Techniques are provided for x-ray onset detection for an intraoral dental sensor. A methodology implementing the techniques according to an embodiment includes calculating a plurality of superpixel values for each of a plurality of rows of detector pixels of a sensor. Each of the superpixel values is based on a sum of pixel values of a set of pixels associated with the superpixel value, the set of pixels selected from the detection row of a current frame of the sensor. The method also includes calculating a difference between each of the superpixel values and a corresponding stored superpixel value generated from a previous sensor frame and determining if the differences exceed a superpixel threshold value. The method further includes incrementing a hit counter in response to the determination and generating a detection signal if the hit counter exceeds a hit count threshold, otherwise proceeding to process the next detection row.