Method and system for 3-D acoustic microscopy using short pulse excitation and 3-D acoustic microscope for use therein

• US 5,615,675 A
• Filed: 04/19/1996
• Issued: 04/01/1997
• Est. Priority Date: 04/19/1996
• Status: Expired due to Fees

• Current Assignee: Board of Regents of the University of Michigan (University of Michigan)
• Original Assignee: Board of Regents of the University of Michigan (University of Michigan)
• Inventors: Hamilton, James D., O'Donnell, Matthew
• Primary Examiner(s): MANUEL, GEORGE C
• Application Number:
  US08/635,361
• Time in Patent Office: 347 Days
• Field of Search: 128/653.1, 128/660.02, 73/627, 73/656, 374/45, 374/17, 374/57, 356/432, 356/436
• US Class Current: 600/425
• CPC Class Codes: G01N 2291/02475 Tissue characterisation
• CPC Class Codes: G01N 2291/0421 Longitudinal waves
• CPC Class Codes: G01N 29/0681 by acoustic microscopy, e.g. scanning acoustic microscopy
• CPC Class Codes: G01N 29/069 Defect imaging, localisation and sizing using, e.g. time of flight diffraction [TOFD], synthetic aperture focusing technique [SAFT], Amplituden-Laufzeit-Ortskurven [ALOK] technique
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046
• CPC Class Codes: G01S 15/8965 using acousto-optical or acousto-electronic conversion techniques

Abstract

A method and system are provided for 3-D acoustic microscopy using short pulse laser excitation. A 3-D acoustic microscope for use in such a system is also provided. In a first embodiment, wherein optical detection is utilized, a focused excitation beam is scanned by a first acoustooptic scanning device across an absorbing layer of an opto-acoustic transducer (if needed) coupled to an object under investigation to create spherical ultrasonic waves within the object which may be living tissue. The reflected spherical ultrasonic waves are detected through the use of an unfocused probe beam and an optical detector array or a focused probe beam and a single photodetector which receive the probe beam from a reflecting surface of the opto-acoustic transducer. A second acoustooptic scanning device scans the probe beam at a plurality of positions on the reflecting surface of the opto-acoustic transducer. In another embodiment, an ultrasound transducer is utilized to acoustically detect the reflected spherical ultrasonic waves. Signals from the optical detector array, the photodetector, or the ultrasound transducer are subsequently digitized and reconstructed via 3-D synthetic aperture beam-forming equations to generate a 3-D representation of the object. Then the 3-D representation of the image is displayed as an image. The microscope includes the first and second acoustooptic scanning devices and the opto-acoustic transducer housed in a housing to define a needle probe such as a conventional biopsy needle.

Claims

1. A method for examining an object internally, the method comprising the steps of:

positioning an opto-acoustic transducer having a reflecting surface and an absorbing layer capable of converting a burst of electromagnetic energy into a thermal pulse relative to the object so that the thermal pulse propagates as an acoustic wave in the object and is internally reflected within the object;

scanning a plurality of bursts of the electromagnetic energy over a surface of the absorbing layer to obtain acoustic waves in the object;

detecting acoustic waves reflected within the object at a plurality of positions on the reflecting surface of the opto-acoustic transducer to generate resultant signals;

calculating a 3-D representation of the object from the resultant signals; and

displaying the 3-D representation as an image.

2. The method as claimed in claim 1 wherein the bursts of electromagnetic energy are pulses of laser light.

3. The method as claimed in claim 2 wherein the acoustic waves have a primary wavelength, λ

, and wherein the method further comprises the step of focusing the pulses of laser light so that each thermal pulse is generated at an area of the absorbing layer approximately equal to λ

² so that the acoustic sound waves are spherical waves.

4. The method as claimed in claim 1 wherein the bursts of electromagnetic energy are pulses of laser light and wherein the absorbing layer is a material layer of the opto-acoustic transducer.

