the inner ear, whereas in other cases the duct narrows considerably just at this junction. Further clinical correlation is needed to ascertain if a wider opening is related to a higher risk of hearing loss, dizziness, or intraoperative cerebrospinal fluid leak, a potential complication of cochlear implantation.

Fig. 7. Axiom Artis dTA with syngo DynaCT (Siemens, Germany, Munich). The rotational arm and flat panel detector are used to acquire images in 10 to 20 seconds. The standard slice thickness is 0.4 mm. After 3 to 5 minutes of processing, the images are available for review and can be processed further or manipulated much like conventional CT.

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MRI MRI advances relevant to imaging the skull base are being made primarily along three fronts: magnet strength, scanning protocols, and coil development. Magnet strength Currently, the most widely available magnet strength is 1.5 T, with many institutions having one or more 3-T magnets (Fig. 8). Higher strength magnets offer several potential advantages, including improved resolution, increased signal-to-noise ratio, and faster scanning times. Higher strength magnets also can create more problems with certain artifacts, so a stronger magnet does not always yield better images (Fig. 9). Higher strength magnets also have the potential to cause tissue injury caused by heating. Current government regulations limit exposure to the radiofrequency energy generated by the MRI scanner. Depending on the scanning protocol, these limits could be reached in high field strength magnets. Scanning protocols From a clinician’s standpoint there seems to be a never-ending number of scanning protocols. We are facing an evolving and expanding list of

Fig. 8. GE Signa 3T MRI scanner (GE Healthcare, Chalfont, St. Giles, UK). Large patients and patients with claustrophobia may not be able to be accommodated or tolerate the narrow bore of high-strength MRI scanner.

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Fig. 9. High-resolution T2 coronal images obtained on a 3T MRI in a patient with narrow internal auditory canals. The images illustrate extensive artifacts (arrows) that render the scan essentially useless. These sorts of artifacts can be reduced greatly by altering scanning sequence parameters. Not only do scanners of different magnet strengths require unique scanning protocols but also each individual magnet can require tweaking of protocols to maximize performance.

protocols (eg, CISS, FIESTA, Flair, FSE T2, diffusion-weighted images, Turbo spin echo). The days of simply sorting through T1, T2, and postcontrast images is gone. It is becoming more difficult to recognize the specific type of scan and recall the particular imaging characteristics of different tissues for the sequence. Increasing the confusion is the fact that some of these protocols are proprietary names. The physics behind the various protocols are exceedingly complex, and at our institution we have several physicists who work full time tweaking protocols and developing coils to maximize performance. It is beyond the scope of this article to delve into the physics. Cunningham and colleagues [10] recently summarized many of the issues related to high strength magnets. Refinement of high-resolution T2 images (Fiesta sequence) has been helpful in evaluating inner ear structure, the status of the auditory nerve, precise location of a small vestibular schwannoma, and delineation of the vasculature in the posterior fossa. Diffusion-weighted images help to distinguish an arachnoid cyst from an epidermoid cyst; however, many diagnostic challenges still remain. For example, we can routinely visualize the spiral lamina but are not yet able to appreciate Reissner’s membrane (Figs. 10–15). Creative manipulation of MRI technology allows for excellent visualization of the arterial and venous vasculature. For example, MRI venography is performed without contrast using a time-of-flight technique. The veins show up as hyperintense, which allows for identification of venous occlusion, malformations, and some dural arteriovenous fistulas. Similar to CT angiography, the phenomenal images of the arterial and venous systems have drastically reduced the number of traditional angiograms performed.

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Fig. 10. Sagittal oblique 3T MRI (Fiesta protocol) image of the right internal auditory canal shows normal cochlear, facial, and vestibular nerve bundles. Confirming the presence of an auditory nerve is a critical step before cochlear implantation.

Coil development Standard head coils provide adequate images through the skull base. Traditional surface coils provide exquisite detail close to the scalp (1–5 cm), but the signal degrades beyond this depth. Combining a surface coil with a standard head coil (hybrid phased array coil) has been shown to improve the spatial resolution in the internal auditory canal (Fig. 16) [11].

