EP1616201A1

EP1616201A1 - Heart wall strain imaging

Info

Publication number: EP1616201A1
Application number: EP04725119A
Authority: EP
Inventors: Odile c/o Société Civile SPID BONNEFOUS
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2003-04-15
Filing date: 2004-04-01
Publication date: 2006-01-18
Also published as: CN100383554C; WO2004092766A1; CN1774646A; JP2006523485A

Abstract

Ultrasound image processing system comprising means for acquiring ultrasound image data of the heart, means for forming a sequence of ultrasound images of the heart during a cardiac cycle and means for providing tissue velocity data of the heart wall, and further comprising means for estimating the time occurrence of a specific event related to heart strain for a point inside the heart wall. This system may comprise means for estimating the temporal variations of heart strain measured at a selected point inside the heart wall. This system may further comprise means for estimating the instants of occurrence of maximal and minimal variations of heart strain and their amplitudes from the measured temporal variations of heart strain at a selected point inside the heart wall and means for estimating the instants of temporal occurrences of the maximal and/or minimal variations of heart strain, along a line representing the heart wall.

Description

HEART WALL STRAIN IMAGING

FIELD OF THE INVENTION The invention relates to an image processing system for acquiring ultrasound image data of the heart of a body, processing the image data for estimating tissue velocities in the heart wall tissue and displaying ultrasound images of the heart with information relating to heart wall displacements during cardiac cycle, which information is derived from the heart wall tissue velocities. The invention further relates to an ultrasound examination apparatus coupled to such an image processing system.

The invention finds a particular application in the field of medical imaging for the examination of the myocardium.

BRIEF DESCRIPTION OF THE DRAWINGS

An ultrasound image processing technique called Tissue Doppler Imaging (TDI) is already known to those skilled in the art. This TDI technique permits of measuring the velocity of displacement of a tissue in a patient body for further estimating tissue parameters, in a non-invasive manner, and of displaying images representing said velocity in a color-coded manner, together with conventional ultrasound intensity images.

It is already known of the patent US 5,938,606 a method of measuring artery wall amplitude of displacement during cardiac cycles. This method permits of displaying a representation of the artery wall with indications of the artery wall motions, which are represented by curves superimposed onto conventional ultrasound intensity images. This method comprises steps of artery wall segmentation in the images of a sequence for localizing the artery wall, processing the image data to determine the amplitude of displacement of the artery wall in function of time, and drawing curves representing the amplitude of displacement of the artery walls. This method further comprises displaying an image sequence of the artery with the superimposed curves moving in function of time.

SUMMARY OF THE INVENTION Regarding an artery, the information of amplitude of displacement of the walls, as provided by the cited patent, is easy to use because the artery is an elongated organ having a longitudinal axis. The wall displacements are merely perpendicular to the axis and propagate under the influence of the cardiac pulses in the direction of the axis.

Now, regarding the heart, which has walls formed of muscles surrounding the heart cavities, the wall motions are very complex. The heart wall displacements induced by the heart pulses can neither be compared to, nor be represented alike the displacements of the artery wall. The complexity of the heart wall motions is due to the fact that the heart walls are submitted to a contraction followed by a relaxation during each cardiac cycle and to the fact that the propagation of these contraction and relaxation inside the heart wall regarded as a muscle is not isotropic. Instead, the propagation of these contraction and relaxation inside the heart wall appears to follow a specific propagation law. As a matter of fact, from a clinical point of view, rhythms disorders or infracted myocardium may change or disturb the contraction/relaxation propagation. A tool to study these specific propagation disorders is needed.

It is an object of the invention to provide an image processing system, which has means of displaying images giving temporal information of the propagation of the displacements of the heart wall during a cardiac cycle; and more particularly means of giving temporal information of the deformations inside the myocardium wall during a cardiac cycle. These images are called strain images.

An image processing system with such means is claimed in Claim 1.

It is a further object of the invention to provide such an image processing system, which has means of displaying images giving information of the instants of occurrences of the maximal strain, corresponding to the relaxation starting instants and minimal strain, corresponding to the contraction starting instants, at each internal point of the myocardium wall during the cardiac cycles, together with said maximal and minimal values. Such an image processing system is claimed in dependent Claims.

