Description:
The following specification particularly describes the invention and the manner
in which it is to be performed:
Title: Apparatus and method of characterising a narrowing in a fluid filled tub5 e
Field of the invention
This invention relates to an apparatus and method of characterising a
10 narrowing in a fluid filled tube.
Background to the invention
An example of a fluid filled tube or vessel formed with a constriction or
15 narrowing is a blood vessel having a stenosis. Assessment or measurement of
the constriction is helpful to review the extent and location of the constriction.
A methodology for assessment of a constriction in a fluid filled tube such as a
coronary stenosis is fractional flow reserve (FFR). This technique measures
20 the drop in pressure at two points along a vessel; see Figure 1 of the
accompanying drawings where example points P1 and P4 identify where
measurements of pressure and flow rate can be taken, under conditions of
maximal achievable hyperemia in a coronary environment. The Pd
measurement comes from a pressure sensor on the wire and the Pa
25 measurement comes from the catheter. A comparison is then made by
expressing the mean distal pressure (Pd), as a proportion of mean proximal
pressure (Pa), wherein the values are mean Pa and Pd over the entire cardiac
cycle, taken over at least one complete cardiac cycle (but usually an average of
3 or more beats):
30 It is an object of the invention to provide an apparatus and method of profiling
or characterising a narrowing in a fluid filled tube.
3
One aspect of the present invention provides system for characterising a
narrowing in a fluid filled tube, the system comprising: a probe having a first
measurement sensor to take an instantaneous measurement at different
locations along the tube; a mechanism to draw the probe through the tube; a
position measure to provide location data relating to the location at which 5 a
respective instantaneous measurement is taken by the first measurement
sensor; a processor to calculate, from the instantaneous measurements, a
characteristic of the tube at different locations along the tube.
10 Another aspect of the present invention provides a probe for assessing a
characteristic of a fluid filled tube comprising two measurement sensors spaced
apart by a known distance and a line between the two sensors, the line being
drawable through the tube to alter the known distance between the first sensor
and the second sensor.
15
A further aspect of the present invention provides a method of characterising a
narrowing in a fluid filled tube using a probe having a sensor, comprising:
drawing the probe within the tube along the tube; recording probe sensor
readings at different locations along the tube; and calculating, from the
20 instantaneous measurements, a characteristic of the tube at different locations
along the tube.
A yet further aspect of the present invention provides a probe for assessing a
characteristic of a fluid filled tube comprising two measurement sensors and a
25 line between the two sensors, the line being drawable through the tube to alter
the distance between the first sensor and the second sensor.
30
Brief description of the drawings
4
In order that the present invention may be more readily understood,
embodiments of the invention will now be described with reference to the
accompanying drawings, in which:
FIGURE 1 is a schematic diagram of a series of constrictions in a fluid fille5 d
tube, where P is pressure, R is a ratio of the pressures and D is the distance
between measurements;
FIGURE 2 is a schematic diagram of a system embodying the present
10 invention;
FIGURE 3 is a schematic diagram of part of the system of figure 2 located in a
fluid filled tube;
15 FIGURE 4 is a plot created using a method embodying the present invention
illustrating the IPR for a length of artery;
FIGURE 5 is a point-by-point constriction intensity map generated following one
embodiment of the present invention and based on the Figure 4 data, in this
example, the point-by-point assessment is of a stenosis in an artery, where D20 0
is the start of a recording, D1 is a point at the start of high stenosis intensity, D2
is a point at the end of high stenosis intensity and D3 is the end of the
recording;
25 FIGURE 6 is a plot created using a method embodying the present invention
illustrating the IPR for a length of artery and a likely site for a stent along the
tube between locations D1 and D2;
FIGURE 7 is a plot illustrating the likely effect on the same characteristic, IPR,
30 on the artery after a hypothetical angioplasty procedure of locating a stent
along the tube between locations D1 and D2 together with a plot of the
measured values of IPR obtained using a method embodying the present
invention; and
5
FIGURE 8 is a flowchart showing operation of a system embodying the present
invention incorporating a feedback procedure.
