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In cardiac diseases or after myocardial infarction, the fibrous layout in the heart can be modified. To determine the local fiber orientation and to characterize the lesion, an imaging method is required. The fiber orientation can be determined by diffusion MRI, but various factors limit its use in a beating heart. It has been demonstrated that ultrafast ultrasound imaging can measure the local fiber orientation of an in vivo heart based on the ultrasound spatial coherence. This method only returns the fiber orientations in planes parallel to the probe surface. We propose a method called 3D coherence function to improve this initial strategy to extract the full 3D local anisotropy. To validate this approach, 3D ultrasound datasets were acquired on a phantom constituted of several wire layers mimicking different fiber layers. The acquisitions were conducted with different angles between the probe surface and the wire layers. For each dataset, the conventional approach and our 3D coherence function were computed to compare the improvement in the fiber angle evaluation. We have demonstrated that when the out-of-plane angle increases, the 3D coherence function allows a better extraction of the angle.

International audience; 3D fast/ultrasound imaging has emerged in the last years in research but still suffers from its poor image quality. Indeed, using plane/diverge waves does not permit to insonify the medium with sufficient energy at each point to get a good signal-to-noise ratio, contrast-to-noise-ratio or even resolution. On the other hand, coded excitation is currently used to increase signal-to-noise ratio and penetration depth. In this work, the objective is to combine 3D fast/ultrafast imaging with coded excitation to achieve better image quality at a high acquisition rate. Promising experimental results are obtained from both wire and cyst phantoms using a chirp excitation signal. The contrast-to-noise ratio and signal-to-noise ratio were improved by 4 dB and 2 dB respectively by the proposed method in comparison to the conventional way to do 3D imaging using a standard transmit. The improvement of the axial resolution of about 17% is the third important result obtained by the developed method still in comparison with the classical method. Experimental results show that an effective implementation on a research scanner of 3D coded excitation using plane wave imaging is possible.

Sparse arrays represent an alternative to full-gridded arrays in 2-D probes realization. Sparse arrays have been used in B-mode imaging tests, but not in spectral Doppler measurements yet. In this paper, we have investigated, by mean of simulations and experiments, how the use of a sparse array instead of a full-gridded 2-D array impacts on spectral Doppler measurements. A 3 MHz, 1024-element gridded array and a sparse array obtained by properly selecting 256 elements from the full array, have been used to interrogate a parabolic flow. Simulations and experiments highlight that the mean Doppler frequency does not change by using the sparse array instead of the full one. Significant differences appear for the bandwidth (17.2% average bandwidth reduction for the sparse array), and for the signal power (22 dB). Possible explanations of this behavior are discussed. Furthermore, it is shown that the empty lines in the 2-D array impact on the Doppler spectra, leading to sidelobes when steering in the direction perpendicular to such lines.

International audience; High frame rate imaging is particularly important in echocardiography for a better assessment of the cardiac function. Several studies showed that Diverging Wave Imaging (DWI) and Multi Line Transmit (MLT) are promising methods for achieving a high temporal resolution. The aim of this study was to compare MLT and compounded motion compensated (MoCo) DWI for the same transmitted power, the same frame rates (image quality and Speckle Tracking Echocardiography-STE assessment) and the same packet size (Tissue Doppler Imaging-TDI assessment). Our results on static images showed that MLT outperforms DW in terms of resolution (by 30% in average). However, in terms of contrast, MLT outperforms DW only for the depth of 11 cm (by 40% in average), the result being reversed at a depth of 4 cm (by 27 % in average). In vitro results on a spinning phantom at 9 different velocities showed that similar STE axial errors (up to 2.3% difference in median errors and up to 2.1% difference in the interquartile ranges) are obtained with both ultrafast methods. On the other hand, the median lateral STE estimates were up to 13% more accurate with DW than with MLT. On the opposite, the accuracy of TDI was only up to ~3% better with MLT, but the achievable DW Doppler frame rate was up to 20 times higher. However, our overall results showed that the choice of one method relative to the other is therefore dependent on the application. More precisely, in terms of image quality, DW is more suitable for imaging structures at low depths, while MLT can provide an improved image quality at the focal point that can be placed at higher depths. In terms of motion estimation, DW is more suitable for color Doppler related applications, while MLT could be used to estimate velocities along selected lines of the image.

In the context of acute myocardial infarction, we propose that Low Energy Ultrasound (LEUS) exposures might attenuate the detrimental effects of ischemia and reperfusion injury. Specifically, our goal is to quantify and monitor the effects of ultrasound using an in vitro cardiac cell model of ischemia-reperfusion. The study was conducted using a mono-layer cell model (H9C2 cardiomyoblasts) exposed to a prolonged hypoxia-reoxygenation (HR) challenge. Two groups were formed: i). HR: cells submitted to hypoxia followed by reoxygenation period, and ii). HR+PostCond-LEUS: LEUS applied repeatedly for 20 min starting at the onset of reoxygenation. Cell death was evaluated by flow cytometry for each group at the end of US exposure. Cell viability was clearly improved in the HR+PostCond-LEUS group, at all time points during reoxygenation. This study suggests a potential protective effect of LEUS on cardiomyoblasts exposed to a prolonged hypoxia-reoxygenation insult. More complex in vitro models exploring potential protective mechanisms (SAFE and RISK signaling pathways and in vivo models will be required for a better comprehension of the underlying mechanisms.

