Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52019—Details of transmitters
  • G01S7/5202—Details of transmitters for pulse systems
  • G01S7/52022—Details of transmitters for pulse systems using a sequence of pulses, at least one pulse manipulating the transmissivity or reflexivity of the medium
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/13—Tomography
  • A61B8/14—Echo-tomography
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/42—Details of probe positioning or probe attachment to the patient
  • A61B8/4272—Details of probe positioning or probe attachment to the patient involving the acoustic interface between the transducer and the tissue
  • A61B8/4281—Details of probe positioning or probe attachment to the patient involving the acoustic interface between the transducer and the tissue characterised by sound-transmitting media or devices for coupling the transducer to the tissue
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/42—Details of probe positioning or probe attachment to the patient
  • A61B8/4272—Details of probe positioning or probe attachment to the patient involving the acoustic interface between the transducer and the tissue
  • A61B8/429—Details of probe positioning or probe attachment to the patient involving the acoustic interface between the transducer and the tissue characterised by determining or monitoring the contact between the transducer and the tissue
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/44—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device
  • A61B8/4444—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device related to the probe
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/44—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device
  • A61B8/4483—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device characterised by features of the ultrasound transducer
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/48—Diagnostic techniques
  • A61B8/485—Diagnostic techniques involving measuring strain or elastic properties
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52023—Details of receivers
  • G01S7/52036—Details of receivers using analysis of echo signal for target characterisation
  • G01S7/52038—Details of receivers using analysis of echo signal for target characterisation involving non-linear properties of the propagation medium or of the reflective target
  • G01S7/52039—Details of receivers using analysis of echo signal for target characterisation involving non-linear properties of the propagation medium or of the reflective target exploiting the non-linear response of a contrast enhancer, e.g. a contrast agent
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52023—Details of receivers
  • G01S7/52036—Details of receivers using analysis of echo signal for target characterisation
  • G01S7/52042—Details of receivers using analysis of echo signal for target characterisation determining elastic properties of the propagation medium or of the reflective target
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52046—Techniques for image enhancement involving transmitter or receiver
  • G01S7/52049—Techniques for image enhancement involving transmitter or receiver using correction of medium-induced phase aberration
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52077—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging with means for elimination of unwanted signals, e.g. noise or interference
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52085—Details related to the ultrasound signal acquisition, e.g. scan sequences
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
  • G01S15/88—Sonar systems specially adapted for specific applications
  • G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
  • G01S15/8906—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
  • G01S15/8993—Three dimensional imaging systems

Definitions

• the present invention is directed to methods and instrumentation for imaging of linear and nonlinear propagation and scattering parameters of an object using transmission of dual frequency pulse complexes composed of a high frequency (HF) and low frequency (LF) pulse. Imaging with acoustic pressure waves is shown as an example, but the methods are also useful for imaging with shear elastic waves and coherent electromagnetic waves.
• HF high frequency
• LF low frequency
• Applications of the invention are for example, but not limited to, medical imaging and therapy, non-destructive testing, industrial and biological inspections, geological applications, SONAR and RADAR applications.
• BACKGROUND OF THE INVENTION [0003] Transmission of dual frequency pulse complexes composed of a high frequency (HF) and low frequency (LF) pulse for imaging of nonlinear propagation and scattering parameters of an object is described in US Patents 1-4 (7,641,613; 8,038,616; 8,550,998; 9,291,493).
• the methods also provide suppression of multiple scattering noise (reverberation noise) and improved imaging of linear and nonlinear scatterers.
• Imaging with coherent acoustic pressure waves is shown as an example, but it is clear that the methods are also useful for imaging with all types of coherent wave imaging, such as shear elastic waves and electromagnetic waves.
• the cited methods require estimation of one or both of a nonlinear propagation delay (NPD) and a nonlinear propagation pulse form distortion (PFD) which both are challenging tasks.
• NPD nonlinear propagation delay
• PFD nonlinear propagation pulse form distortion
• the present invention describes new methods and instrumentation for improved estimation of both the NPD and the PFD, and provides LEGAL ⁇ 50183637 ⁇ 1 scatter images with reduced multiple scattering noise and images of nonlinear scatterers. Combined with measurements with zero LF pulse, the invention also provides estimates of linear propagation and scattering parameters, that combined with the estimates of nonlinear parameters is used to obtain a thermo-elastic description of the object.
• the current invention provides methods and instrumentation for estimation and imaging of linear and nonlinear propagation and scattering parameters in a material object where the material parameters for wave propagation and scattering have a nonlinear dependence on the wave field amplitude.
• the methods have general application for both acoustic and shear elastic waves such as found in SONAR, seismography, medical ultrasound imaging, and ultrasound nondestructive testing, and also coherent electromagnetic waves such as found in RADAR and laser imaging.
• the methods comprises transmitting at least two pulse complexes composed of a co-propagating high frequency (HF) and a low frequency (LF) pulse along at least one LF and HF transmit beam axis, where said HF pulse propagates close to the crest or trough of the LF pulse along at least one HF transmit beam, and where one of the amplitude and polarity of the LF pulse varies between at least two transmitted pulse complexes, where the amplitude of the LF pulse can be zero for a pulse complex and the amplitude of at least one LF pulse of said at least two transmitted pulse complexes is non-zero.
• HF high frequency
• LF low frequency
• the LF pulse produces a manipulation of the propagation velocity of the co-propagating HF pulse.
• the amplitude of the LF pulse drops so much that one can neglect the LF manipulation of the propagation velocity for the scattered signal.
• the LF pulse polarity and/or amplitude hence produces a modification of the HF pulse delay from transmission to reception that we call the nonlinear propagation delay (NPD) that i) accumulates with the propagation distance to the first scatterer, and ii) is proportional to the LF pulse amplitude, and iii) changes polarity with the LF pulse amplitude.
• NPD nonlinear propagation delay
• Variations of the LF pulse along the HF pulse produces a pulse form distortion (PFD) that also i) accumulates with the propagation distance to the first scatterer, and ii) is proportional to the LF pulse amplitude, and iii) changes polarity with the LF pulse amplitude.