FIGURE 9 is a schematic diagram of another system embodying the presen5 t
invention.
Description
10 This invention provides an apparatus and method of profiling or characterising
a narrowing in a fluid filled tube. The apparatus and method of profiling or
characterising is also useful to characterise or profile a series of narrowings in a
fluid filled tube.
15 Referring to Figure 2, a system 1 embodying the invention for characterising a
narrowing in a fluid filled tube comprises haemodynamic equipment 2 including
a processor 3, a catheter 4, a motor drive 5 and an intra-arterial probe 6 such
as an intra-arterial pressure wire (WaveWire or Combowire (Volcano Corp.) or
Radi pressure wire (St Jude Medical) with a pressure measurement transducer
20 or sensor 7 – i.e. a device measuring pressure (P). Preferably, the probe 6
comprises the wire and the sensor 7 integrated in the wire. The sensor 7 is
shown in situ in Figure 3.
The processor 3 analyses and operates on the measurements taken by the
25 sensor 7. A signal line 8 relays the pressure measurement signal from the
sensor 7 to the processor 3. The signal line 8 is illustrated both as a wired
connection 8 and as a wireless connection 8’ from either the motor drive 5, the
catheter 4 or direct from the transducer 7 – any configuration is available.
30 The processor 3 operates on the measurements received from the transducer
7 in accordance with a number of algorithms which are discussed in greater
detail below.
6
The sensor 7 is a pressure measurement sensor but other forms of sensor are
envisaged; flow sensors, for example. Additionally, a capacitive sensor for
measuring or calculating a thickness of an arterial wall is within the scope of the
invention.
5
The system 1 may be provided in the following configurations or combination of
configurations, but these are not an exhaustive list of configurations:
[001] a stand-alone device incorporating a probe with pressure
measurement capacity in wired connection with a processor to provide on10
device analysis;
[002] a device incorporating a probe with pressure measurement
capacity in wireless connection with a processor to provide analysis at the
processor;
[003] a stand-alone device incorporating a probe with pressure
15 measurement capacity and a data storage device operable to record
measurement data for real time or subsequent communication to a processor
to provide analysis at the processor (real time and/or off-line); and
[004] a device incorporating a probe with pressure measurement
capacity in wireless connection with a data storage device operable to record
20 measurement data for real time or subsequent communication to a processor
to provide analysis at the processor (real time and/or off-line).
In the cardiac environment where the system 1 is configured as part of
haemodynamic equipment, the system is configured using the processor 3 in
25 the haemodynamic equipment, such as in McKesson equipment - Horizon
Cardiology™, a cardiovascular information system (CVIS). The processor can
be configured as supplemental to the haemodynamic equipment. Such
configurations are particularly effective for the equipment processor to perform
off-line analysis of the pressure data.
30 The system 1 can be used in combination with other haemodynamic
equipment, medical imaging equipment and/or in-patient marker location
equipment.
7
The system is used for profiling or characterising a narrowing in a fluid filled
tube. An example of the use of such a system is in the cardiac environment
when the tube is an artery and the narrowing/restriction/constriction in the tube
is a stenosis.
5
The basic system components are: the probe 6 having a measurement sensor
7 to take an instantaneous measurement at different locations along the tube;
the motor drive 5 to draw the probe 6 at a predetermined rate through the tube;
and the processor 3 to calculate, from the instantaneous measurements, a
10 characteristic of the tube at different locations along the tube. In this example a
particularly useful measurement to sense is that of pressure as a pressure drop
results following the fluid passing through a restriction.
A profile or assessment of a restriction to flow is made by expressing the ratio
15 of distal to proximal pressures within the tube. This measures the total
restriction to flow across all stenoses along the length of the tube from position
D1 to D3 where the respective pressure measurements are taken and
expressed as a ratio (P4 / P1) either with or without conditions of maximal
hyperaemia.
