Mathieu Pernot

Spatio-temporal analysis of molecular delivery through the blood-brain barrier using focused ultrasound

The deposition of gadolinium through ultrasound-induced blood-brain barrier (BBB) openings in the murine hippocampus was investigated. First, wave propagation simulations through the intact mouse skull revealed minimal beam distortion while thermal deposition simulations, at the same sonication parameters used to induce BBB opening in vivo, revealed temperature increases lower than 0.5 degrees C. The simulation results were validated experimentally in ex vivo skulls (m = 6) and in vitro tissue specimens. Then, in vivo mice (n = 9) were injected with microbubbles (Optison; 25-50 microl) and sonicated (frequency: 1.525 MHz, pressure amplitudes: 0.5-1.1 MPa, burst duration: 20 ms, duty cycle: 20%, durations: 2-4 shots, 30 s per shot, 30 s interval) at the left hippocampus, through intact skin and skull. Sequential, high-resolution, T1-weighted MRI (9.4 Tesla, in-plane resolution: 75 microm, scan time: 45-180 min) with gadolinium (Omniscan; 0.5 ml) injected intraperitoneally revealed a threshold of the BBB opening at 0.67 MPa and BBB closing within 28 h from opening. The contrast-enhancement area and gadolinium deposition path were monitored over time and the influence of vessel density, size and location was determined. Sonicated arteries, or their immediate surroundings, depicted greater contrast enhancement than sonicated homogeneous brain tissue regions. In conclusion, gadolinium was delivered through a transiently opened BBB and contained to a specific brain region (i.e., the hippocampus) using a single-element focused ultrasound transducer. It was also found that the amount of gadolinium deposited in the hippocampal region increased with the acoustic pressure and that the spatial distribution of the BBB opening was determined not only by the ultrasound beam, but also by the vasculature of the targeted brain region.

A Novel Noninvasive Technique for Pulse-Wave Imaging and Characterization of Clinically-Significant Vascular Mechanical Properties In Vivo

The pulse-wave velocity (PWV) has been used as an indicator of vascular stiffness, which can be an early predictor of cardiovascular mortality. A noninvasive, easily applicable method for detecting the regional pulse wave (PW) may contribute as a future modality for risk assessment. The purpose of this study was to demonstrate the feasibility and reproducibility of PW imaging (PWI) during propagation along the abdominal aortic wall by acquiring electrocardiography-gated (ECG-gated) radiofrequency (rf) signals noninvasively. An abdominal aortic aneurysm (AAA) was induced using a CaCl2 model in order to investigate the utility of this novel method for detecting disease. The abdominal aortas of twelve normal and five CaCl2, mice were scanned at 30 MHz and electrocardiography (ECG) was acquired simultaneously. The radial wall velocities were mapped with 8000 frames/s. Propagation of the PW was demonstrated in a color-coded ciné-loop format in all cases. In the normal mice, the wave propagated in linear fashion from a proximal to a distal region. However, in CaCl2 mice, multiple waves were initiated from several regions (i.e., most likely initiated from various calcified regions within the aortic wall). The regional PWV in normal aortas was 2.70 ± 0.54 m/s (r2 = 0.85 ± 0.06, n = 12), which was in agreement with previous reports using conventional techniques. Although there was no statistical difference in the regional PWV between the normal and CaCl2-treated aortas (2.95 ± 0.90 m/s (r2 = 0.51 ± 0.22, n = 5)), the correlation coefficient was found to be significantly lower in the CaCl2-treated aortas (p<0.01). This state-of-the-art technique allows noninvasive mapping of vascular disease in vivo. In future clinical applications, it may contribute to the detection of early stages of cardiovascular disease, which may decrease mortality among high-risk patients.

Single-element focused ultrasound transducer method for harmonic motion imaging.

The harmonic motion imaging (HMI) technique for simultaneous monitoring and generation of ultrasound therapy using two separate focused ultrasound transducer elements was previously demonstrated. In this study, a new HMI technique is described that images tissue displacement induced by a harmonic radiation force using a single focused-ultrasound element. A wave propagation simulation model first indicated that, unlike in the two-beam configuration, the amplitude-modulated beam produced a stable focal zone for the applied harmonic radiation force. The AM beam thus offered the unique advantage of sustaining the application of the spatially-invariant radiation force. Experiments were performed on gelatin phantoms and ex vivo tissues. The radiation force was generated by a 4.68 MHz focused ultrasound (FUS) transducer using a 50 Hz amplitude-modulated wave. A 7.5 MHz pulse-echo transducer was used to acquire rf echoes during the application of the harmonic radiation force. Consecutive rf echoes were acquired with a pulse repetition frequency (PRF) of 6.5 kHz and 1D cross-correlation was performed to estimate the resulting axial tissue displacement. The HMI technique was shown capable of estimating stiffness-dependent displacement amplitudes. Finally, taking advantage of the real-time capability of the HMI technique, temperature-dependent measurements enabled monitoring of HIFU sonication in ex vivo tissues. The new HMI method may thus enable a highly-localized force and stiffness-dependent measurements as well as real-time and low-cost HIFU monitoring.

