ESTRO 37 Abstract book

ESTRO 37

S458

However, the image quality, and the low contrast visibility in particular, can vary significantly between these different protocols and systems, and this may jeopardize the intended use of the images in the clinic. A quantitative analysis of various vendors’ image protocols is therefore useful in order to ensure treatments of similar quality across different systems. There exists no commercial solution to objectively quantify the quality of low contrast images. The purpose of this work has therefore been to develop our own method which takes into account the subjective perception of the image quality while at the same time objectively quantifying it - thus allowing for comparability of the various image protocols. Material and Methods For the evaluation of the image quality, a cylindrical Difference Detail Curve (DDC) phantom with low contrast objects was produced. The phantom (diameter: 257mm) was made from PMMA and consisted of 14 different contrasts (energy independent and bubble-free sugar/gelatin solutions) each featuring 9 different diameters. A major disadvantage of the use of such a phantom is a lack of the measurement’s objectivity, since the contrast detectability threshold is influenced by the observer knowing the position of the contrast objects. One way to establish an objective value for the contrast detectability threshold is to make use of the phantom’s NPS. The 2D NPS and an artificially created phantom image containing a contrast object (with randomized positons) were linked via a Fourier analysis by the use of in-house developed software. Through an observer study, the subjective detectability of the CBCT was hence measured objectively. The influence of artefacts, chosen energy and inhomogeneous noise distribution is hereby neutralized. The objectively determined contrasts can then be tested with the phantom under clinical realistic conditions. Results Figure 1 shows the number of visible contrast objects for different systems and protocols. The tested Varian OBIs had a better image quality than Elekta XVI for all protocols, whereas the differences between Cliniac and Truebeam OBIs were negligible. Figure 2 shows a CT image of the phantom demonstrating the different contrast details.

Conclusion The slightly thickness decrease of the active layer and the different composition configuration of EBT-XD resulted in a reduced film orientation effect and LRA, as well as a sensitivity increase in high dose regions, for both photon and proton beams. However, a green channel evaluation of EBT3 showed a similar response in high dose regions as EBT-XD. The results indicate that obtained quenching corrections from EBT3 films can also be applied for the EBT-XD films. PO-0871 Evaluation of CBCT low contrast resolution using a novel phantom and Difference Detail Curve method K. Eilertsen 1 , N. Icken 2 1 Eilertsen Karsten, Medical Physics, Borgen, Norway 2 Oslo University Hospital, Medical Physics, Oslo, Norway Purpose or Objective Image-guided patient positioning and set-up verification by use of CBCT has become the dominant technology in the field of external beam radiation therapy. Recent advances in the field extend the use CBCT from merely imaging of 3D anatomy, to tumor and organ motion management as well as CBCT as basis for dose tracking in the field of adaptive radiation therapy. CBCT systems from different vendors typically include a number of image acquisition protocols and image reconstruction filters that facilitate these different clinical needs.