Systems and methods for super-resolution ultrasound imaging
Systems and methods for ultrasound imaging capable of achieving spatial resolutions that can resolve objects smaller than 300 μm are described. Ultrasound is transmitted to and steered over a volume-of-interest that contains a microbubble contrast agent to individually excite microbubbles. Signal data is acquired in response to the transmitted ultrasound, and a plurality of images are reconstructed by beamforming the acquired signal data. The spatial resolution of the beamformed images can be further increased using techniques that determine the position of the microbubble within each image to a greater level of accuracy than the point spread function (“PSF”) of the ultrasound system. The images can also be combined to produce a single high resolution image of the volume-of-interest using, for instance, a maximum pixel projection technique.
1. A method for ultrasound imaging with an ultrasound system, the steps of the method comprising:
a) transmitting ultrasound to a focal region in a volume-of-interest that contains at least one microbubble, using the ultrasound system;
b) acquiring signal data in response to the transmitted ultrasound, using the ultrasound system;
c) reconstructing a plurality of initial images by beamforming the acquired signal data;
d) estimating a position of the at least one microbubble in each of the initial images;
e) computing phase and amplitude correction factors using the positions estimated in step d) and the initial images reconstructed in step c);
f) reconstructing a plurality of target images by beamforming the acquired signal data using the phase and amplitude correction factors computed in step e); and
g) producing an image of the volume-of-interest by combining the target images, the image of the volume-of-interest having a higher spatial resolution than the target images.
2. The method as recited in claim 1 in which step g) includes combining the target images using a maximum pixel projection technique.
3. The method as recited in claim 2 in which the target images are normalized before being combined.
4. The method as recited in claim 2 in which a three-dimensional function is fit to the target images to increase the spatial resolution of the target images before they are combined.
5. The method as recited in claim 4 in which the three-dimensional function is a three-dimensional Gaussian.
6. The method as recited in claim 4 in which the three-dimensional function is a three-dimensional point spread function of an array of receive transducer elements that forms a part of the ultrasound system.
7. The method as recited in claim 2 in which step g) includes increasing the spatial resolution of the image of the volume-of-interest using a time-varying nature of the signal data acquired in step b).
8. The method as recited in claim 2 in which step a) includes tuning a transmit pressure to excite a volume that is smaller than a full width at half maximum (FWHM) of the focal region.
9. The method as recited in claim 2 in which step a) includes using an array of transducer elements to transmit the ultrasound, and in which a phase of the transducer elements is rotated along a central axis of the array.
10. The method as recited in claim 2 in which step a) includes transmitting ultrasound using a plurality of different transmit frequencies.
11. The method as recited in claim 2 in which step b) includes acquiring the signal data using multiple different receive frequencies.