Patent Application
20120289813
Photoacoustic imaging is known in the literature but it is typically accomplished using a one dimensional array of detectors recording the signal from an optical transducer that generates a single pulse of 5-50 nanosecond duration. The systems that use this process are complicated to use and manufacture. These systems are not readily useful for applications such as cancer detection due to the requirements for the size of the device required to detect this acoustic chirp, and the processing power required to convert the detected signals into an image.
Another method used to generate the signal for the array of detectors to record is using echo based ultrasound, and employing a phased array of transmitter elements. This is where a signal is generated by an array of acoustic transducers and reflected off an object of interest. Echo based ultrasound is currently used in the medical field.
There is a need for a more elegant method to produce photoacoustic images in areas such as detection of cancer in internal organs where small device size is a major benefit. A system where a single lens assembly focuses the acoustic image onto a single 2 dimensional array of transducers would be a significant improvement to the art.
Additionally, major improvements in the art would be to create an acoustic imaging zoom lens, and other lenses capable of imaging acoustic signals that originate from a scene a large number of meters (feet) from the imaging system.
Acoustic systems generally utilize a process to determine the range to target features based upon time of flight of an acoustic wave. If the velocity of the acoustic wave within the medium (e.g., water) is known, and the time delay from pulse launch to receipt can be accurately measured, the distance can be determined. The issue with this approach is that the signal is restricted to a string of pulses that can be clearly identified so that all ambiguity of correlation can be removed. Moreover, spurious reflections, even multiple reflections can generate erroneous range data.
By comparison, an imaging system based upon acoustic lens imaging develops an image in a manner very similar to an optical camera system. This provides very little range information unless parallax is employed. Parallax is what allows humans to rapidly determine the range to the handle of a coffee cup so that hand motion accurately intercepts it. The binocular vision system generates data with angular content from which the range to an object can be accurately determined. This, in fact, is the principle behind coincidence range finders long used aboard tanks and ships to establish the range to target for the main fire control systems. Laser range finders have taken over this function in recent years, but for decades, the coincidence range finder provided accurate range data using parallax. Parallax is a form of triangulation. However in the imaging system described here, the utilization of parallax, or binocular vision, can allow real time stereoscopic acoustic vision within object space with very little computational overhead.
Magnetic resonance scanning, ultrasounds, and soft x-ray scans are 3 tools currently used to detect breast cancers, all of which are limited in their usefulness. Magnetic resonance images are expensive to create. Many general practitioners do not have direct access to these machines nor do they possess the specialized training necessary to interpret the resulting images, so there is an inefficiency in the implementation of MRI systems.
The images produced by normal ultrasound tend to be both coherent and grainy, so specialized training is required for an individual to be able to recognize the features that are produced by the image. As one who is skilled in the art realizes a coherent image is characterized by spatially varying amplitude distortion whereas an incoherent image is characteristic of normal, healthy human vision. If one integrates (averages) the detection of the image over a long enough period of time, then one should be able to create time averaged coherent image which gives the appearance to a detection device of an incoherent image.
X-rays have radiation issues, especially for women in child bearing years.
The present invention overcomes these obstacles.
Without wishing to be bound to any particular theory, applicant believes that the photoacoustic excitation of blood cells by IR radiation absorption causes rapid thermal expansion and an (related) acoustic pulse over several frequencies. The range of frequencies and the intensity of the acoustic pulse are believed to be due to the concentration of hemoglobin in the blood cells, the type of tissue, and the wavelength of photo excitation.
If the initial IR radiation is transmitted to the hemoglobin containing tissue (arteries, veins, capillaries) via a pulse, the resulting acoustic pulse can be identified and, by extension, filtered or gated so one can differentiate signal from the noise. Moreover, a properly designed acoustic lensing arrangement can be so devised as to enable rapid assessment of depth in the imaged scene by using the parallax or similar method.
With higher amounts of energy used to excite the blood cells, it may be possible to get the blood cells to emit waves at a higher frequency spectral band, but this may be damaging to the blood cells and surrounding tissues. If the blood cells emit waves at a higher frequency spectral band, the resulting image will have higher resolution as the resolution diffraction limited cutoff frequency will increase. The range of acoustic frequencies generated currently falls in the 2 MHz to 10 Mhz range.
