Method and apparatus for obtaining a volumetric scan of a periodically moving object

Method and apparatus for acquiring and processing a volumetric scan of a periodically moving object. A volumetric scan is performed of the periodically moving object which repeats a cycle of movement over time. A time interval of the periodic movement of the object is identified within the volumetric scan, and the volumetric scan is rearranged based on the time interval.

BACKGROUND OF THE INVENTION

This invention relates generally to diagnostic ultrasound systems. In particular, the present invention relates to method and apparatus for obtaining a volumetric scan of a periodically moving object within a body.

A current challenge exists to perform a multidimensional ultrasound scan of a quickly, and more or less rhythmically, moving object within a body, such as a fetal heart. Currently, volume probes having a conventional one-dimensional (1D) array, which is mechanically moved in the elevation direction, as well as electronically steered 2D arrays, may be used for the acquisition. This technique makes it possible to acquire pyramid-shaped volume data sets. To image the fetal heart, high frame rate acquisitions are necessary, no matter whether 2D or 3D data sets are being acquired. For acquiring 3D data sets in real-time, one limitation is the constant speed of sound at 1540 m/s; this limits the amount of data to be acquired per second and thus these acquisitions are a tradeoff between frame rate and image quality. To acquire and achieve high frame rates, the line density has to be decreased, which significantly impairs lateral and elevation resolution.

One approach is to perform ECG-triggered volumetric acquisitions as described in U.S. Pat. No. 5,159,931 to Pini, Nov. 3, 1992, which is hereby incorporated by reference in its entirety, which works well when imaging the adult heart, but is generally not available for the fetal heart due to absence of an appropriate fetal ECG signal. Alternatively, one may acquire data of several heart cycles at several fixed positions which are recorded with a position sensor, and obtain cardiac motion information via Fourier transform methods as described in Nelson et al, “Three Dimensional Echocardiographic Evaluation of Fetal Heart Anatomy and Function: Acquisition, Analysis, and Display”, J Ultrasound Med 15:1–9, 1996, which is hereby incorporated by reference in its entirety. Another problem is experienced when imaging a fetal heart during early pregnancy with 2D fetal echocardiography, when the relationship between fetus and amniotic fluid allows a lot of movement. If the fetus is very active, it may be time consuming or impossible at the time of the scheduled exam to acquire sufficient cardiac data.

Thus, a system and method are desired to obtain multidimensional data sets of a quickly moving object within a body that addresses the problems noted above and others previously experienced.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a method of processing a volumetric scan of a periodically moving object comprises performing a volumetric scan of a periodically moving object. A time interval of a periodic movement of the object is identified within the volumetric scan, and the volumetric scan is rearranged based on the time interval.

In one embodiment, a method of acquiring a diagnostic image of a periodically moving object comprises acquiring a series of scan planes. The series of scan planes comprise a moving object which repeats a cycle of movement over time, and the series of scan planes are acquired over at least two movement cycles. At least one common point of interest is identified within each of the series of scan planes. Intensity values of the common points of interest are compared between the series of scan planes. At least two intensity values are identified based on a result of the comparison, and the series of scan planes are rearranged based on the intensity values.

In one embodiment, an apparatus for acquiring a volumetric scan of a periodically moving object comprises a transducer having an array of elements for transmitting and receiving ultrasound signals to and from an area of interest. The area of interest comprises a periodically moving object. A transmitter drives the array of elements to scan the periodically moving object once in a single direction. A receiver receives the ultrasound signals which comprise a series of adjacent scan planes. A memory stores the series of adjacent scan planes as a volumetric data set, and a processor processes the series of adjacent scan planes. The processor identifies a time interval based on the periodically moving object, and rearranges the series of adjacent scan planes based on the time interval.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a block diagram of an ultrasound system100formed in accordance with an embodiment of the present invention. The ultrasound system100includes a transmitter102which drives an array of elements104within a transducer106to emit pulsed ultrasonic signals into a body. A variety of geometries may be used. The ultrasonic signals are back-scattered from structures in the body, like blood cells or muscular tissue, to produce echoes which return to the elements104. The echoes are received by a receiver108. The received echoes are passed through a beamformer110, which performs beamforming and outputs an RF signal. The RF signal then passes through an RE processor112. Alternatively, the RF processor112may include a complex demodulator (not shown) that demodulates the RE signal to form IQ data pairs representative of the echo signals. The RE or IQ signal data may then be routed directly to RF/IQ buffer114for temporary storage.

