Introduction:
Using computed tomography (CT) is increasing significantly in Iran and worldwide. Reduction of CT's radiation dose is a critical concern. The aim of this study was evaluating the effect of localizer kVp on spiral CT images when the tube current modulation (TCM) is activated.
Materials and Methods:
A special wedding cake phantom with circular intersection and diameters of 15 to 40 cm was scanned on a GE VCT 64 slice scanner. The whole length of phantom (84cm) was scanned with kVps of 80,100,120 and 140. The image noise was measured in SD of Ct numbers and the radiation dose was measured in term of CTDIvol. The tube current variation through the Z-Axis was extracted from DICOM images.
Results:
Function of TCM was affected by changing the scout view parameters. By increasing the localizer kVp the average used tube current was decreased and image noise was increased (8-32%, p<0.05); especially for larger phantom diameters and wide kVp difference between localizer and spiral kVps (up to 32%). On the other hand, the localizer with lower kVp than spiral leads to higher CTDIVOLs and lower image noises. The image noise was closer to chosen Noise Index when the localizer and spiral scan were in the same kVps.
Conclusion:
The attenuation of localizer X-ray is the criteria of tube current changing in spiral CT. The attenuation is variable in different kVps of X-ray. The ability of TCM system was the best in offering the constant image noise when the localizer and spiral scans were in the same kVp. Inappropriate localizer kVp may increase the patients’ radiation dose or degrade the image quality by miscalculations in TCM system