Acute Rhinosinusitis on CT: A Radiologist’s Guide

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The central principle

Acute rhinosinusitis (ARS) is primarily a clinical diagnosis, not an imaging diagnosis. CT findings such as mucosal thickening, sinus fluid, air-fluid levels, and complete opacification support sinonasal inflammation but cannot reliably distinguish viral from bacterial infection; similar abnormalities occur with viral upper respiratory infections and in asymptomatic patients.[1][2][^3]

For the radiologist, the most important question is usually not simply “Is sinusitis present?” but rather:

1. Is there a plausible acute inflammatory pattern that matches the symptoms?
2. Is there an alternative cause, especially dental disease, tumor, trauma, or postoperative change?
3. Has infection extended beyond the sinus into the orbit, bone, intracranial compartment, deep face, or vascular structures?
4. Is the patient at risk for acute invasive fungal rhinosinusitis?

A report should therefore separate intrasinus inflammatory findings from extrasinus complications, state the limitations of CT for assigning bacterial etiology, and communicate immediately when a vision-, brain-, or life-threatening complication is present.[4][5][^6]

Clinical framework

Definition and terminology

ARS refers to inflammation of the nasal cavity and paranasal sinuses lasting less than four weeks. Because the nasal and sinus mucosa form a contiguous system, rhinosinusitis is preferred to sinusitis.[1][7]

Presumed acute bacterial rhinosinusitis (ABRS) is diagnosed clinically when compatible symptoms:

• Persist without improvement for at least 10 days, or
• Worsen after initial improvement within 10 days—the “double-worsening” pattern.

The IDSA framework also recognizes severe onset with high fever and purulent nasal drainage or facial pain for at least three consecutive days. These criteria are based on clinical evolution, not CT appearance.[2][8][^9]

Imaging is usually unnecessary

Initial imaging is usually inappropriate for uncomplicated ARS. AAO-HNS, IDSA, and ACR guidance reserve imaging for a suspected complication, an alternative diagnosis, recurrent or treatment-refractory disease, immunocompromise, or surgical planning.[1][2][^8]

This matters when interpreting an incidental CT head or maxillofacial CT: mucosal thickening alone should not be converted into a clinical diagnosis of “acute bacterial sinusitis.” A more defensible formulation is “paranasal sinus inflammatory change; correlate with symptoms for acute rhinosinusitis.”[9][10][^11]

Choosing the examination

For complicated infection, CT should use thin sections with bone and soft-tissue reconstructions and multiplanar reformations. Coverage must include the entire sinuses, orbits, facial soft tissues, anterior and middle cranial fossae, and cavernous sinuses; extend brain or neck coverage according to symptoms.[12][6]

A systematic CT approach

First: verify adequacy

Before interpreting the sinuses, ask:

• Was IV contrast administered when a complication is suspected?
• Are the orbits and relevant intracranial structures fully included?
• Are thin-section bone and soft-tissue reconstructions available?
• Is motion or dental artifact obscuring the maxillary alveolus, pterygopalatine fossae, orbital apices, or cavernous sinuses?
• Does the clinical history mention immunocompromise, diabetes, neutropenia, recent dental procedure, trauma, surgery, severe headache, visual symptoms, or neurologic deficit?

A limited noncontrast head CT may reveal sinus disease but can inadequately assess the face, orbital floors, teeth, cavernous sinuses, or subtle enhancing intracranial complications. If the symptoms imply complicated infection, recommend the appropriate contrast-enhanced study rather than falsely reassuring from incomplete imaging.[12][6]

Second: characterize each sinus

For each sinus, document:

• Side and involved compartment.
• Mucosal thickening versus dependent fluid.
• Air-fluid level or gas bubbles within secretions.
• Partial versus complete opacification.
• Homogeneous versus heterogeneous contents.
• Hyperattenuating or calcified material.
• Drainage-pathway obstruction.
• Bone response: normal, smooth chronic sclerosis, smooth pressure expansion, focal dehiscence, osteomyelitis, or aggressive destruction.
• Extrasinus inflammatory change.

