An electric toothbrush includes a handle and a brush head attached to one end of the handle. The brush head includes a bristle unit and an ultrasound sensor. The bristle unit cleans teeth and the ultrasound sensor acquires ultrasound data of the teeth. A portable tooth decay detection device includes a handle and an ultrasound sensor attached to one end of the handle. The ultrasound sensor emits and receives ultrasound signal at a frequency of higher than 20 KHz and acquires ultrasound data of teeth.
