A computer-implemented method of calculating at least one map of a parameter of a model of the diffusive behaviour of particles within a target object by analysing nuclear magnetic resonance, NMR, diffusion signal attenuation data of the target object, wherein the method comprises: receiving the NMR diffusion signal attenuation data of the target object; analysing the received NMR diffusion signal attenuation data using the model of the diffusive behaviour of particles within the target object; and calculating the at least one map of the parameter of the model using the model, wherein: the model includes a time parameter and a space parameter, the time parameter describing temporal characteristics of the diffusive behaviour of particles in the model and the space parameter describing spatial characteristics of the diffusive behaviour of particles in the model; and the model comprises a continuous-time random walk model, and wherein: a) the space parameter describes a size of steps made by the particles in the random walk model and the space parameter is an exponent relating to a probability density function of steps made by the particles in the random walk model; and b) the time parameter describes a time between steps made by the particles in the random walk model and the time parameter describes an exponent relating to a probability density function of time between steps made by the particles in the random walk model, characterised in that the model is constrained such that the value of the time parameter and the value of the space parameter are related according to a correlation function. receiving the NMR diffusion signal attenuation data of the target object; analysing the received NMR diffusion signal attenuation data using the model of the diffusive behaviour of particles within the target object; and calculating the at least one map of the parameter of the model using the model, wherein: the model includes a time parameter and a space parameter, the time parameter describing temporal characteristics of the diffusive behaviour of particles in the model and the space parameter describing spatial characteristics of the diffusive behaviour of particles in the model; and the model comprises a continuous-time random walk model, and wherein: a) the space parameter describes a size of steps made by the particles in the random walk model and the space parameter is an exponent relating to a probability density function of steps made by the particles in the random walk model; and b) the time parameter describes a time between steps made by the particles in the random walk model and the time parameter describes an exponent relating to a probability density function of time between steps made by the particles in the random walk model, characterised in that the model is constrained such that the value of the time parameter and the value of the space parameter are related according to a correlation function. the model includes a time parameter and a space parameter, the time parameter describing temporal characteristics of the diffusive behaviour of particles in the model and the space parameter describing spatial characteristics of the diffusive behaviour of particles in the model; and the model comprises a continuous-time random walk model, and wherein: a) the space parameter describes a size of steps made by the particles in the random walk model and the space parameter is an exponent relating to a probability density function of steps made by the particles in the random walk model; and b) the time parameter describes a time between steps made by the particles in the random walk model and the time parameter describes an exponent relating to a probability density function of time between steps made by the particles in the random walk model, a) the space parameter describes a size of steps made by the particles in the random walk model and the space parameter is an exponent relating to a probability density function of steps made by the particles in the random walk model; and b) the time parameter describes a time between steps made by the particles in the random walk model and the time parameter describes an exponent relating to a probability density function of time between steps made by the particles in the random walk model, characterised in that the model is constrained such that the value of the time parameter and the value of the space parameter are related according to a correlation function.