Acute inflammatory CT appearances include mucosal thickening, dependent fluid, air-fluid levels, bubbles in fluid, and partial or complete opacification. Air-fluid levels and complete opacification are more suggestive of active sinus disease than mild mucosal thickening, but none alone proves bacterial infection.[18][3][^19]

Third: inspect beyond the sinuses

The highest-value part of a complicated-sinusitis interpretation is the deliberate search outside the sinus walls:

• Preseptal and postseptal orbital fat.
• Medial and superior orbital subperiosteal spaces.
• Extraocular muscles and orbital apex.
• Optic nerves and superior ophthalmic veins.
• Cavernous sinuses and internal carotid arteries.
• Frontal scalp, frontal bone, and epidural space.
• Dura, extra-axial spaces, frontal lobes, and temporal lobes.
• Dural venous sinuses and cortical veins.
• Premalar and retroantral fat, pterygopalatine fossae, infratemporal fossae, palate, and deep facial spaces.

Sinus opacification establishes a possible source; abnormalities in these adjacent compartments determine urgency and management.[6][20]

Interpreting intrasinus findings

Mucosal thickening

Mild smooth mucosal thickening is nonspecific and commonly incidental. It may accompany viral infection, allergy, chronic inflammation, dental disease, recent instrumentation, or other mucosal irritation; it should not be labeled ABRS without appropriate clinical findings.[9][10][^11]

More extensive circumferential thickening that obstructs an ostium is more meaningful, particularly when accompanied by dependent fluid or complete opacification. However, the report should describe the imaging pattern rather than infer the pathogen.[3][19]

Fluid and air-fluid levels

Dependent fluid or an air-fluid level supports active secretions and, in the correct clinical context, acute rhinosinusitis. It is not specific for bacterial disease and may also occur with viral inflammation, hemorrhage, trauma, recent surgery, prolonged recumbency, nasogastric or endotracheal tubes, and barotrauma.[2][3][^11]

If trauma is present, hyperattenuating sinus fluid adjacent to a fracture should be interpreted as hemosinus rather than automatically called sinusitis. If an intubated inpatient has diffuse dependent sinus fluid, correlate for nosocomial sinusitis but avoid assuming infection based on fluid alone.

Complete opacification

Complete opacification is compatible with acute inflammation but has a broad differential:

• Acute or chronic inflammatory secretions.
• Obstructed retention cyst or mucocele.
• Fungal ball or allergic fungal disease.
• Hemorrhage.
• Polyp or tumor with trapped secretions.
• Postoperative packing or debris.

The radiologist should examine sinus volume, wall remodeling, focal bone destruction, internal density, and the obstructed drainage pathway. Smooth expansion favors a mucocele; focal aggressive destruction or a discrete enhancing mass requires evaluation for invasive infection or neoplasm.

Hyperattenuating contents

Hyperattenuation can reflect inspissated proteinaceous secretions, fungal elements, hemorrhage, or highly cellular material. In a routine immunocompetent patient, central hyperattenuating material with chronic wall sclerosis may suggest a fungal ball rather than uncomplicated ARS; in an immunocompromised patient, hyperattenuation does not by itself establish invasion—extrasinus infiltration and devitalized mucosa are more important.[14][15]

Bone changes

Uncomplicated acute inflammation generally does not cause aggressive bone destruction. Smooth wall thickening and sclerosis suggest chronic or recurrent inflammation, while smooth expansion and thinning suggest a mucocele.[4][21]

Focal cortical erosion, permeative destruction, or adjacent marrow abnormality should raise concern for osteomyelitis, invasive fungal disease, aggressive bacterial infection, or tumor. Importantly, acute invasive fungal infection can spread through vessels and nerves before bone erosion occurs, so intact sinus walls do not exclude invasion.[14][22]

Sinus-by-sinus implications

Maxillary sinus

Assess the maxillary infundibulum and ostiomeatal unit, but also scrutinize the maxillary alveolus. Unilateral maxillary-predominant disease should trigger a dedicated search for periapical lucency, periodontal bone loss, oroantral fistula, extraction site, displaced root, implant protrusion, graft material, or other dental foreign body.[^17]

Odontogenic disease may extend from the maxillary sinus into the ipsilateral anterior ethmoid and frontal sinuses. If a dental source is visible, state it explicitly because antibiotics or sinus treatment alone may not correct the cause.

