Dr. Phillips is an Associate Professor in the Departments of Radiology, Neurosurgery, and Otolaryngology­Head and Neck Surgery; and the Director of the Division of Neuroradiology at the University of Virginia Health System, Charlottesville, VA. He is also a member of the editorial board of this journal.

As has happened in all facets of neuroimaging, the introduction and wide acceptance of cross-sectional imaging has changed the way we "do business" in imaging the sinonasal structures. Our imaging choices now are largely computed tomography (CT) or magnetic resonance (MR) imaging to depict benign and malignant sinus pathology. We have moved away from the relatively subtle findings on plain film radiographs to modern high-resolution sinus CT and MR, which has allowed us to visualize the pathology within the sinuses directly, as well as depict the normal anatomy.

Functional endoscopic sinus surgery (FESS) has driven the requirements of sinus imaging in the realm of benign sinus disease. The endoscopic sinus surgeons must have an accurate depiction of the anatomy, normal and abnormal, to perform sinus surgery to maximal effectiveness and safety. Direct coronal CT, or high-quality reformatted coronal CT images, in combination with endoscopy, remains the most effective method of diagnosing sinus disease requiring surgery. ^1-6 I believe it is crucial that radiology departments that perform sinus imaging provide studies that answer the appropriate clinical questions for the sinonasal endoscopist, and the performance of a high-quality coronal sinus screening CT (SSCT) is of paramount importance in the patient population with chronic sinus disease. This overview of paranasal sinus pathology will discuss chronic sinus disease, detail techniques to obtain very high-resolution images of the sinuses, and outline the patient preparation that is important in obtaining the greatest information from these scans. I will also cover the contribution of MRI in the patient with benign or inflammatory sinus disease and will conclude with a discussion of imaging in malignant sinus pathology.

Chronic sinus disease

Chronic sinus inflammatory disease is a large public health problem in the United States. Over-the-counter sinus preparations are a large market, many workdays are lost due to chronic sinus disease, and there is a corresponding large expenditure of health-care dollars. Some understanding of the function of the normal sinonasal mucociliary pathways is necessary to understand chronic sinus inflammatory disease.

The epithelium that lines the sinonasal cavity produces a significant volume of mucous that subsequently coats the epithelium. This epithelium is a ciliated cuboidal epithelium, and the cilia moves in a rhythmic fashion, directing the mucous "blanket" toward the native ostia of the sinuses, and then posterior toward the nasopharynx. Most of this mucous is swallowed. This system has several points of possible derangement in function. The mucous can be abnormal (too small in volume, too thick, etc.), the cilia may be absent or abnormal in function, and the ostia of the sinuses may be abnormal through development or narrowed by previous infection or trauma, etc.

There are larger patterns of sinus drainage to consider. The sinuses tend to drain as two major groups through an amalgam of drainage pathways. The anterior group of sinuses include the frontal, anterior, and middle ethmoids, and maxillary sinuses. These sinuses drain toward the middle meatus (the airspace lateral and superior to the middle turbinate). The final common pathway of the anterior sinus drainage is referred to as the anterior ostiomeatal complex, and consists of the frontal sinus ostium and frontal recess, maxillary sinus ostium, infundibulum, and middle meatus. The posterior group of sinuses include the posterior ethmoids and the sphenoid sinus. These sinuses all drain toward the superior meatus (the airspace lateral and superior to the superior turbinate). The final common pathway of the posterior sinus drainage is the posterior ostiomeatal complex, which consists of the sphenoid ostium, sphenoethmoidal recess, and superior meatus.

The effect of mucociliary drainage in normal patients is to drive the mucous blanket toward the native ostia of the sinuses. This happens regardless of the presence of other surgical ostia, and explains the rationale behind functional endoscopic surgery. The functional component of the name refers to the placement of surgical ostia at the site of the native ostia, thereby facilitating drainage through an "enhanced" normal drainage pathway.

