Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Patent
1997-10-24
2000-02-22
Jaworski, Francis J.
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
A61B 800
Patent
active
060274476
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to coherent imaging systems, such as ultrasound pulse-echo imaging systems, and, more particularly, it relates to a method of phase and amplitude aberration correction. Use of this invention will enable a significant improvement of image quality. It can also be used for measurement accuracy improvement in media, such as attenuation, velocity, and blood flow velocity.
BACKGROUND OF THE INVENTION
The ultrasound pulse-echo technique is widely used in medical imaging. This imaging method currently uses an array of transducer elements to transmit a focused beam into the body, and each element then becomes a receiver to collect the echoes. The received echoes from each element are dynamically focused to form an image. Focusing on transmission and on reception is done assuming that the velocity inside the body is uniform, and is usually assumed to be 1540 ms.sup.-1. Unfortunately, the velocity inside the body is not constant; it varies from 1470 ms.sup.-1 in fat to greater than 1600 ms.sup.-1 in some other tissues, such as collagen, and 1540 ms.sup.-1 is therefore an approximation. This variation will result in increased side lobes and degraded lateral resolution. Aberration phenomena and their extent in tissue have been evaluated in many works. The degradation is tolerable if the frequency is not very high and the aperture is not very large. Recently, however, higher frequencies and larger apertures have been used to improve lateral resolution of ultrasound images. But the resolution improvement can not be achieved beyond a certain limit, because both larger aperture and higher frequency make the system more sensitive to propagation velocity variations in the body. It is therefore one of the major difficulties in improving lateral resolution of ultrasound imaging systems.
Methods have been developed to correct aberrations in such areas as atmospheric effects in astronomical imaging, antenna-position errors in radar and microwave imaging, weathered-layer-effect in seismic imaging, and subcutaneous body layers effects in medical ultrasound imaging. Some of these methods use the wavefront from a special target such as a dominant point target or a spectacular reflecting plane, to measure the phase-aberration profile. Some methods measure the phase-aberration profile using signals from arbitrary target distributions, and signal redundancy is the basic principle behind these methods. Even though the signal redundancy principle has been successfully used in astronomical imaging to correct aberrations introduced by atmospheric turbulence, its application in medical ultrasound imaging systems has achieved only moderate success because the near-field effect has not been analysed in sufficient detail.
Amplitude aberrations in ultrasound medical imaging have also been previously reported, particularly for imaging complex tissue structures such as the female breast. Amplitude aberration will influence the quality of images, even though it is perhaps not as important as phase. In some cases, amplitude aberration correction is needed in addition to phase aberration correction to obtain a good image. The present invention seeks to also incorporate an amplitude aberration correction with the phase aberration correction algorithm.
Astronomical imaging systems are passive and incoherent. Phase and amplitude aberrations caused by the atmosphere make it difficult to achieve diffraction-limited resolution on the ground. A widely used aberration-correction method is the direct wavefront measurement method. This is used when there is a dominant bright star either present or artificially created [R. K. Tyson, "Principle of adaptive optics." Academic Press, ch. 5, 1991]. A plane wavefront at the aperture should be observed since the dominant point target is in the far field of the imagine aperture. Any departure from a plane wavefront is caused by phase aberration. Many methods can be used to measure a wavefront, one of which is the interferometer wavefront sensing method. After measuring the wa
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Commonwealth Scientific and Industrial Research Organisation
Jaworski Francis J.
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