, half will evolve to fibrosis , 1015% to cirrhosis and 5.4% to hepatic insufficiency .
radiologists must report fatty liver disease because it may have serious long - term consequences if left untreated .
we searched our archive retrospectively , reviewed the literature and identified six patterns of liver steatosis : diffuse , geographic , focal , subcapsular , multifocal and perivascular .
diffuse forms may be graded subjectively according to severity ( mild , moderate , severe ) or quantitatively with magnetic resonance ( mr)-based methods .
therefore , previous knowledge of their cross - sectional appearance may prevent unwarranted investigations or allow non - invasive diagnosis .
ultrasound is the first - line imaging technique for the screening and follow - up of patients with chronic liver disease .
it is , therefore , important for radiologists to be aware of patterns of heterogeneous or focal forms of fatty deposition . on ultrasound , fatty liver
is hyperechoic compared with renal cortex and spleen , whereas fatty sparing is isoechoic or hypoechoic .
on unenhanced computed tomography ( ct ) , liver density less than 40 hounsfield units ( hu )   or a density difference of more than 10 hu between spleen and liver indicates fatty liver .
alternatively , liver parenchyma hypodensity relative to vessels establishes the presence of moderate to severe fatty liver disease . on portal phase contrast - enhanced ct with a standardised protocol
, a density difference greater than 25 hu between spleen and liver suggests fatty liver deposition .
ct with contrast material was previously thought to be less reliable in detecting fatty liver .
however , recent data suggest that portal phase contrast - enhanced ct using blood - subtracted hepatic attenuation may have a similar accuracy to or even greater accuracy than unenhanced ct in the diagnosis of fatty liver .
mr imaging ( mri ) allows measurement of fat and water proton signals and is considered the most accurate non - invasive technique for hepatic fat quantification .
chemical shift gradient - echo imaging is a widely available mri technique for fat detection and quantification .
fatty liver shows a signal drop on gradient echo t1-weighted out - of - phase images compared with in - phase images .
the dual - echo method results in underestimation of the fat fraction by only 2.9% compared with spectroscopy , the non - invasive  gold standard  .
it may have different levels of severity that can be determined by using quantification maps .
the presence of diffuse steatosis in a patient with hepatomegaly and an enlarged caudate - to - right lobe ratio should raise the possibility of nash . 
 fig .
d severe diffuse fatty liver disease . a ultrasound image in a 58-year - old man known for intraductal papillary mucinous neoplasm ( ipmn / ipmt ) of the pancreas and alcoholic pancreatitis showing a hyperechoic liver compared with the right renal cortex .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 42 hu ) compared with the spleen ( 113 hu ) ( > 25 hu difference ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image .
mr images were obtained on a 1.5-t mri system with tes of 2.2 and 4.5 for in- and out - of - phase images respectively a  d severe diffuse fatty liver disease . a ultrasound image in a 58-year - old man known for intraductal papillary mucinous neoplasm ( ipmn / ipmt ) of the pancreas and alcoholic pancreatitis showing a hyperechoic liver compared with the right renal cortex .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 42 hu ) compared with the spleen ( 113 hu ) ( > 25 hu difference ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image .
mr images were obtained on a 1.5-t mri system with tes of 2.2 and 4.5 for in- and out - of - phase images respectively geographic fatty liver disease is a frequently encountered variant .
for example , fat accumulation sometimes occurs only in the right lobe ( fig .  2 ) .
one hypothesis to explain this distribution is that blood from the superior mesenteric vein containing lipogenic alimentary factors is preferentially distributed to the right lobe of the liver .
