Patent Application
20060241454
The present invention relates to compounding ultrasound data. In particular, spatial or frequency compounding is provided to increase detectability of specular targets and/or soft tissue inhomogeneities. Two or more component images are compounded after envelope detection. Each of the component images has a different spatial and/or temporal frequency response so that they are substantially uncorrelated. For example, scan lines associated with different angles relative to a transducer are used to form the two different component images. By summing these two component images (i.e., spatial compounding), the detectability of soft tissue lesions and anisotropic objects increases.
U.S. Pat. No. 6,790,181, the disclosure of which is incorporated herein by reference, discloses spatial compounding where the scan angles associated with each frame of data vary within the frame of data. The variation in angle allows for a fully overlapped scan region without pre-weighting or smoothing to account for regions with different numbers of component images.
Compounding with multiple component images reduces temporal resolution and may introduce temporal discordance artifacts. To increase frame rate, the spatial or temporal frequency response differences may be implemented using only receive parameter variations. However, using both transmit and receive parameter variations increases component image decorrelation, and therefore increases detectability of specular targets and soft tissue inhomogeneities. An alternative way to increase the compounding frame rate is to use receive multibeam with many parallel receive beams. But this requires widening the transmit beam which compromises detail and contrast resolution.
The preferred embodiments described below include methods and systems for spatial and frequency compounding in ultrasound. A transmit multibeam insonifies an object with multiple noncollinear transmit beams fired substantially simultaneously. The noncollinear beams are along different scan lines of same scan geometry, or they belong to scan lines of different scan geometries. For example, a scan line set may constitute unsteered linear scan geometry or steered linear scan geometry. One or more receive beams are formed in parallel in response to each of the noncollinear beams. With different transmit beams with or without different frequencies for each transmit beam, data responsive to a same transmit event is used in different component images. Frequency variation between the transmit beams of a transmit event reduces inter-beam interference, thereby improving contrast resolution. The component images are scan-converted (if scan geometries are different), weighted and combined after envelope detection.
Due to spatial and/or frequency differences, the component images exhibit different spatial and/or temporal frequency responses. Therefore, they are sufficiently uncorrelated. Compounding of uncorrelated images increase detectability of specular targets and soft tissue inhomogeneities. Using simultaneous transmission and reception of beams increases frame rate and thus reduces temporal discordance between the component images of compounding. Distributing the parallel receive beams between the multiple noncollinear transmit beams of a transmit event limits the amount of transmit widening required to support a given number of parallel receive beams.
In a first aspect, a method is provided for increasing detectability of tissue inhomogeneities and specular targets in ultrasound. First and second noncollinear transmit beams are transmitted substantially simultaneously. The first transmit beam has a different angle, different center frequency or both different angle and center frequency than the second transmit beam. A first receive beam is received in response to the first transmit beam, and a second receive beam is received in response to the second transmit beam. A first frame of data including the first receive beam is compounded with a second frame of data including the second receive beam.
In a second aspect, a method is provided for increasing detectability of tissue inhomogeneities and specular targets in ultrasound. Two or more frames of data are formed with different angle or center frequency beam characteristics. The two or more frames of data are formed, at least in part, from data acquired in response to two or more non-collinear, substantially simultaneous transmit beams having the different angle or center frequency beam characteristics. The two or more frames of data are compounded.
In a third aspect, a system is provided for spatial and/or frequency compounding in ultrasound. A transmit beamformer is operable to transmit first and second noncollinear transmit beams substantially simultaneously. The first transmit beam has a different angle, different center frequency or different angle and center frequency than the second transmit beam. A receive beamformer is operable to receive a first receive beam in response to the first transmit beam and a second receive beam in response to the second transmit beam. A compound processor is operable to compound a first frame of data that includes the first receive beam with a second frame of data that includes the second receive beam.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.
The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.
Transmit and receive compounding is performed with minimal temporal resolution loss and temporal discordance artifacts. Non-collinear transmit multi-beams are fired to insonify an object simultaneously along distinct transmit lines with distinct steering angles and/or center frequencies. Non-collinear receive multi-beams are formed to sample the object substantially along each of the transmit lines with substantially the same steering angle and/or related center frequency as the respective transmit beams. The non-collinear transmit beams and their respective receive beams for spatial compounding are on different master line grids. For example, one frame of data is formed using steered lines and another frame of data formed using unsteered lines. The distinct beam groups have different frame indices. For frequency compounding, the non-collinear transmit beams and their respective receive beams have different center frequencies, such as a low and high frequency. Spatial and frequency compounding may be provided together.
