Keywords:
Quality assurance, Dosimetric comparison, Physics, Dosimetry, Acceptance testing, Image manipulation / Reconstruction, CT, Radioprotection / Radiation dose, Radiation physics
Authors:
G. Venturi , S. Farnedi, N. Scrittori, V. Ravaglia; Ravenna/IT
DOI:
10.26044/ecr2019/C-1370
Methods and materials
In this study Catphan 600 was used as a test object.
This phantom is dedicated to image quality assurance in computed tomography and it’s composite of several modules.
We employed the CTP486 homogeneous water equivalent module to evaluate noise properties and the low-contrast CTP515 module to evaluate CNR.
This second module is composed of supra and subslice contrast targets with different diameters ranging from 2 to 25 mm and nominal contrast of 0.3%,
0.5% and 1%.
Images of the 2 modules of the Catphan 600 phantom were acquired on a GE Revolution Evo,
and all were reconstructed by using conventional FBP and different levels of IR from 20% to 60%.
Noise properties was evaluated varying the tube voltage (100-120 and 140 kV),
with a fixed tube loading (9 mAs,
as calculated in the DICOM TAG “Exposure”).
Other noise measurements were performed varying the tube loading (3 – 9 – 13 mAs,
120 kV) and varying the pitch (0.531,
0.974 and 1.375).
Slice thickness was set at 2.5 mm.
The set of uniform images were analyzed using a commercial software,
IQworks.
This program allows to analyze every module of the Cathphan 600 through specific analisys trees.
Noise dedicated analysis tree set an elliptical ROI centered in every image and gives measurement of the standard deviation within the ROI.
CNR was evaluated using the ImageJ software and creating a ROI set to place on the 15 mm supraslice target (nominal contrast 1%) and on the background.
The contrast to noise ratio is defined as:
CNR = (ROIt - ROIbg)/ Stdevbg
Where ROIt is the mean value of the target,
ROIbg is the mean value of the background and Stdevbg is the image noise evaluated as the standard deviation of the background.
Images of the low contrast module were acquired varying the tube loading (50-105.-145-300 mA).
CTDIvol values varied from 7.38 mGy to 44.32 mGy.