Functional magnetic resonance imaging - Essay Example

? Functional magnetic resonance imaging Introduction Functional magnetic resonance imaging or fMRI is a process of mapping brain activities by analysing the modifications in blood flow and oxygenation levels that vary according to neural activities taking place within the brain (Huettel, Song and McCarthy, 2009). With increased activities in a particular area of the brain, there is an increase in blood flow to that particular location. Subsequently more oxygen is necessary for consumption, to meet the rise in blood flow. fMRI techniques can be used for revealing which part of the brain is active (for specific mental activities) through activation maps. The basic fMRI technique, developed by Seiji Ogawa and Ken Kwong in the 1990s, makes use of blood-oxygen-level-dependent (BOLD) signals (ibid, p. 26), which is a scan procedure for mapping brain and spinal cord related neural activities. It produces an image of the modification in hemodynamic responses related to the energy usage by the brain (human and animal) (ibid). Since its inception, fMRI has been popular for brain imaging, as it is non-invasive in nature, easy to handle, no use of radiation makes it safe for the patients, and the process also gives good spatial and temporal resolution (ibid, p.4). fMRI is very similar to MRI with differences lying in the use of magnetization between oxygen-deprived and oxygen-rich blood levels, which form the fundamental measure. BOLD contrast techniques use the concept of deoxygenated or (paramagnetic, which is more magnetic) and oxygenated (diamagnetic or non-magnetic) blood, and their varying magnetic properties, and these variations though subtle in form, are distinguishable through MRI intensity measures. fMRI currently is a popular technique for imaging of normal brain activities, especially in the stream of psychology, as it helps to provide a different outlook on the development of language skills, formation of memories feeling of pain, and developing learning skills within humans, which form the core area of research on this subject. FMRI is used also in the medical arena and for various commercial purposes, combined with other forms of brain diagnoses measures, like EEG. Commercial products like lie detectors developed by some private companies use fMRI techniques, however, further research needs to done on this arena. Discussion fMRI technique Since late nineteenth century, we find that various researches have derived that variations in hemodynamics (sum total of the flow of blood and oxygenation of blood within brain) are related to the various neural activities (Roy and Sherrington, 1890). fMRI is based on the concept derived from MRI technique, where brain is scanned by static and permanent magnetic field, used for formatting nuclei in the region to be scanned. Another magnetic field is then used to raise the nuclei to higher levels of magnetization, and when this second magnetic field is removed, nuclei move back to their former state, while the energy released is measured in order to recreate the positions of the nuclei. Thus, MRI gives a gridlocked anatomical view of the brain, which is of clinical use to the physicians for treating patients with brain and CNS related problems.  When nerve cells in the brain are active, there is an increase in blood flow to those particular areas, and around 2 seconds later oxygenated blood transpose the deoxygenated blood, and this continues to rise till the highest point is reached in 4–6 seconds, after which the level again falls back to the original mark. Deoxygenated haemoglobin is paramagnetic (more magnetic) in nature than oxygenated haemoglobin, which is diamagnetic or nonmagnetic. This variation in magnetic properties gives rise to elevated MRI signals, since diamagnetic blood intervenes less with MR signals. This elevation in signals can be used for showing the activities of specific neurons at any specific time. The currently detected rise in signal varies from 1-5%, as recorded on a 1.5T MR system (Baudendistel, Schad, Friedlinger, Wenz, Schroder, and Lorenz, 1995, 701-705). To transform the neurons from their active state to a depolarized form makes it necessary for ions to be pumped across cell membranes (Baert, Sartor, and Youker, 2000, 3-25). Since glucose is the source of energy for the motor pumps that are producing ions, it is necessary for the blood flow to carry more glucose to the affected areas (this is owing to the fact that brain does not store glucose, despite latter being the chief source of energy). This result in greater amount of oxygen to flow in through the RBCs as oxygenated haemoglobin molecules, and the change in the rate of blood-flow is limited to 2-3 mm around the region of the on-going neural activities (Baert, Sartor, and Youker, 2000, 15-20). The oxygen that flows in is more than the oxygen used for consumption of glucose, and this result in a fall in deoxygenated haemoglobin (dHb) in the blood vessels of that specific area, within the brain. This modifies the blood’s magnetic properties, whereupon it intervenes little with magnetization, along with subsequent consumption that is activated by MRI. The cerebral blood flow in different parts of the brain conforms differently to the levels of the consumed glucose. Incoming blood flow is lower than the consumption rates, in areas like lateral parietal and frontal lobes (cogitative areas of the brain associated with the thinking processes). On the other hand, there is higher inflow of blood than glucose consumption, in regions such as thalamus, basal ganglia, cingulate cortex and amygdala (Ogawa, Menon, Kim and Ugerbil, 1998, 250-465). These brain parts are associated with rapid reactions. This difference in incoming blood flow and glucose