Multimodality imaging to assess immediate response to irreversible electroporation in a rat liver tumor model

Abstract

Purpose: To compare changes on ultrasonographic (US), computed tomographic (CT), and magnetic resonance (MR) images after irreversible electroporation (IRE) ablation of liver and tumor tissues in a rodent hepatoma model.

Materials and methods: Studies received approval from the institutional animal care and use committee. Forty-eight rats were used, and N1-S1 tumors were implanted in 24. Rats were divided into groups and allocated for studies with each modality. Imaging was performed in normal liver tissues and tumors before and after IRE. MR imaging was performed in one group before and after IRE after hepatic vessel ligation. US images were graded to determine echogenicity changes, CT attenuation was measured (in Hounsfield units), and MR imaging signal-to-noise ratio (SNR) was measured before and after IRE. Student t test was used to compare attenuation and SNR measurements before and after IRE (P < .05 indicated a significant difference).

Results: IRE ablation produced greater alterations to echogenicity in normal tissues than in tumors. Attenuation in ablated liver tissues was reduced compared with that in control tissues (P < .001), while small attenuation differences between ablated (42.11 HU ± 2.11) and control (45.14 HU ± 2.64) tumors trended toward significance (P = .052). SNR in ablated normal tissues was significantly altered after IRE (T1-weighted images: pre-IRE, 145.95 ± 24.32; post-IRE, 97.80 ± 18.03; P = .004; T2-weighted images, pre-IRE, 47.37 ± 18.31; post-IRE, 90.88 ± 37.15; P = .023). In tumors, SNR differences before and after IRE were not significant. No post-IRE signal changes were observed after hepatic vessel ligation.

Conclusion: IRE induces rapid changes on gray-scale US, unenhanced CT, and MR images. These changes are readily visible and may assist a performing physician to delineate ablation zones from the unablated surrounding parenchyma.

Figure 1:

Representative images acquired before and after IRE in normal hepatic parenchyma. A, Pre-IRE US image shows normal liver parenchyma. B, Post-IRE US image shows a hypoechoic region (X) in the liver parenchyma and represents the ablation zone. Arrow = IRE probe position (hyperechogenicity related to introduction of air at probe placement). C, Pre-IRE CT image shows normal liver parenchyma. D, Post-IRE CT image shows ablation zones as areas of hypoattenuation (X). E, Pre-IRE T1-weighted MR image shows normal liver parenchyma. F, T1-weighted MR image shows ablation zone as a hypointense region (X). G, Pre-IRE T2-weighted MR image shows normal liver parenchyma. H, T2-weighted MR image shows ablation zone as a hyperintense region (X).

Figure 2:

Representative images acquired before and after IRE in tumor tissues. A, Pre-IRE US image shows the tumor (+) appeared hyperechoic peripherally and isoechoic centrally when compared with normal hepatic parenchyma. B, Post-IRE US image shows normal and tumor tissues adjacent to the IRE probe (X) become hypoechoic. Arrow = probe position. C, CT image shows control tumor (arrow) as a hypoattenuating focus. D, Post-IRE CT image shows the ablation zone (X) as an area of hypoattenuation; a clear tumor boundary is difficult to determine. E, Pre-IRE T1-weighted MR image shows a hypointense tumor (arrow). F, Post-IRE T1-weighted MR image shows the ablation zone as a hypointense region (X) encompassing the tumor (arrow). G, Pre-IRE T2-weighted MR image shows hyperintense tumor (arrow). H, Post-IRE T2-weighted MR image shows a heterogeneously hyperintense region (X) encompassing a central hyperintense tumor (arrow).

Figure 3:

Representative hematoxylin-eosin–stained histologic slides from tumor tissues in, A, B, control rats, C, D, rats sacrificed 30 minutes after IRE, and E, F, rats sacrificed 24 hours after IRE. Nucleus agglutinations can be observed 30 minutes after IRE. Tumor cells are arranged more loosely in F than in B. The nucleus-to-cytoplasm ratio tended to increase 24 hours after IRE. □ = approximate locations for D, E, and F. Original magnifications are ×10 for A, B, and C and ×40 for D, E, and F.