1. Field of the Invention
This invention involves a system and a method for generating three-dimensional (3-D) images of structures from two-dimensional (2-D) images obtained through scanning, in particular, from 2-D images scanned using ultrasound.
2. Description of the Related Art
Imaging of a portion of a patient's body typically involves sensing the strength of one or more signals that have passed through (for example, X-ray), been reflected back from (for example, ultrasound) or been generated within (for example, positron-based imaging) a region of a body. In the context of medical ultrasonic imaging, signals are sensed most strongly from portions of the region where the local change in acoustic impedance is greatest. The relative strengths of the return signals are then converted and processed and displayed in some form, for example, on a monitor, that represents an image of the scanned region.
Existing imaging systems using, for example, ultrasound and positron-based technologies such as Positron Emission Tomography (PET) and Single Positron Emission Computerized Tomography (SPECT), generate images of the body that represent scan planes, that is, 2-D "slices" of the scanned region. These systems display each slice as it is generated so that the user "sees" the 2-D image corresponding to the current position and orientation of the transducer.
One big drawback of such purely 2-D imaging is that most of the imaged structures appear only as cross sections: the user gets no clear image of structures that do not extend in the plane of the "slice" currently being displayed. For example, if an artery is perpendicular to the scan plane, then all the user will be able to see is a small, circular region, and she will not be able to see even sharp "bends" in the artery.
One would think that a solution to this problem would be to simply compile a large number of 2-D image frames, register them in some way, and then display images in any plane of the registered compilation. The problem with this is that, in order to make proper registration possible, one must have accurate information about the distance between adjacent frames. This problem is made worse by the fact that the user normally does not move the transducer at a constant speed, even assuming she moves it in a constant direction; the user may, for example, spend more time "looking" at a particularly interesting portion of the scanned region and move quickly past other portions. Furthermore, different users will normally not move the transducer at the same speed.
One known way of dealing with this problem is to mount the transducer in a motorized bracket arrangement and then move it at a constant speed using the motors. This has several disadvantages: It's expensive; it's bulky; it requires a separate procedure for 3-D scanning than is used for 2-D scanning; and it eliminates much of the user's ability to directly control the scan, especially when using the hand-held transducers commonly used in ultrasonic imaging.
Another way to solve this problem is to mount mechanical (for example, wheels), inertial (accelerometers), magnetic (for example, Polhemus devices) or other types of position sensors on the transducer itself, so that one gets distance information along with the scan information. The drawback of this solution, however, is that such sensors add weight and complexity to the transducers, which makes it difficult to provide them in low-cost machines. Moreover, metallic objects in the examination area can create noise that disturbs magnetic position sensors, and almost every object between the sensor and the transducer will interfere with line-of-sight infrared or ultrasound sensors.
Another known way of creating 3-D images is to use multiple transducers that simultaneous image the same regions from two or more perspectives. The "stereo" imaging data is then processed using known algorithms into a 3-D data set. This solution, however, has an obvious disadvantage: multiple transducers lead to multiplied costs and complexity.
In "Measurement of the Complete (3D) Velocity Vector of Blood Flows," Proceedings of the 1988 Ultrasonics Symposium, pp. 795-99, Bonnefous describes using the distribution of a series of successive scatterers in the scanned region and certain correlation techniques to construct a 3-D model of blood flow. This method presupposes, however, that the scanned region comprises a flowing medium with a given velocity distribution.
What is needed is a system and associated method for generating 3-D images using a single transducer. Three-dimensional imaging should also be possible with little or no change to the flexible and familiar user-directed scan procedures, even for hand-held transducers, and it should be possible to create 3-D representations even of non-moving tissue.