WO2011027253A1

WO2011027253A1 - Ultrasonic elastographic strain imaging with fade control

Info

Publication number: WO2011027253A1
Application number: PCT/IB2010/053733
Authority: WO
Inventors: Unmin Bae; Junzheng Man
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2009-09-04
Filing date: 2010-08-18
Publication date: 2011-03-10
Also published as: CN102483455A; RU2012112934A; JP2013503680A; EP2473866A1

Abstract

An ultrasonic diagnostic imaging system produces a strain image in which a user can delineate a point or region of reference tissue. A strain ratio image is then produced with points in the image being a ratio of the strain value at the point over a strain value for reference tissue. Strain ratio values in the strain ratio image are color-coded in correspondence with the strain ratio values and a color bar indicates the color coding. Strain ratio values above a range or threshold can be distinctively colored to be easily identifiable. A strain ratio image can be overlaid in anatomical registration with a corresponding B mode image, and one or both images faded back and forth to see the B mode image alone, a solid strain ratio image, or a semi-transparent combination of the two images.

Description

ULTRASONIC ELASTOGRAPHIC

STRAIN IMAGING WITH FADE CONTROL

This invention relates to medical diagnostic ultrasound systems and, in particular, to ultrasound systems which assess the stiffness of tissue regions in the body by elastography .

Elastography is the assessment of the elastic properties of tissue in the body. It has been found that the stiffness of tissue in the body can give an indication of whether the tissue may be malignant or benign. The female breast, for instance, can contain a variety of different lumps, cysts, and other growths, some of which may be malignant and some of which may be benign. To spare the patient from needless biopsies, and perform them when needed, ultrasound is frequently used to assess tissue characteristics to determine whether to biopsy suspect tissue. Elastography can be performed to determine whether the breast contains softer or harder (stiffer) regions. Since stiffer tissue correlates more greatly with malignant masses, the identification of regions of stiffer tissue can indicate a need to make a definitive diagnosis by biopsy.

A problem posed by elastography is the need to measure quantifiable properties of tissue

noninvasively within the body. This means that the properties of the target tissue cannot be measured directly at the site of the tissue, but only through measurements made at the surface of the body through intervening tissues. Accordingly it is desirable to simplify the problem and make certain approximations and assumptions that will lead to valid data and analyses. One set of assumptions that is frequently made is that the tissue being examined is homogeneous and isotropic. These assumptions enable certain property of materials equations to be applied to the problem, Poisson's ratio and Young's modulus.

Poisson's ratio is the ratio, when a sample is stretched or compressed in a given direction, of the expansion or contraction (strain) normal to the stretching or compressing force, to the expansion or contraction axially in the direction of the force. A related measure is Young's modulus, which is a measure of stiffness, and is defined as the ratio of the uniaxial stress (pressure) applied to a sample over the resulting uniaxial strain (deformation) . However, the stress component at target tissue is generally unknown and difficult to measure

noninvasively .

Since each of these material properties can be expressed as a function of deformation (strain) , a number of researchers have concentrated on assessing the strain or deformation exhibited by tissue at different applied pressures. While strain has been shown to be a useful parameter, its shortcoming is that it varies with the applied pressure. It is thus technique-dependent, with different results obtained by researchers who apply different pressures to the tissue or use different techniques for applying pressure. A consequence is that strain measurements from patient to patient or from exam to exam are not directly comparable. A patient subjected to one type of pressure will exhibit different strain

(deformation) values as compared to another patient subjected to a different form or amount of pressure.

To compensate for the pressure variation between patients, strain of a reference point indicating the pressure level can be used to normalize strain of a target region. In addition, a strain ratio between normal tissue and tumor can indicate their relative stiffness, assuming similar pressure between two regions within a patient.