• PFD pulse form distortion
• MSN scattered noise
• the co-propagating LF pulse produces a NPD and a PFD of the MSN given by those of the HF pulse at the 1 st scattering, and are hence different from the NPD and a PFD of the 1 st order scattered signal arriving at the same time at the HF receiver transducer.
• High quality estimation of the NPD opens for suppression of multiple scattering noise and imaging of nonlinear scattering, as shown in US Pats 7,641,613; 8,038,616; 8,550,998; 9,291,493.
• US Parent Patent Application 16/258,251 it is also shown that good estimates of the NPD opens for quantitative estimation of the nonlinear bulk elasticity of tissues.
• the signal received at a given arrival time is therefore generally composed of three components: i) a 1 st order scattered signal, and ii) a Class I MSN component, and iii) a Class II MSN component, where all three components have different NPDs and PFDs.
• the NPD at the HF cross-beam observation cell can for example be obtained by transmitting two pulse complexes along the same LF and HF beams and with opposite polarities of the LF pulse. Comparing the difference in arrival time of the HF signal from the two pulse complexes and the same HF cross-beam observation cell, for example through correlation, gives the NPD, as described in more detail below.
• the HF cross-beam receive signals are processed to estimate one or both of i) a nonlinear propagation delay (NPD), and ii) a nonlinear pulse form distortion (PFD) of the transmitted HF pulse for said cross-beam observation cell, where one or both of said NPD and PFD are used in the further processing to estimate one or more of i) a local nonlinear propagation parameter, and ii) a local quantitative nonlinear propagation parameter ⁇ p, and iii) a local value of the linear pulse propagation velocity c0, and iv) a linearly scattered HF signal, and v) a nonlinearly scattered HF signal, and vi) local changes in tissue structure during therapy, and vii) local changes in tissue temperature during HIFU therapy.
• NPD nonlinear propagation delay
• PFD nonlinear pulse form distortion
• said at least one HF receive cross-beam is focused on the HF transmit beam axis forming a cross-beam observation cell as the cross-over region of the HF transmit and HF receive cross-beams.
• a local nonlinear propagation parameter can be estimated through receiving scattered signals from the HF transmitted pulse for at least two HF receive cross-beams with close distance along the HF transmit beam, and estimating a nonlinear propagation delay (NPD) of the transmitted HF pulse at the at least two cross- beam observation cells determined by the cross-over between the HF transmit beam and each of the said at least two HF receive cross-beams.
• NPD nonlinear propagation delay
• Said estimated NPDs from neighboring cross-beam observation cells along a HF transmit beam are combined to form estimates of the local nonlinear propagation parameter.
• Scaling said estimated local nonlinear propagation parameter by an estimate of the LF pulse pressure at the location of the HF pulse gives a quantitative estimate of the nonlinear propagation parameter ⁇ p .
• Both a local nonlinearly and a linearly scattered signal may be obtained through correcting said at least two HF receive signals with one or both of i) the NPD, and ii) the PFD to produce two corrected signals, and combining said at least two corrected signals.
• the invention further devices to also use a HF back-scatter receive beam with the same beam axis as the HF transmit beam to record HF back-scatter receive signals.
• the NPD and the PFDs are determined by the LF and HF transmit beams and the LF pulse polarity and the amplitude, according to Eqs.(12,13). As we are using the same LF and HF transmit beams as for the cross-beam receive signals, we get the same NPD and PFD for the back-scatter as for the cross beam receive signals.
• the estimated PFD and/or the NPD are processed to estimate delay and speckle/ pulse form corrections of the HF back-scatter receive signals for suppression of multiple scattering noise, for example as described in US Pats 7,641,613, 8,038,616, 8,550,998, 9,291,493.
• the at least two HF back-scatter signals from at least two transmitted pulse complexes with different LF pulses are corrected with the estimated delay and speckle/pulse form corrections to produce at least two corrected HF back-scatter signals that are combined to provide HF noise-suppressed back-scatter signals with suppression of multiple scattering noise, for example as described in US Pat 8,038,616, 8,550,998, 9,291,493.
• 2D and 3D images of the estimated parameters and signals may be obtained by scanning the transmit beam and matched HF cross-beam and HF back-scatter beams across a 2D or a 3D region of the object, and recording HF back-scatter and/or cross-beam receive signals and back-scattered signals with further processing according to the invention to produce local estimates of propagation and scattering parameters.
• the size of the cross-beam observation cell may with 2D or 3D scanning be synthetically reduced through spatial filtering of the HF cross-beam receive signals from several neighboring cross-beam observation cells. In this process one can use the estimated linear propagation velocity to correct for wave front aberrations in the spatial filtering.
• the invention further devices to use HF back-scatter receive beams that are equal to the HF transmit beams, and for multiple depths carry through lateral filtering of one of i) the HF back-scatter receive signals, and ii) the HF noise-suppressed back-scatter signals, to produce HF signals from combined HF transmit and receive beams that are synthetically focused for said multiple depths, for example as described in US Pat 9291493.
• the invention also describes instruments for carrying through the practical measurements and processing according to the invention, in particular to obtain local estimates of the PFD and/or the NPD, and one or more of the parameters: i) a local nonlinear propagation parameter, and ii) a local quantitative nonlinear propagation parameter ⁇ p , and iii) a local value of the linear pulse propagation velocity c 0 , and iv) a linearly scattered HF signal with suppression of multiple scattering noise, and v) a nonlinearly scattered HF signal, and vi) local changes in tissue structure during therapy, and vii) local changes in tissue temperature during therapy.
• HF back-scatter and/or cross beam receive signals are generated in dedicated beam forming HW according to known methods, and digital HF receive signals are transferred to the processing structure for storage and further processing in a general SW programmable processor structure of different, known types.
• the individual receiver element signals are digitized and transferred to a general SW programmable processor structure where the receive beam forming and further processing is SW programmed.
• the instrument comprises a display system for display of estimated parameters and images according to known technology, and user input to the instrument according to known methods.
