rel="nofollow" href="#ulink_d2841787-54cf-5946-8d6d-b070a31fe0aa">Figures 2.1 and 2.2). In a similar manner, images are reconstructed using a computer algorithm to optimize bone details as more sharp and defined (Figure 2.3). The lung apex is often imaged in a complete neck evaluation and displayed using lung window settings (Figure 2.4a). Dedicated CT scans of the chest are beneficial in the postoperative evaluation of patients with salivary gland malignancies as lung nodules can be observed, possibly indicative of metastatic disease (Figure 2.4b). Multiplanar reformatted images of the neck are obtained typically in the coronal and sagittal planes (Figures 2.5 and 2.6), although they may be obtained in virtually any plane desired or in a 3D rendering.
The Hounsfield unit (H) (named for Godfrey Hounsfield, inventor of the CT scanner) is the unit of density measurement for CT. These units are assigned based on the degree of attenuation of the X‐ray beam by tissue in a given voxel (volume ele ment) and are assigned relative to water (0H) (Table 2.1). The scale ranges from −1024H for air, to +4000H for very dense bone. The images are created based on a gray scale from black (−1024 H) to white (+4000 H) and shades of gray. Despite the wide range of units, majority of tissues in the human body are between −100 and +100 H. Soft tissues and parenchymal organs are in a range between 20 and 80 H, whereas fat is approximately −100 H. Simple fluid is 0 H, but proteinaceous fluid can be upward of 25 H. Unclotted and clotted blood varies depending on the hemoglobin concentration and hematocrit but average measurements are 50 and 80 H, respectively. CT images are displayed using a combination of “window widths” (WW, range of CT numbers from black to white), and “window levels” (WL, position of the window on the scale), which are based on the attenuation characteristics of tissues. Typically, head and neck images are interpreted using “soft‐tissue windows” (WW 500 H, WL 30 H), “bone windows” (WW 2000, WL 500), or “lung windows” (WW 1500, WL 500). The “soft‐tissue windows” demonstrate the slight density differences of soft tissues, whereas “bone windows” demonstrated cortical and medullary features of bones with sharp detail. “Lung windows” demonstrate the sharp interface of air and the fine soft‐tissue components of lung parenchyma.
Figure 2.1. Axial CT of the neck in soft‐tissue window without contrast demonstrating poor definition between soft‐tissue structures. The blood vessels are unopacified and cannot be easily distinguished from lymph nodes. Note the sialolith (arrow) in the hilum of the left submandibular gland.
Figure 2.2. Axial CT of the neck in soft‐tissue window with IV contrast demonstrates improved visualization of structures with enhancement of tissues and vasculature. Note the small lipoma (arrow) anterior to the left submandibular gland, which distorts the anterior aspect of the gland with slight posterior displacement.
Figure 2.3. Axial CT of the skull base reconstructed in a sharp algorithm and in bone window and level display demonstrating sharp bone detail. Note the sharply defined normal right stylomastoid foramen (arrow).
Figure 2.4. Axial CT of the neck at the thoracic inlet in lung windows demonstrating lung parenchyma (a). Axial image of dedicated CT of chest demonstrating cannon ball lesions in a patient previously treated for adenoid cystic carcinoma of the palate (b). These lesions are representative of diffuse metastatic disease of the lungs, but not pathognomonic of adenoid cystic carcinoma.
Figure 2.5. Coronal CT reformation of the neck in soft‐tissue window at the level of the submandibular glands. Orthogonal images with MDCT offer very good soft‐tissue detail in virtually any plane of interest to assess anatomic and pathologic relationships.
Figure 2.6. Sagittal CT reformation of the neck in soft‐tissue window at the level of the parotid gland. Note the accessory parotid gland (black arrow) sitting atop the parotid (Stensen) duct (thin white arrow). Also, note the retromandibular vein (large white arrow) and external auditory canal.
Table 2.1. CT density in Hounsfield units (H).
| Tissue or structure | Hounsfield unit (H) |
|---|---|
| Water or CSF | 0 |
| Fat | −30 to −100 |
| Soft tissue, musclea | 50–60 |
| Unclotted bloodb | 35–50 |
| Clotted bloodb | 50–75 |
| Parotid glandc | −10 to +30 |
| Submandibular glandc | 30–60 |
| Sublingual glandd | 60–90 |
| Bone | 1000 |
| Lung | −850 |
| Air | −1000 |
| Calcification | 150–200 |
| Gray matter | 35–40 |
|
White
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