CROSS REFERENCE TO RELATED UNITED STATES APPLICATIONS 
   Reference is hereby made to the following U.S. Provisional patent applications whereof the benefit is hereby claimed and the disclosures hereby incorporated by reference: 
   U.S. Provisional patent application No. 60/312,872, entitled MARKING 3D LOCATIONS FROM ULTRASOUND IMAGES and filed Aug. 16, 2001 in the names of Frank Sauer, Ali Khamene, Benedicte Bascle; 
   U.S. Provisional patent application No. 60/312,876, entitled LOCAL 3D RECONSTRUCTION FROM ULTRASOUND IMAGES and filed Aug. 16, 2001 in the names of Frank Sauer, Ali Khamene, Benedicte Bascle; 
   U.S. Provisional patent application No. 60/312,871, entitled SPATIOTEMPORAL FREEZING OF ULTRASOUND IMAGES IN AUGMENTED REALITY VISUALIZATION and filed Aug. 16, 2001 in the names of Frank Sauer, Ali Khamene, Benedicte Bascle; 
   U.S. Provisional patent application No. 60/312,875, entitled USER INTERFACE FOR AUGMENTED AND VIRTUAL REALITY SYSTEMS and filed Aug. 16, 2001 in the names of Frank Sauer, Lars Schimmang, Ali Khamene; and 
   U.S. Provisional patent application No. 60/312,873, entitled VIDEO-ASSISTANCE FOR ULTRASOUND GUIDED NEEDLE BIOPSY and filed Aug. 16, 2001 in the names of Frank Sauer and Ali Khamene. 


   Reference is hereby made to the following copending U.S. patent applications being filed on even date herewith. 
   U.S. patent application Ser. No. 10/222,182, entitled LOCAL 3D RECONSTRUCTION FROM ULTRASOUND IMAGES and filed in the names of Frank Sauer, Ali Khamene, Benedicte Bascle; 
   U.S. patent application Ser. No. 10/222,333, entitled SPATIOTEMPORAL FREEZING OF ULTRASOUND IMAGES IN AUGMENTED REALITY VISUALIZATION and filed in the names of Frank Sauer, Ali Khamene, Benedicte Bascle; 
   U.S. patent application Ser. No. 10/222,284, entitled USER INTERFACE FOR AUGMENTED AND VIRTUAL REALITY SYSTEMS and filed in the names of Frank Sauer, Lars Schimmang, Ali Khamene; and 
   U.S. patent application Ser. No. 10/222,170, entitled VIDEO-ASSISTANCE FOR ULTRASOUND GUIDED NEEDLE BIOPSY and filed in the names of Frank Sauer and Ali Khamene. 
   FIELD OF THE INVENTION 
   The present invention relates to imaging and, more specifically to ultrasound imaging. 
   DESCRIPTION OF THE RELATED ART 
   Ultrasound scanners are commonly utilized to capture live 2D images from within objects or patients. Scanners typically have a standard option to freeze an image in time and display the still image on the screen for evaluation, e.g. for measuring spatial dimensions in the image. 
   Helpful background material on augmented reality and related topics can be found in Proceedings of the IEEE and ACM International Symposium on Augmented Reality 2000, dated Oct. 5-6, 2000; Munich, Germany; IEEE Computer Society, Los Alamitos, Calif., U.S.A. In the above-cited Proceedings, an article of particular interest entitled AUGMENTED WORKSPACE: DESIGNING AN AR TESTBED is published on pages 47-53, and is authored by Frank Sauer, an inventor in the present application, et alii. 
   See also the review article by R. T. Azuma: “A Survey of Augmented Reality”, Presence: Teleoperators and Virtula Environments, 6(4), 355-386, (1997). 
   SUMMARY OF THE INVENTION 
     FIG. 1  shows a schematic block diagram of such an augmented reality system as may be utilized in conjunction with features of the invention. A tracker camera  10  is coupled by way of an A/D (analog to digital) converter  12  to a programmable digital computer  14 . Two scene cameras  15  are coupled to computer  14 . An ultrasound scanner  16 , having a transducer  18 , is coupled by way of an A/D converter  20  to computer  14 . A head-mounted display (HMD) control unit  22  is coupled for signal interchange with computer  14  and to an HMD display  24 . 
   In accordance with an aspect of the present invention, a method is provided for marking (or outlining) of targets on an ultrasound slice in the 2-dimensional (2D) ultrasound slice plane and in 3 dimensions (3D). 
   In accordance with another aspect of the present invention, a system tracks the pose of the transducer used in the ultrasound scanner, and hence the pose of the ultrasound slice, with respect to a fixed external 3D coordinate system. 
   In accordance with another aspect of the present invention, a system provides an interface to the user that enables the user either to directly mark a desired target in an ultrasound image, or trigger the system to (semi)automatically locate a target in a given ultrasound image. 
