For the telescope number and size of the PFI proposal (20 x 8m), the Q-band (around 25 mu m) was shown to be an optimal complement to submm- and optical interferometry, to detect substructures in proto-planetary disks. Here, broadband medium-resolution heterodyne would be an appropriate option, considering recent experimental findings on a quantum-limit heterodyne detection (HD) scheme. Under high thermal background and the very small beam filling factors in interferometry, HD was shown to be more sensitive than direct detection for the same bandwidth for wavelengths larger 3-8 micron, depending on the conditions. A new class of heterodyne mixers based on graphene could extend the instantaneous IF-bandwidth up to hundred GHz, for which photonic correlators may be developed since a resolution of R=30000 would be sufficient at 10 mu m. Such large IF-bandwidths could then even be multiplied in "dispersed heterodyne receivers" using optical frequency combs and photonic techniques derived from the DWDM technology.