1. An E-mode imaging apparatus comprising: an image acquisition system collecting a series of images of a material to be imaged during different stages of deformation by a compressor, each image comprised on multiple image values; an electronic computer receiving the image values of the series of images and executing a stored program held in a non-transient medium to calculate: (i) E-mode data indicating a stiffness of the material determined from displacement of corresponding elements of the material represented in image values between different images; (ii) a quality value indicating a quality of E-mode data wherein the quality value is a combination of two different evaluations of the image values of two images, a first evaluation indicating a similarity of corresponding image values in the two images and a second evaluation indicating a degree of variation in image values of an E-mode image formed of the image values of the two images.