Chemistry: analytical and immunological testing – Composition for standardization – calibration – simulation,...
Reexamination Certificate
2000-11-17
2002-03-05
Wallenhorst, Maureen M. (Department: 1743)
Chemistry: analytical and immunological testing
Composition for standardization, calibration, simulation,...
C073S001840, C073S866400, C600S437000, C252S408100, C516S105000
Reexamination Certificate
active
06352860
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of materials which closely mimic the ultrasonic propagation characteristics of human tissue, and particularly to such materials used in ultrasound phantoms for use with ultrasound scanners.
BACKGROUND OF THE INVENTION
Materials which closely mimic the ultrasonic propagation characteristics of human tissue are employed in imaging phantoms and other test objects for use with ultrasound scanners. These phantoms may be used to carry out performance checks on ultrasound scanners. Phantoms may also be used for training or testing student technologists in the operation of ultrasound scanners or the interpretation of ultrasound images produced by such scanners.
Ideally, such material should be capable of mimicking soft human tissue with respect to at least three characteristics: speed of sound, ultrasonic attenuation, and ultrasonic scattering. The speed of sound in the tissue mimicking material should rest in the range from approximately 1460 m/s, characteristic of human fat tissue, to 1640 m/s, characteristic of the human eye's lens. The attenuation coefficient with respect to frequency of the material should lie in the range from approximately 0.4 dB/cm/MHz, characteristic of human fat tissue, to 2.0 dB/cm/MHz, characteristic of human muscle tissue. Additionally, the attenuation coefficient should be approximately proportional to the ultrasonic frequency. In other words, the attenuation coefficient with respect to frequency, or the attenuation coefficient slope, should remain constant for varying ultrasonic frequencies. These characteristics of human tissue should be maintained at all frequencies in the typical range of ultrasonic scanners, from 1-10 MHz. Moreover, the variation of these characteristics within the range of room temperature should be small. Additionally, these materials should be stable in time and invulnerable to reasonable environmental fluctuations. They should also be free of any pockets of air or gas. Furthermore, the bulk properties of the material should be the same throughout the volume of a particular phantom or phantom section.
A tissue mimicking material satisfying the above characteristics was disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and Methods, in which both the speed of sound and the ultrasonic attenuation properties could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. In one embodiment, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue mimicking material.
Tissue mimicking material is typically used to form the body of an ultrasound scanner phantom. This is accomplished by enclosing the material in a container which is closed by an ultrasound transmitting window cover. The tissue mimicking material is admitted to the container in such a way as to exclude air bubbles from forming in the container. In addition to the tissue mimicking material itself, scattering particles, spaced sufficiently close to each other that an ultrasound scanner is incapable of resolving individual scattering particles, and testing spheres or other targets may be located within the phantom container, suspended in the tissue mimicking material body. Such an ultrasound phantom is useful in evaluating the ability of ultrasound medical diagnostic scanners to resolve target objects of selected sizes located throughout the tissue mimicking material. The objective is for the ultrasound scanner to resolve the testing spheres or other targets from the background material and scattering particles. This type of ultrasound phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al., entitled Ultrasound Phantom.
U.S. Pat. Nos. 5,625,137 and 5,902,748 to Madsen, et al. disclose a tissue mimicking material with very low acoustic backscatter coefficient that may be in liquid or solid form. A component in both the liquid and solid forms is a filtered aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters, which may be based on a combination of milk and water. Hydroxy compounds, such as n-propanol, can be used to control the ultrasonic speed of propagation through the material and a preservative from bacterial invasion can also be included. The use of scattering particles allows a very broad range of relative backscatter levels to be achieved.
In an effort to limit patient exposure to ultrasound, the Food and Drug Administration (FDA) and the American Institute of Ultrasound in Medicine have made recommendations to ultrasound equipment manufacturers that values of two parameters, which are relevant to potential biological effects, be available to clinical users. One parameter is the thermal index (TI), the value of which is a predictor for temperature rise of tissue in an ultrasound beam. The other recognized potential mechanism of biological damage involves cavitation, and the likelihood of patient injury due to cavitation is thought to increase with the value of the parameter referred to as the mechanical index (MI). The definitions and models for the TI and MI are detailed in a standard produced by a joint committee of the American Institute of Ultrasound in Medicine (AIUM) and the National Electrical Manufacturers Association (NEMA).
In acoustic output quantification, a hydrophone is typically used in a water tank to record the temporal acoustic pressure of a propagating wave. The most extreme rarefactional pressure, p
r
, is used for calculating MI, and the pulse intensity integral, PII, is used for calculating the temporal average intensity. The MI and the temporal average intensity are subject to restrictions imposed by the FDA. The temporal average intensity is also used in calculating TI.
According to the presently applicable standards, the water-measured pressures and intensities are made to model the attenuation effects of tissue through the application of a derating factor. Derating is applied to any waveform by multiplying all acoustic pressures, p(t) (at time t) by a factor e
−&agr;f
0
z
where &agr;≡0.3 dB/cm-MHz=0.0345 nepers/cm-MHz is the attenuation coefficient slope,
f
0 is the center frequency of the pulse, and z is the distance from the transducer to the receiver. Use of the deration process assumes that the propagation of the sound pulse in water or in tissue is linear. However, it is well known that most diagnostic ultrasound systems commonly emit pulses which are nonlinear for propagation in both water and tissue. Generation of pulse nonlinearities in the water is a concern because the derating process assumes linear propagation and a result can be significant underestimation of intensity. It is not known to what extent such nonlinearities occur in tissue although it is thought that most tissues are less subject to such an effect than water.
There are two approaches to replacing the presently used water-deration method of measurement with more acceptable methods. One is theoretical, viz, to extend the deration method to account for nonlinear propagation in both water and a hypothetical tissue-mimicking material using the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation. The other approach for accurately determining exposure parameters is direct experimental measurement using a medium which adequately mimics tissue in the relevant ultrasonic parameters, viz, propagation speed, attenuation and nonlinear propagation. The liquid tissue mimicking material disclosed in U.S. Pat. Nos. 5,625,137 and 5,902,748 may be utilized for this purpose. Tests of the accuracy of the water-derating procedure have been made employing a tissue-mimicking liquid as described in U.S. Pat. Nos. 5,625,137 and 5,902,748 with properties recommended by the American Institute of Ultrasound in Medicine (AIUM) and having a B/A value typical of soft
Frank Gary R.
Madsen Ernest L.
Wallenhorst Maureen M.
Wisconsin Alumni Research Foundation
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