WO2007144747A2 - Method for estimating the errors affecting the blood flow velocity measurement in curved vessels - Google Patents

Method for estimating the errors affecting the blood flow velocity measurement in curved vessels Download PDF

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Publication number
WO2007144747A2
WO2007144747A2 PCT/IB2007/001578 IB2007001578W WO2007144747A2 WO 2007144747 A2 WO2007144747 A2 WO 2007144747A2 IB 2007001578 W IB2007001578 W IB 2007001578W WO 2007144747 A2 WO2007144747 A2 WO 2007144747A2
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Prior art keywords
velocity
blood
determining
method
vessel
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PCT/IB2007/001578
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French (fr)
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WO2007144747A3 (en
Inventor
Caterina Guiot
Silvestro Roatta
Tullia Todros
Sonia Balbis
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Universita' Degli Studi Di Torino
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Priority to ITTO20060425 priority patent/ITTO20060425A1/en
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Publication of WO2007144747A2 publication Critical patent/WO2007144747A2/en
Publication of WO2007144747A3 publication Critical patent/WO2007144747A3/en

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    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00Detecting, measuring or recording for diagnostic purposes; Identification of persons
    • A61B5/02Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
    • A61B5/02007Evaluating blood vessel condition, e.g. elasticity, compliance
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B8/00Diagnosis using ultrasonic, sonic or infrasonic waves
    • A61B8/06Measuring blood flow
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B8/00Diagnosis using ultrasonic, sonic or infrasonic waves
    • A61B8/13Tomography

Abstract

A method is described for estimating the error (E) affecting the measurement of the blood flow velocity in a vessel, and in particular the measurement error of the flow velocity of blood in a curved vessel, comprising: determining a first blood flow velocity {Vcmax, Vcmean) in a substantially curved vessel; determining a second blood velocity ( VSmax, VSmean), assuming that said substantially curved blood vessel has a straight shape; and evaluating the error (E) on the basis of said first {VCmaXf Vcmean) and said second ( VSmax, VSmean) blood flow velocities.

Description

METHOD FOR ESTIMATING THE ERRORS AFFECTING THE BLOOD FLOW VELOCITY MEASUREMENT IN CURVED VESSELS

TECHNICAL FIELD

This invention relates to a method for the estimation of the error affecting the measurement of the blood velocity when blood flows in curved vessels and the measurement is performed using Doppler Ultrasound equipments.

BACKGROUND ART

As it is well known, the non-invasive assessment of the blood velocity in vessels, currently performed in the field of the vascular diagnostics to investigate several pathologies, such as vascular stenosis and aneurisms, is mainly based on Doppler or Echo-doppler equipments . Their working principle is the fact that sonic waves change their frequency when emitted or received from reflecting and/or scattering moving structures . According to this principle, Echo-Doppler equipments send to the blood contained in a blood vessel, normally by means of a proper probe, ultrasonic waves of a given — 9 —

fixed frequency, which are back-scattered from the moving blood and diffuse as new ultrasonic waves with a frequency which is different from the original one

(Doppler effect) , and are normally collected by the same probe. Based on the frequency difference between the emitted waves and the scattered waves collected by the probe, an estimation of the velocity of the blood in the insonated vessel follows.

Although widely widespread in clinical diagnostic, and currently being used in hospitals, the measurement of the blood velocity using such Doppler equipments is not fully reliable, as several studies performed in academic and university laboratories show, since the value of the measurement depends on many geometrical characteristics of the insonated vessel, such as, for instance, its radius of curvature, the vessels radius and the mutual distance between the blood vessel and the measuring probe .

The above studies have shown that the measurement of the blood flowing velocity performed using a standard Doppler equipment can be considered acceptably reliable only when the investigated portion of the blood vessel is substantially straight, but the measurement accuracy decreases when the investigated portion of the blood vessel starts bending.

In particular, according to the above studies, when the measurement of the blood flowing velocity is performed over bended portions of blood vessels, the standard Doppler equipment shows a bias, consisting in a overestimation of the maximal blood flowing velocity, and produces non negligible percentage errors, as large as 25-30 %.

