If Pixel Spacing is missing, then you don't know it. If it is a CR/DX image, it may contain Image Pixel Spacing, which is similar but not exactly the same (pixel spacing on imaging plate, not in the patien'ts body). If both are absent, then this information is not available.
The first value is the row spacing in mm, that is the spacing between the centers of adjacent rows, or vertical spacing. The second value is the column spacing in mm, that is the spacing between the centers of adjacent columns, or horizontal spacing.