• the transmit and receive of HF and LF pulses are obtained with known transducer arrays, for example as described in US Pat 7,727,156 and 8,182,428.
• FIG.1 shows example pulse complexes comprising a high frequency (HF) pulse and a low frequency (LF) pulse, where two typical forms of LF pulses are shown.
• FIG. 2 shows a measurement setup to obtain acoustic measurements for estimation of HF received signals for one or both of estimation and imaging of several image signals related to the nonlinear material parameters of the material.
• FIG.3 shows by example typical HF receive signals for different transmitted LF pulses.
• FIG.4 shows examples of an estimated nonlinear propagation delay (NPD) and its gradient along the HF pulse propagation direction.
• NPD estimated nonlinear propagation delay
• FIG. 6 shows a ring array transducer structure and processing system for estimation and imaging of linear and nonlinear nonlinear propagation and scattering parameters according to the invention.
• FIG.7a and b show yet two other example ultrasound transducer systems for estimation and imaging of linear and nonlinear propagation and scattering parameters according to the invention.
• FIG.8 shows a bloch diagram for an instrument according to the invention.
• FIG.9 shows schematically an HF transmit beam, an HF receive cross-beam, and a HF back- scatter receive beam equal to the HF transmit beam.
• FIG.10 Acoustic compression elasticity data for some typical tissues, showing good correlation between the nonlinear elasticity parameter ⁇ p and the linear propagation velocity c 0 .
• the left side terms determine the spatial propagation of the wave from the slowly varying components of the material parameters
• the right side terms represent scattering sources that originate from the rapid spatial variation of the material parameters
• ⁇ presents the rapid, relative variation of the isentropic compressibility
• the linear propagation terms (1) of Eq.(7) guide the linear forward spatial propagation of the incident wave with propagation velocity and absorption given by term (3), without addition of new frequency components.
• the linear scattering source terms (4) produce local linear scattering of the incident wave that has the same frequency components as the incident wave, with an amplitude modification of the components produced by the 2 nd order differentiation in the scattering terms, where is the angular frequency of the incident wave.
• the slow variation of the nonlinear parameters give a value to that provides a nonlinear forward propagation distortion of the incident wave that accumulates in magnitude with propagation distance through term (2) of Eq.(7).
• a rapid variation of the nonlinear material parameters gives a value to that produces a local nonlinear distorted scattering of the incident wave through term (5) of Eq.(7).
• FIG. 1a shows a high frequency (HF) transmit pulse 101 propagating at the crest of a low frequency (LF) pulse 102
• FIG. 1b shows the same transmitted HF pulse 101 propagating at the through of the LF pulse 103, where the example is obtained by inversing the polarity of 102.
• HF high frequency
• LF low frequency
• HF:LF ratio is typically > 5:1.
• the propagation terms (1) and (2a) of the left side of Eq.(9b) can for the manipulation of the HF pulse by the co-propagating LF pulse be approximated as [0053]
• the numerator in front of the temporal derivative in this propagation operator is the square propagation velocity, and we hence see that the LF pulse pressure p L modifies the propagation velocity for the co-propagating HF pulse pH as where s the actual LF field variable along the co-propagating HF pulse.
• the orthogonal trajectories of the HF pulse wave-fronts are paths of energy flow in the HF pulse propagation.
• curves ⁇ (r) as the orthogonal trajectories of the HF pulse wave-fronts that ends at r.
• c 0 (s) be the LF pressure and linear propagation velocity at the center of gravity of the HF pulse at the distance coordinate s along ⁇ (r)
• p is a scaling factor for polarity and amplitude of the LF pulse.
• the propagation time-lag of the HF pulse at depth r along the orthogonal trajectories to the HF pulse wave-fronts can then be approximated as [0055]
• the propagation lag with zero LF pulse is t0(r) given by the propagation velocity c0(r) that is found with no LF manipulation of the tissue bulk elasticity.
• ⁇ ⁇ (r) is the added nonlinear propagation delay (NPD) of the center of gravity of the HF pulse, produced by the nonlinear manipulation of the propagation velocity for the HF pulse by the LF pressure p c (s) at the center of gravity of the HF pulse.
• the PFD can be described by a filter. Defining as the temporal Fourier transform of the transmitted HF pulse field that co-propagates with a LF pulse, and as the HF pulse for zero LF pulse, the PFD filter is defined as and the subscript p designates the amplitude/polarity/phase of the LF pulse.
• V p includes all nonlinear forward propagation distortion, where the linear phase component of V p is separated out as the nonlinear propagation delay (NPD) up to the point r as described in Eq.(12).
• the filter V p hence represents the nonlinear pulse form distortion (PFD) of the HF pulse by the co-propagating LF pulse, and also the nonlinear attenuation produced by the nonlinear selfdistortion of the HF pulse.
• the nonlinear terms (2a, b) in Eq.(9b) produces a propagation distortion of the HF pulse as: i) (2a): A nonlinear propagation delay (NPD) produced by the LF pressure at the center of gravity of the co-propagating HF pulse according to Eq.(12), and is affected by the LF pulse up to the 1 st scattering, where the amplitude of the LF pulse drops so much that the nonlinear effect of the scattered LF pulse is negligible for the scattered HF wave.
• NPD nonlinear propagation delay
• the HF scattering cross section given in the right side of Eq.(9b) is composed of a linear component. term (4), and a nonlinear component, term (5), where term (5a) takes care of the variation of the HF fundamental band scattering produced by the LF pulse, while term (5b) represents self distortion scattering that produces scattered signal in the harmonic and sub-harmonic components of the HF-band.
• This scattering term is however so low that it can be neglected, except for micro-bubbles at adequately low frequency, where the scattering process is described by a differential equation, i.e.
• the scattering from the rapid, relative fluctuations in the compressibility, ⁇ ( r ) is a monopole term that from small scatterers (dim ⁇ ⁇ 1 ⁇ 4 HF wave length) give the same scattering in all directions, while the scattering from the rapid, relative fluctuations in the mass density, ⁇ ( r ) , is a dipole term where the scattering from small scatterers depends on cosine to the cross-angle between the transmit and receive beams [18].
