Method for automatically selecting a depth range for calculating a property of a viscoelastic medium

Abstract

A method for automatically selecting a calculation depth range upon measuring a property of a viscoelastic medium, the depth range being selected from P possible ranges, includes calculating, from the ultrasound signal acquired using a probe for elastography, the property of the viscoelastic medium in at least one of the P depth ranges as well as the distance between the probe and the wall of the viscoelastic medium; determining the validity of at least one of the P calculation depth ranges; determining the validity of the calculation of the property of the viscoelastic medium over the valid calculation depth range or ranges; selecting, from the values of the property of the viscoelastic medium the calculation of which is valid at the valid depth ranges, a depth range fulfilling a selection criterion.

Claims

1. A method for automatically selecting a depth range for calculating a property of a viscoelastic medium, the depth range being chosen from P possible ranges, P being an integer number greater than or equal to 2, said method comprising: calculating, from an ultrasound signal acquired using a probe for elastography, the property of the viscoelastic medium in at least one of the P possible depth ranges and a distance between the probe and a wall delimiting the viscoelastic medium; determining a validity of at least one of the P calculation depth ranges, a calculation depth range being considered as valid if it fulfills a validity criterion calculated from the distance between the probe and the wall delimiting the viscoelastic medium; determining the validity of the calculation of the value of the property of the viscoelastic medium over the valid calculation depth range or ranges, said calculation being considered as valid if it fulfills a validity criterion calculated from the quality of an elastogram; selecting, from among the valid depth ranges comprising at least one valid calculation of the property of the viscoelastic medium, a depth range fulfilling a predetermined selection criterion.

2. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 1, wherein: each depth range is delimited by a first depth and a second depth; the depth range is defined as valid if the distance between the probe and the wall delimiting the viscoelastic medium is less than the first and the second depths.

3. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 1, wherein, during the calculating, the property of the viscoelastic medium is calculated from M measurements made in at least one of the P possible depth ranges, M being an integer number greater than or equal to 2.

4. The method for automatically selecting a calculation depth range for a property of a viscoelastic medium according to claim 3, wherein the selection criterion of the calculation depth range is based only on the last measurement made.

5. The method for automatically selecting a calculation depth range for a property of a viscoelastic medium according to claim 4, wherein the depth range selected during the selecting is that in which the elastogram has the greatest signal-to-noise ratio.

6. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 4, wherein the depth range selected during the selecting is that for which the elastogram is of the best quality.

7. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 4, wherein the depth range selected during the selecting is that which fulfills a criterion determined from the homogeneity of the medium.

8. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 3, wherein the selection criterion is based on the complete set of measurements made.

9. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 8, wherein the depth range selected during the selecting is that which minimises dispersion between the calculated values of the property.

10. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 8, wherein the depth range selected in the selecting is that which maximises the number of valid calculations of the property.

11. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 8, wherein the depth range selected during the selecting is that in which the elastograms calculated have the greatest signal-to-noise ratio.

12. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 8, wherein the depth range selected during the selecting is that for which the elastograms are of the best quality.

13. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 8, wherein the depth range selected is that which fulfills a quality criterion determined from the homogeneity of the medium.

14. The method for automatically selecting a calculation depth range upon measuring a property of a viscoelastic medium according to claim 1 wherein, if the depth range in which the calculation of the value of the property has been performed is not valid, the calculation is performed at a deeper range.

15. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 1, wherein if, during the selecting, at least two depth ranges fulfill the selection criterion, the shallower range is selected.

16. The method for automatically selecting a depth range for calculating a property of a viscoelastic medium according to claim 1, wherein if, during the selecting, at least two depth ranges fulfill the selection criterion, the deeper range is selected.

17. A method of global measurement of at least one property of a viscoelastic medium comprising: automatically selecting the calculation depth range using the method according to claim 1; performing a global calculation of the property of the viscoelastic medium from the values of the property for which the calculation is valid at the selected depth ranges, the calculation being performed using a mathematical function of the median or mean type.

18. The method of global measurement of at least one property of a viscoelastic medium according to claim 17, wherein the property of the viscoelastic medium is selected from a group comprising: elasticity, Young's modulus, shear modulus, shear rate within the viscoelastic medium, an ultrasound attenuation parameter or a combination of these properties.

