Doppler energy-related parameters in an ultrasound imaging system

A method and apparatus for ultrasound imaging of Doppler energy-related parameters is described. An ultrasound imaging system includes a transducer for transmitting an ultrasound signal into a body and receiving a reflected ultrasound signal. The system determines the energy of the reflected signal from tissue within the body. Signal processing circuitry determines a Doppler intensity spectrum of the signal reflected from the tissue. The Doppler spectrum represents energy of the tissue-reflected signal as a function of Doppler frequency and time. Integration circuitry integrates the Doppler spectrum over Doppler frequency to determine the energy of the tissue-reflected signal as a function of time for display in strip mode. The system also determines an energy-velocity product. The integration circuitry may comprise circuitry for raising the Doppler intensity spectrum to a power m to generate a first spectral function and for raising a velocity-related function to a power n to generate a first velocity function. The integration circuitry integrates the product of the first spectral function and the first velocity function to determine the energy-velocity product function as a function of time.

BACKGROUND OF THE INVENTION 
In B-mode medical ultrasound imaging, an ultrasound scanner transmits an 
ultrasound signal into a patient and detects the intensity of the signal 
reflected from different depths. The scanner thus provides an image of 
structures within the body. Conventional Doppler imaging goes one step 
further and detects the velocity of moving tissues and fluids in the 
direction of the transmitted ultrasound signal. The conventional Doppler 
ultrasound machine employs two imaging modalities for measuring velocity: 
color flow imaging and strip Doppler imaging. Ultrasound scanners 
typically implement strip Doppler imaging in one of three different modes: 
PW or pulse wave imaging, HPRF or high pulse repetition frequency imaging, 
and CW or continuous wave imaging. According to the pulse Doppler 
technique, the ultrasound scanner places a range gate over the region of 
interest, and interrogates the region with multiple ultrasound pulses. The 
scanner samples the returning echoes and determines velocities using the 
Doppler principle. The velocity of the target is calculated and displayed 
based upon the sampled echoes. 
HPRF imaging is similar to PW imaging. The difference is that the imaging 
system places and receives echoes from multiple range gates within the 
body. In HPRF mode, echoes from prior transmissions reflected from deep 
structures within the body are received at the same time as echoes from 
shallow structures. The system samples the composite echo from all range 
gates, and displays the frequency spectrum (converted to velocity scale) 
of the composite echo. 
In continuous Doppler imaging, the scanner transmits and receives 
continuous ultrasound signals to and from the body. The scanner calculates 
the Doppler frequency spectrum according to continuous-time techniques. 
All three strip Doppler imaging modes employ the same display mechanism. 
The Doppler frequency spectrum is plotted along the ordinate against time 
along the abscissa. In practice, the frequency axis is converted into a 
velocity axis using the Doppler equation. Velocity of the target toward 
and away from the transducer is displayed along the positive and negative 
halves of the ordinate, respectively. The system modulates the brightness 
of the pixels in each frequency bin to display the energy of that bin. 
In addition to displaying the Doppler frequency spectrum graphically, the 
ultrasound system typically converts the Doppler frequency information to 
audio signals. The system distinguishes between the velocity of the target 
toward and away from the transducer by sending it to different speakers. 
Variance is an inherent attribute of all velocity estimates obtained from a 
Doppler system. The width of the Doppler spectrum is one measure of 
variance. The spectral width obtained using any of the methods outlined 
above is due to one or more of many factors, including flow 
characteristics, signal bandwidth, transit time effects and variance in 
parameter estimates. 
The sensitivity of the Doppler method in measuring the velocity of the 
target is best if the velocity is parallel to the ultrasound propagation 
direction. The velocity estimates tend to have a large variance if the 
angle between the propagation direction and the velocity is larger than 
sixty degrees. In those situations, velocity estimates are not good 
indicators of the presence of flow. 