Nuclear medicine imaging Nuclear medicine modalities use radioligands that either follow metabolic pathways or interact with specific cellular receptors [12]. This property allows for functional imaging of physiologic processes. Known physiologic characteristics of tumors can be leveraged to identify tumor location and extent of disease. The number of radioligands is expanding, and similar to MRI sequences it is getting complex. Close cooperation with your radiologist helps with determining the best study for the clinical situation. Functional imaging is performed by injecting a radioligand with a short half-life followed by imaging within minutes or hours. The time from injection until scanning and the total number of scans acquired depends on the specific ligand and pathology. Although the radiation exposure is low, this advantage is somewhat offset by the high cost of producing the radioligand compounds, the specific equipment required, and the total

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Fig. 11. An axial MRI image (Fiesta protocol) at the level of the internal auditory canal. Even with the benefit of multiple images through the area, it can be difficultdor impossibledto establish the presence or absence of the auditory nerve. On the right, only a single nerve bundle is visualized, and in this patient with normal facial nerve function there is likely complete agenesis of the auditory nerve (A). Auditory brainstem response testing reveals a pattern consistent with auditory neuropathy, a present cochlear microphonic consistent with intact outer hair cell function, and otherwise absent waveforms. The sagittal oblique reconstructions of the left internal auditory canal seem to show an atretic auditory nerve and normal facial and vestibular nerves (B).

Fig. 12. High-resolution axial T2 (Fiesta protocol) and postgadolinium T1 images in a patient with labyrinthitis demonstrate loss of normal fluid signal in the cochlea and vestibule and variable enhancement throughout the labyrinth. Performing MRI on patients with sudden cochlear and vestibular function may help shed light on the underlying pathophysiology: Is it a neuronitis, hemorrhage, or diffuse inflammatory process?

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Fig. 13. These axial T2 images demonstrate the superb resolution possible. The right enlarged endolymphatic sac and vestibular aqueduct malformation are clearly delineated. The spiral lamina and basilar membrane can routinely be visualized. Reissner’s membrane is the next challenge.

Fig. 14. Axial MRI of patient with enlarged vestibular aqueduct syndrome. Three-dimensional reconstructions can be created easilt but have not yet found much use clinically.

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Fig. 15. Axial MRI scan demonstrates a recurrent epidermoid cyst involving Meckel’s cave with extension into the posterior fossa. The diffusion weighted images help to distinguish cerebrospinal fluid from recurrent disease.

Fig. 16. The hybrid phased array coil is a combination of a standard head coil and surface coils. Advances in coil technology are a major factor in improving temporal bone imaging.

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time that is takes to perform the examination. The only radiation exposure is from the material injected. Because the scanner records radiation emanating from the patient, additional images can be obtained without added risk. Positron emission tomography (PET) is commonly used to evaluate patients with squamous cell carcinoma of the head and neck, identify the primary lesion, assess nodal status, and evaluate for evidence of metastatic disease. The nonspecific fluorodeoxyglucose ([18F] FDG) is most beneficial in assessing malignant tumors, particularly dedifferentiated or rapidly growing tumors. Because all cells that have high metabolic activity take up [18F] FDG, it is a nonspecific, but sensitive, test. Tumors approximately R1 cm are identified on a PET scan, whereas smaller tumors may be missed. Prior surgery or radiation may make identification of persistent or recurrent tumor more difficult. Serial imaging may be needed to differentiate recurrent disease from posttreatment inflammatory changes. In some tumor types, using a more specific compound is beneficial. For example, metaiodobenzylguanidine (MIBG) is an aralkylguanidine similar to norepinephrine and is well suited for imaging paragangliomas (Figs. 17 and 18). Confusingly, there are already two different compounds available, iodine I 131 MIBG and the more recently introduced iodine I 123 MIBG, that are showing higher sensitivity, better imaging quality, and offers a lower radiation exposure [12]. Some paragangliomas have somatostatin receptors. Octreotide, an octapeptide analog of somatostatin tagged with indium In 111, has been used to diagnose and follow these tumors. Schwannomas never express somatostatin receptors, and this property can be helpful in the differential diagnosis of a jugular foramen lesion. Unfortunately, meningiomas also can express somatostatin. Newer octreotide analogs are also being studied. Skull base osteomyelitis is a challenging clinical entity. Establishing the diagnosis, extent of disease, and response to therapy is imperfect. Prior surgery or radiation and host factors (immunocompromised, concurrent disorders, such as a chronic pain syndrome and chronic otitis media) complicate interpretation of imaging. A testament to this difficulty is the fact that we continue to rely heavily on the patient’s subjective complaint of pain as an indicator of disease severity and response to therapy. The technetium Tc 99m polyphosphate scan demonstrates increased isotope accumulation in areas of increased blood flow and new bone formation. This finding is a sensitive and specific indicator of an active inflammatory (eg, bacterial or fungal infection, vasculitis) or neoplastic process. Prior surgery or radiation can diminish the specificity. The test is specific for bone involvement. Another radioligand beneficial for diagnosis of skull base inflammation is gallium citrate. It leaks from the blood vessels into areas of inflammation, including bone or soft tissue. Often the technetium and gallium scans are obtained initially to assist with diagnosis and determine extent of disease and then the gallium scan is repeated

