Application of Exogeneous Gaseous Agents in Patients with Brain Tumors using High-Resolution Susceptibility-Weighted Imaging

Introduction: Blood oxygenation level dependency (BOLD) magnetic resonance contrast has been used for functional and anatomic investigations. The oxygenation level of venous blood can be modulated by exogeneous agents, such as carbogen and oxygen. The aim of this study was to employ a high resolution BOLD method to investigate the response of brain tumors to these breathing gases. Methods: Data of six patients [four with glioblastoma multiforme (all grade IV), one with an astrocytoma (grade II), and one with a cerebral metastasis; all histologically confirmed] were acquired using a susceptibility-weighted 3D gradient echo, first order velocity compensated sequence, on a 1.5T MR Siemens Magnetom scanner (TE/TR/alpha=40 ms/67 ms/25°,FOV=256×192×64, matrix=512×192×64). The scan was repeated up to three times while the patients were breathing oxygen, carbogen and/or air. Informed consent was obtained from all individuals. The protocol was approved by the local ethics committee. Susceptibility-weighted images (SWI) of the spatially realigned data were computed by multiplication of the magnitude images with phase masks computed from the same scans. Minimum intensity projections (mIPs) were calculated for visualization of small venous vessels. The relative signal changes between the three breathing situations were evaluated over 3D ROIs of typically 0.5cm³ (i.e., 15×15×10 voxels) in different tumor ROIs. Maps of relative signal changes were computed. Results: An ROI-based evaluation of the signal intensities in the tumors revealed a signal decrease during carbogen breathing compared to oxygen inhalation in 3 cases. The glioblastomas displayed the highest response but also the highest heterogeneity (signal changes from –7% to +28%). The astrocytoma and the cerebral metastasis showed a negative response in all ROIs ranging from –9.3% (edema) to –2.0% (central areas) for the astrocytoma and –8% to –16% for the cerebral metastasis. Edemas or areas of high tumor vascularity were clearly identified on maps of oxygen/carbogen response. Discussion: The method proved its potential to non-invasively probe the human brain response in patients with brain tumors to exogeneous contrast agents and at the same time to visualize venous vascularity at very high spatial resolution. Systematic studies on different tumor types may allow for a better classification of brain tumors or areas of abnormal blood supply.