# Geometric correction method for 3d in-line X-ray phase contrast image reconstruction

- Geming Wu
^{1}, - Mingshu Wu
^{1}, - Linan Dong
^{1}and - Shuqian Luo
^{1}Email author

**13**:105

https://doi.org/10.1186/1475-925X-13-105

© Wu et al.; licensee BioMed Central Ltd. 2014

**Received: **27 May 2014

**Accepted: **14 July 2014

**Published: **29 July 2014

## Abstract

### Background

Mechanical system with imperfect or misalignment of X-ray phase contrast imaging (XPCI) components causes projection data misplaced, and thus result in the reconstructed slice images of computed tomography (CT) blurred or with edge artifacts. So the features of biological microstructures to be investigated are destroyed unexpectedly, and the spatial resolution of XPCI image is decreased. It makes data correction an essential pre-processing step for CT reconstruction of XPCI.

### Methods

To remove unexpected blurs and edge artifacts, a mathematics model for in-line XPCI is built by considering primary geometric parameters which include a rotation angle and a shift variant in this paper. Optimal geometric parameters are achieved by finding the solution of a maximization problem. And an iterative approach is employed to solve the maximization problem by using a two-step scheme which includes performing a composite geometric transformation and then following a linear regression process. After applying the geometric transformation with optimal parameters to projection data, standard filtered back-projection algorithm is used to reconstruct CT slice images.

### Results

Numerical experiments were carried out on both synthetic and real in-line XPCI datasets. Experimental results demonstrate that the proposed method improves CT image quality by removing both blurring and edge artifacts at the same time compared to existing correction methods.

### Conclusions

The method proposed in this paper provides an effective projection data correction scheme and significantly improves the image quality by removing both blurring and edge artifacts at the same time for in-line XPCI. It is easy to implement and can also be extended to other XPCI techniques.

## Keywords

## Background

where δ is the decrement of the real part of the refractive index, and β presents the absorption index of local attenuation. It is proved that δ is three orders of magnitude larger than β within the diagnostic X-ray range for human tissue [4]. Thus XPCI can greatly enhance the contrast for soft tissues in biological sample with high spatial and temporal resolutions than conventional absorption-based X-ray imaging technique.

To reveal the morphology of a thick biological sample by using XPCI, 2D projection images are collected by titling the sample around a fixed axis, which is perpendicular to the beam direction, and computed tomography (CT) technique is used to obtain 3D visualization of the internal structure of the sample. Usually standard filtered back-projection algorithm (FBP) or algebraic reconstruction technique (ART) [5] are employed to build CT slice images from projections. However, collected projections cannot be used for CT reconstruction directly without pre-processing in practice. Due to mechanical system with imperfect or misalignment of X-ray source, rotary stagy and CCD detector, an unavoidable problem encountered before CT reconstruction is how to align 2D projections and make the rotation axis coincident with the center line of each projection. Similar problem has been investigated in electron tomography (ET). A conventional scheme for this issue in ET is to employ gold particles or image derived markers to correctly align projections by computing geometric parameters from tracking and measuring projected positions of them [6, 7]. However, this method needs additional components to generate accurate and measurable markers in projection images and thus makes the imaging procedure and post processing complex. Some marker-free methods are also proposed to handle this issue in ET by using the cross-correlation (CC) of successive projection images [8] or taking specimen features as built-in markers for deriving geometric parameters [9, 10]. These methods are based on pattern matching and have the drawback of accumulating alignment errors by comparing successive projection images. Recently, a mass center (MC) based method [11, 12] is proposed to align 2D projection images before reconstructing 3D tomography image in coherent diffraction imaging (CDI) [13, 14]. This method does not use local features but mass center as the invisible marker. Due to only one marker can be used, MC-based method works well for translational alignment, but poorly for rotational alignment. And it is sensitive to background noise and thus needs a well-designed background subtraction scheme. Some existing software suites developed for XPCI reconstruction usually provide a manual and automatic integrated approach to handle the data correction issue before CT reconstruction. PITRE [15] employs a MC-based sinogram correction scheme to calibrate the location of rotation axis, and the user can refine the result manually. DEIRecontructor [16] calculates geometric parameters by using user-selected markers. In this paper, a new automatic method is presented to solve the data correction problem in in-line XPCI, which is a propagation-based phase contrast imaging technique. This method employs an iterative approach to properly determine optimal geometric parameters. And a two-step method, which includes performing a composite geometric transformation and then following a linear regression process, is used to find out the solution at each iteration. Numerical experiments on synthetic and real image datasets demonstrate that the proposed method provides a fast and reliable image pre-processing scheme for CT reconstruction in in-line XPCI.

The rest parts of this article are organized as follows: Methods section describes the projection data misplacement problem for CT reconstruction in in-line XPCI and its mathematical model with primary geometric parameters at first, and then introduces the proposed method in details. Results section shows numerical experimental results from synthetic and real datasets which are pre-processed by the proposed method, and followed by the study conclusion in Consclusions section.

## Methods

### Projection data correction in in-line XPCI

While reconstructing slice images from acquired projections in in-line XPCI, the center line of any projection is considered to be identical to the rotation axis by default. Following this assumption, reconstruction algorithms, such as FBP, can be employed to build slice images correctly. However, it is usually very hard and time-consuming to align the components of imaging system with micrometer precision. If no data pre-processing step is adapted, the final reconstructed slice images will be blurred and with edge artifacts.

*δ*

_{ x },

*δ*

_{ y },

*δ*

_{ z }) are offsets along X, Y and Z directions respectively.

