Automatic Evaluation of Collimation Field Rotation Angle in Digital Radiographic Images

In this paper we propose a method for automatic collimation border rotation angle detection. Algorithm utilizes pooling of image gradients based on their orientations to form a histogram of oriented gradients (HOG) with the goal of determining the dominant gradient orientation in the image as collimation border rotation angle. To avoid accumulation of lower magnitude gradients only a percentage of the highest magnitude gradients was used during HOG calculation. Algorithm was evaluated on image database consisting of 244 real clinical images. Performance was measured through mean absolute error (MAE) between manually determined collimation field rotation angle and the angle estimated by the algorithm. Algorithm has achieved MAE of 0.388 degrees when using 21 % of the highest magnitude gradients.