Various embodiments of the present invention provide for a photoacoustic imaging probe for use in a photoacoustic imaging system, said probe comprising a cohesive composite acoustic lens incorporating aspheric geometry and exhibiting low or practically no measurable dispersion of acoustic waves constructed of at least one material with a low acoustic impedance and attenuation and a relatively low acoustic velocity and at least one other material with a low acoustic impedance and attenuation and a relatively high acoustic velocity is housed in a conduit preferably filled with a low acoustic velocity and low acoustic impedance fluid such as water or mineral oil. The lens may be designed as a single cohesive composite telecentric lens, an acoustic zoom lens, or a catadioptric lens. The lens focuses acoustic waves on an acoustic imager which detects the image. The acoustic imager may be designed as a 2 dimensional array of transducers. Research to date indicates that within the range of acoustic frequencies of interest, 1 MHz-50 MHz and preferable 2 MHz-10 MHz, there exists little velocity variation of acoustic waves transmitted within the materials of interest, and the lens design approach may currently be considered to be essentially monochromatic. The acoustic waves can be generated when an emitting light source illuminates a test subject comprising materials that generate acoustic waves at differing intensities, frequencies, or a combination of both differing intensities and differing frequencies when illuminated with light, for example tissue containing blood vessels, wherein the blood vessels excite and generate an acoustic pulse. The probe has an acoustic window made of a material with low acoustic impedance which allows the acoustic pulse to enter the probe without distortion and then may be reflected by a mirror onto the acoustic lens. The probe may include the emitting light source and an optical window to allow light emitting from said light source to illuminate the test subject. The probe housing interior wall may also be coated with a material which is essentially absorbing of acoustic waves such that it essentially neither transmits nor reflects acoustic waves. The material will preferably absorb at least 90% of acoustic waves and more preferably absorb at least 98% of acoustic waves. An example of such a material is ISODAMP C-8002 manufactured by E-A-R Specialty Composites. In the embodiment where the probe housing interior wall is coated with an acoustic absorbing material, the coating is not applied to the probe window.
Additional embodiments of the present invention provide for a photoacoustic imaging probe for use in a photoacoustic imaging system, said probe comprising a cohesive composite acoustic lens incorporating aspheric geometry and exhibiting low or practically no measurable dispersion of acoustic waves constructed of at least one material with a low acoustic impedance and attenuation and a relatively low acoustic velocity and at least one other material with a low acoustic impedance and attenuation and a relatively high acoustic velocity is immersed in preferably a low acoustic velocity and low acoustic impedance fluid such as water or mineral oil. The test subject is also immersed in the fluid. The lens may be designed as a single cohesive composite telecentric lens, an acoustic zoom lens, or a catadioptric lens. The lens focuses acoustic waves on an acoustic imager which detects the image. The acoustic imager may be designed as a 2 dimensional array of transducers. The lens design approach may currently be considered to be essentially monochromatic. The acoustic waves can be generated when an emitting light source illuminates a test subject comprising materials that generate acoustic waves at differing intensities and/or frequencies when illuminated with light, for example tissue containing blood vessels, wherein the blood vessels excite and generate an acoustic pulse. The acoustic waves can also be directed to the probe via echo, where the acoustic waves are generated by an external source and then are reflected back toward the probe by a test subject comprised of materials of differing acoustic impedance and/or attenuation and differing acoustic velocities reflecting differing portions of the initial acoustic waves at the probe.
Additional embodiments of the present invention provide for a photoacoustic imaging probe for use in a photoacoustic imaging system, said probe comprising an acoustic reflective lens comprising at least one material possessing a high acoustic impedance and comprising at least 2 reflective surfaces and incorporating aspheric geometry; is immersed in preferably a low acoustic velocity and low acoustic impedance fluid such as water or mineral oil. The test subject is also immersed in the fluid. The lens focuses acoustic waves on an acoustic imager which detects the image. The acoustic imager may be designed as a 2 dimensional array of transducers. The lens design approach may currently be considered to be essentially monochromatic. The acoustic waves can be generated when an emitting light source illuminates a test subject comprising materials that generate acoustic waves at differing intensities and/or frequencies when illuminated with light, for example tissue containing blood vessels, wherein the blood vessels excite and generate an acoustic pulse. The acoustic waves can also be directed to the probe via echo, where the acoustic waves are generated by an external source and then are reflected back toward the probe by a test subject comprised of materials of differing acoustic impedance and/or attenuation and differing acoustic velocities reflecting differing portions of the initial acoustic waves at the probe.