*δ*

_{ y }and

*δ*

_{ z }are caused by the movement biases of rotary stage, and

*θ*and

*δ*

_{ x }are mainly due to the misalignment of CCD detector and rotary stage. The movement biases of rotary stage are controlled by the precision of mechanical design, and thus

*δ*

_{ y }and

*δ*

_{ z }are usually small and can be ignored. So

*θ*and

*δ*

_{ x }are two main factors to be considered and the model is simplified as illustrated in Figure 3. There

*δ*

_{ x }is denoted as

*δ*. The existence of two misalignment parameters results in geometric transforms of the projection images. The parameter

*θ*is related to the rotation transform of projection images while

*δ*to translational transform along X. So the misalignment problem can be solved by applying inverse geometric transform to the projection images if we know the values of these geometric parameters.

*δ*= 2 pixels and rotation transform with

*θ*= 5° are applied to projections respectively. Compared to the result without geometric transform, translational transform makes the slice image blurred while rotational transform introduces obviously edge artifacts.

### Two-step iterative correction method

*P*

_{0}and

*P*

_{ π }the two projections with special tilt angles respectively. According to the physical principle of in-line XPCI,

*P*

_{ π }is the reflection of

*P*

_{0}by taking the rotation axis as the mirror line. So

*P*

_{ π }should be identical to the image which is achieved by applying the following composite geometric transformation to

*P*

_{0}in homogeneous coordinates

*x*,

*y*) is a point in

*P*

_{0}and (

*x*′,

*y*′) is its corresponding position in

*P*

_{ π }. If the rotation angle

*θ*is small enough, then (

*x*′,

*y*′) is approximated by

*P*

_{0}and

*P*

_{ π }are available

*k*th iteration, the basic step is given by the following iteration

The optimizer of Eq. (6) is achieved by using a two-step method which includes performing a composite geometric transformation *T*^{
k − 1} and then following a linear regression process.

*k*th step,

*T*

^{ k − 1}is applied to

*P*

_{ π }to rotate it with the angle − 2

*θ*

_{ k − 1}and then reflect it. Here

*T*

^{ k − 1}is defined as follows

*T*

^{ k − 1}to

*P*

_{ π }. It can be proved that we have

*P*

_{0}and ${P}_{\pi}^{k}$ along X

*P*

_{0}and ${P}_{\pi}^{k}$ row by row, and fitting the positions

*t*

_{ m }(

*y*), which are corresponding to the maximum of correlation coefficients, with the following linear model

The stop criterion of the iterative method is set as both *θ* and *δ* are less than specified tolerance values respectively. *T*^{0} is set to the identity matrix.

## Results

To evaluate the proposed method, we carried out experiments on both synthetic and real in-line XPCI data. MC-based sinogram correction scheme and the proposed method have been implemented using MATLAB (The MathWorks, Inc., Natick, MA, USA). Inverse Radon transform with Ram-Lak filter was taken as CT reconstruction algorithm applied to corrected data. Data processing for this paper was carried out on a Dell workstation system with a 2.4GHz Intel Core i5 processor and 8GB memory.

### Simulation on synthetic data

*X*with respect to its reference

*Y*, SSIM index is defined as follows

*x*and

*y*refer to a local window in the image

*X*and

*Y*respectively,

*μ*

_{ x }(

*μ*

_{ y }) is the mean while

*σ*

_{ x }(

*σ*

_{ y }) is the standard deviation over the window

*x*(

*y*),

*σ*

_{ xy }is the co-variance between

*x*and

*y*,

*C*

_{1}and

*C*

_{2}are two small positive constants. And the mean of SSIM index (mSSIM) is the average over all local windows. Mutual information is defined as

*H*(

*X*) and

*H*(

*Y*) are the Shannon entropy of the image

*X*and the reference

*Y*respectively, and

*H*(

*X*,

*Y*) is the joint Shannon entropy of the image

*X*and its reference

*Y*. We calculated mSSIM and MI values for the above slice images and listed the results in Tables 1 and 2 separately. Comparing the results, we can conclude that CT slice images reconstructed from projections pre-processed by the proposed method achieve much higher quality than popular MC-based scheme.

**Comparisons of reconstruction accuracies with mSSIM**

Slice No. | NC | MC | GC |
---|---|---|---|

1 | 0.57 | 0.69 | 0.92 |

2 | 0.55 | 0.76 | 0.95 |

3 | 0.54 | 0.83 | 0.96 |

4 | 0.51 | 0.68 | 0.91 |

**Comparisons of reconstruction accuracies with MI**

Slice No. | NC | MC | GC |
---|---|---|---|

1 | 1.52 | 1.60 | 2.15 |

2 | 1.63 | 1.83 | 2.60 |

3 | 1.62 | 1.99 | 2.73 |

4 | 1.50 | 1.64 | 2.18 |

### In-line XPCI data correction experiment

## Conclusions

CT reconstruction of X-ray Phase Contrast Imaging enables to investigate internal microstructure of biological samples with high resolution. Mechanical imperfect or misalignment problem makes the reconstructed slice images blurred and with edge artifacts, and thus destroy the features of microstructures and reduce the spatial resolution of in-line XPCI. To restore the images from collected projections by in-line XPCI, a fast geometric correction method is proposed to determine the geometric transform parameters properly. From the results of numerical experiments on synthetic and real datasets, we have the conclusion that the proposed method can significantly improve the image quality by removing both blurring and edge artifacts at the same time. Geometric correction method utilizes the symmetry of projections, and thus provides a simple and fast scheme to correct misplaced projection data.

## Declarations

### Acknowledgements

The authors acknowledge the staffs from beamline BL13W1 of Shanghai Synchrotron Radiation Facility (SSRF) for their kindly support for the experiments. This study was supported by the National Natural Science Foundation of China, Grant No. 61227802, 60532090 and 30770593, and by the 7th Framework Programme of the European Community, Grant Agreement Number PIRSES-GA-2009-269124.

## Authors’ Affiliations

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This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.