Pre-FESS imaging

Screening sinus CT (SSCT) has become widely accepted as a necessary part of the preoperative evaluation of patients who are to undergo FESS. ^3,6,7 Our technique for SSCT is a minor modification of the original technique described by Zinreich, ⁸ which has gained wide acceptance from our referral otolaryngology colleagues.

Typically, the decision to perform FESS is made prior to an imaging examination, based on an appropriate clinical history of chronic sinusitis, recurrent acute sinusitis, sinonasal polyposis (figure 1), etc., although utilization of this surgical technique in acute sinusitis has gained much importance. ^9,10 With this in mind, the goal of the imaging examination is to provide superior anatomic detail to the surgeon.

The performance of the exam is dependent upon an appropriate premedication regimen to treat reversible mucosal abnormalities and acute infections. Imaging an acutely ill patient will rarely provide good anatomic landmarks, as the demineralization of cartilage or bone adjacent to an active inflammatory process and extensive mucosal abnormalities distort the anatomy that will be encountered following the treatment of the acute infection and the shrinkage of acutely inflamed mucosa. The importance of patient preparation in this group has been discussed previously. ^11-16

At our institution, we continue to use maximal medical therapy to address acute infections, shrink inflamed mucosal membranes, and reduce hyperplastic mucosa. Courses of antibiotics, nasal steroids, oral and nasal decongestants and antihistamines, and nasal saline washes are prescribed by the referring service. Following 2 to 3 weeks of this medical "tune-up," the patient is scheduled for SSCT. It is appropriate to question the patient prior to the exam, specifically about active infections or even symptoms of the common cold, which often give rise to
transient, though significant, sinus abnormalities. ¹⁷

On entering the CT area, the patient is asked to clear his/her nose and another dose of a nasal decongestant is administered. After a few minutes, the patient is asked to clear the nose again, and is positioned on the CT couch. In all patients who can tolerate the position, we have used the prone coronal position. The supine, or "hanging-head" coronal position may be used, but if free fluid is present in the maxillary sinuses, it will tend to move toward the maxillary sinus ostia and obliterate it. With a spiral CT, or multidetector spiral CT, these scans can be performed in just a few minutes. If the patient is simply unable to tolerate any form of coronal posturing, we use very thin-section axial images (1.5 mm or thinner) and generate coronal reformatted images on a workstation through the nasal cavity. These images are adequate but generally inferior to direct coronal acquisitions. This technique is also used to generate coronal images in patients undergoing frameless stereotaxic sinus surgery with the VTI device (Visualization Technology Inc., Wilmington, MA). ¹⁸ Following a lateral scout radiograph, the images are acquired as close to perpendicular to the hard palate as achievable. We perform 1.5-mm sections at 3-mm intervals through the entirety of the sinuses. The high spatial resolution of the thin sections minimizes partial volume effects. The interslice gap is not significant in routine clinical imaging. This technique provides excellent depiction of the regional anatomy, but variants of this technique are acceptable.

Our technique provides adequate information for patients with disease either in the anterior or posterior ostiomeatal unit. Some authors advocate the performance of limited axial scans in patients with disease of the posterior ostiomeatal complex. ¹⁹ Coronal images depict the anatomy at the orbital apex well, ¹⁶ and the reflection of an asymmetric sphenoid septum onto the carotid can be evaluated. If the referring physician wishes to view axial images of the sphenoid sinus in this patient subset, these images can be performed in addition to the coronally acquired images. However, with a good-quality coronal spiral CT data set, the technologists can reprocess the data without an interslice gap, or ideally with some overlap of reconstruction intervals, and perform reformatted axial images on a workstation. These are diagnostic-quality images, obtained without additional radiation, and are certainly adequate for the purpose of depicting the bony septum and the location of vascular structures. No intravenous contrast is needed. The images are obtained with as small a field-of-view as is appropriate for each patient.

