Resolution enhancement for ultrasonic reflection mode imaging

• US 5,339,282 A
• Filed: 10/02/1992
• Issued: 08/16/1994
• Est. Priority Date: 10/02/1992
• Status: Expired due to Term

• Current Assignee: University of Utah Research Foundation (State of Utah)
• Original Assignee: University of Utah Research Foundation (State of Utah)
• Inventors: Kuhn, Paul K., Johnson, Steven A.
• Primary Examiner(s): Pihulic, Daniel T.
• Application Number:
  US07/956,036
• Time in Patent Office: 683 Days
• Field of Search: 367/7, 367/902, 367/11, 364/413.25, 358/112, 128/660.06, 73/602
• US Class Current: 367/7
• CPC Class Codes: G01S 15/8918 the array being linear
• CPC Class Codes: G01S 15/8922 the array being concentric or annular
• CPC Class Codes: G01S 7/52004 Means for monitoring or calibrating short-range imaging G01S7/5205
• CPC Class Codes: G01S 7/52006 with provision for compensating the effects of temperature
• CPC Class Codes: G01S 7/52049 using correction of medium-induced phase aberration
• CPC Class Codes: G01S 7/5205 Means for monitoring or calibrating
• CPC Class Codes: Y10S 367/902 Speed of sound compensation

Abstract

An annular array scanner utilizes a synthetic focusing approach to improve imaging results. Image sharpening methods include corrections for the speed of sound of the image, phased array misregistration and separate view misalignment as well as inverse filtering for the scanning system point spread function.

Claims

1. In a reflection mode imaging system, an improved means of reconstructing reflectivity images from acoustic or ultrasonic waves, with the use of speed of sound velocity mapping from measured or estimated means, said improved means comprising:

signal processing means constructed and arranged to use the calculus of variations to effect a correction of variations in propagation time along a path of propagation due to changes in the refractive index and variations in the speed of sound of an imaging target to estimate the error in a straight line path and to remove this error from a predicted time of propagation for said straight line path to calculate an actual time of propagation for an actual path propagation to predict times of arrivals of a reflected energy waveform.

2. The improvement in claim 1 wherein said signal processing means is comprised of a digital, computer oriented means.

3. The improvement in claim 1 wherein said signal processing means is comprised of an analog circuit oriented means.

4. In a reflection mode imaging system utilizing a phased array system, an improved means of reconstructing reflectivity images from acoustic or ultrasonic waves, with the use of signal processing means for correction of inaccurate estimates of the transducer array elements by use of an adaptive minimization technique to reduce an objective functional based on cross-correlation techniques to estimate the misalignment of reflected signals from a given calibration target.

5. The improvement in claim 4 wherein said signal processing means is comprised of a digital, computer oriented means.

6. The improvement in claim 4 wherein said signal processing means is comprised of an analog circuit oriented means.

7. In a reflection mode imaging system utilizing a phased array system or non-phase array transducer system using multiple views in the reconstruction signal processing, an improved means of reconstructing reflectivity images from acoustic or ultrasonic waves, with the use of signal processing means for correction of inaccurate estimates of the relative center of rotation of the various views by use of an adaptive minimization technique to reduce an objective functional based on cross-correlation techniques to estimate the misalignment of the individual views of the image reconstruction.

8. The improvement in claim 7 wherein said signal processing means is comprised of a digital, computer oriented means.

9. The improvement in claim 7 wherein said signal processing means is comprised of an analog circuit oriented means.

10. In a reflection mode imaging system utilizing a phased array system or non-phase array transducer system using multiple views in the reconstruction signal processing, an improved means of reconstructing reflectivity images from acoustic or ultrasonic waves, with the use of signal processing means for correction of inaccurate estimates of the relative angle of rotation of the various views by use of an adaptive minimization technique to reduce an objective functional based on cross-correlation techniques to estimate the misalignment of the individual views of the image reconstruction.

11. The improvement in claim 10 wherein said signal processing means is comprised of a digital, computer oriented means.

12. The improvement in claim 10 wherein said signal processing means is comprised of an analog circuit oriented means.

13. In a reflection mode imaging system utilizing a phased array system or non-phase array transducer system using multiple views in the reconstruction signal processing, an improved means of reconstructing reflectivity images from acoustic or ultrasonic waves, with the use of signal processing means for correction of the point spread function or blurring function of the imaging system by use of a calibration imaging target to perform an estimate of the point spread or blurring function so that said function may be removed from the reconstruction by additional signal processing means.

