J.F. Aubry

Optimal transcostal high-intensity focused ultrasound with combined real-time 3D movement tracking and correction.

Recent studies have demonstrated the feasibility of transcostal high intensity focused ultrasound (HIFU) treatment in liver. However, two factors limit thermal necrosis of the liver through the ribs: the energy deposition at focus is decreased by the respiratory movement of the liver and the energy deposition on the skin is increased by the presence of highly absorbing bone structures. Ex vivo ablations were conducted to validate the feasibility of a transcostal real-time 3D movement tracking and correction mode. Experiments were conducted through a chest phantom made of three human ribs immersed in water and were placed in front of a 300 element array working at 1 MHz. A binarized apodization law introduced recently in order to spare the rib cage during treatment has been extended here with real-time electronic steering of the beam. Thermal simulations have been conducted to determine the steering limits. In vivo 3D-movement detection was performed on pigs using an ultrasonic sequence. The maximum error on the transcostal motion detection was measured to be 0.09 ± 0.097 mm on the anterior-posterior axis. Finally, a complete sequence was developed combining real-time 3D transcostal movement correction and spiral trajectory of the HIFU beam, allowing the system to treat larger areas with optimized efficiency. Lesions as large as 1 cm in diameter have been produced at focus in excised liver, whereas no necroses could be obtained with the same emitted power without correcting the movement of the tissue sample.

Adaptive focusing for transcranial ultrasound imaging using dual arrays

Ultrasonic brain imaging remains difficult and limited because of the strong aberrating effects of the skull (absorption, diffusion and refraction of ultrasounds): high resolution transcranial imaging would require adaptive focusing techniques in order to correct the defocusing effect of the skull. In this paper, a noninvasive brain imaging device is presented. It is made of two identical linear arrays of 128 transducers located on each side of the skull. It is possible to separate the respective influence of the two bone windows on the path of an ultrasonic wave propagating from one array to the other, and thus estimate at each frequency the attenuation and phase shift locally induced by each bone window. The information obtained on attenuation and phase is used to correct the wave fronts that have to be sent through the skull in order to obtain a good focusing inside the skull. Compared to uncorrected wave fronts, the spatial shift of the focal spot is corrected, the width of the focal spot is reduced, and the sidelobes level is decreased up to 17 dB. Transcranial images of a phantom are presented and exhibit the improvement in image quality provided by this new noninvasive adaptive focusing method.

High-intensity therapeutic ultrasound: metrological requirements versus clinical usage

High-intensity therapeutic ultrasound (HITU) is an appealing non-invasive, non-ionizing therapeutic modality with a wide range of tissue interactions ranging from transient permeabilization of cell membranes to thermal ablation. The ability to guide and monitor the treatment with an associated ultrasonic or magnetic resonance imaging device has resulted in a dramatic rise in the clinical use of therapeutic ultrasound in the past two decades. Nevertheless, the range of clinical applications and the number of patients treated has grown at a much higher pace than the definition of standards. In this paper the metrological requirements of the therapeutic beams are reviewed and are compared with the current clinical use of image-guided HITU mostly based on a practical approach. Liver therapy, a particularly challenging clinical application, is discussed to highlight the differences between some complex clinical situations and the experimental conditions of the metrological characterization of ultrasonic transducers.

Réseaux de transducteurs ultrasonores : nouvelles avancées thérapeutiques

Bursts of focused ultrasound energy a billion times more intense than diagnostic ultrasound have become a non-invasive option for tumor ablation, from prostate cancer to uterine fibroid, during the last decade. Despite this progress, many issues still need to be addressed. First, for brain targeting, the correction of distortions induced by the skull remains today a technological achievement that still needs to be validated clinically. Secondly, the problem of motion artifacts for abdominal treatments becomes today an important research topic. For all these issues, the potential of new ultrasonic therapy devices able to work both in Transmit and Receive modes will be emphasized and clinical results on monkeys and pigs will be presented.

Adaptive focusing of transcranial therapeutic ultrasound using MR Acoustic Radiation Force Imaging in a clinical environment

Background: In order to focus ultrasound beams through aberrating layers such as fat or bones, adaptive focusing techniques have been proposed to improve the focusing, mostly based on the backscattered echoes. We recently proposed an energy-based technique with the sole requirement being knowledge of the acoustic intensity at the desired focus. Here, Magnetic Resonance-Acoustic Radiation Force Imaging (MR-ARFI) is used to map the displacement induced by the radiation force of a focused ultrasound beam. As the maximum displacement is obtained with the best corrected beam, such a measurement can lead to aberration correction. Material and methods: Proof of concept experiments were previously shown in a small animal MR at 7 T using a 64-elements linear phased array operating at 6 MHz. Optimal refocusing was then obtained through numerical and physical aberrating layers. This work is extended here in a clinical Philips 1.5 T Achieva scanner. The HIFU beam is generated using a 512 elements US phased array (SuperSonic Imagine, France) dedicated to transcranial human experiments and operating at 1 MHz. Experiments are conducted in phantom gels and ex vivo brain tissues through numerical phase aberrators. A motion-sensitized spin echo sequence (TE = 70 ms, TR = 1200 ms, spatial resolution is 2×2×7 mm3) is implemented to measure displacements induced by the acoustic radiation force of transmitted beams. Results: MR-ARFI allowed mapping the distribution of the radiation force at the focus of the array. After the recording of the MR phase signals for different US emissions, the proposed adaptive focusing technique was able to recover the spatial distribution of the phase aberrations. Total acquisition time for 384 ultrasonic emission channels was 2 hours. Conclusion: Those first results in clinical MR at 1.5 T show that adaptive focusing of a human transcranial brain HIFU system can be achieved within reasonable time under MR guidance for aberrator layers as strong as human skull. Ongoing work is aiming at accelerating the acquisition in order to reach acceptable durations for in vivo protocols.

