WO2009031078A1

WO2009031078A1 - Spectral and color doppler imaging system and method

Info

Publication number: WO2009031078A1
Application number: PCT/IB2008/053455
Authority: WO
Inventors: David Hope Simpson; Jeffry E. Powers; Martin Anderson
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2007-09-04
Filing date: 2008-08-27
Publication date: 2009-03-12

Abstract

An ultrasound imaging system is disclosed for creating spectral Doppler images from previously stored color Doppler image data. A relatively long ensemble of Doppler pulses is received from along a plurality of lines in an image plane or volume. The corresponding echoes are Doppler processed to produce I/Q Doppler data sufficient to enable a full Doppler spectrum to be estimated at every point in the image plane or volume. The Doppler data is saved for every frame. A spectral Doppler sample volume may be specified as a post processing function and a spectral Doppler image created from the saved data.

Description

SPECTRAL AND COLOR DOPPLER IMAGING SYSTEM AND METHOD

[001] This invention relates to systems and methods for creating spectral Doppler displays from previously stored color Doppler data on an ultrasound imaging system.

[002] In a vascular study numerous blood flow characteristics of a patient may be measured and quantified. Typically, a sonographer begins the exam by acquiring a two or three dimensional anatomical image of the heart or a blood vessel such as the carotid artery. The patient's vascular anatomy is displayed in the image on the ultrasound system display and a sample volume cursor is moved to a point in the heart or blood vessel where measurements are to be made. Spectral Doppler data is acquired over time from the sample volume location and displayed as a spectral waveform. Once a steady spectral display is being produced, the sonographer begins to record the continuous spectral waveform. After several minutes of recording of the Doppler waveform, the examination of the patient ends. At a later time a physician typically reviews, analyzes, and makes measurements of the spectral waveform that was acquired by the sonographer. The spectral waveform is associated with and valid for only the sample volume location. The doctor may, however, wish to analyze a spectral Doppler waveform for a different sample volume location. This is not possible, however, with prior art ultrasound imaging systems because the Doppler waveform is created on the fly using data that is available only from the particular sample volume location of the body. It is not possible, therefore, to create a Doppler waveform for an alternative sample volume location without conducting another ultrasound exam with the patient.

[003] Another scenario where spectral Doppler finds application is in an obstetrical exam. In an obstetrical exam the clinician often desires to measure the blood flow of the fetus with spectral Doppler. However, finding the location of interest in the minute vasculature of a fetus can be challenging. Once the appropriate location is found, the clinician must move the sample volume to this point in the image to make the measurement at the indicated location. Often, however, the fetus has moved during this manipulation and the entire process must be repeated. Accordingly it would be desirable to capture the data for a spectral Doppler measurement at points in an image of a moving patient without having to repeat the setup and acquisition procedure or hoping that the fetus will remain stationary for the necessary period. Patients, sonographers, doctors and other clinicians would benefit from a system that could minimize the time and cost associated with conducting such exams.

[004] There is therefore a need for an ultrasound imaging system that permits creation of Doppler waveforms after the conclusion of an ultrasound exam based on stored data, and that will acquire the necessary data from an area of an image field rather than a specified single point in the image, so that measurements can be made in the presence of patient motion. Such a system would allow post-exam correction of sample volume placement errors, subsequent investigation of pathology that was not investigated at the time of the exam, and rapid conduct of broad survey examination of a region of anatomy. It would also permit the acquisition of diagnostically useful exam data when the time available for the examination is fleeting.

[005] In accordance with the principles of the present invention an ultrasound system is described which acquires Doppler data during anatomical imaging of flow or motion which can currently or later be used to create a spectral display for a point in an image where flow or motion occurs. In the colorflow (velocity) imaging mode, ensemble lengths are sufficient to produce a colorflow Doppler image, but may also be processed in real time or in post-processing to produce a temporally resolved spectral Doppler display for any point in the colorflow image.

[006] In the drawings:

[007] Figure 1 is an isometric view of an ultrasound imaging system according to one example of the invention.

