Three-dimensional ultrasound imaging method and apparatus using lateral distance correlation function

Disclosed is a three-dimensional ultrasound image method and apparatus for reducing distortions of ultrasound images by precisely estimating a distance between consecutive two-dimensional frames obtained by a manual scanning. The apparatus comprises a transducer array for transmitting ultrasound signals to a target object and receiving echo signals reflected from the target object; a receive-focusing unit for receive-focusing the echo signals to generate a plurality of consecutive two-dimensional ultrasound image frames; a converter for converting the consecutive two-dimensional ultrasound image frames into a three-dimensional ultrasound image; and a display for displaying the three-dimensional ultrasound image. The method comprises the steps of, a) transmitting and receiving ultrasound signals; b) receive-focusing echo signals to generate a plurality of consecutive two-dimensional ultrasound image frames; c) converting the consecutive two-dimensional ultrasound image frames into a three-dimensional ultrasound image; and d) displaying the three-dimensional ultrasound image.

FIELD OF THE INVENTION

The present invention relates generally to ultrasound imaging, and more particularly, to a three-dimensional ultrasound imaging method and apparatus for reducing distortions in ultrasound images, which constructs three-dimensional ultrasound images by precisely estimating the distance between consecutive two-dimensional frames obtained by manual scanning using a lateral distance correlation function.

BACKGROUND OF THE INVENTION

Conventionally, three-dimensional ultrasound images are obtained by rendering a three-dimensional volume consisting of consecutive two-dimensional frames stacked one-by-one. However, where the distance between stacked consecutive two-dimensional frames is not uniform, the three-dimensional ultrasound images constructed from the two-dimensional frames may be distorted. For example, if a target object is an organ in a human body, then the constructed three-dimensional organ may appear distorted from its actual shape due to the non-uniformity of the distance between consecutive two-dimensional frames that represent the organ.

Such non-uniformity typically results from the variation of the movement speed of the probe. Where a probe scans a target object within a certain diagnostic region, the number of consecutive two-dimensional frames obtained is inversely proportional to the movement speed of the probe in that region. For example, if the probe scans a target object within a diagnostic region at a high movement speed, the number of consecutive two-dimensional frames obtained is less than the probe scans at a low movement speed. Thus, relative to the actual target object, a three-dimensional image of a target object may be contracted in size, if it is constructed from consecutive two-dimensional frames obtained at a high movement speed or enlarged in size, if constructed from consecutive two-dimensional frames obtained at a low movement speed. As the variation of the movement speed of the probe increases, the non-uniformity worsens.

In order to compensate for the non-uniformity, a conventional three-dimensional ultrasound imaging system employs both mechanical and non-mechanical means.

The non-mechanical means comprises a stationary correlation function to estimate the distance between consecutive two-dimensional frames obtained by driving a probe, with respect to an elevation distance of a probe obtained from consecutive reference frames, i.e., a distance between that frames, without using a location detection sensor as in manual scanning means (see M. Li, “System and method for 3-D medical imaging using 2-D scan data,” U.S. Pat. No. 5,582,173, 1996). As is well known in the art, the stationary correlation function means a function that is invariant to the calculated position. This non-mechanical means calculates a reference elevation distance correlation function ρ(d) based on reference frames, which are obtained from a tissue that is similar to a target object, to estimate the distance between consecutive frames.