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Fig. 17. Iodine I 123 MIBG whole-body SPECT images show intense tracer uptake in the right side of the neck extending up to the level of the skull base, consistent with an MIBG avid lesion. Physiologic uptake is visualized in the salivary glands, liver, spleen, and gastrointestinal tract. On the right is a collection of coronal ‘‘slices’’ obtained by a rotating gamma detector. Although the areas of uptake can be appreciated, the exact site of the lesion and relationship to surrounding anatomy is obscure. If better localization is required, the images can be fused with a CT.

during therapy to help assess response. Premature cessation of treatment is a common cause of treatment failure. These imaging modalities may underestimate the extent and severity of disease in immunocompromised patients or individuals with poor blood supply to the area. One limitation of a PET scan or other nuclear medicine scan is the lack of definition of surrounding anatomy. Fusing the PET scan data with a CT or MRI allows for more precise localization of the pathology, which is especially helpful for performing directed biopsies. Combined PET/CT scanners greatly facilitate obtaining both types of imaging expeditiously. The field of nuclear medicine continues to evolve rapidly as experience is gained with an ever-increasing number of radioligands. Close cooperation with a radiologist results in the best choice of imaging strategy. Summary Advances in imaging technology are improving our diagnostic accuracy and understanding of some disease processes. Multidetector CT provides

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Fig. 18. Axial and coronal postgadolinium MRI images from the same patient presented in Fig. 17 depict a large enhancing lesion that extends from the right neck up to the skull base with anterior displacement of the carotid artery. This catecholamines-secreting vagal nerve paraganglioma has been resected, but because of the extent of disease there is some risk of recurrence. Identifying tumor recurrence at the skull can be difficult because of postoperative changes and graft material. Because this tumor showed avid uptake of iodine I 123 MIBG, it may be a more sensitive test for follow-up. This test has the added benefit of screening for other potential distant tumors, such as a pheochromocytoma.

higher quality images in less time with reduced radiation exposure. Similarly, current MRI scanners have improved resolution. The field of nuclear medicine is expanding as our understanding of basic physiologic processes improves, providing for novel ways to diagnose and follow different pathologies. Combining various imaging modalities (PET/CT) provides the clinician with better information that enhances diagnosis and treatment planning. Overuse of advanced imaging techniques is a growing concern. It is imperative that we develop evidence-based guidelines to promote the efficient use of imaging techniques and avoid unnecessary testing. References [1] Leong J, Batra P, Citardi M. CT-MR image fusion for the management of skull base lesions. Otolaryngol Head Neck Surg 2006;134:868–76. [2] McCabe K, Rubinstein D. Advances in head and neck imaging. Otolaryngol Clin North Am 2005;38:307–19. [3] Glenn L. Innovations in neuroimaging of skull base pathology. Otolaryngol Clin North Am 2005;38:613–29. [4] McCollough C. Standardization in CT terminology: a physicist’s perspective. Radiology 2006;241:661–2. [5] Karino S, Hayashi N, Aoki S, et al. New method of using reconstructed images for assessment of patency of intracochlear spaces for cochlear implant candidates. Laryngoscope 2004;114:1253–8.

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[6] Chan C, Saunders D, Chong W, et al. Advancement in post-meningitic lateral semicircular canal labyrinthitis ossificans. J Laryngol Otol 2006;23:1–5. [7] Young N, Hughes C, Byrd S, et al. Postmeningitic ossification in pediatric cochlear implantation. Otolaryngol Head Neck Surg 2000;122(2):183–8. [8] Vrtiska T, Fletcher J, McCollough C. State-of-the-art imaging with 64-channel multidetector CT angiography. Perspect Vasc Surg Endovasc Ther 2005;17(1):3–10. [9] Offergeld C, Kromeier J, Ashendorff A, et al. Rotational tomography of the normal and reconstructed middle ear in temporal bones: an experimental study. Eur Arch Otorhinolaryngol 2006;264(4):345–51. [10] Cunningham P, Law M, Schweitzer M, et al. High-field MRI. Orthop Clin North Am 2006; 37:321–9. [11] Kocharian A, Lane J, Bernstein M, et al. Hybrid phased array for improved internal auditory canal imaging at 3.0-T MR. J Magn Reson Imaging 2002;16:300–4. [12] Ilias I, Shulkin B, Pacak K. New functional imaging modalities for chromaffin tumors, neuroblastomas and ganglioneuromas. Trends Endocrinol Metab 2005;16(2):66–72.