Researchers have reported that the comparative assessment of normal and suspect tissues can be of diagnostic significance. Accordingly the present inventors calculate a strain ratio, which is the ratio of the strain of normal or reference tissue over the strain of a target tissue . (The prior art has used the term "strain ratio" to refer to the ratio of lateral to axial strain in the manner of Poisson's ratio, but that is not how the term is used in this patent.) Malignant lesions tend to be stiffer than benign lesions. Strain ratios for malignant lesions may be considerably higher than those of benign lesions. By measuring the strain ratio, the relative stiffness of the target tissue and likelihood of malignancy can be ascertained.

The calculation of strain ratio in target tissue in an image can be performed by indicating or marking the target tissue with a cursor, indicating or marking normal tissue, measuring the strain at each location in the image field, then calculating the strain ratio. However, this process can be time consuming, as the clinician may need to make the measurement at a number of points in the image field. Furthermore, the clinician may want to comparatively assess the appearance of a tumor or other pathology in a B mode image with its spatially corresponding strain characteristics, a problem made difficult when the pathology is only faintly visible in the B mode image. In accordance with the principles of the present invention, an ultrasound system forms an anatomical strain image of strain values in an image field. The system also forms a corresponding anatomical B mode image, and the two images are overlaid for display. A fade control of the system is operative to vary the relative transparency of the overlaid B mode and strain images so that their boundaries, stiffness, and other characteristics can be comparatively assessed.

In the drawings:

FIGURE 1 illustrates in block diagram form an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention .

FIGURE 2 illustrates the steps of a method for producing strain ratio images of an image field.

FIGURE 3 illustrates a B mode image next to a strain image of the same tissue, with a cursor used to designate a point of normal tissue for

determination of a strain ratio image.

FIGURE 4 illustrates a B mode image next to a strain image of the same tissue, with a circle graphic used to designate a region of normal tissue for determination of a strain ratio image.

FIGURE 5 illustrates a strain image window overlaying a larger B mode image with a square graphic used to designate a region of normal tissue for determination of a strain ratio image.

FIGURE 6 illustrates a strain ratio image in the image window of FIGURE 6 overlaying its corresponding B mode image .

FIGURE 7 illustrates another image display in which the B mode image of the target region and the B mode image with an overlaying strain ratio image are shown side-by-side, along with the stiffness color bar of the strain ratio image.

Referring first to FIGURE 1, an ultrasound system constructed in accordance with the principles of the present invention is shown in block diagram form. An ultrasound probe 10 has an array transducer 12 for transmitting ultrasound waves to and receiving echoes from a region of the body. The array

transducer can be a one-dimensional array of

transducer elements or a two-dimensional array of transducer elements for scanning a two dimensional image field or a three dimensional image field in the body. The elements of the array transducer are driven by a transmit beamformer 16 which controls the steering, focusing and penetration of transmit beams from the array. A receive beamformer 18 receives echoes from the transducer elements and combines them to form coherent echo signals from points in the image field. The transmit and receive beamformers are coupled to the transducer array elements by transmit/receive switches 14 which protect sensitive receive circuitry during transmission. A beamformer controller 20 synchronizes and controls the operation of the beamformers.

The received echo signals are demodulated into quadrature (I and Q) samples by a quadrature bandpass (QBP) filter 22. The QBP filter can also provide band limiting and bandpass filtering of the received signals. The received signals may then undergo further signal processing such as harmonic separation and frequency compounding by a signal processor 24. The processed echo signals are applied to a detector 25 which performs amplitude detection of the echo signals by the equation ( I ² + Q² )¹¹² for a B mode processor 26, and to a Doppler processor 28 for

Doppler shift detection of points in the image field.

