Sequential Contrast Enhancement of Portal Images: Study of the Influence on Image Quality and Clinical Usefulness

Portal images, acquired by electronic portal imaging devices (EPID), suffer from poor spatial resolution and contrast. Since portal images are used to verify the position of the patient within the treatment field, image processing could be applied to improve the visualization of anatomical details. The purpose of the present study is to examine the influence of different sequential enhancement techniques on image quality and clinical usefulness. Portal images of 100 patients were acquired by an a-Si based EPID and imported for enhancement into the PIPS-PRO image processing system. We applied 12 different combinations of 3-steps sequential image processing. The first processing step is the application of a single common contrast enhancement technique. The second processing step is noise reduction. For the final step edge sharpening is applied. The original and processed images were assessed by 3 radiation oncologists. They had to estimate: a) if enhanced images were better than the original (according to the visibility of important anatomical structures) and b) the enhancement usefulness. The majority of the processed portal images were judged to have a superior image quality compared to original portal images and in most cases the observers required less time for identifying important anatomical structures. Observer deviations were smaller in the series of processed images. The clinical usefulness was found to vary depending on the anatomical structure to be observed.