Abstract: Pictorial images of selected volume elements of materials are generated by a contactless, non-destructive, substantially hazard free tomographic technique in which the material is brought within the influence of a relatively low-strength electromagnetic field and subjected to plural preselected frequencies to provide output data which is used to generate an image reflective of the magnetic and/or electric and/or conductance properties of the selected volume elements. This data has a high volume information content which enables accurate and reproducible identification of the material under investigation. The technique has application in the medical diagnostic field such as for example in aiding in the detection of small areas of cancerous tissue present in a larger mass of healthy tissue.