lisasmi

• Smith, Lisa; Skulberg, Vidar Magne; Zhang, Lili; Sjaastad, Ivar & Espe, Emil Knut Stenersen (2025). Feasibility of magnetic resonance elastography in the healthy rat heart. Magnetic Resonance in Medicine. ISSN 0740-3194. 94(2), p. 754–760. doi: 10.1002/mrm.30504. Full text in Research Archive
• Smith, Lisa; Skulberg, Vidar Magne; Zhang, Lili; Sjaastad, Ivar & Espe, Emil Knut Stenersen (2022). The effects of geometry on stiffness measurements in high-field magnetic resonance elastography: A study on rodent cardiac phantoms. Journal of The Mechanical Behavior of Biomedical Materials. ISSN 1751-6161. 133. doi: 10.1016/j.jmbbm.2022.105302. Full text in Research Archive Show summary

  Purpose Cardiac magnetic resonance elastography (MRE) can be used to assess myocardial stiffness in vivo. Rodents play an important role in modern cardiovascular research, and small animal cardiac MRE may reveal important aspects of myocardial stiffness. The aim of this study was to explore the feasibility of small animal cardiac MRE through investigation of stiffness measurements of small cardiac phantoms that have known underlying stiffness. Methods Agarose gel phantoms of three different geometrical designs were used: homogeneous gels, solid hearts, and biventricular phantoms. The size of the heart phantoms was comparable with that of an end-diastolic rat heart. All phantoms were made with different underlying stiffnesses agarose concentration, (7.5, 10.0,15.0)g/l, and MRE acquisition was performed with three different frequencies (360, 380, 400)Hz. Two different post-processing methods were applied to the MRE wave images: local frequency estimate (LFE) and direct inversion (DI). Results The stiffness associated with the different agarose concentrations (7.5, 10.0, 15.0)g/l in the homogenous gels at 400 Hz were (1.80 ± 0.18, 3.13 ± 0.20, 4.13 ± 0.37)kPa for LFE and (2.25 ± 0.24, 4.35 ± 0.45, 6.54 ± 0.44)kPa for DI, respectively. Significant differences in MRE-derived stiffness were observed among phantoms with different agarose concentrations for all geometries. However, biases in the stiffness measurements among the different geometries were observed and could not be explained by the measurement variability. The relative stiffness uncertainty was smallest for the LFE inversion algorithm. Conclusions The stiffness measurements validate the use of the MRE technique to differentiate between various underlying stiffnesses in small cardiac phantoms. The stiffness measurements seemed to be dominated by geometrical effects when the cardiac MRE wavelength was longer than half the size of the heart. LFE was the inversion algorithm that was most sensitive to the changes in underlying stiffness.

• Smith, Lisa; Kolaas, Jostein; Jensen, Atle & Sveen, Johan Kristian (2018). X-ray measurements of plunging breaking solitary waves. European journal of mechanics. B, Fluids. ISSN 0997-7546. 73, p. 112–121. doi: 10.1016/j.euromechflu.2018.03.007. Full text in Research Archive Show summary

  The aim of this study is to examine and measure the characteristics of air cavities generated by breaking solitary waves by utilizing a novel tomographic X-ray system. Small scale experiments of solitary waves that propagate on a (1:10) beach are conducted. Waves with amplitude normalized with the water depth, on a flat bottom are investigated by two perpendicular X-ray systems. Images are captured at locations from the surf zone to the swash zone and at maximum runup. A large air tube is observed right after the plunger impacts the dry beach. Void velocity and the shape of the large air tube, are measured and reported. The large air tube evolves from a symmetrical shape with two large air pockets located close to the walls of the wave tank, to an asymmetrical shape. Contrast enhanced X-ray images reveal that the swash tongue surface is unstable and that secondary mixing of air and water occurs. X-ray images from the maximum runup reveal that the air is still entrapped by the thin swash tongue at times close to maximum runup.

