Clinical fMRI

Abstract

Clinical functional MRI (fMRI), in the context of this chapter, refers specifically to the use of blood oxygenation level dependent (BOLD) contrast to detect localized hemodynamic responses due to specific regional neuronal activity elicited by defined cognitive tasks to which the patient is exposed in a controlled manner. The method was first demonstrated in humans in the early 1990s in a research setting (Bandettini et al., 1992; Kwong et al., 1992; Ogawa et al., 1992), but has rapidly found clinical applications (Lee et al., 1999; Thulborn, 1999). At clinical field strengths of 1.5 T, the signal change is small (1% to 3%) thereby requiring the use of image averaging during the cognitive task. Thus, the stimulus paradigm has a block design, consisting of repetitive cycles of at least two different stimulus conditions. These two conditions differ by the cognitive function being examined. Both the baseline and the active conditions may last from 20 to 60 sec and as many as 10 cycles may be used. Images are acquired continuously across all cycles. This block design allows the signal averaging that is essential to detect the small signal changes induced by the paradigm at 1.5 T, although these changes can be increased at higher field strengths (Gati et al., 1997; Thulborn 1999). The alternative approach of event-related design is being developed in a research setting (Rosen et al., 1998; Richter, 1999), but the lower signal-to-noise performance does not suit the short duration of acquisition times that are important in clinical applications (Marquart et al., 2000). An interesting approach, in which no paradigm is apparently required to demonstrate connectivity between different regions of the brain, may have clinical applications but will not be discussed further until clinical applications (UNIT A6.1) are reported from multiple sites (Biswal et al., 1995).