The excessive variability of doses delivered to patients of the same body size, during the same examination for the same medical purpose led the International Commission for Radiological Protection (ICRP) to make recommendations in 1996  . The goal is to determine the Diagnostic Reference Levels (DRLs) for the most-performed and most-irradiating examinations to optimize the dose and dose-area product in conventional radiology received by the patient. From then on, many states have transposed the ICRP recommendations into their laws. Today the determination of the DRL is part of the technical cooperation project of the International Atomic Energy Agency (IAEA), n˚ RAF 9059  . It is within this framework that we are inscribing our work to determine the DRL in western Côte d'Ivoire for three (3) radiology rooms for frontal chest examination for 90 patients. The final objective is to provide conventional radiology practicians with some reference dose values in order to ensure the management of the doses delivered and the efficient control of the exposure of patients in Côte d’Ivoire. Our work was conducted on patients with a measuring device called DAP-meter. In Côte d’Ivoire, Issa Konaté et al. have published in the same conditions two DRLs studies, one on Abidjan frontal chest (PA) examinations  and the other on Abidjan lumbar spine examinations  . It is worth noting that a preliminary work was conducted by Monnehan et al.  using a phantom (water-filled can) and TLD dosimeters. The reading of the dose at the entrance was made from a Harshaw 4500 reader.
For this work, we described the material, then the results were presented, then the discussions and conclusion closed our presentation.
2. Materials and Methods
2.1. Sampling Methods
Three (3) conventional radiology rooms corresponding to 3 hospitals, all located in three (3) cities of western Côte d'Ivoire, were selected. These are the Regional Hospital Center (CHR) in Daloa, the Regional Hospital Center (CHR) in San Pedro, and the General Hospital (HG) in Bangolo. All these rooms comply with the Ivorian standards of at least 25 m2 of base area and a ceiling height of at least 3.5 m  and have been inspected by the competent body. In each of the rooms, we took into account in our measurements, 30 patients  , all equipped with a bulletin for the examination of the frontal chest and all, 18 years old at least. It should be noted that the examination of the frontal chest is the examination most practiced in these rooms. We excluded from our study patients on bed or chair. In total for our study we had 90 patients.
2.2. Data Collection
Our study began after we received permission from each of the managers of the 3 radiology centers. During the months of May, June, July 2016, we went every day to these centers, one after the other. For each patient in our study, we measured the Dose in air (Dair) and dose-area product (DAP) using a DAP-meter. All of these values were recorded on a data collection sheet along with the age, mass, size, and thickness of the patient portion to be examined. We also noted technical parameters such as source-skin distance, voltage and electrical charge.
In a conventional radiology room, we have equipment such as the X-ray generator, X-ray tube, desk, wall stand and X-ray viewer. For our study, we brought with us in each room, a DAP-meter brand Diamentor M4-KDK and type 11,017 manufactured by the German company PTW. This device has been previously calibrated by the PTW-Freiburg calibration laboratory. It consists of an ionization chamber and an electrometer connected by two cords. The ionization chamber is placed at the exit of the tube at the collimator and the electrometer is placed at the desk behind the screen. When the beam passes through the ionization chamber, it deposits energy that is transferred by the leads to the electrometer. At this level, this energy is transformed into to dose in air (Dair) and DAP  .
2.4. Methods for Determining DRLs
We obtained the Entrance Surface Dose (ESD) for each patient, from the Dair by the following equation:
where Dair is the dose in air
(BSF = backscattering factor) with BSF = 1.35 for voltage values included between 60 and 80 kV or 1.5 above 80 kV  .
We determined the Diagnostic Reference Levels (DRLs) for each examination according to the 75th percentile statistical method in accordance with the recommendations of the European Commission. This is to take as DRL the value of the 75th percentile of ESD or DAP for a given exam on a large number of patients and on a large number of ESD and DAP values. This is a method used in statistics to remove the limit values from the sample. The 75th percentile of n values in ascending order is the value of rank k, given by the following mathematical equation:
 ; where n is the number of patient.
3.1. Determination of the DRL for the ESD per Center for the Examination of the Frontal Chest (PA)
On this Figure 1, we have the values of DRL in ESD for the examination of the frontal chest in postero-anterior incidence. The highest value of ESD is obtained at the CHR of Daloa (0.425 mGy) and the lowest is obtained at the CHR of San-Pédro (0.141 mGy).
Figure 1. Comparison of DRLs values in mGy.
3.2. Comparison of Voltage Values in kV and Electrical Charge in mAs of the Rooms of Our Study for the Examination of the Frontal Thorax (AP)
It is observed on Figure 2, that the highest mean value of 117.2 kV of the voltage is obtained at HG Bangolo and the lowest average value 81.66 kV is obtained at the CHR of Daloa.
