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Imaging of frontal sinus disease
CURRENT CONCEPTS IN THE SURGICAL MANAGEMENT OF FRONTAL SINUS DISEASE
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IMAGING OF FRONTAL SINUS DISEASE Concepts, Interpretation, and Technology Vijay M. Rao, MD, Dinesh Sharma, MD, and Ashok Madan, MD
In patients with suspected sinonasal pathologic conditions, a routine clinical examination, or even detailed endoscopic examination, may not adequately detect sinus abnormalities. Imaging of the paranasal sinuses is therefore an integral part of the patient evaluation to gather the information that is essential for clinical management. Cross-sectional imaging provides a detailed display of the regional anatomy and variations that may predispose to a blockage of the sinus drainage. Imaging is also crucial in differentiating inflammatory from neoplastic diseases and in defining the extent of the disease process.
IMAGING MODALITIES
Conventional Radiography Conventional radiographic films were once the mainstay of diagnosis of sinus disease but now have been replaced by high-resolution computed tomography (CT) imaging for the evaluation of sinus inflammatory disease. Plain radiographic films do not allow adequate evaluation of the disease of the frontal sinuses and the drainage pathways because of overlapping structures? ~~~
From the Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
OTOLARYNGOLOGIC CLINICS OF NORTH AMERICA VOLUME 34 NUMBER 1 FEBRUARY 2001
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Computed Tomography Computed tomography is the modality of choice for examination of the paranasal sinuses, including the frontal sinuses, because of its ability to demonstrate the intricate bony anatomy as well as the soft tissue component of the disease process. Computed tomography is a prerequisite for patients undergoing sinus surgery, because it provides a road map for the surgeons. The optimal technique consists of acquiring direct contiguous sections 3-mm thick in the coronal plane. The patient is placed in the prone position with the neck hyperextended to let the secretions drain away from the region of the anterior ostiomeatal complex. Coronal sections are obtained perpendicular to the hard palate. When a patient cannot tolerate coronal positioning, thin-section contiguous axial scans are obtained and images are reformatted in the coronal plane. Sagittal reformations are often helpful in better defining the drainage pathway of the frontal sinuses and the frontal recess. The authors routinely complete the examination by obtaining images in the axial plane for preoperative studies, which allow evaluation of the anterior and posterior walls of the frontal sinuses. At the Thomas JeffersonUniversity Hospital, bone and soft tissues are visualized separately by using the appropriate settings. Some investigators advocate the use of a single set of images pictured at an intermediate window level and width settings to study both bone and soft tissue structure.’ Intravenous contrast is not routinely used for the evaluation of inflammatory sinus disease. Indications for administration of intravenous contrast include suspected complications of inflammatory disease such as abscess, thrombosis and intracranial or intraorbital spread of infection, and any benign or malignant tumor. Computed Tomographic Scanning for Image-Guided Surgery Otolaryngologists and head and neck surgeons are increasingly using interactive image-guided computer-assisted surgery. At the Thomas Jefferson Hospital, The InstaTrak System (Visualization Technology, Inc., Woburn, MA) is used. During CT examination, the patient wears a headset to which an electromagnetic transmitter is attached. Axial contiguous CT images 1-mm thick are obtained extending from the maxillary alveolar ridge to the top of the frontal sinuses. Alternatively, if CT images are obtained in a spiral or helical scanner, 3-mm axial contiguous sections may be obtained. The volumetric CT data are then reconstructed to produce I-mm-thick axial images, thereby reducing the scanning time. This information is transferred through the system network to the InstaTrak System in the operating room. At the time of endoscopicsinus surgery, the same headset is placed on the patient exactly as it was during the CT scan. The images are calibrated electromagneticallyby a transmitter on the headset and a receiver attached to the InstaTrak probe. The surgeon is able to view the position of the endoscopic probe relative to the CT images in three-dimensional display on the
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computer screen in the operating room during surgery. Such technologic advances are important in minimizing the risk of operative complications in endoscopic sinus surgery.
Magnetic Resonance Imaging Magnetic resonance (MR) imaging is complementary to CT imaging for evaluating intracranial complications of frontal sinus inflammation and in determining the extent of sinus tumors. MR imaging is superior to CT scanning in differentiating tumor from the adjacent inflammatory sinus disease. Computed tomography, on the other hand, is superior to MR imaging in demonstrating subtle bone erosions. MR imaging has severe limitations in demonstrating the normal regional anatomy of the frontal sinus and the frontal recess. In practice, most patients with suspected sinus pathologic conditions undergo unenhanced CT imaging. If complications of sinusitis are suspected or a neoplasm is considered, the next study of choice is contrastenhanced MR imaging. Contrast-enhanced CT scanning is an alternative study if there is an absolute contraindication to MR imaging. Other advantages of MR imaging over CT scanning include its multiplanar capability, superior soft tissue contrast, and lack of ionizing radiation. The standard protocol for MR imaging of paranasal sinuses includes T1-weighted images in the sagittal, axial, and coronal planes and T2-weighted images in the axial plane. Following administration of intravenous gadolinium, T1-weighted images with fat saturation are obtained in the axial and coronal planes. NORMAL ANATOMY The frontal sinuses, air-filled cavities within the frontal bone, arise as small invaginations from an anterior ethmoid cell tract. The frontal sinuses are divided by a bony septum into asymmetric cavities, each with its own drainage system. The anatomy of the frontal sinuses and the drainage pathways is complex but is elegantly displayed by coronal CT images.&11,17,21,25,28 The frontal, anterior ethmoid, and maxillary sinuses drain into the middle meatus, which lies superolateral to the middle turbinate. This region is called the anterior ostiomeatal complex and incorporates the middle meatus, middle turbinate, uncinate process, ethmoid infundibulum, maxillary ostium, ethmoid bulla, and the frontal recess. The frontal sinuses drain into the anterior aspect of the middle meatus by an internal channel which is an integral part of the anterior ethmoid complex and is called the frontal recess (Fig. 1). This channel was previously referred to as the nasofvontal duct, a term no longer used because there is no well-defined tubular structure. The frontal recess has been described as the most complicated structure in the anterior ethmoid complex.” The frontal sinus ostium is located at the most anterosuperior part of the frontal recess. The
Figure 1. Normal anatomy. Coronal CT scan showing the left frontal sinus ostium. The frontal recess drains into the middle meatus (arrows).
frontal recess is bounded medially by the lateral surface of the most anterior portion of the middle turbinate. The lateral boundary of the frontal recess is usually formed by the anterosuperior extension of the lamina papyracea.*' In the sagittal plane, the frontal sinus and the frontal recess have an hourglass configuration. Several ethmoid and uncinate variants may cause anatomic narrowing of the frontal recess and predispose to obstruction. Agger nasi cells are the most anterior ethmoid air cells that are located anterior, lateral, and inferior to the frontal recess and are visualized best on coronal CT scans (Fig. 2). They range from a single cell to a group of as many as seven air cells and may develop from the frontal recess. A pneumatized middle turbinate (concha bullosa) may be in direct continuity with the frontal recess or may encroach on the frontal recess. A pneumatized crista galli may also develop and may drain into the frontal recess. The ethmoid bulla may be large (giant bulla ethmoidalis) and expand into the middle meatus with anatomic compromise of the middle meatus. There is a marked variation in the configuration and attachment of the uncinate process, which is responsible for the variable drainage patterns of the frontal recess into the middle meatus.21The frontal recess may open into the middle meatus medial to the uncinate or into the ethmoid infundibulum. A pneumatized uncinate process (bulla uncinate) may also cause anatomic compromise of the frontal recess. The radiologist must pay attention to these anatomic variants on coronal CT images because they may interfere with the normal air flow and mucociliary clearance of the frontal sinuses. Mucociliary clearance occurs in a predetermined fashion in each of the sinuses. In the frontal sinus, mucus is propelled up along the medial walls, laterally along the roof, down along the lateral wall, and medially along the floor toward the region of the frontal sinus ostium. At this juncture, mucus partly extends down to
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Figure 2. Obstructed frontal recess. Coronal CT image revealing opacification of the left frontal recess (FR). Note that the concha bullosa (CB) is in direct contiguity with the frontal recess. An agger nasi air cell (A) is noted along the lateral aspect of the frontal recess.
