Cerebral oxygen extraction fraction (OEF) measurement correlation in human brain tumors using an oxygen-sensitive MRI sequence and ¹⁵O PET

Objectives To quantify and compare Oxygen Extraction Fraction (OEF) measurement in the normal brain and brain tumors using ¹⁵O positron emission tomography (PET) and oxygen sensitive magnetic resonance (MR)^1,2 imaging.

Methods 30 participants (20 with brain tumors) were recruited. MRI scans included standard clinical sequences (T1, FLAIR, T2*, SWI, etc.) and OEF-MR², a two-dimensional multi-echo gradient spin echo sequence. Concurrent with the MR acquisition, subjects with brain tumors underwent PET scanning, which included 2 sets of 3 scans with serial inhalations of air with radiolabeled carbon monoxide (C¹⁵O) and radiolabeled oxygen (¹⁵O₂), followed by an injection of radiolabeled water (H₂¹⁵O). MR and PET data were post-processed off line and co-registered to the anatomic T1 post-contrast images. Regions of interest (ROI) were defined based upon contrast-enhancing tumor areas, peritumoral (edema), contra-lateral normal white matter (NWM), and normal gray matter (NGM). Ratios of OEF (rOEF) of the lesions to NGM were obtained.

Results There is a high correlation between two rOEF-PET measurements for tumor (R=0.82; slope=1.01), edema (R=0.88; slope=0.99), and NWM (R=0.87; slope=0.99). When lesions with SWI abnormalities (blood cloth, hemorrhage, calcification) are excluded, rOEF-MR and rOEF-PET correlate well for tumor (R=0.68; slope=1.14) and overall ROIs (R=0.85; slope =0.96).

Conclusions Both MR and ¹⁵O PET can measure OEF in brain tumors and in peritumoral areas. BOLD MR fails in regions with signal loss on SWI or T2*. Both techniques have tremendous potential in offering new insights into the underlying physiology of brain tumors and their response to therapy.

Research Support NIH UL1 RR024992; ISMRM 2012 Seed grant; ICTS-WUSTL; American Cancer Society (ACS); Siteman Cancer Center (58-010-52 IRG).