The Planet Formation Imager (PFI) is a future world facility that will image the process of planetary formation. It will have an angular resolution and sensitivity sufficient to resolve sub-Hill sphere structures around newly formed giant planets orbiting solar-type stars in nearby star formation regions. We present one concept for this design consisting of twenty-seven or more 4m telescopes with kilometric baselines feeding a mid-infrared spectrograph where starlight is mixed with a frequency-comb laser. Fringe tracking will be undertaken in H-band using a fiber-fed direct detection interferometer, meaning that all beam transport is done by communications band fibers. Although heterodyne interferometry typically has lower signal-to-noise than direct detection interferometry, it has an advantage for imaging fields of view with many resolution elements, because the signal in direct detection has to be split many ways while the signal in heterodyne interferometry can be amplified prior to combining every baseline pair. We compare the performance and cost envelope of this design to a comparable direct-detection design.