Abstract:
This invention discloses a system for providing an image of at least a portion of a fetus in utero including an imager providing image data for a volume including at least a portion of a fetus in utero, an at least partially computer controlled image processing algorithm based segmenter for defining geometrical boundaries of various objects in the volume including at least a portion of a fetus in utero, and a sculpting tool, utilizing the geometrical boundaries of at least some of the various objects defined by the segmenter, for selectably removing image data relating to at least portions of the objects in order to provide a desired non-occluded image of at least a portion of the fetus in utero based on the image data. 
     A method for providing an image of at least a portion of a fetus in utero is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to facial imaging generally and more particularly to facial imaging in utero. 
     BACKGROUND OF THE INVENTION 
     Various techniques are known for facial imaging in utero using ultrasonic technology. The quality and completeness of such images is generally rather non-uniform and depends inter alia on the position of the face of a fetus relative to ultra-sound imaging apparatus. Conventional systems which provide facial imaging in utero are known inter alia from the following publications: 
     U.S. Pat. No. 5,239,591; 
     InViVo-ScanNT of the Fraunhofer Institut fuer Graphische Datenverarbeitung IGD in Darmstadt, Germany, commercially available; 
     3-D Ultrasound—Acquisition Methods Details, of Life Imaging Systems, Inc. of London, Ontario 
     UCSD radiologists are working on a new ultrasound technology that&#39;s guaranteed to produce much clearer images in three dimensions. by Kate Deely, UCSD Perspectives, Spring 1999; 
     Product literature relating to the following products: 
     Imaging software available from A 1  Alpha Space, Inc, of Laguna Hills, Calif., U.S.A. and from Echotech 3-D of Hallbergmoos, Germany; 
     HDI1500 commercially available from ATL—Advanced Technology Laboratories, Bothell, Wash., U.S.A.; 
     Voluson  530 D commercially available from Kretztechnik AG of Zipf, Austria and from Medison America of Pleasanton, Calif., U.S.A. This ultrasound system includes a scalpel feature which enables manual removal of occlusions blocking full visualization of a fetal face. 
     L 3 -Di commercially available from Life Imaging Systems Inc. of London, Ontario, Canada; 
     Echo-Scan, Echo-View and Compact3-D commercially available from TomTec Imaging Systems GmbH of Unterschleissheim, Germany; 
     NetralVUS, commercially available from ScImage, Inc. of Los Altos, Calif. 94022, U.S.A.; 
     3-Scape commercially available from Siemens AG of Erlangen, Germany; 
     Vitrea, commercially available from Vital Images, Inc of Minneapolis, Minn., U.S.A.; 
     VoxarLib, commercially available from Voxar Ltd. of Edinburgh, UK; 
     LOGIC  700  MR commercially available from GE Ultrasound. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved system for fetal face imaging in utero. 
     There is thus provided in accordance with a preferred embodiment of the present invention a system for providing an image of at least a portion of a fetus in utero including an imager providing image data for a volume including at least a portion of a fetus in utero, an at least partially computer controlled image processing algorithm based segmenter for defining geometrical boundaries of various objects in the volume including at least a portion of a fetus in utero, and a sculpting tool, utilizing the geometrical boundaries of at least some of the various objects defined by the segmenter, for selectably removing at least portions of the objects in order to provide a desired non-occluded image of at least a portion of the fetus in utero, based on the image data. 
     Further in accordance with a preferred embodiment of the present invention the imager is an ultrasound imager. 
     Still further in accordance with a preferred embodiment of the present invention the image data contains speckles. 
     Preferably the segmenter is fully automatic. Alternatively the segmenter is semi-automatic. 
     Additionally in accordance with a preferred embodiment of the present invention the segmenter operates substantially in real time. 
     Further in accordance with a preferred embodiment of the present invention the segmenter defines geometrical boundaries in at least one slice of the volume by employing previously acquired information relating to at least another slice of the volume. 
     Preferably the segmenter operates in a slice-by-slice manner. 
     There is also provided in accordance with a preferred embodiment of the present invention a method for providing an image of at least a portion of a fetus in utero, the method including providing image data for a volume including at least a portion of a fetus in utero, utilizing an at least partially computer controlled image processing algorithm based segmenter to define geometrical boundaries of various objects in the volume including at least a portion of a fetus in utero, and utilizing the geometrical boundaries of at least some of said various objects defined by the segmenter to selectably remove image data relating to at least portions of the objects in order to provide a desired non-occluded image of at least a portion of the fetus in utero, based on the image data. 
