Les maladies cardiovasculaires sont la première cause de mortalité dans le monde. Elles sont le plus souvent provoquées par l'obstruction des vaisseaux par des caillots sanguins entrainant un manque d'oxygène dans les cellules. La thrombolyse ultrasonore extracorporelle constituerait un traitement innovant utilisant des ultrasons focalisés pour détruire les caillots sanguins en tirant parti de l'aspect mécanique de la cavitation acoustique. Un prototype a été conçu et amélioré afin de contrôler l'activité de cavitation. Pour suivre le processus de cavitation en temps réel, un système d'imagerie ultrasonore ouvert est utilisé. Les données brutes sont acquises en utilisant une sonde linéaire de 64 éléments actifs dans un mode d'imagerie passive. Lors du traitement de ces données, sur le principe du retournement temporel, le signal acoustique enregistré par la sonde est retro-propagées afin de localiser l'activité de cavitation

Bera, Jean-Christophe

Boulos, Paul,

Cachard, Christian

Poizat, Adrien,

Varray, François

Boulos, P.

Varray, F.

Poizat, A.

Bera, J.C.

Cachard, C.

HAL-UJM

Passive cavitation imaging using an open ultrasonic system and time reversal reconstruction

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22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
 
Passive cavitation imaging using an open ultrasonic 
system and time reversal reconstruction 
 
P. Boulos
a,b
, F. Varray
a
, A. Poizat
a,b
, J.C. Bera
b
, C.Cachard
a
 
 
a. Université de Lyon, Creatis ;CNRS UMR5220 ; Inserm U1044 ; INSA-Lyon ; Université 
Claude Bernard Lyon 1, France 
b.Université de Lyon, Inserm U1032,LabTau; Université Claude Bernard Lyon 1, France 
 
Résumé :  
 
Les maladies cardiovasculaires sont la première cause de mortalité dans le monde. Elles sont le plus 
souvent provoquées par l’obstruction des vaisseaux par des caillots sanguinsentrainantun manque 
d’oxygènedans les cellules. La thrombolyse ultrasonore extracorporelle constituerait un traitement 
innovant utilisant des ultrasons focalisés pour détruire les caillots sanguins en tirant parti de l’aspect 
mécanique de la cavitation acoustique. Un prototype a été conçu afin de contrôler l’activité de 
cavitation.Pour suivre le processus de cavitation en temps réel, un système d’imagerie ultrasonore 
ouvert est en cours de développement.Les données brutes sont acquises en utilisant une sonde linéaire 
de 64 élémentsactifs dans un mode d’imagerie passive.Lors du traitement de ces données, sur le 
principe du retournement temporel, le signal acoustique enregistré par la sonde estretro-propagées 
afin de localiser l’activité de cavitation. 
 
Abstract: 
 
Cardiovascular diseases are the first cause of mortality in the world. They are mostly caused by blood 
clots obstructing vessels which lead to a lack of oxygen in the cells. Extracorporeal ultrasound 
thrombolysis could be aninnovative treatment usinghigh intensity focused ultrasound to destroy blood 
clots by exploiting the mechanical aspect of the acoustic cavitation.A prototype has beendesigned and 
improved in order to control the cavitationintensity. To monitor the treatment in real time, an open 
ultrasound imaging system has been incorporated.Raw data have been acquired using a linear probe 
of 64 active elements in a passive imaging mode. Usingthe time reversal method, acoustic signal 
recorded by the probeis back propagated in order to estimate the location of the cavitation activity.  
 
Keywords:ultrasound;thrombolysis; acoustic cavitation;monitoring;time 
reversal 
 
 
 
 
1 Introduction  
22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
 
Ultrasound (US) imaging is mostly used for diagnosticimaging thanks to its known advantages as 
noninvasive, ease of use, real time display and cheap. HoweverUS can also be used for therapeutic 
treatmentas for examplehigh intensity focused ultrasound (HIFU)fortissue ablation[1],lithotripsy using 
US shock waves for the destruction of kidney stones[2]or US thrombolysis used for the destruction of 
blood clots[3]. 
 
