Читать книгу Salivary Gland Pathology - Группа авторов - Страница 70

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.