Role of Imaging in Paranasal Sinus Disease

1. Imaging Modalities Overview

Plain Radiography

Computed Tomography (CT)

• The coronal plane is best for assessing the anterior osteomeatal unit (OMU)
• The sagittal plane best demonstrates the frontal sinus drainage pathway
• The axial plane is optimal for the sphenoethmoidal recess

Fig. 61.33 - Normal Paranasal Sinus CT. (A) Coronal: red arrows show mucociliary flow in left antrum toward the infundibulum (green stars); EB = ethmoidal bulla. (B) Sagittal: F = frontal sinus, interrupted white line = frontal drainage pathway, AE/PE = anterior/posterior ethmoid, S = sphenoid, MT/IT = middle/inferior turbinate. (C) Axial: S = sphenoid, PE = posterior ethmoid, white arrow = sphenoid ostium, interrupted line = sphenoethmoidal recess.

Magnetic Resonance Imaging (MRI)

• Differentiating tumour from inflammatory/secretion-filled sinus
• Assessing tumour extent, particularly orbital and intracranial spread
• Characterising fungal disease (signal characteristics of concretions)
• Excluding non-sinonasal causes of anosmia (e.g., subfrontal meningioma, olfactory neuroblastoma)
• Evaluating recurrent disease post-surgery (e.g., recurrent inverted papilloma)

2. Indications for Imaging

1. The patient has failed medical treatment for rhinosinusitis
2. Functional Endoscopic Sinus Surgery (FESS) is being considered - CT acts as the "road map"
3. Acute complications are suspected - orbital cellulitis, subperiosteal abscess, intracranial extension
4. A mucocele is suspected
5. A neoplastic lesion is suspected

• Soft-tissue tumour characterisation is needed
• Orbital or intracranial extension needs to be accurately mapped
• Invasive fungal sinusitis is suspected in immunocompromised patients
• CT findings are equivocal between tumour and secretions

3. What to Assess on CT in Rhinosinusitis

• Deviated nasal septum and septal spur
• Concha bullosa (pneumatised middle turbinate) or paradoxical turbinate turn - both can narrow the middle meatus
• Hypoplasia or enlargement of structures (maxillary antrum, frontal sinus, ethmoidal bulla)
• Anomalous air cells: frontoethmoidal, sphenoethmoidal, and infraorbital (Haller) cells

• Does antral inflammation reach the ostium, infundibulum, or middle meatus?
• Is the whole osteomeatal unit involved?
• Does frontal or sphenoid sinus disease extend to the sinus opening or beyond?
• Are there fluid levels or bubbly secretions (suggesting an acute component)?

• Bony thickening suggesting chronicity
• Bony erosion or destruction suggesting a more aggressive process (tumour, invasive fungal disease)

• Radicular cysts or periapical infection may cause reactive maxillary antral inflammation - the underlying dental cause must be identified

Fig. 61.36 - Coronal CT: near-complete opacification of left antrum due to radicular cyst arising from the palatal root of the left upper molar (long arrow); roof of cyst = short arrows.

• Orbital extension (subperiosteal abscess, orbital cellulitis)
• Intracranial extension (rare)

4. Specific Conditions and Their Imaging Features

Rhinosinusitis

• CT/CBCT: mucosal thickening, opacification, air-fluid levels (acute), bony thickening (chronic)
• MRI: T2 high-signal mucosal thickening; fluid levels clearly demonstrated

Nasal Polyposis

• CT: bilateral soft-tissue density filling the nasal cavity and sinuses, with widening of the middle meati; may show remodelling of bony walls
• MRI: post-gadolinium T1 images show peripheral mucosal enhancement distinguishing polyps from retained secretions; can delineate extent to frontal, ethmoidal, and maxillary sinuses

Mucocele

• An expanding mucus-filled sinus with thinned/dehiscent walls. CT shows a homogeneous soft-tissue density lesion expanding and remodelling the sinus walls; most common in the frontal and ethmoidal sinuses.
• MRI: signal varies with protein content of secretions; T1 and T2 can both be high signal.

Fungal Sinusitis

• Non-invasive (fungal ball/mycetoma): high-attenuation central concretions within a sinus (typically maxillary), with or without calcification. Use soft-tissue window settings (~2000 HU window width, centred at -250 HU) to accentuate this.
• Allergic Fungal Rhinosinusitis (AFRS): heterogeneous high-attenuation material ("double density" sign), sinus expansion, possible lamina papyracea erosion and intracranial extension in severe cases

• T1: central hypointensity with peripheral enhancement
• T2: central hypointensity or signal void (due to heavy metals - iron, magnesium, manganese - and calcium within fungal concretions)
• IV gadolinium: can assess uncertain cases or orbital/intracranial complications

Fig. 61.37 - Fungal Sinusitis. Coronal CT (soft-tissue recon): high-attenuation material fills the sphenoid sinus consistent with fungal infection, eroding the roof (1); note proximity to optic nerve (2).

Osteoma

• CT: well-defined, sessile or pedunculated, high-density bony lesion
• Most common in the frontal sinus (80%), followed by ethmoid (20%); rarely maxillary or sphenoid
• Usually an incidental finding; surgery is considered if >50% of sinus volume is occupied or if drainage is compromised

Inverted Papilloma

• CT: middle meatal soft-tissue mass, usually unilateral; characteristic lobulated outline; 10% show calcification at the site of attachment; bony thickening of the sinus wall at the attachment site
• MRI: characteristic convoluted cerebriform pattern on T2-weighted imaging
• CT is usually satisfactory for primary assessment; MRI is reserved mainly for assessing recurrence post-surgery
• The site of bony attachment must be identified as subperiosteal resection is required to prevent recurrence

Juvenile Angiofibroma

• CT: soft-tissue mass originating at the sphenopalatine foramen; characteristic widening of the pterygopalatine fossa (PPF); erosion of the medial pterygoid plate and vidian canal aperture; erosion of the adjacent skull base
• A nasal mass with widened PPF in an adolescent male is pathognomonic
• Contrast-enhanced MRI is complementary to CT for accurate pre-surgical delineation and embolisation planning

Sinonasal Malignancy

• CT: bony erosion (e.g., lamina papyracea) suggests aggressive disease
• MRI: superior for defining soft-tissue extent; can distinguish preserved orbital periosteum (meaning orbital contents are not invaded) from actual orbital fat/muscle invasion - this distinction is critical for surgical planning
• Both CT and MRI should be used together for staging

5. CT as the Surgical Road Map for FESS

• Confirm the extent and distribution of disease
• Identify all relevant anatomical variants
• Identify any high-risk anatomy: position of the skull base (especially a low or asymmetric cribriform plate), course of the internal carotid artery (dehiscent within the sphenoid), position and dehiscence of the optic nerve, variations in the Onodi cell
• Plan the extent of surgery required (e.g., whether uncinate process removal alone will suffice, or whether a full OMU procedure, frontal recess surgery, or sphenoidal opening is needed)
• Identify intraorbital extension of ethmoidal cells (Haller cells) that may complicate antrostomy

Summary Table