, fatty liver deposition can be distributed in territories previously affected by cholangitis ( fig . 
fig . 2a , bgeographic fat accumulation limited to the right lobe of the liver .
mri of the liver in a 57-year - old woman known for multiple focal nodular hyperplasia .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop in the right lobe of the liver on opposed - phase imagefig
. mri of the liver in a 76-year - old man who had hepatico - jejunostomy for biliary duct trauma and stenosis followed by two episodes of cholangitis in 1997 and 2006 .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop on the opposed - phase image in a geographic distribution , affecting the hepatic dome more severely .
c subtraction confirms severe geographic fatty liver deposition a , bgeographic fat accumulation limited to the right lobe of the liver .
mri of the liver in a 57-year - old woman known for multiple focal nodular hyperplasia .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop in the right lobe of the liver on opposed - phase image a 
mri of the liver in a 76-year - old man who had hepatico - jejunostomy for biliary duct trauma and stenosis followed by two episodes of cholangitis in 1997 and 2006 .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop on the opposed - phase image in a geographic distribution , affecting the hepatic dome more severely .
c subtraction confirms severe geographic fatty liver deposition focal fat deposition is slightly less common and can mimic other hepatic benign or malignant lesions on ultrasound and ct ( fig .  4 ) .
mri is very useful for making the diagnosis of focal hepatic steatosis , which appears isointense or hyperintense to liver on in - phase images and loses signal on out - of - phase images .
fat accumulation does not show diffusion restriction , which may also be useful to differentiate steatosis from other liver abnormalities .
moreover , focal fat accumulation tends to show wedge  shaped margins , no mass effect on adjacent blood vessels or the biliary tree , sharp boundaries , and lobar or segmental distribution . 

a ct , axial contrast - enhanced portal phase image in a 58-year - old woman shows a hypodense , mildly heterogeneous , cuneiform lesion in the posterior aspect of segment iv .
mri , b axial t1-weighted in - phase and c out - of - phase images show a signal drop within this lesion on the opposed - phase image focal fatty liver deposition .
a ct , axial contrast - enhanced portal phase image in a 58-year - old woman shows a hypodense , mildly heterogeneous , cuneiform lesion in the posterior aspect of segment iv .
mri , b axial t1-weighted in - phase and c out - of - phase images show a signal drop within this lesion on the opposed - phase image some regions of the liver are more prone to being affected by focal fat deposition .
those are the perivesicular and subcapsular regions abutting the hepatic hilum or falciform ligament in the area drained by the sapey s vein [ 7 , 11 ] .
the classical patterns of focal fatty liver deposition and sparing are linked to vascular anatomical variants that involve capsular veins , peribiliary veins , the cystic vein ( which drains perivesicular areas ) , and the right gastric vein ( which drains the anterior portion of segment i and the posterior portion of segment iv ) .
these veins can be the source of an aberrant venous draining system without portal inflow causing local haemodynamic anomalies that may lead to focal fatty liver deposition and sparing [ 11 , 13 ] . 

d perivesicular fatty sparing . a ultrasound image in a 77-year - old woman who had nephrectomy for renal cell carcinoma shows a hyperechoic liver with hypoechoic perivesicular foci .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 57 hu ) in comparison with the spleen ( 135 hu ) with a perivesicular spared zone ( in segments iv and v ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image with the exception of a perivesicular spared zone a  d perivesicular fatty sparing . a ultrasound image in a 77-year - old woman who had nephrectomy for renal cell carcinoma shows a hyperechoic liver with hypoechoic perivesicular foci .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 57 hu ) in comparison with the spleen ( 135 hu ) with a perivesicular spared zone ( in segments iv and v ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image with the exception of a perivesicular spared zone subcapsular fat deposition may present as small fat nodules ( fig .  6 ) or as a confluent peripheral region of fat confined to a subcapsular zone .
it may be caused by insulin added to the peritoneal dialysate in certain patients with renal failure and insulin - dependent diabetes .
subcapsular hepatocytes are exposed to a higher concentration of insulin than the rest of the liver in those patients .
because insulin promotes the esterification of free fatty acids into triglycerides , the peritoneal administration of insulin can result in subcapsular fat accumulation .
