THREE-DIMENSIONAL PHOTOACOUSTIC IMAGER AND METHODS FOR CALIBRATING AN IMAGER

• US 20100249570A1
• Filed: 06/11/2010
• Published: 09/30/2010
• Est. Priority Date: 12/12/2007
• Status: Active Grant

• Current Assignee: Jeffrey J.L. Carson, Lynn Keenliside, Michael Barret Roumeliotis, Pinhas Ephrat
• Original Assignee: Multi-Magnetics Incorporated
• Inventors: Roumeliotis, Michael Barret, Carson, Jeffrey J. L., Ephrat, Pinhas, Keenliside, Lynn
• Granted Patent: US 9,128,032 B2
• Time in Patent Office: Days
• US Class Current: 600/407
• 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 holder supporting said subject;

means for illuminating said subject by means of a beam capable of generating PA waves;

a staring sparse array of transducers for receiving said PA waves and converting said PA waves to corresponding analog signals, said sparse array having at least three transducers and having at least two dimensions, at least some of said transducers being 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 closely spaced transducers;

a supporting structure for mounting said sparse array;

means for acoustically coupling said sparse array to said subject;

electronic means for receiving said analog signals, processing said analog signals, and converting said analog signals to corresponding digital signals; and

a programmed computer system operatively connected to said electronic means to receive said digital signals and to process said digital signals by an image reconstruction algorithm to create one or more three dimensional (3-D) images of said subject.

2. Apparatus according to claim 1 wherein said illuminating means is a laser capable of providing a pulsed laser beam.

3. Apparatus according to claim 2 including means for measuring the illumination from said pulsed laser beam and providing digital signals indicative thereof to said computer system.

4. Apparatus according to claim 3 wherein said holder includes a separating structure for separating said subject from said sparse array, said separating structure being made of a relatively thin, optically transparent, acoustically transparent material, wherein, during use of said apparatus, the illumination from said laser beam passes through said separating structure in order to illuminate said subject and said PA waves from said subject pass through said separating structure to said sparse array.

5. Apparatus according to claim 2 wherein said laser is tunable.

6. Apparatus according to claim 1 wherein said illuminating means is located so as to illuminate said subject from the same side of the subject as the side on which said sparse array is located.

7. Apparatus according to claim 1 said sparse array is an annular array, said supporting structure includes 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. 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 sparse 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. Apparatus according to claim 1 wherein said sparse array is a hemispherical array, said 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. Apparatus according to claim 1 wherein said sparse 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. Apparatus according to claim 1 wherein said acoustically coupling means includes a tank and medium held by said tank, said medium being suitable for acoustically coupling said sparse array to said subject mounted on said holder.

12. Apparatus according to claim 3 wherein said means for measuring the illumination comprises one or more photo detectors.

13. Apparatus according to claim 3 wherein said computer system is programmed to normalize said digital signals to the measured illumination.

14. 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 electronic means with the operation of said laser.

15. Apparatus according to claim 1 wherein said image reconstruction algorithm is set up to initialize the acoustic response of the sparse array by means of prior measurements of PA waves from a PA source at a plurality of positions.

16. Apparatus according to claim 1 wherein said image reconstruction algorithm is an iterative forward projecting, back projecting image reconstruction algorithm.

17. A method for calibrating an apparatus for photoacoustic (PA) imaging of a subject, said apparatus including a staring sparse array of transducers for receiving PA waves from said subject and converting said PA waves to corresponding analog signals, said sparse array having at least three transducers and having at least two dimensions, at least some of the transducers being spaced apart from one another in order to provide a wider range of viewing angles of said subject compared to viewing angles of said subject compared to viewing angles achievable with an equivalent number of closely spaced transducers, supporting structure for mounting said sparse array, means for acoustically coupling said transducers of said array to said subject, electronic means for receiving, processing and converting said analog signals to provide corresponding digital signals, and a programmed computer system operatively connected to said electronic means to receive and process said digital signals,

said method comprising:

(a) providing a point source and means for illuminating said point source by means of a pulsed beam, said point source being capable of generating PA waves upon illumination by said beam and being located in said means for acoustically coupling;

(b) raster scanning said point source through a calibration volume so that said point source is scanned to and stopped at a plurality of grid points in a predetermined timed manner;

(c) simultaneously operating said illuminating means so as to produce said beam and transmitting a pulse of said beam to said point source at each of said grid points, thereby generating said PA waves from the point source at each of said grid points;

(d) detecting said PA waves from said point source at each of said grid points by means of each of said transducers and converting said PA waves to corresponding analog signals; and

(e) generating three-dimensional (3-D) characterization maps from said digital signals by means of said electronic means and said computer system.

18. A method according to claim 17 wherein said illuminating means is a laser capable of providing a pulsed laser beam.

19. A method according to claim 18 wherein said laser beam is transmitted to said point source by an optical fiber.

20. A method according to claim 19 wherein a tip of said optical fiber has an opaque coating and said source is provided by said tip and opaque coating.

21. A method according to claim 17 wherein said raster scanning is carried out by a robotic scanning gantry capable of scanning said point source in three directions perpendicular to each other.

22. A method according to claim 17 wherein the said step of generating the 3-D characterization maps includes providing 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 source position.

23. A method according claim 17 wherein the said step of 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.

24. A method according to claim 18 including providing a photo-detector operatively connected to said electronic means, which includes a data acquisition system, directing part of each pulse of the laser beam towards said photo-detector, recording the power of each laser pulse by means of said photo-detector and said electronic means, and monitoring fluctuations in the power of the pulses of the laser beam.

25. An apparatus for calibrating an apparatus for photoacoustic (PA) imaging of a subject, said PA apparatus having a staring sparse array of transducers for receiving PA waves and converting said PA waves to analog PA signals, and electronic means for receiving, processing and converting said analog PA signals to digital PA signals, said sparse array having at least three transducers and having at least two dimensions, at least some of the transducers being 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 closely spaced transducers, said calibrating apparatus comprising:

a laser source for producing a pulsed laser beam;

a point source device capable of receiving said pulsed laser beam and generating said PA waves at one end of said device when a pulse of laser beam illuminated said point source device; and

means for raster scanning at least a portion of said point source device through a calibration volume so that said one end of said point source device is scanned to and stopped at a plurality of grid points in a predetermined manner; and

a programmed computer system for generating three dimensional characterization maps from said digital PA signals based on said PA analog signals provided by each transducer.

26. An apparatus according to claim 25 wherein said point source device includes an optical fiber and wherein said one end of the device comprises an opaque coated tip of the optical fiber.