Sinus cavities are air-filled spaces within the bones of the skull and face, centered around the nasal cavity. They help humidify and warm inhaled air, produce mucus to trap pollutants, and lighten the skull’s weight. The sinuses are lined with a soft, pink tissue called mucosa, which is continuous with the lining of the nasal cavity. This connection means nasal infections can spread to the sinuses.
Anatomy of the Sinus Cavities
Humans have four pairs of paranasal sinuses, named after the bones in which they are located. The largest are the maxillary sinuses, situated in the cheekbones just below the eyes. These are often the first sinuses to become infected. Above the eyes, within the forehead bone, are the frontal sinuses. Between the eyes and behind the bridge of the nose are the ethmoid sinuses, which are a collection of many small air cells.
Located deepest within the skull, behind the nose and ethmoid sinuses, are the sphenoid sinuses. Each of these sinus cavities connects to the nasal cavity through small openings. This network of passages allows for the proper drainage of mucus and circulation of air.
Medical Imaging Techniques for Sinuses
The most common method to visualize the sinuses is a Computed Tomography (CT) scan. A CT scanner uses X-rays to create multiple cross-sectional images of the head, which a computer assembles into detailed views of bones and sinus tissues. This detail is excellent for identifying structural problems, inflammation, and blockages.
Magnetic Resonance Imaging (MRI) is another technology used, though less frequently for general sinus problems. An MRI uses powerful magnets and radio waves to generate images, providing superior detail of soft tissues. This makes MRI useful when a doctor suspects an issue like a tumor or certain fungal infections. Standard X-rays are rarely used for detailed sinus evaluation because they provide less detail than a CT scan.
Interpreting Sinus Images
When a radiologist examines sinus images, they look for the characteristics of normal sinuses. On a CT scan, healthy sinuses appear as dark, air-filled spaces outlined by thin bony walls. The dark appearance signifies that the cavity is clear. The mucosal lining in a healthy state is so thin that it is often not visible on the scan.
Abnormalities present a contrasting picture. In cases of sinusitis, the most common finding is mucosal thickening, where the lining becomes inflamed, appearing as a gray layer on the inside of the cavity wall. If the sinus is blocked and filled with fluid, it will look opaque or white on the scan, a condition known as opacification.
Nasal polyps, which are noncancerous growths, show up as soft, grayish, rounded masses that can fill the nasal cavity or sinuses. Structural problems are also identifiable. A deviated septum, where the wall dividing the nasal passages is off-center, can be clearly seen and may contribute to sinus blockage.
Reasons for Ordering Sinus Imaging
A physician may order sinus imaging after initial treatments fail to resolve a patient’s symptoms. One of the most common reasons is chronic sinusitis, which is inflammation lasting over 12 weeks, or recurrent infections that suggest an underlying anatomical issue.
Other reasons for ordering a scan include:
- A suspicion of nasal polyps, based on symptoms like persistent congestion and a diminished sense of smell.
- Unexplained and persistent facial pain, pressure, or headaches that may have a sinus-related cause.
- Pre-surgical planning to provide the surgeon with a detailed map of the patient’s unique sinus anatomy.
- To assess the extent of damage to the sinuses following facial trauma.