Benjamin Robert

MR-guided adaptive focusing of therapeutic ultrasound beams in the human head.

PURPOSE This study aims to demonstrate, using human cadavers the feasibility of energy-based adaptive focusing of ultrasonic waves using magnetic resonance acoustic radiation force imaging (MR-ARFI) in the framework of non-invasive transcranial high intensity focused ultrasound (HIFU) therapy. METHODS Energy-based adaptive focusing techniques were recently proposed in order to achieve aberration correction. The authors evaluate this method on a clinical brain HIFU system composed of 512 ultrasonic elements positioned inside a full body 1.5 T clinical magnetic resonance (MR) imaging system. Cadaver heads were mounted onto a clinical Leksell stereotactic frame. The ultrasonic wave intensity at the chosen location was indirectly estimated by the MR system measuring the local tissue displacement induced by the acoustic radiation force of the ultrasound (US) beams. For aberration correction, a set of spatially encoded ultrasonic waves was transmitted from the ultrasonic array and the resulting local displacements were estimated with the MR-ARFI sequence for each emitted beam. A noniterative inversion process was then performed in order to estimate the spatial phase aberrations induced by the cadaver skull. The procedure was first evaluated and optimized in a calf brain using a numerical aberrator mimicking human skull aberrations. The full method was then demonstrated using a fresh human cadaver head. RESULTS The corrected beam resulting from the direct inversion process was found to focus at the targeted location with an acoustic intensity 2.2 times higher than the conventional non corrected beam. In addition, this corrected beam was found to give an acoustic intensity 1.5 times higher than the focusing pattern obtained with an aberration correction using transcranial acoustic simulation-based on X-ray computed tomography (CT) scans. CONCLUSIONS The proposed technique achieved near optimal focusing in an intact human head for the first time. These findings confirm the strong potential of energy-based adaptive focusing of transcranial ultrasonic beams for clinical applications.

Application of DENSE-MR-Elastography to the Human Heart

Typically, MR‐elastography (MRE) encodes the propagation of monochromatic acoustic waves in the MR‐phase images via sinusoidal gradients characterized by a detection frequency equal to the frequency of the mechanical vibration. Therefore, the echo time of a conventional MRE sequence is typically longer than the vibration period which is critical for heart tissue exhibiting a short T2. Thus, fast acquisition techniques like the so‐called fractional encoding of harmonic motions were developed for cardiac applications. However, fractional encoding of harmonic motions is limited since it is two orders of magnitude less sensitive to motion than conventional MRE sequences for low‐frequency vibrations. Here, a new sequence is derived from the so‐called displacement encoding with stimulated echoes (DENSE) sequence. This sequence is more sensitive to displacement than fractional encoding of harmonic motions, and its spectral specificity is equivalent to conventional MRE sequences. The theoretical spectral properties of this new motion‐encoding technique are validated in a phantom and excised pork heart specimen. An excellent agreement is found for the measured displacement fields using classic MRE and displacement encoding with stimulated echoes MRE (8% maximum difference). In addition, initial in vivo results on a healthy volunteer clearly show propagating shear waves at 50 Hz. Thus, displacement encoding with stimulated echoes MRE is a promising technique for motion encoding within short T2* materials. Magn Reson Med, 2009.

Parallel-transmission-enabled three-dimensional T2-weighted imaging of the human brain at 7 Tesla

Purpose: A promise of ultra high field MRI is to produce images of the human brain with higher spatial resolution due to an increased signal to noise ratio. Yet, the shorter radiofrequency wavelength induces an inhomogeneous distribution of the transmit magnetic field and thus challenges the applicability of MRI sequences which rely on the spin excitation homogeneity. In this work, the ability of parallel‐transmission to obtain high‐quality T2‐weighted images of the human brain at 7 Tesla, using an original pulse design method is evaluated. Methods: Excitation and refocusing square pulses of a SPACE sequence were replaced with short nonselective transmit‐SENSE pulses individually tailored with the gradient ascent pulse engineering algorithm, adopting a kT‐point trajectory to simultaneously mitigate B1+ and ΔB0 nonuniformities. Results: In vivo experiments showed that exploiting parallel‐transmission at 7T with the proposed methodology produces high quality T2‐weighted whole brain images with uniform signal and contrast. Subsequent white and gray matter segmentation confirmed the expected improvements in image quality. Conclusion: This work demonstrates that the adopted formalism based on optimal control, combined with the kT‐point method, successfully enables three‐dimensional T2‐weighted brain imaging at 7T devoid of artifacts resulting from B1+ inhomogeneity. Magn Reson Med 73:2195–2203, 2015.

