Elodie Breton

Magnetic resonance elastography compared with rotational rheometry for in vitro brain tissue viscoelasticity measurement

Magnetic resonance elastography (MRE) is an increasingly used method for non-invasive determination of tissue stiffness. MRE has shown its ability to measure in vivo elasticity or viscoelasticity depending on the chosen rheological model. However, few data exist on quantitative comparison of MRE with reference mechanical measurement techniques. MRE has only been validated on soft homogeneous gels under both Hookean elasticity and linear viscoelasticity assumptions, but comparison studies are lacking concerning viscoelastic properties of complex heterogeneous tissues. In this context, the present study aims at comparing an MRE-based method combined with a wave equation inversion algorithm to rotational rheometry. For this purpose, experiments are performed on in vitro porcine brain tissue. The dynamic behavior of shear storage (G- and loss (G′) moduli obtained by both rheometry and MRE at different frequency ranges is similar to that of linear viscoelastic properties of brain tissue found in other studies. This continuity between rheometry and MRE results consolidates the quantitative nature of values found by MRE in terms of viscoelastic parameters of soft heterogeneous tissues. Based on these results, the limits of MRE in terms of frequency range are also discussed.

SPECT Low-Field MRI System for Small-Animal Imaging

Localization of regions with increased uptake of radiotracer in small-animal SPECT is greatly facilitated when using coregistration with anatomic images of the same animal. As MRI has several advantages compared with CT (soft-tissue contrast and lack of ionizing radiation) we developed a SPECT/low-field MRI hybrid device for small-animal imaging. Methods: A small-animal single-pinhole γ-camera (pinhole, 1.5 mm in diameter and 12 cm in focal length) adjacent to a dedicated low-field (0.1 T) small MR imager (imaging volume, 10 × 10 × 6 cm3) was used. The animal was placed in a warmed nonmagnetic polymethyl methacrylate imaging cell for MR acquisition, which was followed immediately by SPECT after translation of the imaging cell from one modality to the other. 3-Dimensional T1-weighted sequences were used for MRI. Phantom studies enabled verification of a low attenuation (10%) for 99mTc and 201Tl and a very slight increase in Compton scattering due to the radiofrequency coil and polymethyl methacrylate imaging cell. Results: SPECT/MRI data acquisition and image coregistration of selected examples using different radiotracers for lungs, kidneys, and brain were obtained in 3 nude mice with isotropic spatial resolutions of 0.5 × 0.5 × 0.5 mm3 for MRI and 1 × 1 × 1 mm3 for SPECT. The total acquisition time for combined SPECT and MRI lasted 1 h 45 min. Conclusion: A low-magnetic-field strength of 0.1 T is a simple and useful solution for a small-animal dual-imaging device combining pinhole SPECT with the adjacent MR imager.

Dynamic viscoelastic shear properties of soft matter by magnetic resonance elastography using a low-field dedicated system

A magnetic resonance elastography (MRE) based method that consists in measuring dynamic viscoelastic parameters of soft matter by analysis of propagating shear waves at different frequencies is proposed. Dynamic shear tests were performed on soft gels with a dedicated magnetic resonance imaging system at low field (0.1T) and were compared with results obtained with a mechanical rotational rheometer. Storage and loss moduli were plotted against frequency with both methods and good agreement was found between their results in the shared frequency range. Therefore, it is shown that the described method could be a reliable and accessible tool for three-dimensional dynamic mechanical analysis on soft matter able to overcome dynamic range, sample dimensions and directional limitations of mechanical rheometers. Advantages and limits of the method are discussed.

Interventional MR elastography for MRI‐guided percutaneous procedures

MRI‐guided thermal ablations require reliable monitoring methods to ensure complete destruction of the diseased tissue while avoiding damage to the surrounding healthy tissue. Based on the fact that thermal ablations result in substantial changes in biomechanical properties, interventional MR elastography (MRE) dedicated to the monitoring of MR‐guided thermal therapies is proposed here.

