FR2778574A1 - Method to determine effect of ultrasonic treatment therapy on tissue - Google Patents

Abstract

The method involves measuring ultrasound signals back scattered from the tissue before and after treatment. An Independent claim is included for an ultrasound device for tissue therapy.

Description

METHOD FOR MEASURING THE EFFECT OF TREATMENT ON TISSUE

  The invention relates to a method and a device for measuring the effect of a treatment on a fabric. It uses the measurement of the change in the mean of acoustic tissue attenuation as a function of frequency to determine the

effects of treatment.

  The invention applies in particular to the field of the therapeutic treatment of tissues by focused ultrasound, and more particularly the field of destruction of tissues inside an organism by causing high temperatures by means of focused ultrasound. In the general field of focused ultrasound, as those skilled in the art know, there are different types of treatments: the oldest treatment is lithotripsy treatment, which is applied to the destruction of hard bodies; this type of treatment uses shock waves, i.e. short pulses and high power. It was then proposed to treat the soft tissues by hyperthermia, by heating the tissues at low temperatures, ie below C. The hyperthermia involves sending to the tissues to be treated ultrasound in the form of long pulses and lower power. Finally, high intensity focused ultrasound soft tissue treatments, commonly referred to as High Intensity Focused Ultrasound (HIFU) treatments, are now proposed. HIFU treatments consist of heating tissue at high temperatures, typically above 45 C. High intensity focused ultrasound (HIFU) treatments are an effective way to create coagulation necrosis lesions in biological tissues to treat localized tumors. For the HIFU arise different problems. In general, the goal is to improve the effectiveness of treatment, i. e. the destruction of the chosen tissues. A first problem is that of access to targets or tissues to be treated. Because of the patient's anatomy, the targets are sometimes difficult to access ultrasonic beams. It has been proposed to move the transducer; however, the displacement of the transducer may also be limited by the morphology of the patient. In the case of treatment of the prostate with an endorectal probe, various solutions to this problem have been proposed, see for example FR 91 02 620, FR 93 09 158, FR 96 08 096, FR 94 01 304, FR 94 06 539. These different solutions could be further improved, to ensure better treatment, in specific areas, by hyperthermia or by

HIFU.

  A second problem lies in the choice of the parameters of the ultrasound emission; these, and in particular the frequency must be chosen very precisely. They usually depend on many factors, such as: depth

  target, nature of tissue, type of necrosis sought.

  A third problem lies in the control of the results obtained: although good results are currently obtained, especially in urology, the devices used lack a control system to verify the effectiveness of treatments. It has been proposed to use an optical fiber measurement of the temperature in the target; such a type of measurement involves implanting in the organ to treat fibers. This solution is for example proposed in the prostate treatment devices marketed by Domrnier Medizintechnik under the reference Eurowave. It has also been proposed to use the ultrasound image of the treated zone, which most often presents echogenicity variations at the level of the treated zone. Unfortunately A. Sibille et al., "Characterization of extracorporeal ablation of normal and tumor-bearing liver tissue by High Intensity Focused Ultrasound", Ultrasound in Med. And Biol., Vol 19, No. 9, pp. 803-813, 1993 shows that there is no correlation between the level of echogenicity and the rate of

tissue destruction.

  The invention provides a method and a non-invasive device for local measurement of attenuation variation; this measure, applied to HIFU treatments, makes it possible to characterize the appearance of coagulation necrosis lesions during treatment, and thus provides the therapist with a tool for measuring and controlling

  treatment, non-invasive and in real time.

  More specifically, the invention provides a method of measuring the effect of a therapy on a tissue, comprising: measuring ultrasound backscattered by the tissue prior to treatment;

  the measurement of ultrasound backscattered by the tissue after the treatment.

  Advantageously, the method further comprises calculating the variation of

  the average attenuation of the tissue over a frequency range.

  In one embodiment, the measuring steps include measuring the

  ultrasound emitted by ultrasound and backscattered transducers.

  Advantageously, the average attenuation is calculated by the method known as

frequency bands.

  The tissue can include the prostate.

  The treatment preferably comprises the induction of necrosis in the tissue by a sudden rise in temperature. This can be achieved by applying ultrasound

focused on the fabric.

