Eccentricity mapping of the human visual cortex to evaluate temporal dynamics of functional T 1ρ mapping

Abstract

Recent experiments suggest that T 1 relaxation in the rotating frame (T 1ρ) is sensitive to metabolism and can detect localized activity-dependent changes in the human visual cortex. Current functional magnetic resonance imaging (fMRI) methods have poor temporal resolution due to delays in the hemodynamic response resulting from neurovascular coupling. Because T 1ρ is sensitive to factors that can be derived from tissue metabolism, such as pH and glucose concentration via proton exchange, we hypothesized that activity-evoked T 1ρ changes in visual cortex may occur before the hemodynamic response measured by blood oxygenation level-dependent (BOLD) and arterial spin labeling (ASL) contrast. To test this hypothesis, functional imaging was performed using T 1ρ, BOLD, and ASL in human participants viewing an expanding ring stimulus. We calculated eccentricity phase maps across the occipital cortex for each functional signal and compared the temporal dynamics of T 1ρ versus BOLD and ASL. The results suggest that T 1ρ changes precede changes in the two blood flow-dependent measures. These observations indicate that T 1ρ detects a signal distinct from traditional fMRI contrast methods. In addition, these findings support previous evidence that T 1ρ is sensitive to factors other than blood flow, volume, or oxygenation. Furthermore, they suggest that tissue metabolism may be driving activity-evoked T 1ρ changes.