Qualitative assessment of liver for fatty infiltration on contrast-enhanced CT: is muscle a better standard of reference than spleen?

PURPOSE: Our goal was to determine whether spleen or muscle can be used as a qualitative standard of reference for diagnosing fatty infiltration of liver on contrast-enhanced CT.
METHOD: Qualitative visual comparisons and quantitative region-of-interest measurements of liver, spleen, and muscle were made on scans of 96 patients who underwent dynamic CT before and after injection of intravenous contrast material. As the standard of reference, the portion of liver assessed was considered fatty if its attenuation measured less than spleen on noncontrast CT.
RESULTS: In 16 (17%) scans, the portion of liver assessed was fatty on noncontrast CT. After contrast material administration, the attenuation of that portion of liver measured less than splenic attenuation in 93 (97%) of 96 cases (including all 16 fatty livers). Only four (25%) fatty livers, and no nonfatty livers, were visually judged to be less attenuating than muscle after contrast material; these four were the most fatty shown on noncontrast CT. Comparing hepatic and splenic attenuation on postcontrast CT resulted in a specificity of 30% and a positive predictive value of 20%; comparing hepatic and muscle attenuation on postcontrast CT yielded corresponding values of 100 and 100% but a sensitivity of 25%.
CONCLUSION: For the visual assessment of fatty liver, spleen is not an accurate reference standard on contrast-enhanced CT. However, fatty liver can be diagnosed on contrast-enhanced CT if liver appears less attenuating than muscle-a situation that occurs only if fatty infiltration is pronounced.