The Ideal Paradigm

Before starting any routine clinical application of fMRI on patients, it is useful to perform a survey of the literature in the search for the „ideal" paradigm. In our opinion, a reliable paradigm should have the following characteristics: (1) The activation induced should be specific. Specificity refers to the ability to localize a function: a high specific paradigm should have a high localizing power, i.e. the ability to select and discover all and only the areas appertaining to that function considered. This way, evoked eloquent areas are defined unambiguously as to anatomical location and extent. (2) The activation induced should be reproducible: evoked eloquent areas have to remain unchanged as to location and extent, through different trials, made in the same and/or in different sessions, thus allowing patient follow-up. (3) The paradigm should be easy to learn by patients having different social and cultural backgrounds. If a patient does not understand clearly what to do or what will happen, he or she obviously will not perform the paradigm correctly and the result will be a suboptimal activation. (4) The paradigm should be short-lasting. The length of time of a paradigm should enable the patient to maintain a high attention level all through the trial; otherwise, again a suboptimal activation will result. Unfortunately, an optimal duration is only a compromise between the time spent by the patient in the magnet and the need to acquire enough data for statistically significant mapping. Usually, an fMRI session includes more than one paradigm and lasts for almost an hour.

A Few Words on Sensitivity. Sensitivity is the ability to detect low signals and/or to respond to small physical amounts or differences. As we have just seen („3 T vs 1.5 T") and will see soon („Experimental Design"), sensitivity does not depend only on the biological phenomena at the base of the BOLD contrast effect, but also on the characteristics of acquiring equipment (magnetic field strength, kind of sequence acquired, artefacts, etc.), as well as on the experimental design applied. Unfortunately, sensitivity and specificity are often in inverse relation. As a consequence, caution is needed in comparing results obtained by using different equipment and experimental designs.

It is useful, after the choice from the literature of the paradigms having the above characteristics, to select from them those producing the widest activation areas, with the aim, in a presurgical perspective, to spare as much eloquent tissue as possible. The result of such an operation is to obtain a set of paradigms, which, on the whole, constitute an adequate tool with which to explore the main eloquent cortical areas (cf. [7] for review). After this, a safe strategy to set up the system is to implement and apply all paradigms to healthy volunteers, before proceeding with the routine clinical application on patients. Finally, each patient has to receive a personalized set of paradigms, which are selected on the basis of the specific localization and extension of the existing pathology.

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