Imaging in Sinonasal Inflammatory Disease




While most patients with inflammatory rhinosinusitis are successfully diagnosed clinically, imaging is indicated in patients with recurrent or chronic sinusitis, atypical symptoms and complicated acute sinusitis. Non-enhanced high resolution, thin section computed tomography (CT) is the reference standard in evaluating such patients. It provides superb anatomical details and enables a fairly accurate diagnosis and delineation of the disease, addressing all concerns of the endoscopic surgeon prior to intervention. Contrast MR imaging is preferred for assessing intraorbital or intracranial complications. The radiologist must have a systematic approach to sinonasal CT and generate a clinically relevant report that impacts patient management.


Key points








  • Noncontrast, multiplanar, high-resolution, thin-section CT is the accepted gold standard for imaging sinonasal inflammatory disease.



  • MR imaging is preferred to assess intraorbital and intracranial complications, and is not suitable as the first or sole investigation in patients with inflammatory rhinosinusitis.



  • The nature of sinonasal secretions on CT and MRI and the pattern of bone changes on CT offer important information about the disease.



  • Patterns of inflammatory rhinosinusitis described at CT have a bearing on the prognosis, choice of treatment options, and therapeutic outcomes for the patients.



  • A pre-FESS CT report should describe the disease and address all concerns of the endoscopic surgeon, delineating pertinent anatomy, variants and critical relationships.






Introduction


Rhinosinusitis constitutes a fairly large public health problem across the globe. Chronic rhinosinusitis (CRS) affects about 12% to 16% of the US population and is one of the commonest chronic illnesses in America. According to the statistics from the National Institute of Allergy and Infectious diseases (NIAID), about 1 in 8 Indians have this disorder.


Sinusitis, in its simplest definition, means inflammation of the sinus mucosa. It is almost always accompanied by inflammation of the nasal mucosa or rhinitis. Hence, the term rhinosinusitis is used by most otolaryngologists. Sinusitis can be viral, bacterial, allergic, vasomotor, or reactive in nature. Four basic forms are described based on the duration of symptoms:



  • 1.

    Acute sinusitis: symptoms present for 4 weeks or less


  • 2.

    Subacute sinusitis: continuation of the acute process and lasts anywhere between 4 and 12 weeks


  • 3.

    Chronic sinusitis: persistence of the symptoms for 12 weeks or beyond


  • 4.

    Recurrent acute sinusitis: occurrence of more than 4 episodes of acute sinusitis in 1 year with resolution of symptoms between the episodes





Introduction


Rhinosinusitis constitutes a fairly large public health problem across the globe. Chronic rhinosinusitis (CRS) affects about 12% to 16% of the US population and is one of the commonest chronic illnesses in America. According to the statistics from the National Institute of Allergy and Infectious diseases (NIAID), about 1 in 8 Indians have this disorder.


Sinusitis, in its simplest definition, means inflammation of the sinus mucosa. It is almost always accompanied by inflammation of the nasal mucosa or rhinitis. Hence, the term rhinosinusitis is used by most otolaryngologists. Sinusitis can be viral, bacterial, allergic, vasomotor, or reactive in nature. Four basic forms are described based on the duration of symptoms:



  • 1.

    Acute sinusitis: symptoms present for 4 weeks or less


  • 2.

    Subacute sinusitis: continuation of the acute process and lasts anywhere between 4 and 12 weeks


  • 3.

    Chronic sinusitis: persistence of the symptoms for 12 weeks or beyond


  • 4.

    Recurrent acute sinusitis: occurrence of more than 4 episodes of acute sinusitis in 1 year with resolution of symptoms between the episodes





Mucociliary clearance and pathophysiology of sinusitis


The paranasal sinuses are covered by mucous-secreting ciliated columnar epithelium. The cilia move continuously to propel the mucous toward the sinus ostium, the nasal cavity, and finally into the pharynx, constituting mucociliary clearance of the sinonasal cavities. This pattern of mucociliary flow is specific for each sinus and continues even if alternative openings or ostia are created in the sinus. Any disturbance in this mucociliary clearance causes stagnation of secretions, secondary infection, and subsequent sinusitis. Sinus ostial obstruction causing disruption of the mucociliary flow accounts for majority of the cases of recurrent or chronic inflammatory sinusitis. Commonest cause of acute sinusitis is an upper respiratory viral infection. Mucosal congestion causes apposition of the mucosal surfaces of the sinus ostia and drainage pathways, disturbing the mucociliary clearance and predisposing the sinus to secondary bacterial infection. Symptoms may resolve in about a week or may progress to subacute sinusitis. Although most cases may resolve with conservative treatment, about one-third of patients remain refractory to treatment and progress to chronic or recurrent rhinosinusitis.




