Hypervascular Metastases

Hypervascular metastases derive from highly vascular tumors such as carcinoid, islet cell tumor, renal carcinoma, thyroid carcinoma, pheochromocy-toma, melanoma, and breast carcinoma. On unen-hanced T1-weighted images these lesions are usually hypointense, while on T2-weighted images they are slightly hyperintense and/or heterogeneous in SI compared to the background liver tissue. Some hypervascular metastases may have higher SI on T2-weighted images and thus mimic hemangiomas (Fig. 26) [16].

Whereas hypovascular lesions are best imaged during the portal venous phase, maximum enhancement of vascular metastases usually occurs during the arterial phase after Gd injection. In the portal-venous phase these lesions usually show some rapid peripheral wash-out which, on subsequent images, results in a hypointense halo appearance. Hence, lesion characterization is best performed during the equilibrium phase, when the pattern of enhancement and degree of contrast agent wash-out can be determined [ 16]. Larger hy-pervascular metastases receive arterial blood more in the periphery of the lesion than in the less perfused center, and may therefore demonstrate a peripheral rim of enhancement.

The dynamic enhancement patterns on Gd-BOPTA and Gd-EOB-DTPA-enhanced dynamic MR imaging is the same as that seen with conventional Gd-based contrast agents. However, whereas the SI between liver and lesions thereafter equilibrates with conventional Gd-based agents, on delayed hepatobiliary phase images after Gd-BOPTA and Gd-EOB-DTPA, all metastases demonstrate marked hypointensity compared to the surrounding enhanced normal liver parenchyma (Fig. 27) [2]. Although some retention of contrast agent may occasionally be observed in the necrotic center of the lesion on delayed images, repetition of the delayed sequence at 2 to 3 h post-injection when complete washout from the interstitial space has occurred is usually sufficient to confirm that this central enhancement is not due to hepatocytic accumulation (Figs. 28, 29).

Hypervasuclar metastatic lesions typically appear hypointense compared to the surrounding liver parenchyma also after infusion of man-gafodipir trisodium. However, uptake of Mn++ and a resulting hyperintense appearance has been observed after mangafodipir trisodium infusion in hepatic metastases from non-functioning endocrine tumors of the pancreas [25].

On SPIO-enhanced T2- or T2*-weighted images, hypervascular metastases appear hyperin-tense compared to the low SI of the surrounding normal liver parenchyma due to the lack of any significant uptake of SPIO particles [10,28] (Fig. 30).

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