Operating instructions

67
Perfusion CT
Additional Important
Information
1. Why short injection times are necessary?
The brain has a very short transit time (approx. 3 to
5 seconds) and a relatively small fractional blood volume
(approx. 2 to 5%). This requires a compact bolus for
optimal time resolution and a certain minimum amount
of contrast for optimal signal to noise ratio. Bolus defi-
nition can be significantly improved and the amount
of contrast necessary reduced by using a saline chaser
bolus with the same flow rate directly following the
CM injection.
2. What do normal, contrast-enhanced and Perfusion
CT images show?
In order to interpret Perfusion CT images correctly,
it is essential to understand that they are “functional“
or “parameter“ images that display a different type
of information than standard CT images: Normal CT
images basically show only morphological properties
of tissues by displaying their x-ray attenuation relative
to that of water as CT-values in HU-units.
Standard contrast-enhanced CT extends this limitation
either to make a compartment visible that normally
has low contrast (e. g. vascular structures in CTA) or
to qualitatively display major perfusion differences of
tissues (e. g. tumors or multiphase liver studies).
Perfusion CT tries to utilize all the information hidden
in the temporal changes of contrast enhancement by
fitting a mathematical model to the local time attenu-
ation curves. For each voxel this process yields a variety
of numbers which describe different aspects of tissue
perfusion. As the human eye is so much faster and
better suited to interpret images than large amounts
of numbers it makes sense to display these quantities
in the form of images.