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

Oxygen metabolism is altered in brain tumor regions and is quantified by the cerebral metabolic rate of oxygen consumption (CMRO₂). Direct dynamic ¹⁷O MRI with inhalation of isotopically enriched ¹⁷O₂ gas can be used to quantify CMRO₂; however, pixel-wise CMRO₂ quantification in human brain is challenging due to low natural abundance of ¹⁷O isotope and, thus, the low signal-to-noise ratio (SNR) of ¹⁷O 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 ¹H gradient image to construct a non-homogeneous anisotropic diffusion (AD) filter. AD-constrained reconstruction of ¹⁷O 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 ¹⁷O images with improved resolution of fine brain structures and resulted in higher SNR. CMRO₂ values of 0.78 - 1.55µmol/g_tissue/min (white brain matter) and 1.03 - 2.01µmol/g_tissue/min (gray brain matter) as well as the CMRO₂ maps are in a good agreement with the results of ¹⁵O-PET and ¹⁷O 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 ¹⁷O MRI for non-invasive CMRO₂ quantification in tumor patients.