Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-11-28
2003-02-18
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S443000
Reexamination Certificate
active
06520913
ABSTRACT:
BACKGROUND OF THE INVENTION
The mechanical properties of biological tissue (e.g. parameters of elasticity) are of high interest for the characterization of the state of the tissue. In medical diagnostic, changes in the elastic properties reveal histological and possibly also pathological changes of tissue. Commonly known are the development of palpable swellings and lumps. In agriculture mechanical tissue properties are also of interest for the evaluation of the quality of meat.
PRIOR ART
Examination by palpation is inaccurate and insensitive. Elastography has a much better performance in this regard, because it technically measures the elastic tissue properties and visualizes them quantitatively or qualitatively in the form of cross sectional images [1]. In Elastography ultrasound is used, as it is used as an imaging method in medical diagnostics, but in a modified mode. In a series of sequential ultrasonic images even very small displacements and compressions inside the imaged tissue structures can be measured by the evaluation of the images sequences. A mechanical pressure to the tissue leads to a tissue compression and hence areas of different elastic properties will compress differently. The Elastography system will evaluate these compressions by a numerical comparison of the single images of the series. The strain is displayed in an image. The necessary compression of the tissue is applied externally by the transducer or internally by respiration or the heart beat. The compression is very small, usually some fractions of a millimeter in the order of 0.1%. A quantitative control of the pressure used for the compression is important. The pressure is applied in the propagation direction of the ultrasound.
A method for ultrasonic elastography on biological tissue was first described in 1991 by a paper of J. Ophir et al. [1], [2]. Ultrasound images, or more precisely the high-frequency ultrasound echo signals (HF-data) from which the ultrasound images are created in the ultrasound machine, are evaluated such, that the displacement of the tissue between two images under different tissue compression are calculated. Hence, conclusions about the elasticity can be drawn and even a quantitative reconstruction of the elastic modulus is possible.
The HF-echo signals of a compressed tissue area reach the transducer at a time, that depends on the degree of compression of the compressed tissue area. This causes differences in the propagation time of the echo signals due to the tissue compression, which leads to time shifts in corresponding segments of two echo signals that are acquired with a time delay under different compression of the tissue. Consequently, the main task of the evaluation of the HF-echoes (for the calculation of the strain) is the calculation of the time delay (=time shift) from short time intervals of the HF-echo signals. These time shifts between corresponding echo signals are calculated for at least two locations in the tissue or in the form of a two-dimensional image. For the calculation of time shifts the cross-correlation function of the HF-echo data is used. On one hand, the time shift can be found by maximizing the cross-correlation function [1]. On the other hand, the time shift can found using the phase of the correlation function, normally at zero lag [3,4].
After the calculation of the time shifts, the local strain can be computed by forming a gradient at at least one location in the tissue or also in the form of a two-dimensional image using simple linear filters [6] or by forming a difference, and optionally displayed.
All methods for the calculation of time shift use the cross-correlation function of corresponding intervals of the echo signals of the same tissue area under different compressions. The evaluation is time consuming because the integration has to be performed over the entire interval. The method proposed so far can be divided into two groups:
1) cross-correlation methods: methods, that determine the maximum of the cross-correlation function by a complete search or an iteration procedure. For this method the echo signals have to be sampled at a; very high sampling rates to be able to accurately calculate even very small time shifts. Hence, these methods are very time consuming and can not be implemented in real time or online-systems.
2) phase-based methods: methods, that estimate the time shift from the phase of a value of the cross-correlation function of the complex ultrasonic signals (analytic signals or baseband signals). The disadvantage of these methods are possible inaccuracies and the occurrence of ambiguities for large time shifts. Up to now, these ambiguities could only be prevented by additional, time consuming two dimensional pre or post-processing steps.
Due to the disadvantages of the conventional methods, it is not possible to process echo data with sufficiently accuracy to display strain images in real time. In the past it has been shown, that for ultrasonic imaging like the conventional b-mode or the Doppler sonography the real time capability is of fundamental importance for a wide acceptance of these systems. Another problem is, that the changes in the time-of-flight that result from the tissue compression depend on the distance between the tissue area and the transducer. That means, the time shift is not constant and increases monotonously with distance. The non-constant envelope of the echo signals leads to inaccuracies in the calculation of the time shifts, because for the calculation of the displacements an interval of finite length is used. In the past these inaccuracies have been reduced by a logarithmic amplitude compression of the actual HF-echo data [17]. A disadvantage of this techniques is that the phase of the signals change.
REFERENCES
[1] Ophir J., Céspedes I., Ponnekanti H., Yazdi Y., Li X.: “Elastography: A quantitative method for imaging the elasticity of biological tissues.
Ultrason. Imaging
13, 111-114, 1991
[2] Céspedes I., Ophir J., Ponnekanti H., Maklad N.: Elastography: “Elasticity imaging using ultrasound with application to muscle and breast imaging in vivo.”
Ultrason. Imaging
15, 73-88, 1993
[3] O'Donnell M., Skovoroda A. R., Shapo B. M., Emilianov S. Y.: “Internal displacement and strain imaging using ultrasonic speckle tracking”.
IEEE Trans. Ultrason., Ferroelect., Freq. Contr
, 41, 314-325, Mai 1994
[4] N. A. Cohn, S. Y. Emelianov, M. A. Lubinski, and M. A. O'Donnell, “An elasticity microscope. Part I: methods,”
IEEE Trans. Ultrason., Ferroelect., Freq. Contr
., vol. 44, pp. 1304-1319, 1997
[5] R. W. Schafer, und L. R. Rabiner, “A digital signal processing approach to interpolation,”
Proc. of the IEEE
, vol. 61, pp. 692-702, 1973
[6] F. Kallel, und J. Ophir, “A least-squares strain estimator for elastography,”
Ultrason. Imaging
19, 195-208, 1997
[7] I. C{acute over (e )}spedes, und J. Ophir, “Reduction of image noise in elastography,”
Ultrason. Imaging
, vol. 15, pp. 89-102, 1993
SUMMARY OF THE INVENTION
The object of the invention is to develop a method and a system for processing ultrasonic echo data with sufficient speed and accuracy and/or computing precision (e.g. in real time) for determining values of the tissue distention. This method should yield the same accuracy as other cross-correlation methods using a computational efficient signal processing approach.
The object is solved by a method with the features of claim 1 and an apparatus with the features of claim 6. Phase based algorithms use the phase difference between two echo signals as a measure for the time shift. For single-frequency, time-shifted signals, the phase of the cross-correlation function of two complex echo signals (analytical signals) is a linear function of the time shift. At the actual time shift to be determined, the cross-correlation function has a root (zero crossing). The slope corresponds to the oscillation frequency of the single-frequency sign
Ermert Helmut
Pesavento Andreas
Day Ursula B.
Jaworski Francis J.
Lorenz & Pesavento Ingenieurbüro für Informationstechnik
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