Apparatus and methods for treating body tissue by use of thermal treatment material. The thermal treatment material to be injected into target tissue of a body includes: a carrier substrate; a plurality of first particles operative to generate thermal energy in response to an alternating electromagnetic field applied external to the body; and a plurality of second particles, each of the second particles having a core and a coating surrounding the core. The coating is dissolved at a preset temperature by the thermal energy so that the visibility of the core in an external imaging system is affected as the coating is dissolved to expose the core. The variation of the visibility can be used as an indicator to determine if the material has reached the preset temperature.