MR Spectroscopy An Essential Diagnostic Tool
Magnetic resonance spectroscopy is an essential diagnostic tool, allowing noninvasive examination of the composition of molecules (metabolites) in tissues, thereby identifying physiological or pathological processes. By locating peaks in chemical shift that are the result of a shield formed by an electronic cloud of hydrogen nuclei in molecules, MR spectroscopy can identify metabolites. In order to do so, MR spectroscopy needs a very homogeneous magnetic field, as well as suppression of the water signal and/or suppression of the fat signal, which are located in large amounts in the body. Two basic spectroscopy sequences are STEAM and PRESS. STEAM records only a stimulated echo, weaker in intensity than the PRESS, which records a spin echo.
There are two types of MRS: SVS (Single Voxel Spectroscopy), which gets the spectrum from one voxel only. The second type is CSI (Chemical Shift Imaging), which measures spectra in projection, on a slice or a volume. MR spectroscopy requires the use of the same equipment and software that is similarly used in MRI. Additionally needed are adapted data processing software and radiofrequency system.
MR spectroscopy is used mainly in the evaluation of tumors, inflammatory and infectious pathology and metabolic pathologies. It is mostly used to scan the brain, although many developments are occurring in the study of other organs such as breast, prostate, bones and joints. The use of MRS in the clinical practice setting is growing. Studying the brain, MRS can identify tumoral pathologies such as gliomas, glioblastoma, metastasis, meningioma. Demyelinizing inflammatory pathologies, infectious diseases, encephalopathy. Metabolic pathologies: hepatic encephalopathy, cerebral distress, epilepsy, brain maturation and degenerative disease. MRS is also used in to study musculoskeletal system, assessing for prostate cancer, breast cancer, and liver processes.
MRI with MRS is now playing a critical part in screening for women with a high risk for breast cancer in the United States. While MRI can detect more abnormal findings than mammography, it is still not 100% accurate in distinguishing malignant from benign tumors. To help decrease the number of breast biopsies, MRS is being used along with the standard MRI. MRS may possibly be routinely used for all diagnostic breast MRI procedures, significantly decreasing the need for needle biopsies. Biopsy is often required to distinguish benign non-mass lesions from cancer. MR spectroscopy, adds only 15 more minutes to the routine MRI procedure. The radiologist has the benefit of seeing the chemical structure of a tumor. In the majority of patient’s, the results findings show whether or not the lesion is cancerous without needing to undergo a breast biopsy procedure. The MRS measures the levels of choline compounds, which are markers of an active tumor. A study, done at Memorial Sloan-Kettering Cancer Center in New York City, showed positive choline findings were present in 15 of 32 lesions, including all 12 cancers, giving ¹H MRS a specificity of 85 percent and a sensitivity of 100 percent. If only the lesions with positive choline findings had been biopsied, 17 (68 percent) of 25 lesions may have been spared invasive biopsies and none of the cancers would have been missed.