Hi everyone.. can someone tell me the differences between having an mri and cat
scan?  I have been having bad dizziness and pins and needles.. and they did a
cat scan and not an mri.  can a cat scan show infection, swelling, or plaques
and scarring?  what exactly does it show?  

thank you!

A CAT (computerized axial tomography) uses x-rays. An MRI (magnetic
resonance imaging) doesn't use x-days, rather a large powerful magnet.
X-rays are blocked best by bones and calcium, while MRI is more useful for
soft tissues, like the brain. LR

CAT scans are done with X rays, a kind of ionizing radiation; it
causes chemical bonds to break.  An X ray tech I spoke to at a local
hospital indicated they used higher energy 120kEV X-rays
in CAT scans, compared to  50-70 kEV for conventional X ray pictures.
He said the higher energy x-rays were less absorbed by tissue, but
that the CAT scan machine did 60-70 passes over the same spot with that
energy, and so it was quite a bit more exposure than a conventional X ray.

SPECT scans are done by injecting you with a chemical that emits
ionizing radition, and measuring it from the outside of you.  I've
heard estimates on the order of 600 chest x-rays of radiation exposure
from SPECT.  Not something I want to do.

Ionizing radiation can break chemical bonds, chromosomes, and so on, and
as a result can cause cancer.

MRIs are done with a pulsed magnetic field, that is unlikely to do much
of anything bad to human tissue.  Human tissue emits radio waves in response
to this magnetic pulse, and the MRI machine picks that up.  Medical MRI
machines are tuned to a frequency emitted by water molecules, so they tend
to show areas of high {*filter*} flow.  Chemistry laboratories have a similar
machine (NMRS) that does not produce an image, but can measure a wide range
of frequencies, used for determining what an unknown chemical sample might
contain.

fMRIs (Flow MRIs) are a new technique that can take something like a movie,
where an MRI is a still picture.  Synchronized to your heart pumping, it
can show the expansion and contraction of {*filter*} vessels; hopefully it will
replace SPECT scans for showing {*filter*} vessel inflammation that shows up in
Lyme patients.  A small number of experimental labs have these machines.
Like an MRI machine, they produce no ionizing radiation.

    Arly