  The invention also relates to an apparatus for treating tissue by necrosis formation in tissue, comprising means for measuring ultrasound backscattered by the tissue prior to treatment, and for measuring backscattered ultrasound

by the tissue after the treatment.

  Preferably, the apparatus further comprises means for calculating the

  variation of the average attenuation of the tissue over a frequency range.

  The measuring means advantageously lead to a measurement of the signals

  received on an ultrasound transducer.

  In one embodiment, the apparatus includes means for displaying the average attenuation variation in a plane perpendicular to the processing direction. Preferably, the apparatus comprises means for inducing necrosis in the tissue, by sudden elevation of temperature, typically a focusing transducer.

ultrasound.

  Other characteristics and advantages of the invention will appear on reading

  the following description of embodiments of the invention, given by way of example

  and with reference to the appended drawings which show: FIGS. 1 to 3, the different steps of a focused ultrasound treatment in an apparatus that can implement the invention; FIG. 4 the device for acquiring RF signals backscattered by the tissues; FIG. 5 shows the typical evolution of attenuation as a function of frequency, before and after the treatment; - Figure 6 shows the evolution of the average variation attenuation, normalized compared to the value obtained on the central portion of the treatment; FIG. 7 shows an example of evolution of the average variation of the attenuation in the longitudinal direction; FIG. 8 shows an example of evolution of the average variation of the attenuation in the transverse direction; - Figure 9, a graphical representation of the average variation of the attenuation, in a plane perpendicular to the main direction of propagation of ultrasound; - Figures 10 and 11, representations similar to that of Figure 9, made at different times; FIGS. 12, 13 and 14, the average attenuation before the treatment, the average attenuation after the treatment and the variation of attenuation, on views similar to those of FIG. 9;

  In the remainder of the description, the invention is described in its application to

  control of HIFU treatment of the prostate, for a device marketed under the trademark Ablatherm by the company EDAP TECHNOMED, described in the

application PCT / FR94 / 00936.

  The principle of HIFU treatments consists in inducing in the focal zone of an ultrasonic transducer coagulation necrosis lesions caused mainly by a sudden rise in temperature, even if cavitation phenomena are present. This temperature rise corresponds to a transformation of the acoustic energy into heat related to the absorption of ultrasound in tissues by the relation: (Q) ocaI o <Q> is the sound power density transformed per unit of volume, a is the absorption of the tissue and I is the ultrasonic intensity in the considered region. The rapidity of the phenomenon makes it possible to overcome the problems related to heat conduction and tissue perfusion and to consider each lesion as a

  elementary lesion that just need to be reproduced throughout the volume to be treated.

  The application of the invention to the control of treatments is based on the experimental observation that the appearance of lesions in a tissue, and in particular lesions created by HIFU causes an increase in certain parameters.

  acoustic tissue, including attenuation.

  The Ablatherm device combines an endorectal transducer with a 2.5 MHz therapy transducer and a 5.5 MHz center-frequency mechanical scanning biplane imaging transducer with a bandwidth of 2 MHz and a focal length of 2.75 cm; the ultrasound transducer is connected to an ultrasound system

  reference C311 from KRETZ.

  Under normal conditions of use, the therapy transducer creates individual cylindrical lesions 1.6 mm in diameter and 20 mm in height. The imaging transducer is used to acquire the RF signals for measuring the invention. The steps of a representative treatment are shown in Figures 1 to 3. In a first step shown in Figure 1, the probe 1 is introduced into the patient's rectum, facing the prostate 2; the imaging transducer 3 moves longitudinally in steps of 1.6 mm from the apex to the base of the organ allowing visualization in cross sections 4, 5, 6 that the clinician defines according to the position of the areas to be treated. The displacement of the imaging transducer is symbolized by the arrow in the figure. It should be noted that the probe

  itself remains fixed with respect to the patient.

  Figure 2 shows the treatment step; the imaging transducer is retracted, as symbolized by the arrow, and the therapy transducer 8 is placed parallel to the posterior face of the organ to be treated, to allow the transmission

  of ultrasound focused towards the prostate. Induced lesions are marked in gray.