19. A device for measuring a property of a viscoelastic medium with automatic selection of the calculation depth range using the method according to claim 17, said device comprising: a probe for elastography; a calculation unit comprising at least a memory and a microprocessor; said device being constructed and arranged to: calculate, from the ultrasound signal acquired using the probe for elastography and the calculation unit, the distance between the probe and the wall delimiting the viscoelastic medium; calculate, from the ultrasound signal acquired using the probe for elastography and the calculation unit, the value of the property of the viscoelastic medium in at least one of the P possible calculation depth ranges; determine the validity of at least one of the P calculation depth ranges, a calculation depth range being considered as valid if it fulfills a validity criterion calculated from the distance between the probe and the wall delimiting the viscoelastic medium; determine the validity of the calculation of the property of the viscoelastic medium over the valid depth range or ranges, said calculation being considered as valid if it fulfills a validity criterion determined from the quality of an elastogram; select, among the valid depth ranges comprising at least one valid property calculation, a depth range fulfilling a predetermined selection criterion; calculate, from the values of the property for which the calculation of which is valid at the selected depth ranges, the global value of the viscoelastic property, the global value being calculated using a mathematical function of the median or mean type.

Description

LIST OF THE FIGURES

(1) Further characteristics and advantages of the invention will be clear from the description thereof given below, which is indicative and in no way limiting, with reference to the figures among which:

(2) FIG. 1 illustrates the distribution of the acoustic power of ultrasonic waves propagating inside a viscoelastic medium of interest during a measurement of the transient elastography type, the operator being able to choose between two depth ranges S1 and S2;

(3) FIG. 2 illustrates the measurement of a property of a viscoelastic medium such as a liver according to the state of the art: the calculation depth range may include the wall of the medium;

(4) FIG. 3 schematically illustrates the steps of the method according to the invention;

(5) FIG. 4 illustrates an optimal choice of the calculation depth range upon measuring a property of a viscoelastic medium according to the invention;

(6) FIGS. 5a to 5d illustrate examples of validation of depth ranges for different probe-to-capsula distances PCD;

(7) FIGS. 6a and 6b illustrate an example of validation of a calculation of the value of the property of the viscoelastic medium over P=3 depth ranges;

(8) FIGS. 7a and 7b illustrate an example of validation of a calculation of a value of a property and automatic selection of the calculation depth range, the method according to the invention being implemented with P=3 possible depth ranges, wherein the validity criterion may be based on the last measurement only or on all the measurements made;

(9) FIG. 8 schematically illustrates the steps of the method of global measurement of a property of a viscoelastic medium with automatic selection of the calculation depth range and the global calculation of the value of the property;

(10) FIG. 9 summarises the method PRO for automatically selecting the calculation depth range according to the different selection criteria.

DETAILED DESCRIPTION

(11) FIG. 3 schematically illustrates the steps of the method PRO according to the invention.

(12) The method PRO according to the invention comprises the following steps: A CALC step of calculating, from the ultrasound signal acquired using a probe for elastography, a viscoelastic property in at least one of the P possible calculation depth ranges. In this step the property can be calculated in a single depth range or in several depth ranges; during this step, the ultrasound signal acquired with a probe for elastography is used to calculate the distance PCD between the probe for elastography and the wall of the viscoelastic medium; A step TEST_PCD of validating the calculation depth ranges in which the viscoelastic property has been calculated. A depth range is defined as valid if it fulfills a criterion calculated from the distance between the probe and the wall of the medium or “probe-to-capsula distance”, PCD; A step TEST_VAL of validating the calculations of the value of the viscoelastic medium property value over the valid calculation depth range or ranges. During this step, a validation criterion is applied to each calculation of the viscoelastic property to determine the validity thereof. A calculation is considered as valid if it fulfills a validity criterion calculated from the quality of an elastogram. For example, a calculation is considered as valid if the quality of the elastogram measured in the calculation depth range is sufficiently high. In other words, a calculation is considered as valid if the signal-to-noise ratio of the measured elastogram is sufficiently high; A step SEL of selecting a depth range from the valid depth ranges including at least one valid calculation according to a selection criterion. The selection criterion can be based only on the last measurement made or all measurements made. In other words, if the calculation step CALC includes performing a single measurement, the selection criterion is based only on the last or current measurement. If the calculation step CALC includes making multiple measurements, the selection criterion may take into account the different measurements made, that is the history of measurements made.