a axial contrast - enhanced portal phase ct image in a 58-year - old man with a neuro - endocrine tumour of the pancreas shows two small hypodense subcapsular hepatic lesions in segment v. mri , b axial t1-weighted in - phase and c out - of - phase images show a drop of signal within the two subcapsular lesions on the opposed - phase image a 
a axial contrast - enhanced portal phase ct image in a 58-year - old man with a neuro - endocrine tumour of the pancreas shows two small hypodense subcapsular hepatic lesions in segment v. mri , b axial t1-weighted in - phase and c out - of - phase images show a drop of signal within the two subcapsular lesions on the opposed - phase image multifocal fatty liver deposition is another unusual pattern that can represent a difficult diagnostic challenge .
the differential diagnosis on ct and ultrasound may include metastasis , lymphoma , sarcoidosis , abscesses , candidiasis , haemangiomatosis and biliary hamartomas .
d multifocal fatty liver deposition . a ultrasound image in a 39-year - old woman with lymphoma and malignant thymoma with local pleural recurrence treated with radiotherapy and chemotherapy shows multiple nodular hyperechoic foci throughout the liver .
mri , c axial t1-weighted in - phase and d out - of - phase images show multiple lesions with signal drop on the opposed - phase image a  d multifocal fatty liver deposition . a ultrasound image in a 39-year - old woman with lymphoma and malignant thymoma with local pleural recurrence treated with radiotherapy and chemotherapy shows multiple nodular hyperechoic foci throughout the liver .
mri , c axial t1-weighted in - phase and d out - of - phase images show multiple lesions with signal drop on the opposed - phase image a very unusual pattern of fatty liver disease is perivascular deposition .
halos of fat surround the hepatic veins , the portal veins , or both hepatic and portal veins .
. it may be difficult to differentiate perivascular fat deposition or perivascular sparing from periportal oedema , passive hepatic congestion and budd  chiari syndrome on ct and ultrasound .
mri is useful for making this rare diagnosis by showing a signal intensity loss on opposed - phase images compared with in - phase images .
a ultrasound image in a 39-year - old man with a clinical suspicion of non - alcoholic fatty liver disease based on elevated liver enzymes shows a geographic area of fatty sparing parallel to the right hepatic vein .
b doppler image shows an absence of mass effect on the right hepatic vein a , b perivascular fatty sparing .
a ultrasound image in a 39-year - old man with a clinical suspicion of non - alcoholic fatty liver disease based on elevated liver enzymes shows a geographic area of fatty sparing parallel to the right hepatic vein .
it may have different levels of severity that can be determined by using quantification maps .
the presence of diffuse steatosis in a patient with hepatomegaly and an enlarged caudate - to - right lobe ratio should raise the possibility of nash . 
 fig .
d severe diffuse fatty liver disease . a ultrasound image in a 58-year - old man known for intraductal papillary mucinous neoplasm ( ipmn / ipmt ) of the pancreas and alcoholic pancreatitis showing a hyperechoic liver compared with the right renal cortex .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 42 hu ) compared with the spleen ( 113 hu ) ( > 25 hu difference ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image .
mr images were obtained on a 1.5-t mri system with tes of 2.2 and 4.5 for in- and out - of - phase images respectively a  d severe diffuse fatty liver disease . a ultrasound image in a 58-year - old man known for intraductal papillary mucinous neoplasm ( ipmn / ipmt ) of the pancreas and alcoholic pancreatitis showing a hyperechoic liver compared with the right renal cortex .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 42 hu ) compared with the spleen ( 113 hu ) ( > 25 hu difference ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image .
mr images were obtained on a 1.5-t mri system with tes of 2.2 and 4.5 for in- and out - of - phase images respectively
for example , fat accumulation sometimes occurs only in the right lobe ( fig .  2 ) .
one hypothesis to explain this distribution is that blood from the superior mesenteric vein containing lipogenic alimentary factors is preferentially distributed to the right lobe of the liver .
geographic patterns may be secondary to an insult to the liver parenchyma . for example , fatty liver deposition can be distributed in territories previously affected by cholangitis ( fig . 
