Contrast in an Image

As with any imaging modality, the key variable in producing a meaningful image is contrast. The signal measured at one point in space or time must be higher or lower than the signal at another point, and the variation in signal intensity across the image should systematically follow some variable of interest. In the endeavor of brain mapping, the ultimate variable of interest is neural activity. In order to measure local brain activity with MRI, one must exploit a chain of indirect linkages from neural activity (a constellation of electrical and chemical events) to changes in brain physiology and metabolism and finally to changes in the magnetic properties of substances within the brain.

Anything that causes a change in the NMR signal from a given voxel relative to other voxels at the same time is a source of contrast in the image. The density of protons in a given voxel (due to chemical composition) is one such source of contrast, though not an important one in fMRI. More commonly, it is the variation in rates of relaxation from voxel to voxel that generates contrast in the image.

In the earliest fMRI studies, exogenous contrasts (chemicals injected into the bloodstream of the subject) were used to obtain contrast. These blood-borne chemicals locally distorted the magnetic field, thus allowing increased blood perfusion to be detected. Subsequent studies demonstrated that endogenous contrast agents (i.e., naturally occurring molecules in the body, such as the concentration of deoxyhemoglo-bin in the blood) could also yield sufficient contrast between different states of neural activity. The use of endogenous contrast agents obviated the need for injecting foreign molecules into the bodies of normal (healthy) subjects, and this is one of the key reasons that fMRI has become so popular as a technique for assessing human brain function. In the short history of fMRI, a wide variety of techniques that produce various contrasts have been developed for detecting changes in brain physiology.

Aspergers Answers Revealed

Aspergers Answers Revealed

Learn How to Help, Understand amp Cope with your Aspergers Child from a UK Chartered Educational Psychologist. Before beginning any practice relating to Aspergers it is highly recommended that you first obtain the consent and advice of a qualified health,education or social care professional.

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