The ultrasound system100also includes a signal processor116to process the acquired ultrasound information (i.e., RF signal data or IQ data pairs) and prepare frames of ultrasound information for display on display system118. The signal processor116is adapted to perform one or more processing operations according to a plurality of selectable ultrasound modalities on the acquired ultrasound information. Therefore, the signal processor116may be used to perform the functions of a STIC analyzer and converter42and a volume display processor46, which are described below. Acquired ultrasound information may be processed in real-time during a scanning session as the echo signals are received. Additionally or alternatively, the ultrasound information may be stored temporarily in RF/IQ buffer114during a scanning session and processed in less than real-time in a live or off-line operation. An image buffer122is included for storing processed frames of acquired ultrasound information that are not scheduled to be displayed immediately. The image buffer122may comprise any known data storage medium.

FIG. 2illustrates an ultrasound system formed in accordance with one embodiment of the present invention. The system includes a transducer10connected to a transmitter12and a receiver14. The transducer10transmits ultrasonic pulses and receives echoes from structures inside of a scanned ultrasound volume16. Memory20stores ultrasound data from the receiver14derived from the scanned ultrasound volume16. The volume16may be obtained by various techniques (e.g., 3D scanning, real-time 3D imaging, volume scanning, 2D scanning with transducers having positioning sensors, freehand scanning using a Voxel correlation technique, 2D or matrix array transducers and the like).

The transducer10is moved, such as along a linear or arcuate path, while scanning a region of interest (ROI). At each linear or arcuate position, the transducer10obtains scan planes18. The scan planes18are stored in the memory20, and then passed to a spatial and temporal image correlation (STIC) analyzer and converter42. In some embodiments, the transducer10may obtain lines instead of the scan planes18, and the memory20may store lines obtained by the transducer10rather than the scan planes18. Data output by the STIC analyzer and converter42is stored in volume memory44and is accessed by a volume display processor46. The volume display processor46performs volume rendering and/or other image processing techniques upon the data. The output of the volume display processor46is passed to the video processor50and display67.

The position of each echo signal sample (Voxel) is defined in terms of geometrical accuracy (i.e., the distance from one Voxel to the next), ultrasonic response, and derived values from the ultrasonic response. Suitable ultrasonic responses may include B-flow, gray scale values, color flow values, and angio or power Doppler information.

FIG. 3illustrates a series of projecting scan planes acquired by the ultrasound system100ofFIGS. 1and/or2. Each line150-156represents a scan plane into the page, and the elevation164of the scan is indicated. AlthoughFIG. 3illustrates the scan planes in a fan-shape, it should be understood that the fan-shape is not a limitation, and that other shapes, such as a rectangle with parallel scan planes, for example, may also be acquired. The following discussion will be based on acquiring data representing the fetal heart, but it should be understood that other periodically moving objects may be similarly scanned and processed, such as an adult heart, a heart valve, an artery, a vein and the like. Also, although the modality discussed is ultrasound, the image acquisition and processing techniques may be used with other modalities, such as CT, MRI, and the like.

Similarly, the information acquired by the ultrasound system100is not limited to B-mode information only, but may also contain information gathered from evaluating several lines from the same sample volume (e.g., color Doppler, power Doppler, tissue Doppler, B-flow, Coded Excitation, harmonic imaging, and the like). These data of different ultrasound modalities or scanning techniques may also be acquired simultaneously, and may be used either for analysis, display, or both.

The transducer10is held in one position throughout the acquisition, and is positioned to acquire data representative of the item of interest, such as the fetal heart. The elements104, or array of elements104, are electronically or mechanically focussed to direct ultrasound firings longitudinally to scan along adjacent scan planes, and external position sensing is not necessary.

A single, slow, acquisition sweep acquiring adjacent scan planes18may start, by way of example only, at border158, and end at border160. Other start and end points of the acquisition sweep may also be used. By way of example only, the acquisition sweep may have a sweep angle162of 20 degrees and a time period including several, or at least two, movement cycles of the fetal heart. Other sweep angles162may be used. The acquisition sweep may be accomplished by continuously moving the focus of the ultrasound firings or by changing the focus in small increments.

Alternatively, the acquisition sweep may have an acquisition time period covering multiple movement cycles, and the sweep angle162may be changed to reflect the type and/or size of anatomy being scanned. An acquisition with a longer acquisition time will acquire more data and the spatial resolution will be better when compared to a scan acquired over a shorter acquisition time. An acquisition with a higher frame rate will result in better temporal resolution than a scan acquired with a lower frame rate. The elements104are focussed to acquire the adjacent scan planes18very close to each other spatially.

Each scan plane represented by the lines150–156inFIG. 3is a 2D image having a resolution in time. By way of example only, if the resolution in time is within a range of 50–150 Hz, 50–150 scan planes, respectively, may be acquired for every 2 degrees of sweep angle162, for an acquisition time of 10 seconds and a sweep angle162of 20 degrees. The resolution in time is not limited by this example. For clarity, not all of the scan planes are illustrated.

