Steatosis Hepatis

Accumulation of fat within hepatocytes is commonly found in diabetic or obese patients or in patients who have been exposed to ethanol or other chemical toxins. In addition, in patients with advanced malignant neoplasms, fatty changes of the liver may be present due to poor nutrition and the hepatotoxic effects of chemotherapy. In most cases, fatty liver is associated with elevated levels of hepatic transaminases [48,74].

Fatty changes are distributed either diffusely, giving rise to a patchy pattern, or are present as focal lesions. Frequently, the deposition of fat in the liver reflects regional differences in perfusion. Regions with a decreased portal flow accumulate less fat than areas that have normal or increased perfusion [2].

Generally, fatty changes of the liver have a characteristic pattern. In cases of entire fatty infiltration of the liver, the medial segment of the left lobe adjacent to the falciform ligament tends to accumulate fat, while the other side of the medial segment adjacent to the portal vein usually does not show any pathological changes [78].A characteristic wedge-shape in certain regions of the liver is frequently indicative of fatty infiltration. Unfortunately, these findings are not sufficiently specific to enable a definitive diagnosis. With computed tomography (CT) imaging, the absence of a mass effect and the presence of normal vascular structures within the focal fatty lesions can be considered helpful for diagnosis, particularly in very large lesions. With magnetic resonance imaging (MRI), conventional Tl-weighted spin-echo sequences are relatively insensitive to fatty infiltration, with differences in signal intensity (SI) of only 5-15% between normal liver tissue and tissue containing at least 10% triglycerides.

T2-weighted images, moreover, are especially insensitive to fatty infiltration. Thus, the diagnosis of a diffuse or focal fatty infiltration of the liver is based upon the exclusion of a focal or diffuse pathological process. In particular, comparison of CT scans that show regions of decreased attenuation with T2-weighted MR images that show no pathological changes, and T1-weighted images that depict a slightly bright lesion, may indicate fatty infiltration. With the evolution of chemical shift imaging, an accurate differential diagnosis of fatty lesions in the liver is now possible. Thus, fatty infiltration of the liver no longer represents a major diagnostic problem for MRI [32, 77].

In chemical shift imaging, the signal from water and fat is separated on the basis of differences in resonance frequency. This allows an absolute diagnosis of focal or diffuse fatty infiltration. Apart from the case of lipoma of the liver, out-of-phase images are more sensitive for the detection of fatty liver than fat suppressed images. The intensity on out-of-phase images is determined from the absolute value of water intensity minus the fat intensity. Thus, whereas most tissues (including fat) appear similar on in-phase and out-of-phase images, fatty liver will be noticeably darker. if, for example, in a given case of focal fatty infiltration, fat accounts for 10% of the lesion's SI and water accounts for 90% on in-phase images, fat suppression, assuming that the fat signal is totally sup pressed, will lead to an image in which the lesion still has 90% of the signal. However, if out-of-phase imaging is applied, using the same repetition time (TR) as on in-phase images, the SI of the lesion will drop to 80% of the in-phase signal (90% water signal minus 10% fat signal) thereby making it easier to detect the signal drop on MR images.

In the case of liver lipoma which contains only fat, the signal of the lesion will not change to any noticeable extent on out-of-phase images. On the other hand, the SI of the lesion will drop significantly on fat suppressed images (Fig. 1-3).

Bopta Hepatic Phase

Fig. 1a-f. Diffuse focal fatty infiltration of the liver. A 54-year old female patient six months after high-dose chemotherapy for breast cancer. A follow-up CT study showed multiple hypodense liver lesions suspected to be metastases. Whereas T2-weighted (a) and T1-weight-ed (b) unenhanced images showed hardly any SI variations, out-of-phase images (c) show diffusely distributed hypointense areas (arrows). On dynamic and delayed imaging at arterial phase (d), portal-venous phase (e) and hepatobiliary phase (f) after Gd-BOPTA (0.05 mmol/kg BW), no regional changes of hepatic blood flow and no differences in Gd-BOPTA uptake can be seen, indicating that no focal liver lesions are present. Overall, MRI allows the diagnosis of diffuse focal fatty infiltration of the liver following high-dose chemotherapy