• the fundamental HF band write the scattering coefficient as a sum of a linear scattering coefficient and a nonlinear scattering coefficient as [0062] represents the sum of the linear scattering from fluctuations in compressibility and mass density.
• Group A is the effect of the nonlinear propagation term (2a). It is found in the received signal with transmission through the object with tomographic measurements, and in the linearly scattered signal from term (4). It represents the accumulative nonlinear propagation distortion of the HF incident pulse produced by the LF pulse, i.e. term (2a). This group is split into a nonlinear propagation delay (NPD) according to Eq.(12) (/ ' above), and nonlinear pulse form distortion (PFD) according to Eq.(13) (// above) , and [0065] Group B originates directly in the local nonlinear interaction scattering of the LF on the HF pulse, i.e. term (5a), where the local LF pulse pressure at the co-propagating HF pulse exerts an amplitude modulation of the scattered wave, proportional to It is generally useful for detection of micro-bubbles and micro-calcifications, and
• Group C This is found as local nonlinear self distortion scattering from term (5b) of the forward accumulative nonlinear propagation components in the incident wave. However, typical nonlinear material parameters are so low that this group is negligible, except for micro-bubbles and micro-calcifications in some situations.
• FIG.2 shows by example an instrument setup according to the invention for measurement and estimation of local linear, and nonlinear propagation and scattering parameters in the object 200.
• 201 shows a transmit array system for transmission of pulses into the object.
• Acoustic contact between the transmit and receive (208) arrays can for example be obtained by immersing the object in a fluid (214), e.g. water, which for example is common with breast tomography, (FIG.6), or other types of acoustic stand-off between the receive probe and the object, or direct contact between the probes and the object.
• a fluid e.g. water
• FIG.6 shows a fluid
• With EM waves vacuum forms a good contact between the transmitters and the object, while fluids or soft tissue can provide contact for adequately small dimensions.
• the methods comprises transmitting at least two pulse complexes composed of co-propagating high frequency (HF) and low frequency (LF) pulses, where said HF pulse propagates close to the crest or trough of the LF pulse along at least one HF transmit beam, and where at least one of the amplitude and polarity of the LF pulse varies between at least two transmitted pulse complexes, where the amplitude of the LF pulse can be zero for a pulse complex and the amplitude of at least one LF pulse of said at least two transmitted pulse complexes is non-zero, as described in relation to FIG.1 above.
• HF high frequency
• LF low frequency
• a preferred embodiment would be arranged to estimate both linear and nonlinear parameters, where 201 would be arranged to be able to transmit dual band pulse complexes, also including a zero LF pulse.
• An example HF beam in the object is indicated as 202, with the HF pulse propagating to the right along the beam indicated as 203 at a particular time in the propagation.
• a LF pulse is, at the same time point, indicated as 205 with the positive swings as grey, co-propagating with the HF pulse to the right to manipulate both the propagation velocity and the scattering of the HF pulse, as discussed in Section 5.1B above.
• the HF pulse is indicated to be located at the crest of the LF pulse as indicated in FIG.1a.
• the HF pulse is scattered both linearly and nonlinearly, Eqs.(9b,14,15) as it propagates along the beam, and generates a scattered HF wave field propagating as a Mach-cone indicated by the front waves 206 and 207.
• 208 shows a HF receive array and processing system, where the Figure shows by example one HF receive cross-beam 209, where the HF receive cross-beam axis 212 crosses the HF transmit beam axis 213 at 211.
• An x-y-z Cartesian coordinate system shows the HF transmit beam axis along the z-direction and the HF receive cross-beam in the x-direction, where the x-axis is in the paper plane, and the y-axis is vertical to the paper plane.
• the array 208 can for example be a linear array, or a phased array, receiving in parallel on all elements where the element signals are coupled to a parallel receive beam former, producing in parallel individual receive signals for a set of receive cross-beams, all focused at different locations on the HF transmit beam axis.
• a parallel receive beam former producing in parallel individual receive signals for a set of receive cross-beams, all focused at different locations on the HF transmit beam axis.
• the HF cross-beam receive signals are processed in the receive unit 208 to provide linear and nonlinear propagation and scattering parameters for each measurement point rij in the scanned region with spatial resolution given by the dimension of the cross-beam observation cells R ij , as described below.
• the unit 208 also contains a display system for the images. To give an impression of a continuous image in the display, one typically introduces interpolated image display values between the image measurement points r ij .
• rij e HF cross-beam receive signals are composed of three components: i) a linear scattering component y d ii) a nonlinear scattering component where p c is the LF pulse amplitude defined in Eq.(12,14), and iii) followed by a multiple scattering component lustrated in FIG.3.
• r 0 (x 0 , y 0 , z 0 ) is the scatterer source point, and ⁇ l ( r 0 ) and ⁇ n ( r 0 ) are the linear and nonlinear HF scattering densities from Eqs.(9b,14,15).
• u p( ⁇ ,r ) is the received HF pulse from a point scatterer within the cross-beam observation cell R(r) centered around the image point r, that observes a nonlinear propagation delay (NPD) ⁇ p ( r ) according to Eq.(12), a nonlinear pulse form distortion (PFD) according to Eq.(13), and a propagation nonlinear self distortion according to Eq.(9b, term 2b) for the HF transmit pulse propagation up to depth r.
• NPD nonlinear propagation delay
• PFD nonlinear pulse form distortion
• Eq.(9b, term 2b) propagation nonlinear self distortion
• ⁇ f ( r ⁇ r 0 ,r ) ⁇ t( r ⁇ r 0 ,z ) + ⁇ r( r ⁇ r 0 , x ) is the sum of the HF transmit beam focusing phase delay ⁇ t and the HF receive beam focusing phase delay ⁇ r , and r 0 ⁇ r t + r 0 ⁇ r r is the total propagation distance of the pulse from the transmit aperture centered at r t to the scatterer at r 0 and to the receiver aperture centered at r r . Close to the focus of a HF receive or transmit beam we can approximate ⁇ r or ⁇ t ⁇ 0 .