MR Guidance, Monitoring and Control of Brain HIFU Therapy in Small Animals: In Vivo Demonstration in Rats

In the framework of HIFU transcranial brain therapy, it is mandatory to develop techniques capable of assessing the focusing quality and location before the treatment. Monitoring heat deposition in real time and verifying the extension of the treated area are also important steps. In this study, an imaging protocol is proposed to:1/ locate the US radiation force induced displacement in tissues and quantify the acoustic pressure at focus prior to HIFU; 2/ monitor the temperature rise during HIFU; and 3/ assess the changes in elasticity in the treated area. A 7T MRI scanner was equipped with a home-made stereotactic frame for rats and a US focused transducer working at 1.5MHz. Such a tool is key for the evaluation of the biological effects of HIFU on brain tissue and tumors. The proposed protocol was successfully tested on 12 rats with and without injected tumors. The accurate localization of the focal point prior to HIFU was demonstrated in vivo. Furthermore, the pressure estimation in situ allowed to accurately simulate the heat deposition at focus and to plan the treatment (electrical power, duration). The temperature measurements were in good accordance with the predicted curves. The elasticity maps showed significant changes after treatment in some cases.

MR‐Guided Ultrasonic Brain Therapy: High Frequency Approach

A novel MR-guided brain therapy device operating at 1MHz has been designed and constructed. The system has been installed and tested in a clinical 1.5 T Philips Achieva MRI. Three dimensional time domain finite differences simulations were used to compute the propagation of the wave field through three human skulls. The simulated phase distortions were used as inputs for transcranial correction and the corresponding pressure fields were scanned in the focal plane. At half of the maximum power (10 W/cm2 on the surface of the transducers), necroses were induced 2 cm deep in turkey breasts placed behind a human skull. In vitro experiments on human skulls show that simulations restore more than 85% of the pressure level through the skull bone when compared to a control correction performed with an implanted hydrophone. Finally, high power experiments are performed though the skull bone and a MR-Thermometry sequence is used to map the temperature rise in a brain phantom every 3 s in two orthogonal planes (focal plane and along the axis of the probe).

Skull surface detection algorithm to optimize time reversal focusing through a human skull

Ultrasound has two main medical applications: imaging and therapy. Echographic imaging is particularly suited for soft tissues, such as liver, kidney or fetus. Treatment trials mainly concern liver, prostate, and bladder. In order to develop clinical applications dedicated to the human brain, one has to precisely focus ultrasound through the skull. The human skull induces both phase and amplitude aberrations. It has been shown that time reversal coupled with amplitude compensation is an interesting technique to correct those aberrations. We show here numerically that a more precise focusing can be achieved by compensating the amplitude at the exact location of the skull. To implement this in practice, a skull surface detection algorithm is investigated in order to determine the shape of the skull. It is based on the time of flight of pulsed signals emitted and received by each pair of transducers of an echographic array.

Cavitation bubble generation and control for HIFU transcranial adaptive focusing

Brain treatment with High Intensity Focused Ultrasound (HIFU) can be achieved by multichannel arrays through the skull using time‐reversal focusing. Such a method requires a reference signal either sent by a real source embedded in brain tissues or computed from a virtual source, using the acoustic properties of the skull deduced from CT images. This noninvasive computational method allows precise focusing, but is time consuming and suffers from unavoidable modeling errors which reduce the accessible acoustic pressure at the focus in comparison with real experimental time‐reversal using an implanted hydrophone. Ex vivo simulations with a half skull immersed in a water tank allow us to reach at low amplitude levels a pressure ratio of 83% of the reference pressure (real time reversal) at 1MHz. Using this method to transcranially focus a pulse signal in an agar gel (model for in vivo bubble formation), we induced a cavitation bubble that generated an ultrasonic wave received by the array. Selecting the 1MHz...

Protocole non-invasif de therapie transcranienne par ultrasons focalises base sur des images scanner: campagne sur des singes

Objectifs pedagogiques Presentation d’une experience nouvelle de traitement du cerveau ferme chez l’animal par ultrasons focalises. Nouvelle methode de focalisation des ondes acoustiques au travers du crâne pour corriger des aberrations. Messages a retenir Difficultes particulieres du traitement du cerveau par ultrasons : attenuation osseuse et aberration. Transposition chez l’homme non encore realisee. Creation de lesions intracerebrales par ultrasons.