Investigation of conventional diagnostic X-ray tube housing leakage radiation using ion chamber survey meter in Mizoram, India

Contact us: sciencevision@outlook.com Leakage radiation that transmitted the protected X-ray tube housing was measured and compared with national and international safety standard. To the best of the authors’ knowledge, no tube housing leakage measurement has been done so far in the present study area. The authors considered all the conventional diagnostic X-ray units in Mizoram, India. Ion chamber survey meter was used to measure leakage radiation and it was placed at 5 different positions (left, right, front, back, top) of the X-ray tube. Measurements were done at 1 m focus-todetector distance by projecting X-ray tube vertically downward with collimator diaphragms closed completely. SPSS statistics for windows, version 17.0 (SPSS, Inc., Chicago, IL, USA) was used to derived mean, standard error of the mean etc. The tube housing leakage exposure rates ranged between 0.03 mRh -1 and 500 mR h -1 ; among the 5 positions, rate measured in the front direction has the highest mean at 41.61±8.63 mR h -1 ; whereas the top has the lowest 4.57±1.16 mRh -1 . Tube housing radiation level ranged from 0.01 to 58 mR in one hour. Leakage radiation was minimum at the top position of the tube and maximum in the front direction. All the equipment were in compliance with national and international standard norms, the highest leakage radiation level was 50.43% of the safety limit.


Introduction
Diagnostic X-ray imaging is one of the basic and fastest way for physicians to view the internal organs and structures of the human body, which has no proper substitute till today. 1 The rapid increase in demand of X-ray application has led to unnecessary patient exposure. 2 On the other hand, provision of high-quality healthcare services is the main purpose of using medical devices. 3 In addition, medical exposures are the most considerable source of ionizing radiation not only to the patients and radiation workers but also to the general public. So,  www.sciencevision.org through a shield of any thickness without having an interaction. 7 As early as 1899, WH Rollins, a dental physician in Boston, USA, introduced the X-ray tube housing by using lead material as well as primary beam collimation to enhance image quality and radiation protection. 8 In the early days of medical imaging, lead shielding around the X-ray tube was used but before shielding became mandatory, about three decades had passed. 9 In the present day, tube leakage radiation is not emitted through the X-ray tube portal even though it is created inside the X-ray tube. Rather, leakage radiation is transmitted through X-ray tube housing. 10 This is why diagnostic X-ray tube housing is lined with thin sheets of lead. This shielding is intended to protect both the patients and personnel from leakage radiation. 11 Proper shielding of any X-ray tube, using the standard methodology and leakage limit, is mandatory for the radiation protection of the radiation workers, patients and the public. 12 Studies have been performed on tube housing leakage of conventional diagnostic X-ray equipment in different parts of the world. Sungita et al. 13 in 2006 performed measurement of tube housing leakage on 47 units in Tanzania, and reported 'Most of the X-ray machines tested for tube leakage gave results that were below 0.5 mSv h -1 at 1 m, which complied with safety requirements. In 2012, Hassan et al. 14 studied X-ray diagnostic machines used at different medical diagnostic centers in Egypt; they reported that the measured dose of tube housing leakage was in the range of background values 0.15 µSv h -1 at 1m. Tsalafoutas 12 performed a study in excessive tube housing leakage due to the methodology used by the manufacturer on two separate mobile X-ray equipment. Tsalafoutas reported that even at a distance of 3 m from the tube, the leakage radiation exceeded the maximum permissible dose rate of the equipment. For the second unit, the dose-meter reading at 1 m from the tube was 12.1 µGy; for 1 h with tube current 4 mA, a leakage of 3.5 mGy was derived. The author concluded that after changing the methodology used by the manufacturer, the leakage radiation had been reduced to about 1/8 of its previous value and thus following the existing leakage radiation limit.
To the best of the authors' knowledge, no tube housing leakage measurement has been done so far in the present study area. Keeping this in mind, this study was conducted to quantify leakage radiation with the international standard test procedure to all working conventional diagnostic X-ray machines in the present study area. Further, the results were compared to Atomic Energy Regulatory Board (AERB -India), National Council on Radiation Protection and Measurements (NCRP-USA), European Commission standard norms and including the previous study as well.

