CT-ED conversion on a GE Lightspeed-RT scanner: influence of scanner settings

The influence of tube voltage (kV) and current (mA) on the resulting relationship of computed tomography number to electron density (CT-ED) was investigated for a wide-bore GE scanner. The influence of kV and mA scan settings were examined in combination with a 16-bit image reconstruction algorithm made available via the scanner software and which allowed resolution of CT numbers for high density materials. By using titanium and stainless steel inserts in an electron density phantom, mA variation was found to have minimal impact on the CT-ED relationship, whereas variation in kV led to significant differences in CT number for the high density materials. The scanner is also equipped with automatic tubecurrent modulation capabilities. The influence of automatic tube-current modulation on CT number was investigated for a range of materials in a phantom geometry. It was found that tube current modulation has negligible effect on CT number, though the changing dimension of the phantom did influence CT number of an aluminium insert for scans undertaken with both fixed and modulated tube currents. In light of evidence from other studies examining the influence of CT number on dose calculation, it is recommended that scanner settings and specific CT-ED look-up tables be considered when calculations will be required with high-density materials present.