Scope and Limitations of FMRI - Research Paper Example

Scope and Limitations of fMRI Introduction Functional magnetic resonance imaging or fMRI is a specialized and advanced magnetic resonance imaging that functions by measuring the hemodynamic response in relation to neural activity in the brain or spinal cord of human beings and animals. Infact, it is one of the most recent developments in neuroimaging and has been used extensively since its introduction in 1990s (Robinson, 2004). Currently, fMRI has dominated the mapping of brain mainly because of 3 attributes: low invasiveness, absence of exposure to radiation and wide availability. The scope of fMRI is present in several disciplines including neuroanatomy, physiology, physics, psychology and statisitcs. Despite several of its advantages, the technology has some limitations which limit its use in some fields and indications. In this essay, the scope and limitations of this advanced technology will be discussed with reference to suitable literature . Scope of fMRI The advantages of fMRI are that it can record the signals of the brain without any invasion and also without exposing the patient to radiation. The spatial resolution of fMRI is high, about2-3 mm and can even be 1mm. fMRI can record signals from every region in the brain. The images produced through this technology compel those of brain activation. fMRI is a powerful neuroimaging technology that can create not only an anatomic model but also a functional model of a person's brain. It provides a 3-D view of the topography of the cerebrum, related pathology and also the cortical veins, thus providing an unprecedented view of the relational anatomy that is interesting to the neurophysician or neurosurgeon. Because of the 3-D view, optimal exposure and accurate preoperative planning can be done. Localization of the cortical function can be done non-invasively with reference to a surgical target. Thus, it becomes easier for the surgeon to assess the relative risk due to a planned surgical procedure. Determination of the functional cortex helps in selecting safe incision and trajectory. fMRI can also localize and lateralization language function, thus ascertaining the dominant hemisphere (Cosgrove et al, 2005). fMRI is useful intraoperatively. By this technology, regions of interest can be superimposed on the scalp or any soft tissue with reference to standard anatomical landmarks like glabella, ears, occiput and eyes. After the opening of the dura, images of the fMRI display gyral anatomical aspects and superficial veins which help in the direction of cortical incisions and also definition of margins of resection. Cortical veins are very important stereotactic land marks and visualization of these veins is valuable. Following withdrawal of CSF, resection of tumor or administration of mannitol, the cerebral structures shift, and along with these the cortical veins also shift. Thus, relative anatomy with reference to functional lesions or areas continues to remain intact. fMRI displays subcortical location of the lesion and also its extent with reference to cortical surface. Thus,the information provided by fMRI is more than that provided by inspection of the cortex visually (Cosgrove et al, 2005). Recent research has shown that fMRI can be useful in the prediction of recovery from various neurological events like stroke, tumor surgery, head injury and several other surgical interventions. This is because; fMRI allows following of cortical function over a period of time and ascertains its reappearance, helping neurologists observe and examine various processes of the central nervous system and neuroregeneration (Cosgrove et al, 2005). One of the important areas of in which fMRI can be applied is medical education wherein, students can learn the anatomy and functioning of the central nervous system through a technology called "electronic brain cutting" which is possible only with fMRI. This is a valuable interactive teaching aid to learn neuroanatomy. The students learn by cutting in various directions and remove specific structures anatomically. Even performing neurosurgical operations can be learnt through this technology (Cosgrove et al, 2005). fMRI can provide several physiological information like baseline blood volume in the cerebrum, changes in the cerebral blood volume and changes in the perfusion and blood oxygenation of the cerebrum. The fMRI pulse sequence allows quantitative measurement of the metabolic rate of oxygen of the cerebrum (Bandettini, 2004). Blood Oxygen Level Dependent method, popularly known as BOLD is one of the techniques of fMRI that is frequently used in research in cognitive neuroscience areas (Culler, 2005). BOLD contrast, an advanced form of fMRI allows parametric manipulation of the activation of brain. Most neuroscientists prefer this method of brain