According to this non-mechanical means, each of the input frames is classified into a plurality of blocks and an elevation distance correlation ρznfor each block is calculated. A mean elevation distance correlation for each input frame is estimated by averaging all of the elevation distance correlations ρznwith weights and the distance between the input frames is estimated by applying the mean elevation distance correlation to the reference elevation distance correlation function ρ(d). The elevation distance correlation ρznof each block is defined by:ρzn=∑(x,y)∈Bn⁢[Iz⁡(x,y)-I_zn]⁡[Iz+Δ⁢⁢z⁡(x,y)⁢I_z+Δ⁢⁢zn]∑(x,y)∈Bn⁢[Iz⁡(x,y)-I_zn]2⁢∑(x,y)∈Bn⁢[Iz+Δ⁢⁢z⁡(x,y)-I_z+Δ⁢⁢zn]2(Eq.⁢1)
wherein Iz(x,y) is a brightness value corresponding to coordinates (x, y, z) in a three-dimensional space; Bnis an nthblock of a frame; Īznis an average brightness value within a block; and Δz is a distance from a frame located at a position z to a consecutive frame. ρzis an elevation distance correlation between frames and is obtained from the elevation distance correlation ρznof each block. By applying ρzto an equation {circumflex over (Δ)}z=ρ−1(ρz), a distance {circumflex over (Δ)}z between consecutive two-dimensional frames may be estimated. The equation {circumflex over (Δ)}z=ρ−1(ρz) utilizes an inverse function of the reference elevation distance correlation function ρ(d).

However, the aforementioned means have disadvantages. With the mechanical means, the distance between frames may be accurately obtained. However, since probes must be mechanically fixed or provided with additional devices, patients and operators feel uncomfortable. Further, the associated manufacturing costs of ultrasound imaging apparatuses are increased. In particular, using a three-dimensional probe requires more ultrasound sensor arrays than a two-dimensional probe, and thereby increases the manufacturing cost of the ultrasound imaging apparatus and the size of probe. If the size of probe is increased, an operator may not easily handle the probe when diagnosing a patient.

The non-mechanical means may be more comfortable for operators and patients, and does not require additional sensors or devices. However, since the non-mechanical means employs a fixed elevation distance correlation function, without taking into account the non-stationary characteristics of ultrasound images, obtaining an accurate distance between consecutive two-dimensional frames is very difficult and results in lower reliability of the measured ultrasound images.

Thus, need exists for a method for accurately estimating the distance between frames from images obtained through manual scanning without using mechanical devices or location detection sensors.

SUMMARY OF THE INVENTION

Therefore, an objective of the present invention is to provide a three-dimensional ultrasound imaging method and apparatus for reducing image distortions on three-dimensional ultrasound imaging by accurately estimating a distance between consecutive two-dimensional frames that are obtained through a manual scanning by means of a lateral distance correlation function.

In accordance with the present invention, a three-dimensional ultrasound imaging apparatus is provided, comprising: a transducer array for transmitting ultrasound signals to a target object and receiving echo signals reflected from the target object, wherein the transducer array is comprised of a plurality of transducer elements; means for receive-focusing the echo signals from the transducer array to generate a plurality of consecutive two-dimensional ultrasound image frames; means for converting the consecutive two-dimensional ultrasound image frames into a three-dimensional ultrasound image by aligning the consecutive two-dimensional ultrasound image frames with no offset to each other in a three-dimensional space and interpolating a distance between two adjacent frames among the consecutive two-dimensional frames, to thereby make an inter-frame distance uniform; and a display for displaying the three-dimensional ultrasound image.

Also, in accordance with the present invention, a three-dimensional ultrasound imaging method is provided, comprising the steps of: a) transmitting ultrasound signals to a target object and receiving echo signals reflected from the target object; b) receive-focusing the echo signals to generate a plurality of consecutive two-dimensional ultrasound image frames; c) converting the consecutive two-dimensional ultrasound image frames into a three-dimensional ultrasound image by aligning the consecutive two-dimensional ultrasound image frames with no offset to each other in a three-dimensional space and interpolating a distance between two adjacent frames among the consecutive two-dimensional frames, to thereby make an inter-frame distance uniform; and d) displaying the three-dimensional ultrasound image.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring toFIG. 1, which illustrates obtaining a plurality of consecutive two-dimensional (2D) frames by using a probe in a three-dimensional (3D) space. 3D space is expressed in terms of an orthogonal coordinate system of depth-lateral-elevation (X-Y-Z). Assuming that transducer array5is arranged in a lateral direction along the Y-axis, all frames7for target object6are located in the X-Y plane, and the interface of 2D probe8with target object6is always perpendicular to the X-axis. However, where a plurality of frame sequences are obtained by using 2D probe8, 2D probe8moves non-linearly according to the shape of the curved surface of target object6so that the 3D positions of frames7are offset.