On Figure 3, the higher mean value of the electrical charge is obtained at the CHR of Daloa and the lowest is obtained at HG Bangolo.
3.3. Determination of the DRL for the DAP per Center for the Examination of the Frontal Chest (PA)
On Figure 4, DAP have their highest value obtained at CHR of Daloa (74.02 cGy∙cm2) and the lowest value obtained at Bangolo HG (15.2 cGy∙cm2).
3.4. Characteristics of the X-Ray Tubes in Each of the Radiology Rooms of Our Study
In Table 1, the higher filtration of the tube is observed at the CHR San-Pédro (2 mm Al) and the lowest is obtained at CHR Daloa and HG Bangolo (1 mm Al). The tube at CHR Daloa is almost the same age as that of HG Bangolo. However that of CHR San-Pédro is older than four (4) years. All these devices arrived new in the corresponding radiology rooms.
3.5. Voltage in kV and Electrical Charge in mAs, DRLs in mGy and in cGycm2 for All the Centers of Our Study for the Examination of the Frontal Thorax
We calculated the mean tension and the average electrical charge (arithmetic mean)  . We have adopted the following notations: average (minimum, maximum) in kV and mAs. Finally, we determined the DRL (75th percentile) in ESD and DAP. The values are shown in Table 2.
In Table 2, we have reported the mean values of voltages and mAs with their corresponding intervals. We also reported the DRLs values in ESD (0.40 mGy) and in DAP (54.85 cGy∙cm2) of all 3 rooms in our study.
Figure 2. Comparison of voltage mean  values.
Figure 3. Comparison of electrical charge mean  values.
Figure 4. Comparison of DRL values in cGy∙cm2.
Table 1. Comparison of age and total filtration of X-ray tubes in our radiology rooms.
Table 2. Exposure parameters with mean values and range (in bracket) and DRL of ESD and DAP for all the hospital centers of our study.
3.6. DRL of Our Study and DRL Obtained in the District of Abidjan for the Examination of the Frontal Thorax (PA) 
On Figure 5, we observe that the DRL for ESD in our study in the west of Côte d’Ivoire is higher than the one of Abidjan.
On Figure 6, the voltage use in Abidjan for the frontal chest exam is higher than the one of our study.
On Figure 7, the electrical charge use in Abidjan for the frontal chest exam is smaller than the one use in our study in western Côte d’Ivoire.
We observe on Figure 8, that the DRL for DAP in our study in the west of Côte d’Ivoire is higher than the one of Abidjan.
3.7. DRL from Our Study and DRL Obtained in Other Countries
On Figure 9, we compare the DRL in ESD of our study with those obtained in other countries and institutions outside Africa. Our value 0.40 mGy is close to that of Iran and IAEA   but larger than all other values in the figure  .
On Figure 10, we can compare our value of DRL for ESD with those obtained in other African countries. Our DRL value in ESD is equal to that of Nigeria  , close to that of Cameroon  and higher than all other values   .
On Figure 11, which presents the comparison of the DRL for DAP obtained in our study with those of IRNS and other countries outside Africa, the smallest value is obtained in the United Kingdom (UK), 10 cGy∙cm2  , and the most large, 54.85 cGy∙cm2 in our study   .
According to the results of our study, for the examination of the frontal chest, we note that the highest DRL value in ESD of these radiology centers is obtained at CHR Daloa, it is 0.425 mGy. The lowest DRL value is obtained at CHR San-Pédro, 0.141 mGy (Figure 1). We explain this situation by the fact that the average value of the voltage used at the CHR Daloa, 81.66 kV is the smallest
Figure 5. Comparison of the DRL for ESD of our study to the one of Abidjan.
Figure 6. Comparison of the voltage to the one of Abidjan.
Figure 7. Comparison of the electrical charge to the one of Abidjan.
Figure 8. Comparison of the DRL for DAP of our study to the one of Abidjan.
Figure 9. Comparison of DRL for ESD obtained in our study with those obtained in other countries and institutions.
Figure 10. Comparison of DRL for ESD obtained in our study with those obtained in other African countries.
Figure 11. Comparison of DRL for DAP obtained in our study with those obtained in other countries.