Useful wording: (illustrative report)

Near-complete left maxillary sinus opacification with an air-fluid level and obstruction of the left maxillary infundibulum. Periapical lucency and cortical breakthrough at the left maxillary first molar communicate with the sinus floor, favoring an odontogenic source.

Ethmoid sinuses

Ethmoid disease deserves close orbital inspection because infection can spread through the thin lamina papyracea, congenital or acquired dehiscences, neurovascular foramina, and valveless venous channels. Ethmoid sinusitis is a common source of medial orbital cellulitis and subperiosteal abscess.[6][20]

A small early medial subperiosteal abscess may appear only as lentiform or crescentic soft tissue separating the medial rectus/extraconal fat from the lamina papyracea. Do not require a large rim-enhancing collection before identifying a developing subperiosteal process.[23][24]

Frontal sinus

Frontal sinusitis requires inspection of both the anterior and posterior tables, frontal scalp, frontal bone marrow, epidural space, dura, superior sagittal sinus, and frontal lobes. Intracranial spread may occur directly through bone/dehiscence or through diploic venous channels.[25][26]

Pott puffy tumor is frontal bone osteomyelitis with a subperiosteal scalp abscess. CT may show frontal sinus opacification, frontal soft-tissue swelling or collection, cortical erosion, and an epidural collection; MRI better assesses marrow, dura, venous sinuses, and intracranial infection.[27][28]

Useful wording: (illustrative report)

Acute frontal sinus inflammatory disease complicated by anterior-table erosion, frontal bone osteomyelitis, and a 2.4-cm subperiosteal forehead abscess—Pott puffy tumor. Posterior-table irregularity and adjacent epidural enhancement warrant MRI to assess intracranial extension.

Sphenoid sinus

Isolated sphenoid disease may present with headache or cranial neuropathy and can threaten the cavernous sinus, internal carotid artery, optic nerve, pituitary region, meninges, and central skull base. Carefully assess the cavernous sinuses, superior ophthalmic veins, carotid arteries, orbital apices, optic canals, clivus, and prepontine region.[29][30]

Cavernous-sinus thrombosis may produce expansion, convex lateral margins, heterogeneous enhancement or a filling defect; superior ophthalmic-vein dilation/thrombosis and orbital venous congestion are supporting findings. CTA/CTV or MRI/MRV may be necessary when vascular involvement is suspected.[^20]

Orbital complications

Use an anatomic description

The Chandler framework is useful but the report should also describe the actual anatomy and drainable collections:

Report surgically actionable details

For a subperiosteal or orbital abscess, include:

• Side and compartment: medial, superior, inferior, intraconal, or extraconal.
• Three dimensions when practical.
• Rim enhancement and internal gas.
• Adjacent sinus source.
• Proptosis and globe deformation/tenting.
• Optic-nerve stretch, sheath abnormality, or orbital-apex involvement.
• Extraocular-muscle displacement or involvement.
• Superior ophthalmic-vein and cavernous-sinus patency.
• Coexisting intracranial complication.