(13) These steps can be performed in the order shown in FIG. 3 or in a different order. The steps, all or part of them, can be performed in parallel.

(14) According to an embodiment, the selection criterion for the calculation depth range is based only on the current measurement. In this case, the selection of the optimum depth range only takes into account information provided by the last measurement performed.

(15) The depth range selected in the selection step SEL can be: that for which the best signal-to-noise ratio is observed in the measured elastogram; that for which the best propagation of the shear wave is observed on the elastogram, that is the best quality elastogram; that for which the greatest homogeneity of the medium is observed.

(16) Beneficially, these depth range selection criteria make it possible to select, among the depth ranges within the medium, the depth range in which the shear wave propagates correctly or that corresponding to a greater homogeneity of the organ.

(17) According to another embodiment, the depth range selection criterion is based on the history of the measurements made. In this case, the selection of the optimum depth range takes into account information provided by all the measurements made.

(18) The selected depth range can be: that with the highest number of valid measurements among the M measurements made; that for which the smallest dispersion between the calculated property values is observed among the M measurements made; that which fulfills a criterion calculated from the homogeneity of the environment; that for which the best mean or median signal-to-noise ratio is observed in the elastograms among the M measurements made; that for which the best shear wave propagation on the elastogram is observed, that is the best mean or median quality criterion among the M measurements made; that for which the greatest mean or median homogeneity of the medium is observed among the M measurements made.

(19) The first CALC step includes calculating a property of the medium in at least one of the P possible depth ranges, the calculation being carried out from the ultrasound signal acquired by an ultrasound probe during a measurement. According to an embodiment, the calculation is repeated M times in at least one of the P ranges.

(20) The measured property can be a viscoelastic property such as the speed of propagation of a pulse shear wave or the elasticity of the medium. In this case the measurement is a transient elastography measurement.

(21) The measured property can be an ultrasound property such as a controlled attenuation parameter (CAP). In this case the measurement includes the generation of a series of ultrasound acquisitions.

(22) During the step TEST_PCD, the validity of each calculation depth range is determined. Each calculation depth range is considered as valid only if the validity criterion fulfills a predetermined condition.

(23) The depth range validity criterion can be binary and has a first value corresponding to a valid range and a second value corresponding to a non-valid range.

(24) According to an embodiment, the depth range validity criterion is determined from the reflected ultrasound signal, over which the distance between the ultrasound probe and the wall of the viscoelastic medium is calculated. This distance is also called “probe-to-capsula distance” or PCD.

(25) A depth range is defined as valid if it does not comprise the wall or surface of the viscoelastic medium. For example, if the viscoelastic medium to be characterised is a human or animal liver, a depth range is defined as valid if it does not include the liver capsula.

(26) Beneficially, such a validity criterion makes it possible to keep only the measurements entirely included within the medium to be characterised.

(27) During the step TEST_VAL, the validity of the calculation of the value of the property of the viscoelastic medium over the valid calculation depth range or ranges is determined. In other words, the validity of each of the calculated values is determined at each new measurement M. Each calculation is considered as valid only if the validity criterion fulfills a predetermined condition. The calculation validity criterion can be binary and has a first value corresponding to a valid measurement and a second value corresponding to a non-valid measurement.

(28) According to an embodiment, the measurement validity criterion is defined from a pulse elastogram.

(29) The elastogram is the image used to visualise the propagation of the shear wave during a pulse elastography measurement. The elastogram is defined by a two-dimensional matrix and provides a spatio-temporal representation of displacements generated by the propagation of the shear wave in the medium.

(30) For example, the validity criterion can be established from the quality of the measured pulse elastogram. An estimate of the quality of the pulse elastogram can be provided by the signal-to-noise ratio of the pulse elastogram. The property values for which the calculation is valid are then the measurements with a pulse elastogram having a signal-to-noise ratio above a predetermined threshold.

(31) A calculation of the property of the medium is automatically considered as invalid over an invalid depth range.

(32) Beneficially, such a validity criterion makes it possible to discard measurements corresponding to a poor quality pulse elastogram, for example due to poor positioning of the probe or poor propagation of the transient shear wave within the viscoelastic medium.

