Reconstruction accuracy dependence with induced-shear-wave magnitude in Magnetic Resonance Elastography

Congrès sous l’égide de la Société Française de Génie Biologique et Médical (SFGBM)National audienceSince 1996, Magnetic Resonance Elastography (MRE) holds the promise for absolute quantitation of the mechanical parameters of living tissues [1]. The reproducibility of the technique was challenged [2] while the measurement precision was determined by the uncertainty of the recorded MR-signal phase onto which the inferred motion is encoded [3]. We assumed that the ratio of the resulting total wave amplitude to its related uncertainty, AT/ΔAT,_ _should be considered to validate the acquired set of MRE data. Nevertheless, as long as this ratio is greater than unity, the validity of the extracted mechanical parameters might not be questioned. Here, we extract the complex shear modulus, G=G′+G′′, by inversion of the three-dimensional equation of motion [4] for a wide range of inferred wave amplitude, starting from zero, in a breast phantom. The shear dynamic, G′, and loss, G′′, moduli were found to increase with the wave amplitude before reaching a plateau at ratios AT/ΔAT much greater than one. Experiments were carried with standard motion-sensitized refocused field echo (RFE) [1] and motion fractional-encoding fast field echo (FFE) [5], for which sensitivities largely differ, so the relevance of a MRE-validity threshold based on the ratio AT/ΔAT could be exhibited

Brain MR-Elastography in microgravity analogous conditions

International audienceThe head down tilt (HDT) position is commonly used to simulate vascular and tissue fluid dynamics during spaceflights. In HDT position, the cerebral autoregulation faces difficulties to adjust the vascular tone while the cephalad fluid shifts may yield increased intracranial pressures and altered mechanical properties. Recent MRI T2 mapping in HDT position have shown fluid overpressure in the brain and resulting loss of water contents in the CSF and orbital compartments. Brain MRE was performed here in similar HDT conditions. It was sensitive enough to provide new insights on the overall mechanical response of brain tissues in microgravity analogous conditions. Summary of Main Findings/Short Synopsis Brain fluid overpressure and resulting loss of water contents in CSF and orbital compartments were confirmed by T2 mapping in head down tilt position. The overall brain mechanical response in such microgravity analogous conditions, cerebral tissue stiffening, was revealed by whole brain MRE

Comparison between 3D Supersonic Shear Wave Elastography and Magnetic Resonance Elastography: a preliminary experimental study

Congrès sous l’égide de la Société Française de Génie Biologique et Médical (SFGBM).National audienceUltrasound Supersonic Shear Wave Elastog-raphy (SSWE) as well as Magnetic Resonance Elastography (MRE) allow accessing the mechanical properties of human tissues. SSWE is usually performed using a 2D probe. 3D SSWE is now available but needs to be validated. We compared 3D SSWE with both 2D SSWE and MRE which is inherently 3D on a breast phantom. We found that 3D SSWE is reproducible and provides elasticity estimates comparable to those obtained with the validated 2D SSWE. We also showed that 3D SSWE and MRE exhibit quite different elasticity moduli , but they reveal similar qualitative trends in the phantom. Although no relationship could be drawn between the two modalities , this study provides a first basis for comparison and a guide for potential improvements

Does motion affect liver stiffness estimates in shear wave elastography? Phantom and clinical study

International audienceThis study was undertaken to evaluate the impact of free-breathing (FB) vs. Apnea on Shear-wave elastography (SWE) measurements.Quantitative liver-stiffness measurements were obtained during FB and Apnea for 97 patients with various body-morphologies and liver textures. Quality indexes of FB and Apnea elasticity maps (percentage of non-filling (PNF), temporal (TV) and spatial (SV) variabilities) were computed. SWE measurements were also obtained from an homogeneous phantom at rest and during a mechanically-induced motion.Liver-stiffness values estimated from FB and Apnea acquisitions were correlated, particularly for homogeneous livers (r = 0.76, P < 0.001) and favorable body-morphologies (r = 0.68, P < 0.001). However FB values were consistently 20–25% lower than Apnea ones (P < 0.001). FB also systematically resulted in degradation of TV (P < 0.005) and PNF (P < 0.001) compared to Apnea but had no impact on SV. With the phantom, no differences between SWE measurements at rest and during motion were observed.Apnea and FB measurements are highly correlated, although FB data quality is degraded compared to Apnea and estimated stiffness in FB is systematically lower than in Apnea. These discrepancies between rest and motion states were observed for patients but not for phantom data, suggesting that patient breath-holding impacts liver stiffness

Automatic Assessment of Shear Wave Elastography Quality and Measurement Reliability in the Liver

International audienceA strategy is proposed that accesses the quality of individual shear wave elastography (SWE) exams and the reliability of elasticity measurements in clinical practice. For that purpose, a confidence index based on temporal stability and SWE filling was defined to provide an automatic estimation of each scan quality: high (HG) or low (LG) grade. With this index, the intra-observer acquisition variability assessed by comparing consecutive scans of the same patient was 17% and 32% for HG and LG clips, respectively. The measurement quantification variability assessed by comparing the measurements of a radiologist with those of a trained operator and of two automatic measurements on a same clip averaged 13% and 22% for HG and LG exams, respectively. It was found that SWE measurements depend greatly on the quality of the acquired data. The proposed quality index (HG or LG) provides objective input on the accuracy and diagnostic reliability of SWE measurements

Automatic assessment of Shear Wave Elastography quality and measurement reliability in the liver

accepted manuscript to be published in Ultrasound in Medicine and BiologyA strategy is proposed to access the quality of individual shear wave elastography (SWE) exams and the reliability of elasticity measurements in clinical practice. For that purpose, a quality index based on temporal stability and SWE filling was defined to provide an automatic estimation of each scan quality: high (HG) or low (LG) grade. With this index, the intra-observer acquisition variability assessed by comparing consecutive scans of the same patient was 17% and 32% for HG and LG clips respectively. The measurement quantification variability assessed by comparing the measurements of a radiologist to that of a trained operator and of two automatic measurements on a same clip averaged 13% and 22% for HG and LG exams respectively. It was shown that SWE measurements highly depend on the quality of the acquired data. The proposed quality index (HG or LG) provides an objective input on the accuracy and diagnostic reliability of SWE measurements

Automated 3D measurements of the aortic length using the Hough transform

International audiencen.

Acquisition and reconstruction conditions in silico for accurate and precise magnetic resonance elastography

International audienc