Phase-contrast X-ray Imaging (PCI)
Contents
- Phase-contrast X-ray Imaging (PCI)
Phase-contrast X-ray Imaging (PCI)
X-ray technology has been facing issues of imaging soft tissue since its invention for 100 years. Dr./Prof. Atsushi Momose (Tohoku University) has successfully solved this issue that enable to image beast cancer tissues or cartilaginous tissue clearly. Its resolution is very high quality, and its been expected to use not only medical fields but also other non-destructive inspection fields.
☞ Video 1(Japanese)
☞ Video 2(Japanese, Arabic subtitle)
1. Principle
The X-ray double bare refractive indexes generally display as 
.
Here, 
indicates X-ray amplitude decrement (absorption), and 
indicates an X-ray phase shift.
In the hard X-ray area, an X-ray phase shift becomes bigger than decrement of the amplitude (it becomes around 1,000 times for the weak absorption object in particular). Therefor, it can get images with the high sensitivity using a phase shift for the object consisting of light elements.
The phase technique of CT software, TomoShop® can apply to experiment environment using the following Talbot interference.
Fig. 1 A structure of Talbot interferometer
In the case of using Talbot interferometer for a diffraction grating, it can gain phase images with the X-ray source for laboratories. Therefor, it can be said that it is high utility method.
2. Characteristic of the phase technique of TomoShop®(CT software)
Midorino Research Co., ltd (The developer of TomoShop®) has been conducting algorithm study based on the theory of Prof. Atsushi Momose that achieving optimal quality of imaging results of X-ray CT with reducing the number of times of x-ray photography to several times from dozens of times without reducing the quality.
As an advantage, the quantity of X-ray radiation exposure decreases, reducing the degree of expectation of having highly sophisticated level of precision of the machine operation by hardware and reducing the entire cost of developing the CT system.
- The number of shots can be reduced from dozens of times to several times. By doing so, it is possible to greatly reduce the exposure dose of objects to X-rays.
- The interval of moving the diffraction grating increases. This makes operation easier.
- In order to perform calculation with high accuracy, artifacts can be reduced when image reconstruction is performed with phase CT.
- The requirement for accuracy of machine operation decreases and cost reduction becomes possible.
3. Simulation
Like Fig.1 above, fix the distance from a detector to a diffraction grating at Talbot distance to form a self-image. Divides the N period of the diffraction grating equally and decides movement distance. Then, Move a diffraction grating at movement distance laterally and photograph it afterwards.
Fig. 2 shows a phase sample. Fig. 3 shows one piece of image of the simulation.
Fig.2 Sample
Fig.3 Image of Simulation
4. Result
It processes N images which photographed and can restore the phase difference image of the sample. Fig. 4 and Fig. 5 is the phase difference images restored by using the captured image of the four steps (N = 4).
Fig. 4 Phase-contrast images obtained from the difference image of the conventional
Fig. 5 Phase-contrast images improved by Midorino Research Co., ltd
5. Result (actual data)
The following image is the result of the result (real data)by the conventional method and the phase method.
The phase difference image by the conventional method (left and right).
The phase difference image by Midorino Research Co., ltd (The developer of TomoShop®).
Note: The phase contrast method by Midorino Research Co., ltd (The developer of TomoShop®) is developed with the cooperation with Dr. Momose and Dr. Yashiro. For the details, please refer to the paper.
Non-standard fringe scanning method (Preventing degradation of image quality due to movement error of diffraction grating)
For conventional X-ray imaging using phase method, the analysis-grid-interferometer-method is used with Talbot phenomenon.
However, there are some problems with this method. For example, during X-ray image shooting by the analysis-grid-interferometer-method, shooting is performed by moving the diffraction grating several times (the fringe scanning method). In this case, the image quality of the phase images are reduced due to the movement error. Therefore, in order to prevent this degradation of image quality, highly precise machine operation is required and it is tricky issues in some cases.
The non-standard fringe scanning method developed by Midorino Research(TomoShop’s developer) has been developed so that machine operation is not highly accurate and results of high image quality are obtained even if errors occur in the diffraction grating movement. It is also possible to obtain an accurate phase image even when performing multi-periodic operation that moves the diffraction grating a large number of times with multi cycles.
The image below compares experimental results of real data (Phase differential image). The movement error of the analysis grid is around 30 nm.
by the conventional method
by Midorino Co (TomoShop) ‘s method
Note: These images were cooperated by Midorino Research Co., ltd. (The original images are from Dr. Momose).
CT reconstruction technique for 3D imaging of phase differential images (Phase CT Reconstruction)
TomoShop’s technique made possible to calculates absorption image, phase differential images, small angle scattering image, etc. from image data obtained by fringe scanning method (using X-ray Talbot interferometer) by calculation formulas. And it is possible to reconstruct the CT based on these images and acquire a 3D image of the object. This is because the absorption image and the small angle scattering image are line integrals in the X-ray incidence direction, it is possible to use the existing CT reconstruction technique. Also, it is possible to use the CT reconstruction technique after calculating the integral in the differential direction of the phase differential image.
However, in the case where the differential direction of the CT filter (Ramp Filter) and the phase differentiation image are the same, CT reconstruction can be performed without integrating the differential direction to obtain a 3D CT image. Since it is possible to decompose the CT filter (Ramp Filter) in the frequency domain. the following formula represent it.
*
=CT filter (Ramp filter), 
=differential operator, 
=Hilbert Transform
The figure below shows the cross-sectional result of the simulated differential image and 3D image after reconstruction using the above decomposition formula. (Shepp-Logar head phantom is used.)
Phase CT Reconstruction (Option)
TomoShop provides the functions to generate CT images by CT reconstruction from the phase differential image introduced here as an optional function. This function can be installed in the TomoShop (F Edition Series).
Note:Y ＝ Included
N ＝ Not Included
OPT = Optional Function (Separately price)
| Function | Phase Reconstruction Option |
|---|---|
| CT reconstruction from the differenctial image | Y |
| Brightness correction of differential image | Y |
| Price | |
| Price (End User, JPY) JPY = Japanese Yen | 1000000 |
| Price (OEM, JPY) JPY = Japanese Yen | Please Contact Us |
[1] Momose A, Takeda T, Itai Y and Hirano K, Phase−contrast X−ray computed tomography for observing biological soft tissues, Nature Medicine 2, 473—475, 1996.
[2] Yashiro W, Harasse S, Takeuchi A, Suzuki A, Momose A, Hard-x-ray phase-imaging microscopy using the self-imaging phenomenon of a transmission grating, Physical Review A, vol. 82, Issue 4, 2010.