Wednesday 28 November 2012

Functional MRI

Functional magnetic resonance imaging (fMRI) is a MRI procedure that measures brain activity by detecting associated changes in blood flow. The primary form of fMRI uses the blood-oxygen-level-dependent (BOLD) contrast.
This is a type of specialized brain scan used to map neural activity in brain by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells.
It is non invasive, does not require to ingest substances or be exposed to radiation. The procedure is similar to MRI but uses the change in magnetization between oxygen-rich and oxygen-poor blood as its basic measure. The resulting brain activation can be presented graphically by color-coding the strength of activation across the brain or the specific region studied.
FMRI is used both in the research world, and to a lesser extent, in the clinical world.
Brain activity mapping enables revealing of the areas of neuronal activation in response to tests, motor, sensor, and other stimuli. Until recently, similar mapping was performed with the help of radionuclide methods: PET and SPECT imaging.
Functional MRI (fMRI) is based on increase of brain haemodynamics in response to cortical neuronal activity due to certain stimulus (Ramsey 2002; Pouratian et al. 2003; Sunaert 2006).
BOLD EPI-GRE registers hyperintense MR signal from active areas of the brain cortex. The registration time of one MR image is about 100 ms. fMRI signal intensity, registered by physiological load, is compared with the intensity, registered in the event of its lack. During MRI examination, the stimulation periods (duration of 30 s) alternate with control periods
fMRI neuronal activity maps of cortical motor center activation in a patient with intrinsic tumour of the paracentral area, imposed on a T1 image
(without stimulation) of the same duration. The total number of scans registered during the examination reaches 20,000. This method of stimulus presenting is called a block paradigm. The areas of statistically significant MR signal increasing during activation, revealed in the course of subsequent mathematical processing of images, correspond to areas of neuronal activity. They are marked with colour—this way the neuronal activity maps are built and these maps are imposed on T1 MRI sequences. Map construction methods subtract images obtained during neuron stimulation from control images obtained in the absence of stimulation. The subtracted image is imposed on a control scan according to its location, and areas of increased neuronal activity are marked with colour. The revealed functionally significant areas could be “imposed” on a T1 MRI sequence of the same section or on a three-dimensional (3D) brain model, and thus it is possible to estimate the ratio between the affected area (tumour) and functionally active brain areas, for example, motor, sensory or visual cortex.

Clinical Application of fMRI

Neuronal activity mapping enables planning the surgical approach and studying of the pathophysiological processes in brain. This method is used in neurosurgery in studying cognitive functions. Its perspective is in revealing the epileptic foci. Currently, fMRI is an integral part of MRI protocol in patients with brain tumours located close to the functionally important brain areas. In the majority of cases, the examination results adequately reflect the location of sensomotor, speech and acoustical areas of brain cortex.
In cases in which fMRI can locate active cortical areas, in 87% of cases there is a correspondence with the results of intraoperational electrophysiological methods, within 1-cm limits, and in 13% of cases, within 2 cm. This is evidence of the high accuracy of the fMRI technique (Nennig et al. 2007).
Performing fMRI (currently it is conducted for somatosensory and visual cortices) and tractography with mapping of the functionally active cortical areas, pyramidal or optic tracts.
Imposition of these maps over 3D brain images is promising within the framework of one MRI examination for patients with brain tumours. Based on these data, neurosurgeons plan the interventional approach and estimate the volume of neoplasm resection, and radiologists assess the areas of radiation and its distribution in tumour.

Reference : V. N. Kornienko · I. N. Pronin, Diagnostic Neuroradiology.

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