Such errors are mainly caused by the presence of non-axial velocities, generating re-circulating vortices in the vessel, and by the non-uniformity of the axial velocity directions inside the sample volume. DISCLOSURE OF THE INVENTION

The aim of the present invention is hence to provide a method for estimating the error affecting the measurement of the flowing velocity of the blood, which can be implemented in standard Doppler equipments, in order to inform the operator, who is performing the measurement, about the reliability and the accuracy of the measurement itself.

According to the present invention, a method for estimating the error affecting the measurement of the flowing velocity of the blood is provided, as defined in claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention a preferred embodiment is now described, purely by way of non-limiting example and with reference to the attached plate of drawings, wherein:

- Figure 1 shows a block diagram of a Echo-doppler equipment for the measurement of the flowing velocity of the blood in a vessel;

- Figure 2a shows an example of curved blood vessel on which the measurement of the flowing blood velocity is performed;

Figures 2b and 2c shows the geometric characteristics of the blood vessel described in Figure 2a, in particular the spatial distance between the blood vessel and the probe insonating the vessel itself; and - Figure 3 shows a block diagram of the method for estimating the error affecting the blood flowing velocity according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION In Figure 1 the index 1 indicates the Echo-doppler equipment performing the measurement of the blood flowing velocity as a whole, which includes: a standard probe 2 transmitting an acoustic ultrasonic beam to the sample volume of a blood vessel and receiving the ultrasonic signal backscattered from the corpuscular elements of the blood;

- an user interface 3 for the management of the equipment by the operator (not shown) ;

- a data display unit 4, which shows the input parameter values selected by the user by means of the interface 3 and inserted in 1, the results of the performed measurement and of its error and;

- an electronic control unit 5, connected to the probe 2, to the user interface 3 and to the data dispaly unit 4, configured to calculate the blood flowing velocity in the sample volume inside the insonated vessel and for implementing the method of estimation of the error affecting the above measurement according to the present invention.

The method for estimating the error affecting the measurement of the blood flowing velocity performed by the equipment 1 will be hereafter described with reference to the block diagram of Figure 3 and to the geometrical parameters shown in Figures 2a, 2b and 2c.

In particular, in order to assess the blood flowing velocity in a portion of blood vessel, such the one shown in Figure 2a, and in order to estimate the potential error affecting the computed blood flowing velocity, it is mandatory to define and evaluate some geometrical parameters referring to the spatial relation between the blood vessel 6 and the probe 2, as well as some geometrical variables pertaining to the investigated portion of the vessel, on the basis of which the equipment 1 performs the calculation of the blood flowing velocity and of the related error.

Some of the above parameters, in particular, such as the radius of curvature R of the blood vessel 6 and the diameter d of the ultrasonic beam 7 transmitted from the probe 2 to the vessel 6, both shown in Figure 2a, the radius a of the vessel 6, shown in Figure 2b, and the width h of the portion of the vessel 6 insonated by the ultrasonic beam 7 transmitted by the probe 2, shown in Figure 2c, can be evaluated by means of interactive procedures of known type (not shown in Figure 1) , for instance by means of standard ultrasound imagers, which shows on their monitor an image of the blood vessel and include some standard routines for the direct evaluation of the distances between two or more flags positioned by the operator on the images itself. In particular, while performing the measurement, the operator may select and modify one or more of the geometrical variables that define the volume of the vessel portion being examined. For instance the depth h, may be modified so that the ultrasound beam 7 extends over the full cross section of the vessel 6.

In order to determine the geometrical variables that define the spatial relationship between the blood vessel 6 and the probe 2 the operator needs first to establish a Cartesian frame of reference X, Y, Z. This frame is such that the blood vessel 6 lays on the horizontal plane P defined by the orthogonal axes X and Z.

With respect to this frame of reference the following variables are defined, as shown in fig. 2a and 2b: - an angle a (fig. 2b) , being the incidence angle of the propagation direction of the ultrasound beam 7 on the horizontal plane P; and - an angle γ (fig 2a) found within the projection on the P plane of the propagation direction of the ultrasound beam 7 and the X axis .

