In the year 2014 the questionnaire was sent to all, 1822, X-ray departments in Poland. They were asked to fill appropriate data for the year 2013. The feedback was received from 238 X-ray departments (13.1%). Sixty-four of these departments reported CT examinations (27% of all answers). The distribution of the CT answers by the regions is presented in Fig. 1 . There are 2 regions (Podlaskie, Warmińsko-mazurskie) that did not respond. The total number of examinations performed by respondents equals 267,700. The detailed data are presented in Fig. 2 . As from the NHF, the reported number makes 11% of all CT examinations (2,494,977) reimbursed in Poland in 2013. The number of examinations given by respondents could be used to calculate the total number of examinations performed in Poland and to compare it with the NHF data. Such calculations could approximate the true value if the data obtained in the survey will be representative for the whole country. The representativeness could be proofed when the share of the number of the reported examinations corresponds the share of the responded departments, and when the share of the responded CT departments correspond the share of all CT departments at all. There are no reliable data on the number of CT departments in Poland, but they can be approximated by the number of CT devices. Taking into account the data available in the Chief Sanitary Inspectorate, the number of CT devices installed in 2013 in Poland was 640. It is obvious that the number of CT department cannot be higher than this number. Table 1 summarises the parameters which can be used to confirm the representativeness of the survey data. The total number of CT examinations estimated from the survey agrees with the data obtained from the NHF (Table 2). The observed slight difference may be caused by two components. One of them is the representativeness of the data. The second one results from the fact that the survey data contain examinations which are not reimbursed by the NHF and consequently, they are not included in the NHF database. Finally, Fig. 3 shows the estimated number of CT examinations per 1000 population performed in the regions of Poland.

The other information obtained from questionnaire is the dose. The most useful quantity for the estimation of the effective dose for the patient is DLP. It is one of the dose-indexes available from CT scanners which can be easily calculated on the basis of standard phantoms. DLP is defined as:

where L is the scan length and CTDI_vol is the function of the pitch and of the quantity CTDI₁₀₀ measured in the selected positions in the standard phantom:

where D(z) is the profile of the absorbed dose along the z-axis during one full rotation of the X-ray source, s is the nominal width of the slice and N is the number of slices during one full rotation. To use DLP quantity for the estimation of the effective dose the appropriate coefficients must be known which depend on the type of the examination (body region) and on the age of the patient [7]. As stated above, the survey in DLP question concerned adult patients only, weighting 80±10 kg. The returned question­naires contain 655 values of DLP all together.

The effective dose received by a patient during the examination depends not only on the body region but on the number of phases preformed as well. Fig. 4 demonstrates the share of different numbers of the phases used in 5 groups of CT examinations. The usage of the multiphase examinations is related to the usage of the contract agents. Almost all (94%) of the spine examinations are obtained without the contrast (single phase). On the other hand, only few (16%) abdomen and pelvis examinations were performed in a single phase. The difference between the doses obtained during single-phase and multi-phase examinations is substantial. The ratio between the multi-phase examination and the appropriate single-phase one equals the number of the phases roughly. This relationship is less obvious for angio-CT, dynamic-CT of liver and pancreas, and virtual colonography. The summary of DLP values for different examinations is given in Table 3 . The mean values are weighted by the number of the appropriate examinations performed in the appropriate X-ray department. It should be noticed that the high dispersion of the DLP data are consistent with the data in the literature mentioned above. The highest ratio DLP_max/DLP_min is observed in CTA of chest (45), angio-CT of head (34), pelvis CT with contrast (20) and in thoracic spine without- (18) and with contrast (16). There exist 17 other types of examinations with the max/min ratio close to or below 10. Majority of them is performed rather rare. The obtained results of DLP will be used to analyse the appropriateness of the established DRLs in Poland.

The DLP data collected in the survey could be used to evaluate the effective dose obtained by patients during the CT procedures and, further, the dose to population due to medical exposure from the CT examinations. Table 4 presents the average doses received by patients in different CT examinations and the doses to the population resulted from the CT exposures. The doses are calculated based on the factors given in EUR16262 Report [8]. The highest doses are related to pelvis CT with contrast (28 mSv), to abdomen CT with contrast (22 mSv) and to angiography of abdominal aorta, coeliac trunk and renal arteries (22 mSv). To obtain the doses averaged over the Polish population, the number of examinations and the population of Poland [9] should be taken into account. The total annual effective dose averaged in the population due to CT examinations performed in Poland equals 0.53 mSv. Almost half of the dose originated from abdomen and pelvis examinations (41%). The large number of head-, neck- and chest- examinations which characterise rather small dose, transferred to 39% share in the total CT dose. The distribution of doses is presented in Fig. 5.