   In accordance with another aspect of the invention, a method for marking three-dimensional (3D) locations from images obtained from an ultrasound imaging system including a transducer, comprising the steps of: tracking the pose of the transducer with respect to an external 3D coordinate system; obtaining a two-dimensional (2D) ultrasound image from the transducer; marking a desired target with a marker on the 2D ultrasound image; and calculating the 3D position of the marker utilizing data from the step of tracking. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood from the following detailed description of preferred embodiments, in conjunction with the Drawing in which the  FIG. 1  shows a system block diagram of an augmented reality system. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
   In accordance with a preferred embodiment, an ultrasound scanner, including a transducer, is utilized for imaging. The user is provided with augmented reality visualization, where the ultrasound slices are overlaid onto a view of the patient/object in a registered manner. Tracking apparatus is provided for tracking the transducer and for tracking the user&#39;s viewpoint for the augmented reality application. Such augmented reality application techniques, whereby structures that are visible in the ultrasound images appear in the location of the corresponding physical structures are, per se, known in the art and need not be set forth in detail. 
   Preferably, the augmented view is stereoscopic to provide the user with depth perception. A calibration procedure is provided and the equipment is coupled to a computer for generating the graphics required and providing the necessary image processing. 
   Furthermore, a local user interface is provided in accordance with the principles of the present invention. 
   With the user interface, a user can initiate the target localization and marking. 
   In accordance with the present embodiment, the user interface is equipped with a pointing device, such as a computer-type mouse, coupled to the computer. 
   In operation, a user can use the pointing device, such as a mouse, to mark a 2D location in the image. With the knowledge of the 3D pose of that image, the system calculates the 3D position of the marker. The user&#39;s input may be in an “on-line” mode, where he holds the transducer still in the desired pose, or in an “off-line” mode, where the user first freezes, that is, records the image, together with its pose information, and then places the marker in the recorded still image at his leisure. For the on-line mode, the pointing device is preferably attached to the transducer and can be operated with the same hand that is holding the transducer, or alternatively, it is placed on the floor and operated by way of a foot-pedal. 
   In accordance with another preferred embodiment, the system performs image processing on an ultrasound image and automatically, or semi-automatically, locates a target of a type optionally predetermined by the user. The input required of the user is thereby simplified. For example, without an extra pointing device, the user may place the transducer in a pose where the target structure appears on the vertical centerline of the ultrasound image. The user then triggers the system to locate the target along the image&#39;s centerline such as by using a button on the transducer, by operating a foot-switch, by using voice control, or any other convenient control device. The system in accordance with the present embodiment includes a processor that searches the image along its centerline, which makes locating the target easier than if the search would have to be conducted over the whole image. 
   A preferred search algorithm would be to de-noise the image around the centerline, for example, with a median filter, identify potential target locations in a line scan along the centerline, and verify the existence of a target with, for example, a Hough transform. The Hough transform is known and may be found in various textbooks, such as, for example, “Fundamentals of Electronic Image Processing” by Arthur R. Weeks, Jr., IEEE Press, New York, N.Y.; 1996. 
   When the system proposes a target location, the user then accepts or rejects the proposed location with another trigger input by using a button on the transducer, a footswitch, voice control, etc. 
   As an alternative to using a line on the ultrasound image as pointer, a line other than the vertical centerline can be used, such as a vertical off-center line, or a line that is tilted at an angle with respect to the vertical direction. 
   A target position along a line can be input by user via a thumbwheel at transducer or a similar one-dimensional (1D) pointing device for which no image processing is necessary. 
   In an alternative embodiment, the user uses a line on the ultrasound image to point to target from two different transducer poses, so that the system can calculate the target location as the intersection of the two lines in 3D space. Alternatively two different lines can be used to require less movement between the two pointing transducer poses, for example, two lines that intersect in the image. 
   The image processing is powerful enough to find a target in the 2D image. The user need then only position the transducer so that the target is visible in the image, initiate the automatic target search, and then confirm or reject a target proposed by the system. Several targets can be found in the same image. This embodiment is attractive from a user point of view as it requires the least input; however, robust target detection in ultrasound images is difficult. Therefore, there are advantages to providing the user with a 1D or 2D pointing interface as described in the above embodiments. 
   The system thus graphically marks confirmed target locations and records the respective 3D coordinates thereof. Preferred visualization is in a stereoscopic augmented or virtual reality fashion. 
   Instead of just marking the target location as the target center, the system can also perform image processing to obtain an outline of the target, and show this outline in a graphical representation. 
   Marking of the 3D locations of target structures can be helpful for diagnostic purposes, for guidance of interventional procedures, as well as for registering the ultrasound images to images taken with other imaging modalities like CT (computer tomography) and MR. (magnetic resonance imaging). 
   While the invention has been explained by way of exemplary embodiments, it will be understood by one of skill in the art to which it pertains that various modifications and changes may be readily made without departing from the spirit of the invention which is defined by the claims following. For example, where the claims refer to marking a target, this is to be understood to encompass deriving an outline of a target by image processing to present the outline by graphical representation.
A method for marking three-dimensional (3D) locations from images obtained from an ultrasound imaging system including a transducer, comprising the steps of: tracking the pose of the transducer with respect to an external 3D coordinate system; obtaining a two-dimensional (2D) ultrasound image from the transducer; marking a desired target with a marker on the 2D ultrasound image; and calculating the 3D position of the marker utilizing data from the step of tracking.