Electromechanical imaging of the myocardium at normal and pathological states

The motion of the myocardium is mainly due to the contraction and the relaxation of the cardiac muscle. However besides this slow and large motion component, more rapid displacements occur at several periods of the cardiac cycle. Indeed, the contraction of the myocardium is induced by electrical waves that propagate very fast (about 1 m/s) in the cardiac tissue. Due to the electrical excitation, the fibers contraction results in a strong mechanical wave that propagates in the myocardium. We propose here a method for measuring the electromechanical coupling properties in the myocardium. Our method is based on imaging and analyzing the delay in small tissue displacements resulting from the propagation of this contraction wave. In-vivo experiments are performed in anesthetized normal and ischemic animals. The contraction wave is first observed in dogs using a commercial clinical scanner. The displacement maps are estimated using a 1D cross-correlation- based technique. The displacement maps clearly show the propagation of a strong mechanical wave along the circumference of the myocardium. The speed of the wave is found to be lower in the ischemic region. Then, thanks to the high frame rate capability of an ultrasound scanner for small animals (Vevo 770, visualsonics), some sequences of approximately three cardiac cycles are acquired at a frame rate of 4000 images/s in mice. The wave velocity is found to be approximately 0.87 m/s in the posterior wall. Temporary regional ischemia is then induced by coronary artery ligation. The velocity of the electromechanical wave is found to decrease to approximately 0.66 m/s in the ischemic region. myocardial viability. Using these techniques, the evaluation of the heart function is based on the mechanical interpretation of the global heart contraction (systole) and relaxation (diastole). However, the contraction of the cardiac muscle is an active process that results from electrical excitations. During the cardiac cycle, electrical waves propagate in the myocardium in order to initiate the contraction and relaxation of the myocardium. The fiberscontraction induces a strong mechanical wave that propagates in the myocardium. Since this wave results from coupling of the electrical excitation and the mechanical response of the tissue, it is hereby named electromechanical wave. This electrical activation occurs in a very short time compared to the following contraction or relaxation of the muscle and therefore cannot be detected in most of the conventional imaging devices. Moreover, some evidences of mechanical vibrations have been shown by Kanai et al. (5),(6), in human patients. They demonstrated that several pulsive low frequency mechanical vibrations were obtained around end-systole and end-diastole in the frequency range of 25 to 100 Hz. These waves propagate in the myocardium with high velocities up to 5m/s. We propose here a new approach to image the local mechanical properties of the myocardium. Our method is based on imaging small displacements of the myocardium at high frame rate. This method allows imaging the propagation of the mechanical waves induced both by electrical excitations and mechanical excitation. The wave speed is a function of both electrical and mechanical properties of the myocardium, i.e., the electrical conductivity and the shear modulus, this method could potentially be used for early, noninvasive and simultaneous detection of electrical and mechanical dysfunctions in the heart. The goal of this paper is to present preliminary results demonstrating the feasibility of imaging in vivo the electromechanical coupling mechanisms in the heart.

Single-element focused transducer method for harmonic motion imaging

The feasibility of the harmonic motion imaging (HMI) technique for simultaneous monitoring and generation of focused ultrasound therapy using two separate focused ultrasound transducer elements was previously shown (1). In this study, a new HMI technique is described that images tissue displacement induced by harmonic radiation force excitation using a single focused ultrasound element. First, wave propagation simulation models were used to compare the use of one Amplitude- Modulated (AM) focused beam versus two overlapping focused beams as previously implemented for HMI (2). Simulation results indicated that, unlike the two-beam configuration, the AM beam produced a consistent, stable focus for the applied harmonic radiation force. The AM beam thus offered the unique advantage of sustaining the application of the spatially-invariant radiation force. Experiments were then performed on gelatin gel phantoms and in-vitro tissues. The radiation force was generated by a 4.68 MHz focused transducer using a low-frequency Amplitude- Modulated (AM) RF-signal. A 7.5 MHz single-element, imaging transducer was placed through the center of the focused transducer so that the diagnostic and focused beams were aligned. Consecutive RF signals were acquired with a PRF of 5 kHz and the displacements were estimated using 1D cross- correlation. Finally, taking advantage of the real-time capability of our method, the change in the elastic properties was monitored during focused ultrasound (FUS) ablation of in-vitro tissues. Based on the harmonic displacements, their temperature- dependence, and the calculated acoustic radiation force, the change in the regional elastic modulus was monitored during heating. In conclusion, the feasibility of using an AM radiation force for HMI for simultaneous monitoring and treatment during ultrasound therapy was demonstrated in phantoms and tissues in-vitro. Further study of this method will include stiffness and temperature estimation, ex-vivo and in-vivo.