     Further in accordance with a preferred embedment of the present invention the method employs ultrasound. 
     Additionally in accordance with a preferred embodiment of the present invention, the image data contains speckles. 
     Still further in accordance with a preferred embodiment of the present invention the segmenter operates fully automatically. Alternatively the segmenter operates semi-automatically. 
     Moreover in accordance with a preferred embodiment of the present invention the segmenter operates substantially in real time. 
     Additionally in accordance with a preferred embodiment of the present invention the segmenter defines geometrical boundaries in at least one slice of the volume by employing previously acquired information relating to at least another slice of the volume. 
     Still further in accordance with a preferred embodiment of the present invention the segmenter operates in a slice-by-slice manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
     FIG. 1 is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with a preferred embodiment of the present invention and employing a volume imager and a volume segmenter followed by sculpting and visualization tools; 
     FIG. 2 is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with another preferred embodiment of the present invention employing a volume imager and a slice-by-slice volume segmenter followed by sculpting and visualization tools; 
     FIG. 3 is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with yet another preferred embodiment of the present invention it employing a 2-D imager, a 2-D slice-by-slice segmenter and a 2-D sculpting tool followed by a volume constructor and a visualization subsystem; 
     FIG. 4 is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with still another preferred embodiment of the present invention employing a 2-D imager and a 2-D slice-by-slice segmenter followed immediately by a volume constructor, followed by 3-D sculpting and visualization tools; 
     FIG. 5A is a flow chart illustrating operation of the system of FIG. 1 in accordance with a preferred embodiment of the present invention; 
     FIG. 5B is a flow chart illustrating operation of the system of FIG. 2 in accordance with a preferred embodiment of the present invention; 
     FIG. 5C is a flow chart illustrating operation of the system of FIG. 3 in accordance with a preferred embodiment of the present invention; 
     FIG. 5D is a flow chart illustrating operation of the system of FIG. 4 in accordance with a preferred embodiment of the present invention; 
     FIG. 6 is a flow chart illustrating 3-D image segmentation step of the operation of FIG. 5A in accordance with a preferred embodiment of the present invention; 
     FIG. 7 is a flow chart illustrating a surface edge enhancement step of the operation of FIG. 6 in accordance with a preferred embodiment of the present invention; 
     FIGS. 8A,  8 B,  8 C and  8 D are pictorial illustrations showing the progressive shrinking of an imaging balloon about an image of the head of a fetus; 
     FIGS. 9A and 9B together are a flowchart illustrating a three-dimensional filtering operation performed in accordance with a preferred embodiment of the present invention on an original volume image; 
     FIG. 10 is an illustration useful in understanding the filtering operation illustrated in FIGS. 9A and 9B; 
     FIG. 11 is a flow chart illustrating 3-D image segmentation step of the operation of FIG. 5B in accordance with a preferred embodiment of the present invention; 
     FIG. 12 is a flow chart illustrating a region of interest defining step of the operation of FIG. 11 in accordance with a preferred embodiment of the present invention; 
     FIGS. 13A and 13B together are a flowchart illustrating a two-dimensional filtering operation performed in accordance with a preferred embodiment of the present invention on an original volume image; 
     FIGS. 14A and 14B together are a flow chart illustrating one part of an optimal boundary defining step of the operation of FIG. 11 in accordance with a preferred embodiment of the present invention; 
     FIG. 15 is a flow chart illustrating another part of the optimal boundary defining step of the operation of FIG. 11 in accordance with a preferred embodiment of the present invention; 
     FIG. 16 is an illustration useful in understanding the flowchart of FIG. 12; 
     FIG. 17 is an illustration useful in understanding the flowchart of FIGS. 13A and 13B; 
     FIGS. 18A and 18B are illustrations useful in understanding the flowchart of FIGS. 14A and 14B; 
     FIG. 19 is an illustration useful in understanding the flowchart of FIG. 15; 
     FIGS. 20A and 20B are simplified illustrations showing operation of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1, which is a simplified block diagram illustration of a fetal face imaging system constructed and operative in. accordance with a preferred embodiment of the present invention. 
