Gated ultrasound imaging apparatus and method

An ultrasound imaging system is programmed to acquire first ultrasonic image frames intermittently. These first frames, typically triggered frames synchronized with a selected portion of an ECG cycle, are optimized for high image quality of a contrast agent included in the tissue. The imaging system automatically acquires second ultrasonic image frames between at least some of the first frames. The second image frames are typically locator frames which are optimized for reduced degradation of the contrast agent. More of the second frames are acquired per unit time than first frames, and both the first and second frames are displayed, either superimposed over one another or in side-by-side relationship. In this way the user is provided with substantially continuous transducer locating information, yet contrast agent destruction between acquisitions of the first, triggered frames is reduced or eliminated.

BACKGROUND OF THE INVENTION 
The present invention relates generally to ultrasound imaging systems, and 
more specifically to ultrasound imaging systems which provide improved 
visualization of contrast agents. 
Ultrasound imaging systems usually are operated in a fashion to produce 
real-time moving images of a subject being scanned. These moving images 
are acquired as discrete static images, but at a high enough frame rate 
(typically 20-30 frames/sec) to present the illusion of a continuously 
moving image. Commercial ultrasound systems have also included triggered 
acquisition modes. In these modes, an ultrasound image frame is acquired 
at a specified point in each cardiac cycle, as measured for example by a 
delay from the R-wave of an ECG waveform. Typically the ultrasound system 
is quiescent between acquisition of successive triggered frames, neither 
transmitting nor receiving, and the system display is static, showing the 
last triggered frame. For example, an ultrasound system can be programmed 
to generate a triggered frame at 100 ms after each R-wave. At typical 
human heart rates of 60-120 beats/minute, this results in the image being 
updated at 1-2 frames/second, rather than the 20 or more frames/sec that 
might be possible if scanning were continuous. In other variants, 
triggered frames may be acquired only on selected beats (e.g., 150 ms 
after every 3rd R-wave), or multiple frames may be acquired per beat 
(e.g., 100, 150, and 250 ms after every 2nd R-wave). 
One application where gated imaging modes are useful is imaging of 
ultrasound contrast agents. Contrast agents are injected into the 
bloodstream to increase the brightness of blood and blood-perfused 
tissues. However, these contrast agents (which are typically composed of 
stabilized gas microbubbles a few microns in diameter) are fragile and 
easily degraded (destroyed or altered) by the ultrasound pulses used to 
image them. A first ultrasound frame may show the contrast agent well, but 
subsequent frames often show less and less signal as the contrast agent is 
destroyed. 
Thomas Porter and other researchers have demonstrated that gated imaging 
may be used to advantage where bubble destruction is an issue. A single 
image frame is acquired every cardiac cycle (or every few cardiac cycles). 
During the interval between frame acquisition, while the ultrasound 
transmitters are inactive, fresh contrast agent circulates into the 
tissues and vessels being imaged. Thus, if the interval between successive 
triggered frames is sufficiently long, a second acquired frame presents a 
signal that is as strong as the first. Some researchers have proposed that 
bubbles are not destroyed by ultrasound, but are altered in some way; 
during the interval between frames, the bubbles may recover in some way. 
In this case, the effect is the same: after an interval without 
transmission, the image returns to its initial brightness. 
Another method used to reduce bubble destruction is to transmit at a 
reduced ultrasound intensity. This reduces bubble destruction at the cost 
of a reduced signal-to-noise ratio. 
Another property of contrast agents that should be mentioned is non-linear 
scattering. Many contrast agents, when insonified with an acoustic pulse 
centered at one frequency, reflect or radiate ultrasound containing 
components at harmonics of the insonifying frequency as well as at the 
insonifying frequency. This property has been used to advantage in 
distinguishing contrast agents from normal tissues, which do not tend to 
scatter non-linearly. U.S. Pat. No. 5,255,683 (Monaghan), U.S. Pat. No. 
5,410,516 (Uhlendorf), U.S. Pat. No. 5,456,257 (Johnson) and U.S. Pat. No. 
5,577,505 (Brock-Fisher) disclose techniques for imaging non-linear 
scattering from tissues, and several ultrasound manufacturers are known to 
be developing second harmonic imaging capability (that is, forming an 
image from energy scattered at a harmonic multiple of twice the 
insonifying frequency). Harmonic imaging suffers from the same bubble 
destruction effects as does conventional fundamental imaging, and the same 
techniques of gating and reduced transmit power may be used. 
