Validation of the Monte Carlo simulation software:
We validated the Monte Carlo (MC) software environment (ImpactMC,
Erlangen,
Germany) for our CT scanner (GE revelation CT scanner).
Dose was measured in each hole of the CTDI phantom in an axial acquisition for 70,
80,
100,
120 and 140 kVp,
and 120 mAs.
Tube voltage,
tube current,
scan time,
collimation,
bowtie filter,
and spectrum were implemented in the MC software.
MC simulations were performed with 2×109 number of photons.
The weighted CTDI values,
as well as dose measurements in each hole,
were calculated from the simulations and compared with the measurements.
The error was considered as the average error of CTDI experiment for 80 and 100 kVp spectra.
The measurements were in good agreement with the simulations (percentage error within 5.5 %).
Monte Carlo simulations on experimental and virtual aorta phantoms:
Nine syringes (d = 20 mm) with different iodine concentration ranges (0 – 44.4 mg/ml) were prepared and centered in a cylindrical water phantom (d = 215 mm).
Scans were performed with the same acquisitions which were used for the software validation (80,
120,
and 140 kVp,
and 120 mAs).
In order to see the net physical effect of iodine concentration on the absorbed dose,
a virtual aorta model of the experimental aorta phantom was made.
In the virtual images,
the CT value for air was -1000,
for water was 0 and for the iodine solution was the mean value of CT values (HU) in the region of interest (ROI,
d = 15 mm,
depth = 25.625 mm) in the syringe in CT images obtained from the scans.
Material types and their density properties were included in the MC software and simulations were performed for both the experimental aorta phantom and the virtually reconstructed phantom.
Data and statistical analysis:
Absolute dose values were obtained by averaging the dose values in 3D region of interests (d = 15 mm,
depth = 25.625 mm). The uncertainty in the simulated dose for each concentration was considered as;
UC,i= DoseC,i×E‾rel,
where UC,i is the uncertainty in the iodine concentration "i",
DoseC,i is the simulated dose in the concentration "i" and E‾rel is the average relative error between the measured and simulated dose in all the holes of CTDI phantom excluding the hole positioned close to the table (table was not modeled in the MC simulations).
Dose enhancement factors (DEFs) for 80,
120 and 140 kVp tube voltages were reported as the dose in the presence (DI) over the dose in the absence (DI,0) of iodine contrast agent (DI/DI,0).
The uncertainty in the DEFs and in the iodine concentrations were obtained by the error propagation.
We used a Wilcoxon test in order to investigate the statistical difference between the dose results obtained from experimental and virtual aorta models,
as well as the comparison between DEFs for different tube voltages.