5. The method as claimed in claim 4 wherein the absorbing layer of the opto-acoustic transducer is a thin metallic film.

6. The method as claimed in claim 5 wherein the opto-acoustic transducer includes an optically semi-transparent substrate layer having a thickness of approximately λ

/4 and wherein the metallic film is formed as a surface layer on the substrate layer.

7. The method as claimed in claim 1 wherein the step of detecting includes the step of scanning the reflecting surface of the opto-acoustic transducer with a focused, continuous-wave laser beam operating at a wavelength different than the wavelength of the bursts of electromagnetic energy.

8. The method as claimed in claim 7 wherein the reflecting surface is a surface of the absorbing layer of the opto-acoustic transducer.

9. The method as claimed in claim 1 wherein the acoustic waves have a primary wavelength, λ

, and wherein the step of detecting includes the step of scanning a probe beam having spot size with a diameter less than λ

, over the reflecting surface of the opto-acoustic transducer at a plurality of scan positions.

10. The method as claimed in claim 1 wherein the step of detecting is accomplished with an array of opto-acoustic detectors.

11. The method as claimed in claim 1 wherein the acoustic waves have a primary wavelength, λ

, and wherein the step of detecting is accomplished with an ultrasonic microscope transducer for focusing an acoustic spot on the surface of the object, the spot having an area approximately equal to λ

² and wherein the microscope transducer is utilized to scan in a plane substantially parallel to the surface of the object.

12. The method as claimed in claim 1 wherein the step of calculating is based on 3-D synthetic aperture beam-forming equations.

13. A method for examining an object internally, the object being capable of converting a burst of electromagnetic energy into a thermal pulse which propagates as an acoustic wave in the object and is internally reflected within the object, the method comprising the steps of:

scanning a plurality of bursts of the electromagnetic energy over a surface of the object to generate acoustic waves in the object;

detecting the acoustic waves reflected within the object at a plurality of positions on the surface of the object to generate resultant signals;

calculating a 3-D representation of the object from the resultant signals; and

displaying the 3-D representation as an image.

14. The method as claimed in claim 13 wherein the bursts of electromagnetic energy are pulses of laser light.

15. The method as claimed in claim 14 wherein the acoustic waves have a primary wavelength, λ

, and wherein the method further comprises the step of focusing the pulses of laser light so that each thermal pulse is generated at an area of the object approximately equal to λ

² so that the acoustic sound waves are spherical waves.

16. The method as claimed in claim 13 wherein the step of detecting includes the step of scanning the surface of the object with a focused, continuous-wave laser beam operating at a wavelength different than the wavelength of the bursts of electromagnetic energy.

17. The method as claimed in claim 13 wherein the acoustic waves have a primary wavelength, λ

, and wherein the step of detecting includes the step of scanning a probe beam having spot size with a diameter less than λ

, over the surface of the object at a plurality of scan positions.

18. The method as claimed in claim 17 wherein the step of detecting is accomplished with an array of opto-acoustic detectors.

19. The method as claimed in claim 17 wherein the acoustic waves have a primary wavelength, λ

, and wherein the step of detecting is accomplished with an ultrasonic microscope transducer for focusing an acoustic spot on the surface of the object, the spot having an area approximately equal to λ

² and wherein the transducer is utilized to scan in a plane substantially parallel to the surface of the object.

20. The method as claimed in claim 17 wherein the step of calculating is based on 3-D synthetic aperture beam-forming equations.

21. A system for examining an object internally, the system comprising:

an opto-acoustic transducer adapted to be acoustically coupled to the object and having a reflecting surface and an absorbing layer capable of converting a burst of electromagnetic energy into a thermal pulse so that the thermal pulse propagates as an acoustic wave in the object and is internally reflected within the object;

an excitation beam scanner for scanning a plurality of bursts of the electromagnetic energy over a surface of the absorbing layer to obtain acoustic waves in the object;

a detector for detecting the acoustic waves reflected within the object at a plurality of positions on the reflecting surface of the opto-acoustic transducer to generate resultant signals;

a calculator for calculating a 3-D representation of the object from the resultant signals; and

a display for displaying the 3-D representation as an image.

