Wall shear stress calculation in ascending aorta using phase contrast magnetic resonance imaging. Investigating effective ways to calculate it in clinical practice

Introduction: There is growing evidence that atherosclerosis, as well as endothelial biology, depend on arterial wall shear stress (WSS). Several methods of WSS calculation with varying degrees of complexity have been proposed. This study aimed at investigating whether the most straightforward and easier to apply of these methods give comparable results in clinical practice. Methods: Complete velocity encoding measurements using phase contrast magnetic resonance imaging were performed in 20 patients at a level perpendicular to the long axis of the ascending aorta approximately 2 cm above the aortic valve. WSS was calculated at this location on maximum systole. MR imaging was accomplished on a 1.5 T scanner. Four methods were applied to calculate WSS; three of them are based on the predictions of Poiseuille's theory of flow, while the last one is based on calculations resulting by the application of the definition of WSS. Results: WSS calculated with the above mentioned methods was found to be in the range 4.2 ± 1.8 to 3.5 ± 1.7 dynes/m2. The velocity profile at the site of measurements can be described with a parabolic equation of the form u = a r2 + b r + c with an average r2 = 0.83, which is in good agreement with Poiseuille's theory of flow. Comparison of the results shows no statistically significant differences between WSS measurements calculated with these methods. Discussion: The four methods are equivalent in calculating WSS at the ascending aorta when blood flow velocities have a good parabolic distribution. © 2008 Associazione Italiana di Fisica Medica

The effect of energy weighting on the SNR under the influence of non-ideal detectors in mammographic applications

This work investigates the effect of the energy-weighting technique on the signal to noise ratio (SNR) response of X-ray imaging detectors. So far in the literature all scintillation-detector characteristics (detection efficiency, conversion efficiency, light-attenuation effects, etc) that degrade image quality have been ignored. A theoretical evaluation of the scintillator's SNR output was carried out. An algorithm was produced to describe the variation of the weighting factor, and SNR, with respect to the anode material (Mo or W), in a particular energy range (25-40 keV), typical for mammography, using two different phantoms. Results show that under non-ideal conditions the ratio of the weighted SNR to the original SNR appears to be increasing from values that are close to unity, and under specific conditions, can reach values up to 30. For the further investigation of this method, a more complex, simulated computed tomography breast imaging system was modeled and studied for various parameters such as breast software phantoms, scintillation materials and reconstruction filters. © 2006 Elsevier B.V. All rights reserved