Tissue characterisation by ultrasound strain imaging. Methodological aspects and gastroenterological applications

Real-Time Elastography (RTE) is a new imaging modality that monitors tissue strain during applied stress. The aim of this thesis was to validate if RTE was a reliable modality to reflect pathologically induced differences in elasticity or tissue hardness and to interrogate if assessment of elastic changes by RTE could be used as a diagnostic tool in clinical practice. Particularly, we focused on the ability of RTE to differentiate between benign and malignant disease in intestinal and pancreatic lesions. Two validation studies were performed using a tissue-mimicking phantom containing inclusions with different size, depth and defined elastic properties. We evaluated the influence of different parameter settings on image quality, and intra- and interobserver variation was also assessed. Furthermore, we examined the impact of different size and depth of selected calculation areas in strain ratio (SR) measurements. In a sub-study on resected bowel specimens, elastograms were compared with histology in order to verify the ability to differentiate between benign and malignant disease and study mechanisms leading to changes in elastic properties. In a clinical study on focal pancreatic lesions, RTE was performed using endoscopic ultrasonography (EUS). In the evaluation of strain images from human tissue we applied a previously published visual categorical score (VCS), a visual analogue scale (VAS) and SR. The validation studies confirmed RTE’s ability to assess elastic properties in vitro with fair to good inter- and intraobsever agreement. The level of dynamic range influenced image quality but had no impact on SR measurements. On the other hand, we found that the reference areas’ distance from the US transducer but not the size influenced significantly on SR measurements. In resected bowel lesions, no significant strain difference was found between stenotic lesions caused by Crohn’s disease and adenocarcinomas. EUS-based RTE of pancreatic lesions showed significant strain difference between the entities of benign and malignant lesions, but the variation within the entities was substantial