Perfusion Imaging

The most common MR perfusion imaging techniques exploit the spin dephasing (T2*) effect from the passage of contrast through the parenchymal capillary bed. The SI loss engendered by the passage of gadolinium-based contrast enables cal culation of contrast concentration within each pixel over time, which in turn provides relative measurements of regional cerebral blood volume (rCBV). Rapid imaging is necessary to gather as much data as possible from the entire brain during the first-pass of a bolus injection of gadolinium.3

Perfusion imaging likely reflects tumor angiogenesis, a strong indicator of tumor grade. Studies employing perfusion techniques with histologic correlation agree that high rCBVs mean higher tumor grade (Figure 27.6). Specifically, rCBV correlated with areas highest in mitotic activity and vascularity but not with areas of cellular atypia or high cellularity.29 Su-gahara et al. found a significant correlation between rCBV ratios of gliomas and vascularity of the tumors determined both by angiography and histology (P less than 0.001).30 In a series of 160 patients, Law et al. demonstrated increased sensitivity of detection of high-grade gliomas with rCBV values when compared to conventional imaging only. They also demonstrated a significant difference in the values of rCBV between high-grade and low-grade gliomas (P less than 0.0001).31 Similar results were reached in three other studies.32-34

Perfusion imaging has proved helpful in the preoperative diagnosis of and differentiation between different brain lesions. Recently, Hartmann et al. found significantly lower rCBVs in primary CNS lymphoma (PCNSL) compared to glioblastoma multiforme (GBM) (P less than 0.0001).35 Similarly, low rCBV measurements were found in 17 patients with gliomatosis cerebri, which is in concordance with the lack of vascular hyperplasia found at histopathologic examination in those tumors. The authors concluded that perfusion MR imaging provides useful adjunctive information to conventional MR imaging techniques in the evaluation of this relatively rare but important entity.32 In the classic problem of differentiating toxoplasmosis from lymphoma in patients with acquired immunodeficiency syndrome (AIDS), rCBV was decreased throughout the toxoplasmosis lesions whereas all active lymphomas displayed areas of increased rCBV. The difference in rCBV between those two entities was significant (P less than 0.005). Reduced rCBV in toxoplasmosis lesions is probably due to a lack of vasculature within the abscess compared to the hypervascularity of lymphomas, especially within foci of active tumor growth.36

Finally, in a series of 51 patients, Law et al. found that peritumoral rCBV values in high-grade gliomas were significantly higher than in metastases (P less than 0.001). Mean maximum rCBV in high-grade gliomas was also significantly higher than in low-grade gliomas in a study by Yang et al.32 Whether perfusion imaging can be used to define tumor borders remains an attractive concept to be investigated.

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