Quantitative magnetic resonance (MR) mapping is known to provide additional information compared to the conventional qualitative weighted MR images. Relaxation times such as spin-spin relaxation time (T2) and spin-lattice relaxation time (T1ρ) have been shown enable early detection of human knee cartilage degeneration. Obtaining these relaxation parameter maps, however, requires long acquisition time. Compressive sensing (CS) has been extensively studied over the last decade as one of the possible ways to accelerate MR acquisition. These studies, however, are retrospective. In this study, we perform a prospective downsampling study on T1ρ mapping of knee cartilage using CS, which implements the downsampling pattern into the in vivo MR scan sequence.