An advantage of the image processing system is that said system peπnits of visualizing the myocardium contraction/relaxation phenomenon, which is due to the heart polarization, together with the representation of the propagation of the muscular response. An ultrasonic examination apparatus coupled to such an image processing system is claimed in dependent Claims.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, reference is made to the accompanying schematic drawings, in which:

FIG.l is an ultrasound image of the heart left ventricle with representation of a line of segmentation (ABC) of the left ventricle wall;

FIG.2 shows a velocity (V) map recorded along the segmentation line (ABC) of the left ventricle wall of FIG.1 measured by a technique of Tissue Doppler

Imaging (TDI), in function of time (t), where the horizontal axis represents the time, during several successive cardiac cycles, and the vertical axis represents the segmentation line (ABC)

FIG.3 is a curve of the local strain measured at a selected point of the segmentation line (ABC), from which the times (t) of occurrences of the maximal values (M1-M4) of strain and of the minimal values (ml-m4) of strain for four cardiac cycles are extracted, and together with the amplitudes of strain, where each maximal value and each minimal value of strain respectively corresponds to a start of relaxation and a start of contraction of the myocardium; Fig.4 is an instantaneous map of left ventricle where regions exhibiting their maximum contraction value during the cardiac cycle at this particular time are tagged in color; and where the correspondence with the ECG curve give the time reference of the event.

FIG.5 shows, along the line of segmentation (ABC) represented on the vertical axis, the occurrences of maximal contraction in black and the occurrences of minimal contraction in white of the left ventricle wall of FIG.l, in function of time (t) represented on the horizontal axis, for four cardiac cycles;

FIG.6 is a curve giving the time (t) of occurrence of a maximal contraction on each point of the segmentation line (ABC) during one cardiac cycle; FIG.7 is a block-diagram representing schematically an ultrasound apparatus coupled to the imaging system of the invention;

FIG.8 is a block-diagram representing schematically an ultrasound imaging system for acquiring ultrasound data and providing heart strain information and heart strain images. DESCRIPTION OF EMBODIMENTS

The invention relates to an image processing system for processing echographic signals provided by an ultrasound imaging apparatus. The image processing system has means for acquiring ultrasound data, for image data processing and for performing ultrasound imaging. According to the invention, this image processing system is used to perform measurements of the heart wall strain. This imaging system has processing means to estimate the time of occurrence of the maximal and minimal values of heart wall strain at each point of the heart wall, with respect to the cardiac cycle, together with the amplitudes of heart wall strain. Each maximal value and each minimal value of strain respectively corresponds to a start of relaxation and a start of contraction of the myocardium. This image processing system has means of displaying images, called strain images, giving temporal information of the propagation of the deformations inside the myocardium wall during a cardiac cycle. This imaging system displays images giving information of the precise time of occurrences of the maximal deformation (relaxation starting instant) and minimal deformation (contraction starting instant), at each internal point of the myocardium wall during the cardiac cycles. These images are animated images that show the propagation of the muscular response of the myocardium excitation, as illustrated by Fig 4.

Referring to FIG.8, an ultrasonic examination system 50 comprises a probe or scan-head 10 and display means 40. The operation of the ultrasonic examination system involves setting the probe 10 in contact with a patient under study for transmitting, via periodic excitations, ultrasonic signals to the heart of the patient and for receiving the echoes returned by the obstacles encountered in the medium.

The probe 10 may be composed of ultrasonic transducers 12 that are assembled in a phased array or in a linear array, curved array or other array. The probe may be placed external or internal to a body under study. The array of transducer elements transmits pulses or beams of energy into the body and receives returning pulses of energy as they are reflected from internal structures of the body. The pulses or beams of energy are electronically coupled to the ultrasound system.

The ultrasonic examination system, which is connected to the probe 10, includes a transmitter/receiver stage 14 for generating the excitation signals applied to probe 10. The probe transducers convert these signals into periodic train of ultrasonic pulse signals supplied with a predetermined recurrent frequency. The returned acoustic signals are received and combined by the transmitter/receiver stage 14 to perform beam formation in 16. Control signals are further supplied by the transmitter/receiver stage to control the probe 10, the pulse signals and velocity estimation stages.

