The potential for improving the penetration depth of optical coherence tomography systems by using light sources with longer wavelengths has been known since the inception of the technique in the early 1990s.Nevertheless,the development of mid-infrared optical coherence tomography has long been challenged by the maturity and fidelity of optical components in this spectral region,resulting in slow acquisition,low sensitivity,and poor axial resolution.In this work,a mid-infrared spectral-domain optical coherence tomography system operating at a central wavelength of 4μm and an axial resolution of 8.6 μm is demonstrated.The system produces two-dimensional cross-sectional images in real time enabled by a high-brightness 0.9-to 4.7-μm mid-infrared supercontinuum source with a pulse repetition rate of 1 MHz for illumination and broadband upconversion of more than 1-μm bandwidth from 3.58-4.63 μm to 820-865 nm,where a standard 800-nm spectrometer can be used for fast detection.The images produced by the mid-infrared system are compared with those delivered by a state-of-the-art ultra-high-resolution near-infrared optical coherence tomography system operating at 1.3 μm,and the potential applications and samples suited for this technology are discussed.In doing so,the first practical mid-infrared optical coherence tomography system is demonstrated,with immediate applications in real-time non-destructive testing for the inspection of defects and thickness measurements in samples that exhibit strong scattering at shorter wavelengths.