consumption influences BOLD responses (Huettel, Song and McCarthy, 2009, 199). Haemoglobin, based on the presence of a bound oxygen molecule, varies in its responses to magnetic field, where it is observed that magnetic fields tend to attract a dHb molecule more. This results in a deviation of the magnetic fields that surround it, activated by the scanner used for MRI. Subsequently the nuclei present lose its factor for magnetization, rapidly through the consumption of T2*. Here it can be derived that MRI pulses that are responsive to T2*, display greater MRI signals in cases of highly oxygenated blood and low signals in cases of deoxygenated blood. This effect of the MRI pulses rise with the magnetic strength (square), therefore fMRI signals requires powerful magnetic field (of strength 1.5 T or greater) and pulse waves (EPI), responsive towards T2* contrast (Huettel, Song and McCarthy, 2009, 199; Bandettini, Wong, Hinks, Tikofsky and Hyde, 1992). In BOLD fMRI, blood-flow responses decide the temporal responsiveness (measuring correctly the activities of the neurons), and the parameter for time resolution is taken to be TR that decides how often the neurons within a specific location in the brain are activated and allowed to become demagnetised. In case of fMRI however, the hemodynamic response takes for almost 10 seconds, increasing according to the proportion of the present value, reaching a peak value at around 4 - 6 seconds, and then decreasing to reach the former state of inaction. Once the activity is over, BOLD signal drops below the starting level (‘undershoot’), later slowly recovering to touch the baseline (Huettel, Song and McCarthy, 2009, 208-214). Generally there are other unwanted loud sounds that tend to affect the fMRI signals emanating from normal brain functions, from MR scanner, and other objects, and to remove these disturbances fMRI readings must duplicate the presentation of a stimulus many times (ibid, 243-245). Spatial resolution in fMRI relates to the study that helps to distinguish between closely situated regions within the brain, and is measured in terms of voxel size (3 ?3 ?3 mm3 the typical volume of a voxel, as used in MRI) (Logothetis, 2008; Carr, Rissman and Wagner, 2010). Temporal resolution is referred to studies where the smallest period during which an on-going neural activity can be reliably distinguished by fMRI techniques, is measured. Less than a TR (sampling time) of 1 - 2 seconds, scanning images just produce HDR waves, beyond which it does not reveal much information. However, fMRI images can be improved by careening the presentation of stimulus during analysis. Evaluation of fMRI In the medical field, the technique of fMRI is widely used for diagnosing central nervous system and brain related problems. When compared with X-ray CAT scanners, it is observed that fMRI techniques provide the same resolution level of anatomical features and better contrast (spatial and temporal) resolution. When compared with positron emission tomography (PET) scanners, fMRI techniques produce the same functional information that however gives anatomical features with more details. Since MRI is capable of differentiating between the soft tissues that are found in a healthy body and a sick body, fMRI scanners can produce images that correspond to X-ray pictures (Weiller, May, Sach, Buhmann, and Rijntje, 2006, 840-845). Since fMRI and MRI do not use any ionizing radiation, it is safer for subjects who are undergoing screening for brain or CNS medical problems (except for patients, like workers dealing in sheet metals and having iron shavings close to their eyes, those with aneurysm brain clips and cardiac pacemakers, and those with ear transplants) (ibid). Clinicians primarily make use of fMRI imaging to determine the percentage of risk associated with radiation therapy of the brain, surgery of the brain, or other obtrusive modes of treatment, study the effects of Alzheimer’s on an individual, and analyse after-effects of a stroke or brain tumour. Researchers use fMRI to study how the brain functions in various states (diseased, normal, and injured). fMRI is also used for brain mapping, which helps scientists to identify areas within the brain associated with major bodily functions, like, making body movements, talking, and thinking (Mehagnoul-Schipper et al., 2002, 14- 23). Despite widespread use of fMRI by the clinicians, this area still needs further researches (Rombouts, Barkhof, & Sheltens, 2007, 1). It is harder to use fMRI on those with brain problems than on volunteers that are healthy, while body lesions and tumours can modify the direction of blood flow using methods that have no connection to normal neural activities, thus covering the neural HDR and making it invisible for fMRI studies. Besides this, often caffeine and various other medicines like antihistamines, can influence HDR, posing to be a problem for fMRI studies (ibid, 4-5). Sometimes some kinds of disorders make it impossible for patients to remain without making any body movements for a long period, which makes fMRI studies impossible in such cases. Using bars to prevent bites, or even restraints for the head may end up inflicting injuries on epileptic patients if they have a seizure within the fMRI scanner (ibid, 14). Another major limitation within fMRI is associated with its spatial resolution, as “BOLD measurements are ultimately limited in spatial resolution, because the signal is only an indirect measure of neural activity and limited by, among other things, the spatial scale of the local vascular system” (Sapountzis, Schluppeck Bowtell, and Peirce, 2010, 1632). Even though recent improvements in “imaging hardware and analysis techniques” have resulted in images that are of higher-resolution with an even at augmented “signal-to-noise ratio,” the problem persists and more research needs to be conducted into this area to get a clearer view (ibid). Despite the aforementioned disadvantages associated with fMRI, clinicians tend to use it widely as it helps one to understand many of the brain and CNS related problems. As for example, it helps one to identify how a stroke may originate and how one can help a patient to recover from it, find out the presence of mental depression, locate the starting of Alzheimer's, and analyse the effects of a drug or a therapy on the brain. fMRI images help during brain surgeries and assist the surgeons in not touching the critical areas within the brain, especially during removing tumours and those with epilepsy associated with the temporal lobe in the brain.   