Further, the amount of flow is not readily apparent from the conventional 
strip display mechanism discussed above. As an example, if there are two 
plug flows that are identical in velocity and different in flow volume, 
this difference would be difficult to perceive, especially if the 
difference in volume is small. In that instance, both flows would cause 
the same frequency or velocity band to be bright and the resulting small 
difference in brightness would be the only indication of the difference 
between the two flows. 
Audio processing of Doppler information also has the same disadvantages 
listed above. If the velocity estimates experience a wide variance, the 
audio output will be noisy. Furthermore, the strength of the audio output 
is no indication of the amount of flow in conventional systems. 
To overcome these problems, some ultrasound systems employ Doppler 
techniques to measure reflected energy or power. For example, U.S. Pat. 
No. 5,243,987, issued to Shiba, describes an ultrasound system for 
measuring the backscattering power of blood. Shiba employs high pass 
filtering and thresholding to eliminate reflections from slow-moving 
structures, such as tissue. The Shiba system displays the intensity of the 
echo signal by varying the brightness of a gray scale image or the hue of 
a color display, or as a three dimensional plot of Doppler spectrum 
against frequency and time. U.S. Pat. No. 5,285,788, issued to Arenson, 
and assigned to the assignee of the present invention, provides a Doppler 
tissue imaging (DTI) system that produces a color Doppler image of moving 
tissue representing estimates of velocity and reflected energy. 
U.S. Pat. No. 5,014,710, issued to Maslak, and assigned to the assignee of 
the present invention, describes a color Doppler imaging system that 
processes Doppler-shifted echoes into blood flow information, including 
velocity, variance and power. Further, U.S. patent application Ser. No. 
08/691,204, entitled "Imaging Modality Showing Energy and Velocity," and 
assigned to the assignee of the present invention, discloses a color 
Doppler imaging system having a mixed mode in which luminance is a 
function of the product of the velocity and the energy of the echo signal. 
All of the patents, applications and other references discussed herein are 
incorporated by reference herein. 
One disadvantage of color display techniques is that the display frame rate 
is relatively slow with respect to the cardiac cycle. Further, because of 
the relatively low number of samples used to compute Doppler shift, color 
systems exhibit a relatively poor signal-to-noise ratio. 
The present invention overcomes these disadvantages by providing more 
flexibility in the diagnostic parameters available in ultrasound machines 
implementing other display modes. 
Further, the invention has potential advantages over another imaging 
technique--integrated backscatter imaging. This imaging technique is 
generally used for estimating the echo-density over a region of 
integration. B-mode information is integrated over a region and displayed 
instead of the usual echo intensity. The principle behind this technique 
is the assumption that the energy reflected from a region is related to 
the density of the reflecting medium at that location. A corollary mode 
currently exists whereby the integrated backscatter information can be 
obtained via Doppler processing through the use of Doppler tissue imaging 
(DTI) in the energy mode. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for calculating 
Doppler energy-related parameters in an ultrasound imaging system. An 
ultrasound imaging system includes a transducer for transmitting an 
ultrasound signal into a body and receiving a reflected ultrasound signal. 
The system determines the energy of the reflected signal from structures, 
such as tissue, within the body. Signal processing circuitry determines a 
Doppler squared magnitude (intensity) spectrum of the signal reflected 
from the structures. The Doppler spectrum represents energy of the 
tissue-reflected signal as a function of Doppler frequency and time. 
Integration circuitry integrates the Doppler spectrum over Doppler 
frequency to determine the energy of the tissue-reflected signal as a 
function of time for display in strip mode. 
Alternatively, the energy may be calculated in the time domain by employing 
squaring circuitry to determine the squared amplitude of the 
tissue-reflected signal. Integration circuitry integrates the squared 
amplitude in the time-domain. 
The present invention also determines an energy-velocity product. The 
integration circuitry frequency-integrates the product of the Doppler 
squared magnitude spectrum and a velocity-related function to determine 
the energy-velocity product as a function of time for display in strip 
mode. The velocity-related function is a function of the Doppler 
frequency. The integration circuitry may comprise circuitry for raising 
the Doppler squared magnitude spectrum to a power m to generate a first 
spectral function and for raising the velocity-related function to a power 
n to generate a first velocity function. The integration circuitry 
integrates the product of the first spectral function and the first 
velocity function to determine the energy-velocity product function as a 
function of time. In one embodiment, m may be set to 1-n, where m and n 
are less than 1. The values of m and n may be set by a user. 
