High-resolution Tomographic Imaging Using Coherent Hard x-rays From Compact Laser Driven Accelerators

Abstract

Abstract:Extremely bright coherent femtosecond x-ray pulses are generated in compact laserdriven electron accelerators. Micro-tomography obtained with the Gemini laser indicates the usefulness of these sources in research and clinical applications.OCIS codes: (140.7090) Ultrafast lasers; (170.7440) X-ray imaging. Laser driven x-ray sourceThe acceleration of electrons in gas targets is a core research activity on short pulse (<100 fs) petawatt-class (1PW=10 15 W) laser systems. As an intense laser pulse propagates through a gas, electrons are trapped and accelerated to extremely high energies (~GeV) within a short distance (~1 cm). These electrons also oscillate and emit an extremely short (~femtosecond), bright coherent x-ray pulse in the forward direction [1] that can be used for imaging applications. Modern petawatt lasers can be constructed with a footprint as small as a standard research laboratory, thus this x-ray source has been dubbed a tabletop laser synchrotron (Fig. 1). The electrons generate a broad polychromatic x-ray spectrum with a critical energy up to 50 keV (similar to a synchrotron spectrum) and a peak brightness comparable with large-scale synchrotron light sources. This provides sufficient intensity to obtain an image using a single pulse [2] and so the image acquisition rate is limited only by the repetition rate of the drive laser pulses. The increase in repetition rate of petawatt lasers to 10 Hz in the near future opens up exciting possibilities for applying these extremely bright compact sources for research, pre-clinical and clinical imaging. High resolution x-ray imagingAs shown in Fig. 1, after the laser interaction electrons are swept away using a large permanent magnet and samples are placed in the x-ray beam to obtain point-projection images on a detector placed some distance (several metres) away. The magnification can be changed by altering the position of the sample relative to the source. Because the xray source size is determined by the magnitude of oscillation of the electrons, which is of order 1 µm, the spatial resolution of the imaging is very high. It is therefore suitable for challenging biological imaging that requires resolution at the cellular level. As well as x-ray absorption imaging the spatial coherence of the source enables phase contrast imaging (PCI) by detecting the slight deflections of x-rays by refractive index gradients. The more widespread use of PCI for medical imaging is being pursued [3] because it is superior in distinguishing between soft tissues where attenuation imaging quality is poor (particularly for mammography) and also because it has the potential for a lower patient dose during a scan.