Computed tomography dose index

The computed tomography dose index (CTDI) is the a commonly used radiation exposure index in X-ray computed tomography and is reported by the CT manufacturers to scan personnel for each exam.

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:

$CTDI=\frac{1}{nT}\int_{-7T}^{7T}{D(z)dz}$

where n is the number of slices acquired, T is the slice thickness and D(z) is the radiation dose measured at position z along the scanner's main axis.

This equation is often used in a modified form

$CTDI_{100}=\frac{1}{nT}\int_{-50 mm}^{50 mm}{D_a(z)dz}$.

The dose absorbed in air D_a(z) 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 in still somewhat larger near the skin than in the body center. Therefore, a third measure, the weighted CTDI was introduced:

$CTDI_w=\frac{1}{3} CTDI_{100}^{central} + \frac{2}{3} CTDI_{100}^{peripheral}.$

Similar measures with yet wider chambers are useful for CT systems with large numbers of detector rows.^[1]

Notes

1. ^ Geleijns 2009

References

• Rothenberg, LN and Pentlow, KS (2000). "CT dosimetry and radiation safety", Categorical Course in Diagnostic Radiology Phyics: CT and US Cross-sectional Imaging, LW Goldman and JB Fowlkes, eds., pp171-188, RSNA, Oak Brook, IL
• J Geleijns, M Salvadó Artells, P W de Bruin, R Mather, Y Muramatsu and M F McNitt-Gray (2009). "Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner", Phys. Med. Biol. 54, pp3141-3159