Abstract
BackgroundThe high-resolution X-ray imaging system employing synchrotron radiation source, thin scintillator, optical lens and advanced CCD camera can achieve a resolution in the range of tens of nanometers to sub-micrometer. Based on this advantage, it can effectively image tissues, cells and many other small samples, especially the calcification in the vascular or in the glomerulus. In general, the thickness of the scintillator should be several micrometers or even within nanometers because it has a big relationship with the resolution. However, it is difficult to make the scintillator so thin, and additionally thin scintillator may greatly reduce the efficiency of collecting photons.MethodsIn this paper, we propose an approach to extend the depth of focus (DOF) to solve these problems. We develop equation sets by deducing the relationship between the high-resolution image generated by the scintillator and the degraded blur image due to defect of focus first, and then we adopt projection onto convex sets (POCS) and total variation algorithm to get the solution of the equation sets and to recover the blur image.ResultsBy using a 20 μm thick unmatching scintillator to replace the 1 μm thick matching one, we simulated a high-resolution X-ray imaging system and got a degraded blur image. Based on the algorithm proposed, we recovered the blur image and the result in the experiment showed that the proposed algorithm has good performance on the recovery of image blur caused by unmatching thickness of scintillator.ConclusionsThe method proposed is testified to be able to efficiently recover the degraded image due to defect of focus. But, the quality of the recovery image especially of the low contrast image depends on the noise level of the degraded blur image, so there is room for improving and the corresponding denoising algorithm is worthy for further study and discussion.
Highlights
High-resolution X-ray imaging systems such as Nano-CT based on the third-generation synchrotron sources and X-ray detector with transparent luminescent screen have been widely used to achieve a high spatial resolution in sub-micrometer or nanometer range [1,2]
The scintillator receives the x-ray and converts it into visible light, which is magnified by the microscope optics and collected by the CCD camera [2]
If we desire that the detection system employing transparent luminescent screens should achieve the limit of resolution of visible light (~0.3 μm), the thickness of corresponding scintillator should be ~0.5 μm according to Rayleigh criterion and formula of depth of focus
Summary
Simulated imaging In the high-resolution X-ray imaging system, the detector consists of thin scintillator material, optical lens with magnification and low noise CCD camera (Figure 1). Given the parameters of the optical lens and the relationship between the degraded blur image and the original high-resolution image, we can approximately simulate a blur image caused by inappropriate thickness of scintillator and defect of focus. The only shortage is that this algorithm can not perfectly recover the low contrast parts (shown in part 3 of Figure 6(a)), and the quality of restored image depends on the signal to noise ratio of the degraded blur image. From the local SNR and global PSNR curves for different noise level shown in Figure 7(b) (c), we can quantitatively determine how the noise level impacts the recovery image quality, so the curves can help us in practice to determine which noise level is suitable according to the quality requirement
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