20
In addition to calculation of the total restriction to flow along a vessel, it is
possible to calculate the instantaneous pressure drop across an individual
stenosis from the ratios of pressure in segments D distance apart. For
example the ratio of fall in pressure over distance D3 is:
25
which is approximately identical to the normalised instantaneous pressure ratio
(nIPR):
In one example, there are two measurement sensors displaced from one
another - see Figure 3. This system 1 has a further sensor 9 so that two
30 instantaneous measurements are taken, one by the further sensor 9 at a
substantially constant location along the tube and another by the first sensor 7
at different locations along the tube. The line or wire between the two sensors
is drawable through the tube to alter the distance between the first sensor and
8
the second sensor. One sensor (9 in this example) is fixed at the substantially
constant location. The other sensor (7 in this example) moves relative to the
one sensor 9. The “fixed” sensor 9 is located at the end of the catheter 4 from
which the wire 6 carrying the other sensor 7 emanates. The probe sensor 7
therefore moves relative to the fixed sensor 9. The measurements ar5 e
normalised with respect to the measurements taken at the substantially
constant or fixed location.
The normalised instantaneous pressure ratio is more robust, as each distal
10 value is normalised to the proximal aortic pressure, thus making comparisons
along the length of the vessel more reliable as perturbations in absolute
pressure are minimised.
Systematically moving back along the vessel, at velocity U, and logging the
15 instantaneous measurements alongside the draw distance for the probe create
a pressure ratio (R1, R2, and R3 etc.) for each position (D1, D2, and D3 etc.) as
shown in figure 5. The profiling or assessment of stenosis can be performed
using either the normalised instantaneous pressure ratio or the instantaneous
pressure ratio.
20
In one example, the predetermined rate of draw through the tube of the probe
is a known and preferably constant speed. The draw is a known velocity draw
to allow instantaneous pressure measurements to be taken as the probe is
being drawn along the tube, for those measurements to be recorded as
25 pressure measurements and for a pressure ratio to be calculated for each
position of the probe along the tube.
The motor drive 5 is controlled, preferably by the processor 3, to draw the
probe 6 back toward the catheter 4. The control may involve use of a feedback
30 loop.
The systematic assessment of pressure along a vessel is performed by
withdrawing the pressure sensor, at velocity U. Pressure is recorded at each
9
location. It is possible to minimise error and to speed up the acquisition phase
by using a feedback loop. In this feedback loop, the sensor is positioned in the
tube, and then attached to the variable speed motor drive, or stepper motor.
After sampling for a period of x seconds to establish a baseline for the
measurements being taken and characteristics calculated, in this case NIPR o5 r
IPR mean and standard deviation moving averages, the motor drive
commences pullback of the probe at velocity U. Sampling can also be over a
fraction or specific time point of a beat.
10 Using high sampling frequencies and an appropriate sensor with a suitable
frequency response, the pullback velocity U can be made faster by looking at a
partial cardiac cycle in a single beat over a known distance.
Pressure measurements are fed to the processor in the control console, and
15 IFR or nIFR is calculated. This live pressure is compared against the moving
average mean and standard deviation for the proceeding n beats, in a cardiac
environment. If the live pressure data falls within the tolerance threshold, the
motor continue with the pullback. If however the live pressure data falls outside
of the tolerance threshold, the motor is paused and further measurements of
20 pressure are made. Once pressure measurement falls within the tolerance
threshold the motor continues with the pullback. A serial assessment or profile
is created by this method. The feedback loop example is illustrated in Figure 6.
In another example, the draw is stepped through the tube with at least one
25 instantaneous measurement being taken at each location along the tube. The
probe is then drawn through the tube for a predetermined distance, stopped
and then another at least one instantaneous measurement is taken at the next
location and so on. Preferably but not necessarily, the predetermined distance
is a constant distance.
30
Each instantaneous measurement is logged as being at a respective location or
with respect to a draw distance.
10
An alternative system embodying the invention has a position sensor fitted
which monitors the position of the pressure sensor wire whilst being pulled back
through the tube. In this way, each distance point/position/location would be
linked or cross-referenced to a specific pressure measurement. Specifically,
the position sensor monitors the guide wire holding the pressure sensor5 .
