Difficulty Differentiating Ablative Margin from Necrotic Tumor at MR Imaging with Impaired Ferucarbotran Clearance

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Editor: We read with great interest the article by Dr Mori and colleagues (1) about the determination of the ablative margin (AM) on magnetic resonance (MR) images by using impaired clearance of ferucarbotran, which appeared in the May 2009 issue of Radiology. We have also studied the effectiveness of this technique after radiofrequency (RF) ablation in nine patients with 11 hepatocellular carcinoma (HCC) nodules. In these patients, the results of this approach were compared with those of three-phase dynamic computed tomography (CT). The nodules were 8–33 mm in maximum diameter. All patients underwent RF ablation 1 hour after ferucarbotran (Bayer Schering Pharma, Berlin, Germany) injection. Unenhanced T2*-weighted MR images were acquired after 3–7 days, and dynamic CT imaging was performed at the same time. In five (45%) of the 11 nodules, distinguishing the AM from the necrotic tumor was difficult because the volume of the central hyperintense area had increased, making the necrotic tumor nonvisible on MR images. Because the AM cannot be calculated if the necrotic tumor is not visible, the thickness of the hypointense rim (2 mm) was considered to be the thickness of the AM. In the remaining six nodules (55%), the AM was considered to be distinguishable from the tumor, and the border between the tumor and the ablated peritumoral liver tissue could be identified. A nonenhancing area (the AM) surrounding accumulated lipiodol (the tumor) was visualized in all nodules on CT images. An AM of 5 mm or greater was an important factor for local control of HCC and survival (2). MR imaging with impaired clearance of ferucarbotran can provide a useful approach for determining whether there is an AM surrounding the tumor, but it cannot be used to determine whether the AM is large enough.