Functional Neuroimaging: Fundamental Principles and Clinical Applications

Abstract

Functional imaging modalities, such as functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI), are rapidly changing the scope and practice of neuroradiology. While these modalities have long been used in research, they are increasingly being used in clinical practice to enable reliable identification of eloquent cortex and white matter tracts in order to guide treatment planning and to serve as a diagnostic supplement when traditional imaging fails. An understanding of the scientific principles underlying fMRI and DTI is necessary in current radiological practice. fMRI relies on a compensatory hemodynamic response seen in cortical activation and the intrinsic discrepant magnetic properties of deoxy- and oxyhemoglobin. Neuronal activity can be indirectly visualized based on a hemodynamic response, termed neurovascular coupling. fMRI demonstrates utility in identifying areas of cortical activation (i.e., task-based activation) and in discerning areas of neuronal connectivity when used during the resting state, termed resting state fMRI. While fMRI is limited to visualization of gray matter, DTI permits visualization of white matter tracts through diffusion restriction along different axes. We will discuss the physical, statistical and physiological principles underlying these functional imaging modalities and explore new promising clinical applications.

Neurobiological signatures associated with alcohol and drug use in the human adolescent brain

“…Diffusion tensor imaging (DTI) is a structural technique that enables measurement of the restricted diffusion of water in tissue to examine white matter architecture (directional diffusion, fractional anisotropy (FA), with higher levels typically reflecting better white matter integrity), and average water diffusion (mean diffusivity (MD)) (Mukherjee et al, 2008a; Mukherjee et al, 2008b). Functional magnetic resonance imaging (fMRI) provides information regarding brain responses to passive activity in a resting state (rsfMRI) or to external stimuli (task fMRI), by acquiring blood oxygenation level dependent (BOLD) signals, which reflect hemodynamic responses to transient neural activity that result from changes in the ratio of oxyhemoglobin and deoxyhemoglobin (Khanna et al, 2015). More recent advances in fMRI-based connectivity analyses permit assessment of network-level properties of brain function and intrinsic/resting state functional connectivity (rsFC) (Ernst et al, 2015).…”

Variance decomposition for single-subject task-based fMRI activity estimates across many sessions

“…Finally, such a dataset can also help inform the future development of fMRI clinical protocols. Although, many scientists and clinicians alike foresee resting-state as the primary paradigm for clinical fMRI (Khanna et al, 2015; Shimony et al, 2009), task-based fMRI is also clinically relevant, as clearly evidenced by its inclusion in many existing pre-surgical protocols (Hirsch et al, 2000; Stippich et al, 2007). Low test-retest reliability is often cited as a reason why fMRI has not been widely adopted in clinical practice (Stevens et al, 2016).…”

Resting-state functional magnetic resonance imaging for surgical planning in pediatric patients: a preliminary experience

“…21 Task-based fMRI (t-fMRI) combines standard anatomical imaging with advanced sequences sensitive to the blood-oxygen level dependent (BOLD) signal, which is an indirect measure of neuronal function. 13 Traditional t-fMRI is performed by measuring the BOLD signal while a patient alternates between epochs of task performance and rest. Interleaved epochs of rest are used to model background signal from which the evoked task response is referenced.…”

“…Interleaved epochs of rest are used to model background signal from which the evoked task response is referenced. 3,13 Common tasks include hand or foot movement for motor localization and speaking tasks to localize language. While non-invasive, t-fMRI still depends on patient performance.…”