In a surface acoustic wave element 10, in which IDT electrodes 31 and 32, a grounding electrode 37, etc. A conventional bottom anti-reflective coating is applied over a reflective surface, for example an inter-layer dielectric. Based on the targeted subset of ultrasound images, a displacement of the feature is determined and image parameters used to obtain the targeted subset of ultrasound images are adjusted based on the displacement. Different from the prior design concepts of striving to distribute reactants as uniformly as possible in the whole reaction area, the whole cell in this invention is divided into multiple independent reaction regions, and relevance of the reaction regions is eliminated.