U.S. patent application number 10/450277 was filed with the patent office on 2004-04-15 for gain setting in doppler haemodynamic monitors.
Invention is credited to Smith, Leonard.
Application Number | 20040073116 10/450277 |
Document ID | / |
Family ID | 9905042 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040073116 |
Kind Code |
A1 |
Smith, Leonard |
April 15, 2004 |
Gain setting in doppler haemodynamic monitors
Abstract
This invention relates to a method of automatically setting the
gain of an echo signal in a Doppler ultrasound haemodynamic
monitor. In essence the invention comprises monitoring the strength
of velocity components measured by the apparatus, and identifying a
group of such components occupying a particular band within the
overall velocity spectrum. The signal gain is then adjusted so that
the perceived with of the band falls within predetermined limits.
The monitoring or assessment of the velocity components is
preferably undertaken when the overall flow velocity is high e.g.
during systole.
Inventors: |
Smith, Leonard; (Hampshire,
GB) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
ONE LIBERTY PLACE, 46TH FLOOR
1650 MARKET STREET
PHILADELPHIA
PA
19103
US
|
Family ID: |
9905042 |
Appl. No.: |
10/450277 |
Filed: |
November 6, 2003 |
PCT Filed: |
December 11, 2001 |
PCT NO: |
PCT/GB01/05482 |
Current U.S.
Class: |
600/450 |
Current CPC
Class: |
A61B 8/065 20130101;
A61B 8/5276 20130101; G01S 7/52033 20130101; G01S 15/8979
20130101 |
Class at
Publication: |
600/450 |
International
Class: |
A61B 008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 13, 2000 |
GB |
0030449.3 |
Claims
1) A method of automatically setting signal gain in a Doppler
ultrasound haemodynamic monitor, said method including the steps
of: assessing the strength of components of the blood velocity
spectrum to identify a group of velocity components above a
predetermined level occupying a band within the velocity spectrum;
and adjusting the gain setting so that the perceived width of said
band falls within predetermined limits.
2) A method as claimed in claim 1 wherein said predetermined limits
are derived from clinical testing.
3) A method as claimed in claim 1 or claim 2 including assessing
the strength of components of the velocity spectrum whilst the
overall velocity is high.
4) A method as claimed in claim 3 wherein the assessment is made
during systole.
5) A method as claimed in any one of the preceding claims further
including applying a smoothing step to said band so as to avoid
rapid changes in gain setting.
6) A method as claimed in claim 5 wherein said smoothing step is
achieved by applying a control variable to counts of said velocity
components or bins above said predetermined level.
7) A method as claimed in claim 6 wherein said control variable is
applied to counts of said velocity components above said
predetermined limit but below a further, higher, predetermined
limit i.e. within a band.
8) A method as claimed in claim 7 wherein a decrement or increment
is applied to said control variable according to whether said
counts of said velocity components above said predetermined limit
are above or below said predetermined limits.
9) A method as claimed in claim 8 wherein if the discrepancy or
error between the count and the threshold is large, then a large
correcting decrement or increment is applied to the control
variable and if the discrepancy or error is small then a small
correcting decrement or increment is applied.
10) A method of automatically setting signal gain in a Doppler
ultrasound haemodynamic monitor, said method including the steps
of: establishing a base gain value; and automatically adjusting the
received signal gain in real time to bring the same within a band
defined about said base gain value.
11) A method of detecting movement of a Doppler transducer in a
Doppler ultrasound haemodynamic monitor, said method including:
setting the signal gain according to the method claimed in any one
of claims 1 to 10; storing the width of the velocity band; and
monitoring the distribution of velocities to determine the onset of
non-laminar flow and hence possible transducer movement.
12) A Doppler ultrasound haemodynamic monitor including an
automatic gain setting facility constructed and arranged to operate
according to the method claimed in any one of claims 1 to 10.
