Three-dimensional staring spare array photoacoustic imager and methods for calibrating an imager

• US 9,128,032 B2
• Filed: 06/11/2010
• Issued: 09/08/2015
• Est. Priority Date: 12/12/2007
• Status: Active Grant

• Current Assignee: Multi-Magnetics Incorporated
• Original Assignee: Multi-Magnetics Incorporated
• Inventors: Carson, Jeffrey J. L., Ephrat, Pinhas, Keenliside, Lynn, Roumeliotis, Michael Barret
• Primary Examiner(s): Jung, Unsu
• Assistant Examiner(s): Akar, Serkan
• Application Number:
  US12/814,086
• Publication Number:
  US 20100249570A1
• Time in Patent Office: 1,915 Days
• Field of Search: 600/437
• US Class Current: 1/1
• CPC Class Codes: A61B 5/0059 using light, e.g. diagnosis by transillumination, diascopy, fluorescence A61B5/0093, A61B5/1455, A61B5/02416 and A61B5/0261 take precedence
• CPC Class Codes: A61B 5/0095 by applying light and detecting acoustic waves, i.e. photoacoustic measurements
• CPC Class Codes: A61B 8/483 involving the acquisition of a 3D volume of data
• CPC Class Codes: G01N 2291/106 one or more transducer arrays
• CPC Class Codes: G01N 29/0654 Imaging
• 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: G01N 29/30 Arrangements for calibrating or comparing, e.g. with standard objects

Abstract

A photoacoustic imaging apparatus is provided for medical or other imaging applications and also a method for calibrating this apparatus. The apparatus employs a sparse array of transducer elements and a reconstruction algorithm. Spatial calibration maps of the sparse array are used to optimize the reconstruction algorithm. The apparatus includes a laser producing a pulsed laser beam to illuminate a subject for imaging and generate photoacoustic waves. The transducers are fixedly mounted on a holder so as to form the sparse array. A photoacoustic (PA) waves are received by each transducer. The resultant analog signals from each transducer are amplified, filtered, and converted to digital signals in parallel by a data acquisition system which is operatively connected to a computer. The computer receives the digital signals and processes the digital signals by the algorithm based on iterative forward projection and back-projection in order to provide the image.

Claims

1. Apparatus for photoacoustic (PA) imaging of a subject, said apparatus comprising:

a light positioned to illuminate said subject to generate PA waves emanating from said subject;

a programmed computer system;

a staring array of transducers for receiving said PA waves and converting said PA waves to corresponding analog signals, said staring array having at least three transducers and arranged in at least three dimensions and wherein three-dimensional (3-D) spatial co-ordinates of the transducers is to be determined by the programmed computer system;

an acoustically transmissive medium which surrounds the transducers and acoustically couples said staring array to said subject;

at least one analog to digital converter configured to receive said analog signals and convert said analog signals to corresponding digital signals; and

the programmed computer system operatively connected to said at least one analog to digital converter to receive said digital signals and configured to;

calibrate, at least when 3D spatial co-ordinates of the transducers is to be determined by the programmed computer system, the acoustic response of each of the transducers by measurements of PA waves from a PA source at a known plurality of positions, predetermined by the programmed computer system, through a calibration volume to generate at least three-dimensional (3-D) characterization maps for each transducer, and

process said digital signals, received when the staring array is staring, by an image reconstruction algorithm to create one or more three dimensional (3-D) images of said subject, said image reconstruction algorithm utilizes the three-dimensional (3-D) characterization maps for at least some of the transducers.

2. An apparatus according to claim 1 wherein said light comprises a laser capable of providing a pulsed laser beam.

3. An apparatus according to claim 2 including at least one photo detector, comprising a photo diode, configured to detect the illumination from said pulsed laser beam and providing digital signals indicative thereof to said computer system.

4. An apparatus according to claim 3 further comprising a relatively thin, optically transparent, window formed of acoustically transparent material for separating said subject from said staring array, wherein, during use of said apparatus, the illumination from said laser beam passes through said relatively thin, optically transparent, acoustically transparent window in order to illuminate said subject and said PA waves from said subject pass through said relatively thin, optically transparent, acoustically transparent material to said staring array.

5. An apparatus according to claim 2 wherein said laser is tunable.

6. An apparatus according to claim 1 wherein said light is located to illuminate said subject from a same side of the subject as a side on which said staring array is located.

7. An apparatus according to claim 1 wherein said staring array is an annular array, further comprising a supporting structure including an annular holder having said transducers distributed around an annular surface of said annular holder and defining a central hole, and said central hole is covered by an optical window made of an optically transparent material.

8. An apparatus according to claim 7 wherein said annular holder includes an annular wedge-shaped section having a flat annular side on which said transducers are mounted and creating a focal zone above a center of said staring array and wherein, during use of said apparatus, said PA waves are refracted by said wedge-shaped section before being transmitted to said transducers.

