A real-time image processing algorithm, using a GPU with NVIDIA's CUDA technology, for a 3-D through-clothes detection radar in the THz band has been developed. Traditional CPU-based architectures lack the computing power and parallelization needed to meet the image refresh rates imposed by the radar's scanning. This solution presents a low-cost and flexible alternative that allows image refresh at a rate of more than 8 fps for images composed of 6,000 pixels, corresponding to a scanned area of 50x90 cm². The performance of this code has been profiled, with comparison against a previous CPU-executed version, and some optimizations that would enable even faster refresh have been analyzed.