Functioning of MRI in fMRI
Neuroimaging techniques have been instrumental in increasing our understanding of the brain and its functioning (Kirkman, 2015). There are several non-invasive techniques used for neuroimaging, and fMRI is one of them (Bunge & Kahn, 2009). In this essay, the functioning and mechanism of fMRI has been discussed.
Magnetic resonance imaging (MRI) is a leading magnetoencephalography (MEG) method that studies blood flow and metabolic activity (Buxton, 2013). It is generally divided into two subcategories- structural and functional (fMRI) (Symms, Jäger, Schmierer & Yousry, 2004). FMRI relies on the principle of BOLD (Blood Oxygen-Level Dependency) contrast, i.e. oxygenated blood containing more haemoglobin has less magnetic properties than deoxygenated blood (Glover, 2011). At present, fMRI is the most widely used neuroimaging method due to the availability of MRI scanners and a relatively cheap rate per scan. FMRI doesn't make use of any invasive techniques such as injecting radiolabelled material. FMRI can measure rapid changes in neural activity (Blache & Maloney, 2017). By using the varied magnetic properties of oxygenated and deoxygenated blood, the fMRI gives us an indirect measure of blood flow in the brain regionally (Veldhuizen, 2010). It also has a decent spatial resolution than any other indirect imaging method (Bunge & Kahn, 2009). The spatial resolution ranges from 1 to 3mm3. Because of these qualities and its non-invasive nature, fMRI proves to be a safe but effective neuroimaging technique (Veldhuizen, 2010).
During an fMRI procedure, a significant amount of images are sequentially processed within few minutes. This time taken to process is known as acquisition period, wherein, the subject is presented with various stimuli for brain activation while whole brain’s images are collected (Ogawa, Menon, Kim & Ugurbil, 1998). FMRI, which is based on MRI uses Nuclear Magnetic Resonance (NMR) and gradients in magnetic fields to create the images (Glover, 2011). The NMR pulse sequence used for fMRI is the simplest, but the imaging methods are very erudite (Buxton, 2013). Since the mechanism is based on the BOLD principle, a strong magnetic field, typically of 1.5 teslas (in humans), enables one to measure the oxy-to-deoxygenated blood in the veins and venules. Increase in blood flow is known as hemodynamic response and usually is quite brief and appear after a few seconds of stimulus presentation (Bunge & Kahn, 2009). Even a brief stimulus can generate quite a strong blood flow. Deoxygenated blood becomes paramagnetic and thus changes the magnetic predisposition of blood. Due to this difference in predisposition, blood vessels and tissues create a local magnetic field which decreases the magnetic resonance signal. Thus, it can be observed that the MR signalling of the brain is sensitive to oxygen extraction fraction, i.e., the removal of a fraction of oxygen from the blood while passing through the capillary bed (Buxton, 2013). Along with BOLD, perfusion measurements are also of significant importance to fMRI. The change in perfusion signal is essential to be analysed (Ogawa et al., 1998).
In an fMRI procedure, visual, auditory or any other type of stimuli is presented to the subject while continuously collecting MR images. Two or more cognitive states are induced, wherein one is the experimental and other is the control state. The difference in signalling is to be measured (Glover. 2011).
The fMRI technique is quite advantageous. Primarily, the fMRI has high spatial resolution as compared to other techniques i.e. most accurate data is collected within one image. This is limited by the signal-to-noise ratio because the technique demands rapid acquisition of data in a set time frame (Glover, 2011). The functional mapping maps area from a few millimetres or larger in clusters. The mapping is done using tissue and local vein signals. The temporal response of fMRI signals is dependent on the hemodynamic response.
Conversely, fMRI measures signals real-time and will detect the period of stimulation accurately if it is long enough to be detected. Similarly, the fMRI signals are highly susceptible to movement. By restricting the subject's movement, this limitation can be overcome to some extent (Ogawa et al., 1998). The spatial and temporal limitations can easily be overcome by the use of higher magnetic fields which improve the signal-to-noise ratio (Buxton, 2013). The non-invasiveness of an fMRI procedure makes it ideal for longitudinal studies where the same subject needs to go through the process more than once (Buxton, 2013). It is also being looked as a biomarker for various diseases (Glover, 2011).
In conclusion, fMRI is an essential development in the field of neuroimaging that has facilitated complex investigations. The MR signalling used in fMRI gives the most relevant data while being remarkably safe and non-invasive.
Blache, D., & Maloney, S. K. (2017). New physiological measures of the biological cost of responding to challenges. In Advances in Sheep Welfare (pp. 73-104). Woodhead Publishing.
Bunge, S. A., & Kahn, I. (2009). Cognition: An overview of neuroimaging techniques.
Buxton, R. B. (2013). The physics of functional magnetic resonance imaging (fMRI). Reports on Progress in Physics, 76(9), 096601.
Glover, G. H. (2011). Overview of functional magnetic resonance imaging. Neurosurgery Clinics, 22(2), 133-139.
Kirkman, M. A. (2015). The role of imaging in the development of neurosurgery. Journal of Clinical Neuroscience, 22(1), 55-61.
Ogawa, S., Menon, R. S., Kim, S. G., & Ugurbil, K. (1998). On the characteristics of functional magnetic resonance imaging of the brain. Annual review of biophysics and biomolecular structure, 27(1), 447-474.
Symms, M., Jäger, H. R., Schmierer, K., & Yousry, T. A. (2004). A review of structural magnetic resonance neuroimaging. Journal of Neurology, Neurosurgery & Psychiatry, 75(9), 1235-1244.
Veldhuizen, M. G. (2010). Neuroimaging of sensory perception and hedonic reward. In Consumer-driven innovation in food and personal care products (pp. 597-633). Woodhead Publishing.