Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-10-09
2003-02-25
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S442000
Reexamination Certificate
active
06524248
ABSTRACT:
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CROSS REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to receiving and focusing of a radiating wave field that propagates in a medium, specifically where that medium causes wave field distortions that degrade quality of focus. Example applications in the field of the invention include ultrasonic medical imaging, seismic prospecting, ultrasonic industrial inspection, radar, sonar, and optics.
An ideal medium for propagation of radiating waves is free space where electromagnetic waves propagate at precisely known speed without distortion. Devices that utilize wave propagation in a medium that is less perfect than this ideal, are subject to a variety of limitations. There are many ways that a medium can be imperfect. It can attenuate signals, it can have propagation speeds that are inaccurately known, and it can be inhomogeneous. Wave fields are focused using an aperture where the quality of focus depends on characteristics of the medium and characteristics of equipment that implements the aperture. A variety of measures are needed to compensate for medium effects to improve quality of focus, but an especially difficult problem is presented when the medium is inhomogeneous.
In discussing wave fields, it is common to speak of wavefronts to help in intuitive understanding of a very complicated physical process. A wavefront is defined to be a surface that contains points of equal phase. A simple wave field has many possible wavefronts, but only one is pictured. However, when this discussion refers to wavefront shape or to modifying wavefront shape, all possible wavefronts are assumed to be so shaped or so modified. References made here to a wavefront are intended to be references to a wave or a wave field by implication. Changing amplitude of a wavefront means that the wave system represented by that wavefront is proportionally changed in amplitude. The term phase is also somewhat inadequate where a single frequency is not utilized. Here a wavefront refers to points at a similar position in a function that causes a wave, such as a leading edge of a pulse or a first peak. For purposes of the present specification, wave speed is the speed of a point on the wavefront, under either of these definitions of wavefront. Wavefronts are frequently described here as spherical surfaces which means that they are sections of a surface of a sphere. When shown as a three dimensional drawing these wavefronts are indicated with a wire mesh that describes the surface. In two dimensional drawings they are indicated by a curved line which is intended to mean a spherical surface, unless otherwise stated.
Waves and wavefronts are a form of signals as are electrical voltage variations in wires and samples of voltages that are in computers. All these forms represent information.
Inhomogeneous problems are significant in ultrasonic imaging in a three dimensional volume such as the human body. It is known that sound speed variations in human tissue can cause significant wavefront distortion over an aperture extent. Such spatial variations tend to defeat the basic focusing function of the aperture because this focusing function depends on a predicted wavefront shape for points in the focus zone and well formed wavefront shapes for wavefronts that come from any point outside the focus zone. Wavefront distortion causes amplitude reduction of a focused signal to be reduced in amplitude, widening of a focal zone (a beam), and increased response for points outside the focal zone.
High resolution is critical to viewing disease processes, but it is widely believed in the field of ultrasonic imaging that aberration effects of inhomogeneous media would limit usefulness of high resolution devices. In ideal media, resolution improves with the use of shorter wavelengths or larger aperture transducers, but in human tissue, which is inhomogeneous, the use of such measures is expected to lead to aberrations that would prevent full benefits that might otherwise be realized (M. O'Donnell and P. Li, “Aberration correction on a two-dimensional anisotropic phased array,” 1991 Ultrasonics Symposiun, p1190, IEEE). Although detailed study of past experimental work reveals that spatially uniform attenuation that strongly varies as a function of frequency is also a significant limitation of large aperture devices, the aberration problem remains as a significant barrier to development of high resolution ultrasonic systems.
Inhomogeneous conditions cause degradation of a response function, where that response function describes performance of a transducer system. There can be a transmitting response, which is a measure power as intensity as a function of a spatial dimension or there can be a receiving response, which is a measure of sensitivity as a similar function. Reciprocity usually applies so a transducer response is the same for either direction. Where there is a strong peak in the response function, a main beam is established. In receiver terms, the key degradation issue is the relative strength of a signal that comes from points within a focal zone as compared with the strength of an off-beam signal that comes from points outside the focal zone. Off-beam response is often called sidelobe response, though there can also be a grating lobe response. Terminology tends to be inadequate in practice. It becomes particularly problematic when it leads to use of analytical methods that were developed for ideal media.
Understanding of propagation in a medium that is inhomogeneous requires a meaningful description in terms that can be mathematically analyzed. A common model in the field of medical ultrasonic imaging addresses irregular sound speed variations, where such variations cause wave arrival times at a receiving aperture to deviate from the ideal shape. Arrival time is represented by wavefront shape as it is immediately approaching a receiving aperture. Sound speed variations over the propagation path cause wavefronts to distort, but if accurate time corrections could be ascertained to compensate for the variations of sound speed, the main beam response would be restored. FIG.
1
(
a
) shows as reference an ideal wave
2
propagating from a source that is approximated as a point
11
to a receiver
3
in a homogeneous medium
1
without distortion to cause received signals
4
. FIG.
1
(
b
) is comparative illustration for imperfect media
8
that shows a wave perturbing effect
7
of a localized material
5
that varies wave speed, where compensation material
6
is inserted as a lump that would reverse the initial variation of wave speed. In ultrasonic systems, the actual compensating process would be handled as an electronic process after reception of received signals
4
. In optical systems, the use of corrective material is a common way to correct for lens errors, which have much the same effect.
This comparative illustration of FIG.
1
(
b
) also shows a blockage
9
that distorts the actual wave by leaving a gap
10
where wave amplitude is zero. For relatively large blockage shown the gap
10
projects geometrically such that the propagated wave
70
proportionately contains a similar gap
71
. Artificially filling in the gap is not inconceivable in simple conditions where signals are simply provided to establish uniformity, but in general it is difficult to know what signal is needed. Repairing this gap by time corrections is not conceivable since there is no wave energy to work with. Though it might seem innocuous, the gap is a cause of distortion of significance that is comparative to an uncorrected speed distortion effect. Coherent relationships of multiple gaps make them far more significant than relatively random speed distortion effects.
While this depic
Imam Ali M.
Lateef Marvin M.
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