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KMID : 1149020170190020059
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Journal of Korean Society of Computed Tomographic Technology
2017 Volume.19 No. 2 p.59 ~ p.68
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The Study on optimal angles of scapula bone to reduce beam hardening artifact in neck ct : Self-made shoulder phantom evaluation
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Yoon Hee-Seok
Lee Ki-Baek
Son Bum-Gu
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Abstract
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Purpose : During taking neck CT, there is a large difference of X-ray absorption in the shoulder
structures with a thick tissue density, resulting in a beam hardening artifact. These artifacts degrade image quality and interfere with the ability to observe fine details. The purpose of this study is to reduce the artifact by implementing the optimal shoulder posture, which varies with the angle of the shoulder bone (Scapula) of the patient during CT scan.
Materials and Methods : Phantom experiments were performed with using equipment from two manufacturers (G and S Corps). Neck pig bone phantom (P-Phantom) and Water phantom (W-Phantom) were used to reproduce anatomical positions of thyroid and apex. We performed scan with 5 angles at 10, 20, 30, 40 and 50 degrees with fixed mA of 200, 300 and 400 mA and AEC mode which based on the angle between the shoulder bones (scapula) and the horizontal plane of the table. The degree of artifact generation was evaluated using image noise and setting the average posture at 20 ~ 40 degrees on patients which includes the shoulder posture tilted back to 10 degrees and the maximum posture at 50 degrees. Volunteer image evaluation was performed under the condition of optimal shoulder bone angle according to the results of phantom experiment.
Results : The mean index with noise figure of the three W-Phantoms was significantly different (P = 0.000) from 10°to 40°and decreased from 50°. The minimum value at 10 degrees, the maximum value at 40 degrees and the decrease at 50 degrees which is similar to 30 ~ 40 degrees. The average noise level of the W-Phantom at the thyroid position was measured under the conditions of 400 mA in both instruments. The average noise values of 10 to 50 degrees of G were 6.08, 7.9, 10.76, 14.79 and 12.85, respectively. And that of S was 6.99, 15.76, 18.56, 20.73 and 18.04. The result of the smallest average noise value was obtained at 10 degrees and the device for the optimal posture was produced and evaluated the volunteer image. As a result, it was possible to reduce the occurrence of artifacts at the maximum posture (10 degrees) of the shoulder as in the phantom experiment.
Conclusion : In neck CT scan, both arms are lowered and beam hardening artifact decreases in posture with both shoulders tilted as far as possible (10 degrees).
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KEYWORD
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Neck CT, Beam hardening artifact, self-made shoulder phantom, scapular angle
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