Fig. 1a-f. Diffuse focal fatty infiltration of the liver. A 54-year old female patient six months after high-dose chemotherapy for breast cancer. A follow-up CT study showed multiple hypodense liver lesions suspected to be metastases. Whereas T2-weighted (a) and T1-weight-ed (b) unenhanced images showed hardly any SI variations, out-of-phase images (c) show diffusely distributed hypointense areas (arrows). On dynamic and delayed imaging at arterial phase (d), portal-venous phase (e) and hepatobiliary phase (f) after Gd-BOPTA (0.05 mmol/kg BW), no regional changes of hepatic blood flow and no differences in Gd-BOPTA uptake can be seen, indicating that no focal liver lesions are present. Overall, MRI allows the diagnosis of diffuse focal fatty infiltration of the liver following high-dose chemotherapy

Bopta Hepatic Phase

Fig. 2a-d. Focal fatty infiltration of the liver in the area of the portal vein bifurcation in a 62-year old female patient with a history of sigmoid colorectal cancer. Routine follow-up US detected a focal liver lesion near the liver hilum, indicative of a metastasis of colorectal carcinoma. Both T2-weighted (a) and Tl-weighted (b) MR images reveal a focal hyperintense liver lesion (arrows). The high SI on both T1-weighted and T2-weighted images makes the diagnosis of a liver metastasis of colorectal carcinoma unlikely. Both out-of-phase (c) and fat suppressed Tl-weighted (d) images reveal a significant drop in SI, which allows the accurate diagnosis of focal fatty infiltration of the liver. In this case, the high fat content means that even fat suppressed Tl-weighted images show a significant drop in SI compared to the normal parenchyma

Fig. 2a-d. Focal fatty infiltration of the liver in the area of the portal vein bifurcation in a 62-year old female patient with a history of sigmoid colorectal cancer. Routine follow-up US detected a focal liver lesion near the liver hilum, indicative of a metastasis of colorectal carcinoma. Both T2-weighted (a) and Tl-weighted (b) MR images reveal a focal hyperintense liver lesion (arrows). The high SI on both T1-weighted and T2-weighted images makes the diagnosis of a liver metastasis of colorectal carcinoma unlikely. Both out-of-phase (c) and fat suppressed Tl-weighted (d) images reveal a significant drop in SI, which allows the accurate diagnosis of focal fatty infiltration of the liver. In this case, the high fat content means that even fat suppressed Tl-weighted images show a significant drop in SI compared to the normal parenchyma

Lipoblastoma

Fig. 3a-d. Diffuse fatty liver in a 26-year old obese female patient. Both T2-weighted (a) and Tl-weighted (b) unenhanced images reveal an unusually high SI of the liver parenchyma. In this case, fat suppressed Tl-weighted imaging (c) does not allow the diagnosis of diffuse fatty liver because the SI of the liver is not markedly different from that on Tl-weighted images without fat suppression. However, out-of-phase images (d) readily allow the diagnosis of diffuse fatty liver because of the dramatic reduction of liver parenchymal SI

Fig. 3a-d. Diffuse fatty liver in a 26-year old obese female patient. Both T2-weighted (a) and Tl-weighted (b) unenhanced images reveal an unusually high SI of the liver parenchyma. In this case, fat suppressed Tl-weighted imaging (c) does not allow the diagnosis of diffuse fatty liver because the SI of the liver is not markedly different from that on Tl-weighted images without fat suppression. However, out-of-phase images (d) readily allow the diagnosis of diffuse fatty liver because of the dramatic reduction of liver parenchymal SI

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Responses

  • HILDA
    Can focal fatty liver be mild T2 hyperintense?
    8 months ago

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