International audience; A novel ultrasound (US) high-channels platform is a prerequisite to open new frontiers in diagnostic and/or therapy by experimental implementation of innovative advanced US techniques. To date, a few systems with more than 1000 transducers permit full and simultaneous control in both transmission and receiving of all single elements of arrays. A powerful US platform for implementing 4-D (real-time 3-D) advanced US strategies, offering full research access, is presented in this paper. It includes a 1024-elements array prototype designed for 4-D cardiac dual-mode US imaging/therapy and 4 synchronized Vantage systems. The physical addressing of each element was properly chosen for allowing various array downsampled combinations while minimizing the number of driving systems. Numerical simulations of US imaging were performed, and corresponding experimental data were acquired to compare full and downsampled array strategies, testing 4-D imaging sequences and reconstruction processes. The results indicate the degree of degradation of image quality when using full array or downsampled combinations, and the contrast ratio and the contrast to noise ratio vary from 7.71 dB to 2.02 dB and from 2.99 dB to −7.31 dB, respectively. Moreover, the feasibility of the 4-D US platform implementation was tested on a blood vessel mimicking phantom for preliminary Doppler applications. The acquired data with fast volumetric imaging with up to 2000 fps allowed assessing the validity of common 3-D power Doppler, opening in this way a large field of applications.

Purpose Treatments using high-intensity focused ultrasound (HIFU) in the abdominal region remain challenging as a result of respiratory organ motion. A novel method is described here to achieve 3D motion-compensated ultrasound (US) MR-guided HIFU therapy using simultaneous ultrasound and MRI. Methods A truly hybrid US-MR-guided HIFU method was used to plan and control the treatment. Two-dimensional ultrasound was used in real time to enable tracking of the motion in the coronal plane, whereas an MR pencil-beam navigator was used to detect anterior–posterior motion. Prospective motion compensation of proton resonance frequency shift (PRFS) thermometry and HIFU electronic beam steering were achieved. Results The 3D prospective motion-corrected PRFS temperature maps showed reduced intrascan ghosting artifacts, a high signal-to-noise ratio, and low geometric distortion. The k-space data yielded a consistent temperature-dependent PRFS effect, matching the gold standard thermometry within approximately 1°C. The maximum in-plane temperature elevation ex vivo was improved by a factor of 2. Baseline thermometry acquired in volunteers indicated reduction of residual motion, together with an accuracy/precision of near-harmonic referenceless PRFS thermometry on the order of 0.5/1.0°C. Conclusions Hybrid US-MR-guided HIFU ablation with 3D motion compensation was demonstrated ex vivo together with a stable referenceless PRFS thermometry baseline in healthy volunteer liver acquisitions. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

International audience; Background, Motivation and ObjectiveAfter a myocardium infarction, cell loss is irremediable leading to a progressive local disorganization and change in the tissue structure altering heart function. An imaging method able to render the local tissue directivity would be a powerful tool to characterize the extent of the lesion. In this field, diffusion MRI is the reference. Because of its long acquisitiontime and the difficulty to tackle organ motion, faster imaging strategies such as ultrasound, are mandatory for such clinical applications. One of them is the spatial coherence of US waves, already developed in focused and plane waves (2D and 3D) but never in diverging waves [Papadacci, UFFC 2014]. The advantage of diverging waves is to create an imagewith a higher field of view, which is more appropriate to in vivo cardiac applications. The purpose of this work is to use 2D spatial coherence to determine the fibers orientation and to compare results in 3D steered plane and diverging waves.Statement of Contribution/MethodsSpatial coherence is representative of the tissue structure. In an anisotropic medium, spatial coherence is high in that preferred local direction and the 2D coherence function exhibits an ellipsoidal shape. The main axis of this ellipse corresponds to the local main direction of the underlying tissue structure. Acquisitions have been conducted on a phantom designed from seven angled wires (ø 0.3 mm) embedded in an agar gel. The dataset was obtained using a 1024 channels ultrasound system based on the synchronization of 4 Verasonics Vantage 256 systems. A 1024 elements of a 32x32 elements 3 MHz array (Vermon) was fully controlled to transmit 2D steered plane and diverging waves on the same location. 25 transmission angles from -5° to 5° in x and y direction were used. The 2D coherence function maps were calculated on the spatial points corresponding to the centre of the seven wires and the main axis of ellipsoidal shapes are extracted to render the wires angle.Results/DiscussionThe curves representing the wires angle obtained in both plane and diverging waves are close to the reference. Using diverging waves, a RMSE of 4.8° is obtained which is better than the RMSE of 11.2° obtained with plane wave transmissions. It demonstrates the interest of 3D diverging waves to increase both the field of view and the coherence calculation accuracy. Such results must now be confirmed on heart sample acquisitions.

International audience; Background, Motivation and ObjectiveIn parallel with the increasing interest for 3D ultrasound imaging, different design techniques have been investigated to find the best configurations of 2D sparse arrays to scan an entire volume of interest [Trucco, IEEE UFFC99], [Diarra, IEEE TBME13]. In particular, we recently addressed the issue of driving a full 2D array of 1024 elements with a reduced number of channels (128, 192 or 256): the optimal arrays (opti128, opti192 and opti256) were obtained using simulated annealing to sculpt the radiated wideband pressure field at multiple depths [Roux, IEEE UFFC17]. The aim of the present work is to experimentally validate these optimal configurations by performing 3D focused imaging on phantoms.Statement of Contribution/MethodsThe 1024 elements of a 3 MHz array made by Vermon were individually driven by four synchronized Verasonics Vantage 256 systems. The systems were programmed to transmit 3-cycle sine bursts at 3 MHz focused at 25 mm and to scan a 3D sector with span ±30° in 31×29 steered beams in azimuth and elevation, respectively. Six arrays were compared: the optimal arrays (opti128, opti192 and opti256), an array whose active elements where randomly selected (rand256) and two references (REF716 and REF1024). The circular dense array REF716 corresponds to the full array REF1024 array without the corner elements. The comparison criteria were the lateral resolution (full width at half maximum - FWHM) and the contrast to noise ratio (CNR), measured on the images obtained by scanning a Gammex (Sono410 SCG) and CIRS (054GS) phantoms respectively.Results/DiscussionThe results are reported in Table 1. The opti256 performs the best among all the compared sparse arrays because it presents the same resolution performance as the REF1024 and an acceptable loss of CNR while using only 25% of the active elements of REF1024. The REF716 array is very competitive with only 70% of the active elements of REF1024.

