T_1ρ magnetic resonance: basic physics principles and applications in knee and intervertebral disc imaging

T_1ρ relaxation time provides a new contrast mechanism that differs from T₁- and T₂-weighted contrast, and is useful to study low-frequency motional processes and chemical exchange in biological tissues. T_1ρ imaging can be performed in the forms of T_1ρ-weighted image, T_1ρ mapping and T_1ρ dispersion. T_1ρ imaging, particularly at low spin-lock frequency, is sensitive to B₀ and B₁ inhomogeneity. Various composite spin-lock pulses have been proposed to alleviate the influence of field inhomogeneity so as to reduce the banding-like spin-lock artifacts. T_1ρ imaging could be specific absorption rate (SAR) intensive and time consuming. Efforts to address these issues and speed-up data acquisition are being explored to facilitate wider clinical applications. This paper reviews the T_1ρ imaging’s basic physic principles, as well as its application for cartilage imaging and intervertebral disc imaging. Compared to more established T₂ relaxation time, it has been shown that T_1ρ provides more sensitive detection of proteoglycan (PG) loss at early stages of cartilage degeneration. T_1ρ has also been shown to provide more sensitive evaluation of annulus fibrosis (AF) degeneration of the discs.