Difference between fMRI and ASL - Essay Example

"Difference between fMRI and ASL" is a perfect example of clinical research.
Functional magnetic resonance imaging (fMRI) is a specialized form of MRI scan that "measures changes in blood oxygenation and blood volume," which is the outcome of neural activity (Ogawa et al., 1990; Belliveau et al., 1992 qt. in Boynton et al. 4207). The endogenous paramagnetic agent is the deoxygenated hemoglobin; therefore, the concentration of deoxygenated hemoglobin lessens, while it augments the T2 magnetic resonance signal. A usual fMRI experimentation involves the measurement of the existing interconnection between the fMRI response and the stimulus. In many instances it has been believed that a simple, direct correlation is present between neural activity and fMRI response; however, no clear explanation has been given by scientists regarding the origin of this assumed correlation (Boynton et al. 4207).

On the other hand, Arterial spin labeling (ASL) is a non-invasive MRI technique that measures blood flow "by magnetically tagging the protons in arterial blood" (Wen et al. 1020). The process is made possible through the utilization of instantaneous (PASL) or continuous (CASL) RF pulses. Arterial spin labeling is used for perfusion measurement. ASL has gained the interest of researchers in clinical and basic neuroscience and has been applied for the examination of organs other than the brain. Unlike the radioactive isotopes used in positron emission tomography (PET) and metal ion chelates used in dynamic susceptibility contrast (DSC)-enhanced MRI, ASL makes use of water in arterial blood as its endogenous tracer. The tagging efficiency of this magnetic resonance technology allows it to produce accurate perfusion quantification and perfusion images (Wen et al. 1020).

Compared to fMRI BOLD (blood-oxygen-level dependence) noise, ASL noise is whiter (Wang 2003, qtd. in "Arterial Spin Labeling"), and ASL does not require for any contrast agents, while fMRI uses one. With ASL, the changes taking place in perfusion are confined to the parenchyma, while BOLD changes bind with the veins and venules (Duong 2002, qtd. in "Arterial Spin Labeling"). A single ASL image takes about 2.5 seconds to obtain, but after zinc subtraction, it may take only 1.2 seconds, whereas with fMRI BOLD the images are obtained in a matter of 500 msec. Furthermore, it has been observed that changes in ASL indicate that the outcome from activation is cleaner compared to what is usually observed when one utilizes the average fMRI scan technique ("Arterial Spin Labeling").

fMRI has been widely used in the visualization of regional brain activation as a response to sensorimotor or cognitive activities. Most fMRI studies utilize blood oxygenation level-dependent (BOLD) contrast to identify neural activation; baseline drift effects lead to poor sensitivity in the detection of slow variation in neural activity. Differently, there is a reduction in drift effects, in the application of ASL perfusion contrast due to the succeeding pairwise subtraction between the images that were obtained either with or without labeling. According to recent studies, ASL contrast manifests a stable noise characteristic beyond the whole frequency spectrum, making it an appropriate subject for the study of low-frequency activities in brain function (Wang et al. 796).