International audience; Background, Motivation and ObjectiveA novel ultrasound (US) platform with high channels density that offers flexibility, precision and open access is a pre-requisite to open new frontiers in diagnostic and/or therapy by experimental implementation of new enabled advanced techniques: dual-mode US imaging/therapies in the heart, new approaches to study the myocardial tissue (structure/characterization), fast sparse array strategies, multi-line transmit (MLT), powerful motion correction strategies. To date few systems in the world permit to have a full control both in transmit and receive of all single elements simultaneously of arrays with more than 1000 transducers. This paper presents a powerful US platform for implementing 4D (real-time 3D) advanced US strategies.Statement of Contribution/MethodsAn US platform was developed including a 1024-element US prototype designed for 4D cardiac dual-mode US imaging/therapy (Vermon, 2D 32x32 planar phased-array transducer, fc = 3.4 MHz) and a high channels density (1024-channels) US scanner, made of 4 256-Vantage systems (Verasonics) synchronized together (Fig1e). These systems have per-channel arbitrary waveform transmit/receive generation capability with easy access to RF data. The physical addressing of each US element was properly chosen for allowing various array sparsity combinations while minimizing the number of Vantage driving systems needed. Numerical simulations of US imaging were performed and experimental data of identical configuration were acquired to compare full and sparse array strategies, testing 4D imaging sequences and reconstruction processes.Results/DiscussionReal-time 4D US imaging was successfully performed in full array mode by synchronizing in emission/reception up to 1024 elements independently. The modular US platform could be reconfigured depending on the number of available Vantage (1 to 4 systems) allowing to use dense halves/quarters of the array, or downsampled full array (Fig1b-d). The validationof image sequences involved plane, diverging and focused waves (SPW, SDW, MLT) and the technical feasibility of real-time 4D US for low therapy focused US and imaging was confirmed with this 1024-channels density US system, offering full research access for developing advanced US strategies.

International audience; Background, Motivation and ObjectiveThe emergence of ultrafast imaging allowed new insights into cardiac deformation/motion analysis that enabled new advancements in clinical diagnosis. Several studies showed that a good compromise between the temporal and the spatial resolution can be obtained by transmitting multiple focused beams (MLT) simultaneously. However, the currentimplementation of MLT in 3D cannot be used in dynamic conditions as it was obtained synthetically, by summing the Single Line Transmit (SLT) events before beamforming [Ortega et al., TUFFC, 2016]. The objective of this study is to evaluate for the first time the performance of MLT in 3D ultrasound under dynamic conditions. Statement of Contribution/MethodsThe 3D images were acquired by using four Verasonics research scanners, each of them controlling 8-by-32 elements of a 32-by-32, 3 MHz, Vermon ultrasound transducer. The scan volume was created by stitching 30 triangular sectors (each of angular width = 40°) along the azimuth angular direction. For each triangular sector, 9 transmission events were used for insonifying a full sector by sending simultaneously three focused beams. Thus, the total number of transmissions was 270 corresponding to 27 (elevational) x 30 (azimuthal) focal beams. The Pulse Repetition Frequency (PRF) was set to 2250. Our in vitro model was a tissue mimicking spinning disc having a diameter of 11 cm whose speed could be controlled by a step motor. The disk was placed at approximately 3 cm away from transducer and the image depth was 6.8 cm. Doppler velocities were estimated from the IQ signals using a 2D auto-correlator and a packet length of 7.Results/DiscussionThis study demonstrates the feasibility of Doppler velocity estimation for 3D MLT images. The estimated values presented in the image were in agreement with the expected velocities since the relative mean error was 3.5%. We obtained a three-fold increase in volume rate compared with focus imaging. Analyzing further different MLT/MLA configurations would allow increasing the frequency of volumes and obtaining the optimal compromise between high volume rate and quality color Doppler.

Echocardiography is the most used modality for the evaluation of cardiac function. To obtain a time-resolved volumetric quantification of cardiac motion, ultrafast 3-D imaging is required. Ultrafast ultrasound imaging with diverging waves has demonstrated its potential for clinical 2-D echocardiography. It has been shown that MoCo (motion compensation) strategies based on a triangle transmit arrangement could produce high-contrast cardiac B-mode images at 500 fps [1]. With the purpose of developing high-volume-rate 3-D echocardiography, we introduced a transmit sequence enabling volumetric MoCo for spherical diverging wave imaging.

This work shows the feasibility of performing ultrafast 3D ultrasound imaging by producing diverging waves (DW) with 2D sparse arrays. The 3D volumes were experimentally acquired by individually driving the 1024 elements of a full 32×32 matrix array. The volumes obtained with different sparse configurations were compared to those obtained with two references arrays: the full 32×32 array and a dense array of 716 elements where obtained by de-activating the corner elements of the full array. The comparison criteria were the lateral resolution using the full width at half maximum (FWHM) and the contrast to noise ratio (CNR), measured on the images obtained by scanning a grayscale phantom. The results show that ultrafast 3D ultrasound imaging can be performed with a reduced number of channels using 2D sparse arrays. However, there is still space to find the optimal 2D sparse array configurations to best transmit 3D DWs and potential improvements on image quality may be achieved with dedicated optimization e.g. cost function to homogenize DW wave front.

Ultrafast imaging based on diverging wave (DW) is an active area of research in ultrasound sectorial acquisition because of its capacity of reaching high frame rate. Recently, we have introduced a 3D Fourier-based formalism for the reconstruction of 3D sectorial images with DW insonifications and validated with numerical simulations [Zhang et al. IUS 2016]. The proposed method can provide comparable image quality as DAS but with a much lower computational complexity. This study aims to experimentally verify these theoretical conclusions in vitro.

