Small hypoattenuating hepatic lesions of 15 mm or less in diameter are a common incidental finding at contrast material–enhanced computed tomography (CT) of the abdomen (1).

http://pubs.rsna.org/doi/full/10.1148/radiol.2333031473

The purpose of this study was to determine the prevalence and significance of small low attenuating hepatic lesions (SLAHs) seen on helical CT in preoperative patients...

http://www.ncbi.nlm.nih.gov/pubmed/12439304

Most liver metastases are hypovascular compared with surrounding parenchyma and therefore most lesions appear either hypoattenuating or isoattenuating relative to the surrounding normal liver -

See more at: http://learningradiology.com/archives2007/COW 235-Liver mets...

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Note added at 1 hr (2015-09-29 07:23:45 GMT)
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https://books.google.es/books?id=3PFZX2r3Hj0C&pg=PA909&lpg=P...

'Rounded' is better than 'round'

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Note added at 1 hr (2015-09-29 07:26:18 GMT)
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Two more rounded, hypoattenuating lesions in the liver probably represent cysts and not trauma.

http://radiology.cornfeld.org/ED/Case r/text.htm