Ultrasonic elasticity imaging

• US 6,508,768 B1
• Filed: 09/17/2001
• Issued: 01/21/2003
• Est. Priority Date: 11/22/2000
• Status: Expired due to Term

• Current Assignee: University of Kansas Medical Center
• Original Assignee: University of Kansas Medical Center
• Inventors: Hall, Timothy J., Zhu, Yanning
• Primary Examiner(s): Jaworski, Francis J.
• Application Number:
  US09/955,661
• Time in Patent Office: 491 Days
• Field of Search: 600/437, 600/440, 600/443, 600/447, 600/449, 600/589, 736/02, 735/73, 737/87
• US Class Current: 600/443
• CPC Class Codes: A61B 8/08 Detecting organic movements or changes, e.g. tumours, cysts, swellings A61B8/02 - A61B8/06 take precedence
• CPC Class Codes: A61B 8/485 involving measuring strain or elastic properties
• CPC Class Codes: G01S 7/52036 using analysis of echo signal for target characterisation
• CPC Class Codes: G01S 7/52042 determining elastic properties of the propagation medium or of the reflective target diagnostic techniques involving the measurement of strain A61B8/485
• CPC Class Codes: G01S 7/52061 Plan position indication (PPI display); C-scan display
• CPC Class Codes: G01S 7/52073 Production of cursor lines, markers or indicia by electronic means
• CPC Class Codes: G01S 7/52074 Composite displays, e.g. split-screen displays; Combination of multiple images or of images and alphanumeric tabular information

Abstract

In an ultrasound imaging system, a displacement vector is estimated for a pattern of samples throughout an imaged region of interest (ROI) by comparing two successive B-mode frames. The displacement vector is preferably estimated using block matching. Once displacement vectors are estimated for samples throughout the ROI, corresponding strain values are estimated, which indicate the degree of elasticity of the respective tissue portions. An image is then displayed showing the strain distribution within the ROI as it is stressed, for example, by the user pressing the ultrasound transducer against the patient'"'"'s body. The invention allows for both real-time and post-processed generation of elasticity displays, even based on the same body of acquired frame data. The real-time display is preferably generated using lower quality block matching whereas the post-processed elasticity calculations are carried out using high-quality techniques. Different techniques are included for adjusting the size and location of the search region of the block matching based on different measures of reliability of current displacement estimates.

Claims

1. An ultrasound imaging method comprising the following steps:

repeatedly scanning a region of interest (ROI) of a body with an ultrasound transducer;

applying varying stress to the ROI;

acquiring first ultrasound echo samples at a first stress level, and second ultrasound echo samples at a second stress level that differs from the first stress level, the first and second echo samples each representing the ROI;

selecting a reference sample set among the first ultrasound echo samples;

for each of the first samples in the reference sample set, searching within a first search region for a corresponding second sample;

for selected ones of the first echo samples not in the reference sample set, searching within a second search region for the corresponding second sample, the second search region being smaller than the first search region, measured in number of samples included in the respective search region;

estimating displacement of tissue within the ROI from the difference in estimated positions between the first and second samples and therefrom generating and displaying a first representation of a function of the estimated tissue displacement.

2. A method as in claim 1, further including the following steps:

for each sample in the reference sample set, calculating a displacement vector estimate having first and second displacement components corresponding to estimated displacement of the respective sample in a first and a second direction, respectively;

for selected samples of the first sample set, which comprise non-reference samples;

for at least selected ones of a plurality of non-reference samples selected from the first sample set;

assigning as the second displacement component the calculated second displacement component of the sample in the reference sample set with which the respective non-reference sample is collocated in the second direction; and

calculating the first displacement component corresponding to estimated displacement in the first direction.

3. A method as in claim 2, in which the first search region is two-dimensional and the second search region is one-dimensional.

4. A method as in claim 3, in which the first direction extends axially from the transducer and the second direction is a lateral direction that is perpendicular to the first direction.