The system may also include a filter for filtering out spectral components 
reflected from tissue, and a thresholding circuit for eliminating spectral 
components having an intensity above an upper threshold intensity so as to 
eliminate spectral components from tissue. Further, the system may include 
thresholding circuitry for eliminating spectral components having an 
intensity below a low threshold intensity so as to eliminate spectral 
components from blood. 
The system may further include audio processing circuitry for processing an 
audio output related to Doppler frequency. Tone generation circuitry 
generates a tone related to an energy-related parameter, i.e., energy or 
energy-velocity. Combining circuitry combines the tone with the audio 
output. In one embodiment, the tone generation circuitry modulates the 
frequency of the tone based upon the energy-related parameter. In another 
embodiment, the tone generation circuitry modulates the amplitude of the 
tone based upon the energy-related parameter. In both embodiments, the 
combining circuitry adds the tone to the audio output. 
In yet another embodiment, the audio processing circuitry may comprise gain 
control circuitry for controlling the gain of the audio output in response 
to the energy-related parameter. 
NOTATION AND NOMENCLATURE 
While the term "velocity" is used in this application, it should be 
understood that the velocity can be derived from the Doppler frequency 
shift by use of the well-known Doppler equation: 
EQU v=f.sub.d c/2f.sub.o cos.theta. 
where f.sub.d is the Doppler frequency shift, c is the speed of sound in 
the medium, f.sub.o is the transmitted frequency, and .theta. is the 
Doppler angle (the angle subtended by the ultrasound beam and the 
direction of flow). Because of the relationships among velocity, 
frequency, and wavelength, the term "velocity-related function" or 
"velocity-related parameter" as used in this application, will refer to 
velocity, frequency, or wavelength. Similarly, because of the well-known 
relationships among energy, power, and amplitude, the term "energy-related 
function" or "energy-related parameter" will refer to energy, power, or 
amplitude. The terms "energy-related function" or "energy-related 
parameter" will also refer to the energy-velocity product function 
discussed below. Further, those skilled in the art will recognize that the 
term "circuitry" as used herein may refer not only to hardware, but to 
software and other implementations as well.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a method and apparatus for obtaining Doppler 
energy-related parameters in an ultrasound imaging system. In the 
following description, numerous details are set forth in order to enable a 
thorough understanding of the present invention. However, it will be 
understood by those of ordinary skill in the art that these specific 
details are not required in order to practice the invention. Further, 
well-known elements, devices, process steps and the like are not set forth 
in detail in order to avoid obscuring the present invention. 
FIG. 1 is a block diagram illustrating an ultrasound system 100 for 
implementing the present invention. A transmitter 102 excites an acoustic 
transducer 104, which propagates ultrasonic energy bursts into a body. The 
transducer 104 typically comprises a phased array of ultrasound transducer 
elements. The transducer 104 converts returning ultrasound signals into 
electrical signals. A receiver/beamformer 106 amplifies and focuses the 
electrical signals. A quadrature demodulator 108 downshifts the frequency 
of the focused echo signal by the transducer frequency in order to extract 
the Doppler frequency shift separated into in-phase and quadrature 
components. Those skilled in the art will recognize that the signal path 
may include a high pass clutter removal filter 110 for any Doppler imaging 
mode for imaging blood. As is known in the art, the location of the 
clutter filter depends upon the design employed. 