  Figure 3 shows a control step in which the imaging transducer allows visualization of the treated areas. In the example of these figures, the acquisitions were made during the pre-treatment and post-treatment imaging phases at identical positions. Since the studied tissue is perfectly immobile, the probe does not move, we can say that these

  acquisitions were made on the same transverse slices of tissue.

  Figure 4 shows the device for acquiring the RF signals backscattered by the tissues. An RF output 10 has been implanted on the echograph 11 so that the data can be stored in the memory of a microcomputer 12 (PC 486). An 8-bit digital analog conversion card 13 (STR 8100 D, Sonix) was used for digitizing the RF signals from the ultrasound system with a sampling frequency of 50 IMHz. A 14 FPC 24 counter card made it possible to choose a region of interest (ROI) on the ultrasound image by selecting the RF lines corresponding to the study unit by means of the signaling signals.

  synchronization of lines and images from the ultrasound system.

  On each transverse slice, this device makes it possible to acquire 250 neighboring echo lines corresponding to about 2/3 of the echographic image and containing all the echoes coming from the tissues to be studied. At the end of the displacements one thus acquired a whole volume of tissue whose dimensions 5x4x3 cm3 correspond to

5000 adjacent RF lines.

  The device makes it possible to enter the information provided by the ultrasound transducer on the fly; the combination of this information with the synchronization information makes it possible to determine a region of interest. It should be noted that one could as well proceed to the analysis of the backscattered RF signals by the

  tissues throughout the area covered by the system, if this is necessary.

  The measurement of attenuation according to the invention is now described. This measurement assumes that, in the passband of the transducer, attenuation has tissue following

  a law affine as a function of the frequency a (f) = ao +, l. (f - fr).

  o ca, is the attenuation coefficient at the center frequency of the transducer fc, and [3 is the slope of the attenuation on the passband of the transducer. Under these conditions, the power spectrum of the acoustic signal backscattered by the tissue located at a distance z from the transducer is defined by: S (f, z) = G (f) 2D (f) 2H 2 (z, f) 2 e -2a (f) (1) where G is the transfer function of the ultrasonic source. It groups together the terms of electroacoustic and acoustoelectric conversion of the transducer during the transmission and reception of the signal. D is the backscattering term assumed to be homogeneous and isotropic in the studied volume. H is the term due to the diffraction of the transducer. e- (f), is the transfer function of the attenuation of ultrasound in

tissues.

  In the following are described two modes of measuring the attenuation. One is based on the study of the shift from the central frequency of the spectrum to the low frequencies, called the centroid method; the other one is interested in the decrease of amplitude of the components of the spectrum, method called frequency bands (MNB method). In the first mode, we only estimate the parameter 13, while the second mode also measures the coefficient co. Both methods validate the reproducibility of mitigation measures and confirm the feasibility of implementing

the invention.

  M. Fink, F. Hottier and J. F. Cardoso, "Diffraction effects in pulse-echo measurement", IEEE Trans Sonics Ultrasonics, Vol. SU-31, N 4, pp 313-329, explains that diffraction disturbs the estimation of attenuation, especially when the measurements are in the near field of focused transducers: its effects are contained in the term H2 (z, f) 2 of the Eq. 1. The technique generally used to correct it consists in using a normed spectrum obtained by dividing the real spectrum by a calibration spectrum calculated from the backscattered signals by a homogeneous and isotropic attenuating medium placed at different distances from the transducer, as explained in. This normed spectrum is then used to estimate

  the absolute attenuation (o and or ao) of the tissues.

  The invention proposes to overcome the effects of diffraction by estimating attenuation from records obtained before and after treatment on

  same areas of fabric. During this operation, the calculation of the slope difference Sa -

  Sb of the spectral components Safler (f z) and Sbefore (f z), calculated in accordance with the relation (1), makes it possible to overcome the effects of diffraction: Sa -S = s ope1. log (0'af (il'J> = slope (10 lg Dater (() 2 (aofler (f) -ah.iox (f)) Zjj cc a ,,,,, (ftrf) e- aap, (f aeorf) .f in which the diffraction term H2 (z, f) 2 has disappeared The diffusion terms of the tissues Dbo, (f) (and Daf (f) are assumed to be independent of the distance z. the invention of measuring the variations of the average attenuation as a function of the frequency thus makes it possible to avoid the calibration steps required in the devices of the prior art, the apparatus of FIGS.

  millimetric precision that guarantees the results of the measurement.