FIG. 13illustrates a series of scan planes310of a fetal heart. The beating of the fetal heart during the acquisition causes a rhythmical pattern of changes in the diameter of the heart. The scan planes are acquired in the combined direction of space and time312. Markers314and316are included to illustrate the fetal heart rate. Calculating the fetal heart rate is discussed below.

FIG. 4illustrates a series of adjacent scan planes170, such as the adjacent scan planes acquired inFIG. 3. The scan planes are illustrated over combined space and time176with axial172and azimuth174directions as indicated. Therefore, the elevation is orthogonal to the scan planes170. A 2D image178having data indicative of one scan plane of the fetal heart is represented at the end of the adjacent scan planes170. Referring again toFIG. 13, 2D image318also comprises data of one scan plane of the fetal heart.

FIG. 5illustrates a 2D slice190of the adjacent scan planes170ofFIG. 4. An image correlation technique is used to extract the movement of the fetal heart from the scan plane data.

Returning toFIG. 4, the signal processor116chooses an x,y coordinate, point182, from image acquisition system coordinates. By way of example only, the coordinates may be polar, rectangular, and the like, and are not limited to a specific acquisition geometry. The point182is identified on each scan plane. InFIG. 5, line192is illustrated. Line192corresponds to points182, and therefore runs through the series of adjacent scan planes170at the same x,y coordinate.

FIG. 6illustrates a plot200of intensity194over time and space196for the point182. Therefore, an intensity value is identified for the point182on each image frame and displayed over time as intensity line198. The periodic movement of the fetal heart introduces periodic intensity variation (as illustrated by the rhythmical pattern of the intensity line198). The variations in intensity are analyzed by the STIC analyzer and converter42in order to obtain movement information. It should be understood that the plot200is a representation only, and that other methods, such as storing the identified intensity values in memory20, volume memory44, or image buffer122may be used.

FIG. 7illustrates an autocorrelation184of intensity line198. The autocorrelation184may be calculated by the STIC analyzer and converter42by taking the autocorrelation function of the intensity line198, such as by using the following equation:
A(y)=∫s(x)·{overscore (s(x−y))}dx
wherein A(y) is the autocorrelation function of the signal s, x is the integration variable in the spatio-temporal domain, y is the lag of the autocorrelation function, and s is the intensity line198.

By calculating the autocorrelation184, a peak202at a zero-position is identified. The peak202is the highest peak, or the peak with the most energy. The STIC analyzer and converter42then identifies a first significant local maximum204, which is the peak with the next highest energy. The STIC analyzer and converter42calculates a time interval206between the peak202and the first significant local maximum204. The time interval206identifies the period of the heart cycle. Once the time interval206is known, the STIC analyzer and converter42determines how many adjacent scan planes were acquired within the time interval206.

Alternatively, the heart cycle may be calculated using Fast Fourier Transform Analysis (FFT). The STIC analyzer and converter42or signal processor116may identify the frequency of the movement as the location of the first significant local maximum204in a power spectrum of the intensity line198. In addition, Doppler methods may be used to determine the velocity of tissue movement to identify specific motion states (e.g., Systole, diastole, etc) of the object.

FIG. 8illustrates an autocorrelation210of intensity line208(FIG. 5). The autocorrelation function was taken of the intensity line208, which corresponds to a point186(FIG. 4) identified outside the heart. Therefore, there is little or no periodic movement, and intensity line208may only have one significant peak212. A time interval cannot be calculated.

FIG. 9illustrates a single 2D image220similar to the 2D image178ofFIG. 4. For simplicity, it may be assumed that the heart or other moving object of interest is approximately in the center of the 2D image220. Therefore, the signal processor defines a border222around an interior portion232of the 2D image220. The border222may be a predefined number of pixels in from the top224, bottom226and sides228,230towards the interior portion232of the 2D image220. The border222need not be symmetrical along all edges.

The STIC analyzer and converter42defines a number of points234(similar to points182and186ofFIG. 4) within the interior portion232. The number of points234may be defined by a pattern having a defined size and resolution. For example, the pattern may resemble a chess board in which every other point is chosen. Alternatively, every 4thor 10thpoint may be chosen. The pattern may change based upon user preference, or upon the anatomy being scanned. For example, a different template may be offered for a fetal heart, heart valve, artery, and the like. Optionally, the STIC analyzer and converter42may randomly select a predefined number of points234within the interior portion232. The number of points234and/or the size and resolution of the pattern may vary depending upon factors such as processing speed, image resolution, and the like.

Returning toFIG. 5, a line (similar to lines192and208) is defined for each point in the number of points234(FIG. 9). An intensity line198(FIG. 6), or intensity values, are defined for each of the lines corresponding to the number of points234. The autocorrelation184is then taken of each intensity line198.