In the transmission mode of the transmitter/receiver stage 14, the heart of the patient is scanned along the directions of the excitation lines. In the receiving mode, the image of each excitation line is formed, taking into account the propagation time in the medium and the amplitude of the echoes returned by the obstacles encountered along the considered excitation line. In the transmission mode of the transmitter/receiver stage 14, each excitation line of the probe 10 thus provides acoustic high-frequency signals, which enable the formation of a sequence of intensity images by way of a stage of image formation. The intensity image sequence is referred to as ultrasonic grayscale image sequence, which is displayed in the form of 2-D images comprising images such as the sector image of FIG.1.

The ultrasonic examination apparatus further comprises means to permit operation thereof in a color-coded Tissue Doppler Imaging mode (TDI), which enables determination of the velocities in a tissue of an organ of a patient. Tissue Doppler Imaging is a technique that estimates the component of the tissue velocity vector projected on an acoustic beam. The displacements of structures of interest induce phase shifts on successive high frequency ultrasound echoes backscattered by the moving tissue. These phase shifts are processed through I, Q demodulation stage 18 and tissue/flow separation stage 30 in order to estimate the local velocity of the tissue under examination in tissue motion estimate stage 31. Then, the tissue velocity estimation stage 31 processes Doppler echo signals issued by stage 30 to obtain Doppler shift characteristics such as frequency corresponding to velocity. The Doppler processor 31 processes received echo signals from a same spatial location of the heart and determines the Doppler phase or frequency shift. The Doppler processor 31 may estimate the Doppler shift by a fast Fourier transform (FFT) or auto-correlation operation. Preferably the Doppler estimator 31 employs two-dimensional auto-correlation, which performs autocorrelation in both time and space and produce precise, highly resolved Doppler shift estimates.

The tissue velocity processor stage 32 processes the tissue signals, which may further include scan conversion 33 to the desired image format, if needed. In stage 32, the signals are color mapped to a range of color values. The color map of Doppler signals may then be overlaid on the grayscale image provided the B mode processor 20, using a video processor 34. The colors indicating velocities are superimposed on the grayscale images of intensity thus forming a color-coded tissue velocity sequence of images. This color-coded tissue velocity sequence may be stored in a memory.

The signal processing performed in TDI is similar to the signal processing that is used by Color Doppler Imaging systems to image blood flow. However, modifications of the ultrasound system settings are necessary, because the tissue velocity is low compared to the blood velocity, and the tissue signal amplitudes are high compared to the blood signal amplitudes. These modifications include shorter ensemble lengths, lower pulse repetition frequencies (PRFs) and deactivation of the wall filter if any is present. According to the invention, this TDI technique is used to measure velocities in the heart wall of a patient. The operation of heart wall velocity measurement comprises heart wall motion estimation. The motion of biological structures, such as tissues, seen by TDI, is due to the interaction between internal mechanical forces (arterial pressure, myocardium contraction/relaxation strength) with the tissues (myocardium). Using TDI sequences, it is possible to extract specific physiological parameters, which can be imaged and used to determine the biomechanical functions of the cardiac walls. According to the invention the TDI images are used for calculating heart wall strain from a spatial derivative along the spatial ultrasonic beam, as described below. Referring to FIG.1 , which represents a B-mode sector image of the myocardium, the determination of myocardium strain rate first comprises heart wall segmentation. Heart wall edges are extracted from the B-mode images of the sequence using for example an edge detection technique, which provides a segmentation line SL representing the heart wall position. The position of the wall segmentation line SL is superimposed on the B-mode image of FIG.1.

Still referring to FIG.l, for the estimation of tissue velocity, which velocity is perpendicular to the heart wall, the behavior of the heart wall must be observed over a full cardiac cycle. Therefore, a sequence of a total number N of images covering a time interval that is at least equal to a cardiac cycle must be formed, the images being produced at each instant n, and N being a number superior to 1. Also, time markers that are common to the tissue velocity image sequence and to the cardiac cycle must be identified. Thus, the heart tissue may be observed in relation to the various phases of the cardiac cycle. The position of the wall segmentation line SL is then mapped on the TDI data planes. Based on the location of the moving tissues, a calculation of local time integration of the TDI velocity values is performed. This calculation provides wall displacement values as a function of time throughout the cardiac cycle, and as a function of position. Let z be the coordinate of a point along an acoustic beam and inside the heart wall under examination. The displacement of this point is given by: t M (z,t) = JN(z,u)du to

(1) where V(z,u) is the velocity provided by the TDI, and to is for instance the beginning instant of the cardiac cycle.