  The experimental protocol for validating the results of mitigation measures is now described. This protocol confirms the possibility of implementing the invention in a device of the Ablatherm type. The validation of the measurements was carried out using a reference method in transmission whose experimental principle is described in G. Kossoff, EK Fry, J. Jellings, "Average velocity of ultrasound in the human female breast", J. Acoust . Soc. Am. 53 (6), pp. 1730-1736, 1973. A measuring device consisting of two flat transducers was used.

  identical (Imasonic, France) with a nominal frequency of 10 MHz and a diameter of 10 mm.

  A 50-period sine wave train was used in the 410 MHz band in 500 kHz steps. This method served as a standard for mitigation measures in reflection. Reference ghosts, BULPREN S90 and BULPREN S20 mosses (Rectiel Inc., Kesteren, The Netherlends), were studied with both devices to determine the optimal values of the computational parameters used for the centroid method and frequency. This calibration was also intended to determine the minimum study volume required for a value estimate.

  absolute attenuation with an error of less than 10%.

  The study involved 10 pieces of pork butcher's liver. Each piece was placed in a cylindrical plastic support whose ends, limited by a latex membrane, served as acoustic window. The whole was filled with

  physiological fluid and degassed for 20 minutes using a vacuum pump (Ref).

  For each sample, two series of successive measurements were made: Series A: 7 successive slices processed at zero power (no lesion created) were studied on each piece. The purpose of this series was to test the reproducibility of the measurements on the same area of the tissue. The average time of this

sequence was 10 minutes.

  Series B: 12 successive slices were studied on each piece. The first 7 were treated at nominal power to create a lesion volume of 2.15cm3 (6 lesions per slice). The next 5 slices, processed at zero power,

did not involve any lesions.

  For each piece of liver the evolution of the attenuation function with and without HIFU treatment was studied globally by comparing the measurements of the series A with those of the first 7 slices of the B series. In this study, the function

  attenuation was estimated on average over the entire lesion volume.

  The aim of the series B study was to define the spatial resolution of the method in order to image lesions. To estimate the transversal and longitudinal resolution, the minimum number of RF lines defined during the transmission calibration was used. Longitudinal resolution consisted of an inter-slice study of acoustic parameters. The cross-sectional study looked at the inter-line variation of attenuation on a transverse slice. One of the liver samples was used to study the importance of the effect of temperature on the created images. For this purpose, measurements were made immediately after treatment and then 30 minutes later, by controlling the temperature of the tissues with a thermocouple The parameters a00 and [reference ghosts were estimated in transmission as well as in reflection. The relative error between the two techniques is less than 5% for the following calculation and acquisition parameters: sampling frequency 50 MHz

  450 RF lines of 2048 points (41ps) each.

  size of computation windows 128 points (2.56 ris) with a Hamming windowing Frequency band studied: 4 to 7 MHz These results prove the validity of the method in reflection to estimate the absolute values of attenuation with the imaging transducer used during the treatments. A minimum volume corresponding to 50 RF lines of 2048 points is advantageously used to obtain an error of less than 10% on the estimate of P. Preferably, at least 5 lines are used to estimate a with the same accuracy on the same depth of field; Table I shows the average attenuation coefficient measured in transmission, and the coefficient obtained with 450

  RF lines, 9 slices of 50 RF lines and 90 blocks of 5 RF lines.

  The entire length of the RF lines is preferably used; this improves the accuracy. The attenuation of tissues, expressed in the following, corresponds to a mean estimate of the thickness of the tissue studied, i. e. over the entire depth of the RF line. Insofar as the tissues on either side of the lesion - in front of and behind the lesion - are not affected by the treatment, the results obtained

  are representative of the treatment performed.

  |! Number of lines (MultiNarrowBand) Measurement in 450 RF 50 RF 5 RF Transmission BS 20 (dB / cm) 2.021 2.005 2.026 + 0.13 2.071 + 0.26_ BS 90 (dB / cm) 3.028 2.800 2.85 + 0.07 2.902 + 0.23 | Figure 5 shows the typical evolution of attenuation versus frequency, before and after treatment, for a B-series sample. The attenuation curves before and after treatment show broadly similar general very different average levels. The same characteristics were observed on all the samples of the B series. Table 2 represents the attenuation slope differential AP3 as well as the attenuation differential AU obtained on all the pieces of liver on the treated zones (7). first tranches of Series B) and untreated (Series A). The table confirms that if the values of Af vary significantly during treatment, the values of A [3 do not vary much. The invention therefore proposes to use variations of the attenuation mean to evaluate the effects of the

treatment on the tissues.

