Evaluation of MRI-based Polymer Gel Dosimetry for Measurement of CT Dose Index (CTDI) on ۶۴ slices CT Scanners

Introduction: Computed tomography (CT) has numerous applications in clinical procedures but its main problem is its high radiation dose to the patients compared to other imaging modalities using x-ray. CT delivers approximately high doses to the nearby tissues due to the scattering effect, fan beam (beam divergence) and limited collimator efficiency. The radiation dose from multi-slice scanners is greater than the single-slice scanners and since multi-slice scanners increasingly employ a wide beam, ۱۰۰ mm ion chambers currently used in measuring the CTDI۱۰۰, are not capable of accurately measuring the total dose profile of the slice width. Therefore, the CT dose is underestimated by using them. The purpose of this study is to measure the Computed Tomography Dose Index (CTDI) of a GE multi-slice CT scanner (۶۴-slice) using polymer gel dosimetry based on MRI imaging (MRPD). CTDI is the sum of point doses along the central axis and estimates the average patient dose during CT scanning. Materials and Methods: For measuring CTDI, after designing and fabricating the phantom and preparing the MAGIC gel, MRI imaging using a ۱.۵ T Siemens MRI scanner was performed with the imaging parameters of ST = ۲ mm, NEX = ۱, TE = ۲۰-۶۴۰ ms and TR = ۲۰۰۰ ms. CTDI was measured with a ۱۰۰ mm ion chamber (CTDI۱۰۰) and also the MAGIC gel with MRPD method for ۱۰ mm and ۴۰ mm CT scan nominal widths. Results: Following the measurement of the CTDI۱۰۰ for ۱۰ mm and ۴۰ mm nominal slice widths of the multi-slice scanner using both ion chamber and MAGIC gel, the results showed that the ion chamber underestimates CTDI۱۰۰ by ۲۸.۷۱% and ۱۴.۰۳% compared to gel for ۱۰ mm and ۴۰ mm respectively. Discussion and Conclusion: It was concluded from this study that gel dosimeters have the capability to measure CTDI in wide beams of multi-slice CT scanners whereas ۱۰۰ mm standard ion chamber due to its limited length is not reliable even for a ۱۰ mm beam width. In addition, due to the ۳ dimensional nature of gel dosimetry, by using a MAGIC polymer gel, it is possible to obtain a lot of important information from the mentioned profiles such as the actual slice thickness and z-axis geometric efficiency. In addition to the stated parameters, the percentages of the total and partial homogeneities in the slice plane can be obtained only from gel dosimetry. The results of this study show that MAGIC polymer gel dosimetry based on MRI can be used as a supplementary method to using conventional ion chamber dosimetry especially in measurements for slice widths greater than ۲ mm.

کلیدواژه ها:

CTDI Measurement ، MRI ، Polymer Gel Dosimetry ، ۶۴-Slices CT scanner