In accordance with the principles of the present invention the baseband I and Q echo signals of each image frame (each scan of the image field) are applied to a frame memory 30. The outputs of the B mode processor 26 and the Doppler processor 28 are also coupled to the frame memory 30. The frame memory stores consecutive samplings of the image field on a spatial basis for the calculation of strain by a strain estimator 32 from the frame-to- frame displacement of particles in the image field; strain is calculated as a spatial derivative of displacement. Strain may be calculated from

radiofrequency (RF) or baseband I and Q data, and may also be calculated from amplitude-detected (B mode) or tissue Doppler data. Any form of strain

calculation such as strain, the ratio of lateral to axial strain, and strain velocity estimation may be employed. For instance, the echoes received at a common point in consecutive frames may be correlated to estimate displacement at the point. If no motion is present at the point, the echoes from consecutive frames will be the same. If motion is present, the echoes will be different and the motion vector indicates the displacement. US Pat. 6,558,324 (Von Behren et al . ) describes both amplitude and phase sensitive techniques for estimating strain and employs speckle tracking for strain estimation through block matching and correlation. US Pat.

5,524,636 (Sarvazyan et al . ) also uses speckle tracking to perform elastography and US Pat.

6,527,717 (Jackson et al . ) determines tissue movement by correlating speckle. US Pat. 5,474,070 (Ophir et al . ) estimates tissue compression from time-shifted differences among segments of A-line pairs. US Pat. 6,099,471 (Torp et al . ) describes the estimation of strain velocity calculated as a gradient of tissue velocity. US Pat. 5,800,356 (Criton et al . ) describes the use of the Doppler vector to select points for strain estimation in the direction of the applied force. Preferably a phase-sensitive

technique is used since, as recognized by Von Behren et al . , r.f. data will typically yield the most accurate estimates of strain.

The tissue displacement may be caused by varying the pressure applied to the body by the probe, or preferably by displacement caused by physiological motion of the body. Any other type of compression source can alternatively be used, including

mechanical vibration or acoustic radiation force impulse. For instance, the displacement may be caused by motion of the chest wall during breathing or the blood flow pulsation of a nearby blood vessel such as the carotid or hepatic artery. Another reason for the preference of strain estimation with phase-sensitive techniques is that the slight motion produced by these physiological activities and even from the small, virtually imperceptible motion occurring while holding a probe against the body can be sensed and used to estimate strain by the strain estimator 32.

The strain estimator 32 produces an estimated strain value at each point in the image field, and these values are stored as a strain image of the image field at 34. The strain image is coupled to an image processor 42, as are outputs of the B mode processor 26 and the Doppler processor 28. The image processor processes the image data from these sources, e.g., by scan conversion to a desired image format, image overlay, etc., and produces an image for display on a display 50. As shown in the following drawings, one format for display of a strain image is overlaid on a B mode image for structural orientation.

In accordance with the principles of the present invention the strain image is also used to produce a strain ratio image at 36. The strain ratio image is produced by dividing each strain value of the strain image by a strain value for normal tissue. This value may be provided automatically as by averaging or taking the mean or median value of a plurality of strain values, such as the strain values in a region in a corner of an image (on the assumption that the user will position a suspect tumor in the center of the image.) In an implementation of the present invention the strain value for normal tissue is taken from an indication of normal tissue in an image which is indicated by a user. For this purpose the strain ratio image is responsive to a reference cursor from a control panel 40 manipulated by the user to

indicate a point or region of normal tissue in an image. Each strain value in the strain image is divided by a strain value of normal tissue to produce a strain ratio image at 36.

To better delineate suspect tissue region in the strain ratio image, the user may manipulate a control of the control panel 40 to set a threshold or range of values against which the strain ratio values are compared. Strain ratio values which exceed the threshold or the range of values are uniquely

highlighted in the strain ratio image as by

displaying them with unique colors or brightnesses. For example the user can set the threshold at 5, and all points in the strain ratio display with a value of 5 or greater, indicative of high stiffness, can be displayed in a bright red color. The user can quickly spot suspect regions in the image from the distinctive bright red color in the strain ratio image .