  Without treatment With A- treatment (dB / cm) Method MNB Method MNB

-0.12 + 0.09 3.26 + 1.7

  A [Centroid Method MNB centroid Method MNB (dB / cm / MHiz) -0.01 +0.02 -0.01 +0.02 0.13 + 0.14 0. 02 + 0.11 The average of the average attenuation coefficients measured before treatment on all samples is of 3.8 1.07 dB / cm. This was determined from the series A measurements and the pre-treatment measurements from the B series. The average of the attenuation slopes estimated before treatment is 0.43 0.17 dB / cnm / MHz. In the case without treatment, Aa and A [3 make it possible to determine the reproducibility of the estimate of a and P. The reproducibility errors are respectively 2.3% and 3.15%. In the case with treatment, we note that the average variation of at is of 86% and that of [3 is of 30% and 4.7% respectively with the method of the centroid and the method MNB The evolution of Ad can be studied slice by slice over the entire treated area. This evolution is generally symmetrical around the central slice which presents a maximum. Figure 6 shows the evolution of A- normalized with respect to the value obtained on the central slice of the treatment. Error bars represent standard deviations for all liver pieces. It can be seen in the figure that the measured values are in all cases much greater than the standard deviation, so that the measurement according to the invention remains always representative of the treatment. The invention also proposes a measurement which has a good longitudinal resolution, i. e. in the sense of treatment. Figure 7 shows an example

  evolution of A- on 12 slices (series B) in a plane containing the treated area.

  Slices I to 7 are processed, and slices 8 to 12 are not; Slice number 8, which is not part of the target zone, has an attenuation differential greater than I dB / cm which seems to show that the lesion area has exceeded the user-defined frame. On the other slices, it is found that the evolution of Ad is significantly greater for the tissues actually treated. The spatial resolution in the method according to the invention makes it possible to clearly identify the

treated areas.

  The invention also provides good transverse resolution, i. e. in a direction perpendicular to the main direction of ultrasonic propagation. FIG. 8 shows an example of transverse evolution of Aa on a treated slice as well as on an untreated slice. AU was estimated on a minimum volume of 5 RF lines. Again, the figure shows that AU is representative of the treatment in

The area.

  The invention proposes to use a representation of the coefficient Ai for the visualization of the effects of the processing: FIG. 9 represents a gray scale frontal image of the attenuation differential, hereinafter referred to as "image". A frontal image is an image obtained in plane containing parallel to the therapy transducer. This was obtained by breaking down each transversal slice into 50

  blocks of 5 RF lines on which A- is estimated, leading to a 50x12 matrix.

  Each element of the matrix corresponds to a square of 0.102xl1.60 mm2 which makes it possible to give a metric scale to the "A 'images". A threshold at 12 dB of the maximum appears to be optimal for delimiting lesion areas. In the figure, high levels, greater than -12 dB; are represented in dark, while the other levels are represented in clear. The limit by the white rectangle corresponds to the theoretical limit of the target, as programmed in the treatment. Figure 9 shows that the dark areas almost cover the treated area, and that the

  Visualization of Ad allows a reliable assessment of the treated area.

  FIGS. 10 and 11 show images similar to those of FIG. 9: FIG. 10 is an image made immediately after treatment and FIG. 11 is an image made after cooling the tissues. In both cases, the variation is calculated with respect to the tissues before treatment. The time required for the center of the treated area to return to its original temperature, to within 2 C, was 30 minutes. The attenuation differential decreased slightly with the maximum cooling down to 1 dB / cm less than that calculated immediately after treatment. Comparison of these two figures clearly shows that the method of the invention can be used both in real time, immediately before and after a treatment shot, and later, after the end of the treatment, for an evaluation of the effects of this one. The invention thus applies not only assistance to the conduct of a treatment by providing the practitioner immediate measures allowing him to adapt his treatment; it also applies after the end of a

  treatment, to allow the practitioner to evaluate the effects of a treatment.

