<P> The amplitude of the BOLD signal does not necessarily affect its shape . A higher - amplitude signal may be seen for stronger neural activity, but peaking at the same place as a weaker signal . Also, the amplitude does not necessarily reflect behavioral performance . A complex cognitive task may initially trigger high - amplitude signals associated with good performance, but as the subject gets better at it, the amplitude may decrease with performance staying the same . This is expected to be due to increased efficiency in performing the task . The BOLD response across brain regions cannot be compared directly even for the same task, since the density of neurons and the blood - supply characteristics are not constant across the brain . However, the BOLD response can often be compared across subjects for the same brain region and the same task . </P> <P> More recent characterization of the BOLD signal has used optogenetic techniques in rodents to precisely control neuronal firing while simultaneously monitoring the BOLD response using high field magnets (a technique sometimes referred to as "optofMRI"). These techniques suggest that neuronal firing is well correlated with the measured BOLD signal including approximately linear summation of the BOLD signal over closely spaced bursts of neuronal firing . Linear summation is an assumption of commonly used event - related fMRI designs . </P> <P> Physicians use fMRI to assess how risky brain surgery or similar invasive treatment is for a patient and to learn how a normal, diseased or injured brain is functioning . They map the brain with fMRI to identify regions linked to critical functions such as speaking, moving, sensing, or planning . This is useful to plan for surgery and radiation therapy of the brain . Clinicians also use fMRI to anatomically map the brain and detect the effects of tumors, stroke, head and brain injury, or diseases such as Alzheimer's, and developmental disabilities such as Autism etc...</P> <P> Clinical use of fMRI still lags behind research use . Patients with brain pathologies are more difficult to scan with fMRI than are young healthy volunteers, the typical research - subject population . Tumors and lesions can change the blood flow in ways not related to neural activity, masking the neural HDR . Drugs such as antihistamines and even caffeine can affect HDR . Some patients may be suffering from disorders such as compulsive lying, which makes certain studies impossible . It is harder for those with clinical problems to stay still for long . Using head restraints or bite bars may injure epileptics who have a seizure inside the scanner; bite bars may also discomfort those with dental prostheses . </P>

What does an mri of the brain check for
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