We describe a setup that allows for the measurement of high-quality images at frequencies in the far-infrared (THz) regime of the spectrum. By using water-cooled THz emitters that are biased with a 50-kHz, ±400-V square wave, rapid delay scanning, and differential lock-in detection at 50 kHz, we attain shot-noise-limited detection of THz transients. As a result, THz transients with a large dynamic range of ~5000 can be measured in 20 ms. We show that the THz imaging setup enables the time-resolved detection of the diffusion of gases.