A system and method for monitoring a subject are presented. The system includes a sensing device including at least one magnetic source to generate a magnetic field and an array of magnetic sensors disposed within the magnetic field. The sensor array obtains a plurality of magnetic field measurements at a plurality of locations along a vessel carrying a fluid including one or more magnetic particles. Further, the system includes a processing subsystem communicatively coupled to the sensing device, where the processing subsystem determines variations in the measurements caused by magnetization-relaxation of the magnetic particles based on a coupled model that defines behavior of the fluid in the varying magnetic field based on principles of magnetization-relaxation, bulk motion of the magnetic particles towards a determined gradient of the magnetic field, magnetostatics, and conservation of momentum. The processing subsystem estimates values of one or more desired parameters based on the determined variations.