5. A method as in claim 2, further including the following steps:

for selected ones of the first samples in the reference sample set;

calculating a smoothed displacement vector;

comparing the displacement vector estimate with the smoothed displacement vector; and

if the absolute difference between any component of the displacement vector estimate exceeds a corresponding component of the smoothed displacement vector by a predetermined threshold, setting that individual component of the displacement vector estimate equal to the corresponding component of the smoothed displacement vector.

6. A method as in claim 5, in which the smoothed displacement vector is calculated using curve fitting of the components of the a combined set of individual displacement vectors for the first samples in the reference sample set.

7. A method as in claim 6, in which the smoothed displacement vector for each current base sample in the reference sample set is calculated using regression of the components of the displacement vector estimates for the samples within a sample window in the reference sample set.

8. A method as in claim 2, further comprising generating and updating the first representation with a frame rate at least as fast as a predetermined minimum rate necessary for coordination by a user of transducer movement with the display of the first representation.

9. A method as in claim 8, in which the frame rate is at least one frame per second.

10. A method as in claim 2, further including the following steps:

generating the first representation with a first quality in a real-time mode;

in a post-processing, non-real time mode;

comparing the first and second ultrasound echo data samples and

therefrom calculating a post-processed displacement estimate of the tissue within the ROI; and

generating a second representation of the post-processed displacement estimate with a second quality that is greater than the first quality.

11. A method as in claim 2, further including the following steps:

estimating the tissue displacement as an estimated displacement vector at each of a plurality of reference depths; and

for each successive reference depth, calculating a measure of reliability of the estimated displacement vector and adjusting the search region used to compute a subsequent displacement vector estimate based on the reliability of the displacement vector estimate for the corresponding sample located at the previous reference depth.

12. A method as in claim 11, in which the measure of reliability is temporal.

13. A method as in claim 12, further including the following steps:

storing the first representation of the function of the estimated tissue displacement for a plurality of image frames;

calculating a frame-to-frame displacement difference value for each of a plurality of corresponding samples; and

adjusting the display of each sample having a displacement difference value greater than a predetermined maximum value.

14. A method as in claim 13, in which the step of adjusting the display comprises temporally smoothing displacement difference values greater than the predetermined maximum value.

15. A method as in claim 1, in which the predetermined function of the estimated tissue displacement is tissue strain.

16. A method as in claim 1, in which:

A) the steps of acquiring the first and second ultrasound echo samples comprise;

transmitting into the ROI a first and second series of transmit beams of ultrasound, respectively;

for each transmit beam, receiving a corresponding echo signal;

representing the echo signals as the first and second samples, the ROI thereby being represented as at least two-dimensional, first and second arrays of the first and second sample values, respectively;

B) the step of estimating displacement of tissue within the ROI comprises;

i) for at least one current base sample, chosen as one of selected ones of the first samples, selecting a base sample kernel comprising the first sample values located in a current region surrounding the current base sample;

ii) for each of a selection of second samples located in a search region,

a) selecting a second sample kernel having the same dimensions, measured in numbers of samples, as the base sample kernel;

b) calculating an energy function that is a predetermined function of the second sample values within the second sample kernel and corresponding first sample values in the base sample kernel;

iii) designating as an optimum second sample the second sample for which the energy function is optimized;

iv) calculating a displacement vector estimate between the current base sample and the optimum second sample;

C) calculating a display value of a predetermined function of the displacement vector estimate;

D) repeating steps B) and C) for each base sample, thereby generating a corresponding display value for each of the remaining base samples;

E) displaying the display values;

F) estimating the reliability of the displacement vector estimate for at least a reference one of the base samples; and

G) dynamically adjusting the search region as a function of the estimated reliability.

17. A method as defined in claim 1, in which:

the second search region is dynamically adjusted; and

the step of adjusting the search region comprises adjusting dimensions s_up, s_down, s_left, and s_right of the search region for samples in a neighborhood of a reference base sample as a function of the estimated reliability, where s_up and s_down are numbers of samples above and below the base sample, respectively, in an axial direction, and s_left and s_right samples on either side of the base sample in a lateral direction.