It is believed that the photoacoustic excitation of hemoglobin in blood cells by infrared radiation absorption causes rapid thermal expansion and a related acoustic pulse over several frequencies. The range of frequencies and the intensity of the acoustic pulse are likely due to the concentration of hemoglobin, the type of tissue, and the wavelength of photo excitation.
If the initial infrared radiation is transmitted to the hemoglobin containing tissue (arteries, veins, capillaries) via a properly encoded series of pulses, the resulting acoustic pulses can be identified and, by extension, filtered (the acoustic pulses are essentially tagged) so the signal can be differentiated from the noise.
In various embodiments of the present invention, the acoustic waves can also be directed to the probe via echo, where the acoustic waves are generated by an external source and then are reflected by a test subject back toward the probe with materials of differing acoustic impedance and/or attenuation and differing acoustic velocities reflecting differing portions of the initial acoustic waves at the probe. Systems of this type may be readily adapted to sonar applications involving detection, location, characterization, and identification tasks of targets underwater, as well as structural inspections.
Embodiments of the present invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which:
In describing the present invention, a variety of terms are used in the description. Standard terminology is widely used in the optics and photonic arts. For example, one may refer to Modem Optical Engineering, Warren J. Smith, the disclosure of which is incorporated herein by reference for its general teachings in optical engineering and analogous acoustic engineering.
An acoustic imaging probe for use in an acoustic imaging system comprising a single cohesive composite acoustic lens, a low acoustic impedence fluid, a housing for said acoustic lens, a window through said housing, and an acoustic imager.
An acoustic imaging probe for use in an acoustic imaging system comprising a cohesive composite acoustic lens incorporating aspheric geometry and exhibiting low or practically no measurable dispersion of acoustic waves constructed of at least one material with a low acoustic impedance and attenuation and a relatively low acoustic velocity and at least one other material with a low acoustic impedance and attenuation and a relatively high acoustic velocity is housed in a conduit filled with a low acoustic velocity and low acoustic impedance fluid such as water or mineral oil. The lens focuses acoustic waves on an acoustic imager which detects the image of the acoustic waves. Research to date indicates that within the range of acoustic frequencies of interest, 1 MHz −50 MHz and preferable 2 MHz-10 MHz, there exists little velocity variation within the materials of interest, and the lens design approach may currently be considered to be essentially monochromatic. The acoustic waves can be generated when an emitting light source illuminates a test subject comprising materials that generate acoustic waves at differing intensities, frequencies, or a combination of both differing intensities and differing frequencies when illuminated with light, for example tissue containing blood vessels, wherein the blood vessels excite and generate an acoustic pulse. The probe has an acoustic window made of a material with low acoustic impedance which allows the acoustic pulse to enter the probe without distortion and then may be reflected by a mirror onto the acoustic lens.
As those who are skilled in the art are aware, the acoustic impedance of a material is a frequency dependant parameter defined as the sound pressure divided by the particle velocity divided by the surface area through which an acoustic wave propagates through the material. As those who are skilled in the art are also aware, the acoustic velocity of an acoustic wave is a measure of how quickly the acoustic wave propagates through a specific medium, and when one refers to the acoustic velocity of a material, they are referring to the rate at which acoustic waves propagate through the material.
In
The acoustic waves 105 enter the probe 100 through a window 120 through the housing 180 of the probe 100. The interior wall of the housing 180 is preferentially coated with a material that absorbs acoustic waves of the range of 1 MHz-50 MHz. The window 120 is comprised of a material with low acoustic impedance such as Teflon. The window 120 is designed with a convex curvature that allows the acoustic waves 105 to enter the probe 100 with minimal distortion. The window 120 may be designed to have a toric or non-toric geometry as required to minimize the impact on acoustic wave propagation due to distortion at either the front or rear surface of the window 120 and due to aberrations in the window 120. The impact can be minimized to the first order by ensuring that front and rear surfaces of the window 120 form a uniform thickness, and that the radii of curvature are as generous as the application allows. In the embodiment in which the test subject 110 is a prostate gland, it is preferred to maximize the radii of curvature to minimize patient discomfort while still maintaining non-infinite radii of curvature to minimize acoustic wave attenuation, whereby acoustic wave attenuation may occur due to the probe and the rectal wall not maintaining contact. It is preferred that the radii of curvature be at least about 100 millimeters. The interior of the probe 100 is filled with an acoustic fluid 130 with low acoustic impedance and preferably an acoustic velocity which is similar to the acoustic velocity of the bulk of the test subject 110. It is also preferable, but not essential, for the fluid to have a low acoustic velocity. In an embodiment where the test subject 110 is living tissue, the acoustic fluid 130 might be one which mimics the acoustic properties of the living tissue. Examples of such a fluid include, but are not limited to, mineral oil, water, or Clear Image brand scanning gel manufactured by Sonotech, Inc of Bellingham, Wash.
