Measuring and testing – Instrument proving or calibrating – Apparatus for measuring by use of vibration or apparatus for...
Patent
1998-11-12
2000-11-21
Noland, Thomas P.
Measuring and testing
Instrument proving or calibrating
Apparatus for measuring by use of vibration or apparatus for...
73 186, 424 95, G01N 2900, A61B 800, A61B 814
Patent
active
061486558
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates in general to a method for calibrating diagnostic ultrasound equipment and ultrasound contrast agents and, more particularly, to a method of calibration with which to make absolute measurements of the backscatter and attenuation of tissue or contrast agents.
BACKGROUND OF THE INVENTION
The use of high frequency ultrasound for medical applications has been increasing in recent years with the advent of intravascular ultrasound, acoustic microscopy and second harmonic imaging with ultrasonic contrast agents. Higher frequencies allow new morphologic structures to be examined as well as opening a new area of ultrasonic tissue characterization. In the past, tissue characterization has been hampered by the lack of a standard method of calibration with which to make absolute measurements of the backscatter and attenuation of different types of tissue or contrast agents.
The current standard for calibration of diagnostic ultrasound equipment utilizes the reflection from a polished stainless steel plate. This procedure is disadvantageous because of its sensitivity to alignment of the surface of the plate and the fact that the reflection is much greater than that returned by tissue. The ultrasonic scatter from tissue is often modeled as a collection of scatterers suspended in a water-like medium. The use of calibration standards or "phantoms" that mimic tissue has been of particular interest to the tissue characterization community. Typically these phantoms consist of a collection of scatterers held in an agarose or gelatin-like media such as described in U.S. Pat. Nos. 4,843,866 and 4,277,367, the entirety of each being hereby incorporated by reference. A substantial body of work exists that examines phantoms for ultrasound in the frequency range below 5 MHz. The geometrical shape and physical parameters of the scatterers within tissue can be further derived with the use of an appropriate data reduction method. The use of gelbased phantoms are disadvantageous, however, in that many spatial site averages must be obtained to yield a stable result and requires substantial expertise to construct and maintain.
In the higher frequency world of acoustic microscopy and intra-vascular imaging, however, it is not clear whether the existing phantoms will provide a useful and reproducible method for calibration. There is, therefore, a need in the art for a broadband, in vitro method for calibration of diagnostic ultrasound equipment and for ultrasound contrast agents used therewith that substantially mimics the reflection returned by tissue, is easy to construct and maintain and removes the problems associated with alignment of the calibrating tool.
SUMMARY OF THE INVENTION
The present invention is directed to a method for calibrating diagnostic ultrasound equipment and ultrasound contrast agents is provided that utilizes polystyrene microspheres as a broadband in vitro calibration tool. In accordance with the method, a suspension of polystyrene microspheres are provided in a liquid environment in a container having an acoustic aperture. The suspension of microspheres is continuously agitated during measurement of the backscatter from the suspension of microspheres and the ultrasound equipment or contrast agent is then calibrated or adjusted to a selected backscatter setting.
In one significant aspect of the invention, the method comprises the steps of providing a suspension of microspheres in a liquid environment in a container having an acoustic aperture in a constant temperature water bath, continuously agitating the suspension of microspheres during measurement of the backscatter from the suspension of microspheres, and adjusting the ultrasound system to a selected backscatter setting. Following measurement of the backscatter, various functions such as mean backscatter, backscatter coefficient, backscatter transfer function, or mean backscatter transfer function may be calculated prior to the adjusting step.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1: A diagram of the sp
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Hall Christopher S.
Hughes Michael S.
Mallinckrodt Inc.
Noland Thomas P.
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