18. A method as in claim 17, further including the steps of setting s_up and s_down equal to a predetermined function of (ε

_max1*δ

₁) and setting s_left and s_right equal to a predetermined function of (ε

_max1*δ

₂), where;

ε

_max1 and ε

_max2 are maximum possible strains in the axial and lateral directions, respectively; and

δ

₁, and δ

₂ are distances in the axial and lateral directions, respectively, between the base sample and an assumed optimum second sample.

19. A method as in claim 18, further comprising:

selecting, as the reference sample set, the first samples located in a shallowest portion of the region of interest, where the shallowest portion has a depth of D_ROI, measured in the axial direction, and a width W_ROI, measured in the lateral direction; and

for current base samples within the shallowest portion, selecting the first search region to have the dimensions s_up, s_down, s_left, and s_right, measured in samples.

20. A method as in claim 19, in which the second search region is a sample window centered on the second sample displaced from the current base sample by the amount of the previously estimated displacement of the second sample directly above it in the axial direction.

21. A method as in 20, in which the second search region comprises a 3-by-3 sample window.

22. An ultrasound imaging method comprising the following steps:

A) repeatedly scanning a region of interest (ROI) of a body with an ultrasound transducer;

B) applying varying stress to the ROI;

C) acquiring first ultrasound echo samples at a first stress level, and second ultrasound echo samples at a second stress level that differs from the first stress level, the first and second echo data samples each representing the ROI;

D) selecting a reference sample set among first ultrasound echo samples;

E) for each of the first samples in the reference sample set, which comprise reference samples, searching within a first search region for a corresponding second sample;

F) for the first samples not in the reference sample set, which comprise non-reference samples, searching within a second search region for the corresponding second sample, the second search region being smaller than the first search region, measured in number of samples included in the respective search region;

G) for each reference sample, calculating a displacement vector estimate having first and second displacement components corresponding to estimated displacement of the respective reference sample in a first and a second direction, respectively;

H) for each non-reference sample;

i) assigning as the second displacement component the calculated second displacement component of the reference sample in the reference sample set with which the respective non-reference sample is collocated in the second direction; and

ii) calculating the first displacement component corresponding to estimated displacement in the first direction;

I) estimating displacement of tissue within the ROI from the difference in estimated positions between the first and second samples and therefrom generating and displaying a first representation of a function of the estimated tissue displacement; and

J) generating and updating the first representation with a frame rate at least as fast as a predetermined minimum rate necessary for coordination by a user of transducer movement with the display of the first representation.

23. An ultrasound imaging method comprising the following steps:

A) repeatedly scanning a region of interest (ROI) of a body with an ultrasound transducer;

B) applying varying stress to the ROI;

C) acquiring first ultrasound echo samples at a first stress level, and second ultrasound echo samples at a second stress level that differs from the first stress level, the first and second echo data samples each representing the ROI;

D) selecting a reference sample set among first ultrasound echo samples;

E) for each of the first samples in the reference sample set, which comprise reference samples, searching within a first search region for a corresponding second sample;

F) for the first samples not in the reference sample set, which comprise non-reference samples, searching within a second search region for the corresponding second sample, the second search region being smaller than the first search region, measured in number of samples included in the respective search region;

G) estimating displacement of tissue within the ROI from the difference in estimated positions between the first and second samples and therefrom generating and displaying a first representation of a function of the estimated tissue displacement;

H) for each reference sample, calculating a displacement vector estimate having first and second displacement components corresponding to estimated displacement of the respective reference sample in a first and a second direction, respectively;

I) for each non-reference sample;

i) assigning as the second displacement component the calculated second displacement component of the reference sample in the reference sample set with which the respective non-reference sample is collocated in the second direction; and

ii) calculating the first displacement component corresponding to estimated displacement in the first direction;

J) generating the first representation with a first quality in a real-time mode;

K) in a post-processing, non-real time mode;

i) comparing the first and second ultrasound echo data samples and therefrom calculating a post-processed displacement estimate of the tissue within the ROI; and

ii) generating a second representation of the post-processed displacement estimate with a second quality that is greater than the first quality.

