Preprocessing of control portal images for patient setup verification during the treatments in external radiotherapy

The main problem in processing the control portal images is their poor qual ity. We have developed a way of improving the image quality to allow a segm entation stage. Three items were studied for this purpose the background gr adient of intensity correction, the noise reduction, and the image restorat ion. The background was corrected by subtracting a smoothed version of the image from the original. We tested 15 noise reduction methods. The most app ropriate for control portal images was found to be the truncated average. F inally, four restoration techniques were compared. The maximum a posteriori (MAP) algorithm was the most efficient. The algorithms were tested over a wide range of conditions (image quality). They produced a great improvement in anatomic detail for all the imaging systems, energies, and anatomical z ones tested. For example, the signal-to-noise ratio of a SRI-100 pelvis ima ge, acquired with 4 monitor units (MU) at 10 MV (very low quality image), i ncreased from 0.97 to 42.84 after preprocessing. We found that the improvem ent in image quality facilitated or even enabled segmentation of the contro l portal images. The percentage of segments belonging to a structure increa sed from 30% to 65% in the example cited. The preprocessing of control port al images is the first step in checking the patient setup. (C) 1999 America n Association of Physicists in Medicine. [S0094-2405(99)00912-8].