Imaging of the Paranasal Sinuses and Nasopharynx
Introduction
Imaging of the paranasal sinuses and nasopharynx is essential in the diagnosis of patients with pathology in these regions. In the last three decades we have witnessed a tremendous advance in the therapy of patients with inflammatory, infectious, and neoplastic diseases of the head and neck. During the same period we have also seen revolutionary changes in imaging technology that have greatly influenced the evolving surgical and medical therapies.
The use of computed tomography (CT) imaging technology is well known for the evaluation of inflammatory disease affecting the paranasal sinuses. A combination of CT and magnetic resonance imaging (MRI), and at times positron emission tomography (PET) imaging, is obtained in the work-up of patients with suspected sinonasal and nasopharyngeal malignancy. There are many confounders in imaging, which occasionally make it difficult to distinguish inflammatory and infectious diseases from neoplasms. However, these three main imaging modalities can be used in a complementary fashion to help reduce this uncertainty and to assist with staging, biopsy, and treatment planning for tumors.
The objective of this chapter is to highlight the application of imaging information with regard to the identification of the typical findings in inflammatory, infectious, and neoplastic disorders of the paranasal sinuses and nasopharynx. Additionally, we will describe some of the pitfalls of diagnosis encountered with imaging. We will also review radiological staging of sinonasal (SN) and nasopharyngeal (NP) malignancies as well as the imaging of postsurgical and post–radiation therapy patients.
The efficacy and quality of information provided by radiological assessment primarily depends on the way in which the images are acquired. For example, depending on the plane of acquisition, the slice thickness, and the presence or absence of contrast, a CT image can vary greatly in its utility in assessing a potential sinonasal or nasopharyngeal tumor patient.1 Described in each section are the current standards for imaging of these regions. Occasionally these vary depending on the clinical requisite (i.e., contrast medium is rarely administered in patients with neutropenic fever being assessed for acute sinusitis), but for the most part they are fairly consistent and reliable.2
Paranasal Sinuses
Imaging Protocols
Computed Tomography
Given its superior bony resolution, CT is typically the best imaging modality for the display of the delicate regional bony anatomy as well as of the mucosal changes in the presence of inflammatory disease. It is also superior in the detection of bony involvement by pathology, in particular periosteal reaction and bony erosion. Particularly when intravenous (IV) contrast is used, CT can also provide excellent soft tissue information, although it is inferior to the soft tissue contrast resolution provided by MRI.
Images are acquired in the axial plane, preferably with 0.5 mm slice thickness. The imaging plane should start above the skull base structures and include the entirety of the paranasal sinuses and nasal cavity, the orbits, and the middle and anterior cranial fossae. Coronal and sagittal reformatted images are reconstructed from the axial imaging acquisition. If derived from the thin-section source images, these reformatted images should demonstrate excellent spatial resolution, essentially indistinguishable from the axial source images.
Coronal images represent the optimal plane for endoscopic correlation. Sagittal planes are very helpful in improving the 3D conceptualization of the regional morphology.3 However, when employing these images for ensuring the accuracy of the spatial orientation in one’s mind, the sagittal images should always be correlated with an additional orthogonal plane, that is, a coronal or axial plane image. The application of multiplanar reconstruction and the use of cross-hairs for localization purposes is especially helpful (Fig. 5.1).
If contrast administration is contemplated, one should first consider doing an MRI examination. Intravenous contrast is typically administered for improved soft tissue resolution. Information provided by MRI is superior for this purpose and avoids the radiation dose received with CT.
Magnetic Resonance Imaging
MRI provides excellent soft tissue contrast resolution. It offers multiplanar capabilities, and does not involve ionizing radiation, which is of particular advantage when imaging children and women of childbearing age.
Typically, a surface coil is used. Axial and coronal planes are acquired, pre- and postcontrast T1-weighted (T1W) (typically with fat saturation), noncontrast T2-weighted (T2W), and noncontrast STIR (a fat-saturated fluid-sensitive sequence) images being the standard protocol for evaluation.
Magnetic resonance angiography (MRA) should be considered when dealing with a vascular pathology or a pathological process affecting or in relationship with the regional vascular structures. This is an excellent noninvasive means to obtain information regarding vascular supply to pathological tissue, as well as to display vascular relationships that may be critical for performing surgery safely.