[008] Figure 2 is a block diagram of the electrical components that may be used in the ultrasound imaging system of Figure 1.

[009] Figure 3 illustrates selection of a sample volume in an anatomical image of a blood vessel and a spectral Doppler display of flow velocity at the sample volume location.

[010] Figure 4 depicts a scrolling spectral Doppler display as produced by ultrasound systems of the prior art

[011] Figure 5 illustrates an exemplary color Doppler image.

[012] Figure 6 depicts a process flow diagram of a method for synthesizing spectral

Doppler from stored color Doppler data in accordance with an embodiment of the invention.

[013] Figure 7 depicts a process flow diagram of a possible workflow in accordance with embodiments of the invention. [014] An ultrasound imaging system 10 according to one example of the invention is illustrated Figure 1. The system 10 includes a chassis 12 containing most of the electronic circuitry for the system 10. The chassis 12 may be mounted on a cart 14, and a display 16 is mounted on the chassis 12. An imaging probe or transducer 20 is connected through a cable 22 to one of three connectors 26 on the chassis 12. The chassis 12 includes a keyboard and controls, generally indicated by reference numeral 28, by which a sonographer operates the ultrasound system 10 and enters information about the patient or the type of examination that is being conducted. At the back of the control panel 28 is a touchscreen display 18 on which programmable softkeys may be displayed for supplementing the keyboard and controls 28 to control the operation of the system 10. The chassis 12 generally also includes a pointing device such as the trackball visible on the front of the control panel that may be used to manipulate an on-screen pointer. The control panel 28 may also include one or more buttons which may be pressed or clicked after manipulating the on-screen pointer. These operations are analogous to a mouse being used with a computer.

[015] In operation, the imaging probe 20 is placed against the skin of a patient (not shown) and held stationary to acquire an image of blood or tissues in a region beneath the skin. The image may be presented on the display 16, and it can be recorded by a recorder (not shown) placed on one of the two accessory shelves 30. The system 10 may also record or print a report containing text and images. Data corresponding to the image may also be downloaded through a suitable data link, such as the Internet or a local area network. In addition to using the probe 20 to produce a two-dimensional image of the patient's anatomy, the ultrasound imaging system may also or alternatively provide other types of images using other types of probes (not shown) to provide other types of images such as three-dimensional images of volumetric regions of the body.

[016] One example of the major subsystems of the ultrasound imaging system 10 is illustrated in Figure 2. Ultrasonic signals are transmitted by a transducer array of the ultrasound probe 20 and the resultant echoes are received by the elements of the transducer array. The received echo signals are formed into a single signal or beam by a beamformer 214. The echo signal information is detected by a Doppler detector 216 which produces quadrature I and Q signal components. This basic Doppler I,Q data is processed by a Doppler processor 220, which refines the data by techniques such as wall filtering, gain control, and compression. A number of such signals from the site in the body being diagnosed are applied to a Doppler estimator 218, one form of which is a fast Fourier transform (FFT) processor, which estimates the Doppler frequency shift of the received signals due to motion. Intermittently during the reception of Doppler echoes, B mode echoes may be received. These echoes may also be formed into I and Q components which may then be amplitude detected by taking the square root of the sum of the squares of the I and Q values in a B mode image processor 264. The B mode image processor also arranges the B mode echoes into a desired display format by scan conversion. The resultant two or three dimensional image of the anatomy is coupled to a Doppler measurement processor 230 where the image is prepared for display with spectral and/or color Doppler data and measurement data processed as discussed below. The B mode image can be used to locate and display the point in the patient's anatomy at which the spectral information is acquired or synthesized as will be discussed more fully below.

