mr imaging of liver

MRI OF LIVERDr.Parvathy S Nair

Role of MRI in liver imaging• Suspected Liver Metastases– CT is the imaging modality of choice– Diffuse liver disease and fatty infiltration limit the sensitivity

of CT in lesion detection. If the clinical suspicion of a lesion is high and the CT scan is negative –MR

• Preoperative Evaluation of Liver Tumors– Contrast-enhanced MRI is superior to CT - high sensitivity

for detecting small lesions and its ability to characterize small lesions.

– Tissue characterization with T1- and T2-weighted images, a high spatial resolution, and a high sensitivity for contrast enhancement contribute to the greater accuracy

• Screening for Liver Tumors in Diffuse Hepatic Disease– US can be used for initial screening, CT or MRI should be

used once cirrhosis sets in.– In patients with equivocal findings on CT, MRI is useful

for its ability to accurately identify fatty infiltration and small HCCs

• Incidentally Detected Lesions with Other Imaging Modalities

• Assessing Tumor Response to Surgery or Chemotherapy– The presence of enhancement along surgical margins

usually represents postoperative changes, but nodular enhancement or a discrete soft-tissue mass at the surgical margins indicates recurrence.

• A comprehensive MR imaging examination of the liver should include T1-weighted images,T2-weighted images and dynamic multiphasic contrast-enhanced images, preferably supplemented by chemical-shift images and fat-saturation images.

• Single-shot fast-spin echo (SSFSE) is recommended for coronal localizer images-HASTE

• Each of the images is obtained in <1 second, providing a rapid, motion-independent T2-weighted survey.

• These sequences give an overview of the liver, and differentiate solid lesions from cystic lesions or hemangiomas, particularly with the use of a heavily T2-weighted sequence

• Gradient-echo sequences are generally used for T1-weighted sequences.

• Spoiled gradient-echo (SGE) sequences allow acquisition of sufficient sections in one breath hold (about 20 sec) for complete liver coverage and provide good SNR- – FLASH

• Short tau inversion recovery (STIR) also may be used to acquire T2WI– these images have high soft-tissue contrast and

minimal signal from adipose tissue.

Dual Gradient-Echo In-Phase and Opposed-Phase

• This sequence is primarily used to detect and characterize focal as well as diffuse steatosis of the liver by using the chemical shift cancellation artifact (also known as fat-water cancellation artifact, black lining artifact, or India ink artifact).

• Protons in water and fat precess at different frequencies.

• Chemical shift• At certain TE they are in-phase and at other TE

they are out of phase• In phase TE images will have signals of water and

fat added together-both are bright • Out of phase-fat is subtracted.• In out of phase imaging fat containing lesions will

appear darker when compared to in-phase.• Only when the lesions contain both fat and water.

• The characterization of primary focal liver lesions such as hepatocellular carcinoma and adenoma can be improved by using the chemical shift cancellation artifact .

• These lesions may contain fat and water within the same voxel, which can be detected reliably with gradient-echo in- and opposed-phase MR imaging.

CONTRAST STUDIES

Post-contrast• T1 2D or 3D gradient echo sequences (e.g.

VIBE) at– arterial phase : 20 - 25 seconds– portal venous phase : 60 - 70 seconds– equilibrium phase : 3 - 5 minutes– hepatobiliary delayed phase : 10 - 30 minutes with

and without fat sat

Contrast Agents

• Extracellular agents • Reticuloendothelial agents

• Hepatobiliary agents

• Blood pool agents• Combined agents

Extracellular agents

• Extracellular agents are distributed within the extracellular interstitial space.

• Indications

– Lesion detection– Lesion characterization– Liver vasculature assessment.

• Gadolinium has seven unpaired electrons and is highly paramagnetic.

• Gadolinium shortens the T1 (spin-lattice) and T2 (spin-spin) relaxation times of adjacent water protons.

• These relaxation effects tend to cause signal enhancement at T1-weighted imaging and signal loss at T2-weighted imaging

• Appropriate evaluation of the liver requires imaging during the arterial phase of extra-cellular contrast enhancement, specifically the late hepatic arterial phase, when the portal vein is only slightly enhanced

Reticuloendothelial Agents

• Reticuloendothelial agents target the reticuloendothelial system, particularly the liver and spleen.

• Reflects the number of functioning macrophages

• Currently in clinical use include superparamagnetic iron oxide (SPIO) particles.

• SPIO particles are phagocytosed by macrophages throughout the body but are preferentially entrapped by Kupffer cells, which line the hepatic sinusoids.

• SPIO particles act as a negative contrast agent. Their superparamagnetic properties cause local magnetic field inhomogeneity and result in considerable T2 and T2* shortening.