A goal of the present invention is to maintain a good signal to noise ratio in the incoming acoustic waves 105. The acoustic fluid 130 maintaining the property of low acoustic impedance furthers the goal. The window 120 maintaining the property of low acoustic impedance also furthers the goal.
The acoustic waves 105 travel through the acoustic fluid 130 and reach a flat reflective surface 140. The flat reflective surface 140 functions as an acoustic mirror (and will hereafter be referred to as the mirror 140) deflecting the axis of acoustic wave propagation. As those skilled in the art are aware, the mirror 140 will preferably be constructed of a material with high acoustic impedance, for example aluminum. The mirror 140 may be gimbaled in 1 or more planes to allow for the scanning the entire test subject 110 with minimal movement of the probe 100. As the mirror 140 is rotated, angular scanning of the test subject is effected. By gimbaling the mirror 140 in 2 or more planes, the entire test subject 110 can be scanned with minimal moving of the probe 100 during the scan. Motion of the mirror can be effected through a variety of mechanisms as diverse as a sealed cable and housing or a small stepper motor. Means for mounting and moving the mirror 140 are not depicted in
The acoustic fluid 130 allows for a range of rotation of the mirror 140 to enable scanning of the field of view of the test subject 110.
The acoustic waves 105 will next pass through an acoustic lens 150. The acoustic lens 150 is preferably constructed to be a single, cohesive composite, acoustic lens enabling the acoustic lens to maintain alignment despite subjection to mechanical shock and thermal excursions. The acoustic lens 150 may be of a spheric geometry, a combination of spheric and aspheric geometries, or preferably of an aspheric geometry. One may mount the acoustic lens 150 in the probe 100 by conventional means. One may use any one or more of the means for mounting a lens disclosed in U.S. Pat. No. 7,372,647 (lens mount assembly for optical components), U.S. Pat. No. 5,739,962 (lens mount), U.S. Pat. No. 4,387,968 (zoom lens mount barrel), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
The acoustic lens 150 is shown in greater detail in
The refractive indices of several materials have been calculated for an acoustic frequency of both 10 MHz and 2 MHz and are presented in tabular format in
A commonly useful material pair is considered to consist of one material with a relatively high acoustic velocity and one material with a relatively low acoustic velocity. A minimally commonly useful material pair is considered to consist of one material with a relatively high acoustic velocity and one material with a relatively low acoustic velocity when the index break is at least about 0.02 when the reference material is acrylic, where the index break is the difference between the refractive index of the material with relatively high acoustic velocity minus the refractive index of the material with the relatively low acoustic velocity. However, a far more useful material pair would maximize this index break, e.g., acrylic and silicone rubber.
Referring again to
2 MHz-10 MHz has been chosen as a preferred frequency range because in one preferred embodiment, the probe 100 depicted in
The resolution diffraction limited cutoff spatial frequency is provided by the following equation:
wherein ν is the resolution diffraction limited cutoff spatial frequency, λ is the wavelength, and F/# is the infinity F-number.
By increasing the acoustic frequency of the bulk of acoustic waves generated by blood cells, the resolution of an imaging system capturing these waves will increase as the resolution diffraction limited cutoff frequency increases as those who are skilled in the art are aware. The diffraction limited cutoff spatial frequency is approximately 3.3 cycles per millimeter for an F/0.54 lens.
In the preferred embodiment depicted in
In another embodiment not shown, at least one cavity which exists between at least one pair of rotationally symmetric pucks may be filled with a non-curing fluid with a low acoustic impedance and a relatively low acoustic velocity. The pucks may be rigidly joined at the flange interfaces between adjoining pucks and the combination of rotationally symmetric pucks and non-curing liquid will also be a cohesive composite assembly.
In another embodiment, at least one cavity which exists between at least one pair of rotationally symmetric pucks may be filled with an acoustic fluid surrounding the acoustic lens and the rotationally symmetric pucks are joined together by conventional means. It should be noted that this incorporation of the acoustic fluid as a component of the acoustic lens will lessen the ability to treat the acoustic lens as a rigid composite structure, and will lessen the advantage of very stable alignment.
