Focal fatty change
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These in- and out-of-phase T1-weighted gradient-echo sequences show focal fat infiltration to the right of the fissure for the ligamentum teres. There is signal loss on out-of-phase images. Signal loss occurs when water and fat molecules occupy the same voxel, and the signals they return negatively interfere. This is due to the different precession speeds of fat and water in the same magnetic field. At 1.5 T, the water and fat signal are in phase when TE is an even multiple, and out of phase when TE is an odd multiple of 2.3 ms.
Focal fat infiltration is common. It is frequently seen as increased echogenicity on ultrasound, decreased density on CT, and increased T1 signal on MR. There is typically a geographic border and no distortion or displacement of vessels. Alcohol abuse is a common cause. Fifty percent of patients with poorly-controlled diabetes have evidence of fatty liver. Other causes are obesity, hyperlipidaemia, malabsorption, corticosteroids, total parenteral nutrition and pregnancy. Fatty liver is a reversible change, and will generally resolve once the cause is addressed. If the hepatic insult is ongoing, cirrhosis may result.