We have developed a novel technique that combines MR and gamma detection for measuring T1ρ of radioactive isotopes. Gamma-ray emission is anisotropic with respect to the polarization direction of hyperpolarized radioactive nuclei (spin>1/2). When the spins are flipped into the transverse plane and subjected to a spin-locking field, the emission anisotropy also precesses and gamma detectors in the transverse plane will observe an oscillating count rate. Decay of the oscillation amplitude during SL can be used to fit T1ρ. This technique also vastly reduces the number of spins needed for a meaningful measurement since gamma-ray detection is highly efficient.