  FIGS. 12, 13 and 14 respectively represent, with the same conventions as FIG. 9, and for the same fabric zone, the average attenuation before the treatment, the average attenuation after the treatment and the attenuation variation. In all three figures, the white rectangle represents the target. In Figure 12, the heterogeneities of the fabric show significant variations in gray levels. In Figure 13, a zone of greater attenuation appears in the targeted area, but the heterogeneities do not allow to delimit the treated area. On the other hand, there is no equivocation on the image of FIG. 14, which represents the average attenuation variations. It can also be seen in Figures 12 and 13 that the average attenuation coefficient has not changed outside the target area, which indicates the validity of the acquisition system, the slight differences being

due to estimation errors.

  The invention thus makes it possible to characterize the appearance of coagulation necrosis lesions during HIFU treatments by means of a local variation measurement of the average attenuation in the tissues. The experiment was carried out on pieces of pork liver using an ultrasound therapy apparatus used exactly under the same conditions as for the treatment of the human prostate. For computational accuracy reasons, ultrasound attenuation in the examples was estimated over the entire length of the RF signals thus allowing only an average estimate of tissue thickness. Since it is necessary to have a greater precision in the direction of the depth, one can reduce the length of study on the RF lines and preferably increase their number to keep a small variance on the estimate of attenuation. This

  could be done by increasing the number of acquisition tranches.

  The figures show that the estimation of the average variation of the attenuation, and in particular on frontal sectional images, makes it possible to clearly visualize the. user-defined lesion area, which agrees with the region o Ai is greater than a defined threshold at -12dB of the maximum. The target area in the figures corresponds to a block of 42 lesions (6 by 7). As each lesion is equivalent to a cylinder of 1.6 mm in diameter and 20 mm in height, the frontal surface in the middle of the treated region then represents 1.08 cm 2. The average lesion area estimated on the images is 1.3 cm 2, ie 20% larger. This slight spreading can be explained by the phenomena of thermal conduction during treatments. These are also the cause of a greater temperature accumulation in the central part of the treatment. There is thus a maximum Af 'at the center of the target region on the images of Figures 9 to 13 and in Figure 6. It is likely that these phenomena are less important on the prostates treated clinically because of the vascularity . AU images according to the invention offer several advantages over conventional attenuation images. The interpretation of an absolute attenuation image made on HIFU-treated tissue is not straightforward in that it is difficult to separate the areas of high attenuation due to the lesions from the areas of high attenuation existing in the tissue. as can be seen in Figs. 12 and 13. Some weakly attenuating areas may also see their attenuation level increase when the lesions are created without the latter being different from the average attenuation level of the tissue. A differential measure eliminates these problems. Moreover, absolute attenuation images are limited by the diffraction problems of the imaging probe as well as by the presence of heterogeneities in the tissue (shadow zones, specular echoes, etc.) which make the term Diffusion may vary with depth. The technique used here differs from the diffraction problems inherent in the imaging probe since the measurements before and after treatment are performed in exactly the same positions. A small variation can nevertheless occur because of the voluminal dilatation of the tissues during the creation of the lesions, but this one is negligible vis-a-vis the inter-slice space which is of

  1.6 mm in the embodiment chosen.

  The Ablatherm device, thanks to its probe, makes it possible to avoid the movements of the probe, and thus ensures that the measurement is essentially independent of the

  movements of the patient, including respiratory movements.

  The influence of tissue temperature on the images can be discussed.

  Measurements made here are made only minutes after the creation of the lesions while the treated tissues have a high temperature gradient due to heat accumulation. A measurement performed after cooling a sample showed that temperature was not the determining factor in the average attenuation variations. The maximum difference of A differs only from 1 dB / cm, and the dimensions measured on the images vary only by 3%. The comparison of FIGS. 10 and 11 show that the invention can be

  used at different times.

  The method of the invention can be directly applied to the control of treatments in vivo since the experimental conditions are those used clinically. Its real time implementation is possible because the calculation algorithms (based on FFT calculations) are very fast, not very greedy in memory and it is not necessary to make diffraction correction. Finally, in order to increase the efficiency of the HIFU treatments, an in-depth estimation of AU- is useful: The images in frontal sections of FIGS. 9, 10, 11 or 14, however, give an indication of the thickness of the treated area since the strong levels match

  regions where the treated tissue thickness is maximum.

