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Figure 4.7. Differentiation between DN and well-differentiated HCC by SPIO-enhanced MR imaging. Both well-differentiated HCC in a 64-year-old female (a) and DN in a 55-year-old male (e) exhibit homogeneous signal decrease on T2W-FSE after SPIO administration (b, f; arrows). Differentiation between HCC and DN is therefore difficult with this sequence. On T|W-GRE, well-differentiated HCC exhibits heterogeneous signal decrease (d, arrow), but DN exhibits homogeneous signal decrease (h, arrow). T|W-GRE thus aids differentiation between HCC and DN.

Figure 4.7. Differentiation between DN and well-differentiated HCC by SPIO-enhanced MR imaging. Both well-differentiated HCC in a 64-year-old female (a) and DN in a 55-year-old male (e) exhibit homogeneous signal decrease on T2W-FSE after SPIO administration (b, f; arrows). Differentiation between HCC and DN is therefore difficult with this sequence. On T|W-GRE, well-differentiated HCC exhibits heterogeneous signal decrease (d, arrow), but DN exhibits homogeneous signal decrease (h, arrow). T|W-GRE thus aids differentiation between HCC and DN.

surrounding liver. Small intracellular SPIO clusters in well-differentiated HCCs produce little SNR decrease on T2 W-GRE images, but cause significant SNR loss on T2W-FSE images. Conversely, large intracellular SPIO clusters in DNs cause prominent SNR loss on T2 W-GRE imaging, likely because of magnetic susceptibility effects related to fixed field inhomogeneity (Tanimoto et al., 2001). This is likely due to mechanisms of T2 shortening that are independent of magnetic susceptibility effects, but related to the greater free water interaction made possible by the relatively larger surface area of the small clusters (Tanimoto et al., 1994, 2001).

SPIOs are known to be a negative enhancer of MR images due to its strong T2 and T2 relaxation effects, but can be used as a positive enhancer on T-weighted images due to its strong Tx relaxation effect (Chambon et al., 1993). The diagnostic usefulness of SPIO-enhanced T1W-GRE imaging has been previously documented in tissue characterization (Takahama et al., 2003; Lee et al., 2003). Differentiation between hemangiomas and malignant tumors is sometimes difficult on the basis of their signal intensity on unenhanced or gadolinium-enhanced MR imaging. Differentiation between hemangiomas and solid masses relies on the combination of the T1 blood pool effect, which positively enhances hemangiomas, and the T2 effect, which negatively enhances hemangiomas and the surrounding liver (Montet et al., 2004) (Figure 4.8). Positive ring enhancement of focal hepatic lesions on SPIO-enhanced TjW-GRE images is known to be a sign of malignancy (Reimer and Tombach, 1998; Kanematsu et al., 2003). This feature is most typical of hepatic metastases (Figure 4.9), but is frequently seen in HCC as well (Kim et al., 2002). In our study, positive ring enhancement was noted in 13 of 18 HCCs (72%) (Tanimoto et al., 2005). There have been a few reports on the mechanism of ring enhancement (Kanematsu et al., 2003). Peritumoral Kupffer cell density was increased in the ring enhancement (+) group compared with the ring enhancement (-) group (Tanimoto et al., 2005). In the ring enhancement (+) group, the tumor size measured on T2W was

Figure 4.8. Liver hemangioma in a 60-year-old male. A hemangioma in S7 (arrow) exhibits low intensity on TjW-GRE (a, left) and high intensity on T2W-FSE (b, left). After SPIO administration, the tumor is positively enhanced on TjW-GRE (a, right) and negatively enhanced on T2W-FSE (b, right).

Figure 4.9. Liver metastasis from colonic cancer in a 52-year-old female: Ring enhancement. Contrast-enhanced CT exhibits a well-enhancing mass in S8, mimicking liver hemangioma (a, arrow). The T1W-GRE image exhibits a hypointense mass (b, arrow). After SPIO administration, positive ring enhancement is noted around the tumor (c, arrowheads).

Figure 4.9. Liver metastasis from colonic cancer in a 52-year-old female: Ring enhancement. Contrast-enhanced CT exhibits a well-enhancing mass in S8, mimicking liver hemangioma (a, arrow). The T1W-GRE image exhibits a hypointense mass (b, arrow). After SPIO administration, positive ring enhancement is noted around the tumor (c, arrowheads).

smaller than that measured on either T{W or T2* W, suggesting a sustained T relaxation effect and a decreased T2* relaxation effect on the peritumoral region. Ring enhancement on SPIO-enhanced T{W may correlate with increased Kupffer cell density and decreased SPIO clustering in Kupffer cells in the peritumoral region.

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