Even though fMRI is a safe process, a common risk associated with it is the feeling of claustrophobia (fear of closed spaces) amongst the subjects (Huettel, Song and McCarthy, 2009, 53). Often there are high-pitched sounds during scanning, owing to Lorentz forces that originate in the gradient coils due to fast switch-over of current, which may also cause a tingling sensation in the subject’s body, while pacemakers may not work properly owing to these currents (ibid, 51-52). While magnetic fields have no permanent negative effects on human bodies, damage may be caused when heavy metal objects situated nearby are pulled by the magnetic forces and turned into projectiles, causing injuries to the subject within the scanner. An evaluation of fMRI thus reveals that are a large number of advantages associated with the clinical use of this technique, however more research work must necessarily be conducted on the subject to find out measures that would allow optimal use of it for further clinical and commercial purposes. Cultural neuroscience An increasing number of researches reveal that culture tends to affect ways through which an individual views the surrounding world (Jenkins, Yang, Goh, Hong, and Park, 2010, 1). As the authors further claimed, “Westerners tend to engage in an analytical style of processing marked by a focus on salient objects independent of the context in which they are embedded. In contrast, East Asians process visual information in a more holistic fashion, attending to the relationship between object and context” (ibid). People from the eastern culture tend to focus more on the background of a scenario, while those from the western culture fixed their vision on the focal objects within the same scenario (Freeman, Rule and Ambady, 2009). The differences in perception that arose due to varying cultural background are supported by various researches on the subject of neuroscience and imaging (Han and Northoff, 2008; Goh et al. 2007). A majority of the researches that explore differences in psychological measures that exist owing to varying cultural background are primarily theoretical or social based studies (Lewis, Goto and Kong, 2008). Very few researches (until recently) have explored the link that exists between neural activities and variation in cognition, between people of two cultures. Gutchess, Welsh, Boduroglu, & Park, (2006) conducted a study that explored the link between fMRI and hemodynamic responses of the brain from people belonging to two different cultures (Asians and Americans) by showing images of various background, different objects, and images that combined backgrounds with objects. It was found that US subjects while seeing the focal objects, revealed higher levels of activities in the cortical zones of their brains (an area which takes part in the processing of visual perception), along with increased activity in the temporal and parietal lobes. There was no any significant rise in neural activities in the brain of the East Asian subjects while viewing background areas. From the experiment Gutchess et al. derived that “cultural differences in the encoding of complex scenes result predominantly from additional processing of objects by Westerners” (2006, 107). However, owing to the limitations accorded by poor temporal resolution, the researchers could not exactly locate the point in the cognitive processing, where the cultural variations arose between the subjects. fMRI makes it possible to distinguish specific areas of the brain involved in psychological processes like thinking, decision making, analysing, and consciousness (Lieberman, 2010). With the functions of these areas in the brain more-or-less established, fMRI allows scholars to locate the attributes of various brain processes that form the basis for these psychology procedures (Kitayama and Park, 2010). From such researches, scholars have derived that culture is a powerful instrument in attuning human psychological procedures. In a research carried out by Tang et al., in 2006, using fMRI it was observed that while solving arithmetic sums, subjects that spoke English as their native language used the left part of the perisylvian cortex (these are areas in a brain used for processing languages). Subjects that spoke Chinese as their native language showed significant activities in the pre-motor association region of the brain, instead of the perisylvian cortex (Tang et al., 2006). This experiment strongly supported the theory that individuals do the same work by using different neural activites in the brain, based on their socio- cultural background. Even though in the last few decades there have been increasing number of researches into the subject of neuroscience and culture via fMRI, more researches are necessary to firmly establish the link that exists between neural activities and cultural background of the people. References Baert, A., Sartor, K., and Youker, J. (2000). Functional MRI. NY: Springer. 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