Conventional gated imaging techniques require a user to hold an ultrasound 
probe in a fixed location for as long as several heartbeats without any 
visual feedback from the image, which statically shows the previously 
acquired frame. Furthermore, static images of dynamic structures such as 
the heart may be difficult to interpret and may contain less diagnostic 
information than moving images. Reducing transmit power reduces bubble 
destruction, but makes the image noisier and ultimately limits the 
penetration depth (the maximum depth that may successfully be scanned). 
Transmit power reductions of 15 dB or more (below the maximum attainable 
by a typical diagnostic system under FDA limitations) may be required in 
order to avoid destroying bubbles when imaging perfusion of tissues. 
Transmit power reduction may be particularly disadvantageous in harmonic 
imaging. The harmonic component of the scattered and received signal is 
typically much smaller than the fundamental component (only a small 
fraction of the incident acoustic energy is converted to higher harmonic 
frequencies), while the noise floor remains roughly constant. Further, 
because of the non-linearity inherent in generating higher-order 
harmonics, a given reduction in transmit power results in an even greater 
reduction in the harmonic signal strength. For example, a 3 dB transmit 
power reduction may result in roughly a 6 dB decrease in the level of the 
second harmonic signal. 
SUMMARY OF THE INVENTION 
The present invention is directed to an ultrasound imaging method and 
apparatus as defined by the following independent claims. 
One preferred embodiment of the invention includes an ultrasound beamformer 
which acquires harmonic triggered frames using high transmit power 
responsive to a trigger signal derived from an ECG waveform. During the 
interval between acquisition of these triggered frames, additional image 
frames are acquired at reduced transmit power. In general, the in-between 
frames (referred to as "locator frames") are optimized for low levels of 
bubble degradation, possibly at the expense of image quality, while the 
triggered frames (also referred to as "imaging frames") are optimized 
primarily for image quality. Fundamental imaging is often preferred for 
the locator frames because it has a significantly higher signal-to-noise 
ratio than harmonic imaging, and hence can generate a usable image at much 
lower transmit power levels than would be required for useful harmonic 
imaging. These locator frames are displayed in real-time on a display 
device, providing the user with continuous feedback as to the location of 
the scan plane. The triggered frames may be displayed in real-time along 
with the locator frames in a variety of ways as described below, or may be 
reviewed later from memory with or without the locator frames. 
In a broad sense, the invention includes any technique for alternating 
between two types of frames, one adapted to obtain a high-quality image of 
tissues containing contrast media and triggered intermittently, and a 
second adapted not to destroy the bubbles imaged by the first frame.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 provides a schematic diagram of an 
ultrasound imaging system 10 that incorporates a presently preferred 
embodiment of this invention. The imaging system 10 includes a 
programmable transmit/receive beamformer system 12 that is coupled to an 
ultrasonic transducer 14. The beamformer system 12 provides transmit 
waveforms to the transducer 14 which cause the transducer 14 to emit 
ultrasonic energy into a tissue T containing a contrast agent C. 
Scattering sites within the tissue T, including the contrast agent C, 
return ultrasonic energy to the transducer 14, which transmits receive 
waveforms to the beamformer system 12. The region from which receive 
waveforms are collected will be referred to as the imaged region, and may 
include tissue, blood, and optionally contrast agent. 
A beamformer controller 16 controls operation of the beamformer system 12 
by controlling beamformer parameters such as transmit center frequency and 
bandwidth, receive center frequency and bandwidth, transmit power, receive 
gain, and transmit line spacing. The beamformer controller 16 is 
responsive to user controls 22 and to a trigger signal source 18. The 
trigger signal source 18 is responsive to an ECG signal supplied by an ECG 
device 20, and the trigger signal source 18 preferably includes 
conventional software or hardware which recognizes the R-wave of an ECG 
signal and generates a trigger signal when each R-wave occurs. The trigger 
signal is used as discussed below by the beamformer controller 16 to 
select appropriate beamformer parameters. 
Detected, formed receive beams from the beamformer system 12 are sent to a 
display controller 24 which preferably includes a scan converter and 
generates triggered and locator frames as described below for immediate 
display on a display device 26. The beamformer system 12 is also coupled 
to a cine memory 28 which stores triggered and locator frames for later 
playback. The display controller 24 is capable of displaying superimposed 
locator frames and triggered frames on the same area of the screen of the 
display device 26 as they are acquired. Alternatively, locator frames may 
be displayed on different areas of the display screen from the triggered 
frames, as discussed below in conjunction with FIG. 6. 
