Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-06-27
2001-09-18
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S443000
Reexamination Certificate
active
06292682
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to imaging properties of tissue based upon differential absorption of electromagnetic waves in differing tissue types by photo-acoustic techniques.
It is well established that different biologic tissues display significantly different interactions with electromagnetic radiation from the visible and infrared into the microwave region of the electromagnetic spectrum. While researchers have successfully quantified these interactions in vitro, they have met with only limited success when attempting to localize sites of optical interactions in vivo. Consequently, in vivo imaging of disease at these energies has not developed into a clinically significant diagnostic tool.
In the visible and near-infrared regions of the electromagnetic spectrum, ubiquitous scattering of light presents the greatest obstacle to imaging. In these regions, scattering coefficients of 10-100 mm
−1
are encountered. Consequently, useful numbers of unscattered photons do not pass through more than a few millimeters of tissue, and image reconstruction must rely on multiply-scattered photons. While efforts persist to use visible and infrared radiation for imaging through thick tissue (thicker than a few centimeters), clinically viable imaging instrumentation has not been forthcoming.
In the microwave region (100-3000 MHZ), the situation is different. Scattering is not as important, since the wavelength (in biologic tissue) at these frequencies is much greater than the “typical” dimension of tissue inhomogeneities (≈1 &mgr;m). However, the offsetting effects of diffraction and absorption have forced the use of long wavelengths, limiting the spatial resolution that can be achieved in biologic systems. At the low end of the microwave frequency range, tissue penetration is good, but the wavelengths are large. At the high end of this range, where wavelengths are shorter, tissue penetration is poor. To achieve sufficient energy transmission, microwave wavelengths of roughly 2-12 cm (in tissue) have been used. However, at such a long wavelength, the spatial resolution that can be achieved is no better than roughly ½ the microwave length, or about 1-6 cm.
In vivo imaging has also been performed using ultrasound techniques. In this technique, an acoustic rather than electromagnetic wave propagates through the tissue, reflecting from tissue boundary regions where there are changes in acoustic impedance. Typically, a piezoelectric ceramic chip is electrically pulsed, causing the chip to mechanically oscillate at a frequency of a few megahertz. The vibrating chip is placed in contact with tissue, generating a narrow beam of acoustic waves in the tissue. Reflections of this wave cause the chip to vibrate, which vibrations are converted to detectable electrical energy, which is recorded.
The duration in time between the original pulse and its reflection is roughly proportional to the distance from the piezoelectric chip to the tissue discontinuity. Furthermore, since the ultrasonic energy is emitted in a narrow beam, the recorded echoes identify features only along a narrow strip in the tissue. Thus, by varying the direction of the ultrasonic pulse propagation, multi-dimensional images can be assembled a line at a time, each line representing the variation of acoustic properties of tissue along the direction of propagation of one ultrasonic pulse.
For most diagnostic applications, ultrasonic techniques can localize tissue discontinuities to within about a millimeter. Thus, ultrasound techniques are capable of higher spatial resolution than microwave imaging.
The photoacoustic effect was first described in 1881 by Alexander Graham Bell and others, who studied the acoustic signals that were produced whenever a gas in an enclosed cell is illuminate with a periodically modulated light source. When the light source is modulated at an audio frequency, the periodic heating and cooling of the gas sample produced an acoustic signal in the audible range that could be detected with a microphone. Since that time, the photoacoustic effect has been studied extensively and used mainly for spectroscopic analysis of gases, liquid and solid samples.
It was first suggested that photoacoustics, also known as thermoacoustics, could be used to interrogate living tissue in 1981, but no subsequent imaging techniques were developed. The state of prior art of imaging of soft tissues using photoacoustic, or thernoacoustic, interactions is best summarized in Bowen U.S. Pat. No. 4,385,634. In this document, Bowen teaches that ultrasonic signals can be induced in soft tissue whenever pulsed radiation is absorbed within the tissue, and that these ultrasonic signals can be detected by a transducer placed outside the body. Bowen derives a relationship (Bowen's equation 21) between the pressure signals p(z,t) induced by the photoacoustic interaction and the first time derivative of a heating functions, S(z,t), that represents the local heating produced by radiation absorption. Bowen teaches that the distance between a site of radiation absorption within soft tissue is related to the time delay between the time when the radiation was absorbed and when the acoustic wave was detected.
Bowen discusses producing “images” indicating the composition of a structure, and detecting pressure signals at multiple locations, but the geometry and distribution of multiple transducers, the means for coupling these transducers to the soft tissue, and their geometrical relationship to the source of radiation, are not described. Additionally, nowhere does Bowen teach how the measured pressure signals from these multiple locations are to be processed in order to form a 2- or 3-dimensional image of the internal structures of the soft tissue. The only examples presented are 1-dimensional in nature, and merely illustrate the simple relationship between delay time and distance from transducer to absorption site.
SUMMARY OF THE INVENTION
The present invention improves upon what is disclosed by Bowen in two ways. First, the present invention uses multiple transducers to collect photoacoustic signals in parallel, and then combines these signals to form an image. This approach represents a significant advance over Bowen in that the use of multiple, parallel transducers, substantially reduces the time needed to collect sufficient information for imaging. Furthermore, while Bowen fails to suggest methodologies for creating multidimensional images, the present invention provides specific methodologies for reconstructing multidimensional images of internal tissues through the combination of multiple pressure recordings. As part of achieving these advances over Bowen, the present invention details the frequencies that might be used, the size of the multiple transducers, their geometrical relationship to one another and to the tissue, and structures for coupling sensors to the tissue.
Specifically, in one aspect, the invention provides a method of imaging tissue structures by detecting localized absorption of electromagnetic waves in the tissue. An image is formed by irradiating the tissue with a pulse of electromagnetic radiation, and detecting and storing resultant pressure waveforms arriving at the acoustic sensors. Multiple detected pressure waveforms are then combined to derive a measure of the extent to which pressure waveforms are originating at a point distant from the acoustic sensors. This step can then be repeated for multiple points to produce an image of structures in the tissue.
In a disclosed particular embodiment, the multiple pressure waveforms are combined to form an image at a point by determining a distance between the point and a pressure sensor, and then computing a value related to the time rate of change in the pressure waveform, at a time which is a time delay after the pulse of electromagnetic radiation—this time delay being equal to the time needed for sound to travel through the tissue from the point to the pressure sensor. This process, determining a distance and time delay, and then
Imam Ali M.
Lateef Marvin M.
Optosonics, Inc.
Wood Herron & Evans L.L.P.
LandOfFree
Photoacoustic breast scanner does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photoacoustic breast scanner, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photoacoustic breast scanner will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2466153