4J-3 A New Rheological Model Based on Fractional Derivatives for Biological Tissues

The intuitive use of viscoelastic properties is routinely done by physicians via the palpation. However, such an examination relies on the physician experience and is not quantitative nor reproducible. Thus, elastography has been developed to complement the palpation by reliable and reproducible measurements. The principle of elastography is to image elastic waves propagation in a medium. To evaluate the shear moduli, Helmholtz transforms are applied to the displacement images. Then the waves movie is interpreted using a rheological model. For instance, the Voigt model explains the observed frequency dependence of the measured shear wave speed. It has been shown to be a reliable model for mimicking tissue phantoms such as gelatin based phantoms. However, we show here that neither this model nor the Maxwell model are applicable to biological tissues using in vivo data (breast) and ex vivo data (liver). Although these simple models are widely used in the elastography community, they have been substituted by more relevant models in the micro-rheology community in order to reveal the solid-liquid duality of tissues. Based on these experimental observations, we introduce a new rheological model relying on fractional derivatives closer to the viscoelastic properties of in vivo data. Indeed, we observed that the dynamic modulus (Gd) and the loss modulus (Gl) have the same frequency behavior: a non-integer frequency power law smaller than 1. This frequency behavior is contradictory with the use of the Voigt model or any kind of simple arrangements of dash pots and springs. In order to explain this frequency behavior the concept of spring pot was introduced. Moreover, we observed that the ratio Gl /Gd is constant and not linked to the non-integer power observed, not predicted by the spring pot model. Thus we build a network of spring pots where the basic element is responsible of the frequency power and the network is responsible of the ratio Gl/Gd . The experiments were conducted on fresh liver samples and phantoms between 50 Hz and 100 Hz. The frequency behavior was analyzed by plotting the real and imaginary parts of the complex shear modulus using MR-elastography as well as 3D ultrasound based elastography. By applying the fractional derivatives model to these data sets, we observed that the frequency power law in the liver was equal to 0.75 (a liquid-like behavior), while the ratio parameter of the network was equal to 0.15 (a solid-like behavior). The dispersion curves of Gd and Gl obtained through this model correlates much better with the experimental observations. The model parameters values seem to emphasize the necessity to take into account the solid-liquid duality of tissues in the rheological model choice for elastography reconstructions

Magnetization transfer from inhomogeneously broadened lines (ihMT): Improved imaging strategy for spinal cord applications

Inhomogeneous magnetization transfer (ihMT) shows great promise for specific imaging of myelinated tissues. Whereas the ihMT technique has been previously applied in brain applications, the current report presents a strategy for cervical spinal cord (SC) imaging free of cerebrospinal fluid (CSF) pulsatility artifacts.

Bloch Equations-Based Reconstruction of Myocardium T1 Maps from Modified Look-Locker Inversion Recovery Sequence

Modified Look-Locker Inversion recovery (MOLLI) sequence is increasingly performed for myocardial T1 mapping but is known to underestimate T1 values. The aim of the study was to quantitatively analyze several sources of errors when T1 maps are derived using standard post-processing of the sequence and to propose a reconstruction approach that takes into account inversion efficacy (η), T2 relaxation during balanced steady-state free-precession readouts and B1+ inhomogeneities. Contributions of the different sources of error were analyzed using Bloch equations simulations of MOLLI sequence. Bloch simulations were then combined with the acquisition of fast B1+ and T2 maps to derive more accurate T1 maps. This novel approach was evaluated on phantoms and on five healthy volunteers. Simulations show that T2 variations, B1+ heterogeneities and inversion efficiency represent major confounders for T1 mapping when MOLLI is processed with standard 3-parameters fitting. In vitro data indicate that T1 values are accurately derived with the simulation approach and in vivo data suggest that myocardium T1 are 15% underestimated when processed with the standard 3-parameters fitting. At the cost of additional acquisitions, this method might be suitable in clinical research protocols for precise tissue characterization as it decorrelates T1 and T2 effects on parametric maps provided by MOLLI sequence and avoids inaccuracies when B1+ is not homogenous throughout the myocardium.