The scan technique is quite low (100 mAs), which results in low patient exposure. Lower exposures may be acceptable, and should be evaluated by individual sites. The anatomy depicted on these scans has high inherent contrast and the low technique is quite acceptable. ²⁰ The data is processed with a high-resolution algorithm that sharpens edge definition. Certainly, wide variations in the performance of sinus CT exists, but I believe our established technique is performed easily, acquires high-quality sinus images, is consistent across our patient base, and demonstrates our concern with reducing radiation exposure. Few doubt the necessity for thin-section imaging, and 3 mm or less is believed to be the optimal thickness. ^21,22 We have adopted 1.5 mm, as images between 1 and 2 mm thick seem to be less affected by partial volume effects. We print the images at an extended level and window setting (-200/2000 HU), which provides depiction of the often delicate ethmoid septae, and emphasizes the air-tissue interfaces. Others have described the importance of proper windowing to maximize the depicted information. ^5,3,22,23 We do not routinely print additional images at other level and window settings, although some report this to be necessary. ²² In fact, the techniques we choose for performance of these examinations practically guarantees that, unless the data is reprocessed or additional images are acquired, we obtain poor soft-tissue contrast, and consequently generate low-quality images of the brain, or orbital contents.

Again, this technique is an imaging technique specifically for the patient with chronic sinus disease. It is not an appropriate imaging choice for patients with headaches, a "runny nose," or other nonspecific complaints. The images are adequate for evaluation of various densities within the sinus contents, which can be indicative of fungal sinus disease or concretions within the sinuses. If there is concern for orbital pathology, intracranial pathology, or other extrasinus complications, the SSCT alone is not an appropriate choice of imaging test. These patients will require axial images, intravenous contrast, or likely MRI. Very close attention to detail is critical in assuring the high quality of the studies
consistently.

Bone erosion, abnormalities of the fovea ethmoidalis, cribriform plate, or lamina papyracea, or frank bony destruction are not the imaging picture of sinus inflammatory disease and should be "red flags" for the imaging physician. These findings warrant additional imaging (figure 2), typically with the administration of intravenous contrast. ²⁴ Certain clinical warning signs, such as facial numbness, epistaxis, or orbital or intracranial signs and symptoms are also "red flags," and should increase our vigilance.

Acute sinus inflammatory disease

The typical patient seen with acute sinus inflammatory disease needs no imaging. The clinical diagnosis is made, the patient is treated appropriately, and clinical follow-up dictates the necessity for additional management. In problematic cases, some clinicians rely on plain film radiographs of the sinuses. We have tried to minimize the reliance on these studies, and move to a limited CT study in these patients. Plain film radiographs are at best difficult to interpret, and at worst may confuse the clinical picture. ²⁵ In most active clinical practices, however, the number of patients requiring imaging should be low. The patient with orbital or CNS signs or symptoms is the most important group to consider in the setting of acute inflammatory or infectious disease. In this group, some consideration of the eventual therapeutic approach is important, but the literature seems to be clear that MRI is the most sensitive evaluation we can offer. If CT is readily available, a study including axial and coronal evaluation of the orbit and contiguous intracranial structures is necessary, and the exam should be performed with intravenous contrast. MRI should also include contrast media. The inclusion of MR angiography or MR venography can be determined by the extent of disease visualized, or clinical concerns of venous or arterial involvement. Orbital sequelae of ethmoid or frontal sinus inflammatory disease can include involvement by mucoceles (figure 3), subperiosteal abscesses, orbital phlegmon, venous thromboses, or frank orbital abscesses. Intracranial inflammatory sequelae include involvement by mucoceles, subdural or epidural abscesses, meningitis, parenchymal abscesses, and dural sinus thromboses (figure 4). The benefit of MRI in this setting is its clear superiority in depicting soft tissue. The benefit of CT is its ability to be used as a surgical roadmap when a nasal endoscopic approach is to be used. The endoscopists have moved outside the realm of chronic sinus inflammatory disease, and now routinely approach acute inflammatory disease and its sequelae with primary endoscopy.

