Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2003-02-10
2004-10-26
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06808494
ABSTRACT:
BACKGROUND
This invention relates to ultrasound transmit circuits. In particular, ultrasound transmit circuits for generating bi-polar ultrasound waveforms are provided.
For medical diagnostic ultrasound imaging, high current and high voltage amplifiers generate bi-polar waveforms. A wide bandwidth of operation of the amplifiers is used. However, operation of the transmit amplifiers may generate even order distortion products, i.e., such a components at a second harmonic or subharmonics of the fundamental frequency of the bi-polar waveform. The subharmonics and/or harmonics generate undesirable echoes where an ultrasound system is designed to receive valuable information generated by tissue.
Push-pull amplifiers have been used to reduce even order distortion in transmitted ultrasound waveforms.
FIG. 1
shows a push-pull amplifier
100
disclosed in U.S. Pat. No. 3,895,306. The push-pull amplifier
100
includes two class A cascode amplifiers
102
and
104
connected in a push-pull relationship with an output transformer
106
. Each cascode amplifier includes two transistors
108
,
112
and
110
,
114
. Two transistors
108
,
110
are connected in a common base configuration, and the other two transistors
112
,
114
operate as common emitter stages.
A network
116
of resistors
124
,
126
,
138
and a capacitor
118
provides a feedback loop for the push-pull amplifier
100
. The network
116
detects differences in the output of the two cascode amplifiers
102
,
104
as a function of the current at the center tap
132
of the transformer
106
. Any difference at the center tap
132
generates a voltage at the bases of the transistors
108
,
110
. The voltage is applied to the bases of the transistors
112
and
114
through the capacitor
118
to equalize the cascode amplifiers transfer function. The network
116
provides negative feedback, and the resistors
124
,
126
and
138
establish a DC operating voltage. Two input sources
128
,
130
provide a signal of the same amplitude but
180
° out of phase to the cascode amplifiers
102
,
104
. An inductor
134
isolates the feedback path for the network
116
from a supply voltage
136
.
Harmonic and/or subharmonic distortion produced by the two cascode amplifiers
102
and
104
are substantially identical when the fundamental output of the cascode amplifiers
102
,
104
are of the same amplitude. The signals output by the cascode amplifiers
102
,
104
are equal and opposite. Any even harmonics generated by the cascode amplifiers
102
,
104
are cancelled in the output transformer
106
. However, any difference in the fundamental waveforms generates a feedback signal. The feedback signal is in phase with respect to the branch with the lower output amplitude and out of phase for the branch with the higher output amplitude. The feedback signal tends to equalize the output of the two branches of the push-pull amplifier
100
.
This push-pull amplifier
100
is a Class A amplifier. Class A amplifiers have high quiescent power dissipation, resulting in low efficiency. Higher efficiency is achieved by Class B amplification. For Class B amplification, each path provides output for alternate time periods. The positive and negative portions of the bi-polar waveform are separated for amplification. Consequently, subharmonic and/or harmonic distortions in a Class B amplifier cannot be cancelled by the feedback signal. To reduce these distortions, the two paths are matched in gain and phase.
A high efficiency linear transmit circuit for ultrasound diagnostic imaging is disclosed in U.S. Pat. No. 6,104,673 and is shown in FIG.
2
. The transmit circuit
200
operates over a wide frequency bandwidth. The transmit circuit
200
includes a programmable waveform generator (PWG)
202
, two digital-to-analog converters
210
,
212
, a respective pair of current amplifiers or drivers
214
,
216
and an output amplifier
218
. The output amplifier includes a pair of transistors
222
and
224
, and a transformer
220
.
The PWG
202
generates separate unipolar waveforms representing positive and negative portions of the desired bi-polar ultrasound waveform. One unipolar waveform is output on bus
206
to a digital-to-analog converter
212
, and the other unipolar waveform is output on bus
208
to digital-to-analog converter
210
. A sign bit is output on line
204
to enable operation of the digital-to-analog converters
210
and
212
. The two transistors
222
and
224
are connected in a common gate configuration. An external voltage source
226
provides gate biasing. A center tap of the primary winding of the transformer
220
is tied to a high voltage power supply
228
. Since the transmit circuit
200
includes two open loop signal paths for respective positive and negative portions of the bi-polar transmit waveform, the components in each path should be closely matched to avoid harmonic and/or subharmonic distortion.
In order to transmit a waveform with a Gaussian envelope (FIG.
4
A), the current-output DACs
210
and
212
are intended to produce a pair of signals shown in
FIGS. 4B and 4C
, respectively. Having ideally matched signal paths, transmit signal, U(t), is combined as the algebraic difference of positive, U
+
(t), and negative, U
−
(t), portions in accordance with:
U
(
t
)=
U
+
(
t
)−
U
−
(
t
) (1)
Assume further that there is a gain mismatch between the two signal paths, denoted as &dgr;=&Dgr;G/G. In such a case, a “distorted” transmit signal, U
D
(t), yields
U
D
(
t
)=
U
(
t
)+&dgr;[
U
+
(
t
)+
U
−
(
t
)] (2)
The second term of Equation 2 will produce even order distortion products. For instance, given the waveform with the Gaussian envelope, the resulting spectrum expands as shown in FIG.
4
D.
In practice, the purity of a transmitted waveform is estimated with the Linear Response Rejection Ratio (LRRR). The LRRR is defined as the ratio of the energy under matched filters that are centered at fundamental and the second harmonic frequencies. For a Gaussian envelope, the LRRR can be easily computed. The obtained results (
FIG. 4E
) show that the prior art transmit cell
200
is quite sensitive to the gain mismatch. Using a dual DAC topology has a significant drawback since the level of gain mismatch is twice as much higher. This is particularly meaningful because DACs, even high-resolution DACs, may have gain error up to few % of the full scale.
BRIEF SUMMARY
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and system for generating a bi-polar ultrasound transmit waveform. A digital-to-analog converter with differential outputs is connected to two difference amplifiers by current splitters. The difference amplifiers provide current signals to the push-pull output amplifier for generating a desired bi-polar ultrasound waveform. A resistor connecting between the conventional outputs of two differential amplifiers specifies the voltage-to-current scaling factor for both amplifiers. Employing a single resistor, both positive and negative portions of a waveform are uniformly processed. The current splitters allow the digital-to-analog converter to have a low compliance voltage, such as 0.2 or 0.3 volts for an integrated converter, while the difference amplifiers operate at higher voltages for better signal-to-noise ratio performance.
In a first aspect, an ultrasound transmit circuit for generating a bi-polar waveform is provided. An output of a digital-to-analog converter connects with a first current splitter. An ultrasound transducer operatively connects to receive a signal responsive to the digital-to-analog converter.
In a second aspect, an ultrasound transmit circuit for generating a bi-polar waveform includes an output amplifier having first and second inputs. First and second difference amplifiers have respective first and second supply
Jaworski Francis J.
Siemens Medical Solutions USA , Inc.
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