DIAGNOSTIC IMAGING TECHNIQUES
Diagnostic imaging is an essential component of the evaluation of many otolaryngologic problems. Computed tomography (CT) and magnetic resonance imaging (MRI) are the most commonly used imaging modalities, with positron emission tomography (PET) playing an essential role in oncologic imaging. Ultrasound (US) will not be specifically discussed in this chapter.
CT scanning uses ionizing radiation to generate cross-sectional images based on differences in the x-ray attenuation of various tissues. Modern scanners are typically helical, meaning that x-ray source rotation and patient translation occur simultaneously; this results in the acquisition of a “volume” of data that is then partitioned and reconstructed into individual slices. Helical scanning is significantly faster than traditional slice-by-slice acquisition, thereby diminishing artifacts related to motion (eg, breathing, swallowing, and gross patient motion). The rapid data acquisition also allows a larger number of thinner slices to be obtained, which facilitates diagnosis by decreasing partial-volume averaging effects and allows for improved quality of multiplanar reconstructions.
Multislice scanners are now commonplace and have a variable number of parallel arcs of detectors (typically 8–64, though scanners with 320 are now available) that are capable of simultaneously acquiring volumes of data. The increased speed that results from multislice sampling can be traded for improved longitudinal resolution, an increased volume of coverage, or an improved signal-to-noise ratio.
CT scanning of the head and neck is ideally performed with thin sections, usually ≤ 3 mm, in the axial plane; with multislice scanners, 0.625- or 1.25-mm slices are typically acquired and then combined for ease of viewing into slightly thicker 2.5- to 3-mm slices. Coronal and sagittal reformations are useful in many situations, notably in imaging of the paranasal sinuses and the skull base, but in many practices all CT data sets are reformatted into multiple planes (Figure 3–1). Direct coronal imaging is no longer routinely performed, as images of comparable quality can be obtained by reformatting thin, high-resolution axial images.
Axial (A) contrast-enhanced CT image of the neck was obtained through the level of the cricoid cartilage (arrow). The thin slices from the image set were reformatted into the coronal (B) and sagittal (C) planes. The thyroid (T), internal jugular vein (IJ), and common carotid artery (*) are shown for reference.
CT scanning of the neck is generally performed following injection of iodinated contrast material because opacification of vessels helps to separate them from other structures such as lymph nodes and also helps to delineate and characterize pathology. If bony anatomy is the focus of the imaging study, as in imaging of the paranasal sinuses or temporal bones, then intravenous contrast material is ...