• a preferred system allows for adjustment of the HF receive cross-beam focus onto the HF transmit beam axis, which allows the approximation ⁇ r ⁇ 0 in the observation cell, while for a fixed transmit focus ⁇ t ⁇ 0 for r outside the HF transmit focus.
• Lateral filtering of the HF cross-beam receive signal provides a synthetic focusing in the actual image range, as discussed in relation to Eqs.(18,19) below. This allows the approximation of both ⁇ t , ⁇ r ⁇ 0.
• the upper signal 301 shows the situation when the HF pulse propagates close to the positive crest of the LF pulse as in FIG.1a
• the middle signal 302 shows the situation when the LF pulse is zero
• the lowest signal 303 shows the situation when the LF pulse is inverted so that the HF pulse propagates close to the trough of the LF pulse as in FIG.1b.
• the 304 part of 303 is in the same way a delay of 302, i.e. a positive NPD ⁇ ( r ) > 0 , where the subscript + and - and 0 indicates with reference to FIG.1, measurements with a positive (102), negative (103) and zero transmitted LF pulse.
• the amplitude of the LF pulse drops so much in the 1 st scattering that the advancement/delay of the multiple scattered signal n p( t,r ) (305) is much less than for the 1 st part 304.
• Crossing side lobes, grating lobes and edge waves of the transmit and receive beams can add noise components that arrives before the 1 st order scattered component 304 from the cross-beam observation cell. These noise components are suppressed by proper apodization of transmit and receive beam apertures.
• Temporal Fourier transform of the front part 304 of the received signal in Eq.(16) gives where are the temporal Fourier transforms of the linear and nonlinear scattering components from the cross-beam observation cell centered at r.
• the exponential function arises from the delay components in the temporal argument of .
• the nonlinear self-distortion according to Eq.(9b, term 2b) introduces harmonic components of the fundamental band of the HF transmit pulse that varies with depth in a balance between propagation distance and absorption of the transmitted HF pulse.
• harmonic components of the fundamental band of the HF transmit pulse that varies with depth in a balance between propagation distance and absorption of the transmitted HF pulse.
• the observation region is therefore generally limited by a narrow receive beam, but in the x-direction the observation region is limited by the x-width of the transmit beam.
• the transmit beam operates with a fixed transmit focus, the transmit beam width varies with depth z outside the focus.
• the HF transmit beam can therefore be wide both in the azimuth (x-) and elevation (y-) directions.
• the filter kernel can be obtained from simulation of the transmit and receive beams to obtain Eqs.(31-33) below also present methods of estimating that corrects for wave front aberrations due to spatial variations in the propagation velocity within the object.
• the filter amplitude weighting B can conveniently be proportional to the amplitude of the simulated beams, potentially with added windowing.
• the integration over can be approximated by an integration in the x-direction (azimuth) only, with a filter adapted for use with 2D scanning of the transmit beam in the x-direction.
• One advantage of the synthetic focusing is that the cross beam observation cell is reduced, and the approximation that the NPD and PFD are constant in the observation cell becomes more accurate, i.e. improves the approximation of taking outside the integral in Eqs.(17).
• the synthetic focus filtering also provides images with improved spatial resolution.
• t r (r) From the models in Eqs.(12,16, 17), we can determine t r (r) from the delay between the front parts 304 of two of the signals 301 - 303, or all three signals in combination. This can for example be done through correlation between the 1 st order scattered front part 304 of the HF cross-beam receive signals from pulse complexes with different LF pulses, or by measuring the arrival time difference between the front edge or the maximum amplitude of the signals, or a combination of these.
• An advantage of the method as described is that we have first intervals of the measured signals that comprises mainly 1 st order scattering, which gives a clear definition of in Eq.(12) at depth z along the transmit beam axis starting at A schematic example of is shown as 401 in FIG. 4. [0086] Having measured ⁇ p( r t ,z j ) at a set of points zi with distance ⁇ z we can from Eq.(12) estimate the nonlinear propagation parameter where 402 in FIG.4 shows by example the z-differential of ⁇ . From the description of Eq. the LF pressure approximately at the center of gravity of the HF pulse, and c 0( he linear propagation velocity at the same position, i.e.
• Eqs.(20-22) presents a quantitative spatial imaging of the nonlinear elastic tissue parameter, where FIG. 5 shows by example typical values of for connective (501) , glandular (502), and fat (503) tissues in breast from [10].
• FIG. 5 shows by example typical values of for connective (501) , glandular (502), and fat (503) tissues in breast from [10].
• b r the difference of b r between the different tissues is so large that it makes estimation of the b r parameter very interesting for tissue characterization, for example detection and characterization of cancer tumors and atherosclerotic arterial plaque.
• the separate transmit and receive array systems in FIG.2 can conveniently be substituted with a dual frequency ring array used for combined transmit and receive, shown as 600 in FIG.6.
• the ring array surrounds an object 606 that is in acoustic contact with the ring array through a substance 614, typically a fluid like water or an oil.
• the ring array structure allows through-transmission of the ultrasound, typically used for ultrasound tomography imaging of objects, such as the breast and the male testicles.
• 601 of FIG.6 shows a set of HF transmit/receive elements and 602 shows a set of LF transmit elements of ring array 600, for example described in more detail in [5-7].
• the HF and LF elements are connected to a transmit switching system 603, that selectively connects a group of HF and LF elements to a HF and LF transmit system 604 comprising a HF and LF transmit beam former to generate selectable and/or steerable HF and LF transmit beams, where 605 shows an example HF transmit beam through the object 606.
• the HF array elements are also connected to a HF receive switching system 607, that selectively connects a group of HF elements to a HF receive system 608 comprising a HF receive beam former, that generates selectable and/or steerable HF receive cross-beams, where 609 shows an example crossing the transmit beam 605 at the observation cell 615, and a receive processing system 610 as described in relation to FIG. 2a to present estimates of object linear and nonlinear propagation and scattering parameters.