In accordance with a further aspect of the present invention the user can manipulate a fade control on the control panel 40 which is coupled to the image processor 42. When the image processor displays a strain ratio image overlaying the

corresponding portion of a B mode image, the fade control enables the user to adjust the relative transparency of the B mode and strain ratio images. The user can fade the strain ratio image to be completely transparent to see the corresponding structural image alone, or can fade the B mode tissue image to see only the strain ratio image, or an intermediate transparency setting for the two

overlaid images.

FIGURE 2 illustrates a process for producing strain ratio images using the ultrasound system described above. At 60 a frame of ultrasound data is acquired at a reference time ti. At 60 another frame of ultrasound data is acquired at a reference time t₂ ·

The ultrasound data of the two frames is then used at 64 to estimate strain (displacement) in either two dimensions or three dimensions, depending upon the nature of the data. Any strain estimation technique may be used including any of those described above.

The estimated strain values are spatially mapped to produce a strain image at 66. From the strain image the user designates normal tissue at 68.

Alternatively the user can designate the normal tissue in another image such as a B mode image and the spatially corresponding strain values of the strain image are used. At 70 the user sets the strain ratio threshold above which strain ratio values in the strain ratio image are to be

highlighted. At 72 the strain ratio is calculated from the strain values and a normal tissue strain value and color-coded to produce a color strain ratio image, which is displayed at 74. At 76 the user may optionally fade overlaid strain ratio and B mode images for ultrasound systems so equipped with this capability .

FIGURE 3 is an example of a display of an implementation of the present invention in which a B mode image 80 of a phantom is displayed on the left side of the display and a strain image 82 is

displayed to the right. As can be seen from these images, a hard inclusion is barely visible in the center of the B mode image, but appears distinctly at 84 in the strain image. A cursor 86 is shown in the upper left of the strain image. The user can

manipulate a control of a control panel to move this cursor 86 so it overlays and designates an image region of normal tissue. The ultrasound system will then use one or more strain values from the location of the cursor 86 to produce a normal tissue strain value. For example, the system may average a ten by ten group of pixels at the center of the cursor to produce an averaged normal tissue strain value. The normal tissue strain value is then used to produce a strain ratio image.

FIGURE 4 is similar to FIGURE 3 except in this case the user manipulates a graphic 88 over the strain image to designate a region of normal tissue. In this example the graphic is a circle. The user can manipulate the circle graphic 88 over the image and can also select or control the size of the circle. Graphics of other shapes may alternatively be used. The ultrasound system will take the strain values of the pixels inside the graphic and combine them to produce a representative normal tissue strain value, such as averaging or calculating the mean or median of the normal tissue strain values.

Preferably system computes the normal tissue strain value continuously and may simultaneously show the strain ratio image as the graphic is being

manipulated or indicate its corresponding color value on a color bar.

FIGURE 5 shows a display in which the strain image 82 overlays the B mode image 80 in anatomical registration. In this example the strain image of the hard inclusion 84 overlays the location of the inclusion in the B mode image 80. The user has manipulated a box 90 over normal tissue for the computation of a strain ratio image. FIGURE 6 shows the strain ratio image 92 produced in the same reference box as in the strain image 82 of FIGURE 5. The strain ratio image 92 sharply delineates the tumor and the strain values of the pixels inside the color box in relation to strain values of normal tissue. The color bar 94 at the right side of the image shows the color-coding of the strain ratio values. The color bar represents higher stiffness values in very dark shading in this black-and-white representation of the color image, and the tumor 84 is seen to be very dark in the strain ratio image 92, indicating a relatively high stiffness of the tumor tissue. The user can manipulate the transparency control (s) on the control panel 40 to fade back and forth between solid, semi-transparent, and

transparent B mode and strain ratio images. As the user fades the strain ratio image 92 from a solid (non-transparent) image as shown in FIGURE 6 to fully transparent, the user can then see the tumor in grayscale where it underlies the dark tumor area in the strain ratio image. The user can then assess the appearance of the tumor and its boundaries as they appear in both the strain ratio image and the B mode image, aided by their anatomical registration in the display .

