page_title: Haemodynamic response
section_title: Functional magnetic resonance imaging
hierarchical_section_title: Haemodynamic response / Clinical use / Functional magnetic resonance imaging
caption_attribution_description: English: Sample fMRI data This example of fMRI data shows regions of activation including primary visual cortex (V1, BA17), extrastriate visual cortex and lateral geniculate body in a comparison between a task involving a complex moving visual stimulus and rest condition (viewing a black screen). The activations (yellow-red) are shown (as is typical) against a background based on the average structural images from the subjects in the experiment.
context_page_description: In haemodynamics, the body must respond to physical activities, external temperature, and other factors by homeostatically adjusting its blood flow to deliver nutrients such as oxygen and glucose to stressed tissues and allow them to function. Haemodynamic response allows the rapid delivery of blood to active neuronal tissues. The brain consumes large amounts of energy but does not have a reservoir of stored energy substrates. Since higher processes in the brain occur almost constantly, cerebral blood flow is essential for the maintenance of neurons, astrocytes, and other cells of the brain. This coupling between neuronal activity and blood flow is also referred to as neurovascular coupling.
context_section_description: Functional magnetic resonance imaging (fMRI), is the medical imaging technique used to measure the haemodynamic response of the brain in relation to the neural activities.  It is one of the most commonly used devices to measure brain functions and is relatively inexpensive to perform in a clinical setting. The onset of neural activity leads to a systematic series of physiological changes in the local network of blood vessels that include changes in the cerebral blood volume per unit of brain tissue (CBV), changes in the rate of cerebral blood flow, and changes in the concentration of oxyhemoglobin and deoxyhemoglobin. There are different fMRI techniques that can pick up a functional signal corresponding to changes in each of the previously mentioned components of the haemodynamic response. The most common functional imaging signal is the blood-oxygen-level dependent signal (BOLD), which primarily corresponds to the concentration of deoxyhemoglobin. The BOLD effect is based on the fact that when neuronal activity is increased in one part of the brain, there is also an increased amount of cerebral blood flow to that area which is the basis of haemodynamic response. This increase in blood flow produces an increase in the ratio of oxygenated hemoglobin relative to deoxygenated hemoglobin in that specific area. The difference in magnetic properties of oxygenated and deoxygenated hemoglobin is what allows fMRI imaging to produce an effective map of which neurons are active and which are not. In short, deoxygenated hemoglobin is paramagnetic while oxygenated hemoglobin is diamagnetic. Diamagnetic blood (oxyhemoglobin) interferes with the magnetic resonance (MR) signal less and this leads to an improved MR signal in that area of increased neuronal activity. However, Paramagnetic blood (deoxyhemoglobin) makes the local magnetic field inhomogenous. This has the effect of dephasing the signal emitted in this domain, causing destructive interference in the observed MR signal. Therefore, greater amounts of deoxyhemoglobin lead to less signal.  Neuronal activity ultimately leads to an increase in local MR signaling corresponding to a decrease in the concentration of deoxyhemoglobin.
  If fMRI can be used to detect the regular flow of blood in a healthy brain, it can also be used to detect the problems with a brain that has undergone degenerative diseases. Functional MRI, using haemodynamic response, can help assess the effects of stroke and other degenerative diseases such as Alzheimer's disease on brain function. Another way fMRI could be used is in the planning of surgery of the brain. Surgeons can use fMRI to detect blood flow of the most active areas of the brain and the areas involved in critical functions like thought, speech, movement, etc. In this way, brain procedures are less dangerous because there is a brain mapping that shows which areas are vital to a person's life. Haemodynamic response is vital to fMRI and clinical use because through the study of blood flow we are able to examine the anatomy of the brain and effectively plan out procedures of the brain and link together the causes of degenerative brain disease.
  Resting state fMRI enables the evaluation of the interaction of brain regions, when not performing a specific task.                         This is also used to show the default mode network.
caption: This sample fMRI shows how there are certain areas of activation during stimulation
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