Diffusion-weighted and Gd-EOB-DTPA-contrast-enhanced magnetic resonance imaging for characterization of tumor necrosis in an animal model

Abstract

Purpose: To evaluate the role of diffusion-weighted magnetic resonance imaging (MRI) in determining tumor necrosis and contrast-enhanced MRI using gadoxetic acid disodium (Gd-EOB-DTPA) in determining maximum tumor size measurement and tumor delineation compared with criterion-standard histologic measurements in the rabbit VX2 liver tumor model.

Materials and methods: VX2 tumors were implanted in the livers of 13 rabbits. Magnetic resonance imaging was performed using a 1.5-T MRI scanner and an extremity coil. The imaging protocol included T2-weighted fast spin-echo images, 3-dimensional T1-weighted spoiled gradient-echo with and without fat suppression after administration of Gd-EOB-DTPA, and diffusion-weighted echo planar images. Rabbits were killed, and the tumor was harvested and sliced at 4-mm intervals in the axial plane. The MRI parameters evaluated were tumor size, tumor delineation, and tumor apparent diffusion coefficient (ADC) values. Histologic sections were evaluated to quantify tumor necrosis.

Results: On contrast-enhanced MRI (obtained from 11 rabbits), the mean tumor sizes were 20, 19, and 20 mm in the arterial, portal venous, and delayed phases, respectively. Tumor delineation was most distinguishable in the delayed phase. On diffusion-weighted MRI (acquired in 13 rabbits), the mean tumor ADC value was 1.84 x 10 mm/s. The mean tumor size at pathology was 16 mm. The mean percent necrosis at the tumor's pathologic condition was 36%. The correlation between ADC value and percent necrosis showed an R value of 0.68.

Conclusions: Contrast-enhanced MRI using Gd-EOB-DTPA may provide additional information about tumor outline in the liver. Moreover, we showed a remarkable correlation between ADC values and tumor necrosis. Thus, diffusion-weighted imaging may be useful to assess tumor necrosis; nevertheless, the search for new modalities remains important.

FIGURE 1

A, Typical example of a rabbit VX2 tumor in the left lateral lobe of the liver appearing heterogeneously bright on T2-weighted images. B, Diffusion-weighted MR image of the same lesion. After placing an ROI on the lesion, mean ADC value was calculated to be 2.93 × 10⁻³ mm²/s. C, The same tumor after hematoxylin-eosin staining consisting of necrotic (central) and viable (peripheral) areas.

FIGURE 2

Method of staining of the pathologic specimen to maintain proper anatomical orientation.

FIGURE 3

Correlation between ADC value and percent necrosis. R = 0.68.

FIGURE 4

Correlation between tumor size on contrast-enhanced MRI and tumor size on pathology.