Method and apparatus for echo-ultrasound imaging using compound AM-FM detection with increased dynamic range

A Compound echo-ultrasound imaging system in which the intensity of pixels in an image is a nonlinear function of the amplitude and frequency of echo signals. The dynamic range of a synchronous FM detector in the system is increased by effectively filtering from the echo signals all instantaneous frequencies which would produce intensity levels greater than a corresponding signal of equal amplitude at the carrier frequency of the ultrasound pulses. Filtering may be effectively accomplished by setting the Corner Frequency of the synchronous detector to the carrier frequency of the transmitted pulses.

The invention relates to apparatus which utilizes ultrasound pulse-echo 
techniques to image the internal structures of a body. More specifically, 
the invention relates to apparatus and methods which increase the dynamic 
range of an imaging system in which the brightness of pixels is modulated 
by a nonlinear function of both the amplitude and the frequency deviation 
of echo signals. 
BACKGROUND OF THE INVENTION 
Ultrasound pulse-echo imaging has become an important modality for medical 
diagnosis. Pulses of ultrasound energy are produced in a transducer and 
directed into a body. The energy is scattered from organ boundaries and 
other impedance discontinuities within the body; generating echoes which 
are detected with a transducer (which may be the same transducer used for 
transmission) to produce electrical signals which are then processed to 
form an image of the internal body structures. Most ultrasound pulse echo 
imaging systems of the prior art generate images from information which is 
extracted from the AM envelope of the echo signals. Such systems usually 
make use of a peak detector to extract a video signal from the echoes and 
generate a display by modulating the intensity of each pixel as a function 
of the amplitude of a corresponding portion of the video signal. Regions 
of the body which return strong (i.e.: high amplitude) echoes, for example 
organ boundaries, will thus be depicted as bright areas in the image 
whereas regions which return low amplitude echoes, for example homogeneous 
regions within the liver, will be depicted as darker areas in the image. 
This apparatus is more completely described, for example, in Medical 
Ultrasound Imaging: An Overview Of Principles And Instrumentation, J. F. 
Havlice and J. C. Taenzer; Proceedings of the IEEE, Vol. 67, No. 4, April 
1979, pages 620-640, which is incorporated herein, by reference, as 
background material. 
In B-scan imaging, an ultrasound transducer is translated and/or angulated 
along the surface of a body undergoing examination. A two-dimensional 
image is generated by plotting the detected characteristic of echoes at an 
image point which corresponds to the coordinates of the scatterer which 
produce the echoes. The depth coordinate of the scatterer is determined by 
measuring the time delay between pulse transmission and the receipt of the 
corresponding echo signal. The lateral coordinate of the scatterer is 
determined by measuring the lateral position and/or angulation of the 
transducer. 
More recently, Dr. Leonard Ferrari has described a technique for producing 
images utilizing information which is contained in the FM envelope of an 
ultrasound pulse-echo signal (Dr. Ferrari's U.S. Pat. No. 4,543,826 is 
incorporated herein by reference as background material.) This technique 
maps the instantaneous phase or frequency of an echo signal into intensity 
levels in an image. For example, regions of the body which return echoes 
with higher instantaneous frequencies may be displayed as bright areas 
while regions of the body which return echoes with lower frequencies may 
be displayed as darker areas. A squelch circuit may further be provided 
which turns-off the FM detector and displays a neutral intensity level in 
the event that the amplitude of the echo signal is too low for FM 
detection. 
Dr. Ferrari's patent application describes a system in which the intensity 
of regions in an image is normally independent of the amplitude of the 
echo signal. More recently, imaging systems have been developed wherein 
the intensity of pixels is modulated as a nonlinear function of both the 
amplitude and the frequency deviation of the echo signal (such systems are 
hereafter referred to as "Compound Systems"). 
Compound Systems appear to have the ability to delineate diseased regions 
within certain organs (for example, the liver) which were not heretofore 
discernable in systems which used either pure AM or pure FM detection. 
However, prior art Compound Systems suffered from low dynamic video range 
and, in some cases, when they were adjusted to display diseased areas 
within a particular organ (for example, the liver) they could not, without 
readjustment, clearly image other structures or organs. 
SUMMARY OF THE INVENTION 
The present invention provides methods and apparatus for use with Compound 
Systems which increase the dynamic video range and thus allow delineation 
of diseased regions within a particular organ while at the same time 
displaying other structures and organs. 