The present invention is directed to the structure and operation of the 
beamformer controller 16, and all of the remaining elements of FIG. 1 can 
be formed in any suitable manner, including a wide variety of conventional 
systems. The widest variety of trigger signal sources, beamformer 
controllers, beamformer systems, transducers, display controllers, 
displays, and cine memories can be adapted for use with this invention. 
Both analog and digital beamformer systems are suitable, and a wide 
variety of signals can be provided as inputs to the trigger signal source. 
By way of example, without intending any limitation, the ultrasound 
imaging system marketed by Acuson Corporation under the trade name Sequoia 
is capable of being modified to implement this invention. 
A flow chart illustrating one mode of operation of the beamformer 
controller 16 of FIG. 1 is given as FIG. 2. In this mode, the user selects 
a count of R-waves N and a programmable trigger delay .DELTA.T. A counter 
M is initially set to 0. The controller 16 counts R-waves until N R-waves 
have been detected, then initializes a timer t. When the programmed 
interval .DELTA.T elapses without interruption by an R-wave, the 
controller suspends acquisition of any further locator frames after 
completion of any frame acquisition in progress. After locator frame 
acquisition has been completed, the controller changes beamformer 
parameters (setting the transmit power to maximum and selecting a harmonic 
receive center frequency), and acquires a single triggered frame. 
Following acquisition of the triggered frame, the controller changes 
beamformer parameters again (reducing the transmit power and selecting a 
fundamental receive center frequency) and resumes continuous firing of 
locator frames. The process then begins anew. The latter three steps of 
suspending locator frame acquisition, acquiring a triggered frame, and 
resuming locator frame acquisition are shown in more detail in FIG. 3. 
As used herein, "harmonic" is intended broadly to include sub-harmonics and 
fractional harmonic energy (e.g. 1/2 or 3/2 of the fundamental frequency 
at which the transmit beam is centered) as well as higher harmonics (e.g. 
2 or 3 times the fundamental frequency at which the transmit beam is 
centered). In addition, a harmonic image signal or frame may be acquired 
from a single frame of scan lines that are each fired once, or alternately 
a harmonic image signal or frame may be processed from multiple frames or 
from frames where individual scan lines are fired multiple times. See, for 
example, the harmonic images formed with subtractive processing in the 
above-identified Johnson and Brock-Fisher patents. 
Of course, various modifications to the flowcharts of FIGS. 2 and 3 present 
themselves, and these flowcharts should be taken as exemplary of one 
possible implementation of the invention. In particular, the flowchart of 
FIG. 2 shows one way of counting R-waves and measuring time delays; 
various other ways of counting and measuring are possible and can be 
substituted. The behavior of the system when the time comes to acquire a 
triggered frame can also be modified. Instead of waiting for an ongoing 
locator frame to be acquired, the controller can abort the locator frame 
in progress, discarding the partially acquired frame, so as to be able to 
begin the acquisition of the triggered frame as close to the desired time 
as possible. For cases where the trigger delay .DELTA.T between R-wave and 
triggered frame is greater than the time required to acquire a locator 
frame, the controller may anticipate a coming triggered frame acquisition 
and suspend locator frame acquisition before the trigger delay .DELTA.T 
has elapsed. 
The operation of the system of FIG. 1 is illustrated schematically in FIG. 
4. An ECG waveform 30 is shown across the bottom of the figure. Within the 
trigger signal source 18, conventional software or circuitry detects the 
peak of the R-wave, as illustrated by the vertical lines 32 above the ECG 
waveform 30. In this example, the interval between R-waves is 750 ms 
(heart rate=80 bpm) and the trigger signal source 18 generates a trigger 
signal 150 ms after every second R-wave (N=2,.DELTA.T=150 ms ). At each 
trigger, acquisition of locator frames is suspended, and a single 
triggered frame is acquired using alternate beamforming parameters such as 
those suggested above (harmonic imaging; high transmit power). Following 
acquisition of the triggered frame, acquisition of locator frames resumes 
beamformer appropriate beamformer parameters (fundamental imaging; low 
transmit power). In this implementation, both locator frames and imaging 
frames are directed to the screen as they are acquired, forming an 
apparently continuous image (with a flicker as each triggered frame is 
acquired). On later cine review, the triggered frames may be distinguished 
from the locator frames. Alternately, locator frames may be excluded from 
later cine review. Additionally the locator and image frames may be 
combined (optionally color-coding one image then adding the two together) 
into a single image. The user would then see a superposition of a 
relatively static image (the triggered frames) with a more dynamic one 
(the locator frames). 