activation mapping because of easy implementation and also because of the fact that functional contrast to noise is 2-4 higher than other MRI technologies (Bandettini, 2004). fMRI is also being used answer some psychology related questions like "how do people make decisions", "What’s the best way to treat dyslexia?" and " Why is it so hard to stop smoking?" (APA, 2004). Real time fMRI was available since 1995 and this technology demonstrated that in patients suffering from chronic back pain, demonstration of real time feed back of brain activation allows them to modulate their activation, this inturn affected the perception of pain. Thus, it has been thought that fMRI can be used for therapeutic reasons like chronic back pain (Bandettini, 2004). Parallel imaging is possible in fMRI and this allows higher resolution, rapid acquisition of images and higher sensitivity. It is also possible to use higher field in fMRI which again contributes to higher resolution and higher sensitivity. By observing preundershoot during fMRI, optical imaging correlation has been possible, which is another major advantage with fMRI, although, this is a much debated topic (Bandettini, 2004). Currently, there is some evidence that direct neuronal imaging is possible with fMRI, but it is still in developing stage (Bandettini, 2004). fMRI has a great potential in the field of pharmacology too, because of its excellent temporal and spatial resolutions. fMRI can identify as to which system of brain is influenced by the drug, thus revealing specific patterns and locations of activation subsequent to drug administration. Through these attributes of fMRI, the physiology and anatomy of drug addiction has been understood, evolving to better treatments. It has been thought that fMRI might influence in ascertaining the dosage of the drug on individual basis and also could help in identification of areas in the brain on which a drug acts. It can also be used to follow the pharmacokinetic profile of drugs like psychoactive drugs, because of its excellent temporal resolution (Savoy, 2004). fMRI is currently being used to identify, understand and characterize certain neurological and psychiatric disorders and because of these, many erstwhile defined descriptions of the disorder are refined. This has happened because of the ability of the fMRI to obtain spatially localized maps of function (Savoy, 2004). fMRI has been useful for the study of developmental dyslexia, which has not been a well-understood disorder until now. Through fMRI, it is hoped that it is possible to identify different types of dyslexia and that the modes of action of different types of drugs are ascertained so that specific treatments can be delivered. fMRI has also been useful in the study of the complex and less understood disease, migraine. When compared to PET, fMRI allows measurement of neural activity while the individual is engaging in cognitive tasks. When compared to MRI, in which only a static picture of the brain structure is provided, fMRI provides both structural and functional images of the brain (Culler, 2005). Limitations Though fMRI is considered as a "biological revolution", "an explosive factor of brain studies", etc; it is haunted by several limitations. There are currently specific concerns as to the conclusions that can be made about the relationship between neuronal activity and fMRI and also as to how neuronal activity, fMRI signals and flow of blood in the brain are related. Heeger and Ress (cited in Culler, 2005) opined that there does exist some limitation in the linear transformation model because of indirect measure of brain activation by fMRI. According to this model, "the strength of the fMRI signal is proportional to local neuronal activity that has been averaged over a space of several millimeters and over a time period of several seconds" (Culler, 2005). However, Heeger and Ross (cied in Culler, 2005) opined that the application of this mathematical formula for interpretation of results is possible only in a few recording sites, within specific areas of the brain and only while using some experimental protocols. There are several factors which influence the relationship between fMRI signals and variables of the neural activity and also the blood flow in the brain. These are: techniques used for acquisition, various protocols used for stimulus and behaviour and also the data analysis that is used (Culler, 2005). While the growing excitement with fMRI is mainly because of the temporal and spatial resolutions it offers for the image, some researchers have currently opined that gross localization of cognitive function is not feasible as brain activity is distributed in the networks of neurons even for simple activities concerned to cognition (Culler, 2005). Another concern is the imaging power of fMRI which yields different