Thrombolysis consists in destroying chemically and/or mechanically blood clots obstructing the 
vessels.Currently many thrombolysis techniques are used:surgeries (bypass or balloon angioplasty), 
and drug treatment using thrombolytic agentsas tissue plasminogen activator (tPA),in whichultrasound 
sonoporationcould be used to enhance the permeability of cell membrane relative to thrombolytic 
agent[4]. Yet these techniques have high risk of failure caused almost entirely by bleedings.Focused 
US induces in the medium acoustic cavitation which is related tobubbles created by a high acoustic 
depression. These bubbles can be seen in two phases. First, for low pressure non-inertial 
cavitation,also called boiling or stable cavitation, occurswhen bubbles radially oscillate and induce 
thermal damages.Second, for higher pressure,inertial cavitation can appear which consists in bubble 
collapses generating shock waves with mechanical damages.US thrombolysis mainly uses the inertial 
cavitation for inducing mechanical damages on the blood clots. This inertial cavitation is very 
complex and unstable and research are conducted on this stability problem and managed to regulate 
the cavitation activity[5], [6].  
 
This therapeutic technique still needs a monitoring system which would be essential to ensure the 
perfect and entiredestruction of clots without damaging vessels. US monitoring techniques are 
currently used clinically for other treatments as for prostate cancer[7], computer assisted surgery[8] 
and HIFU.Last treatment canbe monitored by mapping the rising tissue temperature and/or the 
cavitation activity.There is currently some clinically existing monitoring technique as MRI monitoring 
[9] by mapping the temperature rises and B-mode US [10] by turning off the therapy transducer 
between each acquisition.These techniques are not efficient in term of real time monitoring and spatial 
resolution. However some studies have already been made on the passive cavitation imaging[11], [12]. 
Gateau et al.worked on a technique combining passive and active detection of cavitation using a 
conventional ultrasonic array[13]. Passive detection was used to locate the bubble cloud and active 
detection to provide information on the temporal behavior over time of the first bubble induced. 
Coussios et al. are working on a passive acoustic mapping (PAM) technique used for spatial 
monitoring of HIFU therapy They are currently working on the beamforming algorithm by combining 
time exposure acoustic (TEA), delay and sum (DAS) and robust capon beamforming(RCB)algorithms 
to improve resolution and reduce interference artifacts [14]. 
Apassive US imaging system for monitoring in real time the US thrombolysisis under research and the 
goal is to locate spatially and in real time the cavitation activity.Time reversal (TR) method is often 
used to locate acoustic sources. Itis used in our study to back-propagate the raw data acquired by a 
research US advanced open Platform (ULA-OP)[15]in a passive mode. 
 
Thus, this work focuses on the integration of the monitoring system to the therapeutic set up and on 
the TR reconstruction method of the cavitation intensity map. In Part 2, the experimental setup and the 
post-processing method are described. Both the therapeutic and monitoring system are highlighted.The 
experimental results are exposed in Part 3. Finally, in Part 4, discussion about the results and further 
work is provided. 
2 Method 
22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
2.1 Experimental setup 
 
The general experimental setup consists in two separate systems. First, the therapeutic system,where a 
custom-manufactured transducer,a FPGAcard (NI PXIe-7966R) and a needle-hydrophone (ONDA 
HNC-1500) are set up to create and regulate cavitation therapy[5].The spherically-focused therapeutic 
transducer working at 1MHz with a 100 mm active aperture is focalizing at 80 mm.The hydrophone 
and the FPGA card are used to create a feedback loop to regulate the cavitation intensity.A pulsed 
sinusoidal signal at frequency of 1 MHzis transmitted with 250 msof pulse repetition and a duty cycle 
of 0.1. The signal is amplified by a power amplifier (69 dB,PRÂNA GN 1000) then sent to the 
therapeutic transducer. This system and the regulation loop were implemented by Desjouyet 
al.[5].Regulated cavitation is used during this study so bubble cloudsaremuch better observed and 
alsoto get the same results during the whole therapy. So the passive images are less related to the 
cavitation phenomena. 
Second, a monitoring system is designed using the ULA-OP diagnostic open-platformwith a linear 
probe of 192 elements (LA523, Esaote, Florence, Italy) working at 7 MHz central frequency with a -
6dB bandwidth of 4-12 MHz. During the acquisitions, a classical B-mode is used for the image 
reference of the setup and then, passive acquisitions are conducted to detect bubbles activity. The B-
mode is an active imaging diagnostic mode using the whole 192 elements of the probeto form 192 
beamformed lines with a frame rate of 20 images/s. Given the limitation of the ULA-OP system, 64 
active elementsare used for each RF lines.The passive mode isonly using the central 64 active 
elements of the probe whichare simultaneously acquiring the pre-beamformed signals.  The US pulse 
transmission is turned off during passive acquisition so echoes only come from external sources as 
bubbles. The listening time depends on the depth of acquired images and can be modified using the 
open-platform. 
The monitoring system is set with the therapeutic setup in the way that the diagnostic probe is placed 
at 90° relative to the US therapeutic beam. The probe is displayed to image the lateral resolution of the 
cavitation activity.In this experiment, a 10x10x10cm
3
agar gelcube (4%) is placed in the focus of the 
transducer. The full scheme of the experimental platform is presented in Fig.1.  
Using the FPGA system that control the therapeutic transducer, an external trigger signal has been 
created to synchronize the imaging system. Then, using appropriate delay, the passive recording can 
be conducted at any moment of the therapeutic ultrasound shot. 
 