the frontal recess to drain into the middle meatus, and the rest is propelled up to recirculate.21
INFLAMMATORY LESIONS Rhinosinusitis may be classified as acute, subacute, chronic; recurrent acute, or acute excacerbation of chronic rhinosinusitis. The imaging findings are nonspecific and should be correlated with the clinical history and physical examination. If there is no history of trauma, lavage, or reason for spontaneous hemorrhage, an air-fluid level in the frontal sinus or any other sinus usually correlates with acute sinusitis. Mucosal thickening in any sinus is often misinterpreted as chronic sinusitis. Components of mucosal thickening that are acute rather than subacute or chronic cannot be differentiated by imaging alone. There is no correlation between the degree of mucosal thickening and the clinical symptoms. Minimal mucosal thickening at the frontal recess or the ethmoid infundibulum is more likely to cause obstruction and sinusitis than moderate mucosal thickening elsewhere in the sinuses. On imaging studies, it is better to describe the degree of mucosal thickening as mild (< 5 mm), moderate (5-10 mm), or severe (> 10 mm), and the location. Sclerosisof the sinus walls is a good indicator of underlying chronic sinusitis; however, the mucosal thickening may still be acute, subacute, or chronic. The attenuation of sinus secretions on CT imaging varies with the degree of hydration. Hydrated secretions show low attenuation, whereas
desiccated secretions show increased attenuation. Areas of increased attenuation may also be seen with hemorrhage or extramucosal fungal disease. The MR imaging signal characteristics of sinus inflammatory disease vary depending on the state of hydration of the mucosa and the content of the secretions. Sinonasal secretions normally reveal low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, relative to muscle, because of the dominant water content (95% water, 5% protein) of the secretions. In a chronically obstructed sinus, however, the gradual resorption of free water results in an increased protein content. As the protein content rises (up to 25%), the signal intensity on T1weighted images increases, resulting in high signal intensity on both T1and T2-weighted images. With progressive water resorption and increasing protein concentration, a significant cross-linking between protein macromolecules occurs, resulting in increased viscosity. This increased viscosity ultimately results in a complete loss of MR signal intensity on both T1- and T2-weighted images and mimics the appearance of an aerated s i n ~ s . ~ , ’ ~ This loss of signal intensity is a major pitfall in MR imaging of sinus inflammatory disease: the extent of disease may be grossly underestimated in chronic sinusitis, sinonasal polyposis, and mucocele. SINONASAL POLYPOSE Sinus polyps form as a result of folding and hypertrophy of the mucosa with submucosal accumulation of fluid. Sinonasal polyps have been associated with allergies, vasomotor rhinitis, infections, rhinosinusitis, cystic fibrosis, diabetes mellitus, aspirin intolerance, and nickel exposure. The imaging appearance of sinonasal polyps is usually quite dramatic. Nasal polyps normally expand the nasal fossa with softtissue masses. The sinuses reveal varying degrees of opacification and expansion. Although polyps are benign, they may cause significant expansion and bone erosion, which may be confused with aggressive fungal sinusitis or carcinoma of the sinus by inexperienced examiners. Bone erosion caused by polyps in the frontal sinus affects the posterior wall more frequently than the anterior wall because the posterior wall is thinner. Computed tomographic imaging usually demonstrates central high density surrounded by a peripheral rim of low density. This appearance is characteristic of sinus inflammatory disease and is not seen with sinonasal tumors.2O Sinonasal polyps show a mixed signal intensity pattern, on both T1- and T2-weighted MR images, depending on the water content of the polyps, mucosal hypertrophy, and the sinonasal secretions. Polyps show a rippled enhancement pattern unlike the diffuse enhancement seen with tumors (Fig. 3).17 FUNGAL SINUSITIS Fungal sinusitis may be categorized as invasive and noninvasive; the latter category includes allergic fungal sinusitis and fungal ball. Noninvasive fungal disease occurs in immunocompetent individuals, resulting
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Figure 3. Frontal sinus polyps in a patient with diffuse chronic sinonasal polyposis. A, Axial T1-weighted MR image showing opacified frontal sinuses. No erosion of the cortical margins or expansion is noted. The soft tissue in the sinuses is hypointense relative to brain. 6, Coronal contrast enhanced T I -weighted image with fat suppression revealingcircumscribed peripheral, well-defined hypointensepolyps with enhancement opacifying the frontal sinuses and the nasal fossa bilaterally. Note that the nasal turbinates normally enhance.
from colonization of the paranasal sinuses, and its course depends on the local host conditions. The most frequently involved sinuses are the maxillary and the anterior ethmoid. Frontal sinuses are the least commonly involved. Computed tomographic imaging usually reveals a soft tissue mass with calcification and is not associated with bone erosion. MR imaging characteristically shows intermediate signal intensity on T1-weighted images and markedly low signal intensity on T2-weighted images, relative to the surrounding tissue. Allergic fungal sinusitis, another form of noninvasive disease, is most commonly encountered in children and young immunocompetent people. Imaging findings include unilateral or bilateral pansinus opacification and expansion, including the frontal sinuses, and mimic the appearance of chronic sinonasal polyposis. Computed tomographic imaging usually shows a central area of hyperdensity in each sinus, surrounded by a peripheral rim of low attenuation. MR imaging reveals variable or mixed signal intensity on T1-weighted images but marked central hypointensity
with only a peripheral rim of high signal intensity on T2-weighted images (Fig. 4).Explanations that have been postulated for the dramatic hypointensity of the MR imaging signal associated with fungal sinusitis are (1)the markedly increased viscosity of the sinus secretions encountered in allergic fungal sinusitis; (2) the intertwined fungal hyphae in fungal ball that behave like solid material; and (3) the dephasing of spins caused
Figure 4. Allergic fungal sinusitis. A 15-year-old presented with nasal obstruction. A, Noncontrast enhanced coronal CT scan revealing marked expansion of the nasal cavity, ethmoid sinus, and frontal sinuses with central areas of hyperdensity. This appearance is characteristic of inflammatory disease and differentiates it from a tumor. €3, Coronal TP-weighted MR image showing complete lack of signal in the corresponding regions of hyperdensitymimicking an aerated sinus. C, Contrast enhanced coronal T I -weighted MR image revealing mild enhancement of the inflammatory mass.
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by ferromagnetic elements (iron and manganese) that are present on the fungal hyphae.17,27 Invasive fungal sinusitis is a potentially life-threateningcondition that is usually seen in immunocompromised patients. This fulminant form of fungal infection results in bone erosion and vascular invasion, leading to vascular occlusion, ischemia, and infarction. Ethmoid sinuses are most frequently affected, and the disease process rapidly progresses to involve the orbital apex and the cavernous sinus. Isolated involvement of frontal sinus is rare (Fig. 5). FRONTAL MUCOCELE The term rnucocele refers to an expanded, mucoid-filled sinus, resulting from ostial obstruction caused by inflammatory scar, trauma, or tumor. The frontal sinuses are most frequently involved (65%),followed by 23 Computed tomography demonstrates an the ethmoid sinus (25%).9*'7,
Figure 5. lnvasive frontal sinus mucormycosis. A, Contrast-enhancedCT scan in a diabetic patientwith biopsy-provenmucormycosisshowing an opacified mucoid-filledfrontal sinus with an air-fluid level. Note the enhancing mass along the posterior aspect of the left frontal sinus associatedwith bone erosion and epidural extension (arrow).Abnormal enhancement caused by direct spread is noted along the anterior falx (asterisks). B, Axial T1-weighted MR image showing an opacified left sinus that is largely hypointense to the brain. Along the posterior aspect of the sinus, a focal lesion is seen with signal intensity similar to the brain. Note the absence of the black line indicating erosion of the cortical bone and epidural spread (arrow). The soft tissue inflammatory mass extends into the anterior cranial fossa along the falx in the midline. C, Contrast enhanced T1-weighted axial MR image reveals intensity enhancing fungal inflammatorymass with intracranialspread. There is peripheral mucosal enhancement. The remainder of the sinus shows inflammatory exudate that is nonenhancing.
