Ultrasonic imaging has become an important and popular diagnostic tool with a wide range of applications.  Particularly, due to its non-invasive and typically non-destructive nature, ultrasound imaging has been used extensively in the medical profession.  Modern high-performance ultrasound-imaging systems and techniques are commonly used to produce both two-dimensional and three-dimensional diagnostic images of internal features of an object, (e.g., portions of the anatomy of a human patient).  A diagnostic ultrasound-imaging system generally uses a wide-bandwidth transducer to emit and receive ultrasound signals.  The ultrasound-imaging system forms images of the internal tissues of a human body by electrically exciting an acoustic-transducer element or an array of acoustic-transducer elements to generate ultrasonic pulses that travel into the body.  The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities to the propagating ultrasonic pulses.  The various echoes return to the transducer and are converted into electrical signals that are amplified and processed to produce an image of the tissues.  These ultrasonic-imaging systems are of significant importance to the medical field by providing physicians real-time high-resolution images of internal features of a human anatomy without resort to more invasive exploratory techniques such as surgery. 
As described above, ultrasonic-imaging systems employ an acoustic transducer to radiate and receive a plurality of ultrasonic pulses.  The acoustic transducer, which radiates the ultrasonic pulses, typically comprises a piezoelectric element or an array of piezoelectric elements.  As is known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the transmitted ultrasonic pulses.  Similarly, the received echoes cause the piezoelectric element to deform and generate a corresponding receive-electrical signal.  The acoustic transducer is often packaged in a handheld device that allows an operator substantial freedom to manipulate the transducer over a desired area of interest.  The transducer is often connected via a cable to a control device that generates and processes the electrical signals.  In turn, the control device may transmit image information to a real-time viewing device, such as a monitor.  In alternative configurations, the image information may also be transmitted to physicians at a remote location and or stored to permit viewing of the diagnostic images at a later time. 
To generate a three-dimensional image, volumetrically spaced information, such as planar or line information, associated with positional information is obtained by using any of various transducers.  One approach is to use a two-dimensional transducer array to obtain three-dimensional image information directly.  A two-dimensional array can be used to scan electronically in any desired orientation to acquire the desired information.  Another approach is to collect multiple two-dimensional image data frames using a one-dimensional or a 1.5 dimensional transducer array along with relative positional information associated with the image-data frames so that these frames may be sequentially assembled in a three-dimensional volume to form the desired three-dimensional reconstruction. 
Based on echo signals received from the transducer, as described above, the volumetric information, such as assembled from multiple sets of planar information, is generated.  The image information is derived as a function of various imaging modes.  For example, B-mode or brightness mode, or color-Doppler image mode. 
Once the volumetrically spaced information, such as planar information, and associated positional information is provided, standard methods are employed for assembling the image information into a three-dimensional volume of the subject and for providing the desired display, such as a cross-section, a surface rendering, or the like. 
Some prior-art ultrasound-imaging systems were designed with the philosophy that a technician would perform the task of acquiring a “full volume” of an organ of interest within a patient and that a physician or other clinician would review the results of a diagnostic session providing a plurality of images offline.  Under this diagnostic modality, it is imperative that the technician obtains all of the slices and projections necessary for a diagnosis.  As a result, no provisions were provided to permit the technician to reduce the size of a volume-under-observation (VUO). 
However, it takes a considerable amount of time to acquire a large volume scan, which negatively impacts the frame rate in a real-time imaging system.  In non-real-time systems, it is the total time of acquisition that is negatively impacted.  For example, it may take upwards of 5 minutes to acquire a full four-dimensional (space and time) volume of the human heart over a single cardiac cycle. 
Some prior-art imaging systems addressed the issue of frame rate by incorporating a multi-channel parallel beam-formation structure within the hardware.  However, this approach significantly increases the cost and the size of the resulting ultrasound-imaging system.  A multi-channel parallel beamforming hardware solution is illustrated in FIG.  1.  As shown, a prior-art three-dimensional imaging system 10 may comprise a transmit controller 12, a transducer 14, a parallel configuration of receive beamformers 16a, 16b, 16c, . . . ,  16x, a radio frequency (RF) filter 18, both a Doppler-image processor 20 and a B-mode image processor 22.  The prior-art three-dimensional imaging system 10 may further comprise a scan converter 24, a three-dimensional image processor 26, an image-data storage device 28, and a display 30. 
As illustrated in FIG.  1, the prior art three-dimensional imaging system 10 may use a transmit controller 12 to control the operation and timing of multiple excitation signals that may be forwarded to the transducer 14.  The transducer 14 may be configured to emit and receive-ultrasound signals, or acoustic energy, respectively to and from an object-under-test (not shown).  In response to ultrasound-transmit signals, one or more echoes are emitted by the object-under-test and are received by the transducer 14, which transforms the echoes into an electrical signal for further processing.  During a receive mode, an analog waveform is received at the transducer 14 at a number of beam positions.  Each of the plurality of received analog waveforms may be forwarded to a dedicated receive beamformer 16a through 16x.  Each of the set of parallel beamformers 16 may receive a series of analog waveform sets, one set for each separate acoustic line, in succession over time and may process the waveforms in a pipeline-processing manner.  Each of the set of parallel beamformers 16a through 16x may be configured to convert its respective analog-echo waveform into a digital-echo waveform comprising a number of discrete-location points.  Each of the set of parallel beamformers 16a through 16x may delay the separate echo waveforms by different amounts of time and then may add the delayed waveforms together, to create a composite-digital RF-acoustic line. 
