It is hoped that this chapter has shed some light on the issues associated with defining the spatial and temporal resolution limits and the sensitivity in fMRI. New pulse sequences and new imaging hardware are being devel- oped constantly and, combined with a better understanding of the physi- ological changes that occur with brain activation, the ability to obtain high resolution fMRI studies in short exam times will continue to improve. There are many trade-offs to be made in deciding on the imaging sequence and parameters to use, and it is hoped that this brief overview will shed some light on the issues involved. Clearly, because many trade-offs must be made, an understanding of these issues will help the investigator to tailor some of these parameters to the specific brain region or study design of interest.
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Edited by: S.H. Faro and F.B. Mohamed, DOI 10.1007/978-1-4419-1329-6_5
© Springer Science+Business Media, LLC 2010
Introduction
Functional magnetic resonance imaging (fMRI) has revolutionized clinical brain mapping and has become the predominant functional neuroimaging technique since its original report by Belliveau and colleagues.1 The appeal of fMRI is attributable to several advantages that it offers over other func- tional neuroimaging techniques. Functional MRI is noninvasive; it is a rapid technique that offers the opportunity for repeated measurements of the same task to investigate response consistency, to compare activations across tasks, and to measure change over time.
Despite its advantages, fMRI presents several unique challenges, especially in the clinical setting (Table 5.1). Many of these challenges arise from the fact that fMRI does not directly measure neuronal activity. Instead, fMRI detects perfusion-related signals that are coupled to neuronal activity. Many studies make assumptions about the characteristics of neurovascular coupling, and therefore the significance of fMRI activations; these assumptions are more suspect in a clinical setting when pathology may alter normal coupling mechanisms; for example, the presence of intracerebral pathologies [e.g., arteriovenous malformations (AVMs)] can induce field inhomogeneities and also may alter neurovascular coupling mechanisms, both of which may hamper measurement of reliable hemodynamic-based fMRI signals.
Another challenge of clinical fMRI includes the inability of patients to com- ply with imaging protocols. One study2 showed that nearly 30% of subjects with intracranial masses were excluded from the final analysis due to gross motion artifact. This may be a particularly difficult problem if one wishes to study patients with known movement disorders. Impairments in cogni- tion also may alter patients abilities to complete tasks, both with respect to motivation and task difficulty.
Finally, clinical brain mapping emphasizes results for an individual rather than for a group, impacting strongly on choice of analysis methods.