expanded sinus with gradual thinning and erosion of the bony margins. In a frontal sinus mucocele, the posterior wall is particularly prone to erosion because of its inherent thinness (Fig. 6). The density of the sinus contents of a mucocele may be low or mixed relative to that of muscle. MR imaging demonstrates variable signal intensities on both T1- and T2-weighted images, depending on the state of hydration, protein content, and viscosity of the contents of the mucocele (see Fig. 5). The two most frequently observed patterns are (1) moderate-to-marked high signal intensity on both T1- and T2-weightedimages, and (2)moderate-to-markedlow signal intensity on both T1- and T2-weighted images. Contrast-enhanced MR imaging is useful in differentiating mucoceles from sinonasal tumors.9,2o Mucoceles characteristicallyreveal a thin peripheral linear enhancement with central low signal intensity on T1-weightedimages. Sinonasaltumors, on the other hand, demonstrate diffuse enhancement? COMPLICATIONS OF FRONTAL SINUSITIS
Orbital complications of frontal sinusitis usually result from direct extension of sinus infection through the wall of the bony orbit and include preseptal cellulitis, postseptal or orbital cellulitis, subperiosteal abscess, and orbital abscess. The most common source of orbital complications is ethmoidal sinus infection. Intracranial complicationsare uncommon but are usually the result of '* Infectionspreads from the frontal sinusby the emissary frontal sin~sitis.'~, veins between the posterior sinus mucosa and the meninges. Alternate routes of spread include direct extension through congenital or acquired bony dehiscences, extension along pre-existing anatomic neurovascular pathways, or by hematogenous spread as part of septicemia. MR imaging is superior to CT imaging in detecting intracranial complications of sinusitis. Contrast-enhanced MR imaging often reveals meningeal enhancement in patients with sinusitis, but this enhancement is not an indication for craniotomy. Epidural abscess and subdural empyema
Figure 6. Mucocele. Axial CT scan revealing increased attenuation of the left frontal sinus contents. Note marked expansion and smooth erosion of the posterior wall of the frontal sinus (arrows).
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commonly occur as a complication of frontal sinusitis and involve the anterior cranial fossa because it is directly contiguous with the posterior wall of the frontal sinus. Cerebral abscess is a rare complication of the frontal sinusitis and is caused by venous thrombophlebitis. The CT and MR imaging features of cerebral abscess vary depending on the stage of the abscess formation. A ring-enhancing lesion with central pus and surrounding edema represents a frank abscess and is readily diagnosed by CT or MR imaging. POSTOPERATIVE FRONTAL SINUS Imaging studies are made after endoscopic sinus surgery only if complications are suspected or if clinical symptoms persist without improvement. A careful scrutiny of the postoperative CT scan in the coronal plane demonstrates the extent of surgery performed. It is important to look for formation of adhesions or synechiae in the region of the frontal recess Computed tomographic which result in obstruction of the sinus (Fig. 7).6,11 images may reveal lateralization of the middle turbinate resulting in obstruction of the middle meatus or frontal drainage. Computed tomography may detect incomplete exenteration of agger nasi cells compromising the region of the frontal recess. Osteoplastic frontal sinus flaps with sinus obliteration are performed in patients with chronic inflammatory disease, mucocele, trauma, and tumors. In this procedure, a bone flap is cut along the anterior aspect of the frontal sinus wall. The mucosal lining is removed completely, and the frontal sinus is obliterated by packing material such as fat, bone, and
Figure 7. Postendoscopic sinus surgery. A, Coronal CT scan showing an obstructed right frontal sinus. The remnant of right middle turbinate (arrow) is lateralized and is adherent to the remaining agger nasi air cell resulting in complete occlusion of right frontal recess. On the left, the region of the frontal recess is markedly wide and open. B, Coronal T2-weighted MR scan in the same patient demonstrating narrowing and obstruction of the right frontal recess. The right middle turbinate remnant, seen as a bright structure, is adherent to the remaining agger nasi cell that is aerated.
hydroxyapatite cement or methyl methacrylate.4,13, l5 The post-operative complications include recurrent sinusitis, infection of the graft, osteomyelitis of the osteoplastic flap, cellulitis, and mucocele.2,10 Postoperative imaging (CT scanning or MR imaging) is often obtained to assess these complicationsin patients with persistent symptoms following surgery. Interpretation of either of these studies is difficult because of the normally expected changes from postoperative granulation tissue and scar. Postoperative CT scans demonstrate an airless sinus cavity with smooth margins of the bone flap. If the margins of the bone flap are irregular and reveal bone resorption or mixed density with adjacent soft tissue swelling, underlying osteomyelitis is suspected. Postoperative MR imaging shows an airless sinus. The signal intensity of the soft tissue in the frontal sinus can vary, depending on the nature of the packing material that is used. For example, patients who have undergone sinus obliteration with fat autograft show hyperintense signal on T1-weighted images and hypointense signal on TZweighted images. The signal intensity on T1-weighted images decreases gradually as the fat is replaced by soft tissue (Fig. 8). Catalan0 et a1 reported that hyperintensity of the autograft on T2weighted images reflected inflammation.2Loevner et a1 reported that MR imaging findings after osteoplastic frontal sinus flap placement were
Figure 8. Osteoplastic frontal sinus flap. Sixty-year-old man 4 months after surgery for treatment of chronic frontal sinusitis. A, Axial T1-weighted MR image shows high signal intensity fat occupying the large portion of the frontal sinus cavity. Note a small portion of the fat has been replaced by soft tissue in the center (solid arrows) and along the lateral portion of the right frontal sinus (open arrow). B, Axial T2-weighted MR image shows the hyperintensity of the soft tissue replacing fat (arrows). C, Contrast enhanced axial T1-weighted high signal intensity fat. Note that the focal areas of soft tissue showed mild enhancement (arrows).This enhancement can be visualized more easily if the image technique included fat saturation.
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nonspecific and were of limited usefulness in differentiating postoperative scar formation from recurrent sinusitis. In their studylo hyperintensity within the frontal sinuses on T2-weighted images was noted in symptomatic and asymptomatic postoperative patients. Contrast enhancement was seen along the periphery and in any region where fat had been replaced by soft tissue. FRONTAL SINUS FRACTURES In general, patients with frontal sinus fractures have significant associated facial and skull fractures or intracranial injury. Fractures can be classified as linear, comminuted, depressed, nondepressed, displaced, or undisplaced and may involve the anterior or the posterior wall, or both. The facial injuries that are frequently associated with frontal sinus fractures include periorbital and orbital hematoma; the intracranial injuries include frontal lobe contusions, pneumocephalus, and subdural and subarachnoid hemorrhage. It has been observed that fractures of the posterior wall of the frontal sinus are the best predictor of the severity of trauma and are associated with more complications and a worse clinical outcome than fractures that involve only the anterior wall.I4Although a high incidence of brain injury is associated with fractures of the posterior wall of the frontal sinus, brain injury may not be apparent on the initial CT scan of the head. Patients should therefore undergo follow-up CT scans or MR imaging to exclude brain injury if the initial scans were n0rma1.l~ Complications of frontal sinus fractures include meningitis and cerebrospinal fluid leakage. Computed tomographic imaging is quite useful in detecting fractures and also in evaluating associated intracranial and intraorbital complications.Axial and direct coronal CT imaging using 3-mm contiguous sections are required for complete evaluation. If a patient cannot tolerate positioning for direct coronal scans, 1-mm axial CT images are obtained followed by coronal reconstruction. TUMORS AND TUMORLIKE CONDITIONS Although a variety of benign and malignant tumors affect the paranasal sinuses, tumors arising in the frontal sinuses are rare. Less than 1% of sinonasal malignancies occur in the frontal sinuses. Therefore, imaging features of all the tumors are not discussed separately here. Rather, this section focuses on the role of CT scans and MR imaging in evaluating sinus tumors and in differentiating tumors from adjacent sinus inflammatory disease. CARCINOMA OF THE PARANASAL SINUS Squamous cell carcinoma accounts for 80% of all sinus malignancies. Eighty percent of all sinus carcinomas arise in the maxillary antrum.ls
Malignancies commonly encountered in the ethmoid sinuses are adenocarcinoma and undifferentiated carcinoma (Fig. 9).16,18Carcinoma of the frontal sinus is rare. Squamous cell carcinoma is often associated with preexisting chronic sinusitis. Further, the tumor may cause obstruction of the sinus ostia leading to obstructive sinusitis.Therefore, differentiatingtumor from inflammatory disease by imaging becomes important. MR imaging plays a critical role in evaluation of sinonasal tumors because of its ability to differentiatetumor from surrounding soft tissues, inflammatory disease, and retained secretions within the sinuses, because of differences in proton mobility?~17~1s~20 Edema or retained secretions within the sinuses is of low signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images, because of the high fluid content, as seen in inflammatory processes. The tumors generally show intermediate signal intensity on the T1- and T2-weighted images. In long-standing
Figure 9. Undifferentiatedcarcinoma involving ethmoid and frontal sinuses. A, Coronal T1weighted MR image demonstrating a large soft tissue mass in the region of nasal cavity, ethmoid, and frontal sinuses (M). The right orbital roof is deformed. B, Axial TPweighted image showing intermediate signal intensity of the mass and hyperintensity of the adjacent inflammatory change. Note that the posterior wall of the right frontal sinus is eroded (arrow). C,Contrast enhanced axial Ti-weighted image showing diffuse enhancement of the soft tissue mass. The tumor erodes the anterior wall of the frontal sinus with extension into the forehead (M). Also note erosion of the posterior wall of the right frontal sinus depicted by the absence of the black cortical line.