     As seen in FIG. 1, the fetal face imaging system of one embodiment of the present invention preferably comprises a volume imager  10 . Volume imager  10  may be of any suitable type and may employ any suitable technology, such as, for example, ultrasound imaging. It is also possible that magnetic resonance imaging (MRI) could be employed. Currently available ultrasound volume imagers and imaging software include: 
     Imaging software available from A 1  Alpha Space, Inc, of Laguna Hills, Calif., U.S.A. and from Echotech 3-D of Hallbergmoos, Germany; 
     HDI1500 commercially available from ATL—Advanced Technology Laboratories, Bothell, Wash., U.S.A.; 
     Voluson  530 D commercially available from Kretztechnik AG of Zipf, Austria and from Medison America of Pleasanton, Calif., U.S.A. 
     L 3 -Di commercially available from Life Imaging Systems Inc. of London, Ontario, Canada; 
     Echo-Scan, Echo-View and Compact3-D commercially available from TomTec Imaging Systems GmbH of Unterschleissheim, Germany; 
     NetralVUS, commercially available from ScImage, Inc. of Los Altos, Calif. 94022, U.S.A.; 
     3-Scape commercially available from Siemens AG of Erlangen, Germany; 
     Vitrea, commercially available from Vital Images, Inc of Minneapolis, Minn., U.S.A.; 
     VoxarLib, commercially available from Voxar Ltd. of Edinburgh, UK; 
     Conventional 2-D ultrasound images are also available from the following sources: ATL—Advanced Technology Laboratories, Bothell, Wash., U.S.A., Siemens AG, Acuson Corporation of Mountain View, Calif., U.S.A., GE Medical Systems of Milwaukee, Wis., U.S.A., Toshiba America Medical Systems of Tustin, Calif., U.S.A. and Hewlett-Packard Medical Group of Palo Alto, Calif. 
     It is appreciated that most currently available volume imagers operate on a slice-by-slice basis. It is anticipated, however, that volume imagers which do not operate on a slice-by-slice basis will become available in the future and will also be useful in the present invention. 
     In accordance with a preferred embodiment of the present invention, image data from volume imager  10  is supplied to a sculpting subsystem  12  preferably embodied in a workstation  13  including a computer controlled image processing algorithm based volume segmenter, preferably a computer controlled 3-D edge detection algorithm based volume segmenter  14 . The volume imager  10  provides a volume image which may be acquired directly or by acquiring a series of 2-D images and construction a volume image therefrom. Typically segmenter  14  receives the output of volume imager  10  in 3-D form and enables a workstation operator  17  using that output, to readily locate and isolate a fetal face image and, as necessary to remove parts of the image which occlude a full view of the fetal face from a desired perspective. 
     Segmenter  14 , as will be described hereinbelow in detail, is operative in a computer-assisted manner, preferably under the control of the operator  17 , to differentiate between various body parts and to distinguish the fetus or the fetal face from its environment, such as for example, from the amniotic fluid in which it resides and the surrounding placenta and uterus. 
     Preferably modified or annotated image data from segmenter  14  is employed by a 3-D sculpting tool  15 . It is appreciated that sculpting tool  15  may be used not only to remove occlusions but also to otherwise enhance the fetal face image. It is additionally appreciated that there may be cases where operator input in the operation of sculpting tool  15  may be unnecessary. In such a case, the sculpting tool  15  may be entirely computer controlled and operated. 
     It is appreciated that sculpting subsystem  12  may be integrated in the same computer platform which serves to control the operation of volume imager  10 . 
     The output of sculpting subsystem  12 , typically in the form of modified or annotated image data, is preferably supplied to a visualization subsystem  16 , which may comprise, for example, a video display, a video recorder or transmitter, or a printer or even a three dimensional model generator. It is appreciated that the visualization subsystem may include image processing circuitry and software for desired image enhancement or modification. 
     Reference is now made to FIG. 2, which is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with another preferred embodiment of the present invention. 
     As seen in FIG. 2, the fetal face imaging system of another embodiment of the present invention preferably comprises a volume imager  20 . Volume imager  20  may be of any suitable type and may employ any suitable technology, such as, for example, ultrasound and magnetic resonance imaging (MRI). Currently available volume imagers include products listed hereinabove with reference to FIG.  1 . 