22. The system as claimed in claim 21 wherein the bursts of electromagnetic energy are pulses of laser light.

23. The system as claimed in claim 22 wherein the acoustic waves have a primary wavelength, λ

, and wherein the system further comprises an optical component for focusing the pulses of laser light so that each thermal pulse is generated at an area of the absorbing layer approximately equal to λ

² so that the acoustic sound waves are spherical waves.

24. The system as claimed in claim 23 wherein the absorbing layer of the opto-acoustic transducer is a thin metallic film.

25. The system as claimed in claim 24 wherein the opto-acoustic transducer includes an optically semi-transparent substrate layer having a thickness of approximately λ

/4 and wherein the metallic film is formed as a surface layer on the substrate layer.

26. The system as claimed in claim 21 wherein the bursts of electromagnetic energy are pulses of laser light and wherein the absorbing layer is a material layer of the opto-acoustic transducer.

27. The system as claimed in claim 26 further comprising an optical fiber and wherein the probe beam is delivered to the probe beam scanner by the optical fiber.

28. The system as claimed in claim 21 wherein the detector includes a probe beam scanner for scanning the reflecting surface of the opto-acoustic transducer with a focused, continuous-wave laser beam operating at a wavelength different than a wavelength of the bursts of electromagnetic energy.

29. The system as claimed in claim 28 wherein the reflecting surface is a surface of the absorbing layer of the opto-acoustic transducer.

30. The system as claimed in claim 21 wherein the acoustic waves have a primary wavelength, λ

, and wherein the detector includes a probe beam scanner for scanning a probe beam having spot size with a diameter less than λ

, over the reflecting surface of the opto-acoustic transducer at a plurality of scan positions.

31. The system as claimed in claim 30 wherein at least one of the scanners includes an electronically driven acoustooptic device.

32. The system as claimed in claim 31 wherein the acoustooptic device is a Bragg cell.

33. The system as claimed in claim 30 wherein the opto-acoustic transducer including the absorbing layer and the reflecting surface, the probe beam scanner and the excitation beam scanner are housed in a housing to define an optical needle probe.

34. The system as claimed in claim 30 further comprising a single optical fiber and wherein the probe beam is delivered to the probe beam scanner, the excitation beam is delivered to the excitation beam scanner and a detecting beam is delivered to the detector by the single optical fiber.

35. The system as claimed in claim 21 further comprising an optical fiber and wherein the excitation beam is delivered to the excitation beam scanner by the optical fiber.

36. The system as claimed in claim 34 wherein the detector includes a photodetector array and wherein a detecting beam is returned to the photodetector array by the optical fiber.

37. The system as claimed in claim 21 wherein the detector includes an array of opto-acoustic detectors.

38. The system as claimed in claim 21 wherein the acoustic waves have a primary wavelength, λ

, and wherein the detector includes an ultrasonic microscope transducer for focusing an acoustic spot on the surface of the object, the spot having an area approximately equal to λ

² and wherein the microscope transducer is utilized to scan in a plane substantially parallel to the surface of the object.

39. The system as claimed in claim 21 wherein the calculator includes a programmed computer for computing the 3-D representation based on 3-D synthetic aperture beam-forming equations.

40. A system for examining an object internally, the object being capable of converting a burst of electromagnetic energy into a thermal pulse which propagates as an acoustic wave in the object and is internally reflected within the object, the system comprising:

an excitation beam scanner for scanning a plurality of bursts of the electromagnetic energy over a surface of the object to generate acoustic waves in the object;

a detector for detecting acoustic waves reflected within the object at a plurality of positions on the surface of the object to generate resultant signals;

a calculator for calculating a 3-D representation of the object from the resultant signals; and

a display for displaying the 3-D representation as an image.