[017] Figure 3 is an ultrasound display 16 illustrating selection of a sample volume in a two-dimensional image and generation of a spectral Doppler display of velocity at the sample volume location. At the top of the display is a B mode or colorflow image 310 of anatomy containing a blood vessel 314. A cursor line is manipulated over the image 310 until a sample volume cursor 312 on the line is located at the point where spectral Doppler data is to be acquired, in this case in the center of the blood vessel 314. Doppler data is then acquired from this location and displayed as a scrolling spectral display 320 as it is acquired. The spectral display 320 shows blood velocity plotted vertically as a function of time, which scrolls horizontally. The spectral display 320 may be captured and saved for later analysis by a clinician. The captured spectral display 320 is, of course, only valid for the sample volume with which it is associated. Prior art methods do not permit a doctor to produce a spectral display for some other sample volume at a later time.

[018] Referring to Figure 4, a scrolling Doppler spectral display is shown. The illustrated display is developed by repetitively transmitting ultrasonic Doppler waves to, for example, the sample volume illustrated in Figure 3. Echo signals returned by moving blood cells in the heart or a blood vessel are received by the transducer probe 20 which converts the ultrasonic echoes into electrical signals. As was discussed above, the signals may be amplified and phase detected to determine their frequency shift characteristics. Samples of the detected signals are processed in the Doppler processor 230 to determine the intensity versus frequency characteristics of the signals. The spectral frequency characteristics may be translated to velocity equivalents, and the Doppler information of discrete sampling periods is displayed as a sequence of continuous scrolling spectral lines in a real-time, time-versus-velocity display as shown in Figure 4. In the display of Figure 4, newly generated spectral lines may be produced at the left side of the display. The sequence of lines moves or scrolls from left to right, with previously generated spectral data on the right and progressively more current data to the left. Each line conveys the range of flow velocities detected in the blood flow at a particular time of Doppler interrogation. The highest velocities shown by lines 410, 420, and 430 would typically occur during the systolic phase of the heart cycle. The intervals 412, 422, and 432 between the systolic phases represent flow velocity during the intervening phases of heart action, including the diastolic phase.

[019] In a typical diagnostic procedure of the prior art the sonographer manipulates the ultrasonic transducer and steers the ultrasonic beam toward the vessel or organ where flow velocity information is desired. The spectral display is monitored as its scrolls by until the sonographer is satisfied that it has become stable. The spectral display is then frozen on the screen and saved for analysis. The analysis may proceed by stopping the scanning of the patient and manually tracing the spectral peaks with a cursor controlled by a joystick or trackball on the ultrasound system. Calculation software in the system may then operate on the tracing to determine clinical flow parameters such as peak systolic velocity, minimum diastolic velocity, the systolic/diastolic ratio, the pulsatility index and the velocity time integral. The time averaged mean velocity can then be estimated by operating on the peak velocity tracing data in concert with assumptions made as to certain flow characteristics. Alternatively the saved spectral information can be applied to a processor which is capable of operating on the spectral information to automatically determine these and other desired clinical parameters as described, for instance, in US Pats. 5,287,753 (Routh et al.) and 5,634,465 (Schmiesing et al.) In either case, the measurement results and analysis of such information is valid only for the sample volume previously selected. In prior art systems, if a different anatomical location requires analysis, there is no way to generate the required spectral information without conducting another exam.

