Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-03-31
2001-03-13
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06200266
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a medical ultrasound imaging system for producing images of anatomic structures. In particular, the present invention relates to an ultrasound imaging system capable of producing and displaying the acoustic impedance of soft tissue, bone and the like, reconstructed from reflected pulse-echo ultrasound signals.
2. Description of the Related Art
Diagnostic imaging, based on technologies such as MRI, X-ray, CT and ultrasound, is currently performed by large, expensive equipment that often requires the patient to be brought considerable distances to special facilities. Although these techniques are non-invasive, some generate hazardous radiation that necessitates separation of the physician and technician from the patient during the procedure. Often, resolution and the ability to define a specific area under test are not satisfactory, making diagnosis difficult without further invasive exploration.
The availability of a convenient and cost-effective method to image and quantitatively assess the real time status of internal injuries, growths and fractures or other defects, and to identify and define the trauma site and trauma status, would be a valuable clinical tool. It could potentially shorten hospital stays and allow earlier return to normal activities. Further, it could provide early detection of malignancies and delayed fractures or non-union of fractures, thereby allowing early introduction of appropriate therapies. This could have a considerable economic impact in those cases where long-term disability could be avoided or minimized. If such a system were available at a moderate cost, it would potentially find use in the majority of medical offices, clinics and hospitals dealing with fractures and soft tissue injuries as well as in other medical fields, including physical therapy, sports medicine, rehabilitation and geriatrics.
Consequently, there has been a growing interest in recent years to develop higher resolution ultrasound imaging systems designed for specific applications. For example, a non-invasive diagnostic imaging technique capable of identifying malignancy in vivo would have a major impact on the detection and treatment of cancer. In dermatologic diagnostics, high resolution ultrasound systems have been developed utilizing transducer frequencies up to 100 MHz for imaging the layers of the skin, determining margins of small skin lesions, and characterizing non-malignant skin diseases by thickness measurements. In ophthalmology applications, such as characterization of ocular tissue, examination of eye tumors and assessment of corneal diseases, high frequency ultrasound systems have been developed approaching resolutions of 20 &mgr;m. Another ultrasound application of interest is that of imaging the gastrointestinal (GI) tract, where an endoscopic device can potentially be used to image gastrointestinal mucosa layers and layers of the esophageal wall and to detect and evaluate gastric tumors and lesions.
The currently employed pulse-echo method of ultrasound imaging provides a display of signals backscattered from tissue and has proven to be the most useful ultrasound method in medical applications to date. While higher signal frequencies generally yield higher image resolution, further improvement in resolution of ultrasound imaging of biological tissue is a challenging problem because of the increased attenuation suffered by the ultrasound signal with increasing frequency. The propagation of an ultrasound pressure beam through tissue causes the pressure beam to attenuate as a function of depth primarily due to absorption and scattering. Specifically, the propagation of an ultrasound pressure wave through a medium will result in the exponential decrease of the acoustic pressure amplitude parameter as a function of propagating distance. Several factors contribute to attenuation, the most important being absorption and scattering. Neglecting other losses such as beam spreading and diffraction, attenuation is described by the following expression:
A(x)=A
0
e
−&mgr;x
(1)
where x is the propagating distance in cm, &mgr; is the amplitude attenuation coefficient, A
0
is the unattenuated amplitude, and A is the attenuated amplitude. The amplitude attenuation coefficient is a function frequency and is approximately given by:
&mgr;=&agr;f
n
(2)
where &agr; is the weakly frequency dependent amplitude attenuation coefficient of the medium in units of Nepers/cm/Hz, and n is the exponent of the frequency dependence.
The frequency dependence of attenuation has an important effect on the spectrum of the propagating pulse. The higher frequencies are disproportionately attenuated, causing the spectrum of the traveling pulse to shift toward lower frequencies with increasing propagating distance. An approximate expression for the downshifted peak frequency in the spectrum of an ultrasound pulse traveling in water, where the exponent of frequency dependence is n=2, is given by:
f
p
(
x
)
=
f
0
2
α
x
σ
2
+
1
(
4
)
where f
0
is the peak frequency in the spectrum of the unattenuated pulse, and f
p
is the peak frequency in the spectrum of the attenuated pulse after it has propagated a distance x in the medium. The term &sgr; is given by:
σ
=
f
0
B
236
(
5
)
where B is Full Width Half Maximum (FWHM) bandwidth of the unattenuated spectrum expressed as a percentage.
For water, the exponent of frequency dependence is n=2; however, many soft tissues of the body attenuate ultrasound to a similar degree, which is a nearly linear frequency dependence. This gives rise to the general rule of thumb for ultrasound attenuation in tissue which is approximately 1 dB per centimeter per megahertz for most soft tissues.
Absorption results in the conversion of the pressure wave energy to heat and is responsible for the temperature rise made use of in ultrasound-induced hypothermia. The absorption mechanisms of ultrasound in biological tissue are quite complex. The mechanisms by which absorption can occur can be classified in three categories: classical mechanisms, molecular relaxation, and relative motion losses.
Classical absorption describes the frictional loss associated with a viscous medium. It has been shown that, in air or water, classical absorption dominates and the absorption is approximately proportional to f
2
, the square of the sound frequency. However, in biological tissue, it has been postulated by Wells in
Biomedical Ultrasonics,
Academic Press (1977), that the absorption is due to a relaxation mechanism associated with the molecules. The pressure fluctuations associated with the sound wave cause reversible alterations in molecular configuration and, because there are likely to be many such mechanisms simultaneously in action, produce a frequency dependence close to f
1
. Relative motion losses, in which the sound wave induces a viscous or thermally damped movement of small-scale structural elements of tissue, are also possible mechanisms for absorption and could produce a frequency dependence of absorption between f
1
and f
2
. For simple solutions of molecules, increasing molecular complexity results in increasing absorption. For tissues, a higher protein content, especially structural proteins such as collagen, or a lower water content is associated with greater absorption of ultrasound.
Scattering of ultrasound radiation can be classified into three regimes: scattering by particles which have radii, a, much larger than the incident wavelength (a>>&lgr;); scattering by particles with radii on the order of the incident wavelength (a≈&lgr;) or “Mie scattering”; and scattering by particles with radii much smaller than the incident wavelength (a<<&lgr;) or Rayleigh scattering.
For particle sizes much larger than the wavelength (a>>&lgr;), specular reflection of sound will occur between two homogeneous media. The laws of reflection and Snell
Izatt Joseph
Roth Sanford
Shokrollahi Nima
Tobocman William
Case Western Reserve University
Imam Ali M.
Lateef Marvin M.
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