CT data acquisition and Materials
This evaluation was performed by phantom (Multi Slice CT Phantom MHT Type: Low Contrast phantom,
Kyoto Kagaku,
Ltd.) which has 4 low contrast modules (3mm,
5mm,
7mm,
10mm diameters; Reference CT value of low contrast module and base module were 65HU and 50HU.) between 2 metal objects .
(Fig1)
We scanned phantom with 128 Slice CT (Ingenuity CT,
PHILIPS Healthcare).
Scan parameters were fixed to 120kV and 400mAs.
The image was reconstructed by O-MAR and FBP.
In addition,
we scanned Low Contrast phantom without 2 metal objects in same scan parameters.
We used this phantom image as a reference.
Analysis method 1(Visual evaluation of Low contrast detectability)
The detectability of simulated low contrasts was assessed in FBP and O-MAR,
Reference FBP image.
Window center and window width of all images was set at 100/300.
The scoring of low contrast detectability was performed by three staffs independently.
Three staffs were radiographer with experience in CT (T.F.,
13 years; D.S.,
16years; F.N.,
23 years; ) All readers were blinded to the three kinds of images setting.
The scoring of low contrast detectability was defined as follows.
Four low contrast modules (3mm,
5mm,
7mm,
10mm diameters) were evaluated by two scores of visible or invisible.
Analysis method 2(Measurement CT Value and standard deviation of Low contrast Modules)
We placed the region of interest(ROI) to the base module and low contrast module(5mm,
7mm,
10mm).
We measured the CT value and the standard deviation (SD) in the ROI.
CT value and standard deviation of three kinds of images (O-MAR,
FBP,
Reference FBP) were compared low contrast module(5mm,
7mm,
10mm) to base module.[2] [3]
Analysis method 3(Measurement Metal Artifact Index)
In order to analyze the artifact of O-MAR and FBP quantitatively,
we measured the Metal Artifact Index.
The objective noise was defined as the standard deviation (SD) of the measured density.
The Metal Artifact Index (MAI) was defined as: MAI =√(SD²o-SD²r)[4]
' SD²o ' is the desired image (FBP or O-MAR) of ROI,
'SD²r ' is the Reference FBP image of ROI.
Therefore,
in case of few metal artifacts,
Metal Artifact Index approaches zero.
5 ROI were set to the size of 3cm * 3cm.
1 ROI was placed in the center of each images.
4 ROI were placed 5cm from the center.
(peripheral of left,
right,
top,
bottom).
(fig.2)