Experimental setup. (a) Schematic of phase-contrast imaging setup at the Advanced Photon Source. X-rays are produced by an undulator and monochromatized by a Si (111) double crystal monochromator. The partially coherent, monochromatic x-ray beam passes through an ion chamber and then the sample. The x-rays are converted to visible light by a scintillator screen, and the resulting image is recorded by a CCD image sensor. (b) Schematic of respirometry setup. MFC, mass flow valve and electronics control unit; S, CO2 scrubber; RC, respirometry chamber; CO2, CO2 analyzer; MFM, mass flow meter. (c) Typical plexiglass respirometry chamber. Yellow material is Kapton, used to provide an x-ray transparent window to the animal. Internal chamber volume is 0.25 ml.