In addition to changing transmit power and selecting fundamental vs. 
harmonic imaging, the transmit center frequency, spectral shape, and/or 
bandwidth can be different for the locator and triggered frames. Contrast 
agents increase sound scattering through a resonance phenomenon, and the 
center frequency of that resonance varies inversely with bubble size. 
Bubbles of a given size scatter more energy (and may produce higher levels 
of harmonics) at or near the corresponding resonance frequency. At the 
same time, bubbles are more likely to be destroyed by ultrasound at or 
near their resonance frequency than by ultrasound away from their 
resonance frequency. Contrast agents achieve a broad bandwidth of contrast 
enhancement partly because each agent as injected includes many bubbles of 
different sizes and hence different resonance frequencies. 
FIGS. 5a-5d show how these properties of contrast agents may be used 
advantageously to reduce bubble destruction. FIG. 5a shows one preferred 
transmit spectrum for the triggered frames, centered at 2.5 MHz. The 
triggered frames may be acquired in fundamental or harmonic imaging (in 
which case the receive spectrum would be as shown by the dashed line 
curve); in either case, the greatest contribution to the image comes from 
bubbles with resonance near 2.5 MHz. FIG. 5b shows a preferred transmit 
spectrum of locator frames, centered (for example) at 4.0 MHz. While the 
locator frames may be optimized to minimize bubble destruction, some 
bubbles may be destroyed anyway. However, the destroyed bubbles will tend 
to have a resonance frequency at or near 4.0 MHz. Thus the bubbles 
destroyed by the locator frames are not those bubbles primarily imaged by 
the triggered frames. Additional benefit may be derived in this example 
from the fact that higher frequencies may tend to destroy bubbles to a 
lesser extent than lower frequencies. 
In general, the beamformer controller 16 of FIG. 1 may control the transmit 
waveform of the beamformer system 12 during locator frame acquisition to 
minimize or eliminate ultrasonic energy transmitted at and near the 
fundamental frequency f.sub.0 of the triggered frames. Analog filters, 
pulse shapers, or digital filters may be used in the transmit beamformer 
to reduce ultrasonic energy transmitted at or near the fundamental 
frequency of the triggered frames. Preferably, the ultrasonic energy level 
at the fundamental frequency f.sub.0 for each transmit pulse during 
locator frame acquisition is at least 6 dB, more preferably at least 12 dB 
or 20 dB, and most preferably at least 30 dB, below the ultrasonic energy 
level at the same frequency during triggered frame acquisition. In the 
preferred embodiment of FIG. 5c, the transmit waveforms are positioned in 
frequency to ensure that the spectrum 40 of the transmit waveforms for the 
locator frames has substantially no energy in a frequency band centered on 
the transmit frequency f.sub.0 of the triggered frames. This band 
preferably extends on both sides of f.sub.0 over a frequency range of 
one-tenth (more-preferably one-fifth) of f.sub.0. Alternately, the band 
may extend on both sides of f.sub.0 to frequencies at which the transmit 
waveforms for the triggered frames have an amplitude at least 6 dB below 
the amplitude at f.sub.0. In this embodiment, the spectrum 42 of the 
transmit waveforms for the triggered frames has its peak energy level at 
f.sub.0 (FIG. 5d). Of course, it is not critical that the spectrum 40 be 
centered at 2f.sub.0 as shown in FIG. 5c, and other center frequencies can 
readily be chosen. In the preferred embodiment of FIG. 5e, the spectrum 44 
of the transmit waveform for the locator frames has been shaped or 
filtered in the region under the dotted line to substantially eliminate 
ultrasonic energy in a band centered on the transmit center frequency 
f.sub.0 of the triggered frames. 
Several different means of displaying the triggered and locator frames are 
possible. FIG. 6 shows one arrangement, in which the locator frames 34 are 
shown on a spatially separated region of the screen from the triggered 
frames 36. In the example, the locator frames 34 are shown as a small 
moving image situated apart from the relatively static (updated once every 
several cardiac cycles) triggered image 36. 
