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 ever-increasing role.
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. The most recent advance in CT imaging has been the introduction of “multislice” scanners. Multislice scanners have a variable number (typically 8–64, though scanners with 320 are now available) of parallel arcs of detectors 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. Direct coronal imaging or coronal reformations are useful in many situations, notably in imaging of the paranasal sinuses and the skull base, and sagittal reformations can also be useful. Because of growing concerns over medical radiation, coronal reformations are generally preferred to a second acquisition if axial images are also acquired. 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 not required. If a patient has a contrast allergy or renal insufficiency, then contrast administration should be avoided; premedication with steroids and antihistamines can be useful if a patient has a history of a contrast reaction, but contrast administration is necessary.
Magnetic Resonance Imaging
MRI exploits differences in relaxation characteristics and spin density of protons in different tissue environments to produce an image that is exquisitely sensitive to soft tissue contrast. Depending on the parameters that are selected, variable tissue characteristics and contrast are produced. At least two different types of sequences in two planes are generally necessary to characterize lesions of the head and neck. The slice thickness should ...