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 mechanical means comprises: mechanical scanning without using the operator's hands; free-hand scanning using a probe with a location detection sensor (see D. F. Leotta, P. R. Detmer, O. H. Gilja, and J. M. Jong, “Three-dimensional ultrasound imaging using multiple magnetic tracking systems and miniature magnetic sensors,” IEEE Proc. Ultrasonics Symposium '95, vol. 2, pp. 1415, November 1995 and N. Pagoulatos, W. S. Edwards, D. R. Haynor, and Y. Kim, “Interactive 3D registration of ultrasound and magnetic resonance images based on a magnetic position sensor,” IEEE Trans. Inform. Technol. Biomedicine, vol. 34, pp. 278–288, December 1999); and scanning with a three-dimensional probe (see T. White, K. Erikson, and A. Nicoli, “A real-time 3D ultrasonic imager based on a 128/spl times/128 transducer array,” IEEE Proc. 18th Annual International Conference of Engineering in Medicine and Biology Society, vol. 5, pp. 2109–2110, January 1997 and J. M. Bureau, W. Steichen, and G. Lebail, “A two-dimensional transducer array for real-time 3D medical ultrasound imaging,” IEEE Proc. Ultrasonics Symposium '98, vol. 2, pp. 1065–1068, February 1998).
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 ρzn for each block is calculated. A mean elevation distance correlation for each input frame is estimated by averaging all of the elevation distance correlations ρzn with 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 ρzn of each block is defined by:                               ρ          z          n                =                                            ∑                                                (                                      x                    ,                    y                                    )                                ∈                                  B                  n                                                      ⁢                                          [                                                                            I                      z                                        ⁡                                          (                                              x                        ,                        y                                            )                                                        -                                                            I                      _                                        z                    n                                                  ]                            ⁡                              [                                                                            I                                              z                        +                                                  Δ                          ⁢                                                                                                          ⁢                          z                                                                                      ⁡                                          (                                              x                        ,                        y                                            )                                                        ⁢                                                            I                      _                                                              z                      +                                              Δ                        ⁢                                                                                                  ⁢                        z                                                              n                                                  ]                                                                                        ∑                                                      (                                          x                      ,                      y                                        )                                    ∈                                      B                    n                                                              ⁢                                                                    [                                                                                            I                          z                                                ⁡                                                  (                                                      x                            ,                            y                                                    )                                                                    -                                                                        I                          _                                                z                        n                                                              ]                                    2                                ⁢                                                      ∑                                                                  (                                                  x                          ,                          y                                                )                                            ∈                                              B                        n                                                                              ⁢                                                            [                                                                                                    I                                                          z                              +                                                              Δ                                ⁢                                                                                                                                  ⁢                                z                                                                                                              ⁡                                                      (                                                          x                              ,                              y                                                        )                                                                          -                                                                              I                            _                                                                                z                            +                                                          Δ                              ⁢                                                                                                                          ⁢                              z                                                                                n                                                                    ]                                        2                                                                                                          (                  Eq          .                                          ⁢          1                )            wherein Iz(x,y) is a brightness value corresponding to coordinates (x, y, z) in a three-dimensional space; Bn is an nth block of a frame; Īzn is an average brightness value within a block; and Δz is a distance from a frame located at a position z to a consecutive frame. ρz is an elevation distance correlation between frames and is obtained from the elevation distance correlation ρzn of each block. By applying ρz to 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.