Image-to-image fluctuation due to physiological motion is a major limitation to the accurate detection of neuronal activity with functional MRI. Last year, we reported that the use of navigator echoes was able to reduce the physiological fluctuation in functional imaging data. In the past year, we have developed improved techniques for further reducing this fluctuation. These techniques are detailed below. Retrospective technique with physiological monitoring: This technique is a general technique for the estimation and the compensation of the physiological effects. During the acquisition of imaging data the respiration and the heart beat are concurrently monitored. By retrospectively ordering the measured k-space data with both physiological cycles, unit cycle fluctuations associated with both respiration and cardiac pulsation are first estimated. Subsequently, fluctuation induced changes at each time point are predicted according to the present position in both cycles and subtracted from the measured data. Images processed in this manner exhibited substantially reduced physiological fluctuations (by as much as 80%) and improved functional maps. This technique does not rely on the periodicity of the respiration or the heart beat, does not distort the signal changes arising from neuronal activation, and is beneficial to images acquired with any speed. This technique is now routinely utilized in our functional studies. A wireless approach for reduction of physiological noise: This technique is similar to the technique described above except that it does not need the physiological monitoring. In contrast, the physiological information is derived directly from the measured imaging data. In FLASH, respiratory cycles are derived utilizing the phase of the center of a navigator echo, in EPI from the phase of the center k-space point. Cardiac cycles are determined from projections obtained from the navigator echo (FLASH) and the center k-space line (EPI). Since direct extraction of physiological parameters eliminates the need for external monitoring, the method can be more readily implemented. Experimental results illustrate that the technique provides effective compensation for physiology related signal fluctuations in functional MRI and performs as well as the retrospective technique using external physiological monitoring.
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