22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
Transducer
Absorbent
Gel
FPGA
Cavitation regulation
250 ms
25 ms
69dB
ULA-OP
Scanner
Synchronisation
Probe
31dB
Hydrophone
 
Fig.1 : experimental set-up of the full therapeutic and monitoring system.The FPGA card creates a pulsed 
sinusoidal signal which is amplified then sent to the therapeutic transducer. Thanks to the hydrophone a 
feedback cavitation regulation loop is created. An external triggerdelivered by the FPGA synchronizes the 
passive acquisition on the ULA-OP scanner using a linear probe.  
 
2.2 Post-processing 
 
During different moment of cavitation exposure in the agar gel, the pre-beamformed signals from the 
passive probe were recorded and then post-processed using the software Matlab (Mathworks). The 
acquired raw data correspond to acoustic signal emitted by bubbles sources. These raw data are 
filtered by a band-pass filterto smooth the waves and are narrowed to choose the number of waves 
which correspond to the time of cavitation„s exposure.TR method was applied using the toolbox “k-
wave” developed for ultrasound simulation by Treeby et al.[16]. Parameters relative to the medium are 
set as the sound of speed distribution, the ambient density and the medium absorption factor.Each 64 
elementsof the probe are recording the pressure according to a same vector time. In the k-wave 
simulation, this vector time is reversed and then the pressure raw data are emittedto focus the wave 
back to the source.The vectortime of the input signal is chosen long enough such that the back 
propagated wave have enough time to go back to the acoustic source. The wave‟s propagation is 
simulated by solving the first order wave‟sequations: 
0
0
0
2
.
(
1
(1)
3
(
)
2)
u
p
t
p
u
t
p c



 
 

 


 

 
 
 
22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
These equations are based on the conservation of momentum (1), conservation of mass (2) and the 
pressure-density relation (3). u is the acoustic particle velocity, pis the acoustic pressure, ρ is the 
acoustic density, ρ0is the ambient density and c0 is the isentropic sound speed. 
The maximum of intensity is then computed and the cavitation intensity map is found. 
 
3 Results 
 
In this study, only the beginning and the end of the 25 ms cavitation pulse were recorded using the 
external trigger of the FPGA.The passive images were computed from raw data received by the 64 
channels of the passive probe and shown in Fig.2. The black sections correspond to the moment when 
the therapeutic transducer is turned off. The white lines correspond to acoustic signal diffused by 
bubbles.A radio frequency (RF) signal recorded by the 32th element of the probe is shown in Fig2.c 
matching the 32th line of the passive image Fig.2.b.The end of the cavitation activity occurring at 27 
µs can be seen in the RF signal. The Fourier transform of this RF signal is illustrated in Fig.2.d. 
Peaksareobserved at specific frequencycorresponding to harmonics from 1 to 8 MHz. These 
harmonics stresses the non-inertial cavitation activity.Broadband noise is also noticed which is for the 
most part created by inertial cavitation. 
 