Indications for imaging


Sinusitis is largely a clinical diagnosis. Imaging is neither recommended nor performed for every patient who presents with sinusitis. The 1997 Task Force on Rhinosinusitis of the American Academy of Otolaryngology – Head and Neck Surgery recommended diagnosing acute rhinosinusitis (ARS) based on major and minor criteria that were further reviewed and simplified in 2007. The American College of Radiology has also laid down appropriateness criteria for imaging in sinonasal inflammatory disease. Table 1 briefly summarizes the indications for imaging in patients with sinusitis.



Table 1

Sinusitis: Indications for imaging



















Indication Recommended Imaging Option
Acute sinusitis Generally a clinical diagnosis; imaging adds little valuable information
Complicated acute sinusitis Noncontrast and contrast CT or MR imaging of the paranasal sinuses including orbits and brain
Chronic sinusitis a Noncontrast CT
Recurrent sinusitis a Noncontrast CT

a Patients with symptoms of recurrent or chronic sinusitis and those who do not respond to medical treatment generally undergo intranasal endoscopy. A CT scan is requested when medical therapy fails or intranasal endoscopy suggests surgically correctable causes.





Imaging options and protocols


With the developments in endoscopic sinus surgery, there has been a parallel supportive evolution in imaging technology and trends with a gradual but definite shift from the use of plain radiography toward CT for evaluating rhinosinusitis. CT has replaced plain radiography because of its greater precision in depicting sinonasal anatomy and pathology. A brief summary on the use of plain radiography, CT, and MR imaging in the evaluation of inflammatory sinonasal disease is provided in Tables 2–4 .



Table 2

Imaging options for sinusitis: Plain radiography






















Feature Comment
Status Almost obsolete
Advantages


  • Low cost



  • Easy availability



  • Portability



  • Air-fluid levels



  • Low radiation dose

Concerns


  • Superimposition of structures



  • Suboptimal delineation of the anatomy and pathology



  • High false-negatives

Patient preparation None
Sitting position preferred for depiction of air-fluid levels
Protocol


  • Water’s view



  • Caldwell’s view



  • Submentomaxillary view



  • Lateral view


Data from Refs.


Table 3

Imaging options for sinusitis: CT






















Feature Comments
Status


  • Gold standard and primary imaging tool for recurrent and chronic rhinosinusitis



  • Contrast CT for complicated acute sinusitis

Advantages


  • Superb delineation of sinonasal anatomy and variants



  • Excellent depiction of sinonasal relationships with critical regional neurovascular structures



  • Optimal characterization of the location, extent of disease, nature of secretions, patterns of bony changes, and intrasinus calcifications



  • Easy differentiation of air, dessicated secretions, and calcifications, all of which appear the same on MR imaging



  • Addresses all the concerns of the endoscopic surgeon



  • More economical than an MR imaging study

Concerns


  • Differentiation from fungal infections and neoplasms can be challenging



  • Radiation dose to the ocular lens and thyroid gland are perhaps the major concern. However, a single-scan dose from sinonasal CT varies between 1.88 and 64 mGy and this is much lower than the threshold for damage to the lens, which ranges between 0.5 and 2 Gy. Moreover, newer multidetector CT scanners allow use of low-dose CT (20 mAs), which offers radiation doses equivalent to plain radiographs without any compromise of the image quality. There is no clear described threshold for thyroid damage, however a single-scan dose is not considered to increase the risk of thyroid cancer

Patient preparation


  • Considered important in eliminating reversible disease and allows better delineation of the anatomy and pathology. These include




    • Resolution of symptoms of acute disease



    • Completion of course of antibiotics



    • Instillation of nasal decongestants 15 min before the scan and nasal blowing




  • However, some studies have shown that these techniques may offer only a small reduction in mucosal thickening and hence have a limited effect on the outcome of CT

Protocol


  • Multidetector CT scan with multiplanar reformations preferred. Scan is performed in the axial plane with patient supine on the CT table and a neutral position of the gantry. Direct coronal acquisition is no longer performed routinely



  • 0.625 mm collimation, field of view of 180 mm



  • Multiplanar reformations are obtained from the dataset in the coronal, axial and sagittal planes. Coronal images are most preferred by the endoscopic surgeons as they simulate the appearance of sinonasal cavities at endoscopy



  • 0.9-mm-thick images are reconstructed in bone and soft tissue algorithms



  • Images are displayed at a window width of about 1500 and center of 150, that delineate the bony anatomy and inflammatory disease. Narrow windowing in the soft tissue algorithm helps with better assessment of the nature of sinonasal secretions (see Fig. 1 )


Data from Refs.



