functional MRI

Functional magnetic resonance imaging (fMRI) is a technique used to obtain functional information by visualizing cortical activity. fMRI detects subtle alteration in blood flow in response to stimuli or actions. It is used in two broad ways:

  • clinical practice
    • typically in presurgical patients 
      • aimed at localizing eloquent areas (e.g. speech, motor function)
  • research 
    • often cohort of patients (often normals)
    • aimed at elucidating novel neural networks
  • fMRI is technically-challenging to perform as the techniques used to visualize cortical activity (most commonly BOLD imaging) rely on minute changes in a low signal-to-noise ratio (SNR) environment.

    Technical requirements

    • 1.5 T or higher field strength MRI
    • excellent quality assurance
    • appropriate software
    • appropriate paradigms and ability to deliver visual and auditory stimuli and record motor response
    • co-operative patients

    Study design

    Two testing designs are employed most commonly:

    • block design
    • event-related design

    Block design uses repeated blocks of activity (paradigm) separated by blocks of inactivity or alternative activity. This is by far the most frequently used study design in clinical fMRI.

    Event-related design involves individual events rather than blocks, and can be randomly distributed during the study.

    Paradigms

    The activity performed or stimulus received by the patient is termed a paradigm, and each is designed to elicit a specific cortical response. Numerous paradigms have been developed of various complexity. In the clinical setting four paradigms (with modifications according to the clinical situation) suffice for most indications.

    • visual paradigm
    • motor paradigm
    • speech paradigm
    • memory paradigm