The angles a and y may be identified and estimated by the operator.

To this purpose, the operator may decide to choose the above mentioned P plane as the insonation plane. In this case the angle a results to be 0 deg and the angle γ is readily identified as the angle formed by the propagation direction of the ultrasound beam and the X axis .

In order to ascertain that the insonation plane is correct, i.e., that it coincides with the X-Z plane, it may be useful to check that, on the echographic image the vessels maintains a constant radius a along its whole curved pathway. In addition the following conditions should be verified before starting the measurement:

• the curvature radius R of the vessel 6 must be considerably bigger than the radius a of vessel 6, e.g., R > 10a. The equations adopted for the estimation of blood velocity, described below, are valid only if this condition is met;

'the ultrasound beam 7 must insonate the blood vessel 6 entirely; i.e., as shown if fig. 2c, it is necessary that both the depth h of the insonated sample volume and the width d of the ultrasound beam are bigger than the diameter 2a of the insonated vessel. The values of the variables R, a, d, h e γ are entered into the equipment 1 through the user interface 3 as shown in block 10 of fig. 3. Then, the electronic control unit 5 checks whether the above described conditions are met. If not, a message is displayed to the operator through the display unit 4, providing details about the condition that still needs to be fulfilled.

In case all condition are met, the operator is allowed to proceed with the velocity measurement of the blood flow in the vessel 6. To this purpose the ultrasound beam 7, of predetermined frequency f, is emitted from the probe 2 and directed to the vessel 6. A series of computation is then performed by the electronic control unit 5, based on the vessel's curvature radius R, of vessel's radius ar and depth h of the insonated sample volume. First, the insonated sample volume is numerically modelled and subdivided in volume units (quanta) by means of a tri-dimensional matrix, block 20. Then, block 30, the axial, w, radial, u, and tangential, vr components of the velocity are computed for each vectorial velocity value Vi pertaining to each quantum volume P± of the insonated volume, according to the following formulae: wQ 144 V M ;

-C6)

Figure imgf000011_0001

where :

• λ is equal to the ratio a/R;

• W0 is the axial blood velocity for r = 0. It may for example be considered as a normalizing factor and set equal to 1.

• Re is the Reynolds' number, defined as

Re = — ; η being blood viscosity

• ζ is equal to r/a;

• r e θ are the polar coordinates of the quantum volume P± to which the velocity components u, v e w are related. Then the velocity V±c is computed based on the three above described velocity components and according to the following formula, for each quantum volume P±:

Vic - (u s±nθ - v cosθ, u cosθ + v sinθ, w)

In particular, the above formula quantitatively define the transverse (with respect to the axial direction of the vessel) velocity components that do not exist in straight vessels but that appear whenever the blood follows a curved pathway.

Then, for each quantum volume Pi belonging to the sample volume of vessel 6, crossed by the ultrasound beam 7, the electronic control unit 5 computes the projection q± of the blood velocity V±c along the direction t of propagation of the ultrasound beam 7 (shown in fig. 2b) , according to the 'dot product' q±= V±c.tr block 40. The

"dot product' q± may turn out to be positive or negative depending on whether the velocity V±c of the blood in the quantum volume P± points toward or away from the probe 2 The electronic control unit 5 stores all positive values qp or negative values qn of the computed projections q±, corresponding to velocity with which blood quanta are respectively approaching or moving away from the probe 2. The term with highest absolute value is identified within all qp qn values, blocks 50 and 60.

Such maximum value corresponds to the maximum blood velocity component VCmax along the direction of insonation, within the sample volume.

Once Vcmax is determined, block 70, the electronic control unit 5 computes the maximum velocity VSmax, that is the velocity the blood would have, if it were flowing along a straight vessel, all other geometrical characteristics remaining unchanged, block 80.

To this aim, the electronic control unit 5 sets to zero the variable λr appearing in each of the formulae for the computation of the axial wr radial u, and tangential v components of blood velocity V±c. Thus, the radial u, and tangential v components of blood velocity- Vic a^re cancelled and the velocity of the blood flowing in a straight vessel V±s, is obtained from the single axial component w, as results from the formula of the velocity Vχc.