Myocardial perfusion assessment in humans using steady‐pulsed arterial spin labeling

Although arterial spin labeling (ASL) has become a routinely performed method in the rodent heart, its application to the human heart remains challenged by low tissue blood flow and cardiac and respiratory motion. We hypothesized that an alternative steady‐pulsed ASL (spASL) method would provide more efficient perfusion signal averaging by driving the tissue magnetization into a perfusion‐dependent steady state.

G.P.119

Fatty infiltration of muscles is a marker of disease progression in many neuromuscular disorders. Muscle MRI is capable of revealing patterns of muscles involvement that are disease specific and facilitates the diagnostic workup of patients. Although routine T1-weighted (T1w) imaging can give an indication of the presence or absence of muscular fat infiltration, it is difficult to extract quantitative data from these images. On the contrary, Dixon methods provide quantitative measure of fat fraction. Usually, whole-body (WB) exams consist in the acquisition of T1w images, followed by Dixon acquisitions on targeted regions to quantitatively assess fat infiltration. With the aim of improving and accelerating the qualitative assessment of neuromuscular disorders, we propose to avoid the T1w acquisition altogether, by allowing to perform the visual diagnosis workup on a WB Dixon imaging. 20 patients underwent WB MRI at 3T. WB T1w images were acquired with a 2D TSE sequence (resolution=1.1×1.1mm 2 , slice thickness=6mm, T acq =5min 40s). WB Dixon acquisition consisted in a 3D VIBE sequence with 3 echoes (resolution=1×1×5mm 3 , T acq =14min 5s). Quantitative fat fraction maps were derived using a standard 3-points Dixon reconstruction method. A customed lookup table was embedded in the DICOM file to provide a colored lecture of fat fraction maps corresponding to the Mercuris scale. Our results show that the acquisition of a high resolution WB Dixon imaging is possible in less than 15min using an optimized VIBE sequence. This provides quantitative data that are more suitable than T1w images for longitudinal natural history studies, or therapeutic clinical trials. Moreover, the color representation renders the visual grading of the muscles more convenient and less operator dependent as it is based on actual fat fraction measurements. WB Dixon might then overcome the use of WB T1w images for diagnostic of neuromuscular disorders.

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).

0411: Improved spatial resolution and post-processing for myocardial blood flow quantification in humans using steady-pulsed arterial spin labeling

Introduction Arterial spin labeling (ASL) is a powerful tool for the noninvasive assessment of tissue perfusion. In the human heart, however, measuring myocardial perfusion (MBF) is challenging due to strong physiological noise. Steady-pulsed ASL (spASL) under free-breathing had been proposed to improve sensitivity. To improve robustness against respiratory motion, efficiency and spatial resolution, we present an optimization of the postprocessing algorithm by way of a dedicated motion correction (Moco). Methods The spASL sequence was implemented on a Siemens 3T Verio system, based on an ECG-triggered bSSFP acquisition combined with a sliceselective labeling module. ASL was performed at each cardiac cycle to drive tissue magnetization into a perfusion-dependent steady-state. The spASL acquisition was repeated 128 times to accumulate data and a customized 2-step Moco was carried out. First, global correction in a large ROI was performed by spatially shifting every image and minimizing signal difference with a reference. Then, a more precise regional correction based on contour correlation was used. Results When performing global correction, MBF mapping was possible. Fig.1 shows a comparison between two maps in a volunteer without and with Moco. Better homogeneity and delineation of the myocardium can be seen. Using the 2nd described step before carrying out the quantification in myocardial regions led to improved signal stability. Without Moco, signal was found stable when including up to 30% of all images. With Moco, the signal reached a plateau around 50% and remained stable until 80% of included images. Conclusion With this new approach, a greater portion (80% versus 30% formerly) of the acquired data can be used for regional perfusion assessment. This work is also a step towards calculation of myocardial perfusion maps with high spatial resolution using ASL. Download full-size image Abstract 0411 – Figure: MBF maps obtained with spASL MRI.