scan is a two dimensional picture of a three dimensional body.
Internal structures are visible, but all the structures from
different depths are superimposed, as if the body had been
"flattened" to fit on the picture. Depth information is lost
and the outlines of objects are simply superimposed.
Tomography is a technique for constructing
images of the structures at a particular depth within the
body - as if through a "slice" at a particular level within
the body. This is done by taking several x-ray images at different
angles and then using a computer to analyze these images and
produce a reconstruction that only includes structures visible
at a certain level or "slice". This process is known as computed
tomography (CT). An older term is computerized axial tomography
(CAT), which refers to the fact that images were originally
only taken in the axial plane; this is no longer the case.
CT images identify intracranial tumors and
other brain lesions as areas of altered density. Bone, the
densest tissue, appears white; soft tissue and brain matter
appears as shades of gray; cerebrospinal fluid - the least
dense - appears black. Normally, the cerebral blood vessels
don't appear on CT images. Magnetic
resonance imaging (MRI) is the preferred procedure for
imaging cerebral blood vessels. However, because the high
density of blood contrasts markedly with low-density brain
tissue, bleeding (e.g a ruptured aneurysm)
is very easy to detect via CT scan. CT also produces higher-resolution
images than MRI, although the contrast is less. To improve
contrast on the CT scan, patients often receive injection
of a medium such as iodine, which is opaque to x-rays, and
thus sharpens the picture obtained.
Further reading: Illustrated Guide to Diagnostic
Tests, 2nd edition. Springhouse Corporation, Springhouse PA,
by Catherine E. Myers. Copyright © 2006 Memory Loss and the Brain
Digital Imagery © copyright 2000 PhotoDisc, Inc.