Benign Focal Liver Lesions of nonHepatocellular Origin

Cavernous and Capillary Hemangioma

Hemangiomas represent tortuous caverns of blood-filled spaces that are usually well-circumscribed, ranging in size from a few millimeters to more than 20 cm. Microscopically, they are composed of multiple vascular channels lined by a single layer of endothelial cells supported by a thin, fibrous stroma.

Small (typically < 2 cm) hemangiomas that show complete and immediate filling with contrast material during the initial arterial phase have been referred to as capillary hemangiomas, but this distinction is without any clinical indication. Such small 'flash-filling' hemangiomas can simulate a vascular neoplasm if only the arterial phase characteristics are considered. However, the enhancement usually persists into the portal venous and delayed phases in capillary hemangiomas while contrast material wash-out usually occurs from small vascular tumors, rendering these lesions iso-or hypoattenuating/intense on later phases compared to the normal liver.

On cut sections, larger hemangiomas are almost always heterogeneous with areas of fibrosis, necrosis and cystic change; sometimes abundant fibrous tissue completely replaces the lesion.

Marked hyperintensity on T2-weighted images has historically been relied upon to establish the diagnosis of hemangioma particularly when using a long echo time (TE) (~ 120 ms). This is generally a useful technique, although high SI on heavily T2-weighted sequences with long TE may occasionally be seen in cystic tumors and uncommonly in hypervascular metastases from sarcoma, islet cell tumor, pheochromocytoma, carcinoid and renal cell carcinoma. On pre-contrast T1-weighted images hemangiomas are most commonly seen as well-defined and slightly hypointense with lobu-lated borders. In the majority of cases the combination of T2-weighted and serial dynamic T1-weighted Gd images allows a confident diagnosis of hemangioma [19,27].

The dynamic imaging characteristics of hemangiomas after administration of Gd-BOPTA and Gd-EOB-DTPA are the same as with conventional Gd agents. Specifically, peripheral nodular enhancement which progresses centripetally to uniform enhancement is typical for cavernous haemangiomas, whereas rapid and complete enhancement during the arterial phase is typical for smaller capillary haemangiomas. Hemangiomas usually appear hy-pointense compared to the surrounding liver parenchyma on delayed phase images after these agents, as well as on delayed images after man-gafodipir trisodium infusion (Fig. 25).

Because hemangiomas generally do not contain Kupffer cells, reticuloendothelial uptake of SPIO particles does not occur. However, uptake of USPIO particles does occur because of their blood-pool effect, resulting in a loss of SI on US-PIO-enhanced T2-weighted images. Dynamic

Fig. 25a-e. Cavernous hemangioma after Gd-BOPTA. The hemangioma (asterisk) is heterogeneously hyperintense on the SE T2-weighted image (a) and hypointense on the pre-contrast GRE Tl-weighted image (b). During the arterial phase (c) of the dynamic series after Gd-BOPTA injection, multiple focal areas of nodular enhancement (arrows) that are isointense with the aorta are seen in the periphery. Centripetal filling-in is seen during the equilibrium phase (d). During the delayed hepatobiliary phase after Gd-BOPTA administration, the lesion appears slightly heterogeneously hypointense (e)

phase T1-weighted imaging after administration of SH U 555 A reveals the hyperintense globular enhancement and centripetal filling-in that is typical of dynamic phase imaging after administration of Gd contrast agents. Hemangiomas frequently appear hyperintense on delayed images acquired 10 min after SPIO administration [26] (see Chap. 4,Fig.12).

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