The object of the invention is achieved by effectively filtering from echo 
signals those instantaneous frequency components which lie above the 
compound AM-FM detection curve. In the case of a synchronous FM detector 
circuit with squelch, this filtering may be effectively achieved by 
shifting the Corner Frequency of the detector to the nominal carrier 
frequency of the transmitted ultrasound pulses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an echo-ultrasound imaging system of the present 
invention. A transmitter 30 produces a stream of periodic radio frequency 
(RF) electrical pulses having a carrier frequency f.sub.o which energize a 
transducer 10 via a T-R switch 40. The transducer 10 converts the 
electrical pulses into a beam of ultrasound energy 15 which is projected 
from the transducer into a body 20. The ultrasound energy is scattered 
within the body, for example, from the boundaries of organs 22 and 24, and 
portions of the energy are returned, in the form of echoes, to the 
transducer 10 where they are converted into electrical signals and 
transmitted, via the T-R switch 40 to a receiver 50. The receiver detects 
the echo signals producing a video output signal having an amplitude which 
is a nonlinear function of both the instantaneous amplitude of the echo 
signals and the instantaneous frequency of the echo signals. The output of 
the receiver modulates the intensity of pixels in a display 70, typically 
a CRT display. 
The beam of ultrasound pulses is displaced and/or angulated with respect to 
the body 20 by a scanner 15 which is mechanically and/or electrically 
coupled to the transducer 10. In a typical sector-scan system, the scanner 
may be mechanically coupled to the transducer to sweep the ultrasound beam 
through an arc with respect to the body and its organs. The methods and 
apparatus of the present invention are, however, equally applicable to 
linear or phased array systems or to pantograph type B-scan systems. 
Signals from the scanner 15 are transmitted to the display 70 via a 
control circuit 60 which is likewise coupled to the transmitter and 
receiver and which controls the sweep circuitry of the display 70 so that 
the video signals from the receiver 50 modulate the intensity of display 
pixels at positions in the image which correspond to locations in the body 
of the structures which produced the corresponding echo. 
In the prior art, it was found that Compound Systems of the type described, 
when properly calibrated for a liver 22, were particularly useful for 
discerning diseased areas 26 therein, but that, because of the limited 
dynamic range of the detection system, when so calibrated, the system 
might not be suitable for producing a visible image of another organ (for 
example, 24) in the image. 
FIG. 2 is a typical synchronous FM detector stage which is suitable for use 
in the receiver of a Compound System. Echo signals RF from the transducer 
and T-R switch are mixed with a SQUELCH signal in an adder 110. The 
SQUELCH signal is typically a radio frequency sine wave at the nominal 
carrier frequency f.sub.o of the pulses which are transmitted into the 
body. The output of the adder 110 is applied to a hard limiter 115. The 
output of the hard limiter 115 is applied directly to one input of an 
EXCLUSIVE OR gate 125. The output of the limiter 115 is also applied, via 
a delay circuit 120 (having a delay time D) to the second input of the 
EXCLUSIVE OR gate 125. The output of the EXCLUSIVE OR gate 125 is 
transmitted to the input of the display via a low-pass filter 130. The 
low-pass filter is designed to remove radio frequency components from the 
output of the EXCLUSIVE OR gate and to allow only video signals to pass to 
the display. 
The circuit of FIG. 2 operates as a frequency deviation detector whenever 
the amplitude of the signal at the input RF is substantially greater than 
the amplitude of the SQUELCH signal. FIG. 3 is a plot of the output of the 
low-pass filter 130 under these conditions as a function of the frequency 
of the input RF. It may be seen that the output of the low-pass filter 
rises with increasing input frequency until the input frequency is 1/2D. 
The output of the low pass filter then drops linearly as the frequency 
rises from 1/2D to 1/D and alternates thereafter in sawtooth fashion. 
Hereinafter, and in the claims that follow, the frequency 1/2D is termed 
the "Corner Frequency" of this detector. 
In FM echo-ultrasound systems and in Compound Systems of the prior art, the 
value of the delay circuit D was usually chosen to be 1/4f.sub.o where 
f.sub.o is the nominal carrier frequency of the transmitted ultrasound 
pulses. Echoes which were returned without any frequency shift thus 
produced display shades of neutral gray while echoes which contained 
positive or negative frequency shifts produced lighter and darker shades 
in the display. 
FIG. 4 is an analog implementation of a synchronous FM detector stage for 
use in Compound Systems. The echo signal RF and SQUELCH are combined in an 
adder 110 in the same manner as described with respect to FIG. 2. The 
output of the adder 110 is applied to a hard limiter 115 which produces a 
balanced output. The balanced output is applied to one input x of an 
analog multiplier 140. The output of the hard limiter 115 is further 
applied to a RC delay network comprising resistors 150 and 150a and 
capacitors 155 and 155a to derive a shifted signal between nodes P and Q 
which is applied to the y input of the analog multiplier 140. The output 
of the analog multiplier 140 is applied to a low-pass filter 130 in the 
same manner as in FIG. 2 and the output of the low-pass filter is applied 
to the input of the display. 
