Program Information
Manual Estimation of Scatter and HU Effect On Field Length On Cone Beam Computed Tomography
H Erra Sriramulu1*, M Alnaimi2 , a Mousa3 , A Sharma4 , V Lyngampalli5 , (1) ,,,(2) Kuwait Cancer Control Center, Kuwait, Kuwait, (3) Kuwait Cancer Control Center, Kuwait, shuwaikh, PB No 42262, Kuwait, (4) Long Beach Memorial, Long Beach, CA, (5) Long Beach Memorial, Ca, California
Presentations
TU-L-GePD-I-5 (Tuesday, August 1, 2017) 1:15 PM - 1:45 PM Room: Imaging ePoster Lounge
Purpose: Scattered radiation profile can be split into two parts internal field part(internal macro kernel-within the field ) and external field part(external macro kernel) due to its extended scatter tail. Attempt has been made to simplify for the calculation of out of field dose using central axis dose. HU effect studied for change in length of the field.
Methods: CIRS phantom was scanned with varian OBI(on board imager) using pelvis spot light protocol and ABC is turned off. The scattered dose was measured with ion chamber (PTW, 0.125cc) and with diode. The chamber is placed in cylindrical CIRS phantom in center and measured at the center of the field and outside the field starting from, the point of inflexion 40% of the center profile. Out of field points are fit exponentially x axis being distance from center and y axis being response value. Extrapolated this curve to zero distance, gives the power of scattering for that particular field and depth. Scatter to primary ratio also obtained by measuring the response for different field sizes and extrapolated to zero field size. HU variation due to field length also observed.
Results: It was found that the exponent factor of -0.171 and that of for diode is -0.163 is not changing with field size for the external macro kernel . The initial values from the exponential fit for center of cylinder for the field sizes 3, 5.2, 9.6, 14, 18.2 are 0.7486, 0.8992, 1.1929, 1.5314, 2.408 respectively. Change in HU with length of the field also noticed.
Conclusion: For the all the fields it exponentially decreasing with exponent of -0.17 and only initial value varies. Scattering power (initial value) depends much on the variation in field size than the depth of the phantom with respect to central axis dose and HU effect studied.
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