The transducer 12 is an array of a plurality of elements. The elements are piezoelectric or capacitive membrane elements. The array is configured as a one-dimensional array, a two-dimensional array, a 1.5 D array, a 1.25 D array, a 1.75 D array, an annular array, a multidimensional array, combinations thereof or any other now known or later developed array. The transducer elements transduce between acoustic and electric energies. The transducer 12 connects with the transmit beamformer 14 and the receive beamformer 16 through a transmit/receive switch, but separate connections may be used in other embodiments.
The transmit beamformer 14 is shown separate from the receive beamformer 16. Alternatively, the transmit and receive beamformers 14, 16 may be provided with some or all components in common. Operating together or alone, the transmit and receive beamformers 14, 16 form beams of acoustic energy for scanning a one, two or three dimensional region. Vector®, sector, linear or other scan formats may be used. A single receive beam is generated for each transmit beam. Alternatively, two or more receive beams are generated for each transmit beam. Data representing scan lines may be synthesized from coherent receive beam data, such as disclosed in U.S. Pat. No. 5,623,928, the disclosure of which is incorporated herein by reference. Fully populated control data sets for any of the transmit or receive beamformer parameters discussed herein are provided. Alternatively, sparse sets are used for real-time calculation of the control data, such as disclosed in U.S. Pat. No. 5,581,517, the disclosure of which is incorporated herein by reference.
The transmit beamformer 14 is a processor, delay, filter, waveform generator, memory, phase rotator, digital-to-analog converter, amplifier, combinations thereof or any other now known or later developed transmit beamformer components. In one embodiment, the transmit beamformer 14 is the transmit beamformer disclosed in U.S. Pat. No. 5,675,554, the disclosure of which is incorporated herein by reference. The transmit beamformer 14 digitally generates envelope samples. Using filtering, delays, phase rotation, digital-to-analog conversion and amplification, the desired transmit waveform is generated. Other waveform generators may be used, such as switching pulsers or waveform memories.
The transmit beamformer 14 is configured as a plurality of channels for generating electrical signals of a transmit waveform for each element of a transmit aperture on the transducer 12. The waveforms are unipolar, bipolar, stepped, sinusoidal or other waveforms of a desired center frequency or frequency band with one, multiple or fractional number of cycles. The waveforms have relative delay and/or phasing and amplitude for focusing the acoustic energy. The transmit beamformer 14 includes a controller for altering an aperture (e.g. the number of active elements), an apodization profile (e.g., type or center of mass) across the plurality of channels, a delay profile across the plurality of channels, a phase profile across the plurality of channels, center frequency, frequency band, waveform shape, number of cycles and combinations thereof. A scan line focus is generated based on these beamforming parameters.
The transmit beamformer 14 is operable to transmit at least two transmit beams of ultrasound energy substantially simultaneously. Substantially simultaneously accounts for differences in steering direction or depth of focus and associated time of transmission from any given element of the transducer 12. Separately generated waveforms are summed together for each channel, or a complex waveform representing the relatively delayed and apodized waveforms for two beams is generated for each channel. The transmit beams are generated for a same transmit event, but have different angles, different center frequencies or both different angles and center frequencies. For example, transmit beams with angles of greater than 5, 10, 15, 20, 45, 90 or other degree difference from each other with or without the same origin on the transducer 12 are generated. In one embodiment, two transmit beams with about 70 degree difference (each 35 degrees from normal) are generated in a same transmit event. In another embodiment, three transmit beams with about 20 degree difference (one normal and the other two at 20 degrees from normal) are generated in a same transmit event. As another example, transmit beams with center frequency differences in the Kilohertz or Megahertz ranges are provided. In one embodiment, the center frequencies are different by about ¼ of the center frequency of the highest center frequency (e.g., 3 and 4 MHz).
The transmit beams formed at a substantially same time are non-collinear. The transmit beams have different angles and/or different origins. The differences result in insonifying different regions of the patient along at least a portion of the beams. The different regions are relative to an imaging resolution provided by the system 10.
