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 p roton 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 i maging scans of the tumors and histologic correlates obtained at vario us times. Changes in tumor metabolite levels were monitored on a seria l basis using water-suppressed proton spectroscopy. The existence of t umor hypoxia was established using F-19 MRS in combination with a fluo rinated nitroimidazole and subsequently confirmed by immuno histochemi cal staining of tumor sections. RESULTS. Ring-enhancing lesions are pr oduced by RT2 rat brain gliomas approximately 7 days after intracerebr al implantation. Beginning at day 5, marked deviations in brain metabo lite levels are observed on proton MR spectra. However, while the sign al from the fluorinated nitroimidazole is first detected by F-19 MRS a t day 7, immunohistochemical staining of tissue sections reveals bound drug as early as day 5, when the first histologic signs of necrosis b ecome apparent. CONCLUSIONS. Magnetic resonance imaging of RT2 rat bra in glioma exhibits ring-enhancing characteristics similar to those obs erved in clinical studies. The appearance of the ring enhancement corr esponds 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 tum or process early in its development.