3D CMRO2 mapping in human brain with direct 17O MRI: Comparison of conventional and proton-constrained reconstructions

Oxygen metabolism is altered in brain tumor regions and is quantified by the cerebral metabolic rate of oxygen consumption (CMRO2). Direct dynamic O-17 MRI with inhalation of isotopically enriched O-17(2) gas can be used to quantify CMRO2; however, pixel-wise CMRO2 quantification in human brain is challenging due to low natural abundance of O-17 isotope and, thus, the low signal-to-noise ratio (SNR) of O-17 MR images. To test the feasibility CMRO2 mapping at a clinical 3 T MRI system, a new iterative reconstruction was proposed, which uses the edge information contained in a co-registered H-1 gradient image to construct a non-homogeneous anisotropic diffusion (AD) filter. AD-constrained reconstruction of O-17 MR images was compared to conventional Kaiser-Bessel gridding without and with Hanning filtering, and to iterative reconstruction with a total variation (TV) constraint. For numerical brain phantom and in two in vivo data sets of one healthy volunteer, AD-constrained reconstruction provided O-17 images with improved resolution of fine brain structures and resulted in higher SNR. CMRO2 values of 0.78 - 1.55 mu mol/g(tissue)/min (white brain matter) and 1.03 - 2.01 mu mol/g(tissue)/min (gray brain matter) as well as the CMRO2 maps are in a good agreement with the results of O-15-PET and O-17 MRI at 7 T and at 9.4 T. In conclusion, the proposed AD-constrained reconstruction enabled calculation of 3D CMRO2 maps at 3 T MRI system, which is an essential step towards clinical translation of O-17 MRI for non-invasive CMRO2 quantification in tumor patients.