"Determining precise sodium concentrations has been a problem, but MRI can do it at the same time it identifies possible malignant lesions," says Ronald Ouwerkerk, Ph.D., instructor of radiology at the Johns Hopkins Department of Radiology, MRI Research Division. Using proton MRI after injecting contrast agents to enhance areas containing many small blood vessels, the researchers found 50 percent more sodium in lesions that turned out to be malignant tumors than in normal brain tissue.
"Using MRI noninvasively in a single examination to get information about tumor metabolism and physiology improves identification of tumor malignancy and is a big step forward," says Ouwerkerk.
In the Hopkins study, 20 patients with brain malignancies confirmed by biopsy and nine subjects with no brain tumors underwent sodium MRI. Sodium concentrations were determined in malignant tumors, gray brain matter, white brain matter, cerebrospinal fluid and vitreou s humor (fluid within the eye). The image intensities in the sodium images were compared with the signals from a sample with known sodium content. The special scan method the researchers use for three-dimensional sodium MRI makes it possible to directly calculate sodium concentrations.
The researchers found sodium concentrations in cancer cells were elevated by an average of 50 percent compared to the other noncancerous tissues or those tissues in subjects with no tumors.
Elevated sodium, as an indicator of cancer, may be due to several factors. Among them are angiogenesis, development of new micro blood vessels to feed the tumor and cell growth, both of which cause increased sodium. Energy depletion and the death of normal cells also lead to a massive increases in cell sodium content. However, in malignant tumors, there is an increase in sodium in cells that are not energy deprived or dying, but are rapidly dividing. The signaling mechanism that "turns on" cells to divide rapidly in cancer leads to an influx of sodium into the cell.
The study was funded by the National Health, Lung and Blood Institute, the National Center for Research Resources, and the American Cancer Society.
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Radiology