Functional MRI

Running head : FUNCTIONAL MRI

Functional Magnetic Resonance Imaging : BOLD Response

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Functional Magnetic Resonance Imaging : BOLD Response

Magnetic resonance imaging (MRI ) has become a widely used diagnostic modality for

analysis and examination of anatomical structures . With advancements in the field of science and technology , increasing research regarding further diagnostic enhancement of MRI techniques has been accomplished since the past decade . One such advanced MRI technique is functional magnetic resonance imaging (fMRI . While the conventional MRI shows structural and anatomical details , fMRI also helps in examining

br physiological body processes (especially metabolic and haemodynamic processes . fMRI helps in establishing neuro-physiological abnormalities underlying various pathologies which could not have been possible with conventional MRI examination . fMRI measures the responses of large populations of neurons , rather than that of single cells . In humans , the best resolution now available with fMRI is around one cortical column which contains some 105 neurons (Rees , Friston Koch , 2000 Currently fMRI has become the most widely used method for brain mapping and studying the neuronal basis of human cognition and other pathologies associated with haemodynamic dysfunctioning (ischemic stroke , tumors etc . The ultimate usefulness and interpretation of fMRI is dependent on the BOLD (the blood-oxygenation level dependent ) signal and the relationship of BOLD signal with fMRI and with spiking activity of neurons . These aspects of fMRI would be discussed in detail in this br

BOLD response

Conventional MRI examination is scientifically based on behavior of hydrogen atoms or

protons in a magnetic field and signals produced on the interaction of these protons with non-ionizing radio frequency signals . fMRI further elaborates on this principle by also utilizing the paramagnetic properties of deoxygenated hemoglobin (Ogawa , Lee , Kay Tank , 1990 The principle of BOLD (Blood oxygenation-level dependent ) signal is based on the fact that when oxygenated hemoglobin , normally diamagnetic and in low-spin state , gives up its oxygen , the resulting deoxygenated hemoglobin is paramagnetic and in high-spin state (Ogawa , et al Presence of deoxygenated hemoglobin in blood changes the signals which are emitted from the protons (hydrogen ions ) in the water molecules surrounding the deoxygenated hemoglobin . This causes difference between magnetic signals created by oxygenated and deoxygenated blood and are detected by a MRI scanner and manifest as `BOLD signal (Ogawa et al This effect can be accentuated through the use of gradient-echo techniques in high magnetic fields ? 4 Tesla (Ogawa et al . Thus paramagnetic deoxygenated hemoglobin in venous blood acts as a naturally occurring contrast agent for fMRI (Ogawa et al . Since BOLD contrast depends on the state of blood oxygenation , physiological body events that change the ratio of oxygenated and deoxygenated hemoglobin in the blood can be detected non-invasively through BOLD signals of fMRI . In 1990 , Ogawa et al in their study on rats demonstrated the BOLD signal for the first time , by creating changes in blood oxygen levels in rats by inducing them with anesthetics , by creating insulin induced hypoglycemia , and by making them inhale a gas mixture that altered metabolic demand or blood flow in the brain . BOLD...