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inflammatory process caused by neoplasia, absorption of free water and collection of mucoproteins with high water affinity causes shortening of T1 and T2 relaxation times. Such cases show mixed signal intensities on T1and T2-weighted images. In such cases, contrast-enhanced MR imaging may give additional information. Tumors in such cases show enhancement except in the areas of necrosis, whereas obstructed sinuses show peripheral mucosal enhancement. Contrast-enhanced MR imaging is also considered superior to CT imaging in evaluating the intracranial spread of tumorsF2 Computed tomographic imaging is superior to MR imaging in evaluating fine bone detail and the involvement of the anterior and posterior walls of the frontal sinus. Computed tomographic imaging also allows delineation of the extent of the tumor in the anterior cranial fossa or the orbit. The contrast-enhanced CT scan may show enhancement caused by meningeal or brain parenchymal involvement. Thus CT and MR imaging play complementary roles in the evaluation of sinonasal tumors. A variety of other tumors (arising in the nasal cavity such as melanoma, esthesioneuroblastoma,lymphoma, plasmacytoma, and minor salivary gland lesions) either obstruct or extend into the frontal s i n ~ s e s . ' Jayaraj ~~'~~ et~a1~reported a rare case of melanoma arising in the frontal sinuses? FRONTAL SINUS OSTEOMA
Osteomas are benign tumors that may occur in any sinus but are found most frequently along the floor of the frontal sinuses. They occur at any age and reveal a slight male predominance. Osteomas are generally solitary lesions. Multiple osteomas are associated with intestinal polyposis in Gardner 's syndrome. Osteomas may consist of dense compact bone or lamellar bone with intertrabecular fibrous tissue and do not demonstrate osteoblastic activity. These lesions are of little clinical significance unless they obstruct the frontal sinus drainage pathway. A rare complication, erosion of the posterior wall of the frontal sinus by an osteoma resulting in spontaneous pneumocephalus, has been reported in the literature." Computed tomographic imaging reveals a well-circumscribed, dense mass in the frontal sinus, varying in size from a few millimeters to several centimeters.Depending on their composition, osteomas may show a dense homogeneous or heterogeneous appearance. MR imaging may show signal void on all pulse sequences, and therefore these lesions may not be detected. Alternatively, osteomas with lamellar composition may reveal intermediate to slightly hyperintense signal on T1-weighted images. FIBROUS DYSPLASIA
Fibrous dysplasia is a skeletal disorder of unknown origin that is characterized by replacement of medullary bone by disorganized fibro-osseous
tissue. It occurs in two forms, mono-ostotic and polyostotic. Fibrous dysplasia is characterized by marked thickening of the calvarium with expansion of the diploic space. Frontal bone involvement may encroach on the frontal sinus cavity. Alternately, there may be obliteration of the frontal recess leading to mucocele formation. Very rarely, there may be malignant transformation in polyostotic form, usually osteosarcoma. Computed tomographic studies show marked expansion of the diploic space with thickening of the cortical margins. The diploic space itself may be sclerotic or radiolucent, depending upon the ratio of fibrous to osseous components. MR imaging reveals a pattern of mixed signal intensity. T1-weighted images may show intermediate or low signal intensity, and T2-weighted images may show intermediate to high signal intensity. Intense enhancement is seen following contrast administration."
SUMMARY High-resolution CT imaging is the preferred modality for evaluating sinus inflammatory disease. Computed tomographic imaging demonstrates the complex and highly variable anatomy of the frontal sinus drainage in exquisite detail. For this reason, CT imaging serves as a road map for the surgeons in preoperative cases and is crucial in the assessment of patients who have persistent symptoms following endoscopic sinus surgery. MR imaging is important in the assessment of intracranial complications of frontal sinusitis and in differentiating tumors from inflammatory disease and serves as a powerful problem-solving tool.
References 1. Babbel R, Harnsberg HR, Nelson B, et a1 Optimization of techniques in screening CT of the sinuses. AJNR Am J Neuroradiol12:849454,1991 2. Catalan0 PJ, Lawson W, Som P, et al: Radiographic evaluation and diagnosis of the failed frontal osteoplastic flap with obliteration. Otolaryngol Head Neck Surg 104:225-234,1991 3. Dillon WP, Som PM, Fullerton G D Hypointense MR signal in chronically inspissated sinonasal secretion. Radiology 17473-78,1990 4. Friedman CD, Costantino PD, Jones K, et al: Hydroxyapatite cement, 11: Obliteration and reconstruction of the cat frontal sinus. Arch Otolaryngol Head Neck Surg 117385-389, 1991 5. GoldsteinJH, Phillips C D Current indications and techniques in evaluating inflammatory disease and neoplasia of the sinonasal cavities. Curr Probl Diagn Radiol 2741-71,1998 6. Hudgins PA: Complications of endoscopic sinus surgery: Role of the radiologist in prevention. Radiol Clin North Am 31:21,1993 7.Jayaraj SM, Hem JD, Mochloulis G, et al: Malignant melanoma arising in the frontal sinuses. J Laryngol Otol111:376-378,1997 8. Laine FJ,SmokerW E Theostiomeatal unit and endoscopic surgery: Anatomy, variations and imaging findings in inflammatory disease. AJR Am J Roentgen01159:849-857,1992 9. Lanzieri CF, Shah M, Krauss D, et al: Use of gadolinium-enhanced MR imaging for differentiating mucoceles from neoplasms in the paranasal sinuses. Radiology 178425428, 1991 10. Loevner LA, Yousem DM, Lama DC,et al: MR evaluation of frontal sinus osteoplastic flaps with autogenous fat grafts. AJNR Am J Neuroradiol16:1721-1726,1995
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11. Mafee MF, Chow JM, Meyers R Functional endoscopic sinus surgery: Anatomy, CT screening, indications and complications. AJR Am J Roentgen01160:735-744,1993 12. Marras LC, Kalaparambath TP, Black SE, et al: Severe tension pneumocephalus complicating frontal sinus osteoma. Can J Neurol Sci 25:79-81,1998 13. Montgomery WW, Pierce D L Anterior osteoplastic fat obliteration for frontal sinus: Clinical experience and animal studies. Transactions of the American Academy of Ophthalmology and Otolaryngology 6746-57,1963 14. Olson EM, Wright DL, Hoffman HT, et al: Frontal sinus fractures: Evaluation of CT scans in 132 patients. AJNR Am J Neuroradiol13:897-902,1992 15. Owens M, Klotch D W Use of bone for obliteration of the nasofrontal duct with the osteoplastic flap: A cat model. Laryngoscope 103833-889,1993 16. Phillips CD, Futterer S, Lipper MH, et al: Sinonasal undifferentiated carcinoma: CT and MR imaging of an uncommon neoplasm of the nasal cavity. Radiology 202477480,1997 17. Rao VM,El-Noueam KI: Sinonasal imaging: Anatomy and pathology. Radio1 Clin North Am 36:921-939,1998 18. Som P: Tumors and tumor-like conditions of sinonasal cavity. In Som P, Bergeron RT (eds): Head and Neck Imaging, ed 2. St. Louis, Mosby Year Book, 1990, pp 169-227 19. Som PM, Dillon WP, Fullerton GD, et a1 Chronically obstructed sinonasal secretions: Observations on T1 and T2 shortening. Radiology 172:515-520,1989 20. Som PM, Shapiro MD, Biller HF, et al: Sinonasal tumors and inflammatory tissues: Differentiation with MR imaging. Radiology 1678034308,1988 21. Stammberger H R Paranasal sinuses: Anatomic terminology and nomenclature. Ann Otol Rhino1 Laryngol104:7-14,1995 22. Tassel PV, Lee YY: Gd-DTPA enhanced MR for detecting intracranial extension of sinonasal malignancies. J Comput Assist Tomogr 15:387-392,1991 23. Voegels RL, Balbani PS, Santos RC Jr, et a1 Frontoethmoidal mucocele with intracranial extension: A case report. Ear Nose Throat J 77117-120,1998 24. Yamasoba T,Kikuchi S: Intracranial complications resulting from frontal pyocele: Case presentation and review of experience in Japan. Head Neck 15:450453,1993 25. Yousem DM, Kennedy DW, Rosenberg S : Ostiomeatal complex risk factors for sinusitis: CT evaluation. J Otolaryngol20:419424,1991 26. Yousem DM, Li C, Montone KT, et a1 Primary malignant melanoma of the sinonasal cavity: MR imaging evaluation. Radiographics 16:1101-1110,1996 27. Zinreich SJ, Kennedy DW, Malat J, et al: Fungal sinusitis: Diagnosis with CT and MR imaging. Radiology 169:439444,1988 28. Zinreich SJ, Kennedy DW, Rosenbaum AE, et a1 Paranasal sinuses: CT imaging requirements for endoscopic surgery. Radiology 163:769-775,1987
