【Methods and materials】
In this study, the CT scanner used was Revolution GSI (GE Healthcare, Milwaukee, Wis, USA), which employs a single-source rapid-switching method( Fig. 7 ).
The QA phantom was set on the CT scanner, and the water part of the phantom was scanned( Fig. 8 ). Every time, the QA phantom was scanned immediately after the simplified calibration. Scans were performed in single energy mode and dual energy mode. Present scan parameters( Fig. 9 ).
The CT value of water was measured from the obtained single energy image (SEI) and VMI. The method of measuring the CT value is shown( Fig. 10 ). An ROI with a diameter of 100 mm was set at the center of the image reconstructed with display field of view (DFOV) 250 mm, and the CT value within the ROI was measured. CT values were measured using Advantage Workstation Ver. 4.6 (GE Healthcare, Milwaukee, Wis, USA), an image analysis workstation( Fig. 7 ). In each condition, measurement was performed on ten images, and the average value was recorded.
The research period is from April 15, 2019, to October 3, 2019.
The measured CT values were verified, and the accuracy of CT values in SEI and VMI was examined. The contents examined were the mean value, standard deviation, and variation of CT values. In the examination, SEI used an image of 120 kVp, and VMI used images of 40 keV, 70 keV, and 140 keV.
Also, from August 1, 2019, to September 30, 2019, we used VMI images in 10 keV steps from 40 keV to 140 keV and examined the effects of differences in VMI energy.
【Result & Discussion】
The result of the measurement is shown( Fig. 11, Fig. 12, Fig. 13, Fig. 14 ). The fluctuation of CT value was the smallest in SEI. The average CT value of SEI was 1.31, which cleared the IEC standard. The CT value of VMI was 60 keV, and the average CT value was -0.46, which was the closest to Null. The average CT value at each keV deviated from 0 as it moved away from 60 keV, and the fluctuation increased. The variation was more significant for low keV than for high keV. It was 50 keV to 90 keV that satisfied the IEC CT value accuracy range in VMI.
The change of CT value of each energy of VMI draws a curve similar in shape to that of Iodine(Fig. 15). The scanner used in this study creates Iodine density image data and water density image data, based on which VMI images are reconstructed. Therefore, it is suggested that iodine density image data may affect the CT value of water in VMI.
CT value of water in VMI may differ significantly from the CT value of SEI depending on the spectrum energy. Therefore, when a full calibration is performed (e.g., when installing a scanner, after replacing the X-ray tube), measure the CT value of the VMI, set the reference CT value, and then periodically We think that measurement is necessary.
We propose to measure the VMI CT value at the frequency specified in the IEC standard.
【Limitation】
This study cannot study only images of the rapid kV switching method. VMI's created by other scanning methods also need to be considered.