IMPROVEMENT OF DETECTABILITY OF BRAIN ACTIVATION IN FUNCTIONAL MAGNETIC RESONANCE IMAGING

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The purpose of this study was to improve the detectability of brain activated area in functional magnetic resonance imaging (fMRI). In fMRI, influence of cardiac pulsation is the most difficult factor to be eliminated for enhancing the detectability since it induces intracranial vibration. We found that partial volume of cerebrospinal fluid in the activated voxel vibrates owing to cardiac pulsation and then the detectability of activated area deteriorates. PhysionFix is one of the best software packages to make physiological corrections in fMRI. We clarified that the efficacy of PhysioFix depends on correlation between cardiac pulsation or respiration and the task paradigm frequency. A signal increase in fMRI has been believed to be a result of a decrease of paramagnetic deoxygenated hemoglobin (deoxyHb) content in the neutral activation area. However, experimental studies using optical techniques, such as near infrared spectroscopy (NIRS), that directly measure hemoglobin changes show disagreements with the canonical blood oxygenation level dependent (BOLD) theory. To improve the detectability in fMRI, it was necessary to make clear the reason for the disagreements and to understand the true hemodynamics associated with brain activation. We developed a new theory bridging magnetic resonance (MR) signal and hemoglobin changes. It was found that the deviation from the value expected by the canonical BOLD theory is caused by the fact that the BOLD theory ignores the effect of the capillaries. The new theory enables us to explain the paradoxical phenomena of the oxygenated hemoglobin and deoxyHb contributions to the MR signal and to have a new insight into the precise hemodynamics of activation by analyzing fMRI and NIRS data. We concluded that the capillary hemodynamics of activation focus should be taken into account for further improvements of detectability in fMRI, in addition to counter-measures against the influence of physiological fluctuations.