9. An apparatus according to claim 1 wherein said staring array is a hemispherical array, further comprising a supporting structure includes a plurality of transducer mounts each having one or more transducers mounted thereon at predetermined elevation angles, and said transducer mounts are arranged in a circle around a central hole which is covered by an optical window made of an optically transparent material.

10. An apparatus according to claim 1 further comprising a supporting structure for mounting said staring array wherein said staring array and said supporting structure are part of a transportable device which can be held in a user'"'"'s hand during use of the apparatus.

11. An apparatus according to claim 1 further comprising a tank which holds the acoustically transmissive medium.

12. An apparatus according to claim 3 wherein said computer system is configured to normalize said digital signals to measured illumination.

13. An apparatus according to claim 2 wherein said computer system is operatively connected to said laser and programmed to control said laser, and said computer system is also programmed to coordinate the operation of said at least one analog to digital converter with the operation of said laser.

14. An apparatus according to claim 1 wherein said image reconstruction algorithm is an iterative forward projecting, back projecting image reconstruction algorithm.

15. An apparatus according to claim 1, further comprising:

a laser source configured to produce a pulsed laser beam;

an optical fiber configured to receive said pulsed laser beam and generate said PA waves as said PA source at one end of said optical fiber when a pulse of laser beam illuminates said optical fiber; and

a scanner configured to move at least a portion of said optical fiber through the calibration volume.

16. An apparatus according to claim 15 wherein said one end of the optical fiber comprises an opaque coated tip of the optical fiber.

17. An apparatus according to claim 1 wherein said image reconstruction algorithm reconstructs the one or more three dimensional (3-D) images of said subject based on received corresponding digital signals for at least some but not all of the transducers.

18. An apparatus according to claim 17 wherein said image reconstruction algorithm reconstructs the one or more three dimensional (3-D) images of said subject using the three-dimensional (3-D) characterization maps for at least some but not all of the transducers due to additional material located between said subject and which interfere with coupling of said PA waves from said subject to at least one of the remaining transducers.

19. An apparatus according to claim 17 wherein said image reconstruction algorithm reconstructs the one or more three dimensional (3-D) images of said subject using the three-dimensional (3-D) characterization maps for at least some but not all of the transducers due to missing or incomplete data associated with at least one of the remaining transducers.

20. An apparatus according to claim 1 wherein the 3-D characterization maps includes, specific to each pair comprising a respective one of said transducers and said PA source position, at least one or all of:

estimates of time of flight of said PA waves, amplitude of said PA waves, temporal width of said PA waves, shape of said PA waves, dampening of said PA waves, and frequency content of said PA waves.

21. An apparatus according to claim 1 wherein the staring array further comprises a staring sparse array, wherein said transducers are spaced apart from one another in order to provide a wider range of viewing angles of said subject compared to viewing angles achievable with an equivalent number of closer spaced transducers and without said calibration.

22. A method for photoacoustic (PA) imaging of a subject using a staring array of transducers for receiving said PA waves and converting said PA waves to corresponding signals, said staring array having at least three transducers and arranged in at least three dimensions and wherein three-dimensional (3-D) spatial co-ordinates of the transducers is to be determined by a programmed computer system, and an acoustically transmissive medium which surrounds the transducers and acoustically couples said staring array to said subject, said method comprising:

calibrating, at least when 3D spatial co-ordinates of the transducers is to be determined by the programmed computer system, the acoustic response of each of the transducers by measurements of PA waves from a PA source at a known plurality of positions, predetermined by the programmed computer system, through a calibration volume, and generating three-dimensional (3-D) characterization maps for each transducer, and estimating a position of each transducer element relative to the calibration volume;

illuminating said subject to generate PA waves emanating from said subject;

creating, based on signals received from the transducers when the staring array is staring, and utilizing the three-dimensional (3-D) characterization maps for at least some of the transducers, one or more three dimensional (3-D) images of said subject.

23. A method according to claim 22 wherein said PA source comprises a laser capable of providing a pulsed laser beam.

24. A method according to claim 22 wherein the said generating the 3-D characterization maps includes providing at least one or all of:

estimates of time of flight of said PA waves, amplitude of said PA waves, temporal width of said PA waves, shape of said PA waves, dampening of said PA waves, and frequency content of said PA waves, specific to each pair comprising a respective one of said transducers and said PA source position.

25. A method according claim 22 wherein said generating the 3-D characterization maps includes calculating properties of said array, including number of said transducers sensitive to each grid point, angular acceptance of said transducers, and number of said grid points where PA waves are detectable for each transducer.

26. A method according to claim 23 including recording the power of each laser pulse, and monitoring fluctuations in the power of the pulses of the laser beam.

27. A method according to claim 22 wherein the staring array further includes a staring sparse array, wherein said transducers are spaced apart from one another in order to provide a wider range of viewing angles of said subject compared to viewing angles achievable with an equivalent number of closer spaced transducers and without said calibrating.