Referring toFIG. 2, which shows a block diagram of a 3D ultrasound imaging apparatus according to the present invention. Transducer array11transmits ultrasound signals to target object10and receives echo signals reflected from target object10. The echo signals received by transducer array11are amplified to a predetermined level by pre-amplifier12. Time gain compensation (TGC) amplifier13amplifies the signals from pre-amplifier12by varying a gain with respect to time in order to compensate for attenuation of the ultrasound signals within a human body.

Beamformer14receives the amplified signals from TGC amplifier13with different delays. Beamformer14then sums the delayed signals to receive-focus them on a reflection point of target object10in the lateral direction by varying the amount of the delay. Analog-to-digital (A/D) converter15converts the signals processed in beamformer14from analog to digital. 2D/3D converter16receives the digital signals outputted from A/D converter15and constructs 3D ultrasound images from 2D ultrasound image frames. The 3D ultrasound images outputted from 2D/3D converter16are displayed on display17.

Referring toFIG. 3, which shows a block diagram of 2D/3D converter16shown inFIG. 2. 2D/3D converter16comprises pre-processing unit20, 3D volume constructing unit23, and rendering unit24. Pre-processing unit20is comprised of plane location modifying unit21and inter-frame distance estimating unit22. Plane location modifying unit21aligns frames in a three-dimensional manner by estimating movement of two adjacent frames in the upper, lower, right, and left directions with respect to a plane location in 3D space. Inter-frame distance estimating unit22estimates a distance between the two aligned frames and interpolates them to have a uniform inter-frame distance. 3D volume constructing unit23obtains 3D volume information from the uniform aligned 2D frames. Rendering unit24constructs 3D images through a rendering process on the basis of the 3D volume information.

Plane location modifying unit21estimates movement between frames in the upper, lower, right, and left directions and modifies the estimated movement to align the frames in a 3D space. Referring toFIG. 4, which illustrates the offset of two frames in 3D space. Predetermined coordinates (xk, yk) of a kthframe are located at coordinates (x+xk, y+yk, zk) in 3D space. The coordinates (xk, yk) denote a vector representing how far the kthframe is from a starting point in the 3D coordinates system. Vector (xk+1, yk+1) representing the location of the next (k+1)thframe is obtained by summing the coordinates (xk, yk) and a vector sk=(Δxk, Δyk) representing the movement between frames. As can be seen fromFIG. 4, the kthand (k+1)thframes are offset in 3D space. That is, the (k+1)thframe moves toward the lower-right direction in 3D space with respect to the kthframe so that their locations in the X-Y plane are different from each other. If the offset between the two frames is not considered while image reconstruction is performed directly through 3D volume constructing unit23and rendering unit24, a reconstructed 3D image may be distorted.

In order to align the plurality of frames with no offset in the 3D space, plane location modifying unit21estimates a movement vector by using a block matching algorithm (BMA). For the purpose of estimation, plane location modifying unit21divides each of the frames into a plurality of blocks, none of which overlap each other, and matches the blocks up to a half pixel by using a bilinear interpolation method, as shown inFIG. 5, to thereby obtain the movement vector per each block. InFIG. 5, pixels A, B, C, D are represented by the symbol “□” and half pixels b, c, d are represented by the symbol “∘.” Locations of half pixels between pixels A, B, C, D maybe expressed asb=A+B2,c=A+C2,d=A+B+C+D4.
Accordingly, matching the blocks of each of the frames up to the half pixels may improve the accuracy of 3D ultrasound images to be displayed on display17.

Plane location modifying unit21obtains a movement vector per each block and determines vector {tilde over (S)}khaving the highest frequency among the obtained movement vectors to computes a movement vector for each of the frames. Since the moving speed of probe8shown inFIG. 1does not rapidly vary over a short interval, movement vector Ŝkof the kthframe is expressed by the following equation by taking into account correlation of a previous movement vector.
Ŝk=αŜk+(1−α)Ŝk−1, 0≦α≦1  (Eq. 2)
wherein Ŝk−1is the movement vector of the (k−1)thframe; α and (1−α) are weights; and α is defined to a range of 0≦α≦1 in order to obtain previous movement vectors and an average of the weights. Using the computed movement vector, plane location modifying unit21aligns a plurality of frames with no offset by estimating the movement between frames in the upper, lower, right, and left directions in 3D space.