• a HF receive switching system 607 that selectively connects a group of HF elements to a HF receive system 608 comprising a HF receive beam former, that generates selectable and/or steerable HF receive cross-beams, where 609 shows an example crossing the transmit beam 605 at the observation cell 615, and a receive processing system 610 as described in relation to FIG. 2a to present estimates of object linear and nonlinear propagation and scattering parameters.
• [0097] 612 shows a further example HF receive cross-beam that crosses the HF transmit beam at the same cross-beam observation cell 615 as the HF receive cross-beam 609 with a different direction, and hence obtains a different speckle pattern of the front part 304 HF cross-beam receive signal, than for the HF receive cross-beam 609, as discussed in relation to FIG.3, 6, 7 above.
• the HF receive switching system connects selectable groups of HF array elements to the HF receive system 608 that produces a selectable number of HF cross-beam receive signals with different speckle of the front part 304, from selectable HF receive cross-beams in parallel.
• the variation in speckle in these different HF cross-beam receive signals from the same image point r allows for statistical averaging in the signal processing to reduce random errors in the estimates of the nonlinear propagation and scattering parameters.
• the subunits 603, 604, 607, 608 are connected to a processor and display system 610 via the bus 611 for set up, transmission, processing, and display of image data, according to known methods.
• the transmit 604 and receive 608 systems can alternatively be connected to all HF elements, and multiple transmit and receive beams are generated in parallel by for example a SW programmable system.
• a SW programmable system Because the modification of the bulk elasticity of the tissue by the LF pulse severely drops at the first scattering, as discussed following Eq.(13), the methods according to the invention as described above, do not require through-transmission through the object. A full ring array is therefore not necessary for these methods, where one for example could operate with a “horse-shoe” array extraction of the ring array 600 to emulate the functional essence of the array system in FIG.2.
• a 360 deg ring array would however open for more measurements, such as direct transmission through the object, that can be used to improve spatial resolution and estimation accuracy of the NPD and PFD, using tomographic techniques.
• Such a ring array can also be used for tomographic estimation of local tissue absorption and linear propagation velocity according to known methods [13-17], that can be combined with methods according to this invention for improved detection and characterization of disease in tissue, as discussed in relation to Eqs.(28-37) below.
• FIG.7a An example structure for assessment of nonlinear propagation and scattering parameters using a limited access area of the object, is shown in FIG.7a, where 701 shows an array probe arrangement that is in direct contact with only one side of the object 700, for example the surface of a body.
• the probe front comprises a combined LF and HF array, 702, where selectable elements are used to transmit LF and HF pulse complexes, where 703 shows an example, schematic HF transmit beam.
• HF receive elements are selected from the array elements to define HF receive cross-beams, where an example, schematic HF receive cross-beam is shown as 704.705 shows another example, schematic HF receive cross-beam crossing the HF transmit beam at the same cross-beam observation cell 706 as 704 at a different cross-angle, to obtain a HF receive signals with different speckle, for the same purpose as described for the beam 612 in FIG.6.
• both the HF transmit and receive beams can according to known methods be scanned one or both of i) laterally along the probe surface, and ii) angularly from a given selection of elements, to generate crossing HF transmit and receive beams and move the cross- beam observation cell 706 within a region of the object, for estimation of linear and nonlinear object parameters using methods as described in relation to FIG.2.
• the strong angular steering of the HF transmit and receive beams in FIG.7a requires down to ⁇ HF/2 pitch of the HF array, which increases the required number of elements.
• FIG.7b shows a modification that allows the use of wider pitch of the elements, and also the use of annular arrays and even fixed focus transducers, to estimate linear and nonlinear wave propagation and scattering parameters in a cross-beam observation cell 719 of the object 710.711 shows a combined transmit and receive system that comprises a LF and HF transmit array 712 and a HF receive array 713, embedded in a fluid filled region 714 to provide wave propagation contact to the object. Embedment in the fluid allows mechanical rotation of the arrays around the axes 715 and 716, or other mechanical movement of the arrays, potentially in combination with electronic scanning of the beam directions according to known methods.
• the LF-HF transmit array is connected to a transmit beam former inside the system 711 where an example resulting HF transmit beam is shown schematically as 717.
• the HF receive array 713 is connected to a HF receive beam former inside 711 that is used to define HF receive beams, for example schematically as 718, and further connected to a receive processing system according to the methods described in relation to FIG.2.
• the beam formers and processing structures operates and are implemented according to known methods, and details are therefore not shown in the Figure.
• the system allows crossing HF transmit and receive beams with an overlap 719 that defines the transmit-receive cross-beam observation cell 719 that can be scanned within a region of the object, exemplified with 720, by scanning the receive and/or the transmit beams as described for 211 in relation to FIG.2 and to Eqs.(16-25).
• the coupling medium 714 between the transmit and receive arrays (712, 713) and the object allows the transmit and receive arrays to have an angle to the object surface, which hence allows for larger pitch of the array elements. With linear arrays one can scan the transmit and receive beams side-ways for imaging of linear and nonlinear propagation and scattering parameters as presented in relation to FIG.2.
• Mounting the transmit and receive arrays 712 and 713 to rotating shafts 715 and 716, respectively, allows mechanical scanning of the transmit and receive beams that enlarges the number of measurement observation regions that can be obtained.
• the arrays 712 and 713 can then be reduced to annular arrays with sharp symmetric focusing, and even fixed focused transducers for a limited number of cross beam observation regions.
• Mechanical scanning of the beams in an elevation direction also opens for 3D imaging of regions of the object with methods according to this invention.
• the system according FIG. 7b can conveniently be simplified for objects that are approximately homogenous, such as the muscle of a fish or even a human, to measure for example uniform fat content in the muscle.
• the composition of the homogeneous tissue can then for example be obtained from a table of prior data.
• the high correlation between can also be used to generate estimates for example as where s an empirical relation between ⁇ ⁇ q.(30) then implies that c ⁇ 0 itself could be directly obtained from an empirical relation between ⁇ ⁇ r example as shown in FIG.10.
• Such estimates for c ⁇ 0 could be used as initial values for iterative improvement procedures of estimates of c 0( r ) , for example to form start parameters in iterative “bent ray” estimation procedures to estimate the spatially varying linear wave propagation velocity and absorption, for example according to tomographic methods [13-17].
