Detector in ultrasonic holography

Details Detector in ultrasonic holography Detector in ultrasonic holography

2000-06-08

2002-03-05

Lobo, Ian J. (Department: 3662)

Communications, electrical: acoustic wave systems and devices

Acoustic image conversion

Acoustic holography

C367S008000, C073S603000, C073S605000, C359S901000

Reexamination Certificate

active

06353576

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention provides an improved ultrasonic holography or other ultrasonic imaging process that accurately forms the phase and amplitude information of the hologram in a manner that renders the unit insensitive to environment vibrations, and provides long maintenance free functioning lifetime. Specifically, the improved ultrasonic hologram detector component forms an ultrasonic hologram on the surface of a liquid that results from the deformation of the surface. This is due to the reflection of an ultrasound energy profile of a combination of an “object wave” that passes through the object and that of a “reference wave” that is directed to the surface at an off axis angle from the “object wave”.
BACKGROUND OF THE INVENTION
The central element field of holography is fulfilled by combining or interfering an object wave or energy with a reference wave or 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. That is to say, that 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 free-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 faithfully 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, the pulse from the object transducer progresses through the object, then through the 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 system. This is provided by either positioning or rotating the object transducer around the central axis of transmission 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 transmission.
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 for 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 system 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)/f(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 (room temperature). 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, when ultrasound propagating in a liquid encounters a liquid-air interface the entire amount of the energy is reflected back into the liquid. Sin

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