     In accordance with a preferred embodiment of the present invention, image data from volume imager  20  is supplied to a sculpting subsystem  22  preferably embodied in a workstation  23  including a computer controlled image processing algorithm based slice-by-slice segmenter, preferably a computer controlled slice-by-slice segmenter based on 2-D or 3-D edge detection  24 . Typically segmenter  24  receives the output of volume imager  20  in either 2-D or 3-D form and enables a workstation operator  27 , using that output, to readily locate and isolate a fetal face image and, as necessary to remove parts of the image which occlude a full view of the fetal face from a desired perspective. 
     Segmenter  24 , as will be described hereinbelow in detail, is operative slice-by-slice in a computer-assisted manner, preferably under the control of the operator  27 , to differentiate between various body parts and to distinguish the fetus or the fetal face from its environment, such as for example, from the amniotic fluid in which it resides and the surrounding placenta and uterus. 
     Preferably modified or annotated image data from segmenter  24  is employed by a 3-D sculpting tool  25 . It is appreciated that sculpting tool  25  may be used not only to remove occlusions but also to otherwise enhance the fetal face image. It is additionally appreciated that there may be cases where operator input in the operation of sculpting tool  25  may be unnecessary. In such a case, the sculpting tool  25  may be entirely computer controlled and operated. 
     It is appreciated that sculpting subsystem  22  may be integrated in the same computer platform which serves to control the operation of volume imager  20 . 
     The output of sculpting subsystem  22 , typically in the form of modified or annotated image data, is preferably supplied to a visualization subsystem  26 , which may comprise, for example, a video display, a video recorder or transmitter, or a printer or even a three dimensional model generator. It is appreciated that the visualization subsystem may include image processing circuitry and software for desired image enhancement or modification. 
     Reference is now made to FIG. 3, which is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with yet another preferred embodiment of the present invention. 
     As seen in FIG. 3, the fetal face imaging system preferably comprises a two-dimensional (2-D) imager  30 . Two-dimensional imager  30  may be of any suitable type and may employ any suitable technology, such as, for example, ultrasound. Currently available 2-D imagers are listed hereinabove with reference to FIG.  1 . 
     In accordance with a preferred embodiment of the present invention, image data from 2-D imager  30  is supplied to a 2-D sculpting subsystem  32  preferably embodied in a workstation  33  including a computer controlled 2-D image processing algorithm based segmenter, preferably a computer controlled slice-by-slice segmenter based on a 2-D edge detection algorithm  34 . Typically segmenter  34  receives the output of 2-D imager  30  in 2-D form and makes it possible, with or without operator ( 37 ) intervention, to readily locate and isolate a fetal face image and, as necessary, to remove parts of the image which occlude a full view of the fetal face from a desired perspective. 
     Segmenter  34 , as will be described hereinbelow in detail, is operative slice-by-slice in a computer-assisted manner, preferably under the control of an operator  37 , to differentiate between various body parts and to distinguish the fetus or the fetal face from its environment, such as for example, from the amniotic fluid in which it resides and the surrounding placenta and uterus. 
     Preferably modified or annotated image data from segmenter  34  is employed by a 2-D sculpting tool  35 . It is appreciated that sculpting tool  35  may be used not only to remove occlusions but also to otherwise enhance the fetal face image. It is additionally appreciated that there may be cases where operator input in the operation of sculpting tool  35  may be unnecessary. In such a case, the sculpting tool  35  may be entirely computer controlled and operated. 
     The output of sculpting tool  35  is preferably supplied to a volume constructor  36  which is operative to construct a volume image from a plurality of individual slices of two-dimensional image data, while preserving the segmentation and sculpting thereof. 
     It is appreciated that sculpting subsystem  32  may be integrated in the same computer platform which serves to control the operation of 2-D imager  30 . 
     The output of sculpting subsystem  32 , typically in the form of modified or annotated image data, is preferably supplied to a visualization subsystem  38 , which may comprise, for example, a video display, a video recorder or transmitter, or a printer or even a three dimensional model generator. It is appreciated that the visualization subsystem may include image processing circuitry and software for desired image enhancement or modification. 
     Reference is now made to FIG. 4, which is a simplified block diagram illustration of a fetal face imaging system constructed and operative in accordance with yet another preferred embodiment of the present invention. 
     As seen in FIG. 4, the fetal face imaging system of yet another embodiment of the present invention preferably comprises a two-dimensional (2-D) imager  40 . Two-dimensional imager  40  may be of any suitable type and may employ any suitable technology, such as, for example, ultrasound. Currently available 2-D imagers are listed hereinabove, with reference to FIG.  1 . 