Orbital-compartment physiology and vision loss are clinical emergencies; imaging findings such as marked proptosis, posterior globe tenting, optic-nerve stretch, or orbital-apex compression should be communicated immediately.[6][14]

Avoid common errors

• Do not call preseptal edema “orbital cellulitis”; postseptal involvement defines orbital disease.
• Do not miss a small medial subperiosteal abscess by focusing only on the globe.
• Do not stop after finding an orbital abscess; inspect the cavernous sinuses and brain.
• Do not rely on CT alone if vision loss, ophthalmoplegia, meningeal symptoms, or neurologic deficits persist despite nondiagnostic CT; MRI is more sensitive for the orbital apex, optic nerve, meninges, and intracranial infection.[6][26]

Intracranial complications

Search systematically for:

• Pachymeningeal or leptomeningeal enhancement.
• Epidural abscess.
• Subdural empyema.
• Cerebritis.
• Intraparenchymal abscess.
• Frontal or temporal lobe edema.
• Cavernous, superior sagittal, or other dural venous sinus thrombosis.
• Cortical-vein thrombosis.
• Arterial narrowing, infarction, or mycotic aneurysm.
• Hydrocephalus or mass effect.

Frontal sinusitis is a major source of epidural and subdural infection, while sphenoid and posterior ethmoid disease raise concern for cavernous-sinus, carotid, optic, and central skull-base complications.[25][29][^26]

MRI with contrast and DWI is more sensitive than CT for meningitis, cerebritis, early empyema, small abscesses, marrow infection, and venous thrombosis. Restricted diffusion within a rim-enhancing extra-axial or parenchymal collection strongly supports purulent material.[6][13]

Urgent wording: (illustrative report)

Frontal sinusitis complicated by a 6-mm left frontal subdural empyema with restricted diffusion, adjacent cerebritis, and mild local mass effect. No midline shift. Urgent ENT and neurosurgical consultation is recommended.

Osseous and soft-tissue complications

Osteomyelitis

Evaluate the frontal bone, maxilla, central skull base, and clivus for cortical erosion, trabecular rarefaction, sclerosis, marrow signal abnormality, and adjacent subperiosteal or epidural collection. CT is strongest for cortical bone; MRI is better for marrow and intracranial extent.[27][21]

Facial and deep-space spread

Look for inflammatory change or abscess in the premalar tissues, retroantral fat, pterygopalatine fossa, infratemporal fossa, masticator space, palate, and nasopharynx. In an immunocompetent patient this may represent advanced bacterial spread; in a high-risk host it should raise urgent concern for invasive fungal infection.[14][15]

Acute invasive fungal rhinosinusitis

When to suspect it

Acute invasive fungal rhinosinusitis (AIFR) is a separate emergency from routine ARS. Suspicion is highest in patients with neutropenia, hematologic malignancy, transplantation, uncontrolled diabetes or ketoacidosis, corticosteroid exposure, or other major immunosuppression.

Imaging may be subtle early, and absence of bone destruction does not exclude the diagnosis. Endoscopic evaluation and tissue diagnosis remain decisive when clinical suspicion is high.[14][22]

Early CT targets

Actively inspect for:

• Marked asymmetric nasal-cavity mucosal thickening.
• Turbinate ulceration or nonenhancement.
• Periantral or retroantral fat infiltration.
• Pterygopalatine-fossa infiltration.
• Nasolacrimal-duct or lacrimal-sac involvement.
• Premalar, palatal, orbital, or deep-space infiltration.
• Vascular narrowing, thrombosis, or infarction.
• Perineural spread.

Periantral and pterygopalatine-fossa infiltration can precede bone erosion because fungal disease spreads through vessels and nerves. Bone dehiscence is specific but insensitive and therefore is a late, unreliable gatekeeper.[14][33][^22]

MRI targets

MRI better depicts devitalized nonenhancing mucosa, orbital apex and optic nerve involvement, cavernous-sinus disease, perineural extension, dural/intracranial invasion, infarction, and abscess. The “black turbinate” sign must be interpreted cautiously because apparent turbinate nonenhancement can have technical or physiologic mimics; assess the total pattern rather than a single sign.[15][33]

Urgent wording: (illustrative report)

Asymmetric left nasal and maxillary mucosal disease with infiltration of the retroantral fat and pterygopalatine fossa despite intact sinus walls. In this immunocompromised patient, findings are highly concerning for acute invasive fungal rhinosinusitis. Urgent ENT evaluation with endoscopy/biopsy and contrast-enhanced MRI is advised.