Echocardiography is one of the most widely spread modality for non-invasive 2-D imaging of the heart. However, to fully observe and quantify left ventricular function and morphology, 3-D imaging is required. The observation of the whole left ventricle remains limited by the temporal resolution of conventional 3-D echocardiography. Indeed, the only way to get wide-angle volumetric images is to merge small scan volumes during inspiratory breath hold. This limitation makes difficult 3D speckle tracking echocardiography. Increasing the temporal resolution with ultrafast 3-D echocardiography could enable imaging of the whole myocardium in a single heartbeat. It has recently been shown that 3-D ultrafast ultrasound imaging is possible with spherical diverging waves emitted by a matrix-array probe. As ultrafast ultrasound requires backscattered signals to be combined coherently, this approach is sensitive to high-velocity tissue motion, especially when the heart is investigated. Ultrafast ultrasound imaging with diverging waves has demonstrated its potential for clinical 2-D-echocardiography only when motion compensation (MoCo) is integrating during the compounding process. It has been demonstrated that MoCo strategies based on a triangle transmit arrangement could produce high-contrast 2-D cardiac B-mode images at 500 fps. In this study, we proposed a feasibility study of high-frame-rate 3-D echocardiography based on an innovative MoCo approach using steered spherical diverging waves. The method was validated in vitro with a rotating disk. The sequences were generated with four Verasonics scanners combined to get 1024 channels and a 3-MHz matrix phased array of 32×32 elements.

International audience; In this paper a global estimation method based on transverse oscillation to simultaneously extract wall and flow velocities at high-framerate is presented. Several carotid phantoms with various parameters were made to validate the method. All acquisitions were performed at high-framerate (7 500 images per second) using horizontal plane wave with a 3 cycles sinusoidal transmit pulse. Transverse oscillation was introduced in post-acquisition. Finally, velocity vectors were extracted thanks to a phase based estimator with a region of interest of 2 mm (8 axial wavelengths) per 2.96 mm (2 lateral wavelengths) for each pixel. Results are promising, all standard deviations are lower than 10 % and the method is now validated for a deeper study. Indeed, flow and pulse wave velocities computed by the algorithm are in accordance with pressure columns and number of freezethaw cycles.

With the availability of new and more accurate tumour treatment modalities such as high-intensity focused ultrasound or proton therapy, accurate target location prediction has become a key issue. Various approaches for diverse application scenarios have been proposed over the last decade. Whereas external surrogate markers such as a breathing belt work to some extent, knowledge about the internal motion of the organs inherently provides more accurate results. In this paper, we combine a population-based statistical motion model and information from 2d ultrasound sequences in order to predict the respiratory motion of the right liver lobe. For this, the motion model is fitted to a 3d exhalation breath-hold scan of the liver acquired before prediction. Anatomical landmarks tracked in the ultrasound images together with the model are then used to reconstruct the complete organ position over time. The prediction is both spatial and temporal, can be computed in real-time and is evaluated on ground truth over long time scales (5.5 min). The method is quantitatively validated on eight volunteers where the ultrasound images are synchronously acquired with 4D-MRI, which provides ground-truth motion. With an average spatial prediction accuracy of 2.4 mm, we can predict tumour locations within clinically acceptable margins.

Purpose Magnetic resonance-guided high-intensity focused ultrasound is considered to be a promising treatment for localized cancer in abdominal organs such as liver, pancreas, or kidney. Abdominal motion, anatomical arrangement, and required sustained sonication are the main challenges. Methods MR acquisition consisted of thermometry performed with segmented gradient-recalled echo echo-planar imaging, and a segment-based one-dimensional MR navigator parallel to the main axis of motion to track the organ motion. This tracking information was used in real-time for: (i) prospective motion correction of MR thermometry and (ii) HIFU focal point position lock-on target. Ex vivo experiments were performed on a sheep liver and a turkey pectoral muscle using a motion demonstrator, while in vivo experiments were conducted on two sheep liver. Results Prospective motion correction of MR thermometry yielded good signal-to-noise ratio (range, 25 to 35) and low geometric distortion due to the use of segmented EPI. HIFU focal point lock-on target yielded isotropic in-plane thermal build-up. The feasibility of in vivo intercostal liver treatment was demonstrated in sheep. Conclusion The presented method demonstrated in moving phantoms and breathing sheep accurate motion-compensated MR thermometry and precise HIFU focal point lock-on target using only real-time pencil-beam navigator tracking information, making it applicable without any pretreatment data acquisition or organ motion modeling. Magn Reson Med 72:1087–1095, 2014. © 2013 Wiley Periodicals, Inc.

Extracorporeal high-intensity focused ultrasound (HIFU) is a minimally invasive therapy considered with increased interest for the ablation of small tumors in deeply located organs while sparing surrounding critical tissues. A multitude of preclinical and clinical studies have showed the feasibility of the method; however, concurrently they showed several obstacles, among which the management of respiratory motion of abdominal organs is at the forefront. The aim of this review is to describe the different methods that have been proposed for managing respiratory motion and to identify their advantages and weaknesses. First, we specify the characteristics of respiratory motion for the liver, kidneys, and pancreas and the problems it causes during HIFU planning, treatment, and monitoring. Second, we make an inventory of the preclinical and clinical approaches used to overcome the problem of organ motion. Third, we analyze their respective benefits and drawbacks to identify the remaining physical, technological, and clinical challenges. We thereby consider the outlook of motion compensation techniques and those that would be the most suitable for clinical use, particularly under magnetic resonance thermometry monitoring.