• Tissues that accumulate SPIO particles thus show reduced signal intensity, particularly on T2- and T2*-weighted images, and to a lesser extent on T1-weighted images

• Most liver tumors—whether benign or malignant, primary or metastatic—are deficient in Kupffer cells and do not exhibit SPIO particle uptake

• Thus, after an infusion of SPIO particles, – Liver becoes preferentially darker– Tumors become relatively hyperintense

• The primary exception to this rule occurs in focal nodular hyperplasia, in which SPIO particles may accumulate, with a resultant isointense or even hypointense appearance of lesions in comparison with the normal liver parenchyma

Axial T1-weighted spoiled GRE image, obtained during the late hepatic arterial phase of enhancement after the administration of SPIO particles and gadolinium, shows increased conspicuity of the lesion (arrow).

Axial T1-weighted spoiled GRE image, obtained after the injection of SPIO particles and before that of gadolinium, shows negative enhancement of the background liver tissue and clearly depicts the lesion (arrow).

• SPIO particles are most useful when combined with gadolinium to create a double-contrast effect.

• With this technique, the SPIO particles are infused first and are followed later by gadolinium.

• The two agents synergistically improve lesion-to-liver contrast on dynamic T1- images because the background liver is darkened by the SPIO particles while the lesion of interest is lightened by gadolinium

Negative contrast enhancement effect of SPIO particles in a 56-year-old man with cirrhosis and HCC.

• Because mangafodipir is taken up only by hepatocytes, hepatocyte-containing masses (eg, HCC, focal nodular hyperplasia, and hepatic adenoma) appear enhanced in 75% of cases, whereas metastases generally are not enhanced

MRI in Diffuse Liver Diseases• Homogenous- involves

disorders of hepatocytes and reticuloendothelial cells– Hemochromatosis,steatosis

• Segmental- segmental fatty liver and focal confluent segmental fibrosis – subacute hepatitis – Fatty liver

• Nodular- multiple abnormal nodular intensities- includes disorders that cause numerous nodular lesions corresponding to iron deposits in regenerative nodules or granulomas– cirrhosis, Wilson's disease, and

sarcoidosis

• Perivascular involve the periportal lymphatic channel and Glisson's capsules congestive liver and Budd-Chiari syndrome are usually associated with periportal high signal intensity of the liver.

Hemochromatosis

• Hemochromatosis is an iron overload disorder resulting in hepatic parenchymal or reticuloendothelial deposition of iron.

• On T2WI, the superparamagnetic effect of iron causes decreased signal intensity of liver parenchyma in comparison with that of the paraspinal muscle

• Gradient-echo T2* WI are more sensitive to magnetic susceptibility effects. T1WI also show decreased signal intensity of liver parenchyma

DIFFUSE HOMOGENOUS DISTRIBUTION

T2-weighted MR image shows decreased signal intensity of liver, pancreas, and spleen when compared with that of paraspinal muscle.

Steatohepatitis• Pathologically, steatohepatitis demonstrates varying degrees

of steatosis (fatty liver), mixed cellular inflammatory infiltrate across the lobule, the presence of hepatocyte injury, and fibrosis.

• This condition is unrelated to alcohol abuse.

• Radiologically, steatohepatitis cannot be reliably distinguished from other causes of fatty liver.

• The presence of >33% fat on liver biopsy was shown to be optimal for detecting steatosis on radiological imaging

• Nonalcoholic steatohepatitis is sometimes associated with acute hepatic failure, and in a minority of patients, the disease progresses to cirrhosis.

• MR images-• diffuse homogeneous increased signal intensity

on in-phase images and• diffuse homogeneous low signal intensities on

opposed-phase T1-weighted images

Glycogen Storage Diseases

• Patients with this disease may also have hepatocellular adenoma, manifesting as a low-attenuation tumor on conventional CT images and as hyperintensity on T1- and T2-weighted MR images

36-year-old man with glycogen storage disease. T1-weighted spin-echo MR image shows low signal intensity of normal hepatic parenchyma compared with that of bone marrow.

Glycogen storage disease. T1-weighted spin-echo MR image reveals homogeneously increased signal intensity of hepatic parenchyma compared with that of bone marrow. Note round high-signal-intensity tumor (arrow) in lateral segment.

T2-weighted spin-echo MR image shows tumor with high signal intensity

Fatty Liver• Fatty change in hepatocytes occurs in patients with

diabetes mellitus, obesity, transplanted liver, alcohol abuse, and chemical toxicity.

• Segmental fatty liver is characterized by segmental distribution of fatty infiltration, depending on regional differences in perfusion.

• T1-weighted spin-echo MR images show slightