Measurement of Perfusion Using Bolus Tracking T2 MRI

As mentioned in Sect. 4.1, the effect of the contrast agent on the signal intensity on T2*-weighted images can be used to measure perfusion (Villringer et al. 1988; Rosen et al. 1990). This technique, usually referred to as dynamic susceptibility-contrast (DSC) MRI, is also known as bolus tracking. It involves the rapid injection of a bolus of paramagnetic contrast agent and the measurement of the transient signal loss during its passage through the tissue of interest (Fig. 4.1). Since the transit time of the bolus through the tissue is only a few seconds, a rapid MRI technique is required to properly characterise the signal intensity time course. The more commonly used sequences are echo-planar imaging (EPI) (StehlinG et al. 1991) and fast low-angle single-shot (FLASH) (Haase 1990). Although EPI is faster (therefore allowing the acquisition of more slices for a given time resolution), the increased image distortions in regions of high magnetic susceptibility gradients (such as close to tissue-air interfaces) and hardware demands (Fischer and Ladebeck 1998; Bowten and Schmitt 1998) favour FLASH sequences for many applications.

As discussed in Sect. 4.1, T2*-based techniques for measuring perfusion are primarily used in cases where there is a significant compartmen-talisation of the contrast agent (to observe a significant decrease in T2* relaxation). Therefore, they

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