     In accordance with a preferred embodiment of the present invention, image data from 2-D imager  40  is supplied to a sculpting subsystem  42  preferably embodied in a workstation  43  including a computer controlled 2-D image processing algorithm based segmenter, preferably a computer controlled slice-by-slice segmenter based on a 2-D edge detection algorithm  44 . Typically segmenter  44  receives the output of 2-D imager  40  in 2-D form and makes it possible to readily locate and isolate a fetal face image and, as necessary, to remove parts of the image which occlude a full view of the fetal face from a desired perspective. 
     Segmenter  44 , as will be described hereinbelow in detail, is operative slice-by-slice in a computer-assisted manner, preferably under the control of an operator  47 , to differentiate between various body parts and to distinguish the fetus or the fetal face from its environment, such as for example, from the amniotic fluid in which it resides and the surrounding placenta and uterus. 
     Preferably modified or annotated image data from segmenter  44  supplied to a volume constructor  45 , which is operative to construct a volume image from a plurality of individual slices of two-dimensional image data, while preserving the segmentation thereof. 
     The output of volume constructor  45  is preferably supplied to a 3-D sculpting tool  46 . It is appreciated that 3-D sculpting tool  46  may be used not only to remove occlusions but also to otherwise enhance the fetal face image. It is additionally appreciated that there may be cases where operator input in the sculpting tool  46  may be unnecessary or obviated by operation of the segmenter  44 . In such a case, the sculpting tool  46  may be entirely computer controlled and operated. 
     It is appreciated that sculpting subsystem  42  may be integrated in the same computer platform which serves to control the operation of 2-D imager  40 . 
     The output of sculpting subsystem  42 , typically in the form of modified or annotated image data, is preferably supplied to a visualization subsystem  48 , which may comprise, for example, a video display, a video recorder or transmitter, or a printer or even a three dimensional model generator. It is appreciated that the visualization subsystem may include image processing circuitry and software for desired image enhancement or modification. 
     Reference is now made to FIG. 5A, which is a flow chart illustrating operation of the system of FIG. 1 in accordance with a preferred embodiment of the present invention. As seen in FIG. 5A, a series of 2-D ultrasound images or a 3-D ultrasound image may be acquired by volume imager  10  (FIG.  1 ). The relationship of the 2-D ultrasound images is preferably one of adjacent slices of a volume image, such that a volume image is constructed therefrom. Alternatively, where possible, a 3-D volume image may be acquired directly. 
     The operation of segmenter  14  of sculpting subsystem  12  (FIG. 1) is to apply computer-assisted or computer-controlled 3-D image segmentation with a view towards isolating an image of a fetal face from the volume image data received from the volume imager  10  (FIG.  1 ). 
     Following 3-D image segmentation, sculpting tool  15  (FIG. 1) is operative, normally, but not necessarily, without operator intervention, to eliminate portions of the isolated image which occlude a desired image of a fetal face. Following operation of sculpting subsystem  12 , a generally unoccluded image of the fetal face, with or without further image enhancement, is produced. 
     Reference is now made to FIG. 5B, which is a flow chart illustrating operation of the system of FIG. 2 in accordance with another preferred embodiment of the present invention. As seen in FIG. 5B, a series of 2-D ultrasound images may be acquired by volume imager  20  (FIG.  2 ). The relationship of the 2-D ultrasound images is preferably one of adjacent slices of a volume image, such that a volume image is constructed therefrom. Alternatively, where possible, a 3-D volume image may be acquired directly. 
     In this embodiment, the volume image may be converted into a series of 2-D image slices. These slices may correspond to the 2-D image slices originally acquired or alternatively may be sliced in different planes. 
     The operation of segmenter  24  of sculpting subsystem  22  (FIG. 2) in this embodiment is to apply computer-assisted or computer-controlled image segmentation on a slice-by-slice basis with a view towards isolating an image of a fetal face from the volume image data received from the volume imager  20  (FIG.  2 ). 
     Following image segmentation, 3-D sculpting tool  25  (FIG. 2) is operative normally, but not necessarily, without operator intervention, to eliminate portions of the isolated image which occlude a desired image of a fetal face. Following operation of sculpting subsystem  22 , a generally unoccluded image of the fetal face, with or without further image enhancement, is produced. 