24. An ultrasound imaging method comprising the following steps:

A) repeatedly scanning a region of interest (ROI) of a body with an ultrasound transducer;

B) applying varying stress to the ROI;

C) acquiring first ultrasound echo samples at a first stress level, and second ultrasound echo samples at a second stress level that differs from the first stress level, the first and second echo data samples each representing the ROI;

D) selecting a reference sample set among first ultrasound echo samples;

E) for each of the first samples in the reference sample set, which comprise reference samples, searching within a first search region for a corresponding second sample;

F) for the first samples not in the reference sample set, which comprise non-reference first samples, searching within a second search region for the corresponding second sample;

G) estimating displacement of tissue within the ROI from the difference in estimated positions between the first and second samples and therefrom generating and displaying a first representation of a function of the estimated tissue displacement;

in which;

H) the second search region is smaller than the first search region, measured in number of samples included in the respective search region;

I) the steps of acquiring the first and second ultrasound echo samples comprise;

i) transmitting into the ROI a first and second series of transmit beams of ultrasound, respectively;

ii) for each transmit beam, receiving a corresponding echo signal;

iii) representing the echo signals as the first and second samples, the ROI thereby being represented as at least two-dimensional, first and second arrays of the first and second sample values, respectively;

J) the step of estimating displacement of tissue within the ROI comprises;

i) for at least one current base sample, chosen as one of selected ones of the first samples, selecting a base sample kernel comprising the first sample values located in a current region surrounding the current base sample;

ii) for each second sample located in a search region, which comprises the second samples located within a second sample kernel,

a) selecting a second sample kernel having the same dimensions, measured in numbers of samples, as the base sample kernel;

b) calculating an energy function that is a predetermined function of the differences between the second sample values within the second sample kernel and corresponding first sample values in the base sample kernel;

further comprising the following steps;

K) designating as an optimum second sample the second sample for which the energy function is optimized;

L) calculating a displacement vector estimate between the current base sample and the optimum second sample;

M) calculating a display value of a predetermined function of the displacement vector estimate;

N) repeating steps B)-E) for each base sample, thereby generating a corresponding display value for each of the remaining base samples;

O) displaying the display values;

P) estimating the reliability of the displacement vector estimate for at least a reference one of the base samples; and

Q) dynamically adjusting the search region a function of the estimated reliability.

25. An ultrasound imaging system comprising:

means for applying varying stress to a region of interest (ROI) of a body;

an ultrasound transducer forming means for repeatedly scanning the ROI and for acquiring first ultrasound echo samples at a first stress level, and second ultrasound echo samples at a second stress level that differs from the first stress level, the first and second echo data samples each representing the ROI;

processing means;

for selecting a reference sample set among first ultrasound echo samples;

for each of the first samples in the reference sample set, which comprise reference samples, for searching within a first search region for a corresponding second sample;

for the first samples not in the reference sample set, which comprise non-reference samples, for searching within a second search region for the corresponding second sample, the second search region being smaller than the first search region, measured in number of samples included in the respective search region; and

displacement estimation means, included within the processing means, for estimating displacement of tissue within the ROI from the difference in estimated positions between the first and second samples and therefrom for generating and displaying a first representation of a function of the estimated tissue displacement.

26. A system as in claim 25, in which the displacement estimation means is further provided:

for each reference sample, for calculating a displacement vector estimate having first and second displacement components corresponding to estimated displacement of the respective reference sample in a first and a second direction, respectively;

for each non-reference sample;

for assigning as the second displacement component the calculated second displacement component of the reference sample in the reference sample set with which the respective non-reference sample is collocated in the second direction; and

for calculating the first displacement component corresponding to estimated displacement in the first direction.

27. A system as in claim 26, in which the displacement estimation means is further provided:

for generating the first representation with a first quality in a real-time mode;

in a post-processing, non-real time mode;

for comparing the first and second ultrasound echo data samples and therefrom calculating a post-processed displacement estimate of the tissue within the ROI; and

for generating a second representation of the post-processed displacement estimate with a second quality that is greater than the first quality.

28. A system as in claim method as in claim 25, further including:

strain estimation means for computing, for converting the estimated tissue displacement into strain values; and

display means for displaying the strain values.