Dose verifications in several organs in abdominal computed tomography (CT) scan by using Al2O3 OSL dosimeters

The study investigated the dose of several organs in the computed tomography (CT) scan by using Al2O3 optically stimulated luminescent (OSL) dosimeters. The organs of right and left kidney, right pancreas, spleen and liver in an anthropomorphic human phantom were investigated for the dose received during the scan. The OSL dosimeters are embedded into custom-made wax and replace onto two selected slices of human body phantom where the selected organs are located. The phantom was scanned with by using adult abdomen protocol. The percentage differences of OSL doses reading compared to the previous study data and the international dose reference level (DRL). The doses obtained from OSL NanoDots® showed excellent agreement compared to XRQA film dosimeters with percentage differences within 5%. The doses measured by using OSL dosimeters in five organs were also within the range of the national DRL values by the Ministry of Health of Malaysia (MOH). These findings suggest that our results can be utilized to verify the doses received by internal organs during computed tomography abdomen protocols. The overall results indicated the suitability of OSL dosimeters for the indirect dose verification in the CT scan.


Introduction
Due to its extensive use, the computed tomography (CT) became the leading contributor to the collective effective dose among all diagnostic imaging procedures.It has therefore increased the awareness of the potential radiation risks to the population [1].CT would contribute up to 70% of the total dose given to patients [2].The National Commission of Radiation Protection (NCRP) Report 160 indicated that the medical exposure as the highest contributor to the man-made radiation exposure with CT scan as the greatest contributor at 49% of medical exposure [3].The major risks associated with the use of CT include the possibility of cancer induction from x-ray radiation exposure.Therefore, the call for accurate dose verification receive by patients from standard CT protocol used in clinical examination is required to ensure the safety of the patients undergoing CT scans.
The International Commission on Radiological Protection (ICRP) introduced the diagnostic reference levels in 19901 in 1996 as a method to determine the patient's dose at the safe levels [4].Diagnostic reference levels are measures of common practise in a country or region to develop national or regional diagnostic reference values due to different imaging protocols in various nations.DRLs are determined by radiation protection and health authorities along with professional medical organisations of the nation and often used to reduce the patient dose in CT scans while optimizing the quality of the radiographic images [5].
The Al 2O3 Optically stimulated luminescence (OSL) dosimeters are typically used for clinical dosimetry in many medical physics works due to its size as an in vivo dosimeter with good spatial resolution and wide dose range [6,7].Previous study suggested the suitability of the OSL dosimeters for dosimetry works in diagnostic kilovoltage x-rays and investigations of organ doses in general radiography [8,9] The OSL dosimeters also showed good agreement of dose to the CT dose index (CTDI) and dose mapping in CT scans [10,1,11].This study focused on the measurement of dose of 1308 (2024) 012012 IOP Publishing doi:10.1088/1757-899X/1308/1/012012 2 several organs (kidney, spleen, pancreas and liver) by utilise the OSL dosimeters in comparison with the available DRL.The OSL NanoDot® in this study was used to investigate the dose received by several critical organs during the CT abdominal scanning.

Energy calibration of OSL dosimeters
OSL dosimeters are made of aluminium oxide (Al2O3) having a relatively high effective atomic number (Zeff = 11.28).This caused the material to exhibit an exaggerated response of low-energy photons that usually use in diagnostic radiology due to the significant contribution of the photoelectric effect [12].The OSL dosimeters were calibrated by the secondary standard dosimetry laboratory (SSDL), Malaysian Nuclear Agency by using x-rays of 80 kVp energy to set up the dose analysis method by using the OSL reader.The OSL dosimeters were then calibrated with x-ray energies between 80 and 10 kVp to determine their energy dependency on kilovoltage x-ray

Measurement of organ dose in CT abdominal scan
An anthropomorphic human phantom was used to simulate the human anatomical features as shown in Figure 1(a).One slab of the phantom was replaced with paraffin wax for the positioning of the OSL dosimeters as shown in Figure 1(b) [9].This is due to unavailability to position the OSL dosimeters within the slabs of the anthropomorphic phantom.The paraffin wax was shaped based on the replaced slice of the anthropomorphic phantom and the OSL dosimeters were placed at the positions of the critical organs of right kidney, left kidney, pancreas, spleen and liver.The anthropomorphic phantom was scanned by using a CT scanner at abdominal scanning protocol (120 kVp and 250 mAs).The exposed OSL dosimeters were read by using the OSL reader and compared to the dose measured in XRQA film dosimeter and available DRL [13].

Results and discussion
Figure 2 showed the dose response of the OSL dosimeters towards the increased x-ray energies between 80 and 140 kVp.The dose recorded by the OSL showed excellent agreement to the increased x-ray energies shown by the linear regression values, R 2 near to 1.0.This indicated good energy dependent and sensitivity of the OSL dosimeters towards the increase of x-ray energies [14,15].Table 1 showed the dose comparison between the OSL dosimeters and XRQA film dosimeters.The Dose measured by using OSL dosimeters showed excellent agreement to the XRQA film dosimeter at all measured organs with maximum percentage difference lower than 5%.The results are also in good agreement with the previous work by Brady et al., [13] that compared the dose measured in OSL dosimeters and XRQA film dosimeters in both XVI and OBI cone beam systems in the CT scan.  2 showed the dose measured in the critical organs by using the OSL dosimeters during the abdominal CT scanning in comparison with the national DRL and other DRL [16,17].The national DRL did not provide the specific value for individual organs and only gives the dose range for all organs.The Slovenian DRL was referred in this study to as it provides the DRL for individual organs to be compared with the dose readings obtained from the current study.The doses measured by using the OSL dosimeters in this study showed high percentage differences between 31.3 and 38.99% compared to the DRL [17].The dose readings in the OSL dosimeters in this study however showed good consistency of readings within the organs being measured.Kawaguchi et al., [15], suggested that the use of OSL dosimeters for very low energy x-rays such as mammography require adjustments for energy dependence and this could also applied for the CT x-ray energies.

Conclusion
The doses obtained by OSL nanodots showed excellent agreement when compared to the doses obtained by using XRQA films dosimeter.The values of doses assessed in 5 organs undergo CT abdomen protocol are within the range of national DRL values from Ministry of Health Malaysia (MOH).This indicates our results can be used to verify the doses receive by internal organs in computed tomography abdomen protocol.The use of DRL should be in regional level as difference country would have difference value of their national DRL.The overall results indicated the suitability of OSL dosimeters for in-vivo dosimetry works in CT scans.There are also needs for specific DRL values for organs in the national level as part of quality assurance and radiation protection to the patients.

Figure 1 .
Figure 1.(a) The anthropomorphic human phantom used to simulate human body and (b) the paraffin wax used to replace the phantom slice consisting the critical organs.

Figure 2 .
Figure 2. The dose response curve of OSL dosimeter at x-ray energies between 80 and 140 kVp.

Table 1 .
The dose measured in several organs by using OSL dosimeters and XRQA film dosimeter.

Table 2 .
The dose measured in several organs by using OSL dosimeters in comparison to the dose reference levels (DRL).