How can I filter graph from grid?

Hello, 

I have a question about image filtering. I have images of pieces of ecgs. These images contain the graph of the ecg and some other black pixels which represent the background or the grid. Is there an image filtering technique to delete the black pixels of the background and keep only the graph of the image? The images I have are like ecg35_3.png and I want to transform them to images like ecg3_4.png. Also, if a filter like this exists, is there a function in Matlab executing this filtering automatically?

Thank you very much for your time!!!