CT imaging of the paranasal sinuses has become the standard of care for preoperative planning. Helical CT allows for rapid acquisition and multiplanar reformations. Unenhanced axial images are typically obtained at 0.625 mm intervals, with reformatted images in the coronal and sagittal planes obtained at 1–2 mm intervals. Both high-resolution bone and soft-tissue algorithms should be obtained in all three imaging planes.
- MRI: provides an excellent characterization of soft tissues.
MRI can discern secretions and mucosa from masses. It is also useful for determining invasion of the skull base, and perineural spread. It is important to remember that chronic secretions with high protein content may simulate ventilated sinuses.
- CBCT: first applied to dentomaxillofacial radiology.
The multiplanar images acquired by CBCT provide precise three-dimensional visualization of dental, and maxillofacial structures and paranasal sinuses. This technique has several advantages over the CT scan such as higher resolution, lower radiation doses and fewer metal artefacts. The main disadvantage is the poor visualization of soft tissues.
WHEN TO PERFORM IMAGING STUDIES
Table 1 shows the main imaging studies indicated for different sinonasal diseases according to the American College of Radiology (ACR).
1. NASAL CAVITY
The nasal septum separates the nasal cavities and extends from the nares anteriorly to the choanae posteriorly. The nasal septum is comprised of a single nasal cartilage (quadrangular cartilage) and two bones. The cartilage locates anteriorly and meets the anterior margins of the perpendicular plate of the ethmoid (superoposterior) and the vomer (inferoposterior).
- Nasal septum deviation: can be congenital or acquired, frequently secondary to traumas. Nasal septal deviation can be C-shaped, reverse C-shaped, S-shaped or reverse S-shaped. There is an anatomical classification of nasal septum known as Cottle areas: 1, nostril; 2, nasal valve; 3, attic; 4, turbinal; and 5, posterior.
- Nasal septum perforation: defect in the nasal septum with communication between the two nasal cavities. It may affect either the bony or cartilaginous septum. The anterior septal cartilaginous area is affected more commonly.
Turbinates are bony shelves covered by mucosa, located in the medial region of the nasal cavity. Their function is to filter and heat the air while breathing. The nasal meatuses are distinct air passages of the lateral nasal cavity located inferior to each turbinate.
- Superior turbinate: the most posterior and superior turbinate. The air passage between the superior nasal concha and the lateral nasal wall is known as the superior meatus. The superior meatus drains for the posterior ethmoid sinus and the sphenoid sinus drains into the esphenoethmoidal recess.
- Middle turbinate: The air passage between the middle nasal concha and the lateral nasal wall is known as the middle meatus. The middle meatus drains for the maxillary, frontal and anterior ethmoidal sinus. The basal lamella is part of the middle turbinate and separates the anterior and posterior ethmoidal cells, inserting into the lamina papyracea. The basal lamella has a vertical disposition in the sagittal plane. The anterior part of the middle turbinate can be divided into the inferior portion (turbinate tip) and the superior portion (vertical lamella) which inserts into the cribriform plate. Besides, the middle meatus constitutes a part of the ostiomeatal complex.
- Inferior turbinate: the greatest in size. The air passage between the inferior nasal concha and the lateral nasal wall is known as the inferior meatus. The inferior meatus drains for the nasolacrimal duct (through the Hassner valve). The hypertrophic inferior turbinate is associated with allergic rhinitis and/or contralateral nasal septal deviation.
Ostiomeatal complex (OMC): common channel that links the frontal sinus, anterior ethmoid cells and the maxillary sinus to the middle meatus, allowing airflow and mucociliary drainage. The different parts and their names are shown in the following figure.
- Some OMC anatomical variations can originate narrowing and even obstruction. Some examples are concha bullosa (aerated middle turbinate), nasal septal deviation, infraorbital ethmoid cells (Haller cells), oversized ethmoid bulla or agger nasi cells.