An analog-to-digital converter (A/D) 112 digitizes the analog demodulated 
signals. Gating circuitry 114 (for PW and HPRF modes) gates the digitized 
output so that only samples from the region of interest are passed on to 
digital signal processing (DSP) circuitry 116. A range gate integrator in 
the gating circuitry integrates the demodulated sampled echoes arising 
within each gate to obtain an integrated echo sample for each gate, as is 
well known in the art. Such integration reduces the effects of noise. The 
resulting integrated echo samples are passed on to the DSP circuitry 116 
for calculation of the frequency spectrum. For example, 128 consecutive 
integrated echo samples received from the same gate location may be used 
to obtain the Doppler spectrum at one point in space. 
According to the present invention, the DSP circuitry 116 processes the 
sampled demodulated signals to derive energy-related parameters and 
conventional velocity-related parameters. These parameters are, in turn, 
passed to a video processor 118, which converts the Doppler information to 
a raster format for a conventional video display 120. The video processor 
118 of the present invention converts the information to strip Doppler 
format in which the energy-related parameter is plotted against the time 
axis. 
The DSP 116 also passes the demodulated signals to the audio processing 
circuitry 122, which combines these signals and separates the forward and 
reverse components of the flow that represent motion of the reflectors 
toward and away from the transducer, as is well known in the art. Each 
component is sent to one of two speakers 124. 
The DSP 116 also passes signals to the audio processor that are 
proportional to the energy-related parameters. These signals are combined 
with the conventional signals in the audio processor in a manner described 
below. 
FIG. 2 is a block diagram illustrating the DSP circuitry 116 of the present 
invention. For simplicity, the input Doppler signal to the DSP 116 is 
represented as the continuous time signal A(t)e.sup.i2.pi.ft, having 
time-varying amplitude A(t) and frequency f. Those skilled in the art will 
recognize that the actual signal is in discrete-time form and is 
band-limited, and actually comprises a variety of frequency components of 
varying amplitudes. 
Spectrum analyzer circuitry 200 calculates the Doppler energy spectrum 
(energy spectral density) of the Doppler frequency shift signal. The 
spectrum is referred to as an "energy spectral density" herein, rather 
than the more familiar "power spectral density" because the returning 
sampled time-limited echoes have finite energy. In any event, the terms 
are frequently used interchangeably by those skilled in the art. 
Thresholding circuitry 202 may be included to eliminate or pass spectral 
components based upon their amplitude (energy). For example, to enhance 
imaging of blood, an upper energy threshold may be used to eliminate 
high-energy tissue reflections. For tissue imaging, a low-energy threshold 
may be used to eliminate low-energy echoes from blood. 
A parameter estimator 204 of the present invention employs the energy 
density spectrum to calculate a variety of energy-related parameters that 
are based on energy alone or a combination of energy and velocity. The 
energy-related function based on the energy alone can be written as 
EQU E(t)=.intg.S(f,t)df 
where S(f,t) is the Doppler energy spectrum (i.e., the squared magnitude or 
intensity spectrum), E(t) is the total energy of the Doppler intensity 
spectrum S(f,t) at a time t, and f is the Doppler frequency. 
In PW mode, this energy is also a measure of the integrated backscatter 
energy within the range-gate region of interest. In conventional 
integrated backscatter energy calculations, the B-mode intensity images 
are integrated over the entire insonified region to obtain the integrated 
backscatter energy. The principle behind this technique is the assumption 
that the integrated backscatter energy is an estimate of density of the 
reflecting medium within the region. In PW mode, the present invention 
inherently calculates the integrated energy of each returning echo over a 
gated region using Doppler techniques. Because the target is interrogated 
multiple times (e.g., 128), multiple energy samples are collected. The 
energy calculated by this invention is thus the total energy over the 
interrogation time period. Further, although DTI energy mode imaging also 
integrates over a range gate, DTI energy mode uses color processing and 
display techniques. In contrast, the present invention calculates 
parameters for strip display. 
The conventional B-mode calculation of integrated backscatter energy 
requires complex computations, thereby requiring complicated hardware and 
software to perform the computations in real time. The DTI energy mode 
method of obtaining integrated backscatter energy exhibits the slow 
display frame rate and relatively low signal-to-noise ratio of color 
display systems. 
