Steatosis and Steatohepatitis

 Often lobar, segmental, or wedge shaped


image Along hepatic vessels, ligaments, and fissures

image Presence of normal vessels coursing through “lesion” (fatty infiltration)


• NECT: Liver attenuation < spleen

• US: ↑ echogenicity, ↑ attenuation of sound beam

• Best imaging clue
image Decreased signal intensity of liver on T1 opposed-phase GRE images

• Nonalcoholic steatohepatitis (NASH) looks similar to simple steatosis and alcoholic steatohepatitis




TOP DIFFERENTIAL DIAGNOSES




• Lymphoma or metastases
image Diffuse or multifocal lesions can be seen with steatosis or tumor

• Hepatitis 
image Viral or other toxic etiologies

• Opportunistic infection, hepatic


PATHOLOGY




• Focal steatosis or sparing: Most commonly due to variations in hepatic venous drainage


CLINICAL ISSUES




• Most common cause of chronic liver disease in Western countries
image Increasing in prevalence with epidemic of obesity and metabolic syndrome

• NASH will likely become leading cause of cirrhosis and hepatocellular carcinoma in Western countries

image
(Left) Cut section of an explanted liver shows a yellowish, greasy, pale appearance due to steatosis. Steatohepatitis can lead to progressive and irreversible liver failure.


image
(Right) Axial NECT in the same patient shows diffuse low attenuation of the liver relative to the spleen and muscle. Note the area of fatty sparing around the gallbladder fossa image. The liver also shows signs of cirrhosis, with widened fissures and a prominent caudate lobe.

image
(Left) T1WI GRE in-phase MR of diffuse steatosis shows no apparent hepatic abnormality. Note the small right adrenal nodule image.


image
(Right) T1WI GRE opposed-phase MR in the same patient shows selective drop-out of signal from the liver, indicating the presence of excess lipid within the liver and also within an adrenal adenoma image. The most specific imaging test to detect hepatic steatosis is MR, especially GRE in- and opposed-phase images.


TERMINOLOGY


Synonyms




• Hepatic steatosis or hepatic fatty metamorphosis

• Nonalcoholic steatohepatitis (NASH)


Definitions




• Characterized by accumulation of increasing amounts of triglycerides within hepatocytes

• Steatosis is a metabolic complication of a variety of toxic, ischemic, and infectious insults to liver


IMAGING


General Features




• Best diagnostic clue
image Decreased signal intensity of liver on T1WI opposed-phase GRE images

• Location
image Focal, multifocal, or diffuse

• Key concepts
image Diffuse (more common) or focal fatty infiltration

image Often lobar, segmental, or wedge shaped
– More common along hepatic vessels, ligaments, and fissures

image Rarely, unifocal or multifocal spherical lesions, simulating metastases or primary tumor

image Fatty replacement occurs where glycogen is depleted from liver

image Key on all imaging modalities
– Presence of normal vessels coursing through “lesion” (fatty infiltration)

image Variable imaging features of fatty liver based on
– Amount of fat deposited in liver

– Fat distribution within liver

– Presence of associated hepatic disease
image e.g., porta hepatic lymphadenopathy and lymphedema in viral hepatitis

image In general, imaging cannot determine etiology of steatosis

image Nonalcoholic steatohepatitis (NASH) looks similar to simple steatosis and alcoholic steatohepatitis


CT Findings




• NECT
image Diffuse or focal
– Decreased attenuation of liver compared to spleen
image Hepatic attenuation is inversely proportional to degree of steatosis

– Normal: Liver 8-10 HU more than spleen on NECT
image High sensitivity (88-95%) and specificity (90-99%)

image In severe steatosis (> 30%), attenuation of hepatic vessels may be ≥ that of liver

– Normal liver attenuation: 50-65 HU
image Attenuation < 48 HU = steatosis

– Hepatic attenuation index: Calculation of ratio of hepatic to splenic attenuation
image < 0.8 = severe (> 30%) steatosis

image Focal fatty infiltration: Low attenuation
– Common location: Adjacent to falciform ligament

– Due to nutritional ischemia at vascular watershed

image Lobar, segmental, or wedge-shaped fatty infiltration
– May have straight-line margin

– Extending to liver capsule, usually without mass effect

– Often greater in right than left lobe
image Reflects greater portal venous flow to right lobe

• CECT
image Attenuation measurements and comparisons are less reliable than for NECT
– Dependent on timing relative to contrast administration
image Accuracy only ∼ 75-80% for mild to moderate steatosis

– Due to earlier enhancement of spleen (lack of portal blood supply)
image Arterial phase imaging is very unreliable

– On venous phase or delayed CECT, steatotic liver is usually > 35 HU less dense than spleen

image Normal vessels course through “lesion” (fatty infiltration)

image Dual-energy CT
– Scan at different tube currents (e.g., 80 + 140 kVp)

– Steatosis will be accentuated on lower kVp sequence


MR Findings




• T1WI in-phase gradient echo (chemical shift)
image Increased signal intensity of fatty liver > spleen

image On 1.5 Tesla (T) TEs of ∼ 4.6 (in phase) and 2.3 msec (opposed phase)

image On 3 T magnets, shorter TEs can be used
– Can detect + quantitate steatosis better than on 1.5 T MR or CT

• T1WI out-of-phase gradient echo
image Loss of signal intensity from foci of steatosis

image Signal is lost from voxels that contain both fat and water

• T1 C+ out-of-phase GRE image
image Paradoxical decreased signal intensity of liver

• Short T1 inversion recovery (STIR)
image Shows fatty areas as low signal intensity

• MR spectroscopy (MRS)
image Fatty liver demonstrates increase in intensity of lipid resonance peak

image Used for quantitative assessment of fatty infiltration of liver

• Multi-echo gradient echo MR
image Emerging method for reliable quantification of hepatic fat content


Ultrasonographic Findings




• Grayscale ultrasound
image Diffuse fatty infiltration
– Diffuse increased hepatic echogenicity

– Increased attenuation of ultrasound beam

– Normal liver echogenicity is slightly > that of kidney or spleen

– Steatosis may obscure visualization of diaphragm and intrahepatic vessels

– US grading of steatosis is subjective and prone to interobserver variation

image Hepatic steatosis and fibrosis frequently coexist
Nov 16, 2016 | Posted by in GASTROINTESTINAL IMAGING | Comments Off on Steatosis and Steatohepatitis

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