A pulsed radiation beam is directed toward a body structure having a configuration or physical characteristic that changes over time, the radiation being pulsed at a pulse modulation frequency. A detector detects acoustic oscillations set up in the body resultant from the incident pulsed radiation and produces an output signal representative of one or more parameters of the acoustic oscillations. A control system controls operation of the radiation delivery and a processor to process the detector output signal. The pulse modulation frequency of the pulsed radiation is changed over a predetermined range of modulation frequencies and the processor determines the structure resonant frequency from the detector output.