The present invention overcomes these disadvantages. Unlike B-mode systems, 
the present invention determines backscatter energy for a single region of 
interest. This determination is much simpler and more appropriate for 
clinical applications than B-mode systems that compute energy over the 
entire insonified region. Further, the calculation of integrated 
backscatter energy for strip display is much simpler and less costly than 
DTI color mode, and exhibits much better temporal resolution. 
The energy-related function based upon a combination of energy and velocity 
may be expressed as 
EQU EV(t)=K .intg.S.sup.m (f,t)f.sup.n df 
where EV(t) is an energy-velocity product function, and m and n are 
constants that are either preprogrammed or controllable by a user through 
a suitable, conventional user interface. K is a constant of 
proportionality to convert the energy-velocity product function from 
dependence upon frequency to dependence upon velocity, as derived from the 
Doppler equation. Of course, the above expressions are actually 
implemented in discrete form by digital circuitry of the invention. 
The constants m and n can be selected to take on any values that have 
diagnostic utility. In a simple case, m=n=1. For example, setting m=1-n, 
where m and n are less than 1, may be useful for balancing the effects of 
the energy and velocity measurements. The user may wish to emphasize the 
Doppler energy spectrum parameter in the energy-velocity product or 
measure energy alone if the product parameter is only indicating 
relatively small values when it is weighted heavily in favor of the 
velocity. This may occur where the angle between the direction of 
propagation of the ultrasound signal and the direction of movement of the 
scatterer is large, e.g., greater than 60 degrees. Alternatively, 
slow-moving tissue would provide a similar indication. Further, a high 
volume, low-velocity jet of blood may be better diagnosed with energy 
alone or an energy-velocity product weighted in favor of the energy 
spectrum. 
Conversely, a thin, high-velocity jet of blood may be better detected with 
a product parameter emphasizing the velocity component. The advantage of 
using the product parameter in this instance instead of velocity alone is 
that the product parameter may give some indication of the volume of the 
scatterers reflecting the ultrasonic energy. 
In general, a relatively high or low product parameter in one region 
compared to that measured in an adjacent region within the body may 
provide an initial indication of medical problems. Measurements from 
nearby regions should be compared relative to one another to account for 
the effects of angle on the measurements. 
FIG. 4 illustrates an example of a Doppler strip display of a flow that 
increases in velocity from point A to point B, stays constant in velocity 
from B to C and decreases in velocity from C to D. No flow exists from D 
to E. This cycle repeats. The shaded region indicates that the flow 
contains its spectral components within the envelope ABCDE. The energy of 
the flow may look like the dotted curve 12345. From 1 to 2 the energy 
increases, indicating onset of flow. From 2 to 3 the energy remains 
constant, indicating constant flow. From 3 to 4 energy decreases, 
indicating decreasing flow. Finally, no flow is indicated from 4 to 5. 
The embodiment described above computes energy and velocity-based functions 
in the frequency domain. Those skilled in the art will recognize that the 
DSP circuitry can employ equivalent time-domain techniques to calculate 
energy alone, thereby bypassing the spectrum analyzer and parameter 
estimator for the energy calculation. For example, Rayleigh's or 
Parseval's theorem illustrates the equivalence of calculating energy in 
the time and frequency domains. 
Accordingly, referring to FIG. 2, the DSP 116 includes an optional 
time-domain energy calculator comprising a squaring circuit 206 and 
integrator (accumulator) 208 for integrating the squared amplitude of the 
Doppler signal over time in order to obtain Doppler energy. The 
time-domain path may also include thresholding circuitry 210 to eliminate 
or pass time samples based upon their amplitude. Those skilled in the art 
will recognize that the squaring circuit may be a simple multiplier or 
employ more sophisticated rectification and filtering techniques known in 
the art to obtain energy as a function of time. Also, this time-domain 
energy calculation circuitry can be located elsewhere in the system, as 
would be easily appreciated by those skilled in the art. Further 
techniques for calculating Doppler parameters in the time domain are 
described in U.S. Pat. No. 4,928,698, issued to Bonnefous and Bonnefous, 
et al., "Time Domain Formulation of Pulse-Doppler Ultrasound and Blood 
Velocity Estimation by Cross-Correlation," Ultrasonic Imaging B, 73-85 
(1986). Those references are incorporated by reference herein. 
