Available Imaging Modalities Of Sinusitis

Radiological imaging is complementary to the clinical and endoscopic evaluation of patients with rhinosinusitis. The diagnosis of this pathological entity is made clinically and with the help of endoscopy.

Imaging is an essential part of presurgical evaluation and monitoring of difficult-to-treat, recurrent, postsurgical disease. In patients in whom there might be clinical suspicion of a noninflammatory pathology, imaging can be extremely helpful in distinguishing the various pathological entities.

Plain Sinus Radiograph

Plain radiographic evaluation of the paranasal sinuses has fallen out of favor despite its wide availability and comparatively low cost. The overall sensitivity and specificity of plain radiographs for sinusitis is 40 to 50% and 80 to 90%, respectively.

The sensitivity approaches clinically acceptable levels only for the diagnosis of maxillary sinus disease. Generally, for a comprehensive evaluation, four standard views are obtained: lateral view, Caldwell view, Waters’ view, and sub-mentovertex or base view.

The lateral view shows the bony perimeter of the frontal, maxillary, and sphenoid sinus. The Caldwell view shows the bony perimeter of the frontal sinus.TheWaters’ viewshows the outlines of the maxillary sinuses, some of anterior ethmoid air cells, and the orbital outline.

The sub-mentovertex view evaluates the sphenoid sinus and the anterior and posterior walls of the frontal sinuses. Evaluation of the ostiomeatal unit (OMU) and the detail of the ethmoid morphology are precluded by this modality.

Computerized Tomography

Computerized tomography (CT) is the imaging standard for sinusitis. Its ability to display bone, mucosa, and air makes it a perfect tool for the imaging of the paranasal sinuses.

The fine bony architecture of the nasal cavity and the paranasal sinus drainage pathways is accurately depicted with CT examination. CT is very sensitive in detection of mucosal hypertrophy and retained secretions in the paranasal sinuses.

Single channel CT (SC-CT) scanners use either incremental or helical acquisition schemes for paranasal sinus examination. Coronal images optimally display the anterior ostiomeatal unit, the relationship of the orbits and brain to the paranasal sinuses, and also correlate best with the surgical approach.

The slice thickness should be 3mm or less without interslice gap for optimal evaluation of the key structures such as OMU and frontal recess. Image acquisition in the coronal plane requires an extension of the head, which may not be possible for very young patients and patients with airway problems or neck pain.

Thin axial images can be reconstructed in the coronal plane for such patients. Direct sagittal images cannot be obtained with CT. Multichannel CT (MC-CT) (also called multidetector or multislice CT) scanners have been recently introduced.

The single X-ray detector present in SC-CT has been replaced with multiple rows of detectors in MC-CT scanners that allow registration of multiple channels of data with one rotation of the X-ray tube. For example, 16-slice MC-CT equipment has a 16-fold capacity to collect image data per X-ray tube rotation compared to a SC-CT.

This increased capacity affords much thinner slices from larger body parts in shorter periods of time. Thin slices permit isotropic data sets in which the voxels (the smallest elements of a data set) are cuboidal.

Cuboidal voxels offer excellent reconstruction of images in essentially any plane without degradation of quality. Currently, slices as thin as 0.5mm can easily be obtained with near-isotropic voxels. Isotropic imaging created a paradigm shift in CT imaging: we are no longer confined to the plane of acquisition.

Data can be collected from a body part in any desired plane, and two-dimensional images [multiplanar reconstruction (MPR)] can be displayed in any desired plane. Real-time interactive manipulation of image data and three-dimensional reconstructions are made possible by high-performance workstations equipped with special software.

Intravenous contrast administration is not necessary for assessment of uncomplicated inflammatory sinus disease. Different shades of gray inherently present in every CT image can be displayed in various ‘‘windows’’ to enhance visualization of certain tissues (e.g., bones and soft-tissues).

A window width of 2000 Hounsfield units with a level of –200 Hounsfield units is the most advantageous for inflammatory disease of the paranasal sinuses. These ‘‘window levels’’ afford best display of the narrow air channels.

Window settings can easily be changed to highlight certain structures and ‘‘hard-copy’’ films are then printed. Adjustment of window settings is not a post-processing method and does not change the raw data, whereas bone reconstruction algorithms (also called bone filters) manipulate the raw data to best demonstrate the bony detail.

Images reconstructed using bone algorithm are recommended for the evaluation of the paranasal sinuses. Evaluation of the soft tissues on these images, however, is very limited, even when the window settings are adjusted for soft tissues.

The increasing number of CT examinations and desire for high-resolution images inevitably increase the radiation dose to patients. Factors inherent to the individual CT scanner and the patient greatly influence radiation dose.

Among many operator adjustable scanning parameters that affect the radiation dose, tube current (milliamper-second, mAs) has the most direct and profound effect on the final radiation dose received by the patient.

A considerable reduction in radiation exposure can be achieved by lowering mAs. Low mAs results in increased image noise and possible loss of fine detail.

A tube current of 50–80 mAs at 120 kVp tube voltage is a reasonable compromise and diagnostic accuracy of paranasal sinus CT is not affected at these settings compared to higher mAs values.

A very low–radiation dose CT (even lower than four view radiographs) study can be performed with 10 mAs and noncontiguous slices for patients who require multiple repeat studies.

While the radiation exposure to the lens from sinus CT examination is well below the threshold level believed to induce cataracts, there is a theoretical risk of stochastic effects (e.g., carcinogenesis), which is not dependent on a minimum threshold of exposure.

Therefore, judicious use of CT is advised. Multiple examinations with high-dose protocols should be avoided particularly in young patients.

Magnetic Resonance

Imaging Magnetic resonance imaging (MRI)’s exquisite contrast resolution makes it a perfect tool in the imaging of soft tissues. MRI is extremely sensitive to mucosal thickening.

In fact, MRI’s sensitivity may be too high due to the fact that small increases in volume and signal intensity of the mucosa in the ethmoid sinuses and nasal turbinates can be a reflection of physiological nasal cycle.

Due to MRI’s limited ability to display fine bone detail, its use is limited in diagnostic and presurgical evaluation of uncomplicated inflammatory sinus disease.

MRI has proven most helpful in the evaluation of regional and intracranial complications of inflammatory sinus disease and their surgical management, in the detection of neoplastic processes, and in improved display of anatomic relationships between the intra- and extra-orbital compartments.

MRI is also useful in the evaluation of mucoceles and cephaloceles. T1- and T2-weighted MRI obtained in axial and coronal planes provide a satisfactory evaluation of the sinuses.

Contrast [gadolinium– diethylenetriaminepentaacetic acid (Gd–DTPA)]-enhanced, fat-saturated T1- weighted images are recommended for a more comprehensive examination.