Turning speckles into useful images
The new technique, recently published in Scientific Reports
by Wang et al., is a development of a previous two dimensional (2D) raster scan speckle imaging process but one that dramatically reduces the time required to produce the image and, consequently, the X-ray dose required.
The setup is deceptively simple (Fig. 1). A piece of ordinary sandpaper is mounted on a motorised linear translation stage between the X-ray source and the detector. An image is recorded as the sandpaper is moved step by step past the beam, resulting in a series of speckles produced by the randomly orientated grains of abrasive on the paper. The sample is then placed between the sandpaper and the detector and the process repeated giving a second series of speckle image but one that is subtly altered by the sample.
Figure 1: Schematic representation of the experiment setup for (a) synchrotron radiation source and (b) microfocus X-ray source. (a) A stack of phase-based speckle images is resolved using a high resolution X-ray detector by scanning abrasive paper transversely to the monochromatic X-ray beam from a synchrotron radiation source. (b) The corresponding absorption-based “speckle” images recorded by flat panel detector are obtained by placing large grain, abrasive paper close to the polychromatic X-ray microfocus source.
A significant improvement is that previous sandpaper speckle technique required a 2D raster scan, which is more complicated and requires the square of the number of passes needed by the technique presented here. In one example, the new procedure required just 60 steps to produce a useable image whereas an equivalent 2D raster scan would require 60 x 60, ie. 3,600 steps. This a huge reduction in the time required and a full image can be gathered in around 30 seconds rather minutes or hours. It also significantly reduces the X-ray exposure of the sample, important for many biological samples.
Different Images – Different Information
The key to this process is the new theory and algorithm developed by the Diamond-based team to extract the information from the speckles. Each speckle is identified and the images with and without the sample are compared to build a correlation coefficient map. The displacement of the speckles is related to the phase change induced by the sample, while the change in the intensity indicates the speckle distortion, which in turn provides a measure of the scattering angle of the sample. These data are then fed into the algorithm which can then calculate the phase and scattering information. It is this information that is then used to build the different images seen in Fig. 2.
Figure 2 shows the extracted five images of a fish sample, and each image reveals different information about the sample. Image a, the absorption image, shows the bones but very little detail of the soft tissue. Image b is the dark-field image, the scattering of the X-rays. The other three images, c, d and e, are different visualisations of the phase information. This technique is able to pick out differences between tissues with very similar X-ray absorption properties such as muscle and fat, which hugely increases their value for medical and other research.
Figure 2: The different images produced by the single dataset: (a) Absorption, (b) dark-field, (c) vertical and (d) horizontal differential phase gradient, and (e) phase contrast images of a fish. Each image is shown to identify different types of structures within the sample. All five types of image can be simultaneously acquired from a single dataset using our new 1D scanning technique.
“The new technique described here was first developed on a monochromatic X-ray beam here at Diamond, but this paper also demonstrates we can adapt it to common, small scale, polychromatic X-ray lab sources such as those found in labs and hospitals (see Fig. 1).” says Dr Hongchang Wang, Senior Optics Scientist at Diamond and the main author of the paper. “It needs fine tuning for each setup, in particular some work is required to choose suitable sandpaper for a particular source and detector combination, but this is relatively simple to do.”
“The speckle based technique was initially developed to test the quality of the synchrotron’s X-ray optics, and we have been able to extend it to X-ray imaging due to the flexible capabilities of B16” explains Dr Kawal Sawhney, the Principal Beamline Scientist on B16 who is also a co-author on the paper on the new imaging technique. “We wish to develop the technique further so that it can be applied to a wide range of research fields in the future.”
The potential benefits of this technique are obvious. Much more information can be obtained about a sample, including potentially living humans, and in particular images of internal structures that are inaccessible to normal absorption X-ray images. The technique is also applicable for advanced research in material science.
For further information on the B16 beamline or to discuss potential applications, please contact Kawal Sawhney: firstname.lastname@example.org