An apparatus and a method for treating adipose tissue located beneath a patient's skin is disclosed herein. In some embodiments, the apparatus includes a sonotrode and an ultrasound transducer operative to induce longitudinal and/or transversal ultrasound vibrations in a least a portion of the sonotrode. In some embodiments, the apparatus provides a "cold" or "transverse" mode where ultrasound energy delivered to the patient is primarily energy of transverse ultrasound waves, and a "hot" or "longitudinal" mode where ultrasound energy delivered to the patient is primarily energy of longitudinal ultrasound waves. The longitudinal waves may be useful for 'pre-heating' tissue of the patient before delivering the transverse waves.