(33) During the step SEL, the optimum depth range is selected from the property value or values for which the calculation is valid. A depth range is defined optimal if it fulfills the best selection criterion imposed.

(34) Beneficially, this step allows the automatic selection of the optimum depth range for measuring the property of the medium.

(35) FIG. 4 illustrates the selection of the calculation depth range according to the invention when the viscoelastic medium is a liver F and the validity criterion of a depth range is determined from an ultrasound measurement, namely the probe-to-capsula distance PCD between the patient's skin S and the liver wall. The horizontal axis in the figure represents the depth Pr measured from the patient's skin S.

(36) In the case illustrated in FIG. 4 the probe-to-capsula distance PCD measured by virtue of one or more ultrasound acquisitions is 38 mm.

(37) The calculation depth range P has a lower bound Pi and an upper bound Ps. In the case of FIG. 4, Pi=45 mm and Ps=85 mm.

(38) In order to obtain a valid depth range, the calculation depth range P is chosen so that its lower bound Pi is strictly greater than the probe-to-capsula distance PCD, Pi>PCD.

(39) In other words, if Pi>PCD, the validity criterion takes the value corresponding to a valid depth range. Otherwise, the validity criterion takes the value corresponding to a non-valid depth range.

(40) To increase the reliability of the validity criterion of a depth range, it is possible to introduce a transition zone Tr. The validity condition of a depth range in the depth range P then becomes: Pi>PCD+Tr.

(41) In the example of FIG. 4 the transition zone has a thickness of 5 mm, Tr=5 mm.

(42) Beneficially, the use of a validity criterion for a depth range defined from the PCD capsula probe distance ensures that the calculation depth range is entirely included within the medium to be characterised.

(43) In other words, the use of a validity criterion for a depth range defined from the PCD probe-capsula distance enables measurement errors caused by the wall of the viscoelastic medium within the measurement zone to be avoided.

(44) FIGS. 5a, 5b, 5c and 5d illustrate the validation of the measurement depth ranges of an elastic property of a liver F when three depth ranges are possible. The three depth ranges represented have depths comprised: between 35 mm and 75 mm for P[35-75]; between 40 mm and 80 mm P[40-80]; between 45 mm and 85 mm P[45-85].

(45) FIG. 5a corresponds to a probe-to-capsula distance PCD=26 mm. Using the quality criterion of a depth range illustrated with reference to FIG. 4, the result is that 35 mm>PCD+Tr, 35 mm being the lower bound of the shallower range. In this case all three depth ranges correspond to a valid depth range VA.

(46) FIG. 5b corresponds to a probe-to-capsula distance PCD=32 mm. Using the quality criterion illustrated with reference to FIG. 4, the result is that 35 mm<PCD+Tr<40 mm. The depth range P[35-75] has a lower bound equal to 35 mm and therefore corresponds to a non-valid NVA depth range. This is due to the fact that the depth corresponding to PCD+TR=37 mm falls within the range P[35-75]. In other words, the measurement of a liver property made in correspondence of the range P[35-75] would be modified by the presence of the liver wall and the transition zone Tr. On the other hand, the depth ranges P[40-80] and P[45-85] correspond to valid depth ranges because they are entirely included within the environment F to be characterised.

(47) FIG. 5c illustrates the case of a probe-capsula distance PCD=38 mm. The depth PCD+Tr=43 is greater than the lower bounds of the ranges P[35-75] and P[40-80]. These two depth ranges therefore correspond to non-valid NVA depth ranges. In the case illustrated in FIG. 5c, only the range P[45-85] corresponds to a valid VA depth range.

(48) FIG. 5d illustrates the case of a probe-to-capsula distance PCD=60 mm. In this case, none of the depth ranges corresponds to a valid VA depth range.

(49) FIG. 6a illustrates the step TEST_VAL of determining the validity of the calculation of the property value of the viscoelastic medium over the valid calculation depth range or ranges. For each measurement M, the calculation quality criterion is calculated in the valid depth range or ranges.

(50) The validity criterion of a property calculation is established from a quality criterion of the pulse elastogram E.

(51) FIG. 6b illustrates an example of validation of calculations of the values of a property of a viscoelastic medium in the form of a table, constructed from an elastogram such as that represented in FIG. 6a. The construction of a table such as that illustrated in FIG. 6b is an embodiment of the step TEST_VAL of determining the validity of each calculation of the property as a function of valid depth ranges.