[020] As discussed above, a spectral Doppler image is created by interrogating one particular sample volume repeatedly over time. This allows a highly accurate display of the velocity components and motion spectral content in that one location and is typically used for velocity measurements and analysis. A color Doppler image as depicted in Figure 6, on the other hand, is a more qualitative display of one particular motion parameter in color over the regions of the image where that motion occurs. The parameter might be, for example, velocity, Doppler power, or variance. The color at any point within the image denotes the value of the parameter at that particular locality within the image. Figure 5 shows an image 600 that contains a color Doppler region 610, which appears in the drawing as the dark areas in a blood vessel 612.. The color Doppler region 610 of the image 600 may, for example, depict the velocity of blood flow at each point within the color Doppler region by displaying a different color at each pixel location depending on the velocity at that location. The primary difference between spectral Doppler imaging and color Doppler imaging is the length of time the ultrasound beam interrogates the region of interest. In spectral Doppler, the beam stays on the region of interest nearly constantly to provide a long data window for quantitative analysis. For color Doppler, on the other hand, the beam stays in the region of interest only at periodic intervals long enough to calculate a parameter for display and, generally speaking, with somewhat more limited accuracy and precision than that required for a spectral Doppler display. Figure 6 is a process flow diagram 700 depicting a method for synthesizing spectral Doppler from stored color Doppler data. The process starts at 710 when an ensemble of Doppler pulses is transmitted down one or more lines of sight in a 2D image plane or 3D image volume. In one embodiment, 16 or more such pulses may be transmitted down each line of sight, resulting in Doppler ensemble lengths of sixteen or more samples at each point in the image. Ensembles of 32, 64 or 128 samples may be used for even greater temporal resolution. While these ensemble lengths may be greater than needed for colorflow (anatomical Doppler) imaging, and indeed only a subset of the full ensemble may be processed for the colorflow Doppler image, the acquisition of these long ensemble lengths enable the production of a spectral Doppler display of reasonable temporal resolution for any point in the colorflow image, either in real time or by post-processing a stored image. At 720, the echoes from two or more lines of sight are received. In one embodiment, the reception of the echoes may use multiple parallel or angularly displaced receive lines for improved frame rate of display and increased temporal resolution of the spectral display. The process flow continues at step 730 where embodiments of the invention may acquire and process echo data to produce a real time color Doppler image and store sufficient Doppler data to enable a Doppler spectrum to be estimated at points in the plane or volume where motion occurs. At least some of the echo data is typically used to generate and display a color Doppler image such as that depicted in Figure 5. As the Doppler data associated with each echo is acquired and processed for the color (e.g., velocity or power Doppler) display, embodiments of the invention retain all such data at step 740. The Doppler data that is retained may be, for example, the I and Q data produced by the Doppler processor 220 of Figure 2. Alternatively, embodiments of the invention may retain the data generated by the FFT processor 218 of Figure 2. The storage media may be an optical disk, DVD disk, hard drive, CD ROM, or other non-volatile storage media. In other embodiments, any type of data that is capable of being processed to yield a Doppler spectrum may be saved. In some embodiments, as previously mentioned, shorter ensemble lengths will be used to create the displayed color Doppler image but longer ensemble lengths will be stored for spectral display in post-processing.

[022] Prior art ultrasound imaging systems typically use the Doppler data (e.g., I and Q samples) for computing parameters such as mean frequency, power and variance, after which the I and Q data is discarded and only mapped color values are used and saved. Retaining the I and Q samples or Doppler estimate data on non- volatile storage media enables a sonographer, doctor or other clinician to specify a spectral Doppler sample volume position in a post-processing function. As discussed above, this advantageously expands the number of spectral Doppler analysis and measurement points that can be reviewed after an examination has been concluded when the patient is no longer available for further scans.

[023] Figure 7 is a process flow diagram depicting a possible workflow using embodiments of the invention. A typical workflow would start with a sonographer conducting an ultrasound exam at 810. One typical use of an embodiment of the invention might be, as discussed previously, conducting a cardiac scan by imaging portions of the patient's heart or arteries. While conducting the exam, Doppler data is stored for each image frame in accordance with the process depicted in Figure 6. As with prior art ultrasound imaging systems, spectral and/or color Doppler images may be displayed and analyzed in real-time throughout the course of the exam. Embodiments of the invention, however, allow a doctor or clinician to recall and display a color Doppler image from storage media after the exam as shown at step 820. After the image has been recalled, the doctor may select a sample volume location at step 830 in, for example, the manner discussed above in relation to Figure 3. After selecting the sample volume location, the doctor may initiate synthesis of a spectral Doppler curve. This operation may be accomplished by processing the retained Doppler data for a given point in the anatomical image sequence by spectral Doppler processing. At step 840, embodiments of the invention may synthesize and display an estimated spectral Doppler display based on the Doppler data stored in accordance with the process illustrated in Figure 6. Thus, the clinician can move a cursor over the anatomical color Doppler image and cause a spectral Doppler display to be generated for any point of motion where the cursor stops.

