US and MR elastographic techniques have developed into accurate methods for quantitative, noninvasive diagnosis of liver fibrosis in a wide range of etiologies; however, interpretation of results should take into account potential confounding factors, pitfalls, and technical limitations.

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Quantitative Elastography Methods in Liver Disease: Current Evidence and Future Directions.

Neurological disorders are one of the most important public health concerns in developed countries. Established brain imaging techniques such as magnetic resonance imaging (MRI) and x-ray computerised tomography (CT) have been essential in the identification and diagnosis of a wide range of disorders, although usually are insufficient in sensitivity for detecting subtle pathological alterations to the brain prior to the onset of clinical symptoms-at a time when prognosis for treatment is more favourable. The mechanical properties of biological tissue provide information related to the strength and integrity of the cellular microstructure. In recent years, mechanical properties of the brain have been visualised and measured non-invasively with magnetic resonance elastography (MRE), a particularly sensitive medical imaging technique that may increase the potential for early diagnosis. This review begins with an introduction to the various methods used for the acquisition and analysis of MRE data. A systematic literature search is then conducted to identify studies that have specifically utilised MRE to investigate the human brain. Through the conversion of MRE-derived measurements to shear stiffness (kPa) and, where possible, the loss tangent (rad), a summary of results for global brain tissue and grey and white matter across studies is provided for healthy participants, as potential baseline values to be used in future clinical investigations. In addition, the extent to which MRE has revealed significant alterations to the brain in patients with neurological disorders is assessed and discussed in terms of known pathophysiology. The review concludes by predicting the trends for future MRE research and applications in neuroscience.

/pdf/magnetic-resonance-elastography-mre-of-the-human-brain-21pzn7i2kh.pdf

Magnetic resonance elastography (MRE) of the human brain: technique, findings and clinical applications.

Tomoelastography by multifrequency wave number recovery from time-harmonic propagating shear waves

MR elastography of the brain and its application in neurological diseases.

Viscoelastic mechanical properties of the brain assessed with magnetic resonance elastography (MRE) are sensitive measures of microstructural tissue health in neurodegenerative conditions. Recent efforts have targeted measurements localized to specific neuroanatomical regions differentially affected in disease. In this work, we present a method for measuring the viscoelasticity in subcortical gray matter (SGM) structures, including the amygdala, hippocampus, caudate, putamen, pallidum, and thalamus. The method is based on incorporating high spatial resolution MRE imaging (1.6 mm isotropic voxels) with a mechanical inversion scheme designed to improve local measures in pre-defined regions (soft prior regularization [SPR]). We find that in 21 healthy, young volunteers SGM structures differ from each other in viscoelasticity, quantified as the shear stiffness and damping ratio, but also differ from the global viscoelasticity of the cerebrum. Through repeated examinations on a single volunteer, we estimate the uncertainty to be between 3 and 7% for each SGM measure. Furthermore, we demonstrate that the use of specific methodological considerations-higher spatial resolution and SPR-both decrease uncertainty and increase sensitivity of the SGM measures. The proposed method allows for reliable MRE measures of SGM viscoelasticity for future studies of neurodegenerative conditions. Hum Brain Mapp 37:4221-4233, 2016. © 2016 Wiley Periodicals, Inc.

Viscoelasticity of subcortical gray matter structures.

Purpose

To demonstrate the feasibility of in vivo wideband MR elastography (wMRE) using continuous, time-harmonic shear vibrations in the frequency range of 10–50 Hz.



Theory and Methods

The method was tested in a gel phantom with marked mechanical loss. The brains and livers of eight volunteers were scanned by wMRE using multislice, single-shot MRE with optimized fractional encoding and synchronization of sequence acquisition to vibration. Multifrequency three-dimensional inversion was used to reconstruct compound maps of magnitude |G*| and phase φ of the complex shear modulus. A new phase estimation, φ*, was developed to avoid systematic bias due to noise.



Results

In the phantom, G*-dispersion measured by wMRE agreed well with oscillatory shear rheometry. |G*| and φ* measured at vibrations of 10–25 HZ, 25–35 HZ, and 40–50 HZ were 0.62 ± 0.08, 1.56 ± 0.16, 2.18 ± 0.20 kPa and 0.09 ± 0.17, 0.39 ± 0.16, 0.20 ± 0.13 rad in brain and 0.89 ± 0.11, 1.67 ± 0.20, 2.27 ± 0.35 kPa and 0.15 ± 0.10, 0.24 ± 0.05, 0.26 ± 0.05 rad in liver. Elastograms including all frequencies showed the best resolution of anatomical detail with |G*| = 1.38 ± 0.12 kPa, φ* = 0.24 ± 0.10 rad (brain) and |G*| = 1.79 ± 0.23 kPa, φ* = 0.24 ± 0.05 rad (liver).



