INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 35 (2002) R105–R120 PII: S0022-3727(02)25552-7 TOPICAL REVIEW Coherence-enhanced synchrotron radiology: simple theory and practical applications Y Hwu 1 , Wen-Li Tsai 1 , A Groso 2 , G Margaritondo 2 and Jung Ho Je 3 1 Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan 2 Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 3 Department of Material Science and Engineering, Pohang University of Science and Technology, Pohang, Korea Received 15 May 2002 Published 18 June 2002 Online at stacks.iop.org/JPhysD/35/R105 Abstract The advanced characteristics of synchrotron x-ray sources make it possible to implement radiology with powerful and innovative approaches. We review in simple terms the conceptual background of such approaches, then we present a number of selected examples. The practical tests concern life-sciences specimens as well as materials-science systems. 1. A new approach to radiological imaging Ever since their discovery by Wilhelm Conrad R¨ ontgen in 1895 [1], medical radiology has been by far the most important application of x-rays. In the overwhelming majority of cases, the contrast in radiological images is based on the different x-ray absorption by different parts of the specimen. Absorption is very limited for x-rays: this is the basis of the striking success of radiology but also of its limitations. Weak absorption, in fact, also means small absorption differences between different materials—and therefore limited contrast. In some cases, this requires long exposures and a relatively high x-ray dose. The contrast can also be increased by injecting a high-contrast fluid into the imaged specimen. However, the injection procedure is often complicated and may involve non-negligible risks for the patient. These problems have a very significant social impact. In industrialized societies, heart diseases are the biggest killer—but preventive radiological screening is impossible because of the risk involved in iodine injection by a catheter in the heart region. Angiographic radiological examinations are thus typically limited to already ongoing pathologies. Likewise, the use of mammography for systematic screening of breast cancer is still limited due to the weak contrast—that makes x-ray doses unacceptable for a large portion of the women population. Very early detection of breast cancer does, on the other hand, lead to almost 100% successful therapy. How can we enhance the contrast in radiological images? First of all, we should realize that absorption is not the only type of interaction between x-rays and tissues (or matter in general). Can other interaction mechanisms be used to enhance or replace absorption-based radiology? The answer is positive; however, we shall see that x-ray sources with reasonably high coherence are required (see section 2.2.1 for a simple definition of coherence). This is not a trivial requirement: not long ago, coherent x-ray sources were simply not available. The most recent synchrotron light facilities brought into radiology, for the first time after R ¨ ontgen’s discovery, highly coherent x-rays [2,3]. The impact is quite stunning. Figure 1(a), for example, shows the difference between a ‘phase-contrast’ radiological image based on coherence and the corresponding absorption- contrast image. Other examples of this difference can be found, for example, in [4] and [5]. Comments are unnecessary: without increasing the x-ray dose, the visibility of the object becomes much better with phase contrast. Therefore, the dose can be substantially reduced without jeopardizing the practical image quality. The scope of this review is to present a simple introduction to this fascinating new technique. First, we will discuss in elementary terms the mechanisms of coherence-based 0022-3727/02/130105+16$30.00 © 2002 IOP Publishing Ltd Printed in the UK R105