Materials and Methods
The total number of working and out of order diagnostic X-ray machines recorded in Mizoram was 169 in 116 different institutions until June 2016. However, in the present study, the authors considered 111 (65.68%) conventional diagnostic Xray units. In view of the total workloads of all X-ray facilities, conventional X-ray contributed 90.94% and other 9.06% were shared between dental X-rays and other (CT-scan, fluoroscopic & mammographic) procedures; the detail was published in the previous study. [15][16] These workloads were calculated from several parameters such as; patients per day, films per patient, mAs per film and days per week by using formula given by NCRP. [16][17] The authors classified all the working conventional units into fixed, mobile-fixed and mobile unit. Out of all that, 93 (55.03%) working conventional diagnostic X-rays which were installed in 72 different hospitals were studied. The present study area and the location of different hospitals, community health centers and primary health centers were shown in Figure 1. 18 For measuring leakage radiation, pressurized ion chamber survey meter (model 451 P, Fluke Biomedical, Everett, WA, USA) was used (Figure 2). The calibration measurements were traceable to the National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA). The response time of the survey meter was 5 s for 0 µR h -1 to 500 µR h -1 (0 µSv h -1 to 5 µSv h -1 ); 2 s for 0 mR h -1 to 5 mR h -1 (0 µSv h -1 to 50 µSv h -1 ); 1.8 s for 0 mR h -1 to 500 mR h -1 (0 mSv h -1 to 5 mSv h -1 ). The survey meter has accuracy of ± 10% reading between 10% and 100% of full-scale indication on any range with precision within 5% reading. 19 All the measurements were carried out in freeze mode. 20 To measure leakage radiation from X-ray tube, the collimator diaphragms were closed completely and the tube was projected vertically downward. So, the tube is oriented in such a way that the anode is over the head of the table and the cathode is over the foot. When facing the Xray tube assembly, the anode is on the radiographer's left and the cathode is on the right. The tube leakage measurements were done at a 1 m focus-to-detector distance (FDD) by putting detector at five different positions viz. left, right, front, back and top of the X-ray tube. The exposure parameters for the present study were maximum accelerating potential (kVp), maximum tube current (mA) and fixed exposure time (sec). 9,[12][13][14]21 According to the AERB safety code 2001 for 'medical diagnostic X-ray equipment and installations' it is mentioned that 'every tube housing for medical diagnostic X-ray equipment shall be so constructed that the leakage radiation through the protective tube housing in any direction, averaged over an area not larger than 100 cm 2 with no linear dimension greater than 20 cm, shall not exceed an air kerma of 115 mR (1 mGy) in one hour at a distance of 1 m from the X-ray target when the tube is operating at the maximum rated kVp and for maximum rate current at that kVp'. 22 Again, it was reported that the leakage radiation from the tube housing measured at a distance of 1 m from the focus should not exceed 1 mGy (115 mR) in one hour. 23 In addition to that, in the NCRP report No. 147, it was given that the manufacturers were required by regulation to limit the leakage radiation to 0.876 mGy h -1 (100 mR h -1 ) at 1 m. 17 Compliance with this requirement should be evaluated using the maximum X-ray tube potential and the maximum beam current at that potential for continuous tube operation. Furthermore, data presented as mean, range and standard error mean were analyzed by using SPSS statistics for windows version 17.0. (SPSS, Inc., Chicago, IL, USA). T-test was also conducted to check the the existence of significant difference between the amount of leakage radiation measured at different position with respect to the X-ray tube.