value compared to other centers (Figure 2) and the average value of the electrical charge used in this center, 12.8 mAs, is the largest (Figure 3). Indeed, the lower the voltage and the electrical charge, the dose received by the patient is high. According to the French Society of Radiology (FSR)  , the recommended average voltage is 125 kV in a voltage range (115 - 140) kV and the recommended charging interval is (1.5 - 3) mAs for the examination of the frontal chest with a tendency to increase the tension and decrease the electrical charge. But in our work, the electrical charge is 4 times the maximum value of the recommended interval (1.5 - 3) mAs. The DRL of ESD at CHR San-Pédro is 0.141 mGy. This value is slightly lower than that obtained at Bangolo HG which is 0.156 mGy. It is noted that the average value of the voltage at San Pedro is slightly lower than that of Bangolo’s general hospital and that the average electrical charge at San Pedro is slightly larger than at Bangolo’s General Hospital, which can be explained by through filtration (Table 1). Indeed the filtration of the X-ray tube lowers the dose at the entrance of a patient. The greater the filtration, the lower the dose at entry  . FSR recommends total filtration greater than or equal to 3 mm Al. The tube of the CHR of San-Pédro has a total filtration of 2 mm Al while that of HG of Bangolo has a total filtration of 1 mm Al. For DRL in PDS, the highest value is obtained at the CHR of Daloa (74.02 cGy∙cm2) (Figure 4), which is explained by the DRL in ESD which is also the largest in this radiology center. In addition we know that the DAP is proportional to the dose and the surface. Note that the DRL in ESD is lower in the CHR of San-Pédro than the one in HG Bangolo, but the DAP in CHR San-Pédro (16.23 cGy∙cm2) is higher than the one obtained at HG Bangolo (15.2 cGy∙cm2). It is therefore clear that the area of exposure of patients is greater at the CHR of San-Pédro than HG of Bangolo. Operators should properly manipulate the tube diaphragm at the CHR San-Pédro to avoid unnecessary patient exposure to the X-ray beam. We obtained, for all the centers of our study, for the examination of the frontal thorax (PA), the DRL in ESD equal to 0.40 mGy (Table 2). The comparison of this value with that obtained in Abidjan for five (5) centers (0.22 mGy)  , indicates that the DRL in ESD in west of Côte d’Ivoire is larger than that obtained in Abidjan in the south of the same country (Figure 5). An explanation for this result comes from the radiological parameters. The average voltage used in Abidjan (104 kV) is greater than the average voltage used in western Côte d'Ivoire (101.6 kV) (Figure 6) and the average electrical charge used in Abidjan (4.55 mAs) is smaller than that used in western Côte d'Ivoire (6 mAs) (Figure 7). The higher the voltage and the lower the electrical charge, the dose at the entrance is low. We also observe for DRL in DAP that the value obtained in Abidjan (53.26 cGy∙cm2) (Table 2 and Figure 8) is lower than that obtained in our study in western Côte d'Ivoire (54.85 cGy∙cm2). The explanation comes from the fact that the DRL in ESD is weaker in Abidjan than in our study. By comparing the DRL in ESD from our study, obtained in the West of the Côte d’Ivoire to those obtained by the Institute for Radiation Protection and Nuclear Safety (IRNS) in France, the IAEA and other countries (Figure 9, Figure 10) we find that our value is equal to that obtained in Nigeria and by the IAEA, close to those obtained in Iran and Cameroon. However it is larger than those obtained by the IRNS and the other countries. We can reduce the DRL in ESD of our centers of study, if the operators of medical imagery increase the tension more and reduce the charge. The FSR recommends a voltage range of (115 - 140) kV with a tendency to increase the voltage and electrical charge range (1.5 - 3) mAs with a tendency to reduce the charge  . The comparison of DRL in DAP of our study (54.85 cGy∙cm2) with DRL in DAP obtained by the IRNS and other countries, (Figure 11), shows us that our value is greater. It is therefore necessary to take corrective measures by reducing the ESD and the area of exposure of the patients in the rooms of our study for the examination of the frontal chest.
We were able to achieve our goal of determining the DRL for ESD and DAP in western Côte d’Ivoire, for the postero-anterior frontal chest examination. The values that we obtained are for the DRL, in ESD 0.40 mGy and for the DRL in DAP, 54.85 cGy∙cm2. The DRL values obtained for each of the sites in our study are different, which again justifies the need for the establishment of regional and national DRL. This disparity of values is justified by the choice of the radiological parameters by the technicians in the different rooms: voltage and electrical charges but also by the poor focusing of the beam. We also noted the importance of filtration in reducing the dose at the patient's entrance. Our DRL value in De is equal to that of the IAEA but greater than that of Abidjan and those of many countries such as the United Kingdom, Ghana and France. So there are efforts to be made in the rooms of our study to optimize the dose to patients. This involves the appropriate choice of voltage and electrical charge in accordance with IRNS recommendations and also by the equipment and a total equivalent filtration of 3 mm Al.
The value of DRL of DAP obtained in our study in western Côte d’Ivoire for the examination of the frontal chest (54.85 cG∙cm2) is greater than those obtained in Abidjan and in several countries. It is therefore necessary not only to reduce the ESD but also to use the diaphragm of the tube to expose just the part of the patient’s body to examine.
The authors would like to express their gratitude to the Director of the National Public Health Laboratory (NPHL) and the staff of the (NPHL) for their availability and their DAPmeter available to us. The authors send their thanks to the Directors General of the three health establishments for having accepted to participate in this campaign.