# Enhance Soft Tissue Contrast for Cone Beam CT by ill20582

VIEWS: 22 PAGES: 14

• pg 1
```									Enhance soft tissue contrast
for CT cone beam CT

Junan Zhang PhD
Fang-Fang Yin PhD
Duke University Medical Center
Truncation artifacts
When SFOV < Subject Size

SFOV

Appear as bright bands and streaks at the edges of SFOV.
Reduce the contrast of soft tissues inside SFOV.
Purpose/Objective
   Improve algebraic reconstruction
technique for truncated CT
reconstruction

   Reduce truncation artifacts and
enhance the contrast of soft tissues.
What’s the ART?
Interpret CT as linear equations
Column --- Voxels in subject: M>10^4 (2D), >10^6 (3D)
Row --- Rays in projections: N>10^3 (2D), >10^5 (3D)

 A11 A12  A1M                    ln I1 
A                         1         I 0    1 
  21                            
                                        
 M  ln  IN
                                            N 
I0   
 AN 1   ANM                    
Sphere-shape Voxel

   To suppress high frequency noise, sphere-shape
voxels are used (R. Lewitt)
Solve ART iteratively
Take equations row by row
before the n-th equation,
Voxel value α
n 1
            n
 α1n 1  αM 1   
Based on the error of the n-th equation
 n   n  An   n 1

Update
n         n 1       An
α          α                   n
An 2
0   1           Relaxation number
Different ART methods
   Standard ART
   backproject ray by ray
   Simultaneous(S) Iterative (I)RT
   Simultaneous (S)ART
   backproject rays (in a projection) parallel

Less artifact than FBP for small projections number
FBP v.s. ART (1/2)
FBP 663 projs     ART 663 projs

  0.1,   5 iterations
FBP v.s. ART (2/2)
FBP 23 projs      ART 23 projs

  0.1,   20 iterations
Order of back projections
   Sampling rates outside the SFOV are lower
than inside
   Rays away from the center should be
processed earlier than those close to the
center.                           1
3
5
7
6
4
2
ART, SART, SIRT
   Process rays by the order of their projection angles
Progressive ART
   Process rays by the order of their distance from the
center
PART + SART
   PART reduce truncation artifacts but it brings
some new artifacts such as a darkened SFOV
center.
    Therefore SART is called again.
   Since the values outside the SFOV are already
established, SART brings little truncation
artifacts to them.
Results

SART(2 iterations)   PSART+SART   Original

PSAT reduces truncation artifacts and
enhances soft tissue contrast

```
To top