Combined Energy Minimization for Image Reconstruction from Few Views

Reconstruction from few views is an important problem in medical imaging and applied mathematics. In this paper, a combined energy minimization is proposed for image reconstruction. l2 energy of the image gradient is introduced in the lower density region, and it can accelerate the reconstruction speed and improve the results. Total variation of the image is introduced in the higher density region, and the image features can be preserved well. Nonlinear conjugate gradient method is introduced to solve the problem. The efficiency and accuracy of our method are shown in several numerical experiments.


Introduction
Computed tomography CT is one of the most important advance in diagnostic radiology in recent decades.CT uses multiple X-ray images to build up cross-sectional and 3D pictures of structures inside the human body which enable doctors to view internal organs with unprecedented precision.However, the use of ionizing radiation in CT may induce cancer in the exposed individual after a latent period 1-3 .Cancer induction by ionizing radiation is a probabilistic process.Reduction of radiation dose used in CT will therefore lead to a reduction in the number of induced cancer cases.
Some ways can be used to reduce the radiation dose from CT such as decreasing intensity of X-ray beam, handling scattered radiation, restricting exposure area.Reducing the

Reconstruction from Few Views and Total Variation Minimization
There are many approaches about tomographic reconstruction from limited views projection data 17-19 .Algebraic reconstruction technique ART 9, 20, 21 and the expectationmaximization EM algorithm 22, 23 have been widely used in this field.As the image is discretized on the grids, each projection is regarded as a linear equation of the discrete density distribution.Then a system of simultaneous equations can be obtained and ART tends to solve it via iterative method.ART algorithm can find the image that is consistent with the projection data and the sum-of-squares of the density values is minimized.The EM algorithm applies to positive integral equations, seeking to minimize the Kullback-Liebler distance between the measured data and the projection of the estimated image 11 .
However, it is known that the ray does harm to human body and abundant irradiation may lead to cancer 17, 24 .So researchers begin to study the tomographic reconstruction with projection data as little as possible.

Reconstruction from Few Views
Tomographic reconstruction from few views projection data is an efficient way to reduce the harm caused by ray irradiation, and there are some approaches about it 11, 12 .As the gray image to be reconstructed can be denoted by below 2.1 Here r and c mean the size of image.
The projection can be denoted as the following equations: Here f is the vector form rearranged form u. g : g 1 , g 2 , . . ., g k T means the projection data.More exactly, k is the product of the number of views and the number of detector's pixels.M : M 1 , M 2 , . . ., M k T is the projection matrix which can be precomputed.M i i 1, 2, . . ., k is the same size as f.The reconstruction procedure equals to solve 2.2 .
Unfortunately, this equations are indeterminate if the reconstruction was based on few views.In other words, the number of the equations are less than the number of variables k < r • c .In practice, it is more often that k r • c.From the linear algebraic theory, the solution is not unique and the traditional methods cannot be applied.In fact, even if k ≥ r • c is satisfied, it is still compromised to deal with the consistency of the projection data and lead to artifacts in the reconstructed image.
The ART can be applied to solve this equation and it means to solve the following problem: arg min f 2 subject to M • f g.

2.3
Here ||f|| 2 means the l 2 norm of f.Because of the serious insufficiency of the projection data, ART algorithm can hardly provide satisfactory result.The same as to EM algorithm.So some other models should be discussed.

Total Variation Minimization
The total variation TV was first introduced by Rudin et

2.5
Based on these, Sidky et al. developed an iterative image reconstruction algorithm for fanbeam CT in 11 .
The TV minimization can efficiently reduce errors and preserve features in the image reconstruction.In next section, we will concentrate on developing a new model to improve the convergence speed and reduce errors based on this TV model.

Minimization of a Combined Energy for Image Reconstruction
It is known that the convergence speed will be enhanced when the l 2 norm of image gradient is considered as shown in following: But this l 2 result can also blur the image features.So some combined energies can be considered.

A Combined Energy of Image
The natural idea is to combine the l 2 norm and TV directly.The combined energy can be denoted by However, the new result cannot be improved much more than the TV result though the convergence speed may be accelerated in some sense.In fact, the new result is a weighted sum of the l 2 result and TV result.
To improve the TV result of image denoising, Chambolle and Lions 27 proposed a combined functional CL energy where

3.4
Here β is a fixed positive number and it is a threshold of |∇u|.In some way, it means an approximation of the critical value which can be used to distinguish image features and noise.