In another preferred embodiment not shown, an acoustic lens is assembled using pucks of at least two different materials where adjacent pucks may be made of materials possessing differing acoustic velocities.
Referring again to
In another preferred embodiment of the present invention, the acoustic imager comprises two imaging systems within the probe detecting imagery from two different angles simultaneously. The two images detected by the acoustic imager may be converted to a stereoscopic video stream which may be used as the basis for subsequent 3 dimensional (referred to as 3D) reconstruction the test subject by use of readily available means. One may use any one or more of the means for 3D reconstruction of a test subject disclosed in U.S. Pat. No. 6,459,532 (parallax autostereoscopic 3D picture and autostereoscopic 3D display), U.S. Pat. No. 6,459,532 (parallax autostereoscopic 3D picture and autostereoscopic 3D display), U.S. Pat. No. 5,500,712 (method and equipment for printing 3-D stereograph), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. In the embodiment where the test subject is a prostate gland of a living human a virtual 3D reconstruction of the prostate gland with accurately determined positions and proportions may be obtained.
Referring again to
In one embodiment the desired acoustic waves propagate in the 2-10 MHz range, as do the acoustic waves generated when blood cells are excited with a near infrared light. In this embodiment, the bandpass filter will allow signal in that frequency range to pass. In another embodiment, the desired acoustic wave frequency is 20 MHz, which will allow for higher resolution images than those created studying waves at the 2-10 MHz range. As those skilled in the art are aware, the probe may use signal to noise enhancement to improve the resultant image quality. The actual signal to noise ratio will dictate the appropriate frequency band to be transmitted by the bandpass filter.
It is preferable for the probe to possess a relatively low infinity F-number (F/#) of between F/5 and F/0.5. It is more preferable to use an F/# between F/2 and F/0.5 and it is further preferred to use an F/# of approximately F/1 to collect acoustic waves of low intensity. In this embodiment the probe can be used to study a source of the acoustic waves which produces acoustic waves of low intensity since the physical aperture of the lens possesses a large collection area compared to the image area. This allows for acoustic imaging of a test subject when the test subject producing the acoustic waves produces acoustic waves of low intensity.
The probe is nearly telecentric in the back focal region to enable refocusing on various planes in the object region with the same angular subtense. This prevents distortion of the acoustic image which would result in poor mapping from the actual features of the test subject to the observed features of the test subject and will preserve utility in the probe as a detection tool.
An acoustic telecentric lens is designed such that the chief ray of all field positions falls on the image plane at normal incidence. The benefit of this arrangement is that refocusing the lens does not alter the angular field of view presented to the imager. This permits accurate measurements to be made, especially angularly, regardless of focal position. The acoustic telecentric lens has great advantage in those applications where the lens must remain very compact due to space constraints, but in which accurate measurements must be possible. By measuring the defocused position of the imager accurately, for instance with a stepper motor, the position of an object space feature may be accurately computed in x, y, and z coordinates. Likewise, the physical extent of the feature may also be accurately computed, and the resultant 3 dimensional object field may be presented for virtual viewing in a rectilinear 3 dimensional space if images are captured in a range of focus positions. The telecentric lens described below is presented as a representative of the general form of acoustic telecentric lens.
The acoustic telecentric lens preferred configuration possesses a minimal number of elements, and is therefore comprised principally of aspheric surfaces. It also makes use of materials providing large index breaks across refractive surfaces in order to minimize aberrations and to create as compact a unit as possible. It provides a large angular field of view, but is adjusted to optimize the angular resolution based upon the imager with which it is to be employed. The properties of the telecentric lens in the embodiment presented in
The shapes of the optic surfaces is provided by the following equation:
where the terms are as follows:
z is surface contour sag, (departure from a plane surface) at a particular radial distance r from the optic axis, r is the radius from the optical axis, c is the curvature, i.e. the reciprocal of the radius of curvature R, κ is a conic constant, and α1, α2, α3 . . . are the aspheric coefficients. The following Tables 1-4 provide the data for the several embodiments of the present invention. The thicknesses listed therein are the distances between the surface of the identified object, and the surface of the subsequent object which is struck by the rays of acoustic waves passing there through. The coefficients α1, α2, α3 etc. define the departure from the true spheric surfaces of the refractors. Thus in the preferred embodiment depicted in
The window 120 through which the acoustic waves 105 enter the probe 100 may be of toric geometry to provide for anamorphic correction of the acoustic waves 105 and the acoustic lens 150 would be non-rotationally symmetrical. The window 120 may be designed to be non-cylindrical to provide for anamorphic correction of the acoustic waves 105 and would result in the design of the lens 150 to be non-rotationally symmetrical. As those skilled in the art are aware, an anamorphic lens is any lens that generates a scale, magnification, or focal length differing in orthogonal (x and y) axes. Naturally, those x & y axes can then be rotated relative to the global coordinate system as necessary. This approach allows for the acoustic imager 160 to be comprised of 2 dimensional detector array with differing detector intervals as a function of position in the array, such as rectangular pixels. Nonrotationally symmetrical lenses can therefore enable the implementation of rectangular detector arrays.