Objectives: The combination of ultrasound (US) and magnetic resonance imaging (MRI) may provide a complementary description of the investigated anatomy, together with improved guidance and assessment of image-guided therapies. The aim of the present study was to integrate a clinical setup for simultaneous US and magnetic resonance (MR) acquisition to obtain synchronized monitoring of liver motion. The feasibility of this hybrid imaging and the precision of image fusion were evaluated. Materials and Methods: Ultrasound imaging was achieved using a clinical US scanner modified to be MR compatible, whereas MRI was achieved on 1.5- and 3-T clinical scanners. Multimodal registration was performed between a high-resolution T1 3-dimensional (3D) gradient echo (volume interpolated gradient echo) during breath-hold and a simultaneously acquired 2D US image, or equivalent, retrospective registration of US imaging probe in the coordinate frame of MRI. A preliminary phantom study was followed by 4 healthy volunteer acquisitions, performing simultaneous 4D MRI and 2D US harmonic imaging (Fo = 2.2 MHz) under free breathing. Results: No characterized radiofrequency mutual interferences were detected under the tested conditions with commonly used MR sequences in clinical routine, during simultaneous US/MRI acquisition. Accurate spatial matching between the 2D US and the corresponding MRI plane was obtained during breath-hold. In situ fused images were delivered. Our 4D MRI sequence permitted the dynamic reconstruction of the intra-abdominal motion and the calculation of high temporal resolution motion field vectors. Conclusions: This study demonstrates that, truly, simultaneous US/MR dynamic acquisition in the abdomen is achievable using clinical instruments. A potential application is the US/MR hybrid guidance of high-intensity focused US therapy in the liver.

The treatment of liver cancer is a major public health issue because the liver is a frequent site for both primary and secondary tumors. Rib heating represents a major obstacle for the application of extracorporeal focused ultrasound to liver ablation. Magnetic resonance (MR)-guided external shielding of acoustic obstacles (eg, the ribs) was investigated here to avoid unwanted prefocal energy deposition in the pathway of the focused ultrasound beam.Ex vivo and in vivo (7 female sheep) experiments were performed in this study. Magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) was performed using a randomized 256-element phased-array transducer (f∼1 MHz) and a 3-T whole-body clinical MR scanner. A physical mask was inserted in the prefocal beam pathway, external to the body, to block the energy normally targeted on the ribs. The effectiveness of the reflecting material was investigated by characterizing the efficacy of high-intensity focused ultrasound beam reflection and scattering on its surface using Schlieren interferometry. Before high-intensity focused ultrasound sonication, the alignment of the protectors with the conical projections of the ribs was required and achieved in multiple steps using the embedded graphical tools of the MR scanner. Multiplanar near real-time MR thermometry (proton resonance frequency shift method) enabled the simultaneous visualization of the local temperature increase at the focal point and around the exposed ribs. The beam defocusing due to the shielding was evaluated from the MR acoustic radiation force impulse imaging data.Both MR thermometry (performed with hard absorber positioned behind a full-aperture blocking shield) and Schlieren interferometry indicated a very good energy barrier of the shielding material. The specific temperature contrast between rib surface (spatial average) and focus, calculated at the end point of the MRgHIFU sonication, with protectors vs no protectors, indicated an important reduction of the temperature elevation at the ribs' surface, typically by 3.3 ± 0.4 in vivo. This was translated into an exponential reduction in thermal dose by several orders of magnitude. The external shielding covering the full conical shadow of the ribs was more effective when the protectors could be placed close to the ribs' surface and had a tendency to lose its efficiency when placed further from the ribs. Hepatic parenchyma was safely ablated in vivo using this rib-sparing strategy and single-focus independent sonications.A readily available, MR-compatible, effective, and cost-competitive method for rib protection in transcostal MRgHIFU was validated in this study, using specific reflective strips. The current approach permitted safe intercostal ablation of small volumes (0.7 mL) of liver parenchyma.

International audience; Early detection of cardiovascular diseases can be done by assessing the dynamic properties of arteries. In this study, two phenomena related to the carotid wall motion are presented. The 2D motion estimation method employs (i) ultrafast imaging which became a world-wide use modality, (ii) transverse oscillations technics which allow improving the motion estimation in transverse direction i.e. perpendicular to the beam, and (iii) a 2D phase shift estimator. First, using only the radial acceleration wall, the Wall Spatial Periodic Ripples (WSPR) is studied under a cold pressor test. The results show that the WSPR mean maximum amplitude decreased of 22.9% at the systolic wave and 33.2% at the dicrotic notch. So, the WSPR is a possible artery wall rigidity marker. Then, using a very high frame rate imaging, the longitudinal carotid wall motion of one healthy subject is studied. This second experimentation permits to estimate the propagation of a longitudinal motion on an in vivo carotid. Its mean velocity was evaluated at 16.1 m.s-1, which given a ratio of 3 between with the estimated PWV.

