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Diffusion weighted images are not routinely clinically used in the neck or head but are indispensable in the brain. Typical intracranial application is for assessing acute stroke, but can be applied for the assessment of active multiple sclerosis (MS) plaques, and abscesses (Figure 2.15). The concept of DWI is based on the molecular motion of water and the sensitivity of certain MRI sequences to detect the diffusion or movement of water in tissues at the cellular level.

The use of DWI and specifically apparent diffusion coefficient (ADC) values and maps for salivary gland imaging are under investigation and show promise in differentiating benign from malignant tissues (Shah et al. 2003; Abdel‐Razek et al. 2007; Eida et al. 2007; Habermann et al. 2007). The ADC values are affected by technical factors (b‐value setting, image resolution, choice of region of interest, susceptibility artifacts, and adequate shimming) as well as physiologic factors (biochemical composition of tumors, hemorrhage, perfusion, and salivary flow) (Eida et al. 2007). The ADC values of salivary glands change with gustatory stimulation. Although mixed results have been reported, there is generally an increase in the ADC value from pre‐stimulation to post‐stimulation measurements (Habermann et al. 2007). The normal parotid, submandibular, and sublingual glands have measured ADC values of 0.63 ± 0.11 × 10⁻³ mm²/s, 0.97 ± 0.09 × 10⁻³ mm²/s, and 0.87 ± 0.05 × 10⁻³ mm²/s, respectively (Eida et al. 2007). In pleomorphic adenomas, the ADC maps demonstrate areas of cellular proliferation to have intermediate ADC levels and areas of myxomatous changes to have high ADC values (Eida et al. 2007). Warthin's tumor showed lymphoid tissue to have a very low ADC, necrosis with intermediate ADC, and low ADC in cysts among the lymphoid tissue (Eida et al. 2007). Among the malignant lesions, mucoepidermoid carcinoma shows low ADC in a more homogenous pattern whereas the adenoid cystic carcinomas demonstrated a more speckled pattern with areas of low and high ADC likely from multiple areas of cystic or necrotic change (Eida et al. 2007). Lymphoma in salivary glands has been demonstrated to have a diffuse extremely low ADC likely from the diffuse uniform cellularity of lymphoma (Eida et al. 2007). In general, cystic, necrotic, or myxomatous changes tend to have higher ADC and regions of cellularity, low ADC. Malignant tumors tend to show very low to intermediate ADC whereas benign lesions have higher ADC, but with heterogenous pattern. Overlaps do occur for example with Warthin's tumor demonstrating very low ADC regions and adenoid cystic carcinoma with areas of high ADC (Eida et al. 2007).

Figure 2.14. Axial MRI FLAIR image at the skull base demonstrating CSF flow‐related artifactual increased signal in the right prepontine cistern.

Figure 2.15. Axial MRI DWI image at the skull base demonstrating susceptibility artifact adjacent to the left temporal bone (arrow).

Evaluating postoperative changes for residual or recurrent tumors is also an area where DWI and ADC may have a significant impact. In general, (with overlap of data) residual or recurrent lesions have been shown to have ADC values lower than post‐treatment changes (Abdel‐Razek et al. 2007). The lower ADC may be a result of smaller diffusion spaces for water in intracellular and extracellular tissues in hypercellular tumors. The benign post‐treatment tissue with edema and inflammatory changes has fewer barriers to diffusion and increased extracellular space, resulting in a higher ADC (Abdel‐Razek et al. 2007).

Evaluation of connective tissue disorders with DWI has demonstrated early changes with increase in ADC prior to changes on other MRI sequences. This may be a result of early edema and or early lymphocellular infiltration (Patel et al. 2004). Therefore, DWI and ADC may play an important role in early assessment of connective tissue disorders, preoperative evaluation of salivary tumors, as well as surveillance for recurrent disease.