Referring again to
As those who are skilled in the art will realize, the depth of field can be readily controlled with temporal gating of the acoustic imager 160. Temporal gating is well known in the art as a means of enabling a detection device such as an acoustic imager only during a time period correlated to the time of travel of a suspected target. Encoded trains of pulses may also be used in a manner such as NRZ encoding, MLT-3 encoding, or Reed-Solomon encoding. Reference may be had e.g., to U.S. Pat. No. 7,139,294 (multi-output harmonic laser and methods employing same), U.S. Pat. No. 6,690,472 (pulsed laser linescanner for a backscatter absorption gas imaging system), U.S. Pat. No. 5,732,172 (laser pulse sampler), U.S. Pat. No. 5,298,858 (method and apparatus for non-destructive testing of electrically conductive materials), U.S. Pat. No. 5,202,744 (electro-optical measuring instruments), U.S. Pat. No. 5,375,127 (method and apparatus for generating Reed-Soloman error correcting code across multiple word boundaries), U.S. Pat. No. 5,280,500 (method and apparatus for multilevel encoding for a local area network) and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. These techniques provide examples of the means for outputting light pulses in a fashion compatible with time gating the acoustic detector.
As those who are skilled in the art are aware, modest distortion in the resulting image due to the temporal gating is deterministic and can be removed with image processing algorithms which are readily available from any of several software vendors. The depth of field of the imaging system defined by the probe can be controlled, at least to a degree, by gating the captured image. This is due to the relatively slow transit of acoustic waves through the system. Unlike optical waves, the acoustic wave velocity is under 3,000 meters per second, and it could be under 1,000 meters per second depending upon the tissue the waves are generated in. The aggressive aperture of the imaging system also controls the depth of field. The working F/# is approximately F/1.
The conduit 170 through which the light travels may be comprised of readily available means. In the preferred embodiment depicted in
Referring again to
In the preferred embodiment shown in
In the embodiment depicted in
As one who is skilled in the art is aware, the details of the geometry of the probe may and will change depending upon the test subject to be studied. The invention is not avoided by a simple geometry change.
Referring to
Despite this change of focal position, the fidelity or angular resolution of the image is largely unaffected. The Modulation Transfer Function curve of the far point imaging is depicted in
As represented in Modulation Transfer Function curves depicted in
An acoustic imaging probe for use in an acoustic imaging system comprising a single cohesive composite acoustic lens, a low acoustic impedence fluid, and an acoustic imager comprised of a 2 dimensional array of transducers.
An acoustic zoom lens is a lens with multiple components forming multiple groups. This form of lens is preferred to have groups assembled into cohesive composite units in order to preserve alignment precision. In an acoustic zoom lens, movement of some groups of lenses enables the focal length of the overall assembly to be adjusted over a range of focal lengths while maintaining a high level of image fidelity. Within a given image size format, the imaged field of view changes as a function of the focal length. To accomplish the relative motion of the groups of lenses, there exists between certain group assemblies a fluid region that enables coupling of an acoustic wave from one group to the next, but also allowing the relative motion required to achieve net focal length adjustment. The benefit of the acoustic zoom lens is that a comparatively large area may be imaged at modest resolution in order to screen for features of interest in the object. When one is detected, the imaging system may be redirected to center the item of interest in the field of view and the focal length may be adjusted, magnifying the feature to enable more critical examination.
This capability demonstrates the effectiveness of the acoustic zoom lens form in any form of inspection a range of magnifications to screen features throughout a finite volume. The inspection need not be a medical inspection, but may be used for a wide variety of sections and measurement tasks such as examining cracks in welded joints.
