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Investigation of Phantom Misalignment Effect On the Dosimetry of Head Examination

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M Masoudi

M Masoudi1*, S Sina2 , R Faghihi1,2 , (1) School of Mechanical engineering, Shiraz University, Shiraz, Iran (2) Radiation Research Center, Shiraz University, Shiraz, Fars


TU-C2-GePD-I-4 (Tuesday, August 1, 2017) 10:00 AM - 10:30 AM Room: Imaging ePoster Lounge

Purpose: Patient misalignment in CT examinations is occurred due to deficiency of the alignment lasers, lack of radiologist awareness and patient movement. The aim of the present study is to evaluate dose profile along the z-axis in the standard head CT dosimetry phantom and CTDIW variations in the present of phantom misalignment.

Methods: A set of Monte Carlo (MC) simulations were made using GATE toolkit to model a GE LightSpeed 16 slice CT scanner. This model was consisting of: CT x-ray source (x-ray spectra at 80, 100 and 120 kVp, flat and bowtie filters, fan and cone angles for determination of head scan field of view and beam collimation of 10 mm, rotation of the source around the isocenter), head CTDI phantom with its central and peripheral holes and dosimetry calculations. The CT scanner model was validated and was used for more investigation on the misalignment issue.

Results: The results of this study demonstrate that with increasing the phantom position from the isocenter toward the edges of the table: (1) large difference observed in the peripheral CTDI at each misalignment position with the maximum of ~11.5 mGy occurred at 4 cm from the isocenter, (2) peripheral CTDI at 3, 6 and 12 o’clock positions of the phantom were decreased and was increased at 9 o’clock position, (3) a small increase were occurred in the central CTDI value by ~2% on average and for peripheral holes ~9% (4) the CTDIW value was decreased by ~ 1 mGy, (5) small change were observed in only peripheral dose profiles.

Conclusion: In conclusion, patient misalignment in head examinations up to 4 cm from the isocenter has no significant effect on dose profiles and CTDIW values. However, the variation in the peripheral CTDI¬ values should be taking into consideration in CT dosimetry procedures.

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