14. The improvement in claim 13 wherein said signal processing means is comprised of a digital, computer oriented means.

15. The improvement in claim 13 wherein said signal processing means is comprised of an analog circuit oriented means.

16. In an acoustic imaging system comprising source transducers and detector transducers positioned with respect to an imaging target having a volume, said transducers operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) dividing the volume of said target into volume elements of smaller size;

b) estimating the average speed of sound throughout each said volume element;

c) calculating the time of propagation along the straight line path from a plurality of said source transducers to a plurality of reflection points to a plurality of detector transducers, based upon the estimates of step b);

d) calculating the refractive index with respect to a reference speed of sound for each said volume element;

e) calculating the error in the time of propagation calculation of step c), said error being calculated from the normal derivatives of the respective refractive indices of step d) along the straight line paths from said source transducers to reflection points to detector transducers as indicated by the calculus of variations;

f) calculating an improved estimate of the time of propagation of step c) by removing the error in the estimate which was calculated in step e); and

g) generating an improved reflectivity image from the improved estimate of the time of propagation calculated in step f).

17. The improvement according to claim 16 wherein the refractive index of each volume element is the functional equivalent of a ratio of a reference speed of sound and the speed of sound of each said volume element.

18. The improvement of claim 16 wherein said detector transducers are structured and arranged to detect transmission signals.

19. The improvement of claim 16 wherein said reflected signals include refracted signals.

20. In an acoustic imaging system comprising an array of source transducers and detector transducers positioned with respect to an imaging target said array operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) estimating the positions of said source and detector transducers;

b) collecting through said detector transducers a set of spatial representations of reflection signals from a known imaging calibration target, each said representation corresponding to a reflection signal from a said source transducer to a said detector transducer;

c) forming a phased array error function based on cross-correlation techniques to estimate a misregistration of the set of spatially represented reflected signals collected in step b);

d) correcting the misregistration to find improved estimates of the respective positions of said transducers within said array by shifting said set of spatially represented reflected signals collected in step b) to minimize the phase array error function formed by step c); and

e) generating an improved reflectivity image from the improved estimates found by step d).

21. The improvement of claim 20 wherein said detector transducers are structured and arranged to detect transmission signals.

22. The improvement of claim 20 wherein said reflected signals include refracted signals.

23. In an acoustic imaging system comprising source transducers and detector transducers positioned with respect to an imaging target having a volume, said transducers operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) estimating the positions of said source and detector transducers;

b) collecting through said detector transducers a set of time-based representations of reflection signals from a known imaging calibration target, each said representation corresponding to a reflection signal from a said source transducer to a said detector transducer;

c) forming a phased array error function based on cross-correlation techniques to estimate a misregistration of the set of time-based representations of the reflected signals collected in step b);

d) correcting the misregistration of said time-based representations to find improved estimates of the respective positions of said transducers within said array by shifting said set of time-based representations of the reflected signals collected in step b) to minimize the phase array error function formed by step c); and

e) generating an improved reflectivity image from the improved estimates found by step d).

24. The improvement of claim 23 wherein said detector transducers are structured and arranged to detect transmission signals.

25. In an acoustic imaging system comprising an array of source transducers and detector transducers positioned with respect to an imaging target said array operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) using the imaging system to generate a set of reconstructions from different relative view positions of the imaging target;

b) estimating the center of rotation about which each reconstruction of step a) is rotated with respect to another said reconstruction of step a);

c) estimating the angle of rotation that each said reconstruction of step a) is rotated with respect to another said reconstruction of step a);

d) choosing a first reconstruction of step a) corresponding to a first angle of observation;

e) performing a rotational transformation on a second reconstruction of step a) corresponding to a second angle of observation based on the estimated angle of rotation of step c) of said second reconstruction about the estimated center of rotation of step b) of said second reconstruction so that the second reconstruction may be better aligned with said first reconstruction;

f) forming a center of rotation error function based on cross-correlation techniques to estimate the misalignment of said second reconstruction with respect to said first reconstruction in terms of an error in the estimate of the center of rotation of step b) of said second reconstruction;

g) correcting the center of rotation estimate of step b) of said second reconstruction by shifting it to minimize the center of rotation error function of step f);

h) generating an improved rotationally transformed second reconstruction using the improved estimate of step g), whereby to achieve an improved alignment of said improved rotationally transformed second reconstruction with said first reconstruction of step d);

i) repeating steps d) through h) substituting a plurality of alternative reconstructions for said second reconstruction; and

j) generating an improved reflectivity image from aligned reconstructions formed by combining said first reconstruction and said second and alternative reconstructions.

