Display equalization: a new display method for portal images

A method of displaying portal images which permits the user to optimize the display contrast (windowing and leveling) without clipping parts of the image has been developed. The method finds the average signal (brightness) in small regions of the image, generally 1/64th of the total image area. The difference between the average signal in each small region and the global average over the entire image is subtracted from the original image. This eliminates changes in average signal that occur over large spatial dimensions, such as those due to varying patient thickness, without obscuring changes in signal that occur over small spatial dimensions, such as those due to anatomic structures. The technique is conceptually simple, does not introduce any visually distracting noise into the image, and is not computationally intensive. The display method has been shown to work well with images acquired from a TV camera-based imaging system used in radiation therapy.