Introduction

The precise measurement of entrance skin dose (ESD), in diagnostic radiology imaging is helpful in optimizing patient protection and developing standards and reference levels of diagnostic in countries. Many researchers in different countries have studied this parameter in various radiography imaging modalities [ 1 - 5 ]. In addition, risk assessment is not possible except by measuring the dose delivered to different organs. The International Committee for Radiation Protection (ICRP) suggests measuring the dose of various organs to evaluate the biological effects of radiation on the body [ 6 ]. Orthopantomogram (OPG) or panoramic dental imaging is widely used in forensic sciences [ 7 , 8 ] and diagnostic radiology in diagnosis of various malformations of the head, jaw, and face, especially in orthodontic treatments; therefore, the dose reduction in these systems has been the subject of different investigations [ 7 - 11 ]. The children and adolescents dose reduction issue in diagnostic radiology has always been one of the main concerns in health physics science because of their higher radiation sensitivity [ 12 , 13 ] and higher life expectancy than adults. Thus, measuring the dose delivered to patients in various radiographic tests and optimizing the image quality, and dose to reduce the risk of radiation exposure are very important. Thermoluminescence dosimetry has always been one of the effective dosimetry methods for estimating doses in diagnostic radiology, nuclear medicine and radiotherapy [ 14 - 20 ]. The purpose of this study is to estimate the entrance skin dose and the doses received by various organs in OPG imaging.

Material and Methods

Determination of ECC of TLDs

In this experimental study, cubic chips of T1D-100 (LiF, Mg, Ti) with dimensions of 1 × 3 × 3 mm³ were used for dose measurements. At first, a batch containing one hundred TLD-100 chips was selected for dosimetry. All TLDs were heated at 400 °C for 20 hours then at 80 °C for 1 hour. The TLDs were then exposed to an equal value of dose; and the responses of these TLDs were read out using the Harshaw-4500 TLD reader device to obtain the ECC factor of the TLDs according to Equation (1).

${\mathrm{ECC}}_i=\frac{\mathrm{⟨TLE⟩}}{\stackrel{}{\mathrm{⟨TLE⟩}}}$ (1)

In which ECC_i and ⟨TLE_i⟩ are the element calibration coefficient and readout of TLD chip i, respectively, and $\stackrel{}{\mathrm{⟨TLE⟩}}$ is the average readings of TLDs. After obtaining ECCs, 50 chips with ECCs close to 1 were selected to be used for dosimetry, and calibration.

Calibration of TLDs

In order to obtain the TLD calibration curve, 12 TLD chips in 4 plastic packs were used after annealing procedure (1 hour at 400 °C and 20 hours at 80 °C). Three triple packs of these TLDs were exposed to specific doses of 416, 2190, and 5642 µGy. A package was also used to measure background radiation. The calibration process was performed with an X-ray machine with the same X-ray quality of the OPG unit. Subsequently, all exposed chips were read out by the TLD reader, and the corrected readings of each chip (according to Equation 2) were used to plot the calibration curve.

$R_{\mathrm{i-C}}=\left(\frac{R_i}{{\mathrm{ECC}}_i}\right)-\left(\frac{{\sum }_{\mathrm{j=1}}^n\left(\frac{R_{\mathrm{j-b}}}{{\mathrm{ECC}}_{\mathrm{j-b}}}\right)}{n}\right)$ (2)

Where R_i-C and R_i are the primary readout and corrected readout of the i^th chip. R_j-b is the readout of the j^th background chip, ECC_i and ECC_j-b are the ECC values of the TLD chip i and background chip j, respectively and, and finally, n is the number of TLDs assigned to the background measurement.

Since the calibration curve of TLDs is a linear curve, the relationship between the doses of each TLD and its corrected readings is as following:

D_i(μGy)=[a×R_i-C] (3)

Where D_i(µGy) is the dose received by chip i, and a is the calibration coefficient of TLDs.

Dosimetry with TLD chips

In this step, a number of annealed TLD chips were packed in dark plastic envelopes for measurement of the entrance skin dose of the Phantom. Several TLD chips were also placed within the Rando phantom at the position of the target organs (thyroid, eyes, esophagus, sublingual, and parotid, and submandibular salivary glands). Rando phantom was exposed with OPG device, with the routine exposure conditions used for the patients, i.e. 68kv, 8mA, and 13.5 s. The geometry of Rando phantom is shown in Figure 1. Three TLD chips were also used to measure background radiation. The exposed TLDs were read out by Harshaw 4500, TLD-reader. Considering the linear calibration curve, the dose received by each TLD was obtained according to equation (3). The whole process was repeated once more.

Figure 1. Imaging geometry

Results

The calibration curves obtained for the TLD-100 in the two steps are shown in Figure 2. The dose of each TLD was obtained by multiplying the chip readout by calibration coefficient of each step, i.e. 68.72 (µGy/nc), 71.67 (µGy/nc) for the first, and the second steps, respectively. Figures 3a and b compare the dose in different organs obtained in the two steps. Table 1, a comparison of the entrance dose measured in each step is shown. The analysis of the TL dosimetry uncertainty is also shown in Table 2. In Table 3, there is a comparison of the measured values of entrance skin dose and organ doses in this study with the reported values in other investigations.

Figure 2. Thermoluminescence dosimetry(TLD) -100 calibration curve

Figure 3. Comparison of absorbed dose of different organs in Orthopantomogram (OPG) imaging.

Discussion

According to the results shown in Figure 3, and Table 1, the results obtained in both measurements are comparable to each other. The small difference in the results may be due to the uncertainties in TLD, and positioning of the phantom. The total uncertainty in the TLD measurements is about 6%. The uncertainty type-A of the TLD response due to the iteration of the measurements was 4%, while type B uncertainty due to the calibration was 5%. The errors due to the energy dependency of the TLDs were ignorable, as the energy spectrum used for the calibration was similar to energy spectrum used for the measurement. As the calibration TLDs, and those used for the measurement were read out at the same day, the uncertainty for fading of the TLD signals can be ignored.

Conclusion

It can be concluded the dose values can vary based on devices and exposure conditions. The difference observed between the doses values obtained in this study, and those reported by the previous investigators may be due to several reasons such as phantom positioning, the imaging technique, filtration of the x ray beam, and position of the dosimeters inside or on the phantom.

For instance, the large differences observed between the doses of thyroid, and parotid glands may mainly be because of the patient positioning (the parts of the head and neck which is irradiated as the main field size). Another reason for such differences may be because of the different position of the TLDs inside the organ.

Therefore, it can be concluded that we should put more TLD chips inside each organ, when we want to estimate the effective dose accurately.

References

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