Inter-frame distance estimating unit22estimates the distances between consecutive frames among the aligned frames and interpolates a location for each of the aligned frames to even the distances therebetween. Specifically, frame sequences inputted to inter-frame distance estimating unit22from plane location modifying unit21are aligned along the Z-axis as shown inFIG. 6. However, the distance between the frame sequences is not uniform. Referring toFIG. 6, which shows that the next frame, i.e., the (k+1)thframe, is obtained at a distance longer than uniform inter-frame distance D. Thus, in order to align frames at uniform inter-frame distance D, inter-frame distance estimating unit22assumes that a lateral distance correlation function between (x, y, z) and (x, y+Δy, z) is equal to an elevation distance correlation function between (x, y, z) and (x, y, z+Δz), within a short distance of Δz=Δy. Referring toFIG. 7, which shows a partial isotropy with respect to a lateral distance correlation function and an elevation distance correlation function in the Y-Z plane. The reason why make such assumption is such that tissue characteristics of minute parts of human organs may be same and image characteristics of the same are also same as they construct their images by using information received from a same ultrasound array with a same depth.

Inter-frame distance estimating unit22first divides frames into a plurality of blocks without overlapping each other and calculates a lateral distance correlation ηzn(d) per each block by using the following Equation 3. The lateral distance correlation means a correlation that is obtained on Y-axis in a lateral direction in a 3D space.ηzn⁡(d)=∑(x,y)∈Bn⁢[Iz⁡(x,y)-I_zn]⁡[Iz⁡(x,y+d)-I_zn⁡(d)]∑(x,y)∈Bn⁢[Iz⁡(x,y)-I_zn]2⁢∑(x,y)∈Bn⁢[Iz⁡(x,y+d)-I_zn⁡(d)]2(Eq.⁢3)
wherein Īzn(d) is an average brightness of a block moved by a distance din Y-axis; Iz(x, y) is a brightness at a coordinates (x, y, z) in a 3D space; and Bnis an nthblock within a frame.

After calculating the lateral distance correlation ηzn(d), inter-frame distance estimating unit22performs a partial linear approximation on the calculated results as shown inFIG. 8. InFIG. 8, a horizontal axis represents the distance d between pixels and a vertical axis represents a correlation value. In the graph shown inFIG. 8, each point denoted by a symbol “●” indicates a value calculated by using Equation 3 on a pixel-by-pixel basis and correlation of each point is estimated by linearly drawing a line between points. The further the distance d from the starting point, the lower the correlation. Thus, the graph declines downwardly.

Thereafter, inter-frame distance estimating unit22calculates an elevation distance correlation ρznby using Equation 4 as follows:ρzn=∑(x,y)∈Bn⁢[Iz⁡(x,y)-I_zn]⁡[Iz+Δ⁢⁢z⁡(x,y)-I_z+Δ⁢⁢zn]∑(x,y)∈Bn⁢[Iz⁡(x,y)-I_zn]2⁢∑(x,y)∈Bn⁢[Iz+Δ⁢⁢z⁡(x,y)-I_z+Δ⁢⁢zn]2(Eq.⁢4)
wherein Iz(x, y) is a brightness of (x, y, z) in 3D space; Bnis an nthblock within a frame; Īzn(d) is an average brightness of a block; Δz is a distance between a frame located at a position z and the next frame; and ρznis the elevation distance correlation obtained from each block.

