1. No category

Why CT ?
CT scanner physics
Mikael Jensen
Radiation Research Department
Risø National Laboratory
Early CT scanner geometry
Implementation of CT
Single slice
Single projection
Single direction
Very long scan time!
Original system devised by Hounsfield in late 1960ties
Algebraic reconstruction
Io
Io
Io
µ1
µ2
I21
Io
µ3
µ4
I22
I11
I12
proj_demo
1
Algebraic reconstruction(2)
Back projection
I11 = I 0 ⋅ exp(− µ1 ⋅ dx) ⋅ exp(− µ3 ⋅ dx )
I12 = I 0 ⋅ exp(− µ 4 ⋅ dx) ⋅ exp(− µ 2 ⋅ dx)
I 21 = I 0 ⋅ exp(− µ1 ⋅ dx) ⋅ exp(− µ 2 ⋅ dx )
I 22 = I 0 ⋅ exp(− µ 3 ⋅ dx) ⋅ exp(− µ 4 ⋅ dx )
⎧ I0
⎫
−1
⎪ln( I )dx = µ1 + µ3 ⎪
11
⎪
⎪
⎪ I0
⎪
−1
⎪ln( I )dx = µ 4 + µ 2 ⎪
⎪
⎪
12
⎨
⎬4 equations,4 unknown
⎪ln( I 0 )dx −1 = µ + µ ⎪
1
2
⎪ I 21
⎪
⎪
⎪
⎪ln( I 0 )dx −1 = µ + µ ⎪
3
4
⎪⎩ I 22
⎪⎭
1
2
5
3
7
3
4
5
3
2
1
2
3
4
5
6
7
1
For all projections,
the measured values
are added to all
contributing pixels
Filtered back projection
ct_demo
2
Hounsfield unit
Table 1: Typical H.U. Values
Windowing
3
4
Slip Rings
5
Electron Beam CT
Examples from the abdomen
Combination of multiple slices – 3D reconstruction and
segmentation
6
Segmentation
7
CT- fusion with other modalities (PET/CT)
8