In strip Doppler mode, the spectrum analyzer 200 is employed to compute the 
distribution of Doppler frequencies at a particular time t. This 
distribution is necessary to compute and display the distribution of 
velocities in strip mode. Further, the distribution of frequencies is 
necessary for calculation of the energy-velocity product according to the 
present invention. That calculation requires multiplication of the squared 
magnitude spectrum at each frequency by the frequency component itself, 
followed by integration over the range of frequencies. Other Doppler 
display modes, such as the color energy velocity mode described in U.S. 
patent application Ser. No. 08/367,064 do not contemplate calculating an 
energy-velocity product in this manner. 
As mentioned above, these energy-related parameters are provided to the 
video processor 118 for display in strip Doppler mode, i.e., the 
parameters are displayed on the y-axis against time on the x-axis. Strip 
Doppler has the advantage of a higher frame rate than color flow mode. 
Parameters are also provided to the audio processor 122 to provide an 
audio indication of the echo signal. FIG. 3 is a block diagram 
illustrating one embodiment of the audio processor 122 of the present 
invention. A gain control circuit 300 receives the Doppler frequency 
signal and adjusts the gain of the signal based upon the energy-related 
parameter E or EV received from the DSP circuitry 116. The output is 
converted back to an analog signal through a digital-to-analog converter 
(D/A) 302. The analog signal is separated into forward and reverse 
components by a mixer/separation filter block 304 that provides the 
forward and reverse components to the speakers 124. 
FIG. 5 illustrates an alternative embodiment of the audio processor 122 of 
the present invention. A D/A 400 converts the Doppler frequency signal 
into analog form. Mixer/separation filters 402 separate the forward and 
reverse components, as before. In this embodiment, the energy-related 
parameter controls the frequency and/or gain of a tone that is added to 
the conventional audio path. The audio processor 122 includes a D/A 404 
for converting the energy-related parameter to analog form in order to 
control the frequency of a tone produced by a voltage controlled 
oscillator (VCO) 406. The tone is fed into a gain control circuit 408, 
which may be controlled by the energy-related parameter. The output of the 
gain stage is added to the forward and reverse audio components. 
In this design, through preprogramming or user control, the gain or the 
frequency of the added tone can be independently controlled, or 
alternatively controlled at the same time. If only gain is adjusted, then 
the DSP 116 supplies the energy-related parameter to the gain control 408 
while holding the frequency of the VCO 406 constant by supplying a 
constant signal, instead of an energy-related parameter, to the D/A 404. 
Alternatively, the D/A 404 and VCO 406 may be eliminated. Conversely, if 
only frequency is to be modulated, then the DSP 116 will output a constant 
signal, rather than the energy-related parameter, to the control input of 
the gain control circuit 408. Alternatively, the gain control may be 
eliminated. 
The audio processing circuitry 122 of the invention provides a number of 
advantages over conventional systems. For example, sometimes it is 
difficult to hear the Doppler information inherent in small, low-velocity 
jets due to the noise of the velocity measurement. In that case, an energy 
or energy-velocity measurement may provide a better diagnostic indication 
as the jet volume flow increases and decreases. Further, the use of 
energy-related information to control the gain and/or frequency of an 
added tone or the gain of the conventional audio signal provides an 
intuitive indication of the strength of the echo signal due to the volume 
of the scanned medium. 
Although the invention has been described in conjunction with particular 
embodiments, it will be appreciated that various modifications and 
alterations may be made by those skilled in the art without departing from 
the spirit and scope of the invention. For example, those skilled in the 
art will recognize that the techniques described herein can easily be 
modified to apply to continuous and high pulse repetition frequency 
imaging modes. The invention is not to be limited by the foregoing 
illustrative details, but rather is to be defined by the appended claims.