PET-CT
PET is generally reserved for staging and follow-up of sinonasal carcinomas. However, it can also be useful in the work-up of an unknown primary tumor.4 Although PET has poor spatial resolution, this issue is compensated by the simultaneous acquisition and fusion with CT imaging. Therefore, a PET-CT study is the preferred evaluation.5
Imaging for Initial Diagnosis and Staging
CT is the most readily available sectional imaging modality. It provides the best resolution for bone morphology, and the first “glimpse” into the differential diagnosis of the pathology being dealt with.6 The evaluation should focus on the integrity of the bony architecture, and the presence of bone erosion within the sinus morphology as well as the bony perimeter defining the borders of the nasal cavity and paranasal sinuses. The initial questions to be answered with CT are: “Is there a soft tissue pathology?” “Where, exactly, is it?” and “What is its influence on the bony morphology?” “Is there extension beyond the boundaries of the nasal cavity and paranasal sinuses?”7
Important areas assessed by CT in defining the extent of pathological invasion include the cribriform plate and planum sphenoidale; the fovea ethmoidalis; the lamina papyracea; the frontal, maxillary, and sphenoid sinus borders; and the internal bony framework of the sinuses vulnerable to direct invasion and/or destruction. Should the soft tissue and/or bony pathology penetrate the paranasal sinus boundaries, the final step is to assess the extent of “invasion”: into the orbits, or into the intracranial compartment through skull base foramina, and other possible pathways of spread including the pterygomaxillary fissure and pterygopalatine fossa.
CT is also the modality of choice to initially assess bony lesions, such as benign fibro-osseous lesions as well as primary malignant bone tumors. Often the specific diagnosis can be determined by CT.
Having identified the pathology by the CT examination, its etiology as well as the precise extent may still be in question. This is where MRI can provide significant additional help. The soft tissue resolution provided by MRI may be able to further delineate the etiology of the pathological process. It may also help distinguish between the various inflammatory pathologies, and may be able to help narrow the diagnosis to a specific neoplastic entity. The use of contrast and fat suppression can significantly improve the accuracy of assessing subtler perineural spread of tumor, intraorbital invasion, and invasion of intracranial structures.2
In initial diagnosis, PET-CT has little utility except in the case of an unknown primary neoplasm: patients with cervical lymph node metastases in whom a primary tumor cannot be detected on physical examination or other conventional imaging. It is uncommon for sinonasal malignancy to be detected in such a way. Most of these neoplasms have a pharyngeal, hypopharyngeal, laryngeal, or tonsillar origin. However, in a small percentage of cases these tumors are sinonasal, and PET-CT has a high sensitivity for detecting such lesions. The detection of residual or recurrent neoplasm in the postoperative patient can be significantly aided by this modality.
Pathology
Nonneoplastic
Inflammation and Infection
Sinus inflammatory disease typically begins with obstruction of a sinus. This is followed by a mucosal inflammation and fluid exudation. Eventually the fluid is resorbed. However, if the obstruction persists, this mucosal inflammation can continue, at times completely opacifying the sinus.8
On CT, this process is demonstrated by uniform peripheral mucosal thickening and an air–fluid level. The obstruction of a sinus may also be apparent on CT. Following contrast administration, there should be uniform enhancement of the mucosa, which is in general uninterrupted.9 With repeated infections, the inflammatory process may invade the bony framework, producing a uniform bony thickening that usually affects the perimeter of the sinuses and is referred to as “osteitis”10 (Fig. 5.2).
On MRI, the combination of edema and retained secretions associated with inflammatory disease is typically hyperintense on T2W and iso/hypointense on T1W imaging due to the high water content.11 T2W and T1W postcontrast images will demonstrate the uniform appearance of the mucosa, an important “hallmark” of inflammatory disease (Fig. 5.3).
Fungal inflammatory disease, due to its unique qualities including eosinophilic mucus and high concentration of metabolized ferromagnetic elements and calcium, has a totally different appearance, demonstrating low signal on T2W imaging.12 Nevertheless, with this modality, the uniformly “thickened” sinus mucosa also remains intact. Inspissated secretions can also vary in their appearance, demonstrating intermediate signal on T1W and T2W imaging.13 However, on contrast-enhanced MRI, sinuses containing inspissated secretions will still demonstrate intact mucosa with uniform enhancement.14
Neoplasms, on the other hand, tend to be low to intermediate in signal on these sequences. Furthermore, neoplasms penetrate the typically smooth and uniform mucosal thickening seen with inflammatory disease, as they extend beyond the sinus outline (Table 5.1).