(52) The table in FIG. 6b illustrates the case of a sequence comprising measurements #1, #2, #3 for three possible depth ranges: P[35-75], P[40-80] and P[45-85].

(53) The solid line represents the depth of the probe-to-capsula distance PCD+Tr at measurement #i.

(54) The rows in the table show the binary validity results of the property calculations for each depth range P. The lower lines of each line correspond to the boundary depth at which the depth range becomes invalid. When the depth range is not valid, the property calculation validity criterion is not determined and is defined as invalid.

(55) This table illustrates the validity conditions of the calculation of the value of a viscoelastic property according to the variation in PCD values.

(56) The validity criteria of the property calculation in FIG. 6b are binary and can take two values “o” and “x”. The value “o” corresponds to a valid calculation over a given depth range, the value “x” corresponds to a non-valid calculation. The invalidity of a depth range is symbolised by the abbreviation “NVA”, which means that the PCD capsula probe distance does not fulfill the following condition defined previously: Pi>PCD+Tr.

(57) The property values of the validity criterion of the calculation show that measurement #1 is valid in the depth range P[40-80] and non-valid in the other two depth ranges P[35-75] and P[45-85] indicating poor quality of the elastogram.

(58) The values of the depth range criterion indicate that measurement #2 is non-valid in all depth ranges. The position of the solid black line shows the presence of the liver capsula within the possible depth ranges when making the measurement.

(59) The values of the validity criterion of the depth range show that measurement #3 is non-valid in depth range P[35-75], the shallower depth range. The non-validity of the calculation of property #3 in correspondence of the depth range P[35-75] is caused by the presence of the liver capsula in this depth range.

(60) The steps of determining the depth range validity TEST_PCD and measuring TEST_VAL are typically performed using calculation means such as a memory and a microprocessor present in the device used to make the measurement.

(61) The step SEL of automatically selecting the depth range fulfilling the optimal selection criterion when the selection of the optimal range is based only on the current measurement is illustrated in FIG. 7a.

(62) FIG. 7a represents the case of repeated measurement M=6 times over P=3 possible ranges. The two tables represent the history of measurement validity criteria at the times of measurements #3 and #6 in the three depth ranges.

(63) Each column represents the values of the validity criteria for the calculation of the value of the property of the viscoelastic medium calculated for a given measurement #i, each row corresponding to one of the three possible depth ranges.

(64) Within a given depth range, a property calculation may be invalidated either because of an invalid depth range or by an elastogram quality criterion below a predetermined threshold.

(65) For each measurement, the depth range is automatically selected independently of the previous measurements, and appears on a light background in the tables. From one measurement to the next, the history of selecting the optimum depth range does not change.

(66) The mean or median value of the property of the viscoelastic medium is calculated from all the valid values of the property of the viscoelastic medium over potentially different depth ranges.

(67) The step SEL of automatically selecting the depth range fulfilling the optimal selection criterion when the selection of the optimal range is based on the history or all the measurements is illustrated in FIG. 7b.

(68) FIG. 7b considers the example of FIG. 7a again, representing the case of a repeated measurement M=6 times over P=3 possible ranges. The two tables represent the history of measurement validity criteria at the times of measurements #3 and #6 in the three depth ranges.

(69) Each column represents the values of the validity criteria of a calculation of a property value calculated for a given measurement #i, each row corresponding to one of the three possible depth ranges.

(70) Within a given depth range, a property calculation may be invalidated either because of an invalid depth range or by an elastogram quality criterion below a predetermined threshold.

(71) For each measurement, the depth range is automatically selected and appears on a light background in the tables. From one measurement to the next, the history of the depth range selection is updated. The depth range selected from all previous measurements is replaced with the optimum depth range of the last measurement.

(72) For example, at the time of measurement #3, the deeper depth range P[45-85] is selected for all measurements #1 to #3. At the time of measurement #6, the optimum selection criterion is met for the range P[40-80], which updates and replaces all previous range selections with the same depth range P[40-80].

(73) The mean or median value of the viscoelastic medium property is calculated from all valid property values in the same depth range.

