Functional Magnetic Resonance Imaging

The main use for functional magnetic resonance imaging (fMRI) in neuroimaging has been for the noninvasive study of brain activation. The most common fMRI method detects signals based on the blood oxygen level dependence (BOLD) effect. During brain activation, there is increased blood flow to the area of activation, which appears to be a direct consequence of neurotransmitter activity. Blood flow increases over a wider volume and to a greater extent than is necessary simply to provide oxygen and glucose for increased metabolism, so oxygen extraction decreases with greater neuronal activity. Consequently, the ratio of oxygenated (diamagnetic oxyhemoglobin) to deoxygenated (paramagnetic deoxyhemo-globin) blood near the corresponding areas of neuronal activation will increase, resulting in lower T2* (dephasing) effect and increased signal.48 Statistical techniques are employed or baseline images are subtracted from images obtained during activation to generate activation maps that are superimposed on MR images.

In brain tumor imaging, fMRI is generally used for the pre-operative localization of sensorimotor cortex, hemispheric language dominance, and other eloquent (essential) regions, locations that can be perturbed in the presence of a tumor. Cerebral reorganization (plasticity) is defined as the capacity of ipsilateral and contralateral brain regions to assume functions that are normally assumed by the damaged brain. That reorganization puts critical motor regions at risk if the standard anatomic techniques are used to locate motor cortex pre-operatively. Functional MRI can preemptively locate that reorganized cortex (Figure 27.8).

Neurosurgeons routinely perform cortical mapping intra-operatively. Cortical mapping is just that; that is, it determines function only in the cortex, which is a peripheral brain structure. Functional MRI can evaluate subcortical structures in areas far removed from the limited amount of cortex that is exposed and therefore available for intraoperative mapping.3

Functional MRI, however, is incompletely validated. Good, but not perfect, correlation between fMRI and cortical electrical stimulation has been demonstrated in several studies.49,50 Studies comparing PET and fMRI have shown much lower degrees of correlation, usually around 50%, with some patients showing no correlation of activation between the two techniques.51,52 Additionally, it remains unclear whether nonactivated brain regions may be safely resected, in part because of that spatial and temporal dispersion of blood oxygen level changes. Schreiber et al. found that the BOLD contrast can be reduced in the proximity of gliomas, but not affected by nonglial space-occupying lesions, such as vascular malformations, leading to overinterpretation of the inter-hemispheric reorganization in gliomas.53 Similar results were reached by Holodny et al., who suggested that this could result from loss of autoregulation in the tumor vasculature of glioblastomas and venous compression.54

figure 27.8. Cerebral reorganization in right-handed patient with right paracavernous meningioma. Functional magnetic resonance imaging (MRI) demonstrates that the task of reading words activates superiorly displaced speech areas in the right hemisphere. The patient was informed of the risk of losing speech postoperatively.

figure 27.8. Cerebral reorganization in right-handed patient with right paracavernous meningioma. Functional magnetic resonance imaging (MRI) demonstrates that the task of reading words activates superiorly displaced speech areas in the right hemisphere. The patient was informed of the risk of losing speech postoperatively.

Although fMRI is a noninvasive technique with high spatial and temporal resolution, short examination time, and wide availability, it will likely not replace intraoperative cortical mapping.

A Disquistion On The Evils Of Using Tobacco

A Disquistion On The Evils Of Using Tobacco

Among the evils which a vitiated appetite has fastened upon mankind, those that arise from the use of Tobacco hold a prominent place, and call loudly for reform. We pity the poor Chinese, who stupifies body and mind with opium, and the wretched Hindoo, who is under a similar slavery to his favorite plant, the Betel but we present the humiliating spectacle of an enlightened and christian nation, wasting annually more than twenty-five millions of dollars, and destroying the health and the lives of thousands, by a practice not at all less degrading than that of the Chinese or Hindoo.

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