An alternate mode of operation is illustrated in FIG. 7. In this case, 
locator frames are suspended when each triggered frame is fired, and for a 
selected period after acquisition of the triggered frame is completed. In 
FIG. 7, the total duration of suspension of the locator frames is 300 ms . 
The locator and triggered frames are displayed on the same area of the 
screen, so that the user sees a moving image (the locator frames) which 
periodically stops briefly (the triggered frames, each held for a 
persistence interval of 300 ms ). In variations on this approach, the 
triggered or locator frames can be made more easily distinguishable from 
one another by changing their brightness or by color coding. In another 
variation, locator frames may be acquired during the hold period (once the 
triggered frame acquisition is complete), but not displayed. These frames 
are then available for later review. FIG. 7 also illustrates a multiple 
trigger mode of operation, which may be used independently of the 
persistence interval feature discussed above. The multiple trigger mode of 
operation is discussed below. 
The embodiments described above improve upon the technique of using 
intermittent scanning to image fragile contrast agents by displaying a 
moving image even when the interval between triggered frame acquisition is 
large. This greatly eases the operator task of trying to hold the probe in 
a fixed position relative to the tissues being imaged in order to maintain 
a constant and appropriate scan plane. The locator frames are preferably 
formed using very low transmit power and so do not significantly destroy 
the contrast agent being imaged. While the triggered frames may destroy 
the contrast bubbles, the interval between triggered frames is large 
enough for the tissues being imaged to be refreshed with new contrast 
agent. 
A further advantage of these embodiments is that a single ultrasound frame 
(the triggered frame) may be easier to interpret using the dynamic locator 
frames as context. 
Many alternate methods of construction are possible. For example, the 
locator frames may be acquired at an artificially reduced frame rate. For 
example, if acquisition of a frame normally takes 25 ms (40 frames/sec), 
the locator frames can be acquired at 100 ms intervals (10 frames/sec) by 
adding dead time between adjacent scan lines or frames, further decreasing 
bubble destruction associated with acquisition of the locator frames. Even 
in this case, there are preferably more locator frames than triggered 
frames per unit time. Also, one or both of the triggered and locator 
frames may be acquired using a reduced line density. Reduced line density 
(with subsequent loss of resolution and loss of artifact) may be 
acceptable for the locator frames, and can be advantageous if used with an 
artificially reduced frame rate so that the frame rate is not increased as 
the line density is reduced. In this case, the reduced line density 
results in a reduced level of total ultrasound energy delivered into the 
subject being imaged, and hence in reduced bubble destruction. Reductions 
in line density may be beneficial for the triggered frames, as overly high 
line densities may result in excessive overlap of the transmit beams, 
resulting in bubble destruction (one ultrasound line firing destroying 
bubbles which would otherwise contribute to imaging of an adjacent line). 
Another alternative is that triggered frames may be acquired using 
fundamental imaging instead of harmonic imaging. The locator frames should 
be optimized to reduce degradation of the bubbles imaged in the triggered 
frames. The locator frames may be acquired using different transmit center 
frequency, bandwidth, and/or pulse shape than the triggered frames. In 
general, any alteration in the transmit characteristics of the locator 
frames which results in less destruction of the bubbles preferably imaged 
in the triggered frames may be advantageous. In particular, using a 
different transmit center frequency may result in the triggering frames 
selectively destroying a population of bubbles different from those 
primarily contributing to the triggered frame images. In some embodiments, 
this observation may be exploited to fullest advantage by the use of 
hardware or software filtering means in the transmit beamformer to remove 
any components at the triggered frame transmit frequency from the locator 
frame transmit pulses. One or both of the locator frames and triggered 
frames can be acquired using a chirp, swept-spectrum, coded excitation, or 
other high time-bandwidth product transmit pulse. Such signals may attain 
a given signal-to-noise ratio with lower peak pressures than a 
conventional ultrasound pulse, and hence may provide better performance 
for a given level of bubble destruction. In some implementations, such 
techniques may degrade image quality by worsening axial response or by 
worsening focusing. Such tradeoffs may be acceptable for locator frames 
but not for triggered frames. In the event that such transmit signals are 
used, the receive beamformer should include means to restore the axial 
resolution as well as possible. 