(74) According to an embodiment, if two or more depth ranges provide the same optimal selection criterion in the SEL selection step, the shallower depth range is selected.

(75) Beneficially, this allows the depth range closest to the ultrasound transducer and thus that with the highest signal-to-noise ratio to be selected.

(76) Alternatively, according to another embodiment, the deeper depth range is selected.

(77) Beneficially, this allows the depth range furthest from the liver capsula to be chosen.

(78) The selection step SEL is typically performed using calculation means such as a memory and a microprocessor present in the device used to make the measurement.

(79) FIG. 9 graphically summarises the steps of validating the depth range TEST_PCD, validating the calculation of the property value of the viscoelastic medium TEST_VAL and selecting the optimum depth range SEL.

(80) According to an embodiment, the SEL selection of the optimum depth range is based only on the current measurement.

(81) In another embodiment, the SEL selection of the optimum depth range is based on the history of the measurements made.

(82) FIG. 8 schematically illustrates the method steps for the global measurement of a property of a viscoelastic medium with automatic selection of the calculation depth range, the automatic selection of the depth range being carried out using the method PRO according to the invention.

(83) The measurement method comprises a first step A of automatically selecting the calculation depth range using the method PRO according to the invention. In step A, the measured values of the viscoelastic property are stored in a memory.

(84) The global measurement method further includes a second step B of global calculation the viscoelastic property from the values calculated in step A. The global calculation can be performed using a mean or median type function.

(85) According to an embodiment, in step B, the property of the viscoelastic medium is calculated from the values of the property for which the calculation is valid and which have been calculated in step A.

(86) If the method PRO includes the performance of M measurements, the global calculation of the property of the viscoelastic medium is performed from the values of the property the calculation of which is valid and performed in the previously selected depth range or ranges. For example, the value of the property of the viscoelastic medium is the mean or median of the values of the property the calculation of which is valid and obtained over the selected depth range or ranges.

(87) In practice, in the case of depth range selection based on the current measurement only, once the depth range of the last measurement is selected, the M measurements corresponding to independent depth ranges are used to determine the global value of the property measured. For example, the global measured value can be a mean of the values calculated corresponding to the selected range.

(88) In the case of depth range selection based on the history of measurements made, once the depth range of the last measurement is selected, the property values of the M measurements corresponding to the last range chosen are used to determine the global value of the property. For example, the global value may be a mean of the values calculated in correspondence of the selected range. Beneficially, the global measurement method according to the invention makes it possible to improve reproducibility and reliability of the measurement of a viscoelastic property by selecting the property values the calculation of which is valid over the optimum depth range or over the optimum depth ranges for the measurement.

(89) The measurements necessary for the implementation of the methods for automatically selecting a depth range and measuring a property of a viscoelastic medium according to the invention can beneficially be carried out within the scope of a transient elastography measurement using a device such as a Fibroscan®.

(90) In this case the property of the viscoelastic medium is selected from a group of properties comprising: elasticity, Young's modulus, shear modulus, speed of propagation of a shear wave within the viscoelastic medium.

(91) The P possible ranges of calculation depth are set within the range of ultrasound acquisition depth in which the medium is observed. This observation zone depends on the properties of the probe used upon examining. As illustrated in FIG. 1, there are several choices of ranges P of measurement depths.

(92) Each measurement M implementing the method PRO according to the invention includes monitoring the propagation of a transient shear wave within the viscoelastic medium to be characterised. To do this, during measurement #i, a transient elastogram E is constructed from the ultrasound signal acquired during the transient pulse elastography measurement. The elastogram E makes it possible both to measure one of the above-mentioned elastic properties and to determine a validity criterion of the property calculation within a defined depth range.

(93) During the propagation of the transient shear wave, ultrasound acquisitions are generated with a high repetition rate to monitor the propagation of the transient shear wave. The reflected signals can also be used to determine the probe-to-capsula distance. The value of the probe-to-capsula distance makes it possible to calculate the validity criterion for a depth range.

(94) The step of automatically selecting the depth range is performed using the calculation means included in the device used to make the transient elastography measurement. The calculation means include for example a memory and a microprocessor. The memory is configured to store the results of the M measurements made as well as the values of the various validity criteria calculated by the microprocessor.

(95) The device used to perform the transient elastography measurement further includes means for displaying the results of the methods such as the selected calculation depth range or the property value of the viscoelastic medium.