CMOS 3D Hall probe for magnetic field measurement in MRI scanner

This paper presents a 3D Hall probe integrated in a 0.35μm CMOS technology and dedicated to the measurement of the magnetic field gradients in a MRI scanner. It features a 3D Hall device and three instrumentation chains which suppress the MRI main magnetic field and amplify dedicated magnetic field gradients. The unique relationship between the space coordinates in the scanner bore and the magnetic field gradients allows determining accurately the location of the Hall probe inside the bore. First experimental results show that the proposed 3D Hall probe could be used for a MRI tracking system with a sub-millimeter spatial resolution. It is a first step towards the instrumentation of MRI-compatible minimally-invasive surgical tools.

3T MRI scanner magnetic gradient mapping using a 3D Hall probe

The unique relationship between Magnetic Resonance Imaging (MRI) scanner bore space coordinates and magnetic field gradients used in MRI allows building a location system based on the measurement of these gradients. For the first time, these magnetic gradients are accurately measured thanks to a miniature 3D Hall probe integrated in a low cost, low voltage 0.35μm CMOS process. The magnetic gradient 3D map of a 3T MRI scanner has been measured and experimental results show that a sub-millimeter location of the probe is possible. It opens the way for the development of MRI compatible magnetic tracking systems integrable in a surgical tool.

Quantification of left ventricular dyssynchrony in patients with systolic dysfunction: A comparison of circumferential strain MR‐tagging metrics

To define which circumferential strain MR‐tagging metrics of left intraventricular dyssynchrony better identifies patients with systolic dysfunction against control subjects.

Hall-effect magnetic tracking device for Magnetic Resonance Imaging

The unique relationship between the coordinates in the bore of a Magnetic Resonance Imaging (MRI) scanner and the magnetic field gradients used for MRI allows building a localization system based on the measurement of these gradients. We have previously presented a miniature 3D Hall probe integrated in a low cost, low voltage 0.35μm CMOS chip from which we were able to measure the magnetic gradient 3D maps of 1.5T and 3T MRI scanners. In this paper, this 3D Hall probe has been integrated in a magnetic tracking device prototype and an algorithm was built to determine the position of the probe. First experimental results show that the probe gives its position with accuracy close to a few millimeters, and that sub-millimeter localization in a one-shot-3ms-measurement should be readily possible. Such a prototype opens the way for the development of MRI compatible real time magnetic tracking systems which could be integrable in surgical tools for MR-guided minimally-invasive surgery.

Towards a Hall effect magnetic tracking device for MRI

This paper presents the first prototype of a magnetic tracking device for Magnetic Resonance Imaging. The unique relationship between the space coordinates of a MRI scanner bore and the magnetic field gradients used in MRI allows building a localization system based on an accurate measurement of these gradients. These gradients are measured thanks to a 3D Hall device with a footprint of only 50μm2, integrated with its specific conditioning circuit in a low cost, low voltage 0.35μm CMOS process. The first experimental results show that a sub-millimeter localization is possible. It opens the way to the development of MRI compatible magnetic tracking systems integrable in a surgical tool.

K-space data processing for magnetic resonance elastography (MRE)

ObjectiveMagnetic resonance elastography (MRE) requires substantial data processing based on phase image reconstruction, wave enhancement, and inverse problem solving. The objective of this study is to propose a new, fast MRE method based on MR raw data processing, particularly adapted to applications requiring fast MRE measurement or high elastogram update rate.Materials and methodsThe proposed method allows measuring tissue elasticity directly from raw data without prior phase image reconstruction and without phase unwrapping. Experimental feasibility is assessed both in a gelatin phantom and in the liver of a porcine model in vivo. Elastograms are reconstructed with the raw MRE method and compared to those obtained using conventional MRE. In a third experiment, changes in elasticity are monitored in real-time in a gelatin phantom during its solidification by using both conventional MRE and raw MRE.ResultsThe raw MRE method shows promising results by providing similar elasticity values to the ones obtained with conventional MRE methods while decreasing the number of processing steps and circumventing the delicate step of phase unwrapping. Limitations of the proposed method are the influence of the magnitude on the elastogram and the requirement for a minimum number of phase offsets.ConclusionThis study demonstrates the feasibility of directly reconstructing elastograms from raw data.