1 - Push in the frame buffer button «Save Image» (in the form of a 3-inch floppy).
2 - opens the Save As dialog box. In it you choose a convenient format and save.
In any case, do not interrupt and do not close the render 3ds Max, and open the saved image and make sure that he's okay and you can continue to work with him.
If everything is OK, then interrupt the render.
As for quality, it is not as independent as you save images. What do you render, you and treasure. The main format without compression, for example, png .
If you want to continue and not dorenderit dorenderenny piece, just render the region 😉