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We demonstrate terahertz (THz) frequency imaging and sensing using a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilising the effect of self-mixing in the THz QCL, and specifically by monitoring perturbations to the voltage across the QCL induced by light reflected from an external object back into the laser cavity. Self-mixing offers high sensitivity, a potentially fast response, and a simple, compact optical design. We show that it can be used to obtain high-resolution reflection images of exemplar structures, as well as for the measurement of the displacement of a remote target, both with and without opaque (in the visible spectrum) materials in the beam path. We also demonstrate displacement sensing over a stand-off distance of 7m through air.