Table 2.

Comparison of the CT phantom translational shifts and CT/CT registration shifts. The phantom “skin” was used as the 3D volumetric image registration landmark, and the visual color homogeneity was used as registration criterion. The registration shifts were calculated by physical distance $\left(\text{mm}\right)=\left(\text{voxel shift}\right)\times \left(\text{voxel size}\right)$, where $1.0\text{voxel}=0.78\text{mm}$. The registration was cross‐confirmed using the “skull” as landmark. The uncertainty of the CT couch positioning was specified within $\pm 0.5\text{mm}$ in longitudinal direction and within $\pm 0.1\text{mm}$ in lateral direction by the manufacturer. The phantom positioning had $\pm 0.1\text{mm}$ uncertainty by aligning reference lines of 0.13 mm in width under optical amplification.

Experimental Shifts (mm)		Registration Shifts (mm)				Statistical Analysis (mm)
Lateral	${\mathrm{X}}_{\text{Exp}}$	${\mathrm{X}}_1$	${\mathrm{X}}_2$	${\mathrm{X}}_3$	${\mathrm{X}}_4$	${\mathrm{X}}_{\text{Avg}}$	${\mathrm{X}}_{\text{Exp}}$‐${\mathrm{X}}_{\text{Avg}}$	St. Dev.
	$5.0\pm 0.1$	4.92	4.92	4.99	5.07	4.98	0.02	0.08
	$10.0\pm 0.1$	9.92	10.14	9.99	9.99	10.01	$-0.01$	0.09
	$15.0\pm 0.1$ Average	14.91	14.91	14.91	15.08	14.95	0.05	0.10
	Average						0.02	0.09
Longitudinal	${\mathrm{Z}}_{\text{Exp}}$	${\mathrm{Z}}_1$	${\mathrm{Z}}_2$	${\mathrm{Z}}_3$	${\mathrm{Z}}_4$	${\mathrm{Z}}_{\text{Avg}}$	${\mathrm{Z}}_{\text{Exp}}-{\mathrm{Z}}_{\text{Avg}}$	St. Dev.
	$5.0\pm 0.5$	5.00	4.68	4.61	4.68	4.74	0.26	0.04
	$10.0\pm 0.5$	9.76	9.53	9.92	9.59	9.70	0.30	0.21
	$15.0\pm 0.5$	14.84	14.52	15.15	14.44	14.74	0.26	0.39
	Average						0.27	0.21