The measurement of TDI velocities along the segmentation line SL of FIG.l is performed in function of the time t, as illustrated by FIG.2. The velocities are represented along the vertical axis and the time t along the horizontal axis. The points A, B, C of FIG.2 corresponds to the points A, B, C of FIG.l. FIG.2 concerns the time duration of about four cardiac cycles. The white parts are related to high velocities and the dark parts to low velocities.

Now, strain is the representation of the compressibility ability of a tissue.

Given two close positions z and z+dz along the acoustic beam, and inside the tissue under examination, the strain S(z, t) equals: _ M(z + δz, t) - M(z, t) ₌ 5M(z, t) δz δz

(2)

The strain rate represents the velocity of compression or dilation of the tissue.

The strain rate is defined by: sR_(Z)t) =^M

^J dt (3)

From (1) and (2) it follows that the strain rate can be derived directly from the TDI velocity information: σz

(4) Equations (3) and (4) can be combined to derive the strain directly from the TDI velocity information:

S(z,t) = i ^ du σz o

(5) Equation (5) is used hereafter in order to estimate the heart wall strain.

The myocardium wall velocities are measured from a Tissue Doppler velocity image of the myocardium that is formed as illustrated by FIG.2. Then, the velocity gradient, which is the strain rate, is estimated, with corrections that are function of the angle of the ultrasound beam. This computation of corrections is called spatial gradient procedure. These corrections are necessary due to the fact that the myocardium wall is moving with respect to the current ultrasound beam. Then, an image of the gradients is constructed and, at each point of the wall, the gradient values are integrated in time.

Referring to FIG.3, according to the invention, equation (5) is used to estimate a curve of the variations in function of time of the amplitude of strain measured at a selected point, i. e. where the variable z is fixed, on the segmentation line SL passing through the points A, B, C. For each point of the velocity image, a temporal curve of deformation is extracted and called strain curve. In the example shown on

FIG.3, the curve gives the times t of occurrences of the maximal amplitudes Ml, M2, M3, M4, of strain and of the minimal amplitudes ml, m2, m3, m4 of strain for four cardiac cycles, and gives the values of the amplitudes of strain at these instants, where each maximal value and each minimal value of strain respectively corresponds to a start of relaxation and a start of contraction of the myocardium. The maximal and the minimal amplitudes and their time occurrences that are estimated for constructing the strain curve are stored for each point of interest of the heart wall and particularly of the segmentation line SL.

Referring to FIG.4, dynamic sequences showing the location of contraction beginnings corresponding to minimum of strain and relaxation beginnings corresponding to maximum of strain are constructed from the information given by the strain curve of FIG.3. The ultrasound sequence of images, such as illustrated by FIG.l, is now considered. Each point of the myocardium wall in said image receives a respective superimposed color when said point corresponds either to a maximal contraction value or to a minimal contraction value. These images form a contraction/relaxation propagation image sequence, since the location of maximum or minimum strain value change along the time. The maximal amplitudes can be for example represented in red, while the minimal amplitudes can be represented in green. A sequence of images such as the image shown on FIG.4 permits of visualizing the manner according to which an event, such as the maximum or the minimum of strain, corresponding to the maximum or minimum of contraction, propagates in the space represented by the myocardium and in function of time.

Theses events appear to follow specific laws.

To illustrate these representations, in the FIG.4 that is associated to the present description, the white tagged portions correspond to the parts where a maximum of strain (beginning of relaxation) occurs at the instant t. The present invention permits of studying these specific excitation propagation laws using the images of FIG.4,

FIG.5 and FIG.6, which are related to the maximum of contraction propagation. The same study can be applied to the minimum of contraction propagation.