Description
FIELD OF THE INVENTION
[0001] This invention relates to Doppler ultrasound haemodynamic
monitors and, in particular, to the adjustment of received signal
gain in such devices. A byproduct of the invention is that movement
of the ultrasound transducer with respect to the flow being
monitored, can also be detected.
BACKGROUND
[0002] Existing Doppler-based ultrasonic haemodynamic monitors
require significant skill and experience on the part of the
operator to set the signal gain in the signal amplifier, to thereby
ensure a suitable signal is presented to the Fast Fourier Transform
(FFT) analyser for analysis. Even when a suitable signal is
presented, it is still possible for the operator to significantly
influence the output data derived from the machine, by varying the
gain. Further potential for confusion arises from the fact that
these forms of apparatus require the operator interpreting the
displayed waveform and other output data, to be able to distinguish
that a change in displayed data is a result of transducer movement
rather than a change in the patient's cardiac function.
[0003] For apparatus of this type to be truly useful in a clinical
environment, it is important that consistent output data is
produced when a patient is being monitored, and that the quality of
the data is minimally dependent on the skill of the operator in
setting up the machine. If this is achieved then clinicians can
determine and publish a range of `normal` or `acceptable` data for
all patients, thus aiding the process of diagnosis.
[0004] Attempts have been made, in the past, to produce an
`automatic` system for establishing gain. These have not been
successful, however, as they have tended to utilise perceived
signal strength with no regard to the point in the pulsitile flow
at which this signal strength was detected. These known systems
have also incorporated continuous adjustment of the gain rather
than determining a base value and then fixing the gain at that
value. This approach causes particular problems in haemodynamic
monitoring because, as stated above, movement of the flow
transducer with respect to the flow being monitored can result in
incorrect data being presented to the operator. A continuous
automatic gain system has been found to mask probe movement by
increasing the gain as signal strength declines with probe movement
when, of course, the operator should have been informed that the
transducer appeared to be moving out of alignment with the
flow.
[0005] Prior art systems do exist which inform the operator if
movement of the transducer is detected, but these systems typically
incorporate a further transducer and associated electronics to
determine alignment of the transducer with the vessel through which
flow is being monitored.
[0006] It is therefore an object of this invention to provide a
method for automatically setting gain, and apparatus which
incorporates such a gain setting facility, which goes at least some
way in addressing the drawbacks identified above; or which will at
least provide a useful choice.
SUMMARY OF THE INVENTION
[0007] In a first aspect the invention provides a method of
automatically setting signal gain in a Doppler ultrasound
haemodynamic monitor, said method including the steps of:
[0008] assessing the strength of components of the blood velocity
spectrum to identify a group of velocity components above a
predetermined level occupying a band within the velocity spectrum;
and
[0009] adjusting the gain setting so that the perceived width of
said band falls within predetermined limits.
[0010] Preferably said predetermined limits are derived from
clinical testing.
[0011] Preferably said method includes assessing the strength of
components of the velocity spectrum whilst the overall flow
velocity is high.
[0012] Preferably the assessment is made during systole.
[0013] Preferably said method further includes applying a smoothing
step to said band so as to avoid rapid changes in gain setting.
This is preferably achieved by applying a control variable to
counts of said velocity components or bins above said predetermined
level and, more preferably, above said predetermined limit but
below a further, higher, predetermined limit i.e. within a band. A
decrement or increment is applied to said control variable
according to whether said counts of said velocity components above
said predetermined limit are above or below limits. If the
discrepancy or error between the count and the limit is large, then
a large correcting decrement or increment is applied to the control
variable. If the discrepancy or error is small then a small
correcting decrement or increment is applied. After application of
the decrement or increment, the value of the control variable is
assessed and, if this value falls outside acceptable limits, the
gain is changed and the control variable is re-set to zero.
[0014] Preferably the correct gain set point is approached from
below said set point.