2. PARANASAL SINUS
The ethmoid bone is a single midline facial bone that separates the nasal cavity from the brain and is located at the roof of the nose and between the orbits in the anterior cranial fossa.
The different parts of the ethmoid bone are:
- Horizontal plate: cribriform plate.
- Vertical plate: the superior region is known as Crista Galli and the inferior region is the perpendicular plate.
- Roof: fovea ethmoidalis.
- Lateral lamella: the depth of the olfactory fossa is determined by the height of the lateral lamella of the cribriform plate, which can be classified into three categories (Keros classification):
- type 1: depth of 1-3 mm (30%).
- type 2: depth of 4-7 mm (69%).
- type 3: depth of 8-16 mm (1%). This type exposes more of the cribriform plate to potential damage from trauma, tumour erosion, CSF erosion and surgical complications.
The olfactory fossa (intracranial structure) is a variable depression in the cribriform plate that is medially bounded by the perpendicular plate and laterally by the lateral lamella. It contains the olfactory bulb and the anterior ethmoidal artery. The floor is the cribriform plate. The olfactory groove represents a cavity in the nasal roof and it contains olfactory epithelium. The roof is the cribriform plate.
The anterior ethmoid artery is a branch of the ophthalmic artery and supplies the anterior ethmoidal cells, frontal sinus, lateral nasal wall, and septum. A dehicent anterior ethmoidal artery should be reported by the radiologist in pre-operative studies.
- ANTERIOR ETHMOID BONE:
The are two ethmoidal cells important to remember for the International Frontal Sinus Anatomy Classification (IFAC) implicated in the natural drainage of the frontal sinus. The Agger nasi cell is the most anterior ethmoid cell. The ethmoid bulla constitutes the biggest anterior ethmoid cell.
- POSTERIOR ETHMOID BONE:
There are two types of cells with name. Haller cells, also known as infraorbital ethmoidal cells, are ethmoid air cells located lateral to the maxillo-ethmoidal suture along the inferomedial orbital floor. Thet may narrow the ipsilateral OMC, thereby predisposing the obstruction of the ipsilateral maxillary sinus. Sphenoethmoidal air cell, also commonly known as the Onodi cell, is an anatomical variant of the paranasal sinuses, important due to its proximity to the optic nerve and internal carotid artery.
The maxillary sinus is a paired pyramid-shaped paranasal sinus within the maxillary bone. It is the largest of the paranasal sinuses. The anatomical limits are:
- Superior wall: infraorbital nerve (V2 branch), sometimes dehiscent.
- Lateral wall: communicates to infratemporal fossa.
- Inferior wall: communicates to alveolar process.
- Medial wall: constitutes the lateral wall of the nasal cavity.
The most posterior paranasal sinus. The central body of the sphenoid bone is anteroinferior to the sella turcica. It may present different degrees of pneumatization and relation to sella turcica. It is important to report the type of pneumatization of this sinus and its relationship with adjacent neurovascular structures, especially the presence of dehiscences.
- Optic nerve canal (roof).
- Internal carotid artery (lateral wall).
- Foramen rotundum (floor) – V2 nerve.
- Vidian canal (floor).
Drainage from the frontal sinus tends to be more variable than the other paranasal sinuses. In general, the drainage goes along a narrow passage known as the frontal infundibulum. At this point, variant anatomy of the frontal/ethmoidal bone junction (uncinate process) leads to two main variations:
- Indirect drainage: ethmoidal infundibulum.
- Direct drainage: middle meatus.
3. STRUCTURED REPORTING
Some critical variants and landmarks can be quickly recalled and incorporated into the preoperative imaging report through the use of the mnemonic “CLOSE”: Cribriform plate, Lamina papyracea, Onodi cell, Sphenoid sinus pneumatization, and (anterior) Ethmoidal artery.
Structured reporting will enhance the value of the preoperative imaging report for referring otolaryngologists and help reduce the risk of surgical complications.