A system for heating tissue in a subject, has an alternating current (AC) source coupled to a coil; a magnetically permeable passive magnetic concentrator, and positioned to alter an intensity distribution of the field;. The system has a processor with a computer model of coil and concentrator, the memory with code for simulating magnetic fields and configured to generate an intensity map describing the intensity distribution of the AC magnetic field. A method of providing an alternating current (AC) magnetic field to magnetic nanoparticles includes: providing a source of alternating current coupled to a coil; positioning the coil in a vicinity of the nanoparticles; positioning a passive concentrator near the nanoparticles; and energizing the coil to generate the AC field, the field shaped by the concentrator and heats the nanoparticles. A particular embodiment simulates the magnetic field and compares the field to parameters to verify adequate heating of the nanoparticles.