[0015] In a second aspect the invention provides a method of
automatically setting signal gain in a Doppler ultrasound
haemodynamic monitor, said method including the steps of:
[0016] establishing a base gain value; and
[0017] automatically adjusting the received signal gain in real
time to bring the same within a band defined about said base gain
value.
[0018] In a third aspect, the invention provides a method of
detecting movement of a Doppler transducer in a Doppler ultrasound
haemodynamic monitor, said method including:
[0019] setting the signal gain as set forth above;
[0020] storing the width of the velocity band; and
[0021] monitoring the distribution of velocities to determine the
onset of non-laminar flow and hence possible transducer
movement.
[0022] In a fourth aspect the invention provides a Doppler
ultrasound haemodynamic monitor including an automatic gain setting
facility constructed and arranged to operate according to the
methods hereinbefore set forth.
[0023] Many variations in the way the present invention can be
performed will present themselves to those skilled in the art. The
description which follows is intended as an illustration only of
one means of performing the invention and the lack of description
of variants should not be regarded as limiting. Wherever possible,
a description of a specific element should be deemed to include
equivalents thereof whether in existence now or in the future. The
scope of the invention should be limited by the appended claims
alone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention will now be described with reference to the
accompanying drawings in which:
[0025] FIG. 1: shows a received signal waveform from a Doppler
ultrasound haemodynamic monitor with gain set correctly according
to the invention and showing a plot of the acceptable number of
bins in the lower part of the figure;
[0026] FIG. 2: shows a similar view to FIG. 1 but with the gain set
too low;
[0027] FIG. 3: shows a similar view to FIG. 1 but with the gain set
too high; and
[0028] FIG. 4: shows a software flow diagram illustrating the use
of a control variable to influence gain setting according to the
invention.
DETAILED DESCRIPTION OF WORKING EMBODIMENT
[0029] The present invention is primarily concerned with setting
the received signal gain in a Doppler ultrasound haemodynamic
monitor. However, the invention may also be useful in indicating to
the operator that the transducer, which is used to insonate the
blood vessel of interest with ultrasound, has moved out of
alignment with the moving blood stream.
[0030] Apparatus to which the invention is particularly applicable
includes a probe of the general type described in published
International Patent Application WO 00/61005, such a probe carrying
ultrasound transmit and receive crystals on the distal end thereof.
In use, the probe is located in the patient's oesophagus and
aligned so that the crystals are positioned to insonate a section
of the patient's descending aorta with ultrasound. The received
ultrasound signal is conditioned and then processed in a digital
processor to give a reading of laminar flow velocity through the
aorta. This velocity reading is then combined with an estimate of
aorta cross-sectional area to provide an indication of cardiac
output.
[0031] As part of the signal conditioning process, prior to digital
processing, the signal must be amplified, and setting the gain of
this amplification step is not a straightforward task. Whilst some
form of automatic gain setting, under digital control, would be
desirable, traditional automated gain setting methodologies do not
take into account factors inherent in apparatus of this type. For
example, blood through the aorta is pulsatile and the velocity
spectrum varies significantly over the duration of each pulse.
Further, the ultrasound transducer may be influenced by blood flows
in other vessels in the vicinity, and will produce different output
data if moved out of alignment with the flow direction. Thus a
simple continuous automatic gain system, as tried in the past
would, in response to movement of the transducer, merely keep
increasing the gain rather than indicating to the operator that the
probe has moved.
[0032] The present invention, at least in the case of the
embodiment described below, addresses the above problems by first
recognising laminar flow i.e. a flow stream in which the majority
of flow components move at a single velocity or, in practice,
within a narrow velocity band. Accordingly, the first step is to
assess the signal strength of a various components of the velocity
spectrum and identify a band of velocity components above a
predetermined magnitude. In practice, the DSP section of the
processor preferably analyses all forward flow components and
reports those which are above a predetermined magnitude to the main
processor.
