| Diagnostic Imaging Techniques Diagnostic imaging is an essential element in 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. Computed Tomography 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 for more and 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 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. Direct coronal imaging or coronal reformations are useful in some situations, notably in imaging of the paranasal sinuses and the skull base. CT scanning of the neck is usually 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. Magnetic Resonance Imaging MRI exploits differences in tissue relaxation characteristics and spin density 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 be no more than 5 mm. A gadolinium-based contrast agent is generally used to enhance the detection of pathology and improve tissue characterization, and also to aid in the generation of a differential diagnosis. In some circumstances, thinner sections covering a smaller anatomic area may be necessary for more precise diagnosis. In the head and neck, the following imaging sequences are typically obtained: (1) sagittal, axial, and coronal T1-weighted images; (2) axial fast spin-echo T2-weighted images with fat saturation; and (3) axial and coronal postgadolinium T1-weighted images with fat saturation. Additional planes may be useful in some circumstances, such as coronal fast spin-echo T2-weighted images with fat saturation for the assessment of paranasal sinus and anterior skull base pathology. Additional sequences such as magnetic resonance angiography (MRA) may be useful in certain circumstances (eg, paragangliomas and dural fistulas), but are not necessary for evaluating most processes of the head and neck. MR venography may be useful in the assessment of patients with pulsatile tinnitus and in the assessment of the patency of the sigmoid sinus in patients with adjacent neoplastic or inflammatory disorders. Advanced modalities in widespread use in the brain (eg, MR spectroscopy, diffusion-weighted imaging, functional MR imaging) have for the most part not found a place in routine head and neck imaging, with the exception of diffusion-weighted imaging in the evaluation of epidermoid cysts and cholesteatomas. On a T1-weighted image, fat is bright and fluid (eg, cerebrospinal fluid [CSF]) is relatively dark. Muscle and most pathologies are of intermediate signal intensity. The large amount of fat in the head and neck provides intrinsic tissue contrast, which makes the T1-weighted image very sensitive to infiltrative processes that obliterate tissue planes or that replace marrow fat (Figure 31). Some hemorrhagic or proteinaceous lesions cause shortening of T1 relaxation time and appear bright on a T1-weighted image. On a T2-weighted image, fluid is very bright and most pathologies are relatively bright, whereas normal muscle is quite dark. The fast spin-echo technique is very useful in limiting artifacts related to motion and magnetic susceptibility compared with conventional spin-echo T2-weighted imaging. Because fat remains bright on a fast spin-echo image, however, fat saturation should ideally be applied. In the nasal cavity and paranasal sinuses, T2-weighted images are particularly useful in distinguishing neoplastic masses from polyps, thickened mucosa, and retained secretions (Figure 32). Gadolinium is very useful for demonstrating pathology and tailoring a differential diagnosis based on enhancement characteristics. In a patient with head and neck cancer, postgadolinium imaging is also very useful in assessing cavernous sinus invasion, meningeal infiltration, and perineural spread of tumor (Figure 33). Fat saturation should ideally be applied on a postgadolinium T1-weighted image; otherwise, the contrast between an enhancing lesion and the high signal intensity of surrounding fat may actually be reduced compared with the pregadolinium image. Because low-field scanners often do not have fat saturation capability, high-field imaging (1.5 T) is generally preferred for assessing the head and neck and skull base. If a patient is severely claustrophobic, sedation may be necessary to accomplish the scan on a high-field system.
It should be kept in mind that MRI requires more time and more patient cooperation than does CT, and therefore it is not necessarily suitable for acutely ill or uncooperative patients. In addition, there are certain absolute contraindications to MRI, including ferromagnetic intracranial aneurysm clips, cardiac pacemakers, and many cochlear implants. Therefore, patients must be carefully screened for these and other contraindications before undergoing MRI. Positron Emission Tomography PET provides a functional view of tissues rather than simply depicting anatomy. In the head and neck, it is used primarily for oncologic diagnosis and evaluation and is performed with the radiopharmaceutical 18F-fluorodeoxyglucose (FDG). FDG is taken up into tissues in proportion to the glycolytic rate, which is generally increased in neoplastic processes. Focal asymmetric uptake is suggestive of a tumor but is nonspecific, since FDG is also concentrated in areas of inflammation. FDG PET scanning is particularly helpful in the following situations: (1) the search for an unknown primary lesion in a patient presenting with metastatic neck disease (Figure 34), (2) the assessment of residual or recurrent disease after primary therapy, and (3) the search for synchronous or metachronous primary lesions or distant metastases. FDG PET scanning can also be useful for staging the neck, but there may be a significant number of false-negative studies in patients with clinically N0 necks because small tumor deposits (approximately 13 mm) are generally not detectable on an FDG PET scan. These small tumor deposits are found if a neck dissection is performed. At present, most FDG PET scanning is done on dedicated PET-CT scanners, such that precise anatomic localization of FDG uptake can be achieved. AAssar OS, Fischbein NJ, Caputo GR et al. Metastatic head and neck cancer: role and usefulness of FDG PET in locating occult primary tumors. Radiology. 1999;210:177. (FDG PET allows for the effective localization of the unknown primary site of origin in many cases of metastatic head and neck cancer and can contribute substantially to patient care.) [PMID: 9885604]
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