Getting tricked by focal fat infiltration in the liver
Hepatic steatosis is estimated to be present in 20-30% of the Western population. 7 Fatty livers are present with highly variable patterns of fat distribution. Fat can diffusely infiltrate the liver, but other patterns, such as geographical or nodular fat infiltration or sparing, can also be found. Focal fat deposits with a mass-like or nodular appearance that can mimic focal solid liver lesions and pose a diagnostic conundrum.
Common regions of focal fat accumulation are near the gallbladder, the falciform ligament, the posterior segment four and the anterior segment. 1 Focal fat deposition in these areas has been attributed to variations in the venous supply and drainage.8 Alcohol abuse, diabetes, hypoxia, or drug-induced changes have also been proposed as causes of focal fat deposition. 9
More atypical presentations of focal fat are frequently found in US and CT. Diagnosing focal fat can be difficult on these imaging modalities, and an additional MRI for further lesion characterisation is often mandated. Two helpful imaging features are 1) the lack of mass effect on the blood vessels and bile ducts and 2) a geographical shape (figure 4). The latter means that the lesion has irregular contours, resembling the contours of a continent on a map, rather than being mass-like. However, MRI can more definitely solve whether a lesion is real or reflects focal fat. All routine MRI livers include in-phase (IP) and out-of-phase (OOP) T1 images (figure 2), which can demonstrate the presence of microscopic fat or iron.
The liver parenchyma has the same signal intensity in normal livers on both IP and OOP images. However, in hepatic steatosis, triglycerides accumulate within the hepatocytes, resulting in increased intracellular fat, which will be apparent as regions of signal loss on the OOP compared to the IP images. Several hepatic lesions, such as hepatocellular carcinoma or hepatic adenomas, may contain fat. Correlation with other MRI sequences such as diffusion-weighted images (DWI) and contrast-enhanced sequences can help arrive at the proper diagnosis, as focal fat - unlike primary or secondary liver lesions- will not demonstrate diffusion restriction or enhancement.
Ultimately, focal liver fat is highly prevalent and can be present in peculiar patterns, sometimes mimicking sinister lesions. Familiarity with classic areas of focal fat infiltration and typical imaging features can frequently obviate an additional MRI. MRI can quickly and confidently confirm the presence of focal fat in atypical cases.
Figure 4 | Focal fat simulating a liver lesion
(a) CT showed numerous geographical areas (arrows) which were also demonstrated on MRI. (b) On the contrast-enhanced MRI sequences, vessels course through the area and the “lesion” is not enhancing. On T2 (c) and T1 IP (d) the area is hyperintense and shows a signal drop on the T1 OOP images (e) in keeping with fat. DWI (f) is a fat-suppressed sequence and as such the fatty area is hypointense.
Please log in with your myUEG account to post comments.