Comparison of lymph node metastases assessment with the use of USPIO-enhanced MR imaging at 1.5 T versus 3.0 T in a rabbit model

Purpose: To prospectively compare the diagnostic performances of 1.5 T and 3.0 T ultrasmall superparamagnetic iron oxide (USPIO)-enhanced magnetic resonance imaging (MRI) for the detection of lymph node (LN) metastases in a rabbit model. Materials and Methods: Experiments were approved by the animal care committee. VX2 carcinomas were implanted into the thighs of 18 rabbits 3 or 4 weeks before MRI examinations. T2*-weighted 1.5 T and 3.0 T MRI was performed 24 hours after USPIO (2.6 mg/kg iron) administration. Two radiologists calculated signal intensity (SI) ratios (ie, the ratios of postcontrast to precontrast signal intensity) of each LN and also evaluated for the presence of a metastasis in the iliac and retroperitoneal LNs at 1.5 T and 3.0 T MRI. Student's t-test, receiver operating characteristic (ROC) curve analysis, and a Z test were used for the statistical analysis. Results: Metastases were confirmed in 45 of 80 LNs by histopathology. For benign LNs the mean SI ratio at 1.5 T was higher (0.538) than at 3.0 T (0.463) (P = 0.004). In qualitative analysis, 1.5 T and 3.0 T USPIO-enhanced MRI showed similar Az values (0.951 vs. 0.939; P = 0.296). The specificity of 3.0 T USPIO-enhanced MRI was higher (91.4% [32 of 35]) than that of 1.5 T imaging (82.9% [29 of 35]) (P = 0.013); however, the sensitivities (88.9% [40 of 45]) were the same in the both modalities. Conclusion: 3.0 T USPIO-enhanced MRI shows a higher specificity as compared to 1.5 T imaging without a significant difference of sensitivity in a rabbit VX2 model.