Iodine quantification with dual-energy CT: phantom study and preliminary experience with VX2 residual tumour in rabbits after radiofrequency ablation

Abstract

Objective: The purpose of our study was to validate iodine quantification in a phantom study with dual-source dual-energy CT (DECT) and to apply this technique to differentiate benign periablational reactive tissue from residual tumour in VX2 carcinoma in rabbits after radiofrequency ablation (RFA).

Methods: We applied iodine quantification with DECT in a phantom and in VX2 carcinoma in rabbits after incomplete RFA to differentiate benign periablational reactive tissue from residual tumour and evaluated its efficacy in demonstrating response to therapeutic RFA. A series of tubes containing solutions of varying iodine concentration were scanned with DECT. The iodine concentration was calculated and compared with known true iodine concentration. Triple-phase contrast-enhanced DECT data on 24 rabbits with VX2 carcinoma were then assessed at Day 3 (n=6), 1 week (n=6), 2 weeks (n=6) and 3 weeks (n=6) after incomplete RFA independently by 2 readers. Dual-energy postprocessing was used to produce iodine-only images. Regions of interest were positioned on the iodine image over the lesion and, as a reference, over the aorta, to record iodine concentration in the lesion and in the aorta. The pathological specimens were sectioned in the same plane as DECT imaging, and the lesion iodine concentration and lesion-to-aorta iodine ratio of residual tumour and benign periablational reactive tissue were assessed.

Results: There was excellent correlation between calculated and true iodine concentration (r=0.999, p<0.0001) in the phantom study. The lesion iodine concentration and lesion-to-aorta iodine ratio in residual tumour were significantly higher than in benign periablational reactive tissue in the 2-week group during the arterial phase (AP) (p<0.01) and in the 3-week group during both the AP (p<0.05) and the portal venous phase (p<0.05). There was no significant difference between them with respect to the lesion iodine concentration or lesion-to-aorta iodine ratio in the 3-day and 1-week groups.

Conclusion: Iodine quantification with DECT is accurate in a phantom study and can be used to differentiate benign periablational reactive tissue from residual tumour in VX2 carcinoma in rabbits after RFA.

Advances in knowledge: Iodine quantification with DECT may help in differentiating benign periablational reactive tissue from residual tumour in VX2 carcinoma in rabbits after RFA.

Figure 1

Iodine phantom. (a) Photograph shows iodine phantom and test tubes consisting of 10 solutions of varying iodine concentration ranging from 0 to 10 mg I ml⁻¹. (b) Axial dual-energy CT (DECT) iodine overlay image of the phantom with varying iodine concentration ranging from 0 to 10 mg I ml⁻¹ produced by a three-material decomposition algorithm showing 80 kVp and 140 kVp greyscale information combined at a 50:50 ratio with iodine colour-encoded overlay. (c) Scatter plot reveals excellent correlation between the iodine concentration measured with DECT and the true iodine concentration in the phantom (r=0.999, p<0.0001). (d) Bland–Altman plot shows differences between true and observed iodine concentrations vs average of true and observed concentrations. The dashed line represents a mean error of 0.11 mg ml⁻¹. The two solid lines represent the 25th–75th percentiles of the limits of agreement. The error in measured iodine concentration with DECT is less than the mean error for concentrations in the physiological range.

Figure 2

Images of a rabbit with VX2 tumour after radiofrequency ablation (RFA). Contrast-enhanced dual-energy CT image (a) and iodine map (b) show residual tumour (thin arrow) and the inflammation tissue (thick arrow) after RFA. (c) Photomicrograph shows viable tumour, mixture of inflammatory granulation tissue and necrotic areas (haematoxylin–eosin stain, ×200).

Figure 3

Box plots of the lesion iodine concentration (a) and lesion-to-aorta iodine concentration ratio (b) in residual tumours and inflammation tissues in the 2-week group during the arterial phase. The top and bottom of the boxes represents the 25th–75th percentiles of the data values. The line in each box represents the median value.

Figure 4

Changes in iodine quantification of residual tumours and inflammation tissues obtained from Day 3 to Week 3 after radiofrequency ablation (RFA). The lesion iodine concentration and lesion-to-aorta iodine ratio of the residual tumour were significantly higher than that of the benign periablational reactive tissue in the 2-week and 3-week groups during the arterial phase (a,b). The lesion iodine concentration and lesion-to-aorta iodine ratio of residual tumour were significantly higher than that of the benign periablational reactive tissue in the 3-week group during the portal venous phase (c,d). There was no significant difference between them in the 3-day and 1-week groups, respectively. Enhancement of inflammatory reaction appeared to be time-dependent and showed a declining trend.