The circuit of FIG. 4 functions as a frequency deviation detector whenever 
the amplitude of the signal at the input RF is substantially greater than 
the amplitude of the SQUELCH signal. FIG. 5 illustrates the transfer 
function of this circuit under these conditions and is a plot of the 
amplitude of the output of the low-pass filter as a function of the 
instantaneous frequency of the input RF. It may be seen that the amplitude 
of the output rises monotonically with increasing input frequency up to a 
frequency of approximately 1/2T where T=RC is the time constant of the RC 
delay network. At input frequencies above approximately 1/2T, the output 
of the detector saturates and has a substantially constant value. In the 
circuit of FIG. 4, 1/2T is the Corner Frequency which corresponds to the 
Corner Frequency 1/2D in the circuit in FIG. 2. 
When circuits of the type illustrated in FIG. 4 were used in FM systems or 
Compound Systems of the prior art, the RC time constant of the delay 
network was usually chosen to be equal to 1/4f.sub.o, so that echoes which 
were returned without any frequency shift produced neutral gray tones in 
the display. 
The output signal from the circuits of FIG. 2 and FIG. 4 is a nonlinear 
function of both the instantaneous amplitude and instantaneous frequency 
of the input signal RF when the instantaneous amplitude of the signal RF 
is close to the amplitude of the SQUELCH signal. 
FIG. 6 illustrates the nonlinear response of the output of the circuit of 
FIG. 4 in response to variations of the input signal amplitude. The 
instantaneous frequency of a complex waveform may be calculated from the 
length of time between successive zero crossings. FIG. 6 shows the 
instantaneous frequencies of the sum of a low-frequency SQUELCH of fixed 
amplitude and a higher-frequency RF signal of fixed frequency and whose 
amplitude is linearly increasing in time. The output of the systems of 
FIG. 4 or FIG. 2 are equal to the waveform of FIG. 6 averaged by a 
low-pass filter. This averaged output is shown by the dotted curve of FIG. 
6. 
The plot illustrates the operation of prior art synchronous detectors in 
Compound Systems where the nominal RF frequency of the ultrasound pulses 
was set at one half of the Corner Frequency of the detector. 
As can be seen from the plot, the circuit is essentially insensitive to 
input variations when the instantaneous signal amplitude is less than 
approximately one-half the SQUELCH amplitude. The output only changes over 
a narrow range of instantaneous input amplitudes, from approximately one 
half the SQUELCH amplitude to the point where the signal amplitude is 
approximately equal to the SQUELCH amplitude (as depicted by the dashed 
line). For signal amplitudes which are higher than the SQUELCH amplitude, 
the average output amplitude remains constant even thought the 
instantaneous frequencies cause wide variations. 
In accordance with the present invention, the dynamic range of the 
synchronous detector in a Compound System can be greatly increased by 
effectively filtering the signal to eliminate all instantaneous frequency 
variations which produce output signal excursions above the dashed average 
output line in FIG. 6. In a Compound System, this is equivalent to 
eliminating all frequency components which would produce a pixel 
brightness greater than a corresponding signal of equal amplitude at 
f.sub.o. 
FIG. 7 illustrates the operation of the detector of FIG. 4 in which the RC 
network time constants have been adjusted to effectively eliminate 
detection of all frequency components which produce outputs which lie 
above the dashed line in FIG. 6. Except for the network time constant 
values, FIG. 7 corresponds to FIG. 6. It may be seen that the 
amplitude-sensitive dynamic range of the circuit has been extended far 
beyond the earlier saturation amplitude point (at which the signal 
amplitude equaled the SQUELCH amplitude) and that the circuit now 
saturates in the range where the signal amplitude is between three and 
four times the SQUELCH amplitude. 
Filtering to effectively eliminate all frequencies which produce responses 
above the average amplitude output line of FIG. 6 may be accomplished by 
choosing appropriate values for the delay time D for the circuit of FIG. 2 
or the network RC time constant T of FIG. 4. Maximum dynamic range 
improvement is obtained when the Corner Frequency of either detector is 
set equal to the nominal carrier frequency f.sub.0 of the transmitted RF 
pulses. That is when D=1/2F.sub.0 or T=1/2f.sub.0. When the delay or RC 
time constant is much smaller than these values, the dynamic range 
approaches a minimum. Delays in the intermediate range produce 
intermediate levels of dynamic range. 
FIG. 8 shows the inverted output of a simulated prior art synchronous 
detector of FIG. 2 with the delay time D set to 50 nanoseconds (Corner 
Frequency=10 MHz) and the pulse carrier frequency f.sub.o =4 MHz. The 
limited dynamic range is evidenced by the fact that the curve saturates 
near a signal to SQUELCH ratio of 1.0. FIG. 9 shows the same simulated 
curve with the delay D=100 nanoseconds (corner frequency=5 MHz). It can be 
seen that the Compound System signal has a wide dynamic range and does not 
saturate until the signal to SQUELCH amplitude ratio is between 3 and 4.