  In a practical embodiment, the corresponding images can be displayed on the control screen of the treatment device or the ultrasound system, for example superimposed on the ultrasound image; this allows the doctor to

  to easily identify the treated areas, and to adapt its treatment accordingly.

  Throughout this description, the term attenuation has been used.

  One could also use the term of absorption; Strictly, absorption does not

  does not take into account diffusion phenomena and other losses in the environment.

  On the contrary, the attenuation is generally calculated from the overall attenuation of the signal. In practice, the ratio of attenuation and absorption is

  generally constant for a given tissue.

  The measurement of the variations of the attenuation according to the invention allows a control and a follow-up of the formation of the lesions in a focused ultrasound treatment. The

  The foregoing description mentions the application to focused ultrasound for a

  HIFU treatment; the invention also applies to other types of treatments, for example hyperthermia treatments, and more generally all treatments in which the acoustic attenuation is representative of the state of the treated tissues. From this point of view, the invention can also be applied to the measurement of the effects of a hyperthermia treatment using microwaves, or even lasers, or radiofrequency waves. It applies in particular for the control of the treatment of

  endo-urethral prostatic adenoma using devices delivering microparticles

  waves, such as the one marketed by the same company under the Prostatron brand.

  Note that the invention provides only a measure of the variation of the acoustic attenuation, and from this point of view, applies regardless of the treatment envisaged or applied; the invention does not make it possible to immediately suggest the type or the processing power or the total energy to be applied for a given target. In this sense, the measurement of the acoustic attenuation according to the invention is therefore independent of the exercise by a doctor or a surgeon of his art, by the choice of the organs to be treated, the power to be applied, the duration or the type of treatment or other parameters. In fact, the measurement has no functional relationship with the therapeutic effect of treatment, which is determined by the physician

implementing the treatment.

  Of course, the present invention is not limited to the examples and embodiments described and shown, but it is capable of many variants accessible to those skilled in the art. It is clear that if the invention has been described in its application to the example of the prostate, it can also be applied to other tissues. The invention could thus be used for hyperthermia of the breast, liver, or other organs or tissues. It is also clear that the invention is not limited to the embodiment of Figures 1, and can be applied to other endo-cavitary devices or not.

Claims (13)

  A method of measuring the effect of a therapy on a tissue, comprising: measuring the ultrasound backscattered by the tissue prior to treatment; the measurement of ultrasound backscattered by the tissue after the treatment. 2. A process according to claim 1, characterized in that it further comprises   the calculation of the variation of the average attenuation of the tissue over a frequency range.   3. Method according to claim 1 or 2, characterized in that the measuring steps comprise the measurement of the ultrasound emitted by a transducer. ultrasound and backscattered.   4. Process according to claim 2 or 3, characterized in that the attenuation   average is calculated by the so-called frequency band method.   5. Method according to one of claims 1 to 4, characterized in that the fabric includes the prostate.   6. Method according to one of claims 1 to 5, characterized in that the   treatment involves the induction of necrosis in the tissue, by sudden rise in temperature.   7.- Method according to one of claims 1 to 6, characterized in that the   Treatment includes the application of ultrasound focused on the tissue.   8. An apparatus for treating a tissue by forming necroses in the tissue, comprising means for measuring ultrasound backscattered by the tissue before the   treatment, and ultrasound measurement backscattered by the tissue after treatment.   9. Apparatus according to claim 8, characterized in that it further comprises means for calculating the variation of the average attenuation of the fabric on a beach frequency.   10. Apparatus according to claim 8 or 9, characterized in that the measuring means comprise a measurement of the signals received on an ultrasound transducer. 11. Apparatus according to claim 9 or 10, characterized by means for displaying the average attenuation variation in a plane perpendicular to the direction of treatment.   12. Apparatus according to one of claims 8 to 11, characterized by means for inducing necrosis in the fabric, by sudden rise in temperature.   13. Apparatus according to claim 12, characterized in that said means comprise a transducer focusing ultrasound.