ASL perfusion quantification: a comparison with PET and Optical Imaging techniques using double labeled microspheres

Purpose/Introduction: Regional tissue perfusion is a fundamental physiological parameter controlling delivery of oxygen and nutrients to tissue which provides valuable information for cancer research and radiation therapy planning. There do exist established methods in nuclear medicine for perfusion assessment and quantification (e.g. O-15 PET), however, the truly quantitative procedures in general require arterial blood sampling making them ill-suited for routine clinical application.. In contrast, the Arterial Spin Labeling (ASL) MRI technique is absolutely non- invasive, but its quantitative accuracy in small animals is not well investigated. The purpose of this work was to assess accuracy of ASL perfusion measurements in rat brain by a comparison with microspheres derived regional perfusion information using dedicated small animal Positron Emission Tomography (PET) and ex vivo Optical imaging (OI).

Subjects and Methods: 20 µm microspheres were double-labeled with either Cu-64 or Ga-68 for PET and X-sight 670 LSS for OI. They were administered through a catheter to the left ventricle of the heart and a reference blood sample was extracted from the left femoral artery. ASL measurements were performed in a 7T animal system (Bruker Biospin 70/30), using a Flow-sensitive Alternating Inversion Recovery (FAIR) sequence with an adiabatic hyperbolic secant inversion pulse (sech120) and Echo Planar Imaging (EPI) acquisition. Global and selective T1 maps of two measured slices (Hemisphere in caudate putamen region and Cerebellum), PET data and correspondingly cut brain sections were used for calculation of the perfusion. All relevant physiological parameters were monitored. Perfusion of the whole brain (PET, OI) and two dedicated slices (PET, OI, MRI) were analyzed. The results of the different methods were compared.

Results: The perfusion dependency of PET vs. OI was fitted with a linear model resulting in a slope of 0.96 and a Pearson correlation factor (PCF) of 0.97. Since water cannot be considered freely diffusible at elevated perfusion levels, Renkin-Crone formula (y = 1- exp(- PS / x) was used for the fitting of MRI vs. PET data. For the model being used, the sensible value for the surface area product (PS) of 182mL/g/min for water in brain tissue was yielded. .

Discussion/Conclusion: The results of this study show that it is possible to obtain quantitative perfusion values with ASL in the brain of the rat using the EPI-FAIR technique .