Before revision:  Unlike other label-free techniques for imaging microtubules, in TIE microscopy the background is naturally removed when taking the difference of two defocused images, so the need to frequently update a background subtraction image is eliminated.
After revision:  Unlike other label-free techniques for imaging microtubules, in TIE microscopy background removal is a natural consequence of taking the difference of two defocused images, so the need to frequently update a background subtraction image is eliminated.