Referring to FIG.5, an image is further constructed that gives, along the segmentation line SL passing through ABC, developed along the vertical axis, the occurrences of maximal strain, in black, and the occurrences of minimal strain, in white, in the left ventricle wall of FIG.l, in function of time (t) represented on the horizontal axis, for four cardiac cycles. This image permits of visualizing the response to the hearts pulses and to study the manner according to which the strain propagates in the heart wall during the cardiac cycles. Referring to FIG.6, from the values calculated for constructing the image of

FIG.5, now the instants of occurrences of the maximal values of contraction, corresponding to the beginning of relaxation, are represented along the vertical axis of time t, while the points of the segmentation line SL are represented on the horizontal axis. Hence the curve of FIG.6 provides the instant of occurrence of the beginning of relation in each point of SL, during a cardiac cycle. The instantaneous slope of these curves give the propagation direction by the sign of the slope and the propagation velocity by the slope amplitude. For instance, the propagation velocity can be measured in mm/s. Experiments shows the curves obtained over several cardiac cycles are very similar. Equivalent study can provide the instants of occurrences of the minimal values of contraction, corresponding to the beginning of contraction, along the vertical axis of time t, while the points of the segmentation line SL are on the horizontal axis= not represented.

FIG.7 shows a diagram of an ultrasound examination apparatus according to the invention that is coupled the system 50 of FIG.8. The apparatus comprises a probe 10 for acquiring digital image data of a sequence of images, and ultrasound means 53 for processing these data according to the invention. In particular, the data processing device 53 has computing means and memory means to perform the calculations and construct the images as described above. A computer program product having pre-programmed instructions to carry out the calculations and construct the images may also be implemented. The ultrasound computing means can be applied on stored medical images, for example for estimating medical parameters. The system provides the processed image data to display means and/or storage means. The display means 40 may be a screen. The storage means may be a memory of the system 53. Said storage means may be alternately external storage means. This image viewing system 53 may comprise a suitably programmed computer, or a special purpose processor having circuit means such as LUTs, Memories, Filters, Logic Operators, that are arranged to perform the calculations according to the invention. The system 53 may also comprise a keyboard 55 and a mouse 56. Icones may be provided on the screen to be activated by mouse-clicks, or special pushbuttons may be provided on the system, to constitute control means 41 for the user to actuate the processing means of the system at chosen stages of the calculations. This medical viewing system 50 may be incorporated in an ultrasound examination apparatus. This medical examination apparatus may include a bed on which the patient lies or another element for localizing the patient relative to the apparatus. The image data produced by the ultrasound examination apparatus is fed to the medical viewing system 50.

Claims

CLAIMS:

1. An ultrasound image processing system comprising means for acquiring ultrasound image data of the heart, means for forming a sequence of ultrasound images of the heart during a cardiac cycle and means for providing tissue velocity data of the heart wall, and further comprising means for estimating the time occurrence of a specific event related to heart contraction/relaxation corresponding to minimum/maximum strain for a point inside the heart wall.

2. The system of Claim 1, comprising segmentation means for estimating the location of heart wall points to be selected on a segmentation line of the heart wall.

3. The system of one of Claims 1 or 2, comprising means for estimating the temporal variations of heart strain measured at a selected point inside the heart wall.

4. The system of Claim 3, comprising means for estimating the instants of occurrence of maximal and minimal variations of heart strain from the measured temporal variations of heart strain at a selected point inside the heart wall.

5. The system of one of Claims3 or 4, comprising means for estimating the amplitudes of the maximal and minimal variations of heart strain from the measured temporal variations of heart strain at a selected point inside the heart wall.

6. The system of Claim 3, comprising means for estimating the instants of occurrence of maximal and minimal variations of heart strain and their amplitudes from the measured temporal variations of heart strain at a selected point inside the heart wall.

7. The system of Claim 6, comprising imaging means for constructing a dynamic sequence showing the location of contraction beginnings corresponding to minimum of strain and relaxation beginnings corresponding to maximum of strain, (strain maximum).

8. The system of one of Claims 3 to 7, comprising imaging means for constructing an image of the temporal occurrences of the maximal and/or minimal variations of heart strain, along a line representing the heart wall.

9. The system of one of Claims 3 to 8, comprising means for estimating the instants of occurrences of the maximal and/or minimal amplitudes of heart strain, for selected points along a line representing the heart wall.

10. An ultrasound examination apparatus comprising an ultrasound probe of transducer elements for acquiring a sequence of images of the heart, a system as claimed in one of Claims 1 to 9, and display means for displaying ultrasound images.

11. A program product having a set of instructions for operating the functions of the means of the system as claimed in one of Claims 1 to 9.