Estimate bowtie filter shape in PET/CT scan with TLD

Introduction: The CT machine utilizes a bowtie filter to shape the X-ray beam and remove lower energy photons. Configuration of this bowtie filter is complex and its geometry is often not available in detail. It causes the CT dose index (CTDI) be with the different values in measurement versus Monte Carlo simulation studies and other analytical calculations. It is important especially in dosimetry of internal organs. In this study, the bowtie filter shape is extracted by using Thermolumenecence dosimeters (TLDs).   Materials and Methods: In this work, the shape of the bowtie filter of the Biograph 6 PET/CT was derived by using 31 Thermolumenecence chips dosimeters (GR-200 series) made of LiF: Mg, Cu, and P were utilized. These chips calibrated by Cs-137 in SSDL organization and ECC (Element Correction Coefficient) of them by using TLD reader Harshaw-4000. Eventually, to evaluate the accuracy of the body bowtie filter shape as generated by TLDs, Monte Carlo simulation of CT was performed. 16 X-ray sources in various angles were used within the Monte Carlo code (MCNP-4C) to simulate the CT section of the PET-CT Biograph 6 system and to calculate dose. In the simulation code, absorbed dose was determined for CT scan with MeV/gram/source particle units and converted into absorbed dose with mGy/100 mAs units by conversion factor (CF).   Results: The relative difference (RD) between simulated and measured CTDI value for the PET/CT Siemens Biograph 6 at 80, 110 and 130kVp were 4.2, 2.9 and 2.3%, respectively.   Conclusion: In summarize The results showed that it is possible to determine the shape of the bowtie filter in PET/CT using an inexpensive TLDs with acceptable accuracy