The Reconstruction Model
Based on the discrete form of the CL energy and the rearranged vector f, we can get Here It can be found that the l 2 energy of image gradient is considered in the noise part T f − while the TV energy is computed in features part T f .Then the new model for image reconstruction can be denoted as follows: With the Lagrange method applied, this constraint optimization problem can be rewritten as an unconstraint optimization problem of following combined Chambolle-Lions CCL energy:

Conjugate Gradient Descend Algorithm
The gradient of ε CCL f can be computed as where In practice, the parameter λ is set to be 10 −2 .Set a initial value f 0 , the conjugate gradient descend algorithm can be given as Algorithm 1.There the time step is denoted by τ.
From the experiments results, we will find some advantages of the proposed model.These are chiefly due to the different optimization problems and it is related with the algorithm.More exactly, the first term in 3.5 can help to enhance the convergence speed and reduce some artifacts in the smooth region.But the efficiency role of this l 2 energy of image gradient is depended on the well define of β.It is related with the characters of the image to be reconstructed.In our experiments, it is set to be 0.01 times the range of phantoms.The advanced researches about this will be approached in our next work.The general metric % Initialization maxGrad 10 −3 ; maxIter 100; maxTau 10 −3 ; s 4; k 0; τ 0 1; g 0 ∇ε CCL f 0 ; Δf 0 −g 0 ; % Iterations while g k 2 > maxGrad and k < maxIter and τ > maxTau { %Linear search minE ε CCL f k ; opt 0; i −s; Δf k 2 ;      A fruits image with size 256 × 256 is taken to be the true one.The reconstruction views are set to be 72.It costs 25 iterations to finish the reconstruction.Figures 5 b , 5 c , and 5 d show the ART result, TV result, and our CL result.The gray distributions of row 128 and column 128 are shown in Figures 5 e and 5 f .Figure 6 shows the evolutions of PSNR and τ.The PSNR has been improved from 31.8559TV result to 32.2934 CL result .There are few differences between TV result and our CL result though CL result is more satisfactory than TV result in numerical value.It can be found that CL result is more satisfactory than TV result in numerical value, but there are few differences between TV result and our CL result from vision terms.

Conclusion
In this paper, a novel model for image reconstruction from few views in parallel-beam data is proposed.First, the l 2 energy of the image gradient and the total variation of the image are combined to the CL energy.The l 2 energy is applied in the lower density region, and it

Example 4 . 1 .Example 4 . 2 .
Reconstruction of Shepp-Logan phantom from 72 views.The true image is taken to be the Shepp-Logan image shown in Figure 1 a discretized on a 256×256 pixel grid.The computational parameters are set as shown in the algorithm, the same to the following experiments.The reconstruction from 72 views is completed after 31 iterations.Figures 1 b and 1 c show the ART result and TV result while Figure 1 d shows our CL result.It can be found that the ART result is enhanced by TV much more.Many artifacts have been removed or slighted.Figures 1 e and 1 f show the gray distributions of row 128 and column 128.There are few differences between the reconstructed horizontal gray and the real one.It is similar to the reconstructed vertical gray.The evolutions of PSNR and τ are shown in Figures 2 a and 2 b .The PSNR has been improved from 46.4040 TV result to 50.5664 CL result .Though the TV result is almost accurate, our CL result improves it significantly.Reconstruction of Shepp-Logan phantom from 24 views.The true image is still taken to be the same Shepp-Logan image as Example 4.1 while the reconstruction views are reduced acutely from 72 to 24.This reconstruction is completed after 100 iterations.Figures 3 b , 3 c , and 3 d show the ART result, TV result and our CL result.The gray distributions of row 128 and column 128 are shown in Figures 3 e and 3 f .

Example 4 . 4 .
Reconstruction of fruits image from 30 views.The same fruits image as Example 4.3 is taken to be the true one while the views are reduced from 72 to 30.This reconstruction is completed after 59 iterations.ART result, TV result, and our CL result are shown in Figures 7 b , 7 c , and 7 d .The gray distributions of row 128 and column 128 are shown in Figures 7 e and 7 f .The evolutions of PSNR and τ

Example 4 . 6 .
Reconstruction of a synopsis phantom from 20 views.The same synopsis phantom as Example 4.5 is taken to be the true one while the views are reduced from 72 to 20.This reconstruction is completed after 68 iterations.ART result, TV result, and our CL result are shown in Figures 11 c , 11 b , and 11 d .The gray distributions of row 128 and column 128 are shown in Figures 11 e and 11 f .The evolutions of PSNR and τ are shown in Figures 12 a and 12 b .The PSNR has been improved from 31.7194TV result to 36.9629CL result .It can be found that there are more artifacts in TV result than in our CL result.

Figure 9 :
Figure 9: Reconstruction of a synopsis phantom from 72 views.

Figure 11 :
Figure 11: Reconstruction of a synopsis phantom from 20 views.

1 :
Iteration algorithm for CL minimization reconstruction.peak signal to noise ratio PSNR is introduced to evaluate the results and it can be computed as