activity are closely coupled with changes in blood flow in those areas, and knowing this has proved useful in mapping brain functions in humans. the measurement of haemodynamic response, in a clinical setting, can be used to create images of the brain in which especially active and inactive regions are shown as distinct from one another. this can be a useful tool in diagnosing neural disease or in pre-surgical planning. functional mri and pet scan are the most common techniques that use haemodynamic response to map brain function. physicians use these imaging techniques to examine the anatomy of the brain, to determine which specific parts of the brain are handling certain high order functions, to assess the effects of degenerative diseases, and even to plan surgical treatments of the brain. functional magnetic resonance imaging 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. this sample fmri shows how there are certain areas of activation during stimulation 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. pet scan pet scan or positron emission tomography scan is also used alongside fmri for brain imaging. pet scan can detect active brain areas either haemodynamically or metabolically through glucose intake. they allow one to observe blood flow or metabolism in any part of the brain. the areas that are activated by increased blood flow and/or increased glucose intake are visualized in increased signal in the pet image. before a pet scan begins, the patient will be injected with a small dose of a radioactive medicine tagged to a tracer such as glucose or oxygen. therefore, if the purpose of the pet scan is to determine brain activity, fdg or fluorodeoxyglucose will be the medicine used. fdg is a complex of radioactive fluorine that is tagged with glucose. if a certain part of the brain is more active, more glucose or energy will be needed there and more fdg will be absorbed. this increase in glucose intake will be detectable with increased signal in the pet image. pet scanners provide this feature because they measure the energy that is emitted when positrons from the radiotracer collide with electrons in the brain. as a radiotracer is broken down, more positrons are made and there will be an increased signal in the pet scan.