Important mimics

Chronic rhinosinusitis with an acute flare

Underlying chronic disease is suggested by bony wall sclerosis/hyperostosis, sinus contraction, established polyposis, longstanding drainage-pathway obstruction, or prior surgery. Acute fluid or new symptoms may represent superimposed acute inflammation, but CT alone cannot determine whether chronic disease is acutely infected.

Prefer: (illustrative report)

Chronic left maxillary and ethmoid inflammatory disease with superimposed fluid levels that may reflect an acute component in the appropriate clinical setting.

Odontogenic disease

Unilateral maxillary disease, particularly with adjacent dental pathology, should not be reported generically as sinusitis. Identify the responsible tooth and the route of communication whenever possible.[^17]

Hemorrhage and trauma

Hyperdense sinus contents after trauma, surgery, epistaxis, anticoagulation, or biopsy may be blood. Search for fracture and active extravasation; avoid equating hemosinus with infection.

Mucocele

A mucocele is an obstructed, expansile sinus filled with secretions. Smooth expansion and remodeling distinguish it from routine ARS; infection of a mucocele may produce surrounding inflammatory change and is termed a mucopyocele.

Neoplasm

Unilateral opacification associated with a discrete enhancing mass, focal aggressive bone destruction, pterygopalatine-fossa involvement, perineural spread, nodal disease, or unexplained epistaxis should prompt consideration of neoplasm. Obstructed secretions may dominate the CT appearance, so inspect the drainage pathway for the causative lesion.[^4]

Noninvasive fungal disease

A fungal ball typically produces unilateral sinus opacification with internal hyperattenuating or calcific material and chronic bony sclerosis. Allergic fungal rhinosinusitis more often produces heterogeneous hyperdense material, expansion/remodeling, and multisinus disease; neither pattern should be conflated with AIFR without extrasinus invasion and appropriate host factors.

Retention cysts

A dome-shaped maxillary retention cyst is usually incidental and should not be described as acute sinusitis unless accompanied by other inflammatory findings and matching symptoms.

Reporting strategy

Findings section

A useful sequence is:

1. Distribution and severity: identify involved sinuses and nasal cavity.
2. Acute-pattern contents: fluid, air-fluid levels, bubbles, or complete opacification.
3. Drainage pathways: state the obstructed ostiomeatal unit, frontal recess, or sphenoethmoidal recess when relevant.
4. Source: dental, postoperative, obstructing lesion, or no specific source.
5. Bone: no erosion versus osteomyelitis, dehiscence, expansion, or aggressive destruction.
6. Orbit: preseptal/postseptal inflammation and any collection.
7. Intracranial/vascular: meninges, extra-axial spaces, brain, cavernous sinus, superior ophthalmic veins, and dural sinuses.
8. Deep face: periantral and pterygopalatine-fossa infiltration, especially in an immunocompromised patient.

Impression principles

• Do not diagnose bacterial infection from CT alone.
• Lead with complications, not the sinus opacification.
• Name the source sinus and route of spread.
• Distinguish preseptal from postseptal orbital disease.
• Give collection location and size.
• State whether there is bone erosion/osteomyelitis.
• Recommend MRI when CT is negative/equivocal but intracranial, orbital-apex, cavernous-sinus, or fungal invasion remains clinically suspected.
• Directly communicate vision-threatening, intracranial, vascular, or invasive fungal findings.[6][26]

Example impressions

Uncomplicated pattern on incidental CT

Mild scattered paranasal sinus mucosal thickening without fluid level or extrasinus inflammatory change. Findings are nonspecific and should not be used alone to diagnose acute bacterial rhinosinusitis.

