Recent advances in high-performance CMOS-based THz direct detectors enable monolithic integration of detector front ends, signal processing, and readout electronics for room-temperature real-time imaging and focal-plane arrays. A comprehensive cross-layer analysis of device-level nonlinearities, antenna-transistor codesign, impedance matching, noise mitigation and pixel-level readout circuitry is presented for high-sensitivity (sub-)THz imaging applications, particularly for concealed object detection and biosensors. Insights from this analysis enable broadband nonresonant THz detection beyond the transistor ƒT. Embedded amplification and innovative high-Q filtering architectures facilitate better than 500fps video-rate imaging with subnanowatt noise floors.