Abstract:
Photoacoustic imaging is enhanced by scanning ( 61 ) the sensor array ( 10 ) used in photoacoustic imaging laterally relative to the tissue being imaged, gathering multiple tissue images ( 70, 71, 72, 73 ) at multiple relative lateral positions, and generating a photoacoustic image ( 80 ) of the tissue by combining the images taken at multiple relative lateral positions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to photoacoustic computed tomography (PAT) which is also known as photoacoustic computed tomography (PCT) or optoacoustic tomography (OAT). 
       BACKGROUND OF THE INVENTION 
       [0002]    The absorption of light from a modulated light source can be used to stimulate acoustic emissions in biologic tissue, e.g., breast tissue, wherever the light is absorbed. One common light source is a laser operating in the near-infrared region of the electromagnetic spectrum (wavelength=700-1200 nm). The most common type of optical modulation employs short-duration pulses (5-200 nanoseconds), pulsed at a rate of 10-1000 times per second. The 25 subsequent acoustic emissions typically lie in the medical ultrasound frequency range (1,000,000-20,000,000 cycles per second). These emissions propagate throughout the tissue at approximately 1500 meters per second and may subsequently be detected by an array of acoustic sensors placed outside the tissue surface. Typically, these arrays consist of 64-512 sensors mechanically affixed to a curved surface, usually spherical or cylindrical, but other curved surfaces may be used as well. It is possible to form three-dimensional (3D) images of the pattern of optical absorption in the tissue by using mathematical algorithms for image reconstruction applied to the detected acoustic waves, techniques commonly referred to as photoacoustic tomography (PAT). 
         [0003]    Descriptions of such techniques can be found in U.S. Pat. Nos. 5,713,356 and 6,102,857, which are hereby incorporated by reference herein in their entirety. Significantly, 3D-PAT images formed in this way primarily depict blood vessels and tumors, because light in the near infrared is predominantly absorbed by hemoglobin, which is concentrated in blood and malignant tumors. They can also be used to detect optically absorbing contrast agents that are administered intravenously. 
         [0004]    The Applicant herein has previously obtained patents relating to the use of ultrasound and photoacoustic ultrasound in the imaging of tissue, including U.S. Pat. Nos. 5,713,356; 6,102,857; 6,104,942; 48 6,216,025; 6,292,682; 6,490,470; 6,633,774 and 7,774,042. All of these prior patents are hereby incorporated herein by reference. 
       SUMMARY OF THE INVENTION 
       [0005]    Previous embodiments of three dimensional PAT imaging using a hemispherical array employed a stationary beam of light that illuminates part of the tissue being imaged, e.g., a breast. As the hemispherical array is rotated about a vertical axis, the light source is pulsed. In this geometry, each element of the detector array “points” toward the center of curvature of the hemisphere. This common point lies at the intersection of rays passing through the centers of each flat, disk-shaped transducer, whose surfaces are oriented 90 degrees to the rays intersecting their centers. These detectors have the property that they are most sensitive to photoacoustic signals that impinge their front surface from the direction of the center of curvature, the “on-axis” direction. The transducer exhibits decreasing sensitivity off-axis as the off-axis angle increases. Consequently, the PAT imaging system detects photoacoustic signals from tissue located close to the center of curvature of the array with the greatest sensitivity, and that sensitivity decreases as the distance from the center of rotation increases. Thus, the PAT system produces a useful 3D image only within a limited volume, centered at the center of rotation. 
         [0006]    The volume that can be imaged by a PAT system of prior embodiments can be increased by increasing the radius of the hemispherical array, but to image a large volume of tissue, e.g., a 1000 mL breast, the size of the hemisphere would become prohibitively large. One can also decrease the physical size of the transducers (typically 3-5 mm diameter), thereby increasing their off-axis sensitivity, but this results in an undesirable reduction in overall sensitivity, potentially below what is needed to detect the typically weak photoacoustic signals produced within tissue. 
         [0007]    In accordance with principles of the present invention, an alternative strategy is applied to the sensitivity challenges in the prior embodiments of a PAT scanner. Specifically, the sensitivity is improved by scanning the hemispherical array laterally within a plane, e.g., in a rectilinear fashion (left-right, back-forth) as the photoacoustic data are acquired. Importantly, this planar scanning is implemented independently from the rotational scanning of the hemispherical array known in the prior embodiments. In some embodiments, the rotation and scanning occur together, and in other embodiments, the hemispherical array may not be rotated at all during the planar scan. In either case, the net effect is to position the sensitive volume of the scanner variously throughout a larger volume of tissue than can be imaged with the hemispherical array in a fixed location and thus always pointing at the same volume throughout a scan 
         [0008]    Planar scanning in accordance with principles of the present invention can be accomplished either by moving the array beneath a stationary exam table, supporting the patient being imaged, or by moving the exam table supporting the table above a stationary array, which may be allowed only to rotate. 
