Table I.

Measured CT number (in HU) of the ROIs and the background on CT images shown in Figure 1. The indices of ROIs are marked in Fig. 1(a). The indices of scenarios are marked in Fig. 1(b). The R-RMSEs are calculated by Eq. (10) with the ground truth obtained from the full-scan FBP images. The proposed SPIR method and TVR are applied on low-energy CT scans with different numbers of views. Note that, although the contrast rods have different linear attenuation coefficients at two different x-ray energies, their CT numbers in HU may be similar on the CT images of the two scans.

	ROI	High-E CT (First scan)	Low-E CT (Ground truth)	Low-E CT via SPIR				Low-E CT via TVR
Number of views		655	N/A	655	50	20	10	50	20	10
Scenario I	2	−58	−58	−59	−59	−60	−61	−59	−60	−46
	4	127	127	127	127	127	125	128	128	114
	6	62	63	63	63	63	65	63	61	51
	8	−113	−112	−113	−114	−114	−114	−114	−112	−95
II	3	508	1017	1017	1017	1016	1015	1018	1016	1004
	7	508	850	850	850	849	847	851	850	836
III	1	0	−87	−88	−88	−87	−83	−87	−85	−69
	5	1	94	96	96	96	92	95	95	87
Background	9	0	0	0	0	0	4	0	0	6
R-RMSE (%)				0.42	0.50	0.60	1.33	0.48	0.61	6.18