for recurrent disease.
MR spectroscopy
Magnetic resonance spectroscopy (MRS) falls under the category of functional MRI (fMRI), which contains a variety of different exams created to elucidate physiologic functions of the body. DWI, spectroscopy, perfusion weighted imaging (PWI), and activation studies are examples of fMRI. Of these, MRS of brain lesions is the most commonly performed functional study in clinical imaging. Spectroscopy is after all the basis for MRI. MRS attempts to elicit the chemical processes in tissues. Although a variety of nuclei may be interrogated, protons, demonstrating the highest concentration in tissues, are the most practical to evaluate. Most MRS studies are performed for the brain, but several recent studies have evaluated head and neck tumors. The need for a very homogenous magnetic field and patient cooperation (prevention of motion) are the keys to successful MRS. Susceptibility artifact and vascular pulsation artifact add to the challenge of MRS. With higher field strength magnets, MRS shows promise in determining the biochemical nature of tissues (King et al. 2005).
As in brain tumors, the most reliable markers for tumors are choline and creatine. Choline is thought to be an important constituent of cell membranes. Increased levels of choline are thought to be related to increased biosynthesis of cell membranes, which is seen in tumors, particularly those demonstrating rapid proliferation. The choline signal is comprised of signals from choline, phosphocholine, phosphatidylcholine, and glycerophosphocholine. Elevation of the choline peak in the MR spectra is associated with tumors relative to normal tissue. This unfortunately can be seen in malignant lesions, inflammatory processes, and hypercellular benign lesions (King et al. 2005). Another important constituent is creatine, a marker for energy metabolism. Its peak is comprised of creatine and phosphocreatine. The reduction of the creatine peak in neoplasms may represent the higher energy demands of neoplasms. The elevation of choline and, more importantly, the elevation of the ratio of choline to creatine have been associated with neoplasms relative to normal tissue. The elevation of choline is not tumor specific and may be seen with squamous cell carcinomas as well as a variety of salivary gland tumors, including benign tumors. It has been described in Warthin tumors, pleomorphic adenomas, glomus tumors, schwannomas, inflammatory polyps, and inverting papillomas (Shah et al. 2003). In fact, the Warthin tumor and pleomorphic adenoma demonstrate higher choline‐to‐creatine ratios than other tumors (King et al. 2005). King et al. also evaluated choline‐to‐water ratios and suggest that this may be an alternative method (King et al. 2005). Although the role of MRS in distinguishing between benign and malignant tumors may be limited, it nevertheless remains an important biomarker for neoplasms and plays a complementary role to other functional parameters and imaging characteristics (Shah et al. 2003). An area where MRS may play a more significant role is in a tumor's response to therapy and assessment of recurrence. Elevation of the choline‐to‐creatine ratio is seen in recurrent tumors whereas the ratio remains low in post‐treatment changes. Progressive reduction of choline is seen with a positive response to therapy and persistent elevation is seen in failure of therapy (Shah et al. 2003). Use of artificial intelligence and neural network analysis of MR spectroscopy has demonstrated improved diagnostic accuracy of MRS using neural network analysis over linear discriminate analysis (Gerstle et al. 2000). Currently, MRS of salivary gland tumors is under study and not employed clinically.
Dynamic contrast‐enhanced magnetic resonance imaging
Dynamic contrast‐enhanced MRI has demonstrated improved diagnostic capability of tumor masses in the salivary glands and elsewhere in the body. Distinct enhancement curves can be generated based on the time points of acquisition, resulting in improved differentiation of tumors (Yabuuchi et al. 2003; Shah et al. 2003; Alibek et al. 2007). However, data demonstrate similar characteristics in Warthin's tumor and malignant tumors, with a rapid increase in the signal intensity post‐contrast. Pleomorphic adenoma demonstrates a more gradual increase in intensity (Yabuuchi et al. 2003; Alibek et al. 2007). Primary salivary duct carcinomas have also demonstrated the rapid enhancement as well as low ADC values, as are seen with the more common primary malignancies of the salivary glands (Motoori et al. 2005).
Other magnetic resonance imaging techniques
Attempts have been made to develop MR sialography to replace the invasive technique of digital subtraction sialography. The techniques are based on acquiring heavily T2 weighted images to depict the ducts and branches. The lower spatial resolution and other technical factors have not allowed MR sialography to become a standard of care. This may change with newer single‐shot MR sequences and higher field strength magnets (Kalinowski et al. 2002; Shah et al. 2003; Takagi et al. 2005b). Dynamic MR sialography has also been used to assess function of parotid and submandibular glands at rest and under stimulation (Tanaka et al. 2007).
An extension of this concept is MR virtual endoscopy. MR virtual endoscopy can provide high‐resolution images of the lumen of salivary ducts comparable to sialendoscopy (Su et al. 2006). Although this initial experience was a preoperative assessment of the technology, it appears to be a promising method of noninvasive assessment of the ducts. In a similar manner, MR microscopy is a high‐resolution imaging technique employing tiny coils enabling highly detailed images of the glands (Takagi et al. 2005a). This technique was used to demonstrate morphologic changes in Sjögren syndrome (SS).
Use of supraparamagnetic iron oxide particle MR contrast agents has been under investigation for several years. The particles used for evaluation of lymph nodes are 20 nm or smaller. These are intravenously injected and are taken up by the cells in the reticuloendothelial system (RES). Since normal lymph nodes have a RES that is intact, they readily take up the iron oxide agents. MR imaging using T2 and T2* weighted images demonstrates susceptibility to the iron oxide and results in signal loss at sites of iron accumulation. Therefore, normal lymph nodes lose signal whereas metastatic lymph nodes whose RES has been replaced by metastases do not take up the particles and do not lose signal (Shah et al. 2003). Although not a direct imaging technique for the salivary glands, it may prove to be useful in the evaluation of nodal metastases.
ULTRASONOGRAPHY (US)
Ultrasound is performed infrequently for head and neck imaging relative to CT and MRI. Although US can depict normal anatomy and pathology in the major salivary glands, it is limited in evaluation of the deep lobe of the parotid and submandibular gland (Figures 2.16 and 2.17). US is operator dependent and takes significantly longer to perform on bilateral individual salivary glands when compared to contrast‐enhanced CT of the entire neck. US is quite effective at delineating cystic from solid masses, and determining degree of vascularity. US can be used to image calculi and observe the resulting ductal dilatation. Normal lymph nodes and lymphadenopathy can also be reliably distinguished. US can be used to initially stage disease. It is not, however, optimal for post‐therapy follow‐up, be it radiation or surgery. When compared with CT or MRI, US significantly lacks in soft‐tissue resolution and contrast. Because of its real‐time imaging capability and ease of handheld imaging, US is quite good at image‐guided fine needle aspiration and biopsy. The application of color Doppler or power Doppler US can distinguish arteries from veins, which are critical for image‐guided biopsy (Figures 2.18 and 2.19). Eighteen‐gauge core biopsies of the parotid may be safely performed under US guidance (Wan et al. 2004).
Figure 2.16. Ultrasound of the submandibular gland (black arrow) adjacent to the mylohyoid