26. In an acoustic imaging system comprising an array of source transducers and detector transducers positioned with respect to an imaging target said array operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) using the imaging system to generate a set of reconstructions from different relative view positions of the imaging target;

b) estimating the center of rotation about which each reconstruction of step a) is rotated with respect to another said reconstruction of step a);

c) estimating the angle of rotation that each said reconstruction of step a) is rotated with respect to another said reconstruction of step a);

d) choosing a first reconstruction of step a) corresponding to a first angle of observation;

e) performing a rotational transformation on a second reconstruction of step a) corresponding to a second angle of observation based on the estimated angle of rotation of step c) of said second reconstruction about the estimated center of rotation of step b) of said second reconstruction so that the second reconstruction may be better aligned with said first reconstruction;

f) forming an angle of rotation error function based on cross-correlation techniques to estimate the misalignment of said second reconstruction with respect to said first reconstruction in terms of an error in the estimate of the angle of rotation of step c) of said second reconstruction;

g) correcting the angle of rotation estimate of step c) of said second reconstruction by shifting it to minimize the angle of rotation error function of step f);

h) generating an improved rotationally transformed second reconstruction using the improved estimate of step g), whereby to achieve an improved alignment of said improved rotationally transformed second reconstruction with said first reconstruction of step d);

i) repeating steps d) through h) substituting a plurality of alternative reconstructions for said second reconstruction; and

j) generating an improved reflectivity image from aligned reconstructions formed by combining said first reconstruction and said second and alternative reconstructions.

27. In an acoustic imaging system comprising an array of source transducers and detector transducers positioned with respect to an imaging target said array operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) using the imaging system to generate a set of reconstructions from different relative view positions of the imaging target;

b) estimating the center of rotation about which each reconstruction of step a) is rotated with respect to another said reconstruction of step a);

c) estimating the angle of rotation that each said reconstruction of step a) is rotated with respect to another said reconstruction of step a);

d) choosing a first reconstruction of step a) corresponding to a first angle of observation;

e) performing a rotational transformation on a second reconstruction of step a) corresponding to a second angle of observation based on the estimated angle of rotation of step c) of said second reconstruction about the estimated center of rotation of step b) of said second reconstruction so that the second reconstruction may be better aligned with said first reconstruction;

f) forming an angle of rotation error function based on cross-correlation techniques to estimate the misalignment of said second reconstruction with respect to said first reconstruction in terms of an error in the estimate of the angle of rotation of step c) of said second reconstruction;

g) forming a center of rotation error function based on cross-correlation techniques to estimate the misalignment of said second reconstruction with respect to said first reconstruction in terms of an error in the estimate of the center of rotation of step b) of said second reconstruction;

h) forming a combined error function from the functions formed in steps f) and g);

i) correcting the center of rotation estimate of step b) of said second reconstruction by shifting it to minimize the combined error function of step h);

j) generating an improved rotationally transformed second reconstruction using the improved estimate of step i), whereby to achieve an improved alignment of said improved rotationally transformed second reconstruction with said first reconstruction of step d);

k) correcting the angle of rotation estimate of step c) of said second reconstruction by shifting it to minimize the combined error function of step h);

l) generating an improved rotationally transformed second reconstruction using the improved estimates of steps i and k), whereby to achieve an improved alignment of said improved rotationally transformed second reconstruction with said first reconstruction of step d);

m) optionally repeating steps i through l further to reduce the combined error function;

n) repeating steps d) through m) substituting a plurality of alternative reconstructions for said second reconstruction; and

o) generating an improved reflectivity image from aligned reconstructions formed by combining said first reconstruction and said second and alternative reconstructions.

28. In an acoustic imaging system comprising an array of source transducers and detector transducers positioned with respect to an imaging target said array operating to transmit acoustic energy in the direction of the target and to receive reflected energy signals emanating from said target, and further comprising means for processing said signals to generate a reflectivity image representative of the structure of said target, the improvement which comprises:

a) making multiple estimates of the point spread function of the imaging system from a known calibration target;

b) combining the multiple estimates of step a) to generate an improved estimate of the point spread function;

c) calculating an inverse filter from the improved estimate of the point spread function of step b); and

d) performing filtering operations on images generated from said imaging system using the inverse filter calculated in step c).

29. The improvement of claim 28 wherein the multiple estimates of step b) are combined by arithmetic averaging to generate said improved estimate of the point spread function.