Conclusion

wMRE reveals highly dispersive G* properties of the brain and liver, and our results suggest that the influence of large-scale structures such as fluid-filled vessels and sulci on the MRE-measured parameters increases at low vibration frequencies. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.

In vivo wideband multifrequency MR elastography of the human brain and liver.

Purpose
To develop a compact magnetic resonance elastography (MRE) protocol for abdomen and to investigate the effect of water uptake on tissue stiffness in the liver, spleen, kidney, and pancreas.

Methods
Nine asymptomatic volunteers were investigated by MRE before and after 1 liter water uptake. Shear-wave excitation at four frequencies was transferred to the abdomen from anterior and posterior directions using pressurized air drivers. Tomographic representations of shear-wave speed were produced by analysis of multifrequency wave numbers in axial and coronal images acquired within four breath-holds or under free breathing, respectively.

Results
Pre and post water, stiffness of the spleen (pre/post: 2.20 ± 0.10/2.06 ± 0.18 m/s) and kidney (pre/post: 1.93 ± 0.22/1.97 ± 0.23 m/s) was higher than in the liver (pre/post: 1.36 ± 0.10/1.38 ± 0.13 m/s) and pancreas (pre/post: 1.20 ± 0.12/1.20 ± 0.08 m/s), all P < 0.01. Accounting for four drive frequencies, water drinking only changed the splenic stiffness (–6%, P = 0.03), whereas in the frequency range from 50 to 60 Hz the effect became significant also in the pancreas (–6%, P = 0.04) and liver (+3%, P = 0.03). Elastograms of the kidney in coronal view clearly depicted higher stiffness in cortex than in medulla.

Conclusion
Tomoelastography reveals sensitivity of tissue mechanical properties to the hydration state of multiple abdominal organs within one scan and in unprecedented resolution of anatomical details. Magn Reson Med, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

Tomoelastography of the abdomen: Tissue mechanical properties of the liver, spleen, kidney, and pancreas from single MR elastography scans at different hydration states.

Viscoelastic properties of the brain reflect tissue architecture at multiple length scales. However, little is known about the relation between vital tissue functions, such as perfusion, and the ma...

https://journals.sagepub.com/doi/pdf/10.1177/0271678X17691530

Perfusion alters stiffness of deep gray matter.

Purpose
To demonstrate the feasibility of in vivo multifrequency magnetic resonance elastography (MRE) of the prostate using externally placed drivers.

Methods
Three pressurized-air drivers were used to excite shear waves within the prostate at vibration frequencies of 60, 70, and 80 Hz. Full 3D wave fields were acquired by multislice spin-echo echo-planar imaging in conjunction with tomoelastography wave speed recovery for generating full field-of-view stiffness maps. Twelve healthy volunteers were repeatedly scanned to analyze test–retest reproducibility. Five patients with suspected prostate cancer were investigated to demonstrate the clinical feasibility of the method.

Results
In healthy volunteers, the shear wave speed of the entire prostate was 2.24 ± 0.20 m/s with a repeatability coefficient of 0.14 m/s and 88% intraclass correlation coefficient. No significant difference between the peripheral zone (2.27 ± 0.20 m/s) and the central gland (2.22 ± 0.23 m/s) was observed. In patients, wave-speed maps displayed stiff regions consistent with the localization of suspicious masses detected by other imaging markers.

Conclusions
The proposed method provides reproducible quantitative maps of tissue stiffness throughout the pelvic region and can easily be integrated into clinical imaging protocols. Clinical stiffness maps display many details of potential interest for cancer diagnosis. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Florian Dittmann

Papers

Tomoelastography by multifrequency wave number recovery from time-harmonic propagating shear waves

In vivo wideband multifrequency MR elastography of the human brain and liver.

Tomoelastography of the abdomen: Tissue mechanical properties of the liver, spleen, kidney, and pancreas from single MR elastography scans at different hydration states.

Perfusion alters stiffness of deep gray matter.

Tomoelastography of the prostate using multifrequency MR elastography and externally placed pressurized-air drivers.