results for different individuals and thus generalization cannot be done for an entire population (Culler, 2005). fMRI lacks extremely in sensitivity for detecting the concentrations of specific drugs in brain, unlike PET and thus is useless for localizing sites of labeled psychoactivity drugs. According to Hirsch (cited in Robinson, 2004), "fMRI falls short when we want to ask about more detailed brain processes. We're not learning that much about how neurons are doing local computing.” One of the major limitation of fMRI is the fact that fMRI is labor intensive and requires dedication from a group of physicians and radiologists. The average time for image acquisition with fMRI is 60- 90 minutes and another 4-8 man hours are essential for registration of images, segmentation and then rendering of images. Thus, fMRI must be applied only to those patients in whom lesions involve eloquent cortex or in those with lesions lying adjacent to eloquent cortex. fMRI has several limitation like any other neuroimaging modality. The temporal limitations of this technology are ability to detect transient activity only as short as 16ms, ability to create a functional image within 20 seconds for robust activation, thus affecting sensitivity, ability to detect differences in the activation timing of brain across regions of not less than 4 seconds because of temporal lag, and ability to detect inhibitory interaction only without temporal limit. These limits mainly affect the sensitivity of fMRI. The main spatial limit of the technology is that at 3T, only 1.5 mm cubic resolution occurs and the functional point spread goes to 3.5mm. However, it is unknown if a higher resolution is actually needed in practice. At 7T, the resolution becomes 0.5mm cubic, but this is not practical (Bandettini, 2002). Another major limitation with fMRI is the lack of sensitivity of the technology. As of now, the functional contrast to noise ratio is 4:1 when 3T is used. It is very important for the technology to be sensitive in order to extract more subtle information with reference to space and time, thus translating to create a usable functional image in a time frame that is insignificant, which is very important clinically. Infact, researchers are willing to sacrifice, specificity, spatial resolution and temporal resolution for this purpose. Currently, even though the image signal-noise ratio is as high as 800: 1, the temporal signal- noise ratio cannot improve beyond 100: 1, for all field strengths and such a limit is actually influenced by physiological fluctuations that occur over a period of time (Bandettini, 2004). fMRI has some interpretation limits too. For example, it cannot differentiate between excitatory activity and inhibitory activity. Also, a frequently used technology, BOLD, is not a quantitative measure and it is influenced by several hemodynamic factors like neurovascular coupling and baseline blood volume, location, dynamics and magnitiude (Bandettini, 2004). Conclusion fMRI is thus an important invention in the field of neuroimaging that is useful in almost all field of neuroscience. However, some limitations prevent this technology from wider applications. The technique is just 20 years old and every day, newer developments are made which make this a useful tool in the filed of neurology and neurosurgery. References APA. (2004). Functional Magnetic Resonance Imaging. Science Directorate. Retrieved on 13th December, 2010 from www.apa.org/research/tools/fmri-adult.pdf Bandettini, P.A. (2004). Functional MRI Limitations and aspirations. in: Neural Correlates of Thinking. Retrieved on 13th December, 2010 from http://docs.google.com/viewer?a=v&q=cache:tMnQHfYmZWEJ:fim.nimh.nih.gov/files/B18.pdf+functional+MRI+limitations&hl=en&gl=in&pid=bl&srcid=ADGEESjq-sIKsSiqrjUyon2e8VlrJNxEbBkh6PpehN9D-JTkkEsN9732O32PwJJgHe9N_bEIUgkBQMwtWngTGmix3UWDGwU5gqV5N0MB1Tl0somrxqxrSohFoP4JrZv9SjjGllnY21HL&sig=AHIEtbTw2a6CYOGbtm4luyPyzeoH7y2gLA Bandettini, P.A. (2002). Spatial, Temporal, and Interpretive Limits of Functional MRI. Neuropsychopharmacology: The Fifth Generation of Progress, 344- 354. Retrieved on 13th December, 2010 from http://serendip.brynmawr.edu/bb/neuro/neuro05/web2/cculler.html. Cosgrove, G.R., Buchbinder, B.R., Jiang, H. (2005). Functional Magnetic Resonance Imaging for Intracranial Navigation. Massachusetts General Hospital and Harvard Medical School. Retrieved on 13th December, 2010 from http://neurosurgery.mgh.harvard.edu/functional/fmrimage.htm Culler, C. (2005). Functional Magnetic Resonance Imaging (fMRI): Much Ado About What? Spring, 2, 45- 49. Robinson, R. (2004). fMRI Beyond the Clinic: Will It Ever Be Ready for Prime Time? PLoS Biol., 2(6), e150. Savoy, R.KL. (2004). Functional Magnetic Resonance Imaging. Retrieved on 13th December, 2010 from http://www.ohsu.edu/nod/documents/2007/04-25/Savoy.pdf Read More