MAGNETIC-RESONANCE-IMAGING AND SPECTROSCOPY OF SMALL RING-ENHANCING LESIONS USING A RAT GLIOMA MODEL

RATIONALE AND OBJECTIVES. We sought to demonstrate the usefulness of proton and fluorine magnetic resonance spectroscopy (MRS) techniques in characterizing small ring enhancing lesions produced by experimental malignant gliomas. METHODS. The growth characteristics of a rat glioma model (RT2) were studied using contrast-enhanced magnetic resonance imaging scans of the tumors and histologic correlates obtained at various times. Changes in tumor metabolite levels were monitored on a serial basis using water-suppressed proton spectroscopy. The existence of tumor hypoxia was established using F-19 MRS in combination with a fluorinated nitroimidazole and subsequently confirmed by immuno histochemical staining of tumor sections. RESULTS. Ring-enhancing lesions are produced by RT2 rat brain gliomas approximately 7 days after intracerebral implantation. Beginning at day 5, marked deviations in brain metabolite levels are observed on proton MR spectra. However, while the signal from the fluorinated nitroimidazole is first detected by F-19 MRS at day 7, immunohistochemical staining of tissue sections reveals bound drug as early as day 5, when the first histologic signs of necrosis become apparent. CONCLUSIONS. Magnetic resonance imaging of RT2 rat brain glioma exhibits ring-enhancing characteristics similar to those observed in clinical studies. The appearance of the ring enhancement corresponds with the development of central necrosis and could serve as an indicator for rapid growth. Proton and fluorine MRS may be useful in confirming that a small ring-enhancing lesion represents an active tumor process early in its development.