Address reprint requests to Vijay M. Rao, MD Department of Radiology Thomas Jefferson University Hospital 132 South 10th Street, 1072 Main Building Philadelphia, PA 19107
0030-6665/01 $15.00
+ .OO
IMAGING OF FRONTAL SINUS DISEASE Concepts, Interpretation, and Technology Vijay M. Rao, MD, Dinesh Sharma, MD, and Ashok Madan, MD
In patients with suspected sinonasal pathologic conditions, a routine clinical examination, or even detailed endoscopic examination, may not adequately detect sinus abnormalities. Imaging of the paranasal sinuses is therefore an integral part of the patient evaluation to gather the information that is essential for clinical management. Cross-sectional imaging provides a detailed display of the regional anatomy and variations that may predispose to a blockage of the sinus drainage. Imaging is also crucial in differentiating inflammatory from neoplastic diseases and in defining the extent of the disease process.
IMAGING MODALITIES
Conventional Radiography Conventional radiographic films were once the mainstay of diagnosis of sinus disease but now have been replaced by high-resolution computed tomography (CT) imaging for the evaluation of sinus inflammatory disease. Plain radiographic films do not allow adequate evaluation of the disease of the frontal sinuses and the drainage pathways because of overlapping structures? ~~~
From the Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
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Computed Tomography Computed tomography is the modality of choice for examination of the paranasal sinuses, including the frontal sinuses, because of its ability to demonstrate the intricate bony anatomy as well as the soft tissue component of the disease process. Computed tomography is a prerequisite for patients undergoing sinus surgery, because it provides a road map for the surgeons. The optimal technique consists of acquiring direct contiguous sections 3-mm thick in the coronal plane. The patient is placed in the prone position with the neck hyperextended to let the secretions drain away from the region of the anterior ostiomeatal complex. Coronal sections are obtained perpendicular to the hard palate. When a patient cannot tolerate coronal positioning, thin-section contiguous axial scans are obtained and images are reformatted in the coronal plane. Sagittal reformations are often helpful in better defining the drainage pathway of the frontal sinuses and the frontal recess. The authors routinely complete the examination by obtaining images in the axial plane for preoperative studies, which allow evaluation of the anterior and posterior walls of the frontal sinuses. At the Thomas JeffersonUniversity Hospital, bone and soft tissues are visualized separately by using the appropriate settings. Some investigators advocate the use of a single set of images pictured at an intermediate window level and width settings to study both bone and soft tissue structure.’ Intravenous contrast is not routinely used for the evaluation of inflammatory sinus disease. Indications for administration of intravenous contrast include suspected complications of inflammatory disease such as abscess, thrombosis and intracranial or intraorbital spread of infection, and any benign or malignant tumor. Computed Tomographic Scanning for Image-Guided Surgery Otolaryngologists and head and neck surgeons are increasingly using interactive image-guided computer-assisted surgery. At the Thomas Jefferson Hospital, The InstaTrak System (Visualization Technology, Inc., Woburn, MA) is used. During CT examination, the patient wears a headset to which an electromagnetic transmitter is attached. Axial contiguous CT images 1-mm thick are obtained extending from the maxillary alveolar ridge to the top of the frontal sinuses. Alternatively, if CT images are obtained in a spiral or helical scanner, 3-mm axial contiguous sections may be obtained. The volumetric CT data are then reconstructed to produce I-mm-thick axial images, thereby reducing the scanning time. This information is transferred through the system network to the InstaTrak System in the operating room. At the time of endoscopicsinus surgery, the same headset is placed on the patient exactly as it was during the CT scan. The images are calibrated electromagneticallyby a transmitter on the headset and a receiver attached to the InstaTrak probe. The surgeon is able to view the position of the endoscopic probe relative to the CT images in three-dimensional display on the
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computer screen in the operating room during surgery. Such technologic advances are important in minimizing the risk of operative complications in endoscopic sinus surgery.
Magnetic Resonance Imaging Magnetic resonance (MR) imaging is complementary to CT imaging for evaluating intracranial complications of frontal sinus inflammation and in determining the extent of sinus tumors. MR imaging is superior to CT scanning in differentiating tumor from the adjacent inflammatory sinus disease. Computed tomography, on the other hand, is superior to MR imaging in demonstrating subtle bone erosions. MR imaging has severe limitations in demonstrating the normal regional anatomy of the frontal sinus and the frontal recess. In practice, most patients with suspected sinus pathologic conditions undergo unenhanced CT imaging. If complications of sinusitis are suspected or a neoplasm is considered, the next study of choice is contrastenhanced MR imaging. Contrast-enhanced CT scanning is an alternative study if there is an absolute contraindication to MR imaging. Other advantages of MR imaging over CT scanning include its multiplanar capability, superior soft tissue contrast, and lack of ionizing radiation. The standard protocol for MR imaging of paranasal sinuses includes T1-weighted images in the sagittal, axial, and coronal planes and T2-weighted images in the axial plane. Following administration of intravenous gadolinium, T1-weighted images with fat saturation are obtained in the axial and coronal planes. NORMAL ANATOMY The frontal sinuses, air-filled cavities within the frontal bone, arise as small invaginations from an anterior ethmoid cell tract. The frontal sinuses are divided by a bony septum into asymmetric cavities, each with its own drainage system. The anatomy of the frontal sinuses and the drainage pathways is complex but is elegantly displayed by coronal CT images.&11,17,21,25,28 The frontal, anterior ethmoid, and maxillary sinuses drain into the middle meatus, which lies superolateral to the middle turbinate. This region is called the anterior ostiomeatal complex and incorporates the middle meatus, middle turbinate, uncinate process, ethmoid infundibulum, maxillary ostium, ethmoid bulla, and the frontal recess. The frontal sinuses drain into the anterior aspect of the middle meatus by an internal channel which is an integral part of the anterior ethmoid complex and is called the frontal recess (Fig. 1). This channel was previously referred to as the nasofvontal duct, a term no longer used because there is no well-defined tubular structure. The frontal recess has been described as the most complicated structure in the anterior ethmoid complex.” The frontal sinus ostium is located at the most anterosuperior part of the frontal recess. The
Figure 1. Normal anatomy. Coronal CT scan showing the left frontal sinus ostium. The frontal recess drains into the middle meatus (arrows).