Sinus neoplasia

The diagnosis of sinus neoplasia remains a clinical problem for most physicians treating large numbers of patients with sinus complaints. Unfortunately, the signs and symptoms of sinus neoplasia are largely those of sinus inflammatory disease (particularly early in the disease course), and delay in diagnosis is common. This means that the radiologist must keep in mind that any patient imaged for "chronic sinusitis" may instead harbor a neoplasm, and the image interpretation may be key for early diagnosis in the involved patient. There are "red flags" that accompany sinus neoplasia that the radiologist should keep in mind when evaluating sinus CT. These are findings that do not typically accompany benign chronic sinus inflammatory disease, and may point to a tumor. When confronted with these signs, the radiologist may choose to perform the study in a different fashion, administer iodinated contrast, or recommend additional evaluation with MRI. The "red flags" include bony erosion, discernable soft-tissue masses, significant unilateral or focal disease, or the presence of significant bony remodelling. Clinical "red flags" that should also raise suspicions of neoplasm include significant epistaxis, cranial nerve or orbital findings, or facial deformity.

Sinus neoplasia is fortunately an uncommon tumor, representing < 4% of neoplasms of the head and neck. ²⁶ CT and MRI play complementary roles in the evaluation of sinus neoplasia. Clearly, MRI is an important examination in the evaluation of these patients, and few surgeons would contemplate a surgical approach without it. CT continues to play a key role in the evaluation of the bony changes accompanying the tumor and in helping to determine the surgical approach to the tumor. CT studies of these patients should include axial and coronal images with iodinated contrast. We typically screen the suprahyoid neck for nodal disease, but do not routinely examine the entire neck, unless there is clinically palpable cervical disease. If nodal disease is determined to be present in the suprahyoid neck, the exam is continued to evaluate for lower cervical adenopathy. Depiction of the bony anatomy is key, and visualization of the facial skeleton with wide windows must be performed. MRI studies should include both T1- and T2-weighted images of the paranasal sinuses, and include contrast media. We routinely perform the postcontrast images with fat suppression. The choice of axial, coronal, and sagittal images is somewhat dependent upon local opinion, but should typically include all planes. The inclusion of T1- and T2-weighted images increases the specificity of the exam to delineate tumor from contiguous sinus secretions of varying ages. ^27,28 This is a particular problem with CT; there is often no clear distinction from contiguous inflammatory fluid within the sinus and tumor. Enhancement can be of some help, but the distinction on MR is often very clear (figure 5).

Both MRI and CT are of poor diagnostic accuracy in determining the cell type of tumor. ²⁹ The role of the radiologist is to indicate where tumor is and where it is not. A knowledge of the typical pattern of spread of these diseases is important, but beyond the scope of this article. Many of these tumors have rather specific patterns of spread, such as the perineural tumor spread of adenoid cystic carcinoma. The evaluation of the neural structures that could potentially be involved becomes of critical value. Aggressive changes, including bone destruction, are common; but interestingly, many of the tumors also demonstrate chronic bone changes, with sclerosis and occasional smooth remodelling of the bone. The histopathology will be best determined by the pathologist at the time of biopsy or resection. With a skilled Otolaryngology Department, the lesions will most commonly come to the imaging area with a diagnosis already established. Flexible nasal endoscopy and biopsy has already been performed. Benign neoplasms of the nasal cavity are seen occasionally, including the juvenile nasopharyngeal angiofibroma of adolescent males (figure 6), and polyps and papillomas of the nasal cavity in older patients. The lesion seen most commonly is a squamous cell carcinoma of the sinus mucosa. Adenocarcinomas, lympho-
ma, and melanoma are also seen, and the myriad of minor salivary gland neoplasia occur with some frequency. Nasal cavity lesions and paranasal sinus neoplasms can also include many unusual tumors, including the ectodermal derivative tumors of the superior nasal cavity and ethmoid cells, derived from olfactory epithelium. Many of these lesions fall into the category of "little round blue cells," and the diagnosis by light microscopy is difficult; most require immunohistochemistry for diagnosis.

Conclusion

The imaging of sinus pathology, both benign and malignant, has undergone radical changes from the era of plain film radiography. The current application of high-resolution CT and MRI to this anatomic region has benefited patients, reduced unnecessary surgery, and allowed the introduction of a whole new surgical technique into common use. AR