Radiation weighing factors

In order to take into account the biological effects of different kinds of radiation, radiation weighing factors, have been introduced and are given in Table 18.4. Earlier, two similar concepts were used with about the same meaning the "quality factor", Q, and "rela-... 

Because of the varying radiation sensitivities of the different cell types, a tissue weighing factor, Wj, is introduced. It represents the relative contribution of that tissue to the total detriment from uniform irradiation of the body Table 18.6 gives tissue weighing factors. This leads to another dose concept (cf. eqn. 18.1), the effective equivalent dose, 0g, which is the sum of the weighted equivalent doses in all tissues, as given by... 

Each of these factors is dependent on the other two, and the failure of any one of them can affect the accuracy of a weighing. For example, do not expect great accuracy from a balance that is located above a radiator. Likewise, do not expect accuracy from a balance which has just demonstrated poor precision. Finding the source of errors in weighing is a step-by-step process. You must rule out each problem before moving on to the next level. 

An electronic measuring device is used for the measurement of quantities such as illuminance. It is composed of an amplifier, sensing device, filters, and a readout device. The ICH document does not specify which of the several different detector heads and filters available should be used. Photometric measurements are all weighed for human vision and as such do not measure the true amount of incident radiation. Table 1 listed the correction factors that are applied for a photometric detector and Figures 4 to 6 show how these corrections distort the actual data obtained (29). 

With regard to MDCT radiation exposure, several factors must be weighed. There is up to a 4.5% loss of efficiency vs. single-detector row scanners due to absorption of radiation in the z-axis by septa between the detector rows [31]. However, with MDCT, most of the X-ray beam is utilized per rotation due to the increased number of detector rows, with less of a penumbra [31]. Subsequently, these effects balance and the dose efficiency of MDCT has been shown to be comparable to that of single-slice helical CT [31]. 

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