Metastases are moderately high in signal intensity on T2- weighted images and moderately low in signal intensity on T1-weighted images in general. The degree of enhancement of the metastases depends on their vascularity.
Avascular metastases appear as completely cystic or necrotic metastases and include ovarian cancer and posttreatment (chemotherapy, chemoembolization, or ablation) metastases. They are characterized by high signal intensity on T2-weighted images, low signal intensity on T1-weighed images, and lack of enhancement on arterial dominant and interstitial phase images. A thin lesional or perilesional enhancement in the margin of the metastases is often demonstrated in one of the phases post-contrast.
Hypovascular metastases are characterized by near isointensity or high signal intensity on T2-weighted images and low signal on T1-weighted images. After contrast, hypovascular metastases demonstrate minimal enhancement on arterial dominant-phase images, which tends to be more conspicuous on interstitial phase images (Semelka et al. 1994). Primary tumors that commonly result in hypovascular metastases include colorectal carcinoma, transitional-cell carcinoma, pancreatic ductal adenocarcinoma, small bowel adenocarcinoma, pulmonary carcinoma, bladder carcinoma, and prostate carcinoma (Semelka et al. 1994).
Isovascular metastases are characterized by lesional enhancement similar to background parenchyma on arterial dominant-phase images. On interstitial-phase images, isovascular metastases often, but not always, show a decrease in the degree of enhancement (washout), becoming more conspicuous. Isovascular metastases are generally well demonstrated on pre-contrast images, with high signal intensity on T2-weighted images or low signal intensity on T1-weighted images or both. This appearance is most often observed in metastases after chemotherapy, presumably reflecting an antiangiogenic effect. Most commonly, metastases from colon, thyroid, and endometrium may demonstrate isovascularity (Semelka et al. 1994).
Hypervascular metastases are generally high in signal intensity on T2-weighted images, low in signal intensity on T1-weighted images, and possess a moderate or intense peripheral ring of enhancement on early-phase images, comparable with the extent of enhancement of the pancreas and/or renal cortex. On interstitial phase images these metastases are the most likely to show centripetal enhancement and peripheral washout (Semelka et al. 1994). Hypervascular metastases are more conspicuous on arterial dominant phase images due to the great signal difference between intensely enhanced lesions and minimal enhancement of the background parenchyma. Small (<1.5 cm) hypervascular metastases are commonly homogeneously high in signal intensity on T2-weighted images, homogeneously low in signal intensity on T1- weighted images, and show either fading to background or washout.
Often, small hypervascular metastases (especially those <1.0 cm) are only evident on hepatic arterial dominant phase images, i.e., the lesion is isointense on T2- and T1-weighed images and interstitial phase postcontrast images (Semelka et al. 1994). The malignancies that most commonly result in hypervascular liver metastases include breast cancer, renal cell carcinoma, carcinoid tumor, islet cell tumor, thyroid carcinoma, adenocarcinoma of unknown primary site, leiomyosarcoma, and malignant melanoma.