In another preferred embodiment, depicted in
A relatively large field of view is desirable to effectively screen large areas of the test subject 910. The acoustic zoom lens 510 provides for the screening of large areas of the test subject 910. Once an area of specific interest has been identified, the acoustic zoom lens 510 can be used to examine features in more detail as a result of more favorable magnification.
The acoustic zoom lens is shown in greater detail in
In another preferred embodiment not depicted, the acoustic zoom lens is comprised of at least three lens groups and at least one moving portion of the zoom lens is constructed as a single lens element of a single material having low acoustic attenuation and impedance and high acoustic velocity. An example of such material to construct the single lens element is acrylic.
The acoustic zoom lens preferred configuration possesses a minimal number of elements, and is therefore comprised principally of aspheric surfaces. It also makes use of materials providing large index breaks across refractive surfaces in order to minimize aberrations and to create as compact a unit as possible. It provides a reasonable range of focal length adjustment. However, the range of focal lengths can be extended significantly with additional complexity of design. The ratio of focal lengths provided by the zoom lens is a range of at least 1.5 to 1 and is preferably at least 3 to 1 and more preferably is at least 10 to 1.
The properties of the zoom lens in the embodiment presented in
Referring again to
In the embodiment shown in
Referring again to
In the embodiment depicted in
To use the probe 500, the test subject 910 and the probe 500 are coupled acoustically by placing the test subject 910 in contact with an acoustic fluid 930 and by placing the probe 500 in contact with the acoustic fluid 930. The acoustic fluid 930 is preferably chosen to possess a refractive index which is similar to that of the test subject 910. In the embodiment where the test subject 910 is a human breast, the system can have the breast in a container of an acoustic fluid 930 which is closely index matched to the breast tissue where water is chosen as the acoustic fluid 930. This results in few to no distortion issues in the resultant image.
As depicted in
An acoustic imaging probe for use in an acoustic imaging system comprising a reflective acoustic lens, a low acoustic impedence fluid, and an acoustic imager.
The acoustic waves 1105 enter the probe 1100 through a window (not depicted) through the housing (not depicted) of the probe 1100. The interior wall of the housing is preferentially coated with a material that absorbs acoustic waves of the range of 1 MHz-50 MHz. The window is comprised of a material with low acoustic impedance such as Teflon. The window is designed with a convex curvature that allows the acoustic waves 1105 to enter the probe 1100 with minimal distortion. The window may be designed to have a toric or non-toric geometry as required to minimize the impact on acoustic wave propagation due to distortion at either the front or rear surface of the window and due to aberrations in the window. The impact can be minimized to the first order by ensuring that front and rear surfaces of the window form a uniform thickness, and that the radii of curvature are as generous as the application allows.
An acoustic reflecting lens employs strictly reflective surfaces to perform imaging over a relatively restricted angular field of view. The reflectors of an acoustic lens make use of materials possessing a high impedence to acoustic energy rendering them a relatively low loss surface. The folded optical path required by reflective acoustic lenses causes the lens to be relatively compact in the axial dimension. The principle benefit of this form of lens is that the materials are often metallic, and therefore can be readily machined with either spherical or aspheric surface contours. Careful baffling is necessary to ensure that only those waves that strike each reflective surface in the proper sequence can make it to the imaging device.
The acoustic reflective lens preferred configuration possesses a minimal number of elements, and is therefore comprised principally of aspheric surfaces. It also makes use of materials providing large index breaks across refractive surfaces in order to minimize aberrations and to create as compact a unit as possible.
This capability demonstrates the effectiveness of the acoustic reflective lens form in any form of inspection requiring a very short axial length imaging lens to examine a finite volume. The inspection need not be a medical inspection, but may be used for a wide variety of sections and measurement tasks such as examining cracks in welded joints.
It is preferable for the probe to possess a relatively low infinity F-number (F/#) of between F/5 and F/0.5. It is more preferable to use an F/# between F/2 and F/0.5 and it is further preferred to use an F/# of approximately F/1 to collect acoustic waves of low intensity. In this embodiment the probe can be used to study a source of the acoustic waves which produces acoustic waves of low intensity since the physical aperture of the lens possesses a large collection area compared to the image area. This allows for acoustic imaging of a test subject when the test subject producing the acoustic waves produces acoustic waves of low intensity.