Aggressive Infections
Aggressive infections, specifically invasive fungal (Mucor or Aspergillus being the more commonly encountered) diseases, can have a tumorlike appearance (Fig. 5.4). When an aggressive-appearing soft tissue process is identified, it is first important to determine the patient’s history and their immune status as immunocompetent patients are less likely to develop these infections.15 These can be associated with bone destruction, but as the fungi tend to extend along the vasculature, there may be little associated bony destruction or even mucosal inflammation.16 Fungal infection also can have a hyperdense appearance on CT,17 and very low signal on T2W imaging, possibly due to the ferromagnetic elements and calcium within the fungal hyphae.18
Granulomatous Diseases
Granulomatous diseases, such as Wegener’s granulomatosis and sarcoidosis, may also have a tumorlike appearance.19,20 The invasive soft tissue pathology and bony erosive changes can mimic the appearance of a neoplastic process, or the appearance of a patient having had extensive surgical procedures, with persistent inflammatory disease (Fig. 5.5). The presence of associated inflammatory disease within the orbits is not uncommon.
Sinonasal Organized Hematoma
Similarly, sinonasal organized hematoma represents an accumulation of blood products in the sinonasal area, primarily in the maxillary sinus.21 The etiology is unclear but it is believed to be associated with trauma, surgery, bleeding diathesis, and/or a hemorrhagic lesion in the nasal cavity/sinuses. The CT evaluation reveals an expansile lesion eroding bone and penetrating through the sinonasal borders, with the appearance of an aggressive neoplasm. On MRI the lesion is well defined and the mucosa at the periphery is intact and enhances uniformly. Central low-intensity signal changes are characteristic of blood products, and therefore indicative of a peripheral inflammatory process (Fig. 5.6).
Neoplastic
Benign Bony Tumors
Benign bony lesions are the most common neoplasm of the sinonasal region, of which the most common are osteomas. These are well defined and in general homogeneous bone density lesions. They are most commonly found within or associated with the frontal and ethmoid sinuses and less commonly within the sphenoid and maxillary sinuses. These lesions are best displayed on coronal bone-windowed CT images. However, for additional help in establishing their bony adherence, axial images may also be helpful. MRI is rarely helpful in the assessment of these lesions.
Benign fibro-osseous masses are also not uncommonly encountered.22 Ossifying fibromas are well-demarcated lesions with a central fibrous component that is peripherally surrounded by an osseous rim. They are more commonly found in the mandible and maxilla but can also be present in the sinuses.23 Fibrous dysplasia appears as a poorly defined asymmetric expansion of the maxillofacial bony morphology with a classic “ground glass” appearance on CT, which may be homogeneous or have cystic areas (Fig. 5.7). In these cases, special attention is needed with respect to the ostia of the sinuses, as there is a tendency for ostial closure and the creation of mucoceles.24 Other fibro-osseous lesions are much less common. These lesions are best assessed with CT, and often a specific diagnosis can be rendered by CT alone. Imaging evaluation is also essential in determining their site of origin, and their connection/adherence to the bony framework of the paranasal sinuses. This specific determination will have an important effect on the surgery and the postsurgical outcome. Again, MRI is rarely helpful in these cases, and can confound the diagnosis as they can have a variable and sometimes confusing appearance on MRI.
Benign Soft Tissue Tumors
Benign soft tissue tumors of the nose and paranasal sinuses are somewhat less common entities but include papillomas and adenomas, as well as vascular lesions including hemangiomas and angiofibroma.25
Inverted Papillomas
Benign tumors can have a relatively distinctive appearance on imaging and are best assessed with MRI.26 For example, inverted papillomas on postcontrast T1W MRI may demonstrate a unique “serpiginous” enhancing pattern similar to the appearance of the superficial brain gyral pattern (Fig. 5.8). These lesions originate most commonly from the “transitional” epithelium of the lateral nasal wall, but may also arise along the epithelium of the middle turbinate.27 If they originate from the lateral nasal wall, they may erode through the lamina papyracea into the orbit. However, if they arise from the middle turbinate, they extend to the skull base and may be indistinguishable from an esthesioneuroblastoma (olfactory neuroblastoma).28,29
Angiofibroma
The angiofibroma is another lesion that, while histologically benign, can behave in an aggressive fashion. It is an intensely enhancing lesion that originates at the sphenopalatine foramen and can extend into the nasal cavity, the nasopharynx, and the pterygopalatine fossa, and into the intracranial compartment via the foramen rotundum or the vidian canal. It may also extend into the sphenoid, maxillary, and ethmoid sinuses, and may invade the masticator space and the inferior orbital fissure. CT will demonstrate bone destruction, in particular erosion of the upper part of the medial pterygoid plate. MRI with contrast best displays the extent of the lesion (Fig. 5.9). Angiography may be performed to establish the blood supply to the mass and plan for presurgical embolization.30 The blood supply may arise from the external as well as the internal carotid arteries and, even though the mass may appear unilateral, it may also receive blood supply from the contralateral carotid arteries.
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