Ultrasound system and ultrasonic pulse transmission method

1. An ultrasound system comprising:

a probe including an array of CMUT (capacitive micromachined ultrasound transducer) cells, each cell configured to operate in a collapsed state and comprising:

a substrate;

a flexible membrane including a central region, wherein the substrate is spatially separated from the flexible membrane by a gap, wherein the central region is configured to contact the substrate in the collapsed state;

an electrode arrangement comprising a first electrode in contact with the substrate and a second electrode in contact with the flexible membrane; and

a mass element disposed at the central region of the flexible membrane such that at least a portion of the flexible membrane is disposed between the second electrode and the mass element, wherein the mass element is different from the flexible membrane, the first electrode, and the second electrode, wherein the mass element is configured to force the central region to remain in contact with the substrate while the flexible membrane resonates at a frequency in the collapsed state such that the central region does not temporarily release from the substrate while the flexible membrane resonates at the frequency in the collapsed state; and

a voltage supply coupled to said probe and adapted to, in a transmission mode of the ultrasound system, provide the respective electrode arrangement of at least some of the CMUT cells with a voltage including:

a bias voltage component driving the at least some of the CMUT cells into the collapsed state; and

a stimulus component for resonating the respective flexible membrane of the at least some of the CMUT cells at the frequency in said collapsed state, wherein the stimulus component comprises the frequency, wherein the mass element of each of the at least some CMUT cells permits release of said central region from the substrate upon reduction or removal of said bias voltage component.

2. The ultrasound system of claim 1 , wherein each electrode arrangement further comprises a third electrode carried by the substrate, wherein the third electrode is located in between the first electrode and the second electrode and is electrically insulated from the first electrode by a dielectric layer, wherein the voltage supply is adapted to apply the stimulus component across the respective first and second electrodes and to apply the bias voltage component to the respective third electrode of the at least some CMUT cells.

3. The ultrasound system of claim 1 , wherein the voltage supply is further adapted to provide the respective arrangement of at least some of the CMUT cells with a further voltage that forces the at least some CMUT cells in the collapsed state during a reception mode of said probe.

4. The ultrasound system of claim 1 , wherein the voltage supply comprises:

a first stage adapted to generate the bias voltage component of said voltage during said transmission mode, wherein the bias voltage component is sufficient to force the at least some CMUT cells in the collapsed state; and

a second stage adapted to generate the stimulus component of said voltage.

5. The ultrasound system of claim 1 , wherein the flexible membrane comprises a first material and the mass element comprises a second material, the second material having a higher density than the first material.

6. The ultrasound system of claim 5 , wherein the second material is a metal or metal alloy, or a heavy non-metal material.

7. The ultrasound system of claim 1 , wherein the mass element is positioned on the flexible membrane.

8. The ultrasound system of claim 1 , wherein the mass element is integrated in the flexible membrane.

9. The ultrasound system of claim 1 , wherein the mass element has a cylindrical or annular shape.

10. The ultrasound system of claim 1 , wherein the ultrasound system is an ultrasound diagnostic imaging system or an ultrasound therapeutic system.

11. A method of ultrasonic pulse transmission, comprising:

providing a probe including an array of CMUT (capacitive micromachined ultrasound transducer) cells, each cell configured to operate in a collapsed state and comprising:

a substrate;

a flexible membrane including a central region, wherein the substrate is spatially separated from the flexible membrane by a gap, wherein the central region is configured to contact the substrate in the collapsed state;

an electrode arrangement comprising a first electrode in contact with the substrate and a second electrode in contact with the flexible membrane; and

a mass element disposed at the central region of the flexible membrane such that at least a portion of the flexible membrane is disposed between the second electrode and the mass element, wherein the mass element is different from the flexible membrane, the first electrode, and the second electrode, wherein the mass element is configured to force the central region to remain in contact with the substrate while the flexible membrane resonates at a frequency in the collapsed state such that the central region does not temporarily release from the substrate while the flexible membrane resonates at the frequency in the collapsed state; and

providing the respective electrode arrangement of at least some of the CMUT cells with a voltage including:

a bias voltage component driving the at least some of the CMUT cells into the collapsed state; and

a stimulus component for resonating the respective flexible membrane of the at least some CMUT cells at the frequency in said collapsed state, wherein the stimulus component comprises the frequency, wherein the mass element of each of at least some CMUT cells permits release of said central region from the substrate upon reduction or removal of said bias voltage component.

12. The method of claim 11 , wherein each electrode arrangement further comprises a third electrode carried by the substrate, wherein the third electrode is located in between the first electrode and the second electrode and is electrically insulated from the first electrode by a dielectric layer, the method further comprising applying the stimulus component across the respective first and second electrodes and applying the bias voltage component to the respective third electrode of the at least some CMUT cells.

13. The method of claim 11 , further comprising periodically altering the frequency of the stimulus component to periodically alter a resonance frequency of the at least some CMUT cells.

14. The method of claim 11 , further comprising periodically altering the bias voltage component driving the at least some of the CMUT cells into the collapsed state to alter the respective area of the central region of the at least some CMUT cells.

15. The method of claim 11 , further comprising providing, in a reception mode, the respective electrode arrangement of at least some of the CMUT cells with a further voltage forcing the at least some CMUT cells in the collapsed state.

16. The ultrasound system of claim 1 , wherein the mass element is positioned between the portion of the flexible membrane and an additional portion of the flexible membrane.