Fig. 1


Axial CT images in bone ( A ) and soft tissue ( B ) algorithms. The bone algorithm image in a wide window nicely depicts the bony anatomy and mucosal disease in the right maxillary sinus. Minimal mucosal disease is also seen in the left maxillary sinus. ( B ) The adjacent soft tissue anatomy is shown well but is suboptimal to assess the bones that appear thickened on such images. The narrow window in the soft tissue algorithm in ( C ) shows the mucoid density of the disease. The left maxillary sinus disease is not seen well in this image.


Table 4

Imaging options for sinusitis: MR imaging






















Feature Comment
Status Complementary role in assessment of complicated sinusitis when it is preferred over contrast CT. Not preferred for primary evaluation or as a sole imaging tool for inflammatory sinonasal disease
Advantages


  • No radiation



  • Superb soft tissue resolution



  • Assessment of intraorbital/intracranial complications is superior to CT

Concerns


  • Longer scanning time



  • Suboptimal delineation of the intricate sinonasal anatomy compared with CT



  • Normal air, chronic dessicated secretions, and calcifications mimic each other appearing as signal voids, leading to diagnostic errors

Patient preparation


  • None

Protocol


  • Head coil



  • 3 or 4 mm T1-weighted and T2-weighted sequences in coronal and axial planes. Contrast T1-weighted sequences with fat suppression in coronal and axial planes. Sagittal images whenever required



  • Include the adjacent orbits and intracranial cavity along with the sinuses





Imaging appearances


Acute Sinusitis


CT and MR imaging features include nonspecific mucosal thickening, submucosal edema, air-fluid levels, or sinus secretions interspersed with air bubbles ( Fig. 2 ). Acute sinonasal secretions are predominantly water and of a mucoid nature (−10 to 25 HU) on CT. They are hypointense on T1 and hyperintense on T2 sequences. An isolated air-fluid level as the only finding in the sinus is fairly characteristic for acute sinusitis, but may not be seen in all patients. The distribution of disease on CT may provide a clue to the cause of the acute disease. Allergic sinusitis is generally more diffuse and bacterial sinusitis is often fairly localized or asymmetric.




Fig. 2


Axial CT image ( A ) shows the air-fluid level in the left maxillary sinus with overlying air bubbles that are seen better in the coronal CT image ( B ). Nasal septal deviation to left side ( white arrow ), right concha bullosa (C), enlarged right inferior turbinate (T). The narrow window in ( C ) depicts the mucoid density of the secretions.


Chronic Sinusitis


Imaging features include mucosal thickening, sinus opacification, sclerosis of the bony walls of the sinus, and intrasinus calcifications. Air-fluid levels may be seen superimposed on chronic sinusitis ( Fig. 3 ). Retention cysts, polyps, polyposis, and mucoceles may be seen as sequelae to chronic sinusitis.




Fig. 3


Axial ( A ) coronal ( B , C ) CT images show air-fluid levels, air bubbles, and mucosal disease in the maxillary sinuses. ( B ) Opacification of anterior ethmoid air cells, nasal septal deviation ( white arrow ), paradoxic curvature of the left middle turbinate ( elbow arrow ), and mildly enlarged right inferior turbinate (T) are seen. ( C ) Opacified Haller cell ( curved white arrow ), right maxillary ostium and infundibulum ( thick white arrow ), middle meatus ( white asterisks ), and frontal recess ( white arrowheads ) indicate the OMU pattern of chronic sinusitis. Infundibular narrowing ( white outlined arrow ) and frontal recess disease ( black arrowheads ) seen on the left side with patent left middle meatus.


Mucosal thickening


Normal mucosa is generally not seen at CT or MR imaging, and whenever sinus mucosa is seen at the interface between air and bone, that mucosa is presumed to be thickened. Mucosal thickening may be smooth, irregular, or polypoid in morphology. MR is more sensitive to mucosal thickening than CT and depicts the thickened mucosa as a hyperintense signal on T2 images against the black background of sinus air and bone ( Fig. 4 ). In the maxillary sinuses, up to 3 mm mucosal thickening may be seen in healthy individuals and considered normal; any thickening in the frontal and sphenoid sinuses is always considered abnormal and is most likely to be symptomatic. In the ethmoid sinuses, mucosal thickening of up to 2 mm may be attributed to the physiologic phenomenon of the nasal cycle if there is coexistent mucosal swelling of the ipsilateral nasal cavity with prominence of the ipsilateral nasal turbinates ( Fig. 5 ).




Fig. 4


Axial T2 MR image in a patient with chronic headache shows a thin rim of hyperintense mucosal thickening along the walls of the maxillary sinuses.



