Enhance Soft Tissue Contrast for Cone Beam CT by ill20582

VIEWS: 22 PAGES: 14

									Enhance soft tissue contrast
for CT cone beam CT


        Junan Zhang PhD
       Fang-Fang Yin PhD
   Dept. of Radiation Oncology
  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