frontal recess is bounded medially by the lateral surface of the most anterior portion of the middle turbinate. The lateral boundary of the frontal recess is usually formed by the anterosuperior extension of the lamina papyracea.*' In the sagittal plane, the frontal sinus and the frontal recess have an hourglass configuration. Several ethmoid and uncinate variants may cause anatomic narrowing of the frontal recess and predispose to obstruction. Agger nasi cells are the most anterior ethmoid air cells that are located anterior, lateral, and inferior to the frontal recess and are visualized best on coronal CT scans (Fig. 2). They range from a single cell to a group of as many as seven air cells and may develop from the frontal recess. A pneumatized middle turbinate (concha bullosa) may be in direct continuity with the frontal recess or may encroach on the frontal recess. A pneumatized crista galli may also develop and may drain into the frontal recess. The ethmoid bulla may be large (giant bulla ethmoidalis) and expand into the middle meatus with anatomic compromise of the middle meatus. There is a marked variation in the configuration and attachment of the uncinate process, which is responsible for the variable drainage patterns of the frontal recess into the middle meatus.21The frontal recess may open into the middle meatus medial to the uncinate or into the ethmoid infundibulum. A pneumatized uncinate process (bulla uncinate) may also cause anatomic compromise of the frontal recess. The radiologist must pay attention to these anatomic variants on coronal CT images because they may interfere with the normal air flow and mucociliary clearance of the frontal sinuses. Mucociliary clearance occurs in a predetermined fashion in each of the sinuses. In the frontal sinus, mucus is propelled up along the medial walls, laterally along the roof, down along the lateral wall, and medially along the floor toward the region of the frontal sinus ostium. At this juncture, mucus partly extends down to
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Figure 2. Obstructed frontal recess. Coronal CT image revealing opacification of the left frontal recess (FR). Note that the concha bullosa (CB) is in direct contiguity with the frontal recess. An agger nasi air cell (A) is noted along the lateral aspect of the frontal recess.
the frontal recess to drain into the middle meatus, and the rest is propelled up to recirculate.21
INFLAMMATORY LESIONS Rhinosinusitis may be classified as acute, subacute, chronic; recurrent acute, or acute excacerbation of chronic rhinosinusitis. The imaging findings are nonspecific and should be correlated with the clinical history and physical examination. If there is no history of trauma, lavage, or reason for spontaneous hemorrhage, an air-fluid level in the frontal sinus or any other sinus usually correlates with acute sinusitis. Mucosal thickening in any sinus is often misinterpreted as chronic sinusitis. Components of mucosal thickening that are acute rather than subacute or chronic cannot be differentiated by imaging alone. There is no correlation between the degree of mucosal thickening and the clinical symptoms. Minimal mucosal thickening at the frontal recess or the ethmoid infundibulum is more likely to cause obstruction and sinusitis than moderate mucosal thickening elsewhere in the sinuses. On imaging studies, it is better to describe the degree of mucosal thickening as mild (< 5 mm), moderate (5-10 mm), or severe (> 10 mm), and the location. Sclerosisof the sinus walls is a good indicator of underlying chronic sinusitis; however, the mucosal thickening may still be acute, subacute, or chronic. The attenuation of sinus secretions on CT imaging varies with the degree of hydration. Hydrated secretions show low attenuation, whereas
desiccated secretions show increased attenuation. Areas of increased attenuation may also be seen with hemorrhage or extramucosal fungal disease. The MR imaging signal characteristics of sinus inflammatory disease vary depending on the state of hydration of the mucosa and the content of the secretions. Sinonasal secretions normally reveal low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, relative to muscle, because of the dominant water content (95% water, 5% protein) of the secretions. In a chronically obstructed sinus, however, the gradual resorption of free water results in an increased protein content. As the protein content rises (up to 25%), the signal intensity on T1weighted images increases, resulting in high signal intensity on both T1and T2-weighted images. With progressive water resorption and increasing protein concentration, a significant cross-linking between protein macromolecules occurs, resulting in increased viscosity. This increased viscosity ultimately results in a complete loss of MR signal intensity on both T1- and T2-weighted images and mimics the appearance of an aerated s i n ~ s . ~ , ’ ~ This loss of signal intensity is a major pitfall in MR imaging of sinus inflammatory disease: the extent of disease may be grossly underestimated in chronic sinusitis, sinonasal polyposis, and mucocele. SINONASAL POLYPOSE Sinus polyps form as a result of folding and hypertrophy of the mucosa with submucosal accumulation of fluid. Sinonasal polyps have been associated with allergies, vasomotor rhinitis, infections, rhinosinusitis, cystic fibrosis, diabetes mellitus, aspirin intolerance, and nickel exposure. The imaging appearance of sinonasal polyps is usually quite dramatic. Nasal polyps normally expand the nasal fossa with softtissue masses. The sinuses reveal varying degrees of opacification and expansion. Although polyps are benign, they may cause significant expansion and bone erosion, which may be confused with aggressive fungal sinusitis or carcinoma of the sinus by inexperienced examiners. Bone erosion caused by polyps in the frontal sinus affects the posterior wall more frequently than the anterior wall because the posterior wall is thinner. Computed tomographic imaging usually demonstrates central high density surrounded by a peripheral rim of low density. This appearance is characteristic of sinus inflammatory disease and is not seen with sinonasal tumors.2O Sinonasal polyps show a mixed signal intensity pattern, on both T1- and T2-weighted MR images, depending on the water content of the polyps, mucosal hypertrophy, and the sinonasal secretions. Polyps show a rippled enhancement pattern unlike the diffuse enhancement seen with tumors (Fig. 3).17 FUNGAL SINUSITIS Fungal sinusitis may be categorized as invasive and noninvasive; the latter category includes allergic fungal sinusitis and fungal ball. Noninvasive fungal disease occurs in immunocompetent individuals, resulting
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Figure 3. Frontal sinus polyps in a patient with diffuse chronic sinonasal polyposis. A, Axial T1-weighted MR image showing opacified frontal sinuses. No erosion of the cortical margins or expansion is noted. The soft tissue in the sinuses is hypointense relative to brain. 6, Coronal contrast enhanced T I -weighted image with fat suppression revealingcircumscribed peripheral, well-defined hypointensepolyps with enhancement opacifying the frontal sinuses and the nasal fossa bilaterally. Note that the nasal turbinates normally enhance.
from colonization of the paranasal sinuses, and its course depends on the local host conditions. The most frequently involved sinuses are the maxillary and the anterior ethmoid. Frontal sinuses are the least commonly involved. Computed tomographic imaging usually reveals a soft tissue mass with calcification and is not associated with bone erosion. MR imaging characteristically shows intermediate signal intensity on T1-weighted images and markedly low signal intensity on T2-weighted images, relative to the surrounding tissue. Allergic fungal sinusitis, another form of noninvasive disease, is most commonly encountered in children and young immunocompetent people. Imaging findings include unilateral or bilateral pansinus opacification and expansion, including the frontal sinuses, and mimic the appearance of chronic sinonasal polyposis. Computed tomographic imaging usually shows a central area of hyperdensity in each sinus, surrounded by a peripheral rim of low attenuation. MR imaging reveals variable or mixed signal intensity on T1-weighted images but marked central hypointensity
with only a peripheral rim of high signal intensity on T2-weighted images (Fig. 4).Explanations that have been postulated for the dramatic hypointensity of the MR imaging signal associated with fungal sinusitis are (1)the markedly increased viscosity of the sinus secretions encountered in allergic fungal sinusitis; (2) the intertwined fungal hyphae in fungal ball that behave like solid material; and (3) the dephasing of spins caused
Figure 4. Allergic fungal sinusitis. A 15-year-old presented with nasal obstruction. A, Noncontrast enhanced coronal CT scan revealing marked expansion of the nasal cavity, ethmoid sinus, and frontal sinuses with central areas of hyperdensity. This appearance is characteristic of inflammatory disease and differentiates it from a tumor. €3, Coronal TP-weighted MR image showing complete lack of signal in the corresponding regions of hyperdensitymimicking an aerated sinus. C, Contrast enhanced coronal T I -weighted MR image revealing mild enhancement of the inflammatory mass.