     Reference is now made to FIG. 5C, which is a flow chart illustrating operation of the system of FIG. 3 in accordance with another preferred embodiment of the present invention. As seen in FIG. 5C, a series of 2-D ultrasound images may be acquired by 2-D imager  30  (FIG.  3 ). The relationship of the 2-D ultrasound images is preferably one of adjacent slices of a volume image, such that a volume image is constructed therefrom. 
     The operation of segmenter  34  of sculpting subsystem  32  (FIG. 3) in this embodiment is to apply computer-assisted or computer-controlled image segmentation on a slice-by-slice basis with a view towards isolating an image of a fetal face from the volume image data received from the 2-D imager  30  (FIG.  3 ). 
     Following image segmentation, 2-D sculpting tool  35  (FIG. 3) is operative normally, but not necessarily, without operator intervention, to eliminate portions of the isolated image which occlude a desired image of a fetal face. Following operation of sculpting subsystem  32  and volume constructor  36  (FIG. 3) a generally unoccluded image of the fetal face, with or without further image enhancement, is produced. 
     Reference is now made to FIG. 5D, which is a flow chart illustrating operation of the system of FIG. 4 in accordance with another preferred embodiment of the present invention. As seen in FIG. 5D, a series of 2-D ultrasound images may be acquired by 2-D imager  40  (FIG.  4 ). The relationship of the 2-D ultrasound images is preferably one of adjacent slices of a volume image, such that a volume image is constructed therefrom. 
     The operation of segmenter  44  of sculpting subsystem  42  (FIG. 4) in this embodiment is to apply computer-assisted or computer-controlled image segmentation on a slice-by-slice basis with a view towards isolating an image of a fetal face from the volume image data received from the 2-D imager  40  (FIG.  4 ). 
     Following image segmentation, a volume image is constructed by volume constructor  45  (FIG. 4) and supplied to a 3-D sculpting tool  46  (FIG.  4 ), which is operative normally, but not necessarily, without operator intervention, to eliminate portions of the isolated image which occlude a desired image of a fetal face. Following operation of sculpting subsystem  42 , a generally unoccluded image of the fetal face, with or without further image enhancement, is produced. 
     Reference is now made to FIG. 6, which is a flow chart illustrating the 3-D image segmentation step of the operation of FIG. 5A in accordance with a preferred embodiment of the present invention. 
     As seen in FIG. 6, a 3-D image which may have associated therewith initial markings distinguishing between portions of the image which are of interest and portions of the image which it is desired to discard, is supplied to surface edge detection based segmentation circuitry  60  which preferably initially performs surface edge enhancement on the received 3-D image. 
     Following surface edge enhancement, a balloon is defined which is centered on a region of the image which is of interest. The balloon may be defined with the assistance of operator generated markings on the 3-D image, but does not require such markings. 
     The balloon may be subsequently automatically expanded or shrunk until its boundaries lie on or near enhanced edges of the 3-D image or on operator input markings, which may be supplied in the course of 3-D segmentation and not only prior thereto. The final balloon configuration defines one or more surface boundaries. An example of progressive shrinkage of the balloon about a fetal face is illustrated in FIGS. 8A,  8 B,  8 C and  8 D. Shrinkage of the balloon is known from the following prior art publications, the disclosures of which are hereby incorporated by reference: 
     On Active Contour Models and Balloons, Laurent D. Cohen, CVGIP: IMAGE UNDERSTANDING, Vol. 53, No. 2, March, pp 211-218, 1991; 
     Finite-Element Methods for Active Contour Models and Balloons for 2-D and 3-D Images, Laurent D. Cohen and Isaac Cohen, IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, Vol. 1 5 , No. 11, November, 1993, pp 1131-1147; 
     Snakes, Active Contours, and Deformable Models http://www.wpi.edu/ 18 dima/ummed/presentation/index.html. 
     The resultant one or more surface boundaries are superimposed on the 3-D image. An operator may carry out a visual confirmation check to satisfy himself that the indicated boundaries are indeed correct. If so, a closed surface boundary superimposed on the 3-D image is output. 
     Should the operator not be satisfied with the indicated surface boundary or boundaries he can carry out a manual correction or may additionally or alternatively have the boundaries recalculated by edge detection based segmentation circuitry  60 . Whichever method is chosen, the corrected boundaries are superimposed on the 3-D image and a further visual check is conducted repeatedly until the operator is satisfied with the indicated boundaries. 
