A practical method for determining organ dose during CT examination

A practical method, based on depth dose, for determining organ dose during computed tomography (CT) examination is presented. For 4-slice spiral CT scans, performed at radii of 0, 37.5, 75.0, 112.5, and 150.0 mm, measurement of depth dose has been made using thermoluminescent dosimeters (TLDs) inserted into a modified International Electrotechnical Commission (IEC) standard dosimetry phantom and also additional TLDs placed on the surface of the phantom. A regression equation-linking dose with distance from the center of the phantom has been formulated, from which dose to a point of interest relative to the surface dose can also be calculated. The approximation reflects the attenuation properties of X-rays in the phantom. Using the equation, an estimate of organ dose can be ascertained for CT examination, assuming water equivalence of human tissue and a known organ position and volume. Using the 4-slice spiral scanner, relative doses to a patients’ lung have been calculated, the location and size of the lung in vivo being found from the CT scan image, and the lung being divided into 38 segments to calculate the relative dose. Results from our test case show the dose to the lung to have been 69±13% of surface dose.