41. The system as claimed in claim 40 wherein the bursts of electromagnetic energy are pulses of laser light.

42. The system as claimed in claim 41 wherein the acoustic waves have a primary wavelength, λ

, and wherein the system further comprises an optical element for focusing the pulses of laser light so that each thermal pulse is generated at an area of the object approximately equal to λ

² so that the acoustic sound waves are spherical waves.

43. The system as claimed in claim 40 wherein the detector includes a probe beam scanner for scanning the surface with a focused, continuous-wave laser beam operating at a wavelength different than the wavelength of the bursts of electromagnetic energy.

44. The system as claimed in claim 40 wherein the acoustic waves have a primary wavelength, λ

, and wherein the detector includes a probe beam scanner for scanning a probe beam having spot size with a diameter less than λ

, over the surface of the object at a plurality of scan positions.

45. The system as claimed in claim 44 wherein at least one of the scanners includes an electronically driven acoustooptic device.

46. The system as claimed in claim 45 wherein the acoustooptic device is a Bragg cell.

47. The system as claimed in claim 44 further comprising a housing wherein the probe beam scanner and the excitation beam scanner are housed in the housing to define an optical needle probe.

48. The system as claimed in claim 44 further comprising an optical fiber and wherein the probe beam is delivered to the probe beam scanner by the optical fiber.

49. The system as claimed in claim 44 further comprising an optical fiber and wherein the excitation beam is delivered to the excitation beam scanner by the optical fiber.

50. The system as claimed in claim 49 wherein the detector includes a photodetector array and wherein a detecting beam is returned to the photodetector array by the optical fiber.

51. The system as claimed in claim 44 further comprising a single optical fiber and wherein the probe beam is delivered to the probe beam scanner, the excitation beam is delivered to the excitation beam scanner and a detecting beam is delivered to the detector by the single optical fiber.

52. The system as claimed in claim 40 wherein the detector includes an array of opto-acoustic detectors.

53. The system as claimed in claim 40 wherein the acoustic waves have a primary wavelength, λ

, and wherein the detector includes an ultrasonic microscope transducer for focusing an acoustic spot on the surface of the object, the spot having an area approximately equal to λ

² and wherein the transducer is utilized to scan in a plane substantially parallel to the surface of the object.

54. The system as claimed in claim 40 wherein the calculator includes a programmed computer for calculating the 3-D representation based on 3-D synthetic aperture beam-forming equations.

55. A 3-D acoustic microscope adapted to be used in a system for 3-D acoustic microscopy to examine an object internally, the microscope comprising:

an opto-acoustic transducer adapted to be coupled to the object and including an optically reflecting surface and an absorbing layer capable of converting a burst of electromagnetic energy into a thermal pulse so that the thermal pulse propagates as an acoustic wave in the object and is internally reflected within the object;

a first acoustooptic scanning device for scanning a plurality of bursts of the electromagnetic energy over a surface of the absorbing layer to generate acoustic waves in the object;

a second acoustooptic scanning device for scanning a probe beam at a plurality of positions on the reflecting surface of the opto-acoustic transducer; and

a housing for housing the opto-acoustic transducer and the first and second acoustooptic scanning devices to define an optical needle probe.

56. The microscope as claimed in claim 55 wherein at least one of the scanning devices is an electronically driven acoustooptic Bragg cell.

57. A 3-D acoustic microscope adapted to be used in a system for 3-D acoustic microscopy to examine an object internally, the object being capable of converting a burst of electromagnetic energy into a thermal pulse so that the thermal pulse propagates as an acoustic wave in the object and is internally reflected within the object, the microscope comprising:

a first acoustooptic scanning device for scanning a plurality of bursts of the electromagnetic energy over a surface of the object to generate acoustic waves in the object;

a second acoustooptic scanning device for scanning a probe beam at a plurality of positions on the surface of the object; and

a housing for housing the first and second acoustooptic scanning devices to define an optical needle probe.

58. The microscope as claimed in claim 57 wherein at least one of the scanning devices is an electronically driven acoustooptic Bragg cell.