mri of the liver in a 57-year - old woman known for multiple focal nodular hyperplasia .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop in the right lobe of the liver on opposed - phase imagefig
c severe geographic fatty liver deposition . mri of the liver in a 76-year - old man who had hepatico - jejunostomy for biliary duct trauma and stenosis followed by two episodes of cholangitis in 1997 and 2006 . a axial t1-weighted in - phase and
b out - of - phase images show an important signal drop on the opposed - phase image in a geographic distribution , affecting the hepatic dome more severely .
c subtraction confirms severe geographic fatty liver deposition a , bgeographic fat accumulation limited to the right lobe of the liver .
mri of the liver in a 57-year - old woman known for multiple focal nodular hyperplasia .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop in the right lobe of the liver on opposed - phase image a 
mri of the liver in a 76-year - old man who had hepatico - jejunostomy for biliary duct trauma and stenosis followed by two episodes of cholangitis in 1997 and 2006 .
a axial t1-weighted in - phase and b out - of - phase images show an important signal drop on the opposed - phase image in a geographic distribution , affecting the hepatic dome more severely .
focal fat deposition is slightly less common and can mimic other hepatic benign or malignant lesions on ultrasound and ct ( fig .  4 ) .
mri is very useful for making the diagnosis of focal hepatic steatosis , which appears isointense or hyperintense to liver on in - phase images and loses signal on out - of - phase images .
fat accumulation does not show diffusion restriction , which may also be useful to differentiate steatosis from other liver abnormalities .
moreover , focal fat accumulation tends to show wedge  shaped margins , no mass effect on adjacent blood vessels or the biliary tree , sharp boundaries , and lobar or segmental distribution . 

a ct , axial contrast - enhanced portal phase image in a 58-year - old woman shows a hypodense , mildly heterogeneous , cuneiform lesion in the posterior aspect of segment iv .
mri , b axial t1-weighted in - phase and c out - of - phase images show a signal drop within this lesion on the opposed - phase image focal fatty liver deposition . a ct , axial contrast - enhanced portal phase image in a 58-year - old woman shows a hypodense , mildly heterogeneous , cuneiform lesion in the posterior aspect of segment iv .
mri , b axial t1-weighted in - phase and c out - of - phase images show a signal drop within this lesion on the opposed - phase image some regions of the liver are more prone to being affected by focal fat deposition .
those are the perivesicular and subcapsular regions abutting the hepatic hilum or falciform ligament in the area drained by the sapey s vein [ 7 , 11 ] .
the classical patterns of focal fatty liver deposition and sparing are linked to vascular anatomical variants that involve capsular veins , peribiliary veins , the cystic vein ( which drains perivesicular areas ) , and the right gastric vein ( which drains the anterior portion of segment i and the posterior portion of segment iv ) .
these veins can be the source of an aberrant venous draining system without portal inflow causing local haemodynamic anomalies that may lead to focal fatty liver deposition and sparing [ 11 , 13 ] . 

d perivesicular fatty sparing . a ultrasound image in a 77-year - old woman who had nephrectomy for renal cell carcinoma shows a hyperechoic liver with hypoechoic perivesicular foci .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 57 hu ) in comparison with the spleen ( 135 hu ) with a perivesicular spared zone ( in segments iv and v ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image with the exception of a perivesicular spared zone a  d perivesicular fatty sparing .
a ultrasound image in a 77-year - old woman who had nephrectomy for renal cell carcinoma shows a hyperechoic liver with hypoechoic perivesicular foci .
b ct , axial contrast - enhanced portal phase image shows a diffusely hypodense liver ( 57 hu ) in comparison with the spleen ( 135 hu ) with a perivesicular spared zone ( in segments iv and v ) .
mri , c axial t1-weighted in - phase and d out - of - phase images show an important signal drop of the liver on the opposed - phase image with the exception of a perivesicular spared zone
6 ) or as a confluent peripheral region of fat confined to a subcapsular zone .
it may be caused by insulin added to the peritoneal dialysate in certain patients with renal failure and insulin - dependent diabetes .