     Reference is now made to FIG. 7, which is a flow chart illustrating a surface edge enhancement step of the operation of FIG. 6 in accordance with a preferred embodiment of the present invention. As seen in FIG. 7, following input of the volume image from volume imager  10 , the volume image is preferably blurred using a 3-D filter, such as a 3-D Gaussian filter. Thereafter, a 3-D median filter is preferably applied to the blurred volume image. The preceding two steps are examples of noise suppression techniques useful in edge enhancement pre-processing. 
     Following the noise suppression steps described above, a plane enhancement filter is applied to the pre-processed image, thus producing a surface edge enhanced volume image output. 
     Reference is now made to FIGS. 9A and 9B, which together are a flowchart illustrating a three-dimensional filtering operation performed in accordance with a preferred embodiment of the present invention on an original volume image. Such a filtering operation is preferably employed as part of the step of performing surface edge enhancement of a 3-D image forming part of edge detection based segmentation  60 , as shown in FIG. 6, and corresponds to the step in FIG. 7 identified as “APPLY PLANE ENHANCEMENT FILTER”. Reference is also made in this context to FIG. 10, which is an illustration useful in understanding the filtering operation illustrated in FIGS. 9A and 9B; 
     FIGS. 9A and 9B taken together with FIG. 10, describe steps of a filtering operation which is performed on the volume image received from volume imager  10  in accordance with a preferred embodiment of the present invention. The detailed flowchart of FIGS. 9A and 9B describes a plane enhancement operator. The plane enhancement operator is an extension to three dimensions of edge or ridge enhancement operators in 2 dimensions described hereinbelow with reference to FIGS. 13A &amp; 13B as well as FIG.  17 . 
     The plane enhancement operator operates upon a volumetric image and provides a grey-level volumetric image output in which the edges or ridges appear as enhanced surfaces in three dimensions. Stated more generally, the operator provides a volumetric image representation of the intensity of the surface edge property at each image voxel. FIG. 10 is an illustration of the plane enhancement operator whose functionality is detailed in FIGS. 9A &amp; 9B. For the sake of conciseness, in view of the detailed nature of the steps of the operation indicated in FIGS. 9A and 9B with reference to FIG. 10, a further textual explanation of these steps is believed to be unnecessary and thus is not provided. 
     Reference is now made to FIG. 11, which is a flow chart illustrating a 3-D image segmentation step of the operation of FIG. 5B in accordance with a preferred embodiment of the present invention. FIG. 11 presents details of slice by slice segmentation in the operation of FIG.  5 B. As seen in FIG. 11, a 2-D image, which may be sliced from a volume image, is selected by the system or by an operator and is initially operated on by the segmenter using an operator input which applies initial markings, such as boundary markings, to various portions of the 2-D image to distinguish between portions of the image which are of interest and portions of the image which it is desired to discard. 
     Thereafter, fully or partially computerized 2-D segmentation is carried out using edge detection techniques in accordance with an algorithm which is described hereinbelow. The segmenter provides an output which may be stored while additional 2-D image slices are segmented as described hereinabove. 
     For each subsequent 2-D image, the output and/or other characteristics of at least one preceding 2-D image are used as initial markings or in any other suitable manner for determining or partially determining the boundary. It is appreciated that the image may include more than one boundary. Once all of the required 2-D images have been segmented, a segmentation output is provided to the sculpting tool. 
     The segmentation output defines a closed boundary or boundaries distinguishing portions of the image which are of interest and portions of the image which it is desired to discard. 
     The 2-D segmentation step shown in FIG. 11 preferably incorporates the following steps: 
     Initial markings or the preceding boundary are superimposed on the image and a visual check of the boundary may then be carried out. If the boundary appears to need correction and a manual correction is called for, a manual correction is carried out. If, however the boundary does not appear to need correction, it is preferably stored. If the slice being segmented is the last 2-D image slice to be segmented in the 3-D image, the volume having the output boundary or boundaries superimposed thereover is output. If the slice being segmented is not the last 2-D slice to be segmented in the 3-D image, a further 2-D slice is selected. The previous boundary is preferably defined as an initial boundary for the further slice. 
     If, however, the boundary or boundaries are found to need correction and manual correction is selected, a manual correction module applies a manual correction to the boundary or boundaries superimposed on the image. If manual correction is not called for, computerized correction is typically effected by edge detection based segmentation circuitry  90 . 
