Blood volume measurement and diffusion tensor imaging differentiate among tumor types
Neuroradiologists may use MRI blood volume measurement and diffusion tensor imaging to lessen dependence on histopathology for characterizing brain tumors. Preliminary trials from the 2006 International Society for Magnetic Resonance in Medicine meeting demonstrate progress toward the introduction of clinically useful techniques.
Vascular space occupancy (VASO) MRI, a new blood volume measurement technique, shows particular promise for grading brain tumors. The protocol exploits the relationship between the aggressiveness of gliomas and the angiogenetic processes that surround these tumors.
Contrast-enhanced VASO MRI reflects the combined effect of cerebral blood volume and vascular permeability. Areas of high signal on VASO MR images correlate with high relative CBV and high vessel permeability. Both become more pronounced in higher grade gliomas, according to Hanzhang Lu, Ph.D., an assistant professor of radiology at the Advanced Imaging Research Center of University of Texas Southwestern Medical Center.
Lu's study of 39 patients demonstrated the value of using this information to noninvasively grade gliomas. The work was performed during Lu's research studies at New York University. Two calculations of vascular space occupancy produce highly significant measures of tumor grade. Lu found that both VASOtumor (p = 0.0006) and VASOratio (p = 0.0004) accurately differentiated grade 1, 2, and 3 tumors.
"These data suggest that VASO MRI can quantify tumor hemodynamics and complement histopathology for classifying tumor grade," Lu said.
Researchers have also embraced diffusion tensor imaging as a possible way to improve diagnosis and management of glial tumors. Dr. Kristina Szabo of University Hospital Mannheim in Germany took a step back to examine how DTI currently affects surgical planning. She examined therapeutic decisions based on conventional MRI only, then added DTI results for 20 glioma patients.
Szabo discovered that surgical plans were sometimes improved when the evaluation included DTI. The technique improved presurgical assessment, helping reduce the risk of surgery in eight cases, and led to an enlargement of the tissue volume designated for surgical resection in six. DTI should be added to the routine workup to assist surgical planning in these cases, Szabo said.
Isotropic and anisotropic patterns obtained from DTI may predict glial tumor progression and guide radiation therapy, according to a report by Dr. Stephen John Price of the University of Cambridge in the U.K. From DTI studies of 26 patients, Price achieved 92% accuracy when retrospectively identifying glial tumors that would remain stable for at least two years and the anatomic pattern of growth for tumors that would progress. Three patterns emerged from his study at Cambridge's Wolfson Brain Imaging Center:
- The isotropic component was diffusely distributed and larger than the anisotropic component in 13 patients. In 12 of the 13, the recurrence pattern involved diffuse enlargement of the whole tumor.
- Localized abnormalities in eight patients involved the concentration of the isotropic component in a specific region of the tumor relative to the anisotropic component. In all cases, recurrence localized in the direction of this abnormality.
- Minimal abnormalities in five patients correlated with similarities between the isotropic and anisotropic components. The tumor progressed for one patient but remained stable for up to five years for the remaining four.
These findings may have implications for radiation therapy planning, Price said. Radiotherapy could be sculpted to concentrate radiation in the probable starting points for progression and to avoid exposure to normal brain tissue.
At St. George University of London, Dr. Timothy Clark and colleagues have had some success using DTI to noninvasively differentiate among glioblastoma, meningioma, and brain metastasis.