UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

118 8. X-ray images
By placing a different section of the PMMA wedge next to the Al sheet, the contrast in
the phantom can be adjusted. Note that because of the similar absorption properties of
Al and PMMA to human tissue, this selection of the absorber materials simulates a thin
bone (Al) placed next to a thicker muscle (PMMA).
First of all the number of photons per second, the integrator current and the absorbed
spectrum behind 1 mm Al and 1 mm to 10 mm PMMA were simulated. The impinging
X-ray spectrum in the simulation was chosen such that it corresponds to the spectrum of
the Hamamatsu X-ray tube at 90 kVp, including the additional low energy filter consisting
of 1 mm of Al. From these simulation data, the image contrast C can be calculated
according to the following definition:
C = AP MMA − AAl
AP MMA + AAl
(8.1)
The variables AP MMA and AAl represent the signal amplitudes behind the PMMA wedge
and the Al sheet. Fig. 8.5 shows the resulting simulated contrasts. It is evident that
the contrast in both counter and integrator has a minimum around (5.5 - 6.0) mm. This
is because at approximately 5.8 mm PMMA thickness the integrator measures the same
currents behind the PMMA and Al absorbers. At roughly 6 mm PMMA thickness the
contrast in the counter vanishes as both absorbers show the same number of photons per
second.
Apart from these two graphs, the figure also contains two simulations of the contrast based
on the average photon energy. In the simulation labeled ”Avg. energy - Int >10 keV -
Cnt >10 keV ” the absorption spectrum above 10 keV was used to calculate the average
photon energies. This corresponds to the measurements presented in section 7.3, i.e. a
C o n tr a s t
0 .1 2
0 .1 0
0 .0 8
0 .0 6
0 .0 4
0 .0 2
0 .0 0
In te g ra to r (A )
C o u n te r > 1 0 k e V (B )
A v g . e n e rg y - In t > 1 0 k e V - C n t > 1 0 k e V (C )
A v g . e n e rg y - In t > 0 k e V - C n t > 1 0 k e V (D )
1 2 3 4 5 6 7 8 9 1 0
P M M A th ic k n e s s [m m ]
Fig. 8.5: Simulated image contrast in a low contrast PMMA-Al phantom. The phantom
consists of a 1 mm thick Al sheet placed next to a wedge-shaped PMMA absorber with
thicknesses between 1 mm and 10 mm. The average photon energy contrast was calculated
using two different approaches. Once it was derived from the simulated photon spectra with
a 10 keV lower threshold to exclude low energy charge sharing hits. The second curve was
calculated according to (7.6). At a PMMA thickness of approximately 6 mm the average
photon energy measurement should offer a better contrast than the counter or the integrator
measurements.
A
B
C
D