     The operation of edge detection based segmentation circuitry  90  may be summarized as follows: The boundary or boundaries initially superimposed on the image are supplied to circuitry  90  separately from the image and are broadened in order to define a strip-shaped region or regions of interest (ROI). Edge enhancement is performed on the image, preferably, but not necessarily, within the ROI. As seen in FIGS. 18A &amp; 18B, referred to hereinbelow, a multiple vertex geometrical construction is provided within the region of interest and includes a multiplicity of vertices interconnected by line segments, wherein each line segment is assigned a weight. As described in greater detail hereinbelow, an optimal boundary is constructed from the line segments. The optimal boundary is then superimposed onto the image. 
     The foregoing segmentation method continues until it is decided that the boundary on the last 2-D image of the volume does not require correction. 
     Reference is now made to FIG. 12, which is a flow chart illustrating a strip-shaped region of interest defining step of the operation of FIG. 11 in accordance with a preferred embodiment of the present invention and to FIG. 16 which shows such a region of interest. 
     As seen in FIG. 12, the initial marking or boundary is received and any closed loops, as illustrated in FIG. 16, along the initial marking or boundary are deleted. A strip-shaped region of interest is defined about each initial marking or boundary, for example by employing a convolution having filled circles of constant pixel value as a convolution kernel. The circles need not all have the same diameter. A thresholding function is then applied to discard pixels located exteriorly to the region of interest. The thus-corrected initial marking or boundary is then output together with the strip-shaped region of interest. 
     Reference is now made to FIGS. 13A and 13B, which together are a flowchart illustrating a two-dimensional edge enhancement filtering operation performed in accordance with a preferred embodiment of the present invention on a slice of the original volume image and to FIG. 17, which is an illustration useful in understanding the flowchart of FIGS. 13A and 13B. It is appreciated that the operation of FIGS. 13A and 13B provides a grey level edge enhanced image. Stated more generally, the operation provides an image representation of the intensity of the edge property at each image pixel. 
     The operation illustrated in FIGS. 13A,  13 B and  17  is carried out at each pixel location in each slice of the volume image and searches for a candidate edge segment at every such pixel location, preferably by searching for the direction of a candidate edge segment. For the sake of conciseness, in view of the detailed nature of the steps of the operation indicated in FIGS. 13A and 13B with reference to FIG. 17, a further textual explanation of these steps is believed to be unnecessary and thus is not provided. 
     Reference is now made to FIGS. 14A and 14B, which together are a flow chart illustrating one part of an optimal boundary defining step of the operation of FIG. 11 in accordance with a preferred embodiment of the present invention which includes providing a multiple vertex geometrical construction within the region of interest having a multiplicity of vertices interconnected by line segments, wherein each line segment is assigned a weight. 
     Reference is also made to FIGS. 18A and 18B, which are useful in the understanding of the flow chart of FIGS. 14A &amp; 14B. The functionality of FIGS. 14A &amp; 14B provides information for use in defining a closed boundary within the region of interest. The closed boundary is determined at each point therealong inter alia based on the following characteristics: proximity to an initial marking or a boundary already determined for an adjacent or other slice, the degree of similarity in direction to the initial marking or previously determined boundary and the degree of overlap with the initial marking or previously determined boundary. 
     Optionally, not only the configuration of the previously determined boundary for another slice or other slices, but also some or all of the above-listed characteristics of the said slice or slices, may be employed in subsequently determining the boundary for the current slice. 
     For the sake of conciseness, in view of the detailed nature of the steps of the operation indicated in FIGS. 14A and 14B with reference to FIGS. 18A and 18B, a further textual explanation of these steps is believed to be unnecessary and thus is not provided. 
     Reference is now made to FIG. 15, which is a flow chart illustrating a further part of the optimal boundary defining step of the operation of FIG. 11 in accordance with a preferred embodiment of the present invention which provides an optimal boundary by employing dynamic programming based on the operations described hereinabove with reference to FIGS. 14A,  14 B,  18 A and  18 B. Reference is also made to FIG. 19, which is an illustration useful in understanding the flowchart of FIG.  15 . 
     The overall operation of the present invention may be understood from a consideration of FIGS. 20A and 20B. FIG. 20A schematically depicts the head of a fetus partially occluded by the uterus and placenta. The operation of the present invention provides an image of the head of the fetus without such occlusions (FIG.  20 B). 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various elements described hereinabove as well as modifications and variations thereof which would occur to a person skilled in the art upon reading the foregoing description and which are not in the prior art.