General Medical Information

Sinus Imaging

Barbara A. Zeifer

Imaging studies of the paranasal sinuses provide the otolaryngologist with invaluable information. The role of the radiologist is to define anatomy, to describe the location and extent of the abnormality, and to suggest a tissue diagnosis. Plain radiography, computed tomography (CT), and magnetic resonance imaging (MRI) are readily available.

Plain radiography is of limited usefulness in sinonasal imaging. The presence or absence of moderate to severe mucosal thickening and air-fluid levels can be ascertained for the maxillary and frontal sinuses. Evaluation of the ethmoidal labyrinth is unreliable, however, and subtle disease is not depicted at all.

Computed tomography is a well-established method for paranasal sinus imaging. It produces excellent spatial resolution for both bone and soft-tissue structures far exceeding that of plain radiography. Images are generated by means of detection of a transmitted x-ray beam, and tissue contrast results from differences in electron density. The images can be viewed at various window settings. The bone window is best for evaluating bony structures; the soft-tissue window provides soft-tissue detail; and a modified lung window maximizes mucosal detail while producing adequate visualization of bony structures. Computed tomography is an essential investigational tool in the evaluation of all sinonasal abnormalities and is a preoperative requirement for sinus surgery.

Intravenous injection of a contrast agent is used to improve tissue contrast and specificity. For CT, several iodinated preparations are available for use, including both ionic and nonionic (lower osmolality) agents. The nonionic preparations, although more expensive, are better tolerated by patients and are associated with a lower incidence of adverse reactions. Intravenous contrast agents enhance normal vascular structures and accumulate in abnormal tissues with increased vascularity. These changes are most pronounced when a power injector is used to deliver a rapid bolus. This enhancement is particularly useful for evaluating neoplastic disease and complicated inflammatory processes. Soft-tissue (neoplastic or inflammatory) disease becomes enhanced, whereas retained secretions do not.

Absolute contraindications to intravenous injection of contrast material include previous adverse reaction and severe renal insufficiency, particularly with multiple myeloma, diabetes mellitus, and concurrent use of therapeutic nephrotoxic agents. Relative contraindications include atopy, asthma, mild renal insufficiency, advanced age, dehydration, cardiac disease, and the use of b-adrenergic blockers. An oral hypoglycemic medication, metformin hydrochloride, has been associated with lactic acidosis and acute renal failure after administration of iodinated contrast material, according to the package insert. In imaging of patients considered at high risk of reaction to a contrast agent, pretreatment with a two-dose regimen of 32 mg methylprednisolone 12 hours and 2 hours before injection of an ionic contrast agent decreases the severity of all reactions.

Magnetic resonance imaging has become increasingly popular for the evaluation of head and neck disease as image sharpness and spatial resolution have improved. As does CT, MRI provides cross-sectional images of the body. Magnetic resonance images are produced by means of detection and measurement of emitted energy after rapid pulsation of a radiofrequency wave into a high-strength magnetic field (up to 1.5 Tesla). Therefore, the findings are described in terms of the signal intensity of the various tissues. Four physical properties of matter are responsible for producing tissue contrast during MRI—T1 and T2 relaxation times, proton density, and flow. Information about these properties can be obtained in different ways by varying the rapidity of the radiofrequency excitations and the timing of data measurement. A complete study entails production of different imaging sequences in the three orthogonal planes. T1, proton density, and T2 sequences routinely are performed; other sequences are available for use in specific situations.

Magnetic resonance imaging provides better soft-tissue contrast and tissue characterization than does CT. Images can be produced directly in any anatomic plane with the patient positioned supine, whereas CT is limited to the axial and coronal planes. Direct coronal CT necessitates full extension of the neck, a position often poorly tolerated by an ill patient. A good-quality MRI study requires complete cooperation from the patient, however, because even minimal movement degrades the image. Large quantities of dental amalgam interfere with image production during MRI. Normal cortical bone produces no signal on MR images; the thin, bony plates of the midface are seen only because of their investing mucosa. Dependence on these bony structures for definition of spatial relations and anatomic orientation limits the utility of sinonasal MRI in some situations. Magnetic resonance imaging should be performed in addition to initial CT in the evaluation of soft-tissue masses, complicated inflammatory disease, and extension of disease beyond the confines of the sinonasal cavity. Magnetic resonance imaging clearly differentiates soft tissue from fluid and helps to differentiate neoplasia from inflammatory disease. Magnetic resonance imaging is ess useful than CT for the evaluation of trauma, typical inflammatory disease, and delineation of the ostiomeatal complex.

Although MRI is generally considered a safe, noninvasive imaging modality, for some patients MRI examination involves risks and hazards. The most common contraindication to MRI is the presence of a cardiac pacemaker. Persons with cochlear implants, pacer wires, or Swan-Ganz catheters likewise cannot undergo MRI. A metallic intraocular foreign body can deflect in the magnetic field and cause vitreal hemorrhage. Plain radiography of the orbits is considered an adequate screening examination for metallic foreign bodies. Particles too small to be detected with this method are considered unlikely to pose any risk of injury. Metallic fragments in the spinal canal also can be dangerous. Intracranial aneurysm clips are of concern because many are ferromagnetic and deflect. Death due to intracranial hemorrhage has been reported; therefore any patient with an aneurysm clip should not undergo MRI unless the type of clip is known to the physician and the manufacturer states that it is nonferromagnetic. All vascular clips, heart valve prostheses, and orthopedic implants are considered safe in a magnetic field, as are intravascular implants at least 6 weeks after insertion.

Gadolinium–diethylenetriamine pentaacetic acid, the intravenous contrast agent for MRI, is safe and well tolerated, although there have been isolated reports of severe reactions, including anaphylaxis. As do the iodinated agents used for CT, gadolinium accumulates in tissues with increased vascularity, resulting in increased signal intensity (enhancement) on T1-weighted images. Unlike the findings at CT, vascular structures with moderate to high flow have complete lack of signal owing to the flow of blood through the section during data acquisition. This phenomenon is called the flow void.