Contrast-Enhanced CT (CECT) of the Paranasal Sinuses

Overview

Indications for Contrast Enhancement

1. Unilateral pathology - raises suspicion for neoplasm rather than bilateral inflammatory disease
2. Suspected neoplasm - to assess tumor extent, vascular involvement, and enhancement characteristics
3. Aggressive or invasive infections - especially invasive fungal sinusitis with orbital or intracranial spread
4. Vascular pathology - vascular tumors (e.g., juvenile nasopharyngeal angiofibroma), carotid involvement
5. Intracranial extension - epidural abscess, meningitis, cavernous sinus thrombosis as complications of sinusitis
6. Intraorbital extension - orbital cellulitis, subperiosteal abscess
7. Severe nasal polyposis - contrast demonstrates the characteristic "cascading" enhancement of polyps and helps characterize associated mucoceles
8. Fungal sinusitis - to identify soft tissue and bony changes and extrasinus spread

Patient Preparation

• IV access: Insert a peripheral IV cannula (18-20G) in the antecubital fossa or dorsum of hand
• Check renal function: serum creatinine/eGFR before iodinated contrast, especially if there is renal disease, diabetes, or dehydration
• Screen for contrast allergy history: prior reactions, iodine allergy, asthma
• Premedication: if prior mild/moderate reaction, consider corticosteroid + antihistamine premedication
• Hydration: ensure adequate oral or IV hydration, especially in patients at risk for contrast-induced nephropathy
• Metformin: withhold 48 hours before and after contrast if significant renal impairment
• Informed consent and IV contrast injection instructions

Scanner Setup and Technical Protocol

Contrast Agent and Injection Protocol

• Agent: Non-ionic iodinated contrast (e.g., iohexol, iopamidol, iodixanol)
• Dose: ~1.5-2 mL/kg body weight (typically 80-100 mL in adults)
• Concentration: 300-370 mgI/mL
• Injection rate: 2-3 mL/s via IV
• Scanning delay: 60-70 seconds (portal venous/soft tissue phase) for most indications; arterial phase (~25-30 s) if a vascular lesion is suspected
• For suspected tumor invasion: a delayed phase (3-5 minutes) may be added to assess contrast retention vs. washout

Scan Acquisition

1. Survey/scout image (topogram): Lateral and AP scouts to confirm positioning and coverage (from top of frontal sinuses to hard palate)
2. Pre-contrast scan: Recommended baseline, especially for tumors or vascular lesions, to identify intrinsic hyperdensity (calcification, hemorrhage)
3. Post-contrast scan: Acquire in the axial plane, volume acquisition, no gantry tilt
4. Coverage: From the floor of the frontal sinus superiorly to the hard palate/inferior maxillary alveolus inferiorly

Image Reconstruction and Viewing

• Multiplanar reconstruction (MPR): The axial volume dataset is reconstructed in all three orthogonal planes:
  • Coronal plane: Most useful for OMC anatomy, extent of mucosal disease, pre-FESS planning, and cribriform plate assessment
  • Sagittal plane: Best for frontal sinus drainage pathway (frontal recess), anterior ethmoid air cells, lateral nasal wall
  • Axial plane: Best for optic nerves, pterygopalatine fossa, fossa of Rosenmuller, posterior table of frontal sinus, orbital walls
• Window settings:
  • Bone window: WW 2000-4000 HU, WL 400-700 HU - for bony detail, sinus walls, OMC anatomy
  • Soft tissue window: WW 350-400 HU, WL 40-60 HU - for mucosal enhancement, soft tissue lesions, intracranial/intraorbital extension
  • Custom windows: Gradually lowering the window level (WL) makes air spaces more conspicuous; lung-window-like settings help delineate air cell boundaries. Do NOT try to evaluate soft tissue details from bone window images - this degrades image quality unacceptably.

Interpreting CECT Findings

Post-Procedure Care

• Monitor patient for 15-30 minutes post-injection for delayed contrast reactions
• Encourage oral hydration after the procedure
• Document contrast dose, lot number, and any adverse reactions
• Resume metformin 48 hours post-procedure after confirming stable renal function

Key Points

• Routine pre-FESS CT is always non-contrast - contrast adds no benefit for anatomic mapping of the OMC
• CECT is reserved for unilateral, neoplastic, vascular, or complicated infectious pathology
• Isotropic sub-millimeter acquisition with MPR in all three planes is essential; any ≥16-row MDCT can achieve this
• Soft tissue windows on CECT are essential for assessing mucosal enhancement and extrasinus spread; bone windows alone are insufficient
• MRI with gadolinium is preferred over CECT for intracranial extension, perineural spread, and distinguishing tumor from obstructed secretions - CECT is a complementary modality

Patient Position

• Supine, head first into the gantry
• Head in neutral/straight position - no flexion, no extension
• No gantry tilt (0°) - mandatory, especially for image-guided navigation systems

Centering Point

• Center at the nasion (bridge of the nose, between the two eyes)
• Coverage: From the top of the frontal sinuses superiorly → down to the hard palate inferiorly

Reconstruction Slice Thickness

Anatomy of Paranasal Sinuses on CT

1. Nasal Cavity - Coronal + Sagittal CT

• Cribriform plate = roof of nasal cavity
• Nasal septum = medial wall
• Lateral wall = contains the turbinates
• Palatine process = floor
• Middle and inferior conchae (turbinates)
• Choanae - posterior openings into the nasopharynx

2. Coronal CT - All Three Planes (Textbook: Cummings Otolaryngology)

• Ethmoid cells (roof) - between orbits, honeycomb appearance
• Cribriform plate - at the top, separates ethmoid from anterior cranial fossa
• Basal lamella - divides anterior from posterior ethmoid
• Middle and inferior turbinates
• Maxillary sinuses - bilateral large air spaces below the orbits
• Nasal septum
• Hard palate (floor)

• Nasal septum - midline
• Anterior and posterior ethmoid cells
• Lamina papyracea - paper-thin medial orbital wall (important surgical landmark)
• Intersinus septum
• Sphenoid sinus - posterior midline structure

• Frontal sinus - anterior superior
• Frontal recess (frontal beak) - drainage pathway of frontal sinus
• Anterior and posterior ethmoid cells
• Bulla ethmoidalis - largest anterior ethmoid cell
• Sphenoid sinus - posterior
• Sella turcica - above sphenoid sinus
• Basal lamella
• Middle meatus (asterisks)
• Clivus
• Hard and soft palate

3. Sagittal + Axial Views (More Detail)

• Agger nasi cell - most anterior ethmoid cell, just anterior to the frontal recess
• Frontal recess - drainage pathway between frontal sinus and middle meatus
• Hiatus semilunaris - crescent-shaped gap between uncinate process and bulla
• Bulla ethmoidalis
• Sphenoethmoidal recess - drains sphenoid sinus into superior meatus
• Sphenoid sinus, Nasopharynx

• Nasal bones
• Lamina papyracea - medial orbital wall
• Nasolacrimal duct - anterolateral
• Anterior and posterior ethmoid cells
• Intersinus septum
• Sphenoid sinus with its intersinus septum

4. Ostiomeatal Complex (OMC) - Coronal CT

5. Axial CT - Nasal Cavity Detail

• Middle turbinate and middle meatus
• Uncinate process - thin curved bony plate, key FESS landmark
• Infundibulum - the drainage channel lateral to uncinate
• Hiatus semilunaris - the crescent gap opening
• Superior turbinate and superior meatus
• Sphenoethmoidal recess - drains the sphenoid sinus

Summary: What to Identify on Each Plane