It should be understood that althoughFIG. 9represents a 2D image, the above method may be applied to data acquired using other acquisition modes, such as Doppler, B-flow, and the like.

FIG. 10illustrates a summed autocorrelation240. The STIC analyzer and converter42sums the autocorrelations taken for each point in the number of points234. By summing the autocorrelations, noise214(FIG. 7) is effectively removed from the summed autocorrelation240. In addition, points located in areas not experiencing movement, such as above or below the heart (point186) do not prevent the STIC analyzer and converter42from calculating the average time interval246. Alternatively, a filter or windowing function may be used in addition to remove noise214.

The STIC analyzer and converter42identifies a peak242at zero (having the highest intensity) and a first significant local maximum244as discussed previously. The time interval calculated between the zero peak242and the first significant local maximum244is the average time interval246of one movement cycle. The STIC analyzer and converter42now determines the number of scan planes occurring within the average time interval246, or heart cycle.

FIG. 11illustrates a series of adjacent scan planes250similar to the adjacent scan planes170ofFIG. 4. Time intervals252–258separate the series of adjacent scan planes250into the number of slices determined by the STIC analyzer and converter42. The location of time intervals252–258within the series of adjacent scan planes250need not correspond to the start and end of the heart cycle. Therefore, the STIC analyzer and converter42may choose any scan plane within the series of adjacent scan planes250to begin sectioning the data into time intervals252–258. The STIC analyzer and converter42may move forward and backward through the series of adjacent scan planes250to section the data into the time intervals252–258. By way of example only, the series of adjacent scan planes250comprise 4 scan planes within each time interval252–258. Time interval252comprises scan planes260–266, time interval254comprises scan planes268–274, time interval256comprises scan planes276–282, and time interval258comprises scan planes284–290. When imaging a fetal heart or other anatomy as discussed previously, however, many more scan planes would be acquired within each time interval252–258.

The STIC analyzer and converter42rearranges the order of the scan planes260–290and combines the scan planes acquired at the same phase, or point in time within the heart cycle, but from a different lateral position, into a volume. As illustrated inFIG. 11, scan planes A1260, B1268, C1276, and D1284were acquired during the same phase within the heart cycle. Similarly, each of the following subsets of scan planes [scan planes A2262, B2270, C2278, D2286], [scan planes A3264, B3272, C3280, D3288], and [scan planes A4266, B4274, C4282, D4290] were acquired during the same phase within the heart cycle.

FIG. 12illustrates a series of volumes292corresponding to the subsets of scan planes260–290ofFIG. 11. While the volume250ofFIG. 11incorporates two purely spatial dimensions and one combined spatio-temporal dimension, each of the volumes294–300ofFIG. 12has purely 3 spatial dimensions and covers data from a single point in time (relative to the heart cycle).

The STIC analyzer and converter42combines each subset of scan planes260–290into one volume. Therefore, volume1294comprises the image data of scan planes A1260, B1268, C1276, and D1284. Volume2296comprises the image data of scan planes A2262, B2270, C2278, and D2286. Volume3298comprises the image data of scan planes A3264, B3272, C3280, and D3288. Volume4comprises the image data of scan planes A4266, B4274, C4282, and D4290. Each volume294–300comprises a snapshot of the fetal heart during one single beat.

The series of volumes292may be displayed in three orthogonal planes in a cycle, such as a cineloop, which allows a user to navigate through the volumes294–300and view individual volumes. When imaging a fetal heart, for example, approximately 40–60 volumes294–300may be created and displayed on display67. Each of the 40–60 volumes294–300represents a fixed point within the heart cycle.

Alternatively, the data may be processed and displayed in other ways. For example, the volume display processor46may render the image data to show the inner 3D structure of the heart. For example, maximum intensity projection, minimum intensity projection, average projection, and the like may be calculated and displayed. Also, a single volume294–300, or a portion or slice of a volume294–300, may be selected for display. The selected portion may be rotated on the display67or further processed separate from the remaining volume data. In addition, an anatomic M-mode image representing a single selected point within the volumes may be displayed over time. The heart rate and/or other data may also be displayed.

Now that the series of volumes292has been created, it may be stored in a memory, such as ultrasound data memory20, image buffer122, a hard drive, a floppy, CD, or DVD drive, or on a server on a network. The series of volumes292and/or the unprocessed volumetric data may also be transferred via a network or one of the aforementioned portable discs to be further processed and reviewed at a different location after the patient has left the examination. Having the data available to be reviewed and processed later is advantageous, especially during early pregnancy, when the relationship between fetus and amniotic fluid allows a lot of movement. Once the data is acquired, fetal movement is no longer an issue, as previously discussed in relation to 2D fetal echocardiography.