         [0009]    The above-described and other advantages will be apparent in light of the following figures and detailed description of principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
           [0011]      FIG. 1  is a cross-sectional diagram showing the overall structure of a PAT scanner; 
           [0012]      FIG. 2  illustrates details of the geometry of the hemispherical sensor array of the scanner of  FIG. 1 ; 
           [0013]      FIG. 3  is a graphical depiction of the angular sensitivity of the hemispherical array of  FIG. 2 ; 
           [0014]      FIG. 4  is an illustration of a target used for uniformity testing of a PAT scanner; 
           [0015]      FIG. 5  is a slice of an image of the target of  FIG. 4  taken with the PAT scanner of  FIG. 1  centered over the target; 
           [0016]      FIG. 6  is a schematic diagram showing the lateral displacement that may be applied to the hemispherical sensor array of  FIG. 2  to expand the field of view in accordance with principles of the present invention; 
           [0017]      FIGS. 7A ,  7 B,  7 C and  7 D illustrate four independent PAT images taken at each laterally displaced position shown in  FIG. 6 ; and 
           [0018]      FIG. 8  is a composite image of the target of  FIG. 4  formed by the sensor array at the four laterally displaced positions shown in  FIG. 6 , showing greater detail of the target than in  FIG. 5 . 
       
    
    
       [0019]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, functions and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments may have been enlarged or distorted relative to others to facilitate visualization and clear understanding. 
       DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates the basic elements that comprise a PAT scanner. A liquid-filled, hemispherical, detector array  10  detects photoacoustic signals that are emitted from tissue in response to a pulsed laser  11  that produces a light beam  12  that illuminates the tissue  13  being imaged. The tissue is restrained by an acoustically and optically transparent, plastic membrane  14  affixed to a tabletop  15  upon which the patient lies. The laser  11  is pulsed at a typical rate of 10 times per second (10 Hertz) as the detector array  10  is rotated about the vertical axis, completing a full rotation in 3-24 seconds. 
         [0021]      FIG. 2  illustrates details of the hemispherical detector array  10 . An optically clear aperture  20  at the base of the hemisphere allows the light beam  12  to illuminate the tissue placed above the array. This hemispherical bowl rotates about the light beam as shown at  21  during data acquisition. Photoacoustic signals are detected by each transducer  22  that comprises the array following each pulse of light. These transducers are flat-faced, and “point” to the center of curvature  24  of the array, where on-axis rays  23  from all the transducers converge. 
         [0022]    The graph  30  in  FIG. 3  of Far-Field Angular Response describes the angular sensitivity, relative to an on-axis ray ( 23 ,  FIG. 2 ) for a typical transducer element of which the hemispherical array is comprised. In this case, the transducer is a 2-mm diameter disk with peak acoustic sensitivity at 2 MHz (2,000,000 cycles per second). As is illustrated, this particular transducer has a sensitivity of at least 50% of its peak sensitivity over an angular range of ±15 degrees from perpendicular to the disk. Photoacoustic signals detected within this range of angles of the perpendicular axis of the disk are the most useful for three dimensional PAT imaging. 
         [0023]    The effective field of view of a three dimensional PAT scanner can be assessed by placing within the scanner, a uniformity target  40 , such as the one illustrated in  FIG. 4 . This target consists of a sheet of clear plastic upon which a pattern of black dots, spaced 5-mm apart radially, have been printed. This target is placed within the liquid-filled, plastic membrane ( 14 ,  FIG. 2 ) and a three dimensional PAT image is acquired and reconstructed for viewing in accordance with the imaging methods disclosed in the above referenced patents which are incorporated herein. 
         [0024]    One slice ( 50 ) from a three dimensional PAT image of the uniformity target ( 40 ) is shown in  FIG. 5 . As is apparent, the field of view is only about +/−20 mm wide (four dots from the center point of the target). 
         [0025]    Alternative data acquisition in accordance with principles of the present invention uses four sets of PAT data of the uniformity phantom  40 , where the light field  60  has been displaced laterally in a 2×2 rectilinear fashion  61  between the four scans, in accordance with the pattern illustrated in  FIG. 6 , resulting in a composite field of view  62  of greater extent than accomplished without rectilinear scanning. 
         [0026]    Four independent PAT images  70  (as shown in  FIG. 7A ),  71  (as shown in  FIG. 7B ),  72  (as shown in  FIG. 7C) and 73  (as shown in  FIG. 7D ), one for each position of the light beam, are shown in  FIG. 6 . These were generated by displacing the light field to four locations are arranged in a square pattern 32 mm on a side. 
         [0027]      FIG. 8  demonstrates how the field of view of the uniformity phantom ( 40 ,  FIG. 4 ) has been increased by the use of rectilinear scanning of the light beam  12  coupled to the hemispherical array  10 . Specifically, a composite image  80  is assembled from the four component images ( 70 ,  71 ,  72  and  73 ,  FIG. 7 ) by shifting each of the component images to compensate for the rectilinear shift  61  from the center of the uniformity phantom  40  used during data acquisition, and then summing the resulting image data together. The field of view seen in  FIG. 8  is clearly superior and more uniform in contrast than accomplished without rectilinear scanning. 
         [0028]    While embodiments of the present invention have been illustrated by a description of the various embodiments and the examples, and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, although the array surface has been described as “hemispherical,” other curved or piecewise linear surfaces could also be used. Moreover, rotation and movement of the curved surface to multiple locations may be used to gather data from more virtual transducer locations than there are physical transducers in the apparatus, as elaborated in the above-referenced patents. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.