Determining material stiffness using multiple aperture ultrasound
Changes in tissue stiffness have long been associated with disease. Systems and methods for determining the stiffness of tissues using ultrasonography may include a device for inducing a propagating shear wave in tissue and tracking the speed of propagation, which is directly related to tissue stiffness and density. The speed of a propagating shear wave may be detected by imaging a tissue at a high frame rate and detecting the propagating wave as a perturbance in successive image frames relative to a baseline image of the tissue in an undisturbed state. In some embodiments, sufficiently high frame rates may be achieved by using a ping-based ultrasound imaging technique in which unfocused omni-directional pings are transmitted (in an imaging plane or in a hemisphere) into a region of interest. Receiving echoes of the omnidirectional pings with multiple receive apertures allows for substantially improved lateral resolution.
1. An ultrasound imaging system, comprising:
a first ultrasound transducer array configured to transmit an init wavefront that induces a propagating shear wave in a region of interest;
a second ultrasound transducer array configured to transmit circular waveforms into the region of interest and receive echoes of the circular waveforms; and
a processor configured to form a plurality of B-mode images of the region of interest from the circular waveforms to detect the propagating shear wave in the region of interest;
wherein the processor is configured to form a first frame of the plurality of B-mode images by combining sub-images formed by echoes received by multiple elements of the second ultrasound transducer array;
wherein the processor is configured to form a second frame of the plurality of B-mode images by:
transmitting a first unfocused ultrasound ping from a first transmitter transducer element of the second ultrasound transducer array;
forming a first image layer using echoes of only the first ultrasound ping received by elements of a first receive aperture of the second ultrasound transducer array;
forming a second image layer using echoes of only the first ultrasound ping received by elements of a second receive aperture of the second ultrasound transducer array; and
combining the first image layer and the second image layer with the electronic controller to form the first frame;
wherein the processor is further configured to calculate a speed of the propagating shear wave by identifying a first position of the shear wave in the first frame of the plurality of B-mode images, identifying a second position of the shear wave in the second frame of the plurality of B-mode images, determining a distance traveled by the shear wave between the first frame and the second frame, determining a time elapsed between the first frame and the second frame, and dividing the distance traveled by the time elapsed.
2. The system of claim 1 , wherein the first ultrasound transducer array comprises an array of phased-array elements.
3. The system of claim 1 , wherein the first ultrasound transducer array comprises an annular array of piezoelectric rings, and the processor is further configured to focus the init wavefront at various depths by adjusting phasing delays.
4. The system of claim 3 , wherein the first ultrasound transducer array comprises a switched ring transducer.
5. The system of claim 1 wherein the first ultrasound transducer array comprises a single piezoelectric transducer.
6. The system of claim 1 , wherein the plurality of B-mode images of the region of interest are formed at a frame rate between 500 fps and 4,000 fps.
7. The system of claim 1 , wherein the processor is configured to identify the propagating shear wave as a speckle pattern moving through the region of interest.
8. The system of claim 1 , wherein the processor is configured to define an image window identifying a section of the region of interest with a combination of zooming, panning, and depth selection.
9. The system of claim 8 , wherein the system is configured to display a contemporaneous B-mode image of a selected image window while calculating and displaying a modulus of elasticity of an object in the region of interest.