High intensity focused ultrasound (HIFU) under MRI guidance may provide minimally invasive treatment for localized prostate cancer. In this study, ex vivo and in vivo experiments were performed using a prostate-dedicated endorectal phased array (16 circular elements arranged on a truncated spherical cap of radius 60 mm) and a translation-rotation mechanical actuator in order to evaluate the lesion formation and the potential interest of dual-modality (electronic and mechanical) interleaved displacement of the focus for volumetric sonication paradigms. Different sonication sequences, including elementary lesions, line scan, slice sweeping and volume sonications, were investigated with a clinical 1.5 T MR scanner. Two orthogonal planes (axial and sagittal) were simultaneously monitored using rapid MR thermometry (PRFS method) and the temperature and thermal dose maps were displayed in real time. No RF interferences were detected in MR acquisition during sonications. The shape of the thermal lesions in vivo was examined at day 5 post-treatment by MRI follow-up (T2w sequence and Gd-T1w-TFE) and postmortem histological analysis. This study suggests that electronic displacement of the focus (along the ultrasound propagation axis) interleaved with mechanical X-Z translations and rotation around B(0) can be a suitable modality to treat patient-specific sizes and shapes of a pathologic tissue. The electronic displacement of focus (achieved in less than 0.1 s) is an order of magnitude faster than the mechanical motion of the HIFU device (1 s latency). As an example, for an in vivo volumetric sonication with foci between 32 and 47 mm (7 successive line scans, 11 lines/slice, 4 foci/line) with applied powers between 17.4 and 39.1 Wac, a total duration of sonication of 408.1 s was required to ablate a volume of approximately 5.7 cm(3) (semi-chronic lesion measured at day 5), while the maximum temperature elevation reached was 30 °C. While electronic focusing is necessary to speed up the procedure, one should consider as a potential drawback the non-negligible risk for generating secondary lobes with full steering in 3D. Reference-free PRFS thermometry accurately removed the effects of B(o) dynamic perturbation in the vicinity of the moving transducer. Therefore, the dual-modality volumetric sonication paradigm represents a cost-effective technological compromise to induce the desired shape of the lesion in the prostate through the limited endorectal space, in a reasonable period of time and without side effects.

Magnetic resonance imaging (MRI) and ultrasonography have been used simultaneously in this ex vivo study for the image-guidance of high intensity focused ultrasound (HIFU) treatment in moving tissue. A ventilator-driven balloon produced periodic and non-rigid (i.e. breathing-like) motion patterns in phantoms. MR-compatible ultrasound (US) imaging enabled near real-time 2D motion tracking based on optical flow detection, while near-harmonic reference-free proton resonance frequency shift (PRFS) MR thermometry (MRT) was used to monitor the thermal buildup on line. Reference-free MRT was applied to gradient-echo echo-planar imaging phase maps acquired at the frame rate of 250 to 300 ms/slice with voxel size 1.25×1.25×5 mm(3). The MR-US simultaneous imaging was completely free of mutual interferences while minor RF interferences from the HIFU device were detected in the far field of the US images. The effective duty-cycle of the HIFU sonication was close to 100 % and no off-interval was required to temporally decouple it from the ultrasonography. The motion compensation of the HIFU sonication was achieved with an 8 Hz frame rate and sub-millimeter spatial accuracy, both for single-focus mode and for an iterated multi-foci line scan. Near harmonic reference-less PRFS MRT delivered motion-robust thermal maps perpendicular or parallel to the HIFU beam (0.7 °C precision, 0.5 °C absolute accuracy). Out-of-plane motion compensation was not addressed in this study.

Purpose: Endocavitary high intensity contact ultrasound (HICU) may offer interesting therapeutic potential for fighting localized cancer in esophageal or rectal wall. On-line MR guidance of the thermotherapy permits both excellent targeting of the pathological volume and accurate preoperatory monitoring of the temperature elevation. In this article, the authors address the issue of the automatic temperature control for endocavitary phased-array HICU and propose a tailor-made thermal model for this specific application. The convergence and stability of the feedback loop were investigated against tuning errors in the controller’s parameters and against input noise, throughex vivo experimental studies and through numerical simulations in which nonlinear response of tissue was considered as expected in vivo. Methods: An MR-compatible, 64-element, cooled-tip, endorectal cylindrical phased-array applicator of contact ultrasound was integrated with fast MR thermometry to provide automatic feedback control of the temperature evolution. An appropriate phase law was applied per set of eight adjacent transducers to generate a quasiplanar wave, or a slightly convergent one (over the circular dimension). A 2D physical model, compatible with on-line numerical implementation, took into account (1) the ultrasound-mediated energy deposition, (2) the heat diffusion in tissue, and (3) the heat sink effect in the tissue adjacent to the tip-cooling balloon. This linear model was coupled to a PID compensation algorithm to obtain a multi-input single-output static-tuning temperature controller. Either the temperature at one static point in space (situated on the symmetry axis of the beam) or the maximum temperature in a user-defined ROI was tracked according to a predefined target curve. The convergence domain in the space of controller’s parameters was experimentally exploredex vivo. The behavior of the static-tuning PID controller was numerically simulated based on a discrete-time iterative solution of the bioheat transfer equation in 3D and considering temperature-dependent ultrasound absorption and blood perfusion. Results: The intrinsic accuracy of the implemented controller was approximately 1% inex vivo trials when providing correct estimates for energy deposition and heat diffusivity. Moreover, the feedback loop demonstrated excellent convergence and stability over a wide range of the controller’s parameters, deliberately set to erroneous values. In the extreme case of strong underestimation of the ultrasound energy deposition in tissue, the temperature tracking curve alone, at the initial stage of the MR-controlled HICU treatment, was not a sufficient indicator for a globally stable behavior of the feedback loop. Our simulations predicted that the controller would be able to compensate for tissue perfusion and for temperature-dependent ultrasound absorption, although these effects were not included in the controller’s equation. The explicit pattern of acoustic field was not required as input information for the controller, avoiding time-consuming numerical operations. Conclusions: The study demonstrated the potential advantages of PID-based automatic temperature control adapted to phased-array MR-guided HICU therapy. Further studies will address the integration of this ultrasound device with a miniature RF coil for high resolution MRI and, subsequently, the experimental behavior of the controllerin vivo.

