I'm not familiar with medical imaging equipment specifically, but generally speaking without knowing a lot of details about the systems, there is no formula because the resolution limit (actually it's not a number, it's a curve like a point-spread function or a modulation transfer function) of any imaging system depends on a number of factors. Wavelength is often not the most important factor, you need to consider what element does the imaging and how it works, how well does it image (for example even in a telescope the angular resolution is rarely determined solely by diffraction, it's also determined by alignment, mirror/lens quality, atmospheric turbulence, etc.), what feature contrast you're trying to see, detector spatial resolution and detected photon statistics, background statistics, and so on.
So there's no simple answer to your question, except to go to the manufacturer of whatever instruments you're using and see if they can provide you with something useful. I use x-ray imaging systems all the time for other applications, but the spatial resolution I get depends entirely on all these details, and might vary from 5-500 microns, vastly larger than the wavelengths used - I could do better but only in some specialized applications, and I generally don't have the photon statistics to make use of it anyways. If I was imaging a static object at a synchrotron, the game would change entirely, and so would all the numbers.