[0033] Whilst this assessment of laminar flow could, conceivably,
be undertaken at a number of points in the flow cycle, the optimum
point at which to undertake the assessment is when the blood is
moving at relatively high velocity i.e. during systole, the period
in which the heart is contracting. The start and end of systole is
determined by the main processor and, in the case of the invention
herein described, velocity analysis is undertaken over the whole of
systole, although it is conceivable that the analysis might be
restricted to a shorter time period between the start and end of
systole, for example about the time in which the blood velocity
increases rapidly after the start of systole.
[0034] Having set the time period over which the blood velocity
components are to be analysed, it is then necessary to establish
the predetermined upper and lower numbers of velocity components
which are taken to indicate correct gain setting. These are
determined, empirically, through clinical trial and experience, and
are indicated in the Figures. In practice the processor counts the
number of frequency bins (derived from the velocity-time plot)
above a particular minimum level and sets the gain to ensure that
these are kept between limits. Broadly, if the number of bins falls
too low the processor moves the gain setting upwards whilst if the
number of bins exceeds the preset maximum the processor moves the
gain setting downwards.
[0035] Referring to the Figures, all plots show a graph of blood
velocity against time, on which the start and end of systole has
been marked. Beneath the primary plot are plots showing the number
of acceptable velocity frequency bins (representing "acceptable"
velocity components) for each time slice of the plot appearing
above. It will be noted that the variation between that which is
considered a correct gain setting (FIG. 1) and that which is
considered to be an excessive gain setting (FIG. 3) is quite
small.
[0036] It will be appreciated that the blood velocity is being
analysed on a continuous basis and instantaneous fluctuations in
flow velocity could lead to instantaneous rapid changes in gain
setting unless some form of "damping" is included within the
processing. To this end a smoothing technique is applied to the
velocity band. The smoothing technique is preferably applied using
a control variable, a simple number in the processing software.
[0037] Each time the processor identifies a heartbeat complex, it
analyses the data between the start and end of systole to determine
the maximum number of frequency bins above the predetermined limit
in any single sample. If the number of bins falls below a
predetermined threshold, then an increment is applied to the
control variable. This increment is a large number if the
discrepancy or error is large; and is small if the error or
discrepancy is small. If the number of bins exceeds the threshold,
then a decrement is applied to the control variable. Again the
decrement may be large or small, depending on the magnitude of the
error or discrepancy.
[0038] After application of the increment or decrement to the
control variable, the value of the control variable is then
assessed. If it falls within prescribed limits then the gain
setting is left unchanged. If, however, the value of the control
variable falls outside the limits then the gain is changed upwards
or downwards as appropriate. With any change of gain the control
variable is reset to zero and the process then repeated.
[0039] As stated above, the control variable is simply a number,
and the thresholds and limits, in relation to which it is set, are
determined empirically, through practice and experience.
[0040] In order to achieve repeatable results and remove possible
hysteresis from the system, the gain set point is always approached
from below. To this end, if an increment step has not yet been
applied to the gain, then a decrement step is applied to the
control variable, if there is found to be insufficient error in the
velocity band.
[0041] Once the gain set-point has been incremented once, the
system must see a number of complexes (i.e. heart beats) eg 10
which are in limits before concluding that the gain is now set
correctly and that the optimisation process can now be terminated.
The counter which controls this is re-set every time the gain is
changed.
[0042] As stated above, the present invention is also useful to
indicate when the transducer has moved out of alignment with the
flow direction. In this situation the transducer will not be
indicating such a peaky type of velocity profile. The flow will not
be laminar but will be more in the nature of turbulent with
velocity components being spread over a broader range.
[0043] Thus, with the gain set correctly and the transducer
correctly aligned with the flow stream, the width of the velocity
band can be stored. By then continually monitoring the distribution
of velocity components, preferably using an averaging smoothing
filter, any divergence of the velocity distribution from the stored
profile, into a turbulent condition, may indicate transducer
movement and can be notified to the operator as such.
* * * * *