Introduction to Imaging

Introduction

Rapid advancement of medical imaging in the last few decades has significantly enhanced the role of imaging in medicine. Imaging plays an integral role in the evaluation of many of the diseases confronted by the otolaryngologist. It is performed for diagnosis, staging of tumors, assessing efficacy of therapy, follow-up, and guidance for procedures.

Imaging in the head and neck, like no other area of the human body, differs considerably based on the specific anatomic area of interest. In fact, unique imaging protocols have been designed for at least 9 specific areas of the head and neck. These include from superior to inferior the cerebellopontine angle-internal auditory canal (CPA-IAC), skull base, temporal bone, orbit, sinus and nose, suprahyoid neck, infrahyoid neck, pharynx, and larynx. Clinical history and physical findings in combination with high-resolution imaging available today create the possibility of a highly specific clinical-radiologic diagnosis. Computed tomography (CT) and magnetic resonance (MR) form the backbone of the imaging work-up of patients with diseases of the head and neck.

Imaging Modalities

In the past, radiography ("plain films" and "tomography") played a significant, but not very effective, role in imaging of the head and neck. With the emergence of advanced imaging, the role of radiography has virtually disappeared. Currently, CT, MR, and PET/CT are the most commonly performed imaging modalities for diseases of the head and neck. Modern multidetector, multiplanar CT now creates exquisite anatomical detail allowing the diagnosis of very small lesions, such as temporal bone otosclerosis, not thought possible even a decade ago. MR techniques, such as multiplanar thin-section enhanced, fat-saturated T1, allow the imager to identify subtle findings, such as perineural tumor spread along extracranial portions of cranial nerves. The use of a variety of MR sequences (T1, T2, FLAIR, GRE, enhanced T1 fat-saturated) permit the imager to differentiate tissue types, such as normal muscle from tumor. Other unique MR sequences can suggest specific diagnoses, such as the case of diffusion weighted imaging (DWI) MR sequence being highly specific for the diagnosis of epidermoid.

Likewise, fluorodeoxyglucose positron emission tomography (FDG-PET)/CT imaging has a distinct role in the imaging of patients with squamous cell carcinoma (SCCa) staging and follow-up. Metabolically active primary tumor and malignant nodes mapped against an underlying contrast-enhanced CT can help differentiate normal tonsillar tissue and reactive nodes from invasive SCCa primary and metastatic nodes. Ultrasound and color Doppler are useful in evaluating the head and neck vasculature. Ultrasound is also used to evaluate neck masses and localize them for fine-needle aspiration.

CT

CT technology relies on the same physical principles as x-rays. The differential absorption of the x-ray beam by different tissues produces varied levels of density in the image (see Table 1), which on CT scans are measured in Hounsfield units (HU). This can be displayed in cross-sectional format or in multiple planes. Multidetector CT has enlarged the capability of CT with faster scans, greater spatial resolution, and multiplanar reformations.