(96) Alternatively, the property of the medium can be an ultrasound attenuation parameter such as a controlled attenuation parameter (CAP).

(97) A third object of the present invention is a device for measuring a property of a viscoelastic medium with automatic selection of the calculation depth range using the method according to the invention, said device comprising: a probe for elastography; calculation means comprising at least a memory and a microprocessor;

(98) said device being constructed and arranged to: Calculate, from the ultrasound signal acquired using the probe for elastography and the calculation means, the distance between the probe and the wall delimiting the viscoelastic medium (PCD); Calculate, from the ultrasound signal acquired using the probe for elastography and the calculation means, the property of the viscoelastic medium in at least one of the P possible calculation depth ranges; Determine the validity of at least one of the P calculation depth ranges, a calculation depth range being considered as valid if it fulfills a validity criterion calculated from the distance between the probe and the wall delimiting the viscoelastic medium (PCD); Determine the validity of the calculation of the property value over the valid depth range or ranges, a measurement being considered as valid if it fulfills a validity criterion determined from the quality of an elastogram; Select, from among the valid depth ranges including at least one valid measurement, a depth range fulfilling a predetermined selection criterion. Calculate, from the values of the property the calculation of which is valid made at the selected depth ranges, the global value of the viscoelastic property, the calculation being performed using a mathematical function of the median or mean type.

(99) By probe for elastography it is meant a probe with at least one ultrasound transducer. An example of a probe for elastography is a probe for carrying out a transient elastography method.

(100) The device is configured to implement the method for automatically selecting the calculation depth range according to the invention and calculating the global value of a property of the medium within the selected depth ranges.

(101) According to an embodiment of the device according to the invention, the calculation means are included in the probe for elastography.

(102) According to an embodiment, the device according to the invention further comprises means for displaying the results of the measurements. For example, the display means are configured to display the measured elastogram, the depth range chosen for the measurement and the measured viscoelastic property.

(103) FIG. 9 summarizes the method PRO for automatically selecting a calculation depth range according to the invention.

(104) During the step CALC, an ultrasound probe or a probe for elastography is used to calculate a property of the viscoelastic medium as well as the distance between the probe and the wall of the viscoelastic medium PCD. The property of the viscoelastic medium and the PCD are calculated from ultrasound shots emitted by the probe and ultrasound waves reflected by the medium and detected by the probe.

(105) During the step TEST_PCD, the validity of the calculation depth ranges of the viscoelastic property is checked. A range is considered as valid if it is entirely included within the viscoelastic medium, that is if the distance from the probe to the medium wall is less than the bounds of the depth range PCD<Pmin.

(106) According to an embodiment, if no depth range is valid, the calculation of the value of the property of the viscoelastic medium is performed at the next depth range. Alternatively, the method PRO is stopped.

(107) During the step TEST_VAL, the validity of the calculations of the property values of the viscoelastic medium corresponding to the valid depth ranges is checked. A calculation is defined as valid based on the quality of the elastogram associated with the measurement.

(108) In the step SEL, a depth range is selected from the valid depth ranges with at least one valid property calculation. The selection of the depth range is made according to a predefined criterion.

(109) FIG. 9 illustrates two embodiments of the selection step SEL.

(110) According to the first embodiment, the selection of the calculation depth range is based only on the current or last measurement performed. In this case, if several depth ranges include values of the property the calculation of which is valid, one of the following selection criteria is applied to select a depth range: Best signal-to-noise ratio of the elastogram among all depth ranges; Best shear wave propagation on the elastogram (quality criterion) among all depth ranges; Best homogeneity criterion (LTT) among all depth ranges.

(111) This embodiment is also illustrated in FIG. 7a.

(112) In a second embodiment, the selection of the calculation depth range is based on the history or all the measurements made. In this case, the selected depth range is that fulfilling one of the following criteria: Maximum of the number of values of the property the calculation of which is valid; Minimum dispersion of the mean or median value of the property; Best mean or median signal-to-noise ratio of the elastogram among all depth ranges; Best shear wave propagation on the elastogram (mean or median quality criterion) among all depth ranges; Best mean or median homogeneity criterion (LTT) among all depth ranges.

(113) This embodiment is also illustrated in FIG. 7b.