 
 
Fig.2 : a) passive image of cavitation during the beginning of the therapeutic pulse, b)passive image 
cavitation during the end of therapeutic pulse, c)RF signal recorded for element 32 corresponding to the 
32th line of the image b, c) frequency spectrum of RF signal of figure c 
 
The raw data of the image Fig.2.b were filtered with a 5.5-6.5MHz band-path filter and were narrowed 
to6 acoustic waves corresponding to 10 µsusing the TR method on these acoustic waves, the cavitation 
intensity map is obtained and illustrated in Fig.3.a. The reference B-mode image is also acquired in 
22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
Fig.3.b. The same probe aperture,192 elements corresponding to 47 mm aperture,was used for both 
images.In the cavitation intensity map, the cavitation spot is noticed at 60mm deep with 20mm length 
and 1mm wide. In the B-mode image, the cavitation spot is tracked by searching hyper-echoes 
structures which correspond to bubbles observed at 70mm deepmatching the theoretical focused 
therapeutic beam. Taking into account the possible cavitation damage to the diagnostic probe, it is 
inserted as close as possible to the bubble cloud. This problem could be solved by integrating the 
probe coaxially to the therapeutic transducer. 
 
 
Fig. 3 : a) cavitation intensity map, b) B-mode reference image. 
 
 
4 Discussionand Conclusion 
 
The lateral localization of the cavitation spot observed in the cavitation intensity map and the B-mode 
image are highly similar.However the depth resolution of the cavitation spot needs improvements. In 
Fig.2.dthe harmonics 5 to 8 MHz belong to the probe‟s bandwidth (5-9 MHz) so their observation is 
normal. The remaining harmonics (1 to 4 MHz) are observed because the energy delivered is so strong 
that they are still present in the recorded signal. The use of higher frequency during the TR led to 
better lateral resolution. Time between the creation of a bubble and his collapse was not considered 
during processing and was neglected. The acoustic wave signals are sharper in the end (Fig.2.b) than 
at the beginning of the therapeutic pulse (Fig.2.a). This can be explained by the fact that in the 
beginning of the therapeutic pulse the cavitation regulation is less efficient than in the end where it 
have reached the best outcome. Stack of acoustic waves were processed by TR. Instead of sending the 
entire stack, they can be separated in individual acoustic waves, back-propagated and finally the 
intensity averaged. Inertial and non-inertial cavitation activity could be separated so two specific maps 
could be computed.Work still need to be done on the precision and resolution of the intensity map. In 
further studies the probe could be integrated coaxially to the therapeutic transducer so the axial 
resolution of the therapeutic beam is processed. The utilization of nonconsecutive element in passive 
receive will also be investigated. Recalling the goal to get a real-time monitoring system, the signal 
processing stage should be fast enough. TR method is too much time consuming and so is not the best 
way to resolve this real-time problem. However it is a first step in our study to calibrate the cavitation 
focal region. Other passive detection technique will be the focus of future work.  
22
ème
Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 
 
 
5 Acknowledgment 
This work was supported by the Labex CeLyA (ANR-10-LABX-0060) of Universitéde Lyon within 
the program “Investissements d‟Avenir‟ (ANR-11-IDEX-0007) and by the Frenchnational research 
agency project “ULysSE” (ANR-11-JSV5-0008).  
 
6 References 
 
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61, no. 3, pp. 481–495, Mar. 2014. 
[2] J. E. Lingeman, J. A. McAteer, E. Gnessin, and A. P. Evan, “Shock wave lithotripsy: advances in 
technology and technique,” Nat. Rev. Urol., vol. 6, no. 12, pp. 660–670, Dec. 2009. 
[3] C. D. Adrien Poizat, “Regulation of cavitation activity in pulsed sonication with a real-time 
feedback loop system,” 2013. 
[4] H.-D. Liang, J. Tang, and M. Halliwell, “Sonoporation, drug delivery, and gene therapy,” Proc. 
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[5] C. Desjouy, A. Poizat, B. Gilles, C. Inserra, and J.-C. Bera, “Control of inertial acoustic cavitation 
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[6] A. Poizat, D. Cyril, I. C, G. Bruno, and B. Jean-christophe, “Régulation temporelle de l‟activité de 
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[12] K. J. Haworth, T. D. Mast, K. Radhakrishnan, M. T. Burgess, J. A. Kopechek, S.-L. Huang, D. D. 
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[13] J. Gateau, J.-F. Aubry, M. Pernot, M. Fink, and M. Tanter, “Combined passive detection and 
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Passive cavitation imaging using an open ultrasonic system and time reversal reconstruction

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