Fig. 5


Coronal T2 MR image in a patient with vertigo shows prominence of the left middle and inferior turbinates without nasal septal deviation or sinonasal disease; likely attributable to the nasal cycle.


Bone changes


The presence of a sclerotic thickened bone is a fairly characteristic feature of chronic sinusitis but may not be seen in all patients. Sclerosis of the bony walls of the sinuses reflects the chronic nature of the disease process ( Fig. 6 ). However, with progressive obstruction and chronicity, there may eventually be gradual pressure erosion and deossification of the marginal bone that is more commonly seen with mucoceles and polyps (see Fig. 19 ).




Fig. 6


Axial CT image shows chronic maxillary sinusitis with mucosal disease bordering the thickened sclerotic walls.


Secretions and opacification


The density of the secretions at CT and their signal intensity on MR imaging is largely dependent on the proportion of proteins in the secretions. With chronic obstruction, there is a gradual increase in the protein content of the secretions and the thin watery secretions become progressively more viscous and thick. This is seen as a progressive increase in the density of the secretions on CT and the chronic inspissated secretions are hyperdense on CT. A thin hypodense line is usually seen separating the dense sinus secretions from the bony wall and represents the thickened mucosa and submucosal edema ( Fig. 7 ). On MR imaging, various combinations of signal intensities on T1 and T2 images have been described with increasing protein concentration of the secretions (see Fig. 7 ; Figs. 8 and 9 ). These are tabulated in Table 5 . On contrast administration, if the thickened mucosa does not enhance, it is probably not actively infected and is fibrotic and scarred. Active infection has a thin zone of mucosal enhancement with a zone of submucosal edema separating this mucosa from the bony wall ( Fig. 10 ).




Fig. 7


Coronal CT image ( A ) shows hyperdense secretions and scattered calcifications in the left maxillary sinus with a peripheral hypodense rim. The secretions are hyperintense on axial T1 MR images and hypointense on T2 MR images ( B , C ). Minimal mucosal disease seen in right maxillary sinus.



Fig. 8


Axial T1 MR image ( A ) shows hyperintense signal in the right sphenoid sinus that is hypointense on the T2 image in ( B ) ( white curved arrow ). The outlined white arrow points to the sphenoid sinus ostium in ( B ). Coronal CT image in ( C ) shows thickening of the walls of the right sphenoid sinus. Chronic sphenoid sinusitis with chronic inspissated secretions.



Fig. 9


Coronal CT image ( A ) shows opacified sphenoid sinuses with marked thinning of the left sinus wall ( black arrowheads ). T2 MR image ( B ) and contrast T1 image ( C ) show a signal void in the sinus with enhancing peripheral sinus mucosa in ( C ). Chronic sphenoid sinusitis with inspissated secretions mimicking normal sinus on MR imaging.


Table 5

Sinonasal secretions at MR imaging




























Protein Content T1 Images T2 Images
Up to 5% Hypointense Hyperintense
5%–25% Hyperintense Hyperintense
25%–30% Hyperintense Hypointense (see Figs. 7 and 8 )
30%–35% a Hypointense Hypointense
>35% a Signal void Signal void (see Fig. 9 )

a These chronically dessicated secretions can be easily misinterpreted as normal aerated sinuses on MR imaging, but CT allows easy identification because of their hyperdense attenuation (see Fig. 9 ).




Fig. 10


Axial T2 image ( A ) shows polypoid mucosal disease with submucosal edema underlying the thin mucosa in both maxillary sinuses. Contrast T1 image ( B ) shows mucosal enhancement outlining the nonenhancing submucosal edema suggesting active infection.


Calcifications


Calcifications are uncommon in CRS and, when seen, are more peripheral and scattered. Yoon and colleagues found that round or egg-shell calcifications are more common in patients with chronic inflammatory rhinosinusitis (see Fig. 7 A).


Sequelae or Local Complications with Chronic Sinusitis


Retention cyst


Retention cyst is the most common finding in patients with chronic sinusitis and is also seen as an incidental finding in the general population. Two forms of retention cysts have been described: serous and mucous retention cysts; the mucous variety is more common. Mucous retention cysts are formed as a result of obstruction of a submucous mucinous gland and serous cysts occur as a result of accumulation of fluid in the submucosal layer of the sinus mucosa. They are most commonly found in the maxillary sinuses. Both forms cannot be distinguished from one another and are seen as mucoid, low-density, well-defined, outwardly convex lesions on CT, with low signal on T1-weighted images and high signal on T2-weighted MR imaging sequences ( Fig. 11 ).


Mar 13, 2017 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Imaging in Sinonasal Inflammatory Disease
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