Coronary heart disease

Common to liquids, solids and substances in between the former two is that if a stress is applied to them, they will strain. Stress may be visualized by placing a small amount of fluid between two parallel plates. When one plate slides over the other, forces act on the fluid dependent upon the rate of the plate movement. This causes a shear stress on the liquid. Recall laminar flow of fluids through a tubular vessel. Strain is the response to the stress. If solids are elastic, they deform and return to their original shape. Since fluids are not elastic and, hence, viscous, their deformation is irreversible.

Introduction to rheology

An abdominal aortic aneurysm (AAA) is a localized dilatation of the infrarenal aorta. AAA is a multifactorial disease, and genetic and environmental factors play a part; smoking, male sex and a positive family history are the most important risk factors, and AAA is most common in men >65 years of age. AAA results from changes in the aortic wall structure, including thinning of the media and adventitia due to the loss of vascular smooth muscle cells and degradation of the extracellular matrix. If the mechanical stress of the blood pressure acting on the wall exceeds the wall strength, the AAA ruptures, causing life-threatening intra-abdominal haemorrhage — the mortality for patients with ruptured AAA is 65–85%. Although AAAs of any size can rupture, the risk of rupture increases with diameter. Intact AAAs are typically asymptomatic, and in settings where screening programmes with ultrasonography are not implemented, most cases are diagnosed incidentally. Modern functional imaging techniques (PET, CT and MRI) may help to assess rupture risk. Elective repair of AAA with open surgery or endovascular aortic repair (EVAR) should be considered to prevent AAA rupture, although the morbidity and mortality associated with both techniques remain non-negligible. An abdominal aortic aneurysm (AAA) is an enlargement of the abdominal aorta. As the AAA grows, the aortic wall becomes progressively thin, and the risk of rupture increases; AAA rupture causes severe intra-abdominal haemorrhage and has a very high mortality.

Abdominal aortic aneurysms.

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.

/pdf/quantitative-elastography-methods-in-liver-disease-current-2s9a55qzsg.pdf

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.

Nonlinear multiscale regularisation in MR elastography: Towards fine feature mapping

The simulation of magnetic resonance elastography through atherosclerosis.

Inflammation detected through the uptake of ultrasmall superparamagnetic particles of iron oxide (USPIO) on magnetic resonance imaging (MRI) and finite element (FE) modelling of tissue stress both hold potential in the assessment of abdominal aortic aneurysm (AAA) rupture risk. This study aimed to examine the spatial relationship between these two biomarkers. Patients (n = 50) > 40 years with AAA maximum diameters > = 40 mm underwent USPIO-enhanced MRI and computed tomography angiogram (CTA). USPIO uptake was compared with wall stress predictions from CTA-based patient-specific FE models of each aneurysm. Elevated stress was commonly observed in areas vulnerable to rupture (e.g. posterior wall and shoulder). Only 16% of aneurysms exhibited co-localisation of elevated stress and mural USPIO enhancement. Globally, no correlation was observed between stress and other measures of USPIO uptake (i.e. mean or peak). It is suggested that cellular inflammation and stress may represent different but complimentary aspects of AAA disease progression.

/pdf/exploring-the-biological-and-mechanical-properties-of-1x7bzhhavs.pdf

Exploring the Biological and Mechanical Properties of Abdominal Aortic Aneurysms Using USPIO MRI and Peak Tissue Stress: A Combined Clinical and Finite Element Study

Magnetic resonance elastography (MRE) utilises an inversion algorithm to create maps of material properties (elastograms) from displacements caused by mechanically in- duced shear waves. This study aimed to assess the accuracy of measurements from multi-dual elasto-visco (MDEV) inversion in comparison to direct inversion (DI) in simplistic geometries. Finite element analysis (FEA) was used to simulate wave propagation in cuboid geometries with embedded cylindrical inserts. Accuracy of both algorithms was dependent upon insert length, diameter and shear modulus. Whilst MDEV reduced artefacts in the elastograms in comparison to DI it was unable to improve accuracy of the measurements. imply that in reality, measurements using DI are also depen- dent upon the geometry of the medium under investigation (14). An extension of DI more recently introduced is the multi-frequency dual elasto-visco (MDEV) inversion (15). Again this utilises the Laplacian but combines information acquired at multiple frequencies to reduce frequency de- pendent artefacts and achieve increased spatial resolution. This approach has been shown to improve image quality (16), however the ability of the technique to accurately resolve material properties in comparison to DI has not been assessed. This paper aims to investigate whether MDEV is able to achieve greater accuracy of material property measurements than DI in a variety of simplistic geometries. To achieve this finite element analysis (FEA), a commonly used technique in the field (17)-(19), has been employed. Since the methods of assessing newly developed software varies across the litera- ture, the 3-D datasets from the models used in this study have been made publicly available with the intention that future developers can utilise these and compare the performance of their software with the other available techniques.

Finite Element Analysis to Compare the Accuracy of the Direct and MDEV Inversion Algorithms in MR Elastography

Oxygenation defects may contribute to renal disease progression, but the chronology of events is difficult to define in vivo without recourse to invasive methodologies. Blood oxygen level-dependent...

https://www.physiology.org/doi/pdf/10.1152/ajprenal.00113.2013

L. Hollis

Papers

Nonlinear multiscale regularisation in MR elastography: Towards fine feature mapping

The simulation of magnetic resonance elastography through atherosclerosis.

Exploring the Biological and Mechanical Properties of Abdominal Aortic Aneurysms Using USPIO MRI and Peak Tissue Stress: A Combined Clinical and Finite Element Study

Finite Element Analysis to Compare the Accuracy of the Direct and MDEV Inversion Algorithms in MR Elastography

An anatomically unbiased approach for analysis of renal BOLD magnetic resonance images