Instead of acquiring only a single triggered image frame, a selectable 
number of frames can be acquired in quick succession (continuous frame 
acquisition). Various complex trigger schemes can be used to determine 
when to acquire triggered frames. As a first example, multiple 
independently selectable trigger delays can be selected. An example is 
shown in FIG. 8, where two closely spaced triggered frames are acquired, 
300 and 450 ms after every 3rd R-wave. In this example, the controller is 
programmed not to fire any locator frames during the time interval between 
the two triggered frames (which occur fairly close together). Trigger 
delays can be varied following each triggered frame to obtain sequences of 
triggered frames at various points in the cardiac cycle ("swept 
triggers"), as shown in FIG. 7. As an example, the first triggered frame 
is acquired immediately following the first R-wave. Subsequent triggered 
frames are acquired at 150, 300, 450, and 600 ms after the second through 
fifth R-waves, respectively. The cycle then repeats itself following the 
seventh R-wave. As a second example, triggered frames can be acquired at 
different time intervals after different R-waves; for example at 20 ms 
after every 1st, 4th, 7th, . . . R-wave; and at 500 ms after every 2nd, 
5th, 8th . . . R-wave. 
Various alternatives to ECG R-wave detection are possible: triggering can 
be based on a different feature of an ECG signal; an externally provided 
trigger signal; some other physiological measured signal such as 
respiration; or on a combination of signals (such as triggering on the 
first R-wave after the peak of a measured respiration signal, so as to 
compensate for breathing). 
Another aspect of the invention relates to the acquisition of multiple 
triggered frames, even in the absence of locator frames. For example, in 
the foregoing discussion two triggered frames are acquired after every 
third R-wave. FIG. 9 shows a flowchart for another mode of operation of 
the beamformer controller 16. In this mode, the user selects a count of 
R-waves N and two programmable trigger delays .DELTA.T1 and .DELTA.T2, 
with .DELTA.T1 less than .DELTA.T2. A counter M is initially set to zero. 
The controller 16 counts R-waves until N R-waves have been detected, then 
initializes a timer T. When the first programmed interval .DELTA.T1 
elapses without interruption by an R-wave, the controller acquires a first 
triggered frame. When the total interval .DELTA.T2 has elapsed since the 
timer was initialized (that is an additional time interval of 
.DELTA.T2-.DELTA.T1 after the first frame was acquired), the controller 
acquires a second triggered frame. The R-wave counter is then reset, and 
the entire process repeats. In this example, N is greater than 2. 
This mode of operation may be generalized so that more than two frames are 
acquired, each specified by a respective time delay. in an alternate mode 
of operation, after the first triggered frame is acquired, the counter may 
count a second number of R-waves N2, and a second time delay .DELTA.T2 
before acquiring the second triggered frame. Again, this mode of operation 
may be generalized to more than two frames per sequence. For example, in 
the swept trigger sequence of FIG. 7, triggered frames are acquired with 
five different trigger delays after five respective R-waves. Following the 
fifth acquisition, the controller counts two R-waves before beginning the 
sequence anew. 
In general, the R-wave can be considered as one example of a marker signal 
that occurs at a specified portion of an ECG wave. This aspect of the 
invention generates a marker signal such as an R-wave signal at a 
specified portion of a plurality of cycles of an ECG wave. Then a sequence 
of a plurality of triggered ultrasonic image frames is acquired, each 
frame timed to follow a respective marker signal by a respective time 
interval. In some cases two or more triggered ultrasonic image frames may 
share the same marker signal, in which case multiple ones of the triggered 
ultrasonic image frames will occur within a single cycle of the ECG wave. 
Following acquisition of this sequence of triggered ultrasonic image 
frames, acquisition of image frames is interrupted for at least one cycle 
of the ECG wave (two marker signals), before a next sequence of image 
frames is acquired. 
As used herein, the term "responsive to" is intended broadly to cover any 
situation where a first element alters its operation in response to a 
signal generated by a second element, whether directly or indirectly. 
Thus, the first element is said to be responsive to the second when the 
first element responds directly to an output signal of the second element. 
Similarly, the first element is responsive to the second if intermediate 
elements or processors alter or modify a signal of the second element 
before it is applied as an input to the first element. 
Many alternate methods of construction or use of the invention will be 
obvious to one skilled in the art, and the invention should not be limited 
to the specific examples or combinations discussed above. It is only the 
following claims, including all equivalents, which are intended to define 
the scope of this invention.