FIGURE 7 illustrates another strain ratio display of the present invention, this time with the B mode image 80 shown alone at the left side of the display, and the B mode image with a registered overlay of the strain ratio image 92. In this example greater stiffness is color-coded with what appears as a whiter shade in this black-and-white representation of the color image. The stiffness of the colors of the color bar 94 is indicated

qualitatively from "soft" to "hard." In this example it is seen that the tumor 84 is relatively stiffer than the surrounding tissue. This drawing also shows the normal tissue being indicated in the B mode image of the overlaid image pair, with the I-shaped cursor located to the right of the strain ratio image 92. Strain values calculated at the normal tissue

location of the cursor 96 are then used to produce the strain ratio image 92.

Other display variations will readily occur to those skilled in the art. For example, in FIGURE 7 a strain image could be shown alongside the strain ratio image, enabling the user to delineate a region of normal tissue in the strain image and see the resulting strain ratio image adjacent the strain image. One or both of the strain and strain ratio images can be shown overlaying a B mode image in anatomical registration. The fade control may be used to vary the relative transparency of a strain image which overlays its corresponding registered B mode image. Strain ratio values may be computed as the inverse of those described above, that is, the reference value over the strain value at a point. Strain ratio images may be produced in real time or in post-processing image review.

Claims

WHAT IS CLAIMED IS:

1. An ultrasonic diagnostic imaging system for producing strain images comprising:

an ultrasound probe with an array transducer operable for acquiring a plurality of frames of image data of an image field;

a strain estimator, responsive to the image data for producing estimated strain values at a plurality of points in the image field;

a strain image processor, responsive to the estimated strain values at points in the image field for producing an anatomical strain image of the image field;

a B mode processor, responsive to the image data for producing an anatomical B mode image which spatially corresponds to the anatomical strain image; a display coupled to the strain image processor and the B mode processor for producing an overlay of an anatomical strain image and an anatomical B mode image in anatomical registration; and

a user-operable fade control for varying the relative transparency of the overlaid anatomical strain image and anatomical B mode image.

2. The ultrasonic diagnostic imaging system of Claim 1, wherein the transparency of the overlaid anatomical strain image is variable over a range from fully transparent to fully opaque.

3. he ultrasonic diagnostic imaging system of Claim 1, wherein the strain image processor produces color-coded strain image display values in

correspondence to estimated strain values; and

wherein the display is further operable to produce a color bar of the color-coded strain image display values.

4. The ultrasonic diagnostic imaging system of Claim 1, wherein the strain estimator is further operable to detect displacement over a plurality of frames of image data by speckle tracking.

5. The ultrasonic diagnostic imaging system of Claim 1, further comprising a Doppler processor responsive to the image data and coupled to the strain estimator for providing Doppler shift data or tissue Doppler velocity data for strain estimation.

6. The ultrasonic diagnostic imaging system of Claim 1, further comprising a B mode processor responsive to the image data and coupled to the strain estimator for providing amplitude-detected image data for strain estimation.

7. The ultrasonic diagnostic imaging system of Claim 1, further comprising a user control operable to designate reference tissue in a strain image,

wherein the strain image processor is further operable to produce an anatomical strain image of the ratio of estimated strain values at point in the image field to a reference tissue strain value.

8. The ultrasonic diagnostic imaging system of Claim 1, wherein the strain estimator is further operable to produce strain values as local estimates of frame-to-frame image deformation.

9. The ultrasonic diagnostic imaging system of Claim 8, wherein the image deformation is produced by physiological motion of the subject being imaged.

10. The ultrasonic diagnostic imaging system of Claim 8, wherein the image deformation is produced by user compression of the subject being imaged.

11. The ultrasonic diagnostic imaging system of Claim 1, further comprising a reference tissue selector operable to automatically designate

reference tissue in a strain image,