An ultrasound system comprising: - an ultrasonic array of CMUT cells , each CMUT cell comprising a substrate , a first electrode coupled to the substrate, a flexible membrane at least partially spatially separated from the first electrode, and a second electrode coupled to the flexible membrane; - a voltage supply coupled to the array, wherein the voltage supply is arranged to operate the CMUT cells of the array in a collapsed mode and wherein the voltage supply is adapted to: - provide a bias voltage and a stimulus voltage to the electrodes of each CMUT cell ; and a capacitance sensing circuit adapted to derive an indication of capacitance of at least one of the CMUT cells and to determine an indication of a drift voltage of the CMUT cell based on the indication of capacitance, wherein the drift voltage is a shift or change in the collapse voltage of the CMUT cell, wherein the capacitance sensing circuit is adapted to: generate a test signal, the test signal having a predetermined voltage; measure an attenuated signal of the test signal, the test signal being attenuated by the impedance of at least the CMUT cell; determine an impedance of the CMUT cell based on the attenuated signal and the test signal; and determine the drift voltage of the CMUT cell based on the determined impedance; and wherein, responsive to an absolute value of the drift voltage being above a predetermined value, the voltage supply is further adapted to reverse the polarity of at least one of the bias voltage or the stimulus voltage. - an ultrasonic array of CMUT cells , each CMUT cell comprising a substrate , a first electrode coupled to the substrate, a flexible membrane at least partially spatially separated from the first electrode, and a second electrode coupled to the flexible membrane; - a voltage supply coupled to the array, wherein the voltage supply is arranged to operate the CMUT cells of the array in a collapsed mode and wherein the voltage supply is adapted to: - provide a bias voltage and a stimulus voltage to the electrodes of each CMUT cell ; and a capacitance sensing circuit adapted to derive an indication of capacitance of at least one of the CMUT cells and to determine an indication of a drift voltage of the CMUT cell based on the indication of capacitance, wherein the drift voltage is a shift or change in the collapse voltage of the CMUT cell, wherein the capacitance sensing circuit is adapted to: generate a test signal, the test signal having a predetermined voltage; measure an attenuated signal of the test signal, the test signal being attenuated by the impedance of at least the CMUT cell; determine an impedance of the CMUT cell based on the attenuated signal and the test signal; and determine the drift voltage of the CMUT cell based on the determined impedance; and wherein, responsive to an absolute value of the drift voltage being above a predetermined value, the voltage supply is further adapted to reverse the polarity of at least one of the bias voltage or the stimulus voltage. a capacitance sensing circuit adapted to derive an indication of capacitance of at least one of the CMUT cells and to determine an indication of a drift voltage of the CMUT cell based on the indication of capacitance, wherein the drift voltage is a shift or change in the collapse voltage of the CMUT cell, wherein the capacitance sensing circuit is adapted to: generate a test signal, the test signal having a predetermined voltage; measure an attenuated signal of the test signal, the test signal being attenuated by the impedance of at least the CMUT cell; determine an impedance of the CMUT cell based on the attenuated signal and the test signal; and determine the drift voltage of the CMUT cell based on the determined impedance; and wherein, responsive to an absolute value of the drift voltage being above a predetermined value, the voltage supply is further adapted to reverse the polarity of at least one of the bias voltage or the stimulus voltage. <EOS>