Real-Time Monitoring Of Regional Tissue Elasticity During FUS Focused Ultrasound Therapy Using Harmonic Motion Imaging

The feasibility of the Harmonic Motion Imaging (HMI) technique for simultaneous monitoring and generation of focused ultrasound therapy using two separate focused ultrasound transducer elements has previously been shown. In this study, a new HMI technique is described that images tissue displacement induced by a harmonic radiation force induced using a single focused ultrasound element. First, wave propagation simulation models were used to compare the use of a single Amplitude‐Modulated (AM) focused beam versus two overlapping focused beams as previously implemented for HMI. Simulation results indicated that, unlike in the two‐beam configuration, the AM beam produced a consistent, stable focus for the applied harmonic radiation force. The AM beam thus offered the unique advantage of sustaining the application of the spatially‐invariant radiation force. Experiments were then performed on gelatin gel phantoms and tissue in vitro bovine liver. The radiation force was generated by a 4.68 MHz focused transduc...

2I-4 Pulse Wave Imaging in Murine Abdominal Aortas: A Feasibility Study

One of the most crucial aspects of abdominal aortic aneurysm (AAA) diagnosis lies in the early detection of the aneurysm and its propensity for rupture. This study aims at determining whether the estimation of the aortic wall stiffness of the natural mechanical, pulsating motion of the aorta is feasible using high-resolution and high-frame-rate imaging in a murine model. Twelve wild-type (WT) mice were anesthetized, and underwent laparotomy. The abdominal aortas of five normal mice were scanned using a high-resolution (30 MHz) Vevo 770 (Visualsonics, Inc., Ontario, Canada) system and acquiring the RF signals. A composite frame rate of 8 kHz was achieved through ECG triggering on the RF acquisition over several cardiac cycles, which increased the tracking ability of the elastographic technique. Cross correlation techniques using windows of 150-micron size and 90% overlap were applied to estimate motions on the order of tens of microns between successive frames. The strain and the velocity of the pulsive wave in the aorta were estimated and imaged using the gradient and phase shift estimation techniques, respectively. Finally, the Youngs modulus of the aortic wall was derived using the pulsive wave velocity based on the Moens-Korteweg equation. The pulsive wave during pulsatile flow was imaged by mapping the wall displacements consecutively in a cine-loop format. High wall displacements were observed 10.3 ms after the R-wave peak of the ECG and a pulsive wave was initiated traveling from the hearts side along the aortic wall in less than 3 ms within the image view. The phase velocity of the pulse wave was computed at the frequency of 200 Hz and a velocity of 2.60 plusmn 0.57 m/s was found. The radius of the aorta and the wall thickness (measured on the B-mode image) were found equal to R = 0.47 mm and h = 0.12 mm, respectively. Using these measured parameters, the Youngs modulus of the aortic wall was found equal to E = 50.9 plusmn 20.0 kPa. This value is in agreement with previously reported Youngs moduli in the adventitia layer of the porcine descending aorta. In this study, we demonstrated that pulse wave imaging is feasible and that it can be used for mapping the propagation of the pulsive wave along the wall of the abdominal aorta in a murine model in order to determine its elasticity. Ongoing studies aim at determining the potential for this technique to be used in the detection of aneurysms based on their associated change of mechanical properties of the aortic wall

Noninvasive Blood‐Brain Barrier Opening in Live Mice

Most therapeutic agents cannot be delivered to the brain because of brain’s natural defense: the Blood‐Brain Barrier (BBB). It has recently been shown that Focused Ultrasound (FUS) can produce reversible and localized BBB opening in the brain when applied in the presence of ultrasound contrast agents post‐craniotomy in rabbits [1]. However, a major limitation of ultrasound in the brain is the strong phase aberration and attenuation of the skull bone, and, as a result, no study of trans‐cranial ultrasound‐targeted drug treatment in the brain in vivo has been reported as of yet. In this study, the feasibility of BBB opening in the hippocampus of wildtype mice using FUS through the intact skull and skin was investigated. In order to investigate the effect of the skull, simulations of ultrasound wave propagation (1.5 MHz) through the skull using μCT data, and needle hydrophone measurements through an ex‐vivo skull were made. The pressure field showed minimal attenuation (18% of the pressure amplitude) and a w...