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by ferromagnetic elements (iron and manganese) that are present on the fungal hyphae.17,27 Invasive fungal sinusitis is a potentially life-threateningcondition that is usually seen in immunocompromised patients. This fulminant form of fungal infection results in bone erosion and vascular invasion, leading to vascular occlusion, ischemia, and infarction. Ethmoid sinuses are most frequently affected, and the disease process rapidly progresses to involve the orbital apex and the cavernous sinus. Isolated involvement of frontal sinus is rare (Fig. 5). FRONTAL MUCOCELE The term rnucocele refers to an expanded, mucoid-filled sinus, resulting from ostial obstruction caused by inflammatory scar, trauma, or tumor. The frontal sinuses are most frequently involved (65%),followed by 23 Computed tomography demonstrates an the ethmoid sinus (25%).9*'7,
Figure 5. lnvasive frontal sinus mucormycosis. A, Contrast-enhancedCT scan in a diabetic patientwith biopsy-provenmucormycosisshowing an opacified mucoid-filledfrontal sinus with an air-fluid level. Note the enhancing mass along the posterior aspect of the left frontal sinus associatedwith bone erosion and epidural extension (arrow).Abnormal enhancement caused by direct spread is noted along the anterior falx (asterisks). B, Axial T1-weighted MR image showing an opacified left sinus that is largely hypointense to the brain. Along the posterior aspect of the sinus, a focal lesion is seen with signal intensity similar to the brain. Note the absence of the black line indicating erosion of the cortical bone and epidural spread (arrow). The soft tissue inflammatory mass extends into the anterior cranial fossa along the falx in the midline. C, Contrast enhanced T1-weighted axial MR image reveals intensity enhancing fungal inflammatorymass with intracranialspread. There is peripheral mucosal enhancement. The remainder of the sinus shows inflammatory exudate that is nonenhancing.
expanded sinus with gradual thinning and erosion of the bony margins. In a frontal sinus mucocele, the posterior wall is particularly prone to erosion because of its inherent thinness (Fig. 6). The density of the sinus contents of a mucocele may be low or mixed relative to that of muscle. MR imaging demonstrates variable signal intensities on both T1- and T2-weighted images, depending on the state of hydration, protein content, and viscosity of the contents of the mucocele (see Fig. 5). The two most frequently observed patterns are (1) moderate-to-marked high signal intensity on both T1- and T2-weightedimages, and (2)moderate-to-markedlow signal intensity on both T1- and T2-weighted images. Contrast-enhanced MR imaging is useful in differentiating mucoceles from sinonasal tumors.9,2o Mucoceles characteristicallyreveal a thin peripheral linear enhancement with central low signal intensity on T1-weightedimages. Sinonasaltumors, on the other hand, demonstrate diffuse enhancement? COMPLICATIONS OF FRONTAL SINUSITIS
Orbital complications of frontal sinusitis usually result from direct extension of sinus infection through the wall of the bony orbit and include preseptal cellulitis, postseptal or orbital cellulitis, subperiosteal abscess, and orbital abscess. The most common source of orbital complications is ethmoidal sinus infection. Intracranial complicationsare uncommon but are usually the result of '* Infectionspreads from the frontal sinusby the emissary frontal sin~sitis.'~, veins between the posterior sinus mucosa and the meninges. Alternate routes of spread include direct extension through congenital or acquired bony dehiscences, extension along pre-existing anatomic neurovascular pathways, or by hematogenous spread as part of septicemia. MR imaging is superior to CT imaging in detecting intracranial complications of sinusitis. Contrast-enhanced MR imaging often reveals meningeal enhancement in patients with sinusitis, but this enhancement is not an indication for craniotomy. Epidural abscess and subdural empyema
Figure 6. Mucocele. Axial CT scan revealing increased attenuation of the left frontal sinus contents. Note marked expansion and smooth erosion of the posterior wall of the frontal sinus (arrows).
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commonly occur as a complication of frontal sinusitis and involve the anterior cranial fossa because it is directly contiguous with the posterior wall of the frontal sinus. Cerebral abscess is a rare complication of the frontal sinusitis and is caused by venous thrombophlebitis. The CT and MR imaging features of cerebral abscess vary depending on the stage of the abscess formation. A ring-enhancing lesion with central pus and surrounding edema represents a frank abscess and is readily diagnosed by CT or MR imaging. POSTOPERATIVE FRONTAL SINUS Imaging studies are made after endoscopic sinus surgery only if complications are suspected or if clinical symptoms persist without improvement. A careful scrutiny of the postoperative CT scan in the coronal plane demonstrates the extent of surgery performed. It is important to look for formation of adhesions or synechiae in the region of the frontal recess Computed tomographic which result in obstruction of the sinus (Fig. 7).6,11 images may reveal lateralization of the middle turbinate resulting in obstruction of the middle meatus or frontal drainage. Computed tomography may detect incomplete exenteration of agger nasi cells compromising the region of the frontal recess. Osteoplastic frontal sinus flaps with sinus obliteration are performed in patients with chronic inflammatory disease, mucocele, trauma, and tumors. In this procedure, a bone flap is cut along the anterior aspect of the frontal sinus wall. The mucosal lining is removed completely, and the frontal sinus is obliterated by packing material such as fat, bone, and
Figure 7. Postendoscopic sinus surgery. A, Coronal CT scan showing an obstructed right frontal sinus. The remnant of right middle turbinate (arrow) is lateralized and is adherent to the remaining agger nasi air cell resulting in complete occlusion of right frontal recess. On the left, the region of the frontal recess is markedly wide and open. B, Coronal T2-weighted MR scan in the same patient demonstrating narrowing and obstruction of the right frontal recess. The right middle turbinate remnant, seen as a bright structure, is adherent to the remaining agger nasi cell that is aerated.
hydroxyapatite cement or methyl methacrylate.4,13, l5 The post-operative complications include recurrent sinusitis, infection of the graft, osteomyelitis of the osteoplastic flap, cellulitis, and mucocele.2,10 Postoperative imaging (CT scanning or MR imaging) is often obtained to assess these complicationsin patients with persistent symptoms following surgery. Interpretation of either of these studies is difficult because of the normally expected changes from postoperative granulation tissue and scar. Postoperative CT scans demonstrate an airless sinus cavity with smooth margins of the bone flap. If the margins of the bone flap are irregular and reveal bone resorption or mixed density with adjacent soft tissue swelling, underlying osteomyelitis is suspected. Postoperative MR imaging shows an airless sinus. The signal intensity of the soft tissue in the frontal sinus can vary, depending on the nature of the packing material that is used. For example, patients who have undergone sinus obliteration with fat autograft show hyperintense signal on T1-weighted images and hypointense signal on TZweighted images. The signal intensity on T1-weighted images decreases gradually as the fat is replaced by soft tissue (Fig. 8). Catalan0 et a1 reported that hyperintensity of the autograft on T2weighted images reflected inflammation.2Loevner et a1 reported that MR imaging findings after osteoplastic frontal sinus flap placement were
Figure 8. Osteoplastic frontal sinus flap. Sixty-year-old man 4 months after surgery for treatment of chronic frontal sinusitis. A, Axial T1-weighted MR image shows high signal intensity fat occupying the large portion of the frontal sinus cavity. Note a small portion of the fat has been replaced by soft tissue in the center (solid arrows) and along the lateral portion of the right frontal sinus (open arrow). B, Axial T2-weighted MR image shows the hyperintensity of the soft tissue replacing fat (arrows). C, Contrast enhanced axial T1-weighted high signal intensity fat. Note that the focal areas of soft tissue showed mild enhancement (arrows).This enhancement can be visualized more easily if the image technique included fat saturation.
