Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-01-28
2001-06-19
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S455000
Reexamination Certificate
active
06248071
ABSTRACT:
FIELD
This patent specification relates to the field of ultrasound information processing systems. In particular, it relates to a method and system for the demodulation of ultrasound signals for use in ultrasound systems.
BACKGROUND
In recent decades ultrasonic imaging technology has played an increasing role in examining the internal structure of living organisms. The technology has applications in diagnosis of various medical ailments where it is useful to examine structural details in soft tissues within the body. Ultrasound imaging systems are advantageous for use in medical diagnosis as they are non-invasive, easy to use, and do not subject patients to the dangers of electromagnetic radiation. Instead of electromagnetic radiation, an ultrasound imaging system transmits sound waves of very high frequency (e.g., 2 MHz to 10 MHz) into the patient and processes echoes reflected from structures in the patient's body to form two dimensional or three dimensional images.
More recently, ultrasonic imaging systems have been able to detect blood flow as well as tissue amplitude along the axis of the interrogating ultrasonic wave. These blood velocities are related by the Doppler effect movement within the body and blood flow is information of high diagnostic significance for certain diseases. This and other ultrasound background information is disclosed in Zagzebski,
Essentials of Ultrasound Physics
(Mosby 1996), the contents of which are hereby incorporated by reference into the present disclosure.
Signal demodulation represents a key preliminary step in converting reflected ultrasound signals into a usable representation on an output display.
FIGS. 1A and 1B
show exemplary plots of ultrasonic waves as transmitted into the body and received from the body, respectively, by an ultrasound transducer (not shown). In a typical ultrasound system operating in pulse-echo mode, the transducer transmits a signal
100
comprising a series of time-limited modulated bursts as shown in
FIG. 1A
, and then receives a reflected signal
102
as shown in FIG.
1
B. The plot
100
of
FIG. 1A
is of a generic voltage signal which, as known in the art, is converted into sound waves by piezoelectric devices in the transducer for transmission into the body. The plot
102
of
FIG. 1B
is likewise a generic voltage signal resulting from the conversion of reflected sound waves into voltages by the piezoelectric devices in the probe.
It is to be appreciated for purposes of the present disclosure that system specifics such as frequency ranges, pulse durations, and the like are given by way of example only and are not intended to limit the scope of the preferred embodiments. By way of example and not by way of limitation, typical parameters for real-time ultrasound imaging applications would include: a carrier frequency F
c
of 8 MHz (depending on the application, ultrasound implementations may use F
c
selected from the range 2 MHz-15 MHz); a burst repetition frequency BRF of 2000 Hz, i.e., 1 burst is sent every 500 &mgr;s (real-time imaging systems may use a BRF selected from the range 2000 Hz-4000 Hz); and a burst duration BD of 1 &mgr;s. As shown in
FIG. 1B
, the reflected signal x(t) in plot
102
can be expressed as a sinusoid at the carrier frequency F
c
modulated by an envelope signal A(t) and a time-varying phase term &phgr;(t), as can be expressed by Equations (1)-(3):
x
(
t
)=
A
(
t
)cos{&ohgr;
c
t+&phgr;
(
t
)} {1}
&ohgr;
c
=2&pgr;
F
c
{2}
x
(
t
)=
Re{A
(
t
)
e
j&phgr;(t)
e
j&ohgr;ct
} {3}
Depending on the ultrasound implementation, it is the content of the envelope signal A(t) and/or the content of the phase signal &phgr;(t) that is of interest in generating the ultrasound system display output. For example, in B-mode imaging it is the envelope signal A(t) that is of interest because A(t) is proportional to the reflective index of the target tissue. As another example, in Doppler mode imaging it is the phase signal &phgr;(t) that is of interest because as the velocity of fluid flow in the target area is proportional to its first derivative d&phgr;/dt. Because the basic analytic signal A(t)e
j&phgr;(t)
modulated by the carrier frequency F
c
in Equations (1)-(3) is usually found to contain frequency content in the several-megahertz range, which is not significantly less than the carrier frequency F
c
, the resultant signal x(t) is generally classified as a wideband signal.
FIG. 1C
shows frequency spectra
104
,
106
, and
108
of the wideband signal x(t) corresponding to differing exemplary target depths of 1 cm, 10 cm, and 20 cm, respectively. The spectra
104
,
106
, and
108
correspond to the amplitude of the Fourier transform X(f) of the signal x(t) taken over small intervals of time around points corresponding to the expected arrival time of burst reflections from depths of 1 cm, 10 cm, and 20 cm, respectively. As illustrated by
FIG. 1C
, the frequency spectrum X(f) is time varying in its characteristics. As known in the art, this is due to a frequency and depth attenuation factor of about 0.6 dB/MHz-cm for a typical ultrasound system on a typical human target, which causes the spectrum to be increasingly skewed toward lower frequencies in the far field (i.e., at greater depths) than in the near field (i.e., at lesser depths). Accordingly, there are different half-power bandwidths W of the analytic signal A(t)e
j&phgr;(t)
as a function of depth, and in general W
depth=d1
>W
depth=d2
for d
2
>d
1
. For purposes of clarity of disclosure, and not for purposes of limiting the scope of the preferred embodiments, the analytic signal A(t)e
j&phgr;(t)
is characterized herein as having a generic upper frequency limit W/2, where W is a half-power bandwidth for a depth in the near field. It is to be understood, however, that in practical systems a modified value for the upper frequency limit may be used without departing from the scope of the preferred embodiments.
FIG. 2
shows a block diagram of a conventional ultrasound system
200
in accordance with prior art. Ultrasound system
200
comprises a transducer
202
, a front end processor
204
, a demodulator
206
, and an amplitude detector and display apparatus
208
. As described supra, transducer
202
transmits focused acoustic signals into the body and sends an analog response signal to front end processor
204
. Front end processor
204
performs preliminary operations such as depth gain compensation and then transmits the result, commonly called the RF signal, to demodulator
206
. For purposes of clarity and simplicity, the analog signal x(t), shown as plot
102
of
FIG. 1B
, is referred to herein as the RF signal, with depth gain compensation and/or other preliminary processing having already been performed by front end processor
204
.
In the system of
FIG. 2
, the demodulator
206
performs processing steps directed to extracting the envelope signal A(t) and the phase signal &phgr;(t) from the RF signal x(t), and then transmits the result to amplitude detector and display apparatus
208
for downstream processing and eventual image display. In a simplest form, for B-mode processing such a demodulator could simply comprise an analog envelope detector having a full-wave rectifier and a low-pass filter, as described in Carlson,
Communication Systems: An Introduction to Signals and Noise in Electrical Communication
(McGraw-Hill, 3
rd
ed. 1986), the contents of which are hereby incorporated by reference into the present disclosure. However, in most practical ultrasound systems in which both the amplitude signal A(t) will be desired for a B-mode processing mode, and where phase signal &phgr;(t) will be desired for Doppler processing mode, a coherent detection scheme is used wherein demodulator
206
comprises a quadrature mixer. While older prior art implementations were analog in nature, most newer implementations are digital. For purposes of the present disclosure, and without loss of gene
Cooper & Dunham LLP
Lateef Marvin M.
Patel Maulin
U-Systems Inc.
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