Diffuse optical imaging (DOI) offers valuable insights into scattering mediums, but the quest for high-resolution imaging often requires dense sampling strategies, leading to higher imaging errors and lengthy acquisition times. This work introduces Space-Time Encoded Modulation (STEM), a novel light modulation scheme enabling low-noise, high-resolution imaging with single-pixel detectors. In STEM, a laser illuminates the sample, and the transmitted light is detected using a single pixel detector. The detected image is partitioned into a two-dimensional array of sub-images, each encoded with a unique quasi-orthogonal code. These coded sub-images represent light transmission at specific locations along the sample boundary. A single-pixel detector then measures their combined transmission. By virtue of their quasi-orthogonality, the relative strength of each sub-image can be measured, enabling image formation. In this paper, we present a comprehensive mathematical description and experimental validation of the STEM method. Compared to traditional raster scanning, STEM significantly enhances imaging quality, reducing imaging errors by up to 60% and yielding a 3.5-fold increase in reconstruction contrast.