CT dose index

An important parameter, CT dose index (CTDI, defined as the cumulative dose along the patient s axis for a single tomographic image), should be evaluated at least semiannually at different values of kVp and mA. It is measured by using an ionization chamber or a thermoluminescent dosimeters placed in a tissue-equivalent acrylic phantom simulating head or body. ACR (2007) recommends maximum doses CTDI) of 6 rad (60 mGy) for brain, 3.5 rad (35 mGy) for adult abdomen and 2.5 rad (25 mGy) for pediatric (5-year-old) abdomen. The details of these measurements are available in standard CT physics books. 

To compare exposure between different CT protocols, one can use the absorbed dose, usually reported as the CT dose index (CTDI) in mGy. This concept was introduced by Jucius and Kambic to predict the multiple slice average dose at the center of a set of axial scans [35], The product of the CTDI j and the z-axis coverage for the particular scan then yields the dose-length product (DLP), where CTDI j is the average dose in the standard head CT dosimetry phantom [36, 37],... 

CTDlvol volumetric CT dose index DLP dose length product... 

Pitch is also correlated to dose delivery to patients, expressed in mGy, as CT Dose Index (CTDI) (see Appendix). 

The dosimetric quantities typically used in CT are the CT dose index (CTDI) and the dose length product (DLP). The CTDI is defined for an axial CT scan (one rotation of the X-ray tube) by dividing the integral of the absorbed dose along the z axis by the nominal beam width. As shown in Fig. 4.2, this value is equivalent to the dose within the nominal width of the slice assuming that the absorbed dose has a rectangular profile with a constant dose inside the nominal width and zero dose outside. 

CT dose index (CTDI oi) 15 mGy (10-30 mGy depending on patient size)... 

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