• Smith, Lisa; Kolaas, Jostein; Sveen, Kristian & Jensen, Atle (2018). Investigation of surface structures in two phase wavy pipe flow by utilizing X-ray tomography. International Journal of Multiphase Flow. ISSN 0301-9322. doi: 10.1016/j.ijmultiphaseflow.2018.06.004. Full text in Research Archive
• Smith, Lisa; Jensen, Atle & Pedersen, Geir Kleivstul (2017). Investigation of breaking and non-breaking solitary waves and measurements of swash zone dynamics on a 5° beach. Coastal Engineering. ISSN 0378-3839. 120, p. 38–46. doi: 10.1016/j.coastaleng.2016.11.004. Full text in Research Archive Show summary

  This study presents an experimental investigation of plunging breakers on a sloping beach with an inclination of 5.1°. The incident waves are solitary waves with various amplitudes from non-breaking waves to plunging breakers, and the area investigated is the swash zone. PIV (Particle Image Velocimetry) is performed on images captured at four different field of views (FOV). Shoreline position and maximum runup are measured, and are repeatable in both time and height, although cross-sectional variations of the shoreline shape are observed at maximum runup. For non-breaking waves the runup and fluid flow is computed by a boundary integral technique combined with a boundary layer model. Then, there is excellent agreement between the experimental and the computed velocity profiles at the lower region of the beach, while the boundary integral technique overpredicts the maximum runup height severely. For breaking waves the experiments indicate that the motion becomes more irregular as we move further up the beach. In addition, there are more irregularities present for waves with larger amplitude. Length and velocity of air bubbles entrapped by the plunging breakers are extracted from an image series captured with a large FOV. The images showed that a large air bubble remains intact for a time period during runup for the breaking waves.

• Kolaas, Jostein; Smith, Lisa; Jensen, Atle & Sveen, Johan Kristian (2016). Bichromatic Synthetic Schlieren for surface measurement, Proceedings of the 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics. Lisbon Simposia. ISSN 9789899877788. p. 210–221. doi: 10.1007/s00348-018-2580-6. Full text in Research Archive Show summary

  We introduce Bichromatic Synthetic Schlieren (BiCSS) for instantaneous surface measurements, in where we use two simultaneous images taken at blue and near infrared wavelengths, making use of that refractive index variation depends on wavelength, rather than using a reference/dynamic image pair to measure the surface gradient. Experiments on solitary and periodic waves were used to test the method, showing that new method can produce good results. Using this method it might be possible to extend the wave steepness range of Synthetic Schlieren surface measurements by overcoming the Invertibility Condition caused by light ray crossing, but due to compromises in design of real lenses, it might be difficult to achieve in practice. It might also serve as a first step towards a synthetic Schlieren method without a reference image, making it possible to use these methods in experiments where taking the reference image is difficult or impossible.

• Smith, Lisa; Hu, Bin; Kolaas, Jostein; Jensen, Atle & Sveen, Johan Kristian (2016). X-ray PTV Measurements of Solitary Waves, Proceedings of the 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics. Lisbon Simposia. ISSN 9789899877788. p. 2604–2612. Full text in Research Archive Show summary

  The aim of this study is to measure velocity fields and accurate surface elevation in solitary waves utilizing a fast Xray PTV system. It is difficult to measure the free surface of water waves with the optical measurement techniques available today. This is partly due to refraction and reflection of light rays by the surface of the waves, or by reflections of light rays by the wall of the wave tank. The interface between water and air will then get an inaccurate location in images captured by visual light. The use of microelectronics in image sensors, have made it possible to capture images with both high spatial and temporal resolution. In this study, X-ray measurements with high resolution in space and time have been performed on solitary waves. Two perpendicular X-ray systems are utilized. Surface elevation measurements with accuracy of 0.12 mm are presented, and cross-waves due to wall effects are observed when the average wave height is subtracted from the images. Particles that absorb X-ray are seeded into the water and PTV is used to find velocities in the flow field. The two X-ray systems provided enough information to produce a 3D reconstruction of the flow field. The velocities measured are compared to velocities computed by a Boundary Integral Model (BIM). There seems to be discrepancy between the computed and the measured velocities, which may be linked to inaccuracy in the particle’s location or linked to the large size of the particles.

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