gms | German Medical Science

Objective: To describe the dynamic nature of contrast enhancement in high-grade gliomas, the challenges for intraoperative imaging and initial methods to compensate for the phenomenon of “spread enhancement”. Intraoperative MRI (iMRI) has been employed for various indications. While technical issues have been solved, and microsurgery is possible in an iMRI environment, particularly surgery of high-grade gliomas poses a major imaging challenge. While we know, that tumours extend beyond the margins of MRI contrast enhancement, this lesion is taken as the „active“ portion. Subsequently the surgical result is described by removed percent of contrast-enhancing lesion. Nevertheless, contrast enhancement merely reflects the local breakdown of the blood brain barrier and does not remain constant. Imaged in various time intervals, the contrast diffuses into the surrounding. While this is inconsequential in conventional diagnostics, it becomes of major importance in intraoperative MRI.

Methods: We operated on 225 patients in our intraoperative MRI unit between 09/2005 to 09/2007. Of these 83 patients had either primary (43) or recurrent gliomas (40), and received contrast intraoperatively. Initial resection was guided by neuronavigation with contrast-enhanced MRI, and more recently augmented by fluorescence microscopy (5ALA). Subsequently the patients were scanned according to a standard protocol, which includes comparable sequences with specified time interval. In recurrent gliomas fast scans were undertaken, to show the in-flow into different tissues in order to differentiate valid tumor from necrosis (“dynamic imaging”).

Results: Contrast enhancement shows non-specific, time dependent characteristics. Images taken immediately after contrast administration reliably depict tumour extension. However even within the short time course of post-contrast orthogonal images contrast dilutes, and spreads into the surrounding area, yielding ambiguous information. “Dynamic imaging” indicates a differentiation between residual tumour and necrosis.

Conclusions: Contrast enhancement is non-specific and time dependant. Knowledge of this phenomenon, and specific imaging protocols compensate for this effect during surgery in an iMRI. Furthermore, analyses of this problem emphasizes the non-specific enhancing nature of current contrast agents, which pose our current basis to define surgical goals and results, stressing the necessity for more specific contrast media.