In a method for preparing a hyperpolarised sample, for example for a magnetic resonance procedure, a starting solution comprising alpha-ketoglutaric acid and 13C-labelled molecules is frozen to form a frozen solution. The solution is irradiated with ultraviolet and/or visible radiation, to generate free radicals. The frozen solution is then hyperpolarised by applying a magnetic field to the solution while irradiating it with frequency-modulated microwave radiation.