Background Magnetic Resonance-guided High Intensity Focused Ultrasound (MRgHIFU) is a hybrid technology that aims to offer non-invasive thermal ablation of targeted tumors or other pathological tissues. Acoustic aberrations and non-linear wave propagating effects may shift the focal point significantly away from the prescribed (or, theoretical) position. It is therefore mandatory to evaluate the spatial accuracy of ablation for a given HIFU protocol and/or device. We describe here a method for producing a user-defined ballistic target as an absolute reference marker for MRgHIFU ablations. Methods The investigated method is based on trapping a mixture of MR contrast agent and histology stain using radiofrequency (RF) ablation causing cell death and coagulation. A dedicated RF-electrode was used for the marker fixation as follows: a RF coagulation (4 W, 15 seconds) and injection of the mixture followed by a second RF coagulation. As a result, the contrast agent/stain is encapsulated in the intercellular space. Ultrasonography imaging was performed during the procedure, while high resolution T1w 3D VIBE MR acquisition was used right after to identify the position of the ballistic marker and hence the target tissue. For some cases, after the marker fixation procedure, HIFU volumetric ablations were produced by a phased-array HIFU platform. First ex vivo experiments were followed by in vivo investigation on four rabbits in thigh muscle and six pigs in liver, with follow-up at Day 7. Results At the end of the procedure, no ultrasound indication of the marker’s presence could be observed, while it was clearly visible under MR and could be conveniently used to prescribe the HIFU ablation, centered on the so-created target. The marker was identified at Day 7 after treatment, immediately after animal sacrifice, after 3 weeks of post-mortem formalin fixation and during histology analysis. Its size ranged between 2.5 and 4 mm. Conclusions Experimental validation of this new ballistic marker method was performed for liver MRgHIFU ablation, free of any side effects (e.g. no edema around the marker, no infection, no bleeding). The study suggests that the absolute reference marker had ultrasound conspicuity below the detection threshold, was irreversible, MR-compatible and MR-detectable, while also being a well-established histology staining technique.

A novel architecture for a phased-array high intensity focused ultrasound (HIFU) device was investigated, aiming to increase the capabilities of electronic steering without reducing the size of the elementary emitters. The principal medical application expected to benefit from these developments is the time-effective sonication of large tumours in moving organs. The underlying principle consists of dividing the full array of transducers into multiple sub-arrays of different resonance frequencies, with the reorientation of these individual emitters, such that each sub-array can focus within a given spatial zone. To enable magnetic resonance (MR) compatibility of the device and the number of output channels from the RF generator to be halved, a passive spectral multiplexing technique was used, consisting of parallel wiring of frequency-shifted paired piezoceramic emitters with intrinsic narrow-band response. Two families of 64 emitters (circular, 5 mm diameter) were mounted, with optimum efficiency at 0.96 and 1.03 MHz, respectively. Two different prototypes of the HIFU device were built and tested, each incorporating the same two families of emitters, but differing in the shape of the rapid prototyping plastic support that accommodated the transducers (spherical cap with radius of curvature/aperture of 130 mm/150 mm and, respectively, 80 mm/110 mm). Acoustic measurements, MR-acoustic radiation force imaging (ex vivo) and MR-thermometry (ex vivo and in vivo) were used for the characterization of the prototypes. Experimental results demonstrated an augmentation of the steering range by 80% along one preferentially chosen axis, compared to a classic spherical array of the same total number of elements. The electric power density provided to the piezoceramic transducers exceeded 50 W cm(-2) CW, without circulation of coolant water. Another important advantage of the current approach is the versatility of reshaping the array at low cost.

We propose an unconventional approach for transferring of information between multi-modal images. It exploits the temporal commonality of multi-modal images acquired from the same organ during free-breathing. Strikingly there is no need for capturing the same region by the modalities. The method is based on extracting a low-dimensional description of the image sequences, selecting the common cause signal (breathing) for both modalities and finding the most similar sub-sequences for predicting image feature location. The approach was evaluated for 3 volunteers on sequences of 2D MRI and 2D US images of the liver acquired at different locations. Simultaneous acquisition of these images allowed for quantitative evaluation (predicted versus ground truth MRI feature locations). The best performance was achieved with signal extraction by slow feature analysis resulting in an average error of 2.6 mm (4.2 mm) for sequences acquired at the same (a different) time.

Magnetic resonance (MR) imaging and ultrasound (US) imaging are complementary and synergetic noninvasive imaging modalities. US imaging is practically free of geometric distortion and provides high temporal resolution and direct visualization of acoustic obstacles. MR imaging offers excellent tissue contrast and a confirmed method for near-real-time thermometry. Their combination may help increase the intraoperative control and assessment in image-guided therapies. The added value of this dual-modality imaging would consist of a more complete description of the anatomy investigated, more accurate targeting, efficient motion tracking, and reliable immediate assessment of the therapeutic results. This chapter focuses on truly simultaneous US/MR technology integration and early applications. A prototype setup designed to clinical standards is described together with the preliminary evaluation of the hybrid imaging performance in the abdomen. In particular, the precision of image coregistration was assessed in healthy volunteers. Furthermore, a complex study investigated the thermal cavitation effects produced by radio-frequency ablations. Simultaneous US imaging/MR imaging acquisition permitted us to demonstrate that the latter effects induce subsequent magnetic-susceptibility-mediated errors in proton resonance frequency shift thermometry, both in ex vivo models and in patients treated for hepatic malignancies. The foreseen perspectives of the hybrid US imaging/MR imaging technique are included in the final section.

