A receiver includes a low noise amplifier and multiple pairs of mixers. Each of the image forming units has a rectangular concave portion along its longitudinal direction which contains a handle made of an elastic rectangular-shaped plate. The controller controls the amplifier based on the received control information so as to selectively energize the associated windings in the module. A shift in a position of a focal spot of the x-ray tube is then determined utilizing the read z-ratio. The first and second transmitting electrodes are respectively disposed on two sides of the first receiving electrode, the third and fourth transmitting electrodes are respectively disposed on two sides of the third receiving electrode, and the second and third transmitting electrode are respectively disposed on two sides of the second receiving electrode. A date and time when the battery is charged may be included in the charging-history.