A RF filter 18 may be coupled to the output of the parallel beamformers 16 and may be configured to receive and process digital-acoustic lines in succession.  The RF filter 18 may be in the form of a bandpass filter.  As further illustrated in FIG.  1, the filtered image data may be forwarded to a Doppler image processor 20 and a B-mode image processor 22 for two-dimensional image mode processing.  As further illustrated in FIG.  1, the Doppler-image processor 20 and the B-mode image processor 22 may be coupled to a scan converter 24 to convert the image data into a format suitable for display.  The scan converter 24 may process the data once an entire data frame (i.e., a set of all acoustic lines in a single view, or image/picture to be displayed) has been accumulated. 
Next, the prior-art three-dimensional imaging system 10 may forward the converted image data to a three-dimensional image processor 26 for performing the necessary mathematical manipulations to generate volumetric information from a series of planar (i.e., two-dimensional) ultrasound images.  As further illustrated in FIG.  1, the three-dimensional image processor 26 may be coupled to an image-data storage device 28 and a display 30.  The image-data storage device 28 may permit both still frame and video image storage for offline-image manipulation and viewing.  The display 30 may take the form of a specialized cathode-ray-tube (CRT) or other suitable image-creating device that may permit real-time image viewing by an operator. 
As previously described, volumetric information consisting of multiple planes, may be collected by a prior-art three-dimensional imaging system 10 (FIG.  1) as illustrated in FIG.  2.  For example, the planar information 40 may be collected by using the transducer 14 to transmit a plurality of ultrasonic-transmit planes 13a, 13b, 13c, . . . ,  13f as shown.  The plurality of transmit planes 13 may generate a plurality of response planes (not shown) that may be received by the transducer 14.  The plurality of response planes, together with positional information, may be processed by the prior-art three-dimensional imaging system 10 of FIG.  1 to generate a three-dimensional image.  As further illustrated in FIG.  2, volumetric information may be scanned over a 60° by 60° footprint at a depth of 16 cm.  As also illustrated in FIG.  2, the plurality of response planes 13 may span a length and a width of 16 cm, thus forming a volumetric information pyramid.  As is evident by observing FIG.  2, a VUO (e.g., an organ or a portion of an organ of the human anatomy) must lie within the three-dimensional “scan” pyramid formed by the plurality of ultrasonic transmit planes 13.  The planar information 40 collected by the prior art three-dimensional imaging system 10 (FIG.  1) as illustrated in FIG.  2 is representative of planar information 40 that may be collected with a stationary transducer 14. 
To achieve a large volume)(60°×60°) in real-time (i.e., better than 15 Hz), the prior-art three-dimensional imaging system 10 (FIG.  1) was forced to use 16x parallel beamformers.  This 16x parallel beamformer architecture is undesirable as the realizable three-dimensional resolution comes at a significant cost, especially when compared with prior art two-dimensional imaging systems.  First, the cost for each beamformer makes the prior art three-dimensional imaging system relatively expensive.  Second, to achieve 16x parallel (1 transmit firing for 16 simultaneous receive acquisitions) operation, the prior art three-dimensional imaging system 10 uses a broadened transmit beam of approximately 4°×4°.  Then within the transmit beamwidth, 16 receive beams (each 1° apart) are interrogated using a 4°×4° receive beamwidth.  The “round-trip” resolution is effectively a multiplication of the transmit and the receive beamwidths.  As a result, of the relatively broad transmit and interrogation beamwidths, the prior-art three-dimensional imaging system 10 loses significant resolution when compared to prior-art two-dimensional only imaging systems. 
Other prior art systems have been devised that use various devices to control the relative position of the transducer 14 with respect to a VUO.  It will be appreciated that planar information 40 may be collected by a three-dimensional imaging system configured to vary the position of the transducer 14.  The planar information 40 resulting from a plurality of two-dimensional views acquired with a position-varying transducer may take the form of slices.  In return for increased complexity, ultrasound-imaging systems capable of varying the relative position of the transducer 14 can acquire a larger volume than those systems that use a fixed-position transducer 14.  Regardless of the two-dimensional imaging methodology selected, appropriate algorithms are known for combining the image information with positional information associated with each of the image slices acquired to develop a three-dimensional rendering of a VUO. 
Having generally described two prior art methods for acquiring a three-dimensional volume using a plurality of two-dimensional images, reference is now directed to FIG.  3, which illustrates prior-art performance characteristics that may be expected using a relatively large beamwidth and a multi-channel parallel-beamforming system. 
In this regard, FIG.  3 further describes the operation of the prior-art three-dimensional imaging system 10 of FIG.  1.  More specifically, FIG.  3 illustrates a plot of expected-performance characteristics 50 such as transmit plane 52, receive plane 54, and round-trip 56 sensitivity versus transmitted beamwidth as may be expected with the prior-art three-dimensional imaging system 10.  FIG.  3 will be further discussed in relation to the plot of FIG.  6 where a comparison will be made with expected performance characteristics for a three-dimensional imaging system in accordance with the present invention. 
In addition, to the increased cost and size of various prior-art ultrasound-imaging systems, another problem associated with acquiring full volumes is that the target volume's location is referenced to the probe.  As a result, references to the anatomy must be translated accurately to accurately identify and diagnose a tissue volume under observation.  References to the anatomy are typically minimal and either involve technician “labeling” or relying on the diagnosing clinician to identify the anatomy.  As such, it often becomes difficult for the reviewing clinician to understand what they are looking at.  Unless they are very skilled and experienced with a particular-imaging system and typical images that are produced, the clinician often becomes “lost in the volume.” 
As a result, there is a need for an improved four-dimensional (space and time) ultrasound-imaging system that permits an operator to acquire a volume in a time-critical fashion, that is capable of referencing the volume rendering to a standard two-dimensional imaging mode, and permits the operator to selectively choose a number of display-mode parameters that result in a user-directed view within a VUO.