Fine Biomedical Imaging Using X-Ray Phase-Sensitive Technique

Abstract

X-ray imaging is widely used for non-destructive observations of the inner structures of samples in many fields, such as biological, clinical, and industrial ones. The transparency of X-rays is much higher than that of visible light, and therefore the spatial distribution of Xray intensity passing through a sample (radiography) can visualize the mass-density distribution inside the sample. However, X-ray intensity barely changes when passing through samples consisting of a light element, such as carbon, oxygen, or nitrogen, because of the extremely high transmittance of X-rays. Therefore, the sensitivity of absorptioncontrast X-ray imaging is not sufficient for carrying out fine observations of samples such as biological soft tissues and organic materials. Contrast agents, including heavy elements such as iodine, and long exposure to X-rays are ordinarily used to improve sensitivity. However, these supplementary methods may cause allergic reactions and expose subjects to extremely high X-ray dosages. A fundamental solution to this problem is use of the phase information of X-rays. X-rays are electromagnetic waves having very short wavelength and are mainly characterized by their amplitude and phase. When they pass through samples, their amplitude is decreased and the phase is shifted. In the hard X-ray region, the cross-section of phase shift for light elements is about 1000 times larger than that of absorption (Momose & Fukuda, 1995). Therefore, phase-contrast X-ray imaging, which uses phase shift caused by the sample as image contrast, provides a way of conducting fine observations of biomedical samples without the need for contrast agents or excessive X-ray dosages. For phase-shift detection, it is essential to convert the phase shift into the change in X-ray intensity because we can only detect the intensity of X-rays by using current-detecting devices. Many conversion methods, such as interferometry with an X-ray crystal interferometer (Momose & Fukuda, 1995; Momose, 1995; Takeda et al., 1995), diffractometry with a perfect analyzer crystal (Davis et al., 1995; Ignal and Beliaevskaya, 1995; Chapman et al., 1997), a propagation-based method with a Fresnel pattern (Snigirev et al., 1995; Wilkins et al., 1996), and Talbot interferometry with a Talbot grating interferometer (Momose et al., 2003; Weitkamp et al., 2005), have been developed recently. The principle difference between these methods is in the detection of physical values; that is, interferometry detects the phase shift directly, while the other methods detect the first or second spatial derivation of the phase shift.