Clustering of imaging modalities by resolution and energy. Imaging modalities span a range of resolutions and energies, which both contribute to differences in tissue penetration. Very high and very low energy imaging modalities yield high tissue penetration and, in most cases, macroscopic imaging resolution. At intermediate energies, such as fluorescence and bioluminescence, both macroscopic and microscopic imaging resolutions are achievable, but tissue penetration is limited. NIR = near-infrared, UV = ultraviolet, Vis = visible.