Definitions
It is defined by the US Department of Health and Human Services (HHS) as the average dose imparted by a single axial acquisition to a standard 100-mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices:
where is the number of slices acquired, is the slice thickness and is the radiation dose measured at position along the scanner's main axis.
This equation is often used in a modified form
.
The dose absorbed in air can be conveniently assessed with a 100-mm standard pencil dose chamber. Typically, the dose distribution within the body cross section imparted by a CT scan is much more homogeneous than that imparted by radiography, but is still somewhat larger near the skin than in the body center. Therefore, a third measure, the weighted CTDI was introduced:
Similar measures with yet wider chambers are useful for CT systems with large numbers of detector rows.
Read more about this topic: Ctdi
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