Referring now to figure 9 another embodiment of the system is described which
may operate with or without a motor drive 5. In the embodiments shown in
figure 2, the system relies upon the motor to operate in a known way to
10 determine the distance x along the line 6 to the sensor 7. Other mechanisms
for determining the distance x to the sensor from a known point, usually on the
catheter, may be used to take measurements at different known positions of x.
In a purely manual version of the system, the line 6 may be drawn back
through the catheter 4 manually and markings on the line 6 in the form of
15 physical indicia can convey the distance x to the user. The system takes the
position measure by reading the markings or marker on the probe. The marker
may be a visible indicator read by a laser position indicator.
A semi-automatic version of the system can use a manually drawn line 6
20 through the catheter 4 and a combination of i) an RF reader 10 positioned
preferably at the head of the catheter 4 from which the line 6 projects a
distance x out of the catheter 4 and ii) multiple RF tags 11 positioned along the
line 6. The line 6 is provided with a series of equispaced passive RF tags 11
each having an individual identifier which is read when in close (if not only
25 immediate) proximity to the reader 10. In one embodiment, the RF tag reader
10 is in a coincident position with the second sensor 9 mounted at the head of
the catheter 4. Coincidence of these two elements is not essential. More than
one RF tag reader 10 can be used on the catheter.
30 A lookup table stored locally or in the processor 3 takes the read information
from the reader 10 and identifies the tag adjacent the reader 10 for example as
tag 110 and identifies from the lookup table that tag 110 which is positioned at
the reader 10 is a distance x away from the sensor 7 along the line 6 meaning
11
that the sensor 7 is at known position P12 The line is then drawn through until
another RF tag 11 is read by the reader 10 at which point that tag is identified,
its position is known as being at the reader 10 and the distance from that tag to
the sensor 7 is also known so the position of the sensor 7 is known. This
process is repeated and tags 11 are identified, the sensor 7 position i5 s
identified as known and at least one measurement is taken at the known
position.
Preferably, the RF tags 11 are equispaced along the line 6 but they need not be
10 equispaced as their positions along the line 6 relative to the sensor 7 is the only
essential data to be associated with each tag. This essential data need not be
present at the time the measurements are taken. Measurements can be taken
and logged against each RF tag identifier and then subsequently the line can
be measured to provide the relative position information for each tag and then
15 that position information is associated with the measurement taken at each tag.
Preferably, the RF tags 11 are passive RF tags. The RF tags 11 could be
active RF tags powered by a conductor in the line 6.
20 Examples of the invention allow a serial assessment of pressure ratio along a
vessel. A rate of change of pressure or a rate of change of pressure ratio is
further calculated to provide a measure of stenosis intensity. The rate of
change in pressure or stenosis intensity at any position is calculated as
which can be plotted as a point-by-point stenosis intensity map as shown in
25 Figure 4.
A systematic assessment is made at rate U over time t, (known velocity
example) so it is possible to calculate the withdrawal distance and thus the
physiological stenosis length. In this example, this is the length (D2-D1) a
segment which has the greatest physiological impact. The characteristic of the
30 tube or further characteristics derived from the characteristic of the tube can be
assessed and thresholded. This process can be automated using a search
algorithm which looks for points at which the IPR or nIPR exceeds a given
threshold (in this example D1 and D2).
12
The characteristics and/or derived characteristics are used to assess or profile
the tube to identify the length and/or location of a narrowing of the tube along
the tube length. The use of thresholding techniques for the various
characteristics and/or derived characteristics identifies regions of the tub5 e
where the thresholds are exceeded allowing identification and locating of
stenosis and their length.
An example of a derived characteristic of the tube is the cumulative burden on
10 the tube caused by a narrowing in the tube. It is possible to calculate the
individual stenosis burden or stenosis occlusive value (with time points D1 start
of a stenosis, and D2 end of a stenosis):
Or,
15
and total stenosis burden (over time points D0 to D3) for the entire vessel,
Or
Virtual angioplasty assessment is enabled by examples of the present
20 invention. Referring to Figure 6, a systematic assessment approach is applied
and the measured profile is displayed. The segment of tube to which a stent or
other angioplasty is to be applied (having a high stenosis grade (D1-D2)) has its
profile characteristic estimated with the stent applied and then subtracted away
on an individual segment basis to give a compensated profile as shown in
25 Figure 7. It is therefore possible to assess the effects of angioplasty on IPR of
nIPR prior to treatment.
