Communications – electrical: acoustic wave systems and devices – Acoustic image conversion – Acoustic holography
Reexamination Certificate
2000-06-08
2003-07-08
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Acoustic image conversion
Acoustic holography
C073S603000, C073S605000, C359S901000, C600S437000
Reexamination Certificate
active
06590830
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention provides a series of process and apparatus modifications to ultrasonic holography imaging systems that each or together function to enhance image quality of ultrasonic holography images. Specifically, the present invention provides a process and apparatus for generating multiple exposure ultrasonic holography images generated from selected orientations, each of which insures multiple images, multiple intensities and multiple frequencies from each orientation. The inventive process and apparatus for providing multiple view, multiple angle, and multiple frequency or intensity transmissive ultrasound imaging of the internal structures of an object provides an object sound (ultrasound or ultrasonic energy) intensity of equal or near equal intensity across the entire field of the object, such as a human breast.
BACKGROUND OF THE INVENTION
A central element field of holography is fulfilled by combining or interfering an object wave or ultrasonic energy with a reference wave or ultrasonic energy to form an interference pattern referred to as the hologram. A fundamental requirement for the forming of the hologram and the practice of holography is that the initial source of the object wave and reference wave or energy are coherent with respect to the other wave. All parts of both the object wave and the reference wave are of the same frequency and of a defined orientation (a fixed spatial position and angle between the direction of propagation of the two sources). When performing holography the object wave is modified by interference with structure within the object of interest. As this object wave interacts with points of the object the three-dimensional features of the object impart identifying phase and amplitude changes on the object wave. Since the reference wave is an unperturbed (pure) coherent wave, its interference with the object wave results in an interference pattern which identifies the 3-D positioning and characteristics (ultrasonic absorption, diffraction, reflection, and refraction) of the scattering points of the object.
A second process, (the reconstruction of the hologram) is then performed when a coherent viewing source (usually light from a laser) is transmitted through or reflected from the hologram. The hologram pattern diffracts light from this coherent viewing or reconstructing source in a manner to represent the 3-D nature of the object, as seen by the ultrasonic object wave.
To reiterate, to perform holography coherent wave sources are required. This requirement currently limits practical applications of the practice of holography to the light domain (e.g., a laser light) or the domain of acoustics (sometimes referred to as ultrasound due to the practical application at ultrasonic frequencies) as these two sources are currently the only available coherent energy sources. Thus, further references to holography or imaging system will refer to the through transmission holographic imaging process that uses acoustical energies usually in the ultrasonic frequency range. In the practice of ultrasound holography, one key element is the source of the ultrasound, such as a large area coherent ultrasound transducer. A second key element is the projection of the object wave from a volume within the object (the ultrasonic lens projection system) and a third is the detector and reconstruction of the ultrasonic hologram into visual or useful format.
Although other configurations can be utilized, a common requirement of the source transducers for both the object and reference waves is to produce a large area plane wave having constant amplitude across the wave front and having a constant frequency for a sufficient number of cycles to establish coherence. Such transducers will produce this desired wave if the amplitude of the ultrasound output decreases in a Gaussian distribution profile as the edge of the large area transducer is approached. This decreasing of amplitude reduces or eliminates the “edge effect” from the transducer edge, which would otherwise cause varying amplitude across the wave front as a function distance from the transducer.
In the process of through transmission ultrasonic holographic imaging, an ultrasonic energy pulse from the object transducer progresses through the object, then through an acoustic focusing lens and at the appropriate time, the pulse of ultrasound is generated from the reference transducer such that the object wave and reference wave arrive at the detector at the same time to create a interference pattern (the hologram). For broad applications, the transducers need to be able to operate at a spectrum or bandwidth of discrete frequencies. Multiple frequencies allow comparisons and integration of holograms taken at selected frequencies to provide an improved image of the subtle changes within the object.
A hologram can also be formed by directing the object wave through the object at different angles to the central imaging axis of the lens means. This is provided by either positioning or rotating the object transducer around the central axis of the lens means or by using multiple transducers positioned such that the path of transmission of the sound is at an angle with respect to the central axis of the lens means.
With a through-transmission imaging system, it is important to determine the amount of resolution in the “z” dimension that is desirable and achievable. Since the holographic process operates without limits of mechanical or electronic devices but rather reconstructs images from wave interactions, the resolution achievable can approach the theoretical limit of one half the wavelength of the ultrasound used. However, it may be desirable to limit the “z” direction image volume so that one can “focus” in on one thin volume slice. Otherwise, the amount of information may be too great. Thus, it is of value to develop a means for projecting a planar slice within a volume into the detector plane. One such means is a large aperture ultrasonic lens means that will allow the imaging system to “focus” on a plane within the object. Additionally, this lens system and the corresponding motorized, computer controlled lens drive will allow one to adjust the focal plane and at any given plane to be able to magnify or demagnify at that z dimension position.
The image is detected and reconstructed at the detector. Standard photographic film may be used for the recording of light holograms and the 3-D image reconstructed by passing laser light through the film or reflecting it from the hologram pattern embossed on the surface of an optical reflective surface and reconstructing the image by reflecting light from the surface. However, there is no equivalent “film” material to record the intricate phase and amplitude pattern of a complex ultrasonic wave. One of the most common detectors uses a liquid-air surface or interface to record, in a dynamic way, the ultrasonic hologram formed. The sound energy at the frequency of ultrasound (above range of human hearing) will propagate with little attenuation through a liquid (such as water) but cannot propagate through air. At these higher frequencies (e.g., above 1 MHz) the ultrasound will not propagate through air because the wavelength of the sound energy is so short (&lgr;(wavelength)=v/(velocity)/ƒ(frequency)). The density of air (approximately 0.00116 g/cm
3
) is not sufficient to couple these short wavelengths and allow them to propagate. On the other hand the density of a liquid (e.g., water) is a favorable media to couple and propagate such sound. For example, the velocity of sound in air is approximately 330 meters/second whereas in water it is approximately 1497 meter/second. Thus, for water, both the density (1 g/cm
3
) and the wavelength (~1.48 mm at 1 MHz) are significantly large such that ultrasound can propagate with little attenuation. Whereas, for air both the density (0.00116 g/cm
3
) and wavelength (0.33 mm at 1 MHz) are sufficiently small such that the energy at these ultrasonic frequencies will not propagate.
Thus, whe
Garlick George F.
Shelby Jerod O.
Shelby Ronald L.
Advanced Imaging Technologies, Inc.
Lobo Ian J.
SEED IP Law Group PLLC
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