Claims

WHAT IS CLAIMED IS:

1. A method for creating a spectral Doppler image on an ultrasound imaging system display, comprising: transmitting an ensemble of Doppler pulses down at least one line of sight in an image plane or volume; receiving echoes from the at least one line of sight; acquiring echo data for every point in the image plane or volume; saving the echo data; recalling and displaying a sequence of images of the image plane or volume; selecting a point in the image plane or volume; and creating the spectral Doppler image for the selected point by processing the saved echo data corresponding to the point.

2. The method of claim 1 wherein transmitting an ensemble of Doppler pulses comprises transmitting at least sixteen Doppler pulses.

3. The method of claim 1 wherein receiving echoes from the at least one line of sight further comprises receiving echoes from at least two lines of sight using multiple parallel or angularly oriented receive lines.

4. The method of claim 1 wherein the echo data comprises at least one of: I/Q quadrature data, Doppler processed I/Q quadrature data and FFT processed I/Q quadrature data.

5. The method of claim 1 wherein selecting a point in the image plane or volume comprises selecting a sample volume from a B-mode image of the image plane or volume.

6. The method of claim 1 wherein selecting a point in the image plane or volume comprises selecting a sample volume from a color Doppler image of the image plane or volume.

7. A method for creating a spectral Doppler image, comprising: accessing echo data for a plurality of points in an ultrasound image plane or volume; displaying the image plane or volume; using the displayed image to select one of the points in the ultrasound image plane or volume; and creating the spectral Doppler image for the selected point by processing the saved echo data corresponding to the selected point.

8. The method of claim 7 wherein the echo data comprises at least one of: UQ quadrature data, Doppler processed UQ quadrature data and FFT processed UQ quadrature data.

9. The method of claim 7 wherein the act of using the displayed information to select one of the points in the ultrasound image plane or volume comprises: displaying a B-mode image of either the image plane or a plane within the volume; and selecting one of the points using the B-mode image.

10. The method of claim 7 wherein the act of using the displayed information to select one of the points in the ultrasound image plane or volume comprises: displaying a color Doppler image of either the image plane or a plane within the volume; and selecting a sample volume from the color Doppler image.

11. An ultrasound imaging system comprising: a display; a processor coupled to the display; a user interface coupled to the display; a transducer operable to transmit and receive an ensemble of Doppler pulses down at least one line of sight in an image plane or volume; a data storage device operable to store echo data received by the transducer; and an analysis package operatively connected to the processor, the analysis package providing a user the ability to create a spectral Doppler image for a selected point from previously stored echo data, the analysis package being operable to process stored echo data corresponding to a selected point in an ultrasound image with a spectral Doppler processor to create a spectral Doppler image for the selected point.

12. The ultrasound imaging system of claim 11 wherein the ensemble of Doppler pulses comprises at least sixteen Doppler pulses.

13. The ultrasound imaging system of claim 11 wherein the transducer is further operable to simultaneously receive echoes from multiple receive lines.

14. The ultrasound imaging system of claim 11 wherein the echo data comprises at least one of VQ quadrature data, Doppler processed I/Q quadrature data or FFT processed I/Q quadrature data.

15. The ultrasound imaging system of claim 11 wherein the selected point is a sample volume selected from a B-mode image of the image plane or plane within the volume.

16. The ultrasound imaging system of claim 11 wherein the selected point is a sample volume selected from a color Doppler image of the image plane or plane within the volume.

17. The ultrasound imaging system of claim 16 wherein the color Doppler image is a colorflow Doppler image with different colors depicting different velocities.

18. The ultrasound imaging system of claim 16 wherein the color Doppler image is a power Doppler image with different colors depicting different Doppler signal intensities.

19. The ultrasound imaging system of claim 11 wherein the selected point is a sample volume selected from a B mode image of the image plane or plane within the volume.