The electronic control unit 5 then stores the Vχs values, computed for each quantum volume, and identify the maximum Vi3 value.

Such maximum value corresponds to the maximum blood velocity component VSmax along the direction of insonation, within the sample volume of the hypothesized straight vessel. Finally, in order to quantify the difference between the blood velocity VCmax computed for the curved vessel 6 and the blood velocity VSmaχ computed hypothesizing a straight vessel 6, the electronic control unit 5 computes, block 90, a percentage error E according to the following formula:

E = VC max ~ VS max ^ ^ Q Q ^S inax

In particular, this percentage error E provides an indication about the extent the blood velocity VCmaxr computed by the electronic control unit 5 for the case of curved vessel 6, needs to be attenuated in order to match the blood velocity VSmaχr computed for the hypothetical case of straight vessel 6.

The error E is then displayed on the display unit 4.

Based on the error E the operator performing the measurement may then judge the reliability of the blood velocity being measured, block 100.

The technique described in this invention provides obvious improvements .

In particular, it allows the operator to perform a blood velocity measure in a curved portion of a blood vessel while accounting for the error generated by the occurrence of transverse blood motion with velocity components along tangential and radial directions . These transverse blood velocity components, that develop due to the vessel curvature, are responsible for alteration of the velocity measure provided by the eco-Doppler device. In summary the technique presented in the present invention, thank to the quantification of the error introduced by the vessel curvature, keeps the operator informed about the reliability of the measures performed and allows him to repeat the measure in different portions of the vessel and finally chose the most reliable measure, i.e., the one affected by the smallest error. Thank to the technique of the present invention more reliable investigations and improved diagnoses can be achieved in pathologies related to circulatory dysfunctions .

Clearly, changes may be made to the method as described and illustrated herein without, however, departing from the scope defined in the attached Claims .

For instance, the computation of a percentage error E could be based on the average blood velocity VCmean, as obtained from the average of all stored values of the projections gp and qn, and the average blood velocity

Vsmean calculated, under the assumption of straight vessel

6, from the average value of the of the stored velocities Vis.

In this case, the formula for the percentage error E would change as follows :

