Ground glass opacification may be caused by partial filling of the alveolus with cellular material with partial replacement of air with solid material with the net density being gray rather than white if the alveolus were fully filled. The black of the airway nor the white of the vessels may blend with the gray density and hence they are not visualised in ground glass opacities. The replacement may be due to cellular infiltration including inflammatory ,benign or malignant cells without or with fluid.
Ashley Davidoff MD TheCommonVein.net lungs-0707ad

The focal nodules (middle row) and segmental consolidation in the right upper lobe (lower row) in this case is caused by total filling of the alveoli with malignant cells. This results in opacification of the alveoli and the “white” density in contrast to the “black” airways, enable the airways to be visualised as air bronchograms
Ashley Davidoff MD TheCommonVein.net 87770c01

The Ground Glass Opacity (GGO) in this case is caused by partial filling of the alveolus with malignant cells Ground glass opacification may be caused by partial filling of the alveolus with cellular material resulting in partial replacement of air with solid material. The net density is gray rather than white in the situation where the alveolus is fully replaced with cells or fluid. There is blending of the black of the subtending airways and the white of the vessels with the gray density of the cellular infiltrate and hence the normal vessels are not visualized in ground glass opacities.
Ashley Davidoff MD TheCommonVein.net 134375b01