• the phase of g represents both the standard focusing delay for beam forming in a homogeneous medium, and together with the amplitude of g an optimal correction for the wave front aberrations due to the spatially varying propagation velocity [8,9].
• the major component of this filter is however the linear component of the phase that represents a delay correction for the wave front aberrations.
• This first method requires a large numerical simulation capacity, which has a practical solution using Graphics Processing Units (GPUs). A less computer intensive approach can be obtained with ray acoustics techniques.
• c as the spatial average of c 0 ver the actual region in front of the array.
• the beam steering delay for element #k is then calculated as where s the acoustic length of the acoustic ray rfk(s) from rf to rk.
• the array should with mechanical elevation scanning be of the 1.75D type with larger elements in the elevation direction used for beam focusing and aberration corrections. With a full matrix array, one obtains electronic focusing and beam steering both in the azimuth and elevation directions.
• the aberration corrections can be included in the filter functions W( ⁇ ,r - r 0 , r) of Eqs.(18,19) for synthetic focusing of the observed beams.
• W( ⁇ ,r - r 0 , r) of Eqs.(18,19) for synthetic focusing of the observed beams.
• the NPD and the PFD estimated with the crossing transmit and receive beams can also be used in the processing of back-scattered HF signals obtained with a HF receive beam axis along or close to the HF transmit beam axis, as for example described in US Pats 7641613, 8038616, 8550998, 9291493 to suppress multiple scattering noise and estimate nonlinear scattering for received HF back-scatter signals.
• the back scatter images have better spatial resolution in range, and is also the type of images currently in general use, while the cross beam method provides more accurate estimation of the spatial variation of the NPD and PFD with less influence from multiple scattering noise.
• the HF back-scatter receive signals can then be processed for suppression of multiple scattering noise and enhancement of nonlinear scattering signals, for example as described in US Pats 7641613, 8038616, 8550998, 9291493.
• For each HF back-scatter receive beams we estimate from the NPD and PFD one or both of i) time delay corrections, and ii) pulse form and speckle corrections, for the at least two HF back-scatter receive signals from at least two transmitted pulse complexes with different LF pulses, followed by combinations of the corrected HF back-scatter receive signals to produce HF back-scatter receive signals with suppression of multiple scattering noise.
• a useful method is therefore to couple the array elements to a receive beam former that produces both receive cross-beams that crosses the transmit beams like in in FIG.2, 6, 7a, and 7b, and HF back-scatter receive beams close to equal to the HF transmit beams.
• the HF cross-beam receive signals are used for estimation of at least the spatially varying NPD, ⁇ p ( r ) , as described above, and potentially also the PFD described in Eqs.(18- 33) above.
• the spatially varying PFDs and/or the NPDs obtained from the cross-beam signals are then used to estimate correction of the HF back-scatter signals from equal HF transmit and HF back-scatter receive beams for efficient suppression of the multiple scattering noise, as for example described in US Pats 9291493.
• the HF back-scattered receive signals can be synthetically focused at multiple depths through lateral filtering as one of before and after the suppression of the MSN, for example as described in Eqs.(34-37) and US Pat 9291493.
• the invention also devices a method for combined HF cross-beam and back-scatter imaging where the HF receive signals for the individual HF array elements are digitized and stored for each transmit pulse complex and each transmit beam. The stored receive element signals are then first processed to i) generate focused HF receive beams crossing the HF transmit beams for example as shown in FIG.2, 6, 7a, and 7b, according to known methods.
• the receive HF cross-beam signals are then used for estimation of at least the spatially varying NPD, ⁇ and potentially also the PFD described above, and further described in Eqs.(20 - 24) and Eqs.(29-33) above.
• the method applies further processing on the stored element signals to ii) form a set of HF back-scatter receive signals from HF receive back-scatter beams with axis along or close to the HF transmit beam axis and aperture, focus, and apodization that are equal to that of the HF transmit beams, and iii)
• the HF back-scatter receive signal contains multiple scattering noise that can be strongly suppressed by delay corrections and potentially also speckle and pulse form corrections of the HF back-scatter receive signals, and combining the corrected HF back-scatter signals from transmitted pulse complexes with different LF pulses as described in US Pat 8,038,616, 8,550,998, 9,291,493, to produce noise suppressed HF
• the methods hence allow formation of 2D and 3D back-scatter images with suppression of multiple scattering noise and enhancement of nonlinear scattering and also with focus corrections for the wave front aberration effect from spatial variations in ultrasound propagation velocity.
• the strong angular steering of the HF transmit and receive beams used for cross-beam estimation of the NPD as in FIG.7a requires down to ⁇ HF/2 pitch of the HF array, which increases the required number of elements.
• the LF is so low (e.g.
• ⁇ 500 kHz that it is feasible to obtain ⁇ LF /2 pitch for the LF array, while ⁇ HF/2 pitch of the HF array can require undesirable large number of HF elements.
• a larger HF pitch could be allowed by using an angled HF transmit beam, for example as 704, at a low frequency, say at the lower end of the effective frequency band of the HF array, and utilizing for processing a harmonic component of the received signal at the upper end of the effective frequency band of the array with a direct forward HF receive beam, for example as 703.
• the direct forward receive beam sets weaker requirements on the pitch of the HF array, for example ⁇ 1.5 times the harmonic wavelength in the tissue.
• Lateral filtering of the received HF harmonic signal for many lateral HF receive beams can then provide a HF receive beam with focus within a large range of depths. This gives a narrower receive beam that provides improved estimates of the PFD and/or NPD with strong suppression of effects from multiple scattering noise on the estimates, both from the cross-beam effect and the harmonic suppression of multiple scattering noise in the receive signal.
• the estimated PFD and/or NPD can also be used for increased suppression of multiple scattering noise and enhancement of nonlinear scattering in the received harmonic signal.
• the NPDs and the PFDs at the different image points depend on both the transmitted LF amplitude and transmit LF/HF beam directions.
