Nodular Fatty Deposition of the Liver Mimicking Hepatic Metastases

A 27-year-old Asian man with no significant medical history was found to have multiple liver lesions on routine ultrasound. Non-contrast computed tomography (CT) showed multiple ill-defined hypodense nodules measuring up to 3.5 cm (Figure 1a). Fluorodeoxyglucose positron emission tomography/CT (FDG PET/CT) revealed hypermetabolic nodules with a maximum standardized uptake value (SUV_max) of 3.8-4.2 and no identifiable primary tumor (Figure 1b), raising suspicion for metastases.

Figure 1.

Multifocal nodular fatty deposition mimicking hepatic metastases. Arrows mark only representative examples. (a) Non-contrast computed tomography (CT) shows multiple ill-defined hypodense nodules in the liver. (b) 18F-fluorodeoxyglucose positron emission tomography/CT demonstrates multiple hypermetabolic nodules with SUV_max 3.8-4.2. (c) T2-weighted fat-suppressed magnetic resonance imaging (MRI) shows multiple slightly hyperintense nodules. (d) DWI shows multiple hyperintense nodules. (e) The corresponding ADC map demonstrates high signal intensity within the lesions (ADC values approximately 1.8-2.2 × 10⁻³ mm²/s), consistent with T2 shine-through effect rather than true diffusion restriction. (f) Pre-contrast T1-weighted MRI shows the lesions as isointense to slightly hypointense relative to the surrounding liver parenchyma. (g) Contrast-enhanced venous phase shows mild enhancement with signal intensity lower than liver parenchyma. (h) Contrast-enhanced arterial phase shows lesions with higher enhancement than the surrounding liver parenchyma. (i) Hepatobiliary phase after administration of a liver-specific contrast agent shows no definite uptake in the lesions. (j) T1-weighted in-phase MRI shows some lesions slightly hyperintense and some isointense relative to liver parenchyma. (k) T1-weighted opposed-phase MRI is diagnostic: the background liver shows mild signal drop (mild diffuse steatosis), while the lesions demonstrate marked signal drop, confirming benign fatty deposition. Biopsy confirmed moderate fatty degeneration of hepatocytes with no malignancy. (l) Fat fraction image demonstrates high signal intensity of the lesions, confirming intracellular fat. (m) Histopathology of the liver biopsy specimen (hematoxylin and eosin staining, original magnification ×200) shows macrovesicular fatty degeneration of hepatocytes with clear cytoplasmic vacuoles (arrows) displacing nuclei to the periphery. No malignant cells or significant inflammatory changes are identified. SUV_max, maximum standardized uptake value; DWI, diffusion-weighted imaging; ADC, apparent diffusion coefficient.

Magnetic resonance imaging (MRI) was performed before biopsy for further characterization. T2-weighted fat-suppressed imaging demonstrated slightly hyperintense nodules (Figure 1c), whereas diffusion-weighted imaging showed hyperintense nodules (Figure 1d) with elevated apparent diffusion coefficient values (1.8-2.2 × 10^-³ mm²/s) and no evidence of true diffusion restriction, consistent with the T2 shine-through effect (Figure 1e). Precontrast T1-weighted imaging showed the lesions to be isointense to slightly hypointense (Figure 1f). Contrast-enhanced arterial-phase imaging revealed enhancement greater than that of the surrounding liver parenchyma (Figure 1g), whereas the venous phase showed mild enhancement, with signal intensity lower than that of the liver parenchyma (Figure 1h). The hepatobiliary phase after administration of a liver-specific contrast agent demonstrated no definite uptake in the lesions (Figure 1i). Based on these findings, the lesions were still interpreted as suspicious for metastases, and the patient was referred for an ultrasound-guided liver biopsy.

Biopsy revealed moderate fatty degeneration of hepatocytes with no evidence of malignancy. Retrospective review of the MRI subsequently revealed key diagnostic findings that had initially been overlooked. On in-phase imaging, some lesions appeared slightly hyperintense, whereas, others were isointense relative to the liver parenchyma (Figure 1j). On opposed-phase imaging, the lesions demonstrated a marked signal drop in the lesions, confirming the presence of intracellular fat and establishing the diagnosis; this critical finding had initially been overlooked (Figure 1k); and the fat-fraction image showed high signal intensity in the lesions, confirming intracellular fat (Figure 1l). Histopathological examination of the ultrasound-guided liver biopsy specimen confirmed moderate macrovesicular fatty degeneration of hepatocytes with no malignant cells, fibrosis, or significant inflammatory infiltrates (Figure 1m). Follow-up MRI performed at 3 months showed no interval change in the lesions.

In young patients without risk factors for malignancy, nodular fatty deposition should be considered in the differential diagnosis of multiple hepatic lesions, as it can closely mimic metastases on CT and PET/CT.¹ The presence of even mild FDG uptake (SUV_max, 3.8-4.2) can be misleading because benign steatotic lesions may show hypermetabolism due to inflammatory cell activity.² Opposed-phase MRI is highly diagnostic, with a marked signal drop on opposed-phase images confirming the presence of intracellular fat and reliably distinguishing benign fatty deposition from malignant lesions.³ When accurately interpreted, MRI alone may be sufficient for diagnosis and can help avoid unnecessary invasive procedures, as demonstrated in this case. Gadoxetic acid-enhanced MRI may provide additional supportive information; however, it is not always required for diagnosis.⁴