Junit, Syarifah Nurfajrina Syed
(2015)
CT number changes within depth in CT and CBCT images.
Project Report.
Universiti Sains Malaysia.
(Submitted)
Abstract
This study was carried out to determine the effect of increased depth to CT
number on CT and CBCT images. In addition, this study was done to establish
scanning method on CT and CBCT modalities in order to provide more precise
dosimetry calculation involving CT and CBCT. An electron density phantom with
tissue equivalent plugs was used in this study. The plug phantoms of various tissue
equivalent materials were placed at different depths between the centre and
peripheral of the phantom and scanned using CT and CBCT modalities. Images for
both modalities were acquired and CT numbers at different depth in the phantom
were analyzed. The results showed that on CT images, water equivalent plug
phantom showed highest changes of CT number from peripheral position to central
position with 146.4% and the lowest was air cavity with 0.55%. The results in CBCT
images showed that muscle tissue equivalent plug phantom gave the highest change
of CT number at increased depth with 6842.17% and the lowest difference was given
by dense bone tissue equivalent plug phantom with 11.72%. A comparison of
change in CT number between CT and CBCT showed that CBCT images having
higher rate of change of CT number from peripheral position to central position
compared to that in CT images. As conclusion, the CT number was found to be
significantly increase as the depth increased. Thus, dosimetry measurement using
CT number is suggested to be done at central position of phantom considering the
contribution of x-ray scattering to the point of measurement. A calibration of CT
number is required for dosimetry measurement at positions nearer to the surface of
medium.
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