The study investigates a new sonication strategy with high-intensity focused ultrasound (HIFU), aiming for improvement of the original Ablatherm procedure in the prostate cancer treatment. The currently implemented and clinically used method (defined as reference) uses a single-element transducer, operated with 60% duty cycle. To implement the novel strategy, the active surface was split into two sectors, which can be powered either sequentially (for temporal switching) or simultaneously (equivalent to a single-element transducer). Numerical simulations were used to predict the lesion shape and to determine for the novel strategy the best set of treatment parameters among the 99 explored cases. The same pattern for the focal point trajectory was executed irrespectively to the sector activating mode. The theoretical duty cycle reached 100% for the sector switching strategy. The HIFU device was built MRI compatible, and consisted of two mirror symmetrical sectors operating at 3 MHz, shaped as a truncated spherical cap. The two sonication strategies were experimentally tested on fresh samples of degassed porcine liver, using fast MR thermometry (proton resonance frequency shift method with voxel size 0.85 x 0.85 x 4.25 mm (3), 2 s/dynamic, 0.5 ( degrees ) C temperature accuracy, two orthogonal slices). A practical value of 87.5% overall duty cycle could be experimentally implemented. The performance of the two sonication strategies was comparatively assessed based on: cumulated thermal dose derived from MR temperature maps, postoperatory MR morphological images sensitive to tissue contrast changes (inversion-recovery T1-weighted turbo spin-echo, voxel size 0.5 x 0.5 x 4 mm (3)) and postoperatory macroscopic tissue examination. Using a sector-switching sonication strategy for prostate cancer treatment-induced lesions of similar size and shape as for the reference approach. When considering the available reserve of duty cycle and the exact lesion size, we concluded the treatment time was reduced by 20% with the new sector switching strategy at equal performance. Further in vivo studies are considered mandatory for preclinical validation.

A new high intensity focused ultrasound (HIFU) probe has been designed and tested by using MRI. The probe is intended to be used by physicians to correct valvular dysfunction in the saphenous vein, which is known to be the cause of superficial venous insufficiency (SVI) and varicose veins. Treating SVI with HIFU is possible, since venous tissue undergoes localized partial shrinkage when subjected to HIFU. In vitro experiments have demonstrated that diameter shrinkage should be sufficient to restore valvular function, as is done in the more aggressive approach known as external valvuloplasty. Numerical simulations and optimization have led to a probe design with two HIFU elements that focus sound uniformly over a line of length 7 mm, at a depth of 15 mm from the skin. A prototype of the probe has been constructed, with a holder that provides space for an MRI-imaging antenna. The probe has been tested by measuring pressure and temperature fields. Results are in good agreement with those predicted by an analytical approach and numerical simulations.

Objective. To demonstrate the technical feasibility and the potential interest of using a digital optical camera inside the MR magnet bore for monitoring the breathing cycle and subsequently gating the PRFS MR thermometry, MR-ARFI measurement, and MRgHIFU sonication in the upper abdomen. Materials and Methods. A digital camera was reengineered to remove its magnetic parts and was further equipped with a 7 m long USB cable. The system was electromagnetically shielded and operated inside the bore of a closed 3T clinical scanner. Suitable triggers were generated based on real-time motion analysis of the images produced by the camera (resolution 640×480 pixels, 30 fps). Respiratory-gated MR-ARFI prepared MRgHIFU ablation was performed in the kidney and liver of two sheep in vivo, under general anaesthesia and ventilator-driven forced breathing. Results. The optical device demonstrated very good MR compatibility. The current setup permitted the acquisition of motion artefact-free and high resolution MR 2D ARFI and multiplanar interleaved PRFS thermometry (average SNR 30 in liver and 56 in kidney). Microscopic histology indicated precise focal lesions with sharply delineated margins following the respiratory-gated HIFU sonications. Conclusion. The proof-of-concept for respiratory motion management in MRgHIFU using an in-bore digital camera has been validated in vivo.

The emergence of ultrafast imaging allowed new insights into cardiac deformation/motion analysis that enabled new advancements in clinical diagnosis. Several studies showed that a good compromise between the temporal and the spatial resolution can be obtained by transmitting multiple focused beams (MLT) simultaneously. However, the current implementation of MLT in 3D cannot be used in dynamic conditions as it was obtained synthetically, by summing the Single Line Transmit (SLT) events before beamforming [Ortega et al., TUFFC, 2016]. The objective of this study is to evaluate for the first time the performance of MLT in 3D ultrasound under dynamic conditions.

International audience; Background, Motivation and ObjectiveAchieving a high frame rate in echocardiography is highly important for quantifying the short phases of the cardiac cycle that contain valuable information for medical diagnosis. Additionally, the 3D quantitative assessment of the heart would significantly improve the current measurements used in daily clinical routine. Nevertheless, obtaining ultrafast imagesremains a challenge due to the trade-off between the image quality and a high frame rate, especially when volumetric data is acquired. Among the current ultrafast imaging methods, multi-line-transmit imaging (MLT) provides an increased frame rate but in the same time mostly preserves the image quality. However, the current implementation of this methodin 3D proposed by Ortega et al. [IEEE TUFC 2016] is based on generating the MLT data synthetically by summing up the raw data before beamforming. In this paper we present the first real-time implementation of the MLT in 3D ultrasound.Statement of Contribution/MethodsMulti-line-transmission was performed by dividing the angular aperture into three regions. Then, due to practical limitations, three equally spaced focused beams were transmitted simultaneously in the first YZ plane of the first region. Once the transmission for a full YZ plane was completed, the process was repeated for the first YZ plane of the second region. The same steps were followed till the full volume was insonified. Data acquisition was performed using four Verasonics systems synchronized to drive a 32x32 matrix probe. A transmit frequency of 2.97 MHz was used and the images were acquired using a sampling frequency of 11.9 MHz. The focal point was set to 6.7 cm depth along 27 different angles in elevational direction (YZ) and 30 angles in azimuthal direction (XZ) between -20º and 20º.Results/DiscussionThe contrast and the resolution assessment, performed on a CIRS ultrasound phantom (Figure 1) showed a contrast of 6.36 dB and a mean axial (lateral) resolution of 0.9 mm (1.77 mm) measured for different depths. These values are comparable with those obtained for a 3D focused sequence obtained by using the same emission parameters. The results indicate the potential of MLT 3D for achieving high contrast and resolution while increasing the frame rate. This study thus demonstrates the feasibility of 3D MLT in real-time and extends its possible applications to dynamic cardiac imaging.