subcapsular hepatocytes are exposed to a higher concentration of insulin than the rest of the liver in those patients .
because insulin promotes the esterification of free fatty acids into triglycerides , the peritoneal administration of insulin can result in subcapsular fat accumulation .
a axial contrast - enhanced portal phase ct image in a 58-year - old man with a neuro - endocrine tumour of the pancreas shows two small hypodense subcapsular hepatic lesions in segment v. mri , b axial t1-weighted in - phase and c out - of - phase images show a drop of signal within the two subcapsular lesions on the opposed - phase image a 
a axial contrast - enhanced portal phase ct image in a 58-year - old man with a neuro - endocrine tumour of the pancreas shows two small hypodense subcapsular hepatic lesions in segment v. mri , b axial t1-weighted in - phase and c out - of - phase images show a drop of signal within the two subcapsular lesions on the opposed - phase image
multifocal fatty liver deposition is another unusual pattern that can represent a difficult diagnostic challenge .
the foci are usually round or oval and may mimic multiple nodules . the differential diagnosis on ct and ultrasound may include metastasis , lymphoma , sarcoidosis , abscesses , candidiasis , haemangiomatosis and biliary hamartomas .
d multifocal fatty liver deposition . a ultrasound image in a 39-year - old woman with lymphoma and malignant thymoma with local pleural recurrence treated with radiotherapy and chemotherapy shows multiple nodular hyperechoic foci throughout the liver .
mri , c axial t1-weighted in - phase and d out - of - phase images show multiple lesions with signal drop on the opposed - phase image a  d multifocal fatty liver deposition . a ultrasound image in a 39-year - old woman with lymphoma and malignant thymoma with local pleural recurrence treated with radiotherapy and chemotherapy shows multiple nodular hyperechoic foci throughout the liver .
mri , c axial t1-weighted in - phase and d out - of - phase images show multiple lesions with signal drop on the opposed - phase image
halos of fat surround the hepatic veins , the portal veins , or both hepatic and portal veins .
. it may be difficult to differentiate perivascular fat deposition or perivascular sparing from periportal oedema , passive hepatic congestion and budd  chiari syndrome on ct and ultrasound .
mri is useful for making this rare diagnosis by showing a signal intensity loss on opposed - phase images compared with in - phase images .
a ultrasound image in a 39-year - old man with a clinical suspicion of non - alcoholic fatty liver disease based on elevated liver enzymes shows a geographic area of fatty sparing parallel to the right hepatic vein .
b doppler image shows an absence of mass effect on the right hepatic vein a , b perivascular fatty sparing .
a ultrasound image in a 39-year - old man with a clinical suspicion of non - alcoholic fatty liver disease based on elevated liver enzymes shows a geographic area of fatty sparing parallel to the right hepatic vein .
fatty liver deposition is a very common finding but it has many atypical patterns of distribution that can represent diagnostic pitfalls .
when an atypical form of fatty liver deposition is suspected on ultrasound or ct , mri may be used to confirm the diagnosis because of its ability to break down fat and water signal components .
 objectivefatty liver deposition is a very common finding , but it has many atypical patterns of distribution that can represent diagnostic pitfalls . the purpose of this pictorial essay is to review different patterns of fatty liver deposition and sparing.methodswe searched our archive retrospectively , reviewed the literature , and identified six patterns of liver steatosis.resultssteatosis may be diffuse , geographic , focal , subcapsular , multifocal or perivascular.conclusionsprevious knowledge of atypical patterns of steatosis distribution may prevent misdiagnosis of infiltrative disease or focal liver lesions . 
 when an unusual form of fatty liver deposition is suspected on ultrasound or computed tomography , magnetic resonance imaging may be used to confirm the diagnosis . 
Introduction
Diffuse fat deposition
Geographic fat deposition
Focal fat deposition and focal fatty sparing
Subcapsular fat deposition
Multifocal fat deposition
Perivascular fat deposition and perivascular fatty sparing
Conclusion