Motion Compensation in dual energy X-ray imaging based on Deformable Image Registration

The aim of the study was to explore the use of dual energy X-ray imaging for motion management in external beam radiotherapy. We specifically developed a method to increase the quality of chest radiography in dual energy acquisition mode, when fast kVp switching is used to acquire sequential frames at low and high energy. Dual-energy imaging offers advantages such as higher spectral separation with enhanced contrast and detection quantum efficiency, but resulting images are affected by artifacts caused by motion of the anatomical structures in between sequential frames. A quality improvement method was designed and tested, relying on deformable image registration to align the high energy and low energy images from the detector. The non-rigid multi resolution deformable image registration algorithm reported here operates in multi-level, starting from the lower resolution towards a finer resolution scale. In order to separate the bony anatomy structures from the soft tissues, the registered images were processed through a dual energy weighted log subtraction algorithm. For this study, the data were acquired using the ImagingRing System that supports fast kV switching. The potential for motion compensation and weighted log subtraction are clearly characterized in the soft tissue images. The accuracy of the registration was quantified using the absolute mutual information and around 5 percent improvement was seen in the registered image compared to the image with motion artifacts. This resulted in visually improved soft tissue images, especially in terms of rib removal. Further studies will extend the initial results to improve the computation latency and accuracy. The implemented algorithm will be applied for lung tumor tracking, relying on interleaved registration of high energy and low energy images to provide motion compensation at the imaging frame rate.