13
Where D0 is distance=0, D1 the distance at the start of the high stenosis grade,
and D2 the distance at the end of the high stenosis grade.
Such virtual assessment or profiling of a tube or stenosis in a tube using either
IPR or nIPR allows the effects of removing a stenosis to be assessed prior to
performing the procedure itself5 .
There are particular needs in the cardiac environment for simplified equipment
having the smallest possible footprint (or being the least invasive requiring the
smallest possible entry site) so the provision of a known position probe to
10 assess or profile stenoses along the length of the tube represents a significant
technical advance in that field.
When used in this specification and claims, the terms "comprises" and
"comprising" and variations thereof mean that the specified features, steps or
15 integers are included. The terms are not to be interpreted to exclude the
presence of other features, steps or components.
The features disclosed in the foregoing description, or the following claims, or
the accompanying drawings, expressed in their specific forms or in terms of a
20 means for performing the disclosed function, or a method or process for
attaining the disclosed result, as appropriate, may, separately, or in any
combination of such features, be utilised for realising the invention in diverse
forms thereof.

I/We CLAIM:
5
1. A system for characterising a narrowing in a fluid filled tube, the
system comprising:
a probe having a first measurement sensor to take an
instantaneous measurement at different locations along the tube;
10 a mechanism to draw the probe through the tube;
a position measure to provide location data relating to the location
at which a respective instantaneous measurement is taken by the first
measurement sensor;
a processor to calculate, from the instantaneous measurements, a
15 characteristic of the tube at different locations along the tube.
2. A system according to claim 1, wherein the mechanism is a
motorized mechanism.
20 3. A system according to claim 1, wherein the mechanism is a manual
mechanism to draw the probe through the tube.
4. A system according to any preceding claim, wherein the position
measure is a reader to read a marker on the probe.
25
5. A system according to claim 4, wherein the marker on the probe is
an RF tag.
6. A system according to claim 4 or 5, wherein the marker is read by
30 an RF reader.
15
7. A system according to any preceding claim, wherein the position
measure provides a relative location of the first measurement sensor with
respect to a known datum.
8. A system according to any preceding claim, wherein the positio5 n
measure provides an absolute location of the first measurement sensor.
9. A system according to any preceding claim, wherein the
instantaneous measurements are pressure measurements.
10
10. A system according to any preceding claim, wherein the
characteristic of the tube is a ratio of instantaneous measurements taken at
different locations along the tube, the characteristic of the tube being
variable along the tube.
15
11. The system of any preceding claim, wherein a further sensor is
provided so two instantaneous measurements are taken, one by the further
sensor at a substantially constant location along the tube and another by
the first sensor at different locations along the tube.
20
12. The system of claim 11, wherein the calculated characteristic is
normalised with respect to the substantially constant location.
13. The system of any preceding claim, wherein the probe is drawn at
25 a predetermined rate of draw through the tube of the probe is a constant
speed.
14. The system of claim 13, wherein the predetermined rate is a
constant speed.
30
15. The system of claim 13 or 14, wherein the predetermined rate is a
stepped draw through the tube with instantaneous measurements being
16
taken at one location along the tube and the probe is then drawn through
the tube for a predetermined distance for the next set of instantaneous
measurements to be taken at the next location and so on.
16. The system according to claim 15, wherein the predetermine5 d
distance is a constant distance.
17. The system of any preceding claim, wherein each instantaneous
measurement is logged at a respective location or with respect to a draw
10 distance.
18. The system of any preceding claim, wherein the characteristic
represents a restriction to flow along the tube in terms of a pressure drop.
15 19. The system according to any preceding claim, wherein the draw is
a known velocity draw.