Acute-pattern findings without complication

Left maxillary and anterior ethmoid inflammatory disease with maxillary sinus fluid level and obstruction of the left ostiomeatal unit, compatible with acute rhinosinusitis in the appropriate clinical setting. No orbital or intracranial complication identified.

Odontogenic source

Acute-pattern right maxillary sinus disease with extension into the anterior ethmoid air cells. Periapical abscess of the right maxillary first molar breaches the sinus floor, favoring odontogenic sinusitis. No facial, orbital, or intracranial abscess.

Orbital complication

Ethmoid-predominant sinusitis complicated by right postseptal orbital cellulitis and a 1.8-cm medial subperiosteal orbital abscess. Mild proptosis without posterior globe tenting or orbital-apex involvement. Superior ophthalmic vein and cavernous sinus remain patent.

Intracranial complication

Frontal sinusitis complicated by posterior-table osteomyelitis, a 7-mm frontal epidural abscess, and adjacent pachymeningitis. MRI brain with contrast and DWI is recommended to define full intracranial extent; urgent ENT/neurosurgical consultation is warranted.

Sphenoid complication

Complete sphenoid sinus opacification with abnormal enhancement and filling defect in the left cavernous sinus and thrombosis of the left superior ophthalmic vein, consistent with septic cavernous-sinus thrombosis. Mild narrowing of the cavernous left internal carotid artery; CTA/MRA correlation is advised.

Quick reference

When to image

• Do not image routine uncomplicated ARS.
• Use contrast-enhanced CT for suspected orbital, intracranial, osseous, facial, or vascular complication.
• Add MRI with contrast and DWI for neurologic signs, meningeal concern, orbital-apex/optic findings, equivocal CT, or suspected invasive fungal disease.

What CT can show

• Mucosal thickening, fluid, air-fluid level, bubbles, or complete opacification.
• Obstructed drainage pathway and potential dental source.
• Bone erosion, osteomyelitis, mucocele, or obstructing mass.
• Orbital, facial, intracranial, and vascular extension.

What CT cannot prove

• Viral versus bacterial etiology.
• That incidental mild mucosal thickening explains symptoms.
• Absence of early invasive fungal disease merely because bone is intact.
• Absence of subtle meningitis, cerebritis, empyema, orbital-apex disease, or cavernous-sinus disease when clinical suspicion remains high.

Each sinus

• Maxillary: inspect teeth, sinus floor, infundibulum, and retroantral fat.
• Ethmoid: inspect lamina papyracea, medial orbit, and subperiosteal space.
• Frontal: inspect both tables, frontal bone, scalp, epidural/subdural spaces, and frontal lobes.
• Sphenoid: inspect cavernous sinuses, carotids, optic nerves, orbital apices, clivus, and meninges.

Orbital checklist

• Preseptal or postseptal?
• Cellulitis or drainable abscess?
• Medial/superior subperiosteal collection?
• Proptosis, globe tenting, optic-nerve stretch, or orbital-apex involvement?
• Superior ophthalmic-vein or cavernous-sinus thrombosis?

Intracranial checklist

• Pachymeningitis or leptomeningitis?
• Epidural abscess or subdural empyema?
• Cerebritis or brain abscess?
• Dural venous thrombosis?
• Arterial narrowing, infarct, or aneurysm?
• Need for MRI with contrast and DWI?

Red flags

• Forehead swelling plus frontal sinusitis: consider Pott puffy tumor.
• Ethmoid sinusitis plus orbital symptoms: seek a medial subperiosteal abscess.
• Sphenoid sinusitis plus cranial neuropathy: assess cavernous sinus, carotid artery, and orbital apex.
• Immunocompromised host plus unilateral nasal disease or periantral/PPF infiltration: suspect acute invasive fungal rhinosinusitis, even without bone erosion.

Best reporting sentence

CT demonstrates acute-pattern sinonasal inflammatory disease, but imaging cannot distinguish viral from bacterial rhinosinusitis; the clinically important finding is the presence or absence of extrasinus complication and an identifiable source.