iluvcats
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#1
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I get that brain activity involves oxyhemoglobin & deoxyhaemoglobin and so fMRI measures the uptake of oxygen by lighting up in brighter areas. However I've been told that deoxyhaemoglobin absorbs more radio waves and then re-emits it. Why would areas with more oxyhemoglobin (active area) be brighter then?
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macpatgh-Sheldon
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Hi,

You have an interesting Q, but one that could have quite a complex answer - to keep it simple, remember the following basic facts;
:-

1. An MRI scan, in general, shows dense tissue as dark and less dense tissue as lighter (opposite of an X-ray (radiograph), in which dense items like bone appear white).

2. There are two main types of MRI scan: T1 weighted and T2 weighted - just try to remember that these two types will show different tissues as dark or light, the main distinction being between fat- and water-based tissues.

3. An fMRI in reality measures what is called the haemodynamic response function (HRF), which is like a measure of compensatory blood flow [which in turn depends on blood vessel diameter], and the latter will be a response to varying levels of oxyhaemoglobin to deoxyhaemoglobin ratio.

In short, there are several factors that determine the "lightness" of any part of an MR image - if you are doing A level biology, just memorize what you see in your books without getting too deep into the rationale.

If, on the other hand, you are a medical student, read up the MRI chapter in a good introductory book on radiology.

M (ex medic)
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