p _ Cmean Smean ^ -I Q Q

Smean

Claims

1. Method for the evaluation of the error (E) affecting the blood flow velocity in a vessel and, in particular, method for the evaluation of the blood flow velocity in a curved vessel, comprising:
• determining a first blood flow velocity ( Vcmaxi Vcmean) in a substantially curved vessel;
• determining a second blood velocity ( VSmaxι VSmean) , assuming that said substantially curved blood vessel has a straight shape; and
• evaluating the error (E) on the basis of said first (VcmaxiVcmean) and said second ( VSmax, VSmean) blood flow velocities .
2. The method according to claim 1, wherein determining the first blood velocities ( VCmaχ, VCmean) comprises :
• determining a first plurality of geometrical quantities of the substantially curved blood vessel (6) and a second plurality of geometrical quantities relating to a spatial relationship between said vessel (6) and transmitting-receiving means (2) configured to transmit a first signal into said curved vessel (6) and to receive a second signal as a function of said first signal; • transmitting said first signal into said blood vessel (6) ;
• receiving said second signal in response to said first signal;
• determining a volume of said blood vessel crossed by the said first signal on the basis of said first and said second geometrical quantities; • determining a plurality of points belonging to the said volume crossed by said first signal;
• determining, for each of said points belonging to said volume, a third blood flow velocity (ViC) in each of said points; • determining, for each of said points belonging to said volume, a projection {q±) of said third velocity (Vic) in a transmission direction (t) of said first signal;
• determining a maximum absolute value or a mean value of said projections (q±) ; and
• determining said first velocity ( VCmaxι VCmean) , on the basis of said maximum absolute value or said mean value .
3. The method according to claim 2 , wherein said first velocity (VCmaχ) is equal to said maximum absolute value of said projections {q±) .
4. Method, according to claim 2, wherein said first velocity (VCmax) is equal to said mean value of said projections {q±) .
5. The method, according to claim 2, wherein determining, for each of said points belonging to said volume, a third blood flow velocity (ViC) in each of said points comprises :
• determining an axial component (w) , a radial component (u) and a tangential component (v) of said third velocity (Vi); and • determining said third velocity (Vi) on the basis of said axial (w) , radial (u) and tangential
( v) components .
6. The method, according to claim 5, wherein determining a second blood velocity (VSmaχ,Vsmean) comprises, : • setting to zero said radial (u) and said tangential (v) components of said third velocity (Vi0) for each of said points belonging to said volume crossed by the said first signal;
• determining, for each of said points, a fourth blood flow velocity (V±s) on the basis of said axial component (w) and said third velocity (V±c) ;
• determining a maximum or a mean value of said fourth velocities (Vi3) ; and
• determining said second blood velocity ( VSmax, VSmean) on the basis of said maximum or said mean value of said fourth velocities (V±s) •
7. The method, according to claim 6, , wherein said second blood velocity (VSmaκ) is equal to said maximum value of said fourth velocities (Vis) .
8. The method, according to claim 6 , wherein said second blood velocity (VSmax) is equal to said mean value of said fourth velocities (Vi3) .
9. The method, according to the claims 1-3,5-7, wherein said error (E) is evaluated on the basis of said first velocity {VCmax) equal to said maximum absolute value of said projections (gjand of said second blood velocity {VSmax) equal to said maximum value of said fourth velocities (ViS) •
10. The method, according to claim 9, wherein said error (E) is calculated on the basis of the formula:
E = Vemax — ^s-L* ioO s max
wherein Vcmax and Vsmax are said first velocity equal to said maximum absolute value of said projections Cg1) and said second blood velocity equal to said maximum value of said fourth velocities [V13) , respectively.
11. The method, according to the claims 1,2,4,5,6,8, wherein said error (E) is evaluated on the basis of said first velocity {Vcmean) equal to said mean value of said projections (gjand said second blood velocity (VSmean) equal to said mean value of said fourth velocities (V1S) .
12. The method, according to claim 2, wherein said error (E) is calculated on the basis of the formula:
E = V<=mean VSmean_ ^ 1 Q Q ^Smean
wherein Vcmean and VSmean are said first velocity equal to said mean value of said projections (q±) and said second blood velocity equal to said mean value of said fourth velocities (V±s) , respectively.
13. The method, according to any one of the preceding claims, further comprising:
- supplying a piece of information relating to said error.
14. A computer product that can be loaded into the memory of a digital processor, said computer product comprising portions of software code that are able to implement the method according to any one of Claims 1 to 5 when said computer product is run on said digital processor.
15. Blood flow velocity measurement instrument configured to implement the method according to any one of the claims 1 to 13.
PCT/IB2007/001578 2006-06-13 2007-06-12 Method for estimating the errors affecting the blood flow velocity measurement in curved vessels WO2007144747A2 (en)

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ITTO20060425 ITTO20060425A1 (en) 2006-06-13 2006-06-13 Method for the estimation of measurement of speed 'of blood flow in curvilinear vessels

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20160361040A1 (en) * 2014-02-28 2016-12-15 Hitachi, Ltd. Ultrasonic image pickup device and method

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
BALBIS S ET AL: "Assessment of the effect of vessel curvature on Doppler measurements in steady flow" ULTRASOUND IN MEDICINE AND BIOLOGY, NEW YORK, NY, US, vol. 30, no. 5, May 2004 (2004-05), pages 639-645, XP004515006 ISSN: 0301-5629 *
BALBIS S ET AL: "Curvature affects Doppler investigation of vessels: Implications for clinical practice" ULTRASOUND IN MEDICINE AND BIOLOGY, NEW YORK, NY, US, vol. 31, no. 1, January 2005 (2005-01), pages 65-77, XP004711526 ISSN: 0301-5629 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20160361040A1 (en) * 2014-02-28 2016-12-15 Hitachi, Ltd. Ultrasonic image pickup device and method

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ITTO20060425A1 (en) 2007-12-14

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