• FIG.8 shows a block diagram of an example instrument for carrying out imaging according to this method.800 shows a 3D ultrasound probe comprising a dual frequency linear array 801 with a set of M LF elements and N HF elements in an azimuth direction indicated by the arrows 806.
• the dual frequency band linear array can be made according to known methods, for example as described in US Pat 7727156 [5].
• the LF and HF elements of the array are via a cable 802 connected to a transmit/receive unit 803 that connects each LF array element to LF transmit amplifiers, and each HF element to HF transmit/receive circuits comprising a HF transmit amplifier and a HF receive amplifier where the output of the HF receive amplifier is further connected to an analog to digital converter (A/D) presenting a digital representation of the HF received signals from all HF receive elements, according to known methods.
• A/D analog to digital converter
• the AD converter can in a modified embodiment present digital representations of the I-Q components of the HF receive signals from each HF element that represents the same information as the radio frequency (RF) HF signal, according to known methods.
• the linear array 801 can in this example embodiment be rotated around the long axis 804 that provides a mechanical scanning of the LF/HF beam in an elevation direction, indicated by the arrows 805.
• For each elevation position of the array one does electronic scanning of a combined LF/HF transmit beam in an azimuth direction indicated by the arrows 806, through electronic selection of transmitting LF and HF elements, and transmitting combined LF/HF pulse complexes similar to what is shown in FIG. 1, with selected beam directions and focus.
• An example HF transmit beam is illustrated schematically as 807 within a 2D azimuth plane 808 with given elevation position within a total 3D scan volume 809.
• At least two pulse complexes with different LF pulses are transmitted for each transmit beam direction.
• the LF pulse might be zero in one pulse complex per HF transmit beam, but must be non/zero in at least one pulse complex for each HF transmit beam.
• Two versions of the instrument are useful, where in the first version 803 comprises beam former for HF receive cross-beams, illustrated as 814 in the 2D scan plane 808, and HF back scatter receive beams with the same axis as the HF transmit beam 807.
• the HF back-scatter receive beam is equal to the HF transmit beam as this improves suppression of multiple scattering noise in the HF back-scatter receive signal, as discussed in US Pat 9,291,493.
• the HF cross-beam and back-scatter receive signals are via the high speed bus 810 transferred to the processor 811 for storage and further processing.
• the processor 811 comprises a multicore central processing unit (CPU) and a graphics processor unit (GPU) that are SW programmable.
• the processor receives user inputs from a user/operator input unit 813 that operates according to known methods, and displays image data and other information necessary for communication with the user/operator through a combined display and audio unit 812, according to known methods.
• the digital HF receive signals from each HF receive elements and each transmitted pulse complex are via the high speed bus 810 transferred to the processor 811 for storage and further processing.
• a SW program in the processor 811 combines HF receive signals from multiple HF receive elements and produces a set of HF receive cross-beams crossing each HF transmit beam in the 2D set, for example as described in relation to FIG.7a.
• a SW program also produces a set of HF back-scatter receive signals from HF back-scatter receive beams with the same axis as the HF transmit beams, and preferably also equal to the HF transmit beams.
• Example transmit and receive beams are shown in FIG.9, where 901 shows by example a combined HF transmit beam, H t , and HF back-scatter receive beam, H br .
• the spatial frequency responses of the HF transmit beam and the HF back-scatter receive beam are Ht(r - rt, ⁇ ) and Hbr(r - rt, ⁇ ).
• the position vector r t (i,j) defines the origin of the HF transmit and receive beam axes, where i defines the azimuth aperture center element position, and j defines the 2D scan plane elevation position in a 3D scan.
• 902 shows an example HF receive cross-beam focused at the HF transmit beam axis at depth z k , H cr (r - r r , ⁇ ), where r r (i,j,k) defines the origin of the HF receive cross-beam axes, where i, j defines the azimuth and elevation position, and k defines the depth of the cross-over image point z k that is also the focus point of H cr .903 shows a cross-beam observation cell by the cross-over region between a HF transmit beam and the HF receive cross-beam, and 904 shows an indication of a HF cross beam receive signal from the whole cross-over region.
• the effective range of the observation cell is reduced along the cross-beam axis to the hatched region 905.
• a schematic form of the HF back-scatter observation cell is shown as the hatched region 906, along the HF combined transmit and back-scatter receive beams 901.
• the dimensions of the observation cells can be reduced by filtering of the received signals, as shown in Eqs.(18,19). Estimates of the NPD and PFD can then be obtained from the HF cross-beam receive signals at several observation cells along each transmit beam, f.ex. as described in relation to FIG.3 and Eq.(23).
• the magnitude of suppression of HF cross-beam MSN compared to back scatter imaging depends amongst other on the cross-angle between the HF receive cross-beam and the HF transmit beam, in relation to the object structures that produces the MSN, and the type of arrays used.
• the MSN is often given by large, plane fat-layers, parallel to a linear ultrasound transducer array. In this case one can get a ⁇ 6 dB suppression of the MSN with a cross-angle down to ⁇ 10 deg.
• These aberration correction estimates can then be included in the filter kernels of Eqs.(18,19) to provide improved beam focusing with corrections for these aberrations, that reduces the dimension of the observation range cells.
• the back-scatter observation cell is defined by the HF pulse length in the z direction, and is hence short.
• a very useful synthetic focusing of the HF back-scatter receive signal can then be obtained by only transversal filtering of the HF back- scatter receive signal at fixed depth as
• W rt the filter kernel W rt so that the phase gradient of the Fourier transform Hf in the transversal direction is zero [18].
• k t is the Fourier coordinates in the transversal plane
• a rt is an apodization function to reduce sidelobes.
• matched filter useful which includes both phase correction and apodization.
• Wave front aberrations can be included in our model of the HF transmit beam frequency response Ht according to known methods, and the focus filtering in Eqs.(35-37) then also corrects for wave front aberrations.
• the processing one might also use all LF/HF array elements to transmit LF/HF beams that are approximately plane in the azimuth direction. Transmitting azimuth plane waves in several directions one can combine the received signals from the different directions to produce synthetic transmit beams focused at different locations within the 2D plane, according to known methods [4].

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