What does an MRI of the head make images of exactly, and what is it used to look at/diagnose?

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Is there sometimes dye used in an MRI? What is this test used to diagnose/see versus for example a PET scan? TIA.

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MRI scans are physically mapping the concentration of hydrogen, what this means for the human body is mostly measuring water and fat. They produce a structural image for example in the brain you can see all of the grey and white matter (and they appear different which is useful diagnostically). They’re using for diagnosing pretty much anything which causes a structural change in the brain like Alzheimer’s.

MRI does occasionally use contrast agents but it’s rare as most MRI imaging doesn’t require it.

A PET scan on the other hand measures the distribution of the tracer you inject, eg some sugar analog with a radioactive isotope. This lets you map metabolic processes, seeing where that tracer gets concentrated in the body. For example cancer cells use a tonne of sugar because they are growing so much, so if you image them with a PET scan then they can be easily identified.

My experience in this is from studying physics involving medical imaging so someone with a medical background may be able to elaborate on exactly what they can be used for.

You can think of MRI as a tool which captures cross-sectional images which show the locations of different tissues (and fluids). It’s just that while if you were to slice a cross section (you sick f*ck), you’d see the colors of different tissues and fluids, and in an MRI image, everything is grayscale, and shade is determined by how “solid” or “rigid” the thing is, with the least solid stuff being darkest.

This is useful because, for instance a tumor, or a blood clot, or an active hemorrhage is thereby discernable from brain tissue, and the space inside the skull is very difficult and risky to perform exploratory surgery on just to see, or take a biopsy sample of.

The same way in which an X-Ray basically shows you the densities of stuff the radiation passes through, and you can use that to get a clear image of a broken bone without cutting flesh to get to it. Only MRI can help you differentiate between tissues.