20. The system according to any preceding claim, wherein a rate of
change of pressure or a rate of change of pressure ratio is further
20 calculated to provide a measure of stenosis intensity.
21. The system of any preceding claim, wherein the characteristic of
the tube or further characteristics derived from the characteristic of the tube
can be assessed and thresholded.
25
22. The system of any preceding claim, wherein a length and location
of a narrowing of the tube beyond a predetermined threshold is identified.
23. The system of any preceding claim, wherein a derived
30 characteristic of the tube is a cumulative burden on the tube caused by a
narrowing in the tube.
17
24. The system of any preceding claim, wherein instantaneous
pressure measurements are recorded and a pressure ratio calculated for
each position of the probe along the tube.
25. A probe for assessing a characteristic of a fluid filled tub5 e
comprising two measurement sensors spaced apart by a known distance
and a line between the two sensors, the line being drawable through the
tube to alter the known distance between the first sensor and the second
sensor.
10
26. A probe according to claim 25, wherein the first sensor is fixed and
the second sensor moved relative to the first sensor.
27. The probe according to claim 26, wherein the first sensor is at a
15 substantially constant location in the tube and the second sensor moves
along the tube at varying distances from the first sensor
28. A method of characterising a narrowing in a fluid filled tube using a
probe having a sensor, comprising:
20 drawing the probe within the tube along the tube;
recording probe sensor readings at different locations along the tube;
and
calculating, from the instantaneous measurements, a characteristic of
the tube at different locations along the tube.
25
29. The method according to claim 28, wherein the instantaneous
measurements are pressure measurements.
30. The method according to claim 28 or 29, wherein the characteristic
30 of the tube is a ratio of instantaneous measurements taken at different
locations along the tube, the characteristic of the tube being variable along
the tube.
18
31. The method of any one of claims 28 to 30, wherein a further sensor
is provided and the method further comprises taking two instantaneous
measurements, one by a further sensor at a substantially constant location
along the tube and another by a first sensor5 .
32. The method of claim 31, comprising normalising the calculated
characteristic with respect to the substantially constant location.
10 33. The method of any one of claims 28 to 32, comprising drawing the
probe at a constant speed through the tube of the probe.
34. The method of any one of claims 28 to 33, comprising drawing the
probe at a stepped draw through the tube with instantaneous
15 measurements being taken at one location along the tube and drawing the
probe for a predetermined distance to a next location for a next set of
instantaneous measurements to be taken at the next location and so on.
35. The method of claim 34, wherein the predetermined distance is a
20 constant distance.
36. The method of any one of claims 28 to 35, comprising logging the
draw distance between measurements and associating the draw distance or
location of the probe with each instantaneous measurement taken at that
25 location.
37. The method of any one of claims 28 to 36, comprising drawing the
probe and taking instantaneous pressure measurements as the probe is
being drawn along the tube, recording those measurements as pressure
30 measurements and calculating a pressure ratio for each position of the
probe along the tube.
19
38. The method of any one of claims 28 to 37, comprising calculating a
rate of change of pressure or a rate of change of pressure ratio to provide a
measure of stenosis intensity.
39. The method of any one of claims 28 to 38, comprising thresholding
the characteristic of the tube or further characteristics derived from th5 e
characteristic of the tube to provide a measure of stenosis intensity.
40. Use of an apparatus according to any one of claims 1 to 27 to carry
out the method of any one of claims 28 to 39.
10
41. A probe for assessing a characteristic of a fluid filled tube
comprising two measurement sensors and a line between the two sensors,
the line being drawable through the tube to alter the distance between the
first sensor and the second sensor.
15
42. A probe according to claim 41, wherein the first sensor is fixed and
the second sensor moved relative to the first sensor.
43. A data storage medium operable to carry out the method steps of
20 any one of claims 28 to 39.
44. A processor operable to carry out the processing steps of any one
of claims 28 to 39.
25 45. An apparatus, method, processor or data storage medium
substantially as described herein and/or as shown in Figures 2 to 6 and 9.
46. Any novel feature or combination of features disclosed herein.