Toshinori Hirai

Perfusion MR imaging with arterial spin-labeling (ASL) is a method using magnetically labeled blood water protons as an endogenous tracer to allow visualization and measurement of perfusion. Over the last decade, this technique has been used to evaluate perfusion in various pathologic conditions. The usefulness of perfusion MR imaging with ASL for the assessment of brain tumor angiogenesis and glioma grading has been evaluated.

Among ASL techniques, we chose the quantitative STAR labeling of arterial regions (QUASAR) sequence for the following reasons. The bolus saturation techniques and the acquisition of images at multiple time points as used in the QUASAR sequence render ASL less sensitive to the transit time. In addition, the QUASAR sequence can quantify relative cerebral blood flow (rCBF) user-independently because the arterial input function (AIF) is calculated automatically voxel-by-voxel, based on crushed and noncrushed control-label pairs.¹ Thus, QUASAR is a particular application of the ASL method. To our knowledge, however, the reliability and reproducibility of glioma perfusion values obtained by ASL with QUASAR have not been reported. Therefore, we evaluated the intermodality agreement of tumor blood flow (TBF) measurements obtained with QUASAR and dynamic susceptibility contrast-enhanced (DSC) MR imaging and the inter- and intraobserver reproducibility of glioma TBF measurements acquired by QUASAR at 3T.

Although the relative cerebral blood volume (rCBV) is usually used for evaluating the perfusion of gliomas, we used rCBF values. DSC perfusion MR imaging can provide both rCBF and rCBV of the brain. In contrast to DSC perfusion MR imaging, ASL methods measure CBF only. Because earlier reports of using DSC perfusion MR imaging showed a strong

correlation between the rCBV and rCBF ratios in high- and low-grade gliomas,² we used rCBF ratios to compare tumor perfusion obtained by DSC and ASL methods. On the basis of our finding that ASL at 3T yielded measurements similar to those of DSC perfusion MR imaging, we suggest that measuring rCBF ratios with ASL at 3T is useful for evaluating the perfusion of gliomas.

ASL has the advantage of being noninvasive, and because it also does not require the administration of an extrinsic tracer, there is no contrast medium to affect the physical, chemical, or physiologic properties of the blood. Therefore, ASL may be especially useful in patients with glioma and renal dysfunction, because they may be at risk for contrast-related nephrogenic systemic fibrosis, and in children in whom the rapid bolus injection of contrast materials into the vein may be difficult.

University of Miyazaki
toshinorh@med.miyazaki-u.ac.jp

References

1. Petersen ET, Lim T, Golay X. Model-free arterial spin labeling quantification approach for perfusion MRI. Magn Reson Med 2006;55:219–32, 10.1002/mrm.20784
2. Shin JH, Lee HK, Kwun BD, et al. Using relative cerebral blood flow and volume to evaluate the histopathologic grade of cerebral gliomas: preliminary results. AJR Am J Roentgenol 2002;179:783–89, 10.2214/ajr.179.3.1790783

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