Quality assurance in functional MRI.

Over the past 20 years, functional magnetic resonance imaging (fMRI) has ben- efited greatly from improvements in MRI hardware and software. At the same time, fMRI researchers have pushed the technical limits of MRI systems and greatly in- fluenced the development of state-of-the-art systems. Minimizing image noise and maximizing system stability is critical in fMRI because the blood oxygenation level- dependent (BOLD) signal changes that are used for most fMRI studies represent only a small fraction of the total MR signal. In addition, multiple imaging volumes must be acquired over time to track cognitive processes. As a result, MRI scanners must have excellent time-series stability to accurately measure BOLD signal changes over the course of a long time series (typically on the order of 10 min per scan). fMRI studies are particularly demanding on the scanner hardware because they utilize fast imaging methods such as echo-planar imaging (EPI) or spiral acquisitions that can push the scanner hardware to its limits. High temporal stability sustained over the course of a scan session (up to 2 h) is required to obtain quality data within a single subject, sustained stability is required to obtain consistent data throughout the course of a study, and consistent stability across time and sites is needed to allow data from different time periods or acquisition sites to be optimally integrated.