Assuming the elevation distance correlation and the lateral distance correlation are partially isotropic, i.e., ρzn(Δz)=ηzn(Δy), with respect to the short distance of Δz=Δy, inter-frame distance estimating unit22estimates an nthdistance {circumflex over (Δ)}znbetween blocks by using the following Equation 5 (FIG. 7shows the assumption ρzn(Δz)=ηzn(Δy) with respect to Δz=Δy).Δ^zn=ηzn-1⁡(ρzn)(Eq.⁢5)
wherein the nthdistance {circumflex over (Δ)}znis calculated by applying an nthblock elevation distance correlation ρznto an inverse function of an nthblock lateral distance correlation ηzn. Then, inter-frame distance estimating unit22estimates a distance {circumflex over (Δ)}zbetween consecutive 2D frames from the distance {circumflex over (Δ)}znby using Equation 6 as follows:Δ^z=1Nd⁢∑n∈Nd⁢Δ^z,Nd={n:Δ^zn-m⁢〈σ}(Eq.⁢6)
wherein m and σ are an average and a standard deviation of {circumflex over (Δ)}znwith respect to all blocks, respectively; and |Nd| is a size of a set Nd.

Finally, inter-frame distance estimating unit22constructs consecutive frames with a uniform inter-frame distance shown inFIG. 9by using a linear interpolation on the basis of the estimated inter-frame distances. The linear interpolation can be expressed as follows:I^zk+δ⁡(x,y)=Izk+1⁡(x,y)-Izk⁡(x,y)Δ^zk·δ+Izk⁡(x,y),0<δ<Δ^zk(Eq.⁢7)
wherein Î is a new frame generated during the linear interpolation; δ is a predetermined value satisfying a relationship of zk+δ=mD with respect to a positive integer m, where D is a desired uniform inter-frame distance; and zkis a coordinates value in an elevation axis of a kthframe.

As described above, plane location modifying unit21aligns a plurality of consecutive frames with no offset in 3D space and inter-frame distance estimating unit22estimates a distance between two frames among the aligned consecutive frames and interpolates a location of each frame to make the distance uniform. Therefore, 2D ultrasound frame sequences inputted to 2D/3D converter16are constructed as a 3D image having a minimum image distortion.

In addition, a rotation angle of a present frame with respect to a previous frame on the X-Y-Z axis can be computed as follows. First, the present frame is divided into a plurality of blocks and a rotation angle of each block is calculated by using the following Equation 8 defined by:(αn,βn,γn)=arg⁢⁢minα,β,γ⁢D⁡[Bn′⁡(α,β,γ),Bn](Eq.⁢8)
wherein Bnis an nthblock of a present frame; B′n(α, β, γ) is a block of which an nthblock of a previous frame is rotated by (α, β, γ); and D[B′n(α, β, γ), Bn] is an error between the nthblocks of the present and previous frames. An angle that produces the smallest error is selected as the rotation angle of each block. By averaging the rotation angles of the blocks in the present frame, the rotation angle of the present frame is obtained.

If a present frame has been rotated by (α, β, γ), an error which occurred, is eliminated through the rotation of the present frame by (−α, −β, −γ). Coordinates (x′, y′, z′) for a frame having a corrected rotation error is calculated from coordinates (x, y, z) within a frame having a rotation error, by using the following Equation 9 defined by:[x′y′z′]=Rz⁢Ry⁢Rx⁡[xyz]⁢⁢Rz=(cos⁢⁢γsin⁢⁢γ0-sin⁢⁢γcos⁢⁢γ0001),Ry=(cos⁢⁢β0-sin⁢⁢β010sin⁢⁢β0cos⁢⁢β),(Eq.⁢9)Rx=(1000cos⁢⁢αsin⁢⁢α0-sin⁢⁢αcos⁢⁢α)

As described above, the 3D ultrasound imaging method and apparatus in accordance with the present invention is capable of accurate estimation of a distance between consecutive 2D frames obtained by a manual scanning method, which does not require any conventional mechanical devices or location detection sensors, thereby minimizing image distortions caused during the generation of a 3D ultrasound image. Thus, the reliability of measured 3D ultrasound images is improved.

Further, the 3D ultrasound imaging apparatus in accordance with the present invention employs a manual scanning method, which can resolve inconveniences in actual use of those conventional mechanical devices and remarkably lower the manufacturing cost of an ultrasound imaging apparatus.

While the present invention has been shown and described with respect to the particular embodiments, those skilled in the art will recognize that many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.