T1 rho, also known as T1ρ or "spin lock" ('ρ' is the symbol for the Greek letter rho), is an MRI sequence that is being developed for use in musculoskeletal imaging. At the moment it is mostly investigational and does not yet have widespread clinical use.

The "rho" in the sequence name refers to a "ro"tating frame and the sequence has elements of both T1 and T2 weighting. After the initial 90° RF pulse, tipping the magnetization vector into the transverse plane, a second pulse is applied parallel to the tipped magnetization vector. This effectively locks the magnetization vector into the transverse plane ("ro"tating frame) without phase decay (as with T2 decay). The decay of this locked magnetization to 0 is the T1 rho time.

Normal T1 relaxation time is dependent on longitudinal magnetization interactions with surrounding environment (e.g. "spin-lattice"). T1 rho effectively manages to make transverse magnetization interact with the surrounding environment (sort of a T2 "spin-lattice").

T1 rho is dependent on the T1 and T2 of the tissue, but changing the amplitude of the spin locking pulse can also select for different properties within the tissue (e.g. slow motion in the lattice such as proteins) ³.

The sequence is sensitive for low-frequency interactions between macromolecules and bulk water, and is predominantly under investigation for use in cartilage imaging. Applications in other areas (e.g. brain, cardiac and liver imaging) are areas of active research.