In one embodiment the desired acoustic waves 1105 propagate in the 2-10 MHz range, as do the acoustic waves generated when blood cells are excited with a near infrared light. In another embodiment, the desired acoustic waves 1105 propagate in the 1-25 MHz range. In another embodiment, the desired acoustic wave frequency is 20 MHz, which will allow for higher resolution images than those created studying waves at the 2-10 MHz range. As those skilled in the art are aware, the probe may use signal to noise enhancement to improve the resultant image quality. The actual signal to noise ratio will dictate the appropriate frequency band to be transmitted by the bandpass filter.
The reflective lens provides significant acoustic energy collection capabilities since the apertures can be constructed of comparatively thin sheets of material with very large apertures. These apertures can be many times larger than the acoustic detector array. This presents the opportunity to collect very weak acoustic signals, and to do so with no significant loss of spatial resolution provided that the source subtends a relatively small angular extent. However, the addition of a mirror, as described below, can be used to extend the angular extent of the source object.
A flat reflective surface 1140 may be used in conjunction with the acoustic lens system 1100. The flat reflective surface 1140 functions as an acoustic mirror (and will hereafter be referred to as the mirror 1140) deflecting the axis of acoustic wave propagation. As those skilled in the art are aware, the mirror 1140 will preferably be constructed of a material with high acoustic impedance, for example aluminum. The mirror 1140 may be gimbaled in 1 or more planes to allow for the scanning the entire test subject 1110 with minimal movement of the probe 1100. As the mirror 1140 is rotated, angular scanning of the test subject is effected. By gimbaling the mirror 1140 in 2 or more planes, the entire test subject 1110 can be scanned with minimal moving of the probe 1100 during the scan. Motion of the mirror can be effected through a variety of mechanisms as diverse as a sealed cable and housing or a small stepper motor. Means for mounting and moving the mirror 1140 are not depicted in
The properties of the acoustic reflective lens in the exemplary embodiment presented in
The acoustic reflective lens will possess a maximum half field of view of 10 degrees or less in order to minimize the impact of aberrations on the fidelity of the resultant image.
An acoustic imaging probe for use in an acoustic imaging system comprising a catadioptric acoustic lens, a low acoustic impedence fluid, and an acoustic imager.
In another embodiment, a probe uses a combination of at least one reflective lens and at least one refractive lens. One such embodiment of a probe 1200 is depicted in
An acoustic catadioptric lens employs a combination of refracting and reflecting surfaces in order to create a very compact design in the axial dimension, and one in which one or more of the surfaces perform multiple imaging functions (refraction or reflection). These systems will often be characterized by a central aperture obscuration, however this is not necessarily the case if off-axis designs are employed. Due to the inclusion of reflective surfaces, catadioptric designs are often limited in their field of view. They can exhibit very low F/#s, however, and are therefore excellent in those applications where axial compactness is important, but where a long focal length and large physical aperture are required. Their resolution can often be designed to approach the diffraction limit.
The acoustic catadioptric lens preferred configuration possesses a minimal number of elements, and is therefore comprised principally of aspheric surfaces. It also makes use of materials providing large index breaks across refractive surfaces in order to minimize aberrations and to create as compact a unit as possible.
This capability demonstrates the effectiveness of the acoustic catadioptric lens form in any form of inspection requiring a very short axial length imaging lens to examine a finite volume. The inspection need not be a medical inspection, but may be used for a wide variety of sections and measurement tasks such as examining cracks in welded joints.
The properties of the acoustic catadioptric lens in the embodiment presented in
In another embodiment of the present invention not shown, the photoacoustic detection method is used in conjunction with an echo detection method to provide different modalities of images.
While illustrative embodiments have been presented and described, it will be clear to those proficient in the art that various changes can be made therein without departing from the spirit and scope of the invention. In one embodiment this apparatus could be used in non-destructive testing. In another embodiment this apparatus could be used in sonar applications. All one needs is to determine the photoacoustic properties of what they are looking for and what surrounds it.
Priority for this patent application is based upon non-provisional patent application 12/218,659 (filed on Jul. 16, 2008) which claims the benefit of provisional patent application 60/959,652 (filed on Jul. 16, 2007). The disclosure of these United States patent applications are hereby incorporated by reference into this specification.
| Number | Date | Country | |
|---|---|---|---|
| 60959652 | Jul 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12218659 | Jul 2008 | US |
| Child | 13556887 | US |