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nonspecific and were of limited usefulness in differentiating postoperative scar formation from recurrent sinusitis. In their studylo hyperintensity within the frontal sinuses on T2-weighted images was noted in symptomatic and asymptomatic postoperative patients. Contrast enhancement was seen along the periphery and in any region where fat had been replaced by soft tissue. FRONTAL SINUS FRACTURES In general, patients with frontal sinus fractures have significant associated facial and skull fractures or intracranial injury. Fractures can be classified as linear, comminuted, depressed, nondepressed, displaced, or undisplaced and may involve the anterior or the posterior wall, or both. The facial injuries that are frequently associated with frontal sinus fractures include periorbital and orbital hematoma; the intracranial injuries include frontal lobe contusions, pneumocephalus, and subdural and subarachnoid hemorrhage. It has been observed that fractures of the posterior wall of the frontal sinus are the best predictor of the severity of trauma and are associated with more complications and a worse clinical outcome than fractures that involve only the anterior wall.I4Although a high incidence of brain injury is associated with fractures of the posterior wall of the frontal sinus, brain injury may not be apparent on the initial CT scan of the head. Patients should therefore undergo follow-up CT scans or MR imaging to exclude brain injury if the initial scans were n0rma1.l~ Complications of frontal sinus fractures include meningitis and cerebrospinal fluid leakage. Computed tomographic imaging is quite useful in detecting fractures and also in evaluating associated intracranial and intraorbital complications.Axial and direct coronal CT imaging using 3-mm contiguous sections are required for complete evaluation. If a patient cannot tolerate positioning for direct coronal scans, 1-mm axial CT images are obtained followed by coronal reconstruction. TUMORS AND TUMORLIKE CONDITIONS Although a variety of benign and malignant tumors affect the paranasal sinuses, tumors arising in the frontal sinuses are rare. Less than 1% of sinonasal malignancies occur in the frontal sinuses. Therefore, imaging features of all the tumors are not discussed separately here. Rather, this section focuses on the role of CT scans and MR imaging in evaluating sinus tumors and in differentiating tumors from adjacent sinus inflammatory disease. CARCINOMA OF THE PARANASAL SINUS Squamous cell carcinoma accounts for 80% of all sinus malignancies. Eighty percent of all sinus carcinomas arise in the maxillary antrum.ls
Malignancies commonly encountered in the ethmoid sinuses are adenocarcinoma and undifferentiated carcinoma (Fig. 9).16,18Carcinoma of the frontal sinus is rare. Squamous cell carcinoma is often associated with preexisting chronic sinusitis. Further, the tumor may cause obstruction of the sinus ostia leading to obstructive sinusitis.Therefore, differentiatingtumor from inflammatory disease by imaging becomes important. MR imaging plays a critical role in evaluation of sinonasal tumors because of its ability to differentiatetumor from surrounding soft tissues, inflammatory disease, and retained secretions within the sinuses, because of differences in proton mobility?~17~1s~20 Edema or retained secretions within the sinuses is of low signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images, because of the high fluid content, as seen in inflammatory processes. The tumors generally show intermediate signal intensity on the T1- and T2-weighted images. In long-standing
Figure 9. Undifferentiatedcarcinoma involving ethmoid and frontal sinuses. A, Coronal T1weighted MR image demonstrating a large soft tissue mass in the region of nasal cavity, ethmoid, and frontal sinuses (M). The right orbital roof is deformed. B, Axial TPweighted image showing intermediate signal intensity of the mass and hyperintensity of the adjacent inflammatory change. Note that the posterior wall of the right frontal sinus is eroded (arrow). C,Contrast enhanced axial Ti-weighted image showing diffuse enhancement of the soft tissue mass. The tumor erodes the anterior wall of the frontal sinus with extension into the forehead (M). Also note erosion of the posterior wall of the right frontal sinus depicted by the absence of the black cortical line.
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inflammatory process caused by neoplasia, absorption of free water and collection of mucoproteins with high water affinity causes shortening of T1 and T2 relaxation times. Such cases show mixed signal intensities on T1and T2-weighted images. In such cases, contrast-enhanced MR imaging may give additional information. Tumors in such cases show enhancement except in the areas of necrosis, whereas obstructed sinuses show peripheral mucosal enhancement. Contrast-enhanced MR imaging is also considered superior to CT imaging in evaluating the intracranial spread of tumorsF2 Computed tomographic imaging is superior to MR imaging in evaluating fine bone detail and the involvement of the anterior and posterior walls of the frontal sinus. Computed tomographic imaging also allows delineation of the extent of the tumor in the anterior cranial fossa or the orbit. The contrast-enhanced CT scan may show enhancement caused by meningeal or brain parenchymal involvement. Thus CT and MR imaging play complementary roles in the evaluation of sinonasal tumors. A variety of other tumors (arising in the nasal cavity such as melanoma, esthesioneuroblastoma,lymphoma, plasmacytoma, and minor salivary gland lesions) either obstruct or extend into the frontal s i n ~ s e s . ' Jayaraj ~~'~~ et~a1~reported a rare case of melanoma arising in the frontal sinuses? FRONTAL SINUS OSTEOMA
Osteomas are benign tumors that may occur in any sinus but are found most frequently along the floor of the frontal sinuses. They occur at any age and reveal a slight male predominance. Osteomas are generally solitary lesions. Multiple osteomas are associated with intestinal polyposis in Gardner 's syndrome. Osteomas may consist of dense compact bone or lamellar bone with intertrabecular fibrous tissue and do not demonstrate osteoblastic activity. These lesions are of little clinical significance unless they obstruct the frontal sinus drainage pathway. A rare complication, erosion of the posterior wall of the frontal sinus by an osteoma resulting in spontaneous pneumocephalus, has been reported in the literature." Computed tomographic imaging reveals a well-circumscribed, dense mass in the frontal sinus, varying in size from a few millimeters to several centimeters.Depending on their composition, osteomas may show a dense homogeneous or heterogeneous appearance. MR imaging may show signal void on all pulse sequences, and therefore these lesions may not be detected. Alternatively, osteomas with lamellar composition may reveal intermediate to slightly hyperintense signal on T1-weighted images. FIBROUS DYSPLASIA
Fibrous dysplasia is a skeletal disorder of unknown origin that is characterized by replacement of medullary bone by disorganized fibro-osseous
tissue. It occurs in two forms, mono-ostotic and polyostotic. Fibrous dysplasia is characterized by marked thickening of the calvarium with expansion of the diploic space. Frontal bone involvement may encroach on the frontal sinus cavity. Alternately, there may be obliteration of the frontal recess leading to mucocele formation. Very rarely, there may be malignant transformation in polyostotic form, usually osteosarcoma. Computed tomographic studies show marked expansion of the diploic space with thickening of the cortical margins. The diploic space itself may be sclerotic or radiolucent, depending upon the ratio of fibrous to osseous components. MR imaging reveals a pattern of mixed signal intensity. T1-weighted images may show intermediate or low signal intensity, and T2-weighted images may show intermediate to high signal intensity. Intense enhancement is seen following contrast administration."
SUMMARY High-resolution CT imaging is the preferred modality for evaluating sinus inflammatory disease. Computed tomographic imaging demonstrates the complex and highly variable anatomy of the frontal sinus drainage in exquisite detail. For this reason, CT imaging serves as a road map for the surgeons in preoperative cases and is crucial in the assessment of patients who have persistent symptoms following endoscopic sinus surgery. MR imaging is important in the assessment of intracranial complications of frontal sinusitis and in differentiating tumors from inflammatory disease and serves as a powerful problem-solving tool.
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Address reprint requests to Vijay M. Rao, MD Department of Radiology Thomas Jefferson University Hospital 132 South 10th Street, 1072 Main Building Philadelphia, PA 19107