Quality Assurance of Registration of CT and MRI Data Sets for Treatment Planning of Radiotherapy for Head and Neck Cancers

Abstract

Abstract: Background and Purpose: We are implementing the use of MR images for head and neck radiotherapy planning, which involves their registration with CT. The quality assurance (QA) of the registration process was an initial step of this program.
Methods and Materials: A phantom was built and appropriate materials were identified to produce clinically relevant MR T1 and T2 contrast for its constituent ‘anatomy’. We performed a characterisation of the distortion detectable within our phantom. Finally, we assessed the accuracy of image registration by contouring structures in the registered/fused data sets using the treatment planning system. Each structure was contoured using each modality, in turn, blind of the other. The position, area and perimeter of each structure were assessed as a measure of accuracy of the entire image registration process.
Results: Distortion effects in the MR image were shown to be minimised by choosing a suitable (³ ± 30kHz) receiver bandwidth. Remaining distortion was deemed clinically acceptable within ±15 cm of the magnetic field isocentre. A coefficient of agreement (A) analysis gave values to be within 9% of unity, where A = (Ra ´ Rp) and Ra/p is the ratio of the area/perimeter of a particular structure on CT to that on MR.. The centre of each structure of interest agreed to within 1.8 mm.
Conclusion: A QA process has been developed to assess the accuracy of using multi-modality image registration in the planning of Radiotherapy for the Head and Neck; we believe its introduction is feasible and safe.
Keywords: radiotherapy, treatment planning, multi-modality image registration