Probing ischemic tissue fate with BOLD fMRI of brief oxygen challenge

It has been recently shown that at-risk tissue exhibits exaggerated T ₂ ^*-weighted MRI signal increases during transient oxygen challenge (OC), suggesting that the tissue is still metabolically active. This study further characterized the effects of transient OC on T ₂ ^*-weighted MRI in permanent focal stroke rats (N = 8) using additional quantitative measures. The major findings were: i) the ischemic core cluster showed no significant response, whereas the mismatch cluster showed markedly higher percent changes relative to normal tissue in the acute phase. ii) Many of the mismatch pixels showed exaggerated OC responses which became hyperintense on T ₂-weighted MRI at 24 h. The area with exaggerated OC responses was larger than the mismatch, suggesting that some tissue with reduced diffusion were potentially at risk. iii) Basal T ₂ ^*-weighted intensities on the perfusion-diffusion contourplot were high in normal tissue and low in the core, with a sharp transition in the mismatch. iv) OC-induced changes on the perfusion-diffusion contourplot dropped as perfusion and diffusion values fell below their respective viability thresholds. v) Basal T ₁ increased slightly in the ischemic core (P < 0.05). OC decreased T ₁ in normal (P < 0.05) but not in mismatch and core pixels. vi) OC decreased CBF in normal (P < 0.05) but not in mismatch and core pixels. T ₂ ^*-weighted MRI of OC has the potential to offer unique clinically relevant data.