The receive beamformer 16 is a preamplifier, filter, phase rotator, delay, summer, base band filter, processor, buffers, memory, combinations thereof or other now known or later developed receive beamformer components. In one embodiment, the receive beamformer is one disclosed in U.S. Pat. Nos. 5,555,534, 5,921,932 and 5,685,308, the disclosures of which are incorporated herein by reference. Other analog or digital receive beamformers capable of receiving two or more beams in response to a transmit event may be used.
The receive beamformer 16 is configured into a plurality of channels for receiving electrical signals representing echoes or acoustic energy impinging on the transducer 12. A channel from each of the elements of the receive aperture within the transducer 12 connects to an amplifier and/or delay for applying apodization amplification. An analog-to-digital converter digitizes the amplified echo signal. The digital radio frequency received data is demodulated to a base band frequency. Any receive delays, such as dynamic receive delays, and/or phase rotations are then applied by the amplifier and/or delay. A digital or analog summer combines data from different channels of the receive aperture to form one or a plurality of receive beams. The summer is a single summer or cascaded summer. The summer sums the relatively delayed and apodized channel information together to form a beam. In one embodiment, the beamform summer is operable to sum in-phase and quadrature channel data in a complex manner such that phase information is maintained for the formed beam. Alternatively, the beamform summer sums data amplitudes or intensities without maintaining the phase information.
Beamforming parameters including a receive aperture (e.g., the number of elements and which elements used for receive processing), the apodization profile, a delay profile, a phase profile, imaging frequency and combinations thereof are applied to the receive signals for receive beamforming. For example, relative delays and amplitudes or apodization focus the acoustic energy along one or more scan lines. A control processor controls the various beamforming parameters for receive beamformation. The values provided for the beamformer parameters for the receive beamformer 16 are the same or different than the transmit beamformer 14. For example, an aberration or clutter correction applied for receive beam formation is different than an aberration correction provided for transmit beam formation due to differences in signal amplitude.
The receive beamformer 16 is operable to form receive beams in response to the transmit beams. For example, the receive beamformer 16 receives one or two receive beams in response to each transmit beam. The receive beams are collinear, parallel and offset or nonparallel with the corresponding transmit beams. Since two or more transmit beams are substantially simultaneously formed, two or more responsive receive beams are substantially simultaneously formed in a same receive event. Alternatively, the data received at the elements of the receive aperture in a receive event is stored for sequentially forming the two or more receive beams in response to the same transmit event.
The receive beamformer 16 outputs image data, data representing different spatial locations of a scanned region. The image data is coherent (i.e., maintained phase information), but may include incoherent data. The data may be formed by processing received data, such as synthesizing scan lines (i.e., coherent combination), or other processes for generating data used to form an image from received information. For example, inter-beam phase correction is applied to one or more beams, and then the phase corrected beams are combined through a coherent (i.e., phase sensitive) filter to form synthesized ultrasound lines and/or interpolated between beams to form new ultrasound lines. Once the channel data is beamformed or otherwise combined to represent spacial locations of the scanned region, the data is converted from the channel domain to the image data domain.
The detector 18 is a general processor, digital signal processor, application-specific integrated circuit, control processor, digital circuit, summer, filter, finite impulse response processor, multipliers, field programmable gate array, combinations thereof or other now known or later developed processors for forming incoherent image data from received signals. The detector 18 includes a single or multiple processors with or without log compression. The detector 18 detects the amplitude, intensity, log-compressed amplitude or power of the beamformed signals. For example, the detector 18 is a B-mode detector of the signal envelope. One or more filters, such as spatial or temporal filters may be provided with the detector 18. The detector 18 outputs incoherent image data. Additional processes, such as filtering, interpolation, and/or scan conversion, may be provided by the detector 18 or other component before the compounding processor 20.
The compounding processor 20 is a general processor, control processor, digital signal processor, application specific integrated circuit, field programmable gate array, analog circuit, digital circuit, summer, combinations thereof or other now known or later developed device for compounding. The compounding processor 20 is operable to compound two or more frames of data. Data representing the same or similar spatial locations from the two or more frames of data are averaged. Weighted averaging, selection of the maximum or other functions are alternatively used. Low pass filtering or other processes may be used to remove any artifact from compounding frames of data where the number of frames compounded varies within a scanned region. Where the scan geometries for each frame of data are different, scan conversion may be used prior to compounding.
Where the frames of data have different center frequency characteristics, frequency compounding is provided. Where the frames of data have different scan angles, angle combinations or scan geometry, spatial frequency compounding is provided. Both spatial and frequency compounding may be used. By forming the frames of data from transmit beams transmitted substantially simultaneously, temporal resolution is maintained. By using non-collinear transmit beams, multiple frames of data with different characteristics are formed at substantially the same time, further limiting temporal discordance artifacts.
Other characteristics of the transmit beams 30 may vary as a function of the transmit beam 30. For example, the different transmit beams have different temporal frequency response. A different center frequency, band width or frequency response shape of a transmit beam 30 relative to another transmit beam 30 in a same transmit event is provided. As another example, different focal depths are provided. As yet another example, a different aperture size, apodization type (e.g. Gaussian and an equally weighted apodization profile), apodization center of mass (e.g. center of the aperture and off-set from the center of the aperture) or combinations thereof. As yet other example, the transmit beams 30 transmitted at substantially the same time have complimentary codes. Spread spectrum, phase shift keying, chirp or other now known or later developed coding may be used. The receive beams 32 are decoded. The coding provides for additional rejection between spatially distinct scan lines.
In response to a given transmit event, one or more receive beams 32 are formed for each of the transmit beams 30. In the examples shown in
The receive beams 32 are formed using the same or different beamforming parameters as the transmit beams 30. Other than receiving along spatially different scan lines shown in
The beamforming parameters for received beams 32 formed in response to a same transmit event may similarly be the same or different. For example, receive beams 32 formed in response to different transmit beams 30 have different aperture size, apodization type, apodization center of mass or combinations thereof. The receive beams 32 formed in response to a same transmit beam may have the same or different beamform parameters other than focusing along different scan lines adjacent to the transmit beam 30.
The transmission of multiple transmit beams 30 substantially simultaneously and the associated received events are repeated along at least a lateral dimension 34. In the example of
Other than focusing along different scan lines, the transmit and/or receive beamforming parameters may vary as a function of lateral position or scanning. For example, the angle or scan pattern varies throughout a two or three dimensional region as disclosed in U.S. Pat. No. 6,790,181, the disclosure of which is incorporated herein by reference. As another example, scan lines associated with greater angles away from normal to the transducer 12 have a lower frequency, such as disclosed in U.S. Pat. No. 5,549,111, the disclosure of which is incorporated herein by reference. The center frequency used for a given transmit and receive beam 30, 32 is a function of the element spacing, effective element width and the angle away from the normal. For frequency compounding, the center frequency varies as a function of angle within a given frame of data 40, 42. Similar variation is provided for each of the frames 40, 42 such that a frequency difference is provided for any given spatial location represented by both frames of data 40, 42.
After a series of transmit and associated receive events, each frame of data 36, 38 or 40, 42 is formed. Received beams 32 represented in each frame 36, 38, 40, 42 are formed as part of a same receive events as receive beams 32 in other sets of data 38, 36 or 42, 40. Each frame of data represents an at least overlapping spatial region, such as a two or three dimensional region. The data for each of the frames of data 36, 38 or 40, 42 are detected after beam formation and prior to compounding. For example, envelope or B-mode detection is provided. Alternatively, harmonic, Doppler or another form of detection is provided. In yet other alternative embodiments, the detection is performed after combining the sets of data 36, 38 or 40, 42. For example, data synthesis is provided.
After two or more frames of data associated with different angle and/or center frequency characteristics are formed, the frames of data are compounded. For data from different frames representing a same spatial location, the data is averaged. If the data represents adjacent but different spatial locations, interpolation converts the data in different frames of data to a same grid for compounding. Alternatively, weighted averaging is provided. Spatial regions associated with a single frame of data may be discarded or included in a compound frame. Low pass filtering or other techniques may be used to remove or minimize compounding artifacts. Since each frame of data includes received beams 32 from the same transmit events, the spatial or frequency compounding with limited temporal discordance artifacts is provided.
While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.