Results and Discussion
The tube housing leakage exposure rates measured for 93 diagnostic X-ray machines in each five different positions (i.e. left, right, front, back, and top) of the X-ray tube were shown in Table 1. Exposure rate 0.03 mR h -1 was the lowest leakage exposure measured and it was found in back and top positions of the X-ray tube. Leakage exposure rate, 500 mR h -1 was the highest leakage radiation rate from all 93 X-ray machines and it was measured in the front direction of the X-ray tube ( Table 1).
Comparing radiation exposure rates measured at different positions; rates measured at the front direction of the tube has the highest mean±SEM of 41.61±8.63 mR h -1 and rate measured at the top position of the tube has the lowest mean ± SEM of 4.57±1.16 mR h -1 . Therefore, it can be said that radiation leakage in the present study was high in the front position of the tube, whereas, it was low at the top position of the X-ray tube. In addition to that, t-test was performed between leakages exposure rates measured at these five different positions, and the results showed that there was a significant difference (0.01 level) between the top position and the other four directions of the X-ray tube. X-ray tube leakage at the top direction was significantly less than the other four directions. Tsalafoutas 12 reported that there was an excessive leakage radiation from each position except for one position on the top of the new mobile X-ray tube housing. So, similar case was found in the present study, when compared to the others, the top position showed relatively low leakage radiation rate ( Table 2).
From each five different positions of measurement, the authors selected the highest leakage exposure rates from all the X-ray machines. Then, the maximum leakage radiation level at 1 meter from the tube (mR in one hour) of the X-ray equipment were calculated by using the given equation ( Figure 3); 24 The calculated maximum tube housing leakage radiation from 93 X-ray machines ranged between 0.01 mR in one hour to 58 mR in one hour with 5.39±0.97 mR (mean±SEM) in one hour. Leakage radiation levels from 93 X-ray machines were compared to the national and international standard norms; it was found that all the machines complied with the safety standard. [22][23] The highest leakage radiation level was 50.43% of the standard limit. The present result is more or less similar to previous studies conducted by Sungita et al. 13 , in Tanzania (2006) and Hassan et al. 14 , in Egypt (2012). However, in the present study, leakage radiations appeared to be relatively higher than the previous studies.
According to AERB type approval machine, 66 machines were AERB type approved units where 27 machines were unknown approval due to lack of information as these machines were so old. The minimum leakage radiation level in AERB type approved units was 0.02 mR in one hour and the maximum was 58 mR in one hour having 6.97±1.31 mR (mean±SEM) in one hour. Further, minimum leakage radiation level in not known approval units was 0.01 mR in one hour and the maximum was 8.40 mR in hour with mean±SEM 1.51±0.46 mR in one hour. It appears that the leakage radiation level was higher in AERB type approved machines than the unknown approval type. The t-test also showed that the existence of significance difference (0.01 level) between AERB type approved unit and unknown approval status (Table 3). Besides, as already mentioned, both types of all the machines were within safety standard.
Regarding fixed and mobile X-ray machines, 52 machines were mobile X-rays, where, 41 were fixed X -ray equipment. Leakage radiation in fixed X-rays ranged between 0.02 mR in one hour to 58 mR in one hour with mean ± SEM 6.04±1.53 mR in one hour. In mobile X-rays, leakage radiation ranged from 0.01 mR in one hour to 30 mR in one hour with mean ± SEM 4.55±1.04 mR in one hour. It was found that the leakage radiation level was relatively higher in mobile X-ray than fixed X-ray machines even though fixed X-ray can operate at relatively high input parameters. However, there was no significant difference (0.01 level) between fixed and mobile Xray machines ( Table 4). Further, the correlation between the tube housing leakage and the age of the X-ray machine was only 0.15, therefore, the X-ray tube age is not one of the important reasons for the present tube housing leakage radiation.

Conclusion
In the present study it was found that the tube housing leakage radiation level among 5 different positions (i.e. left, right, front, back, top) was highest in the front direction of the tube and lowest at the  6.04 Table 4 | Comparison between maximum leakage radiation in fixed and mobile X-ray machines.
top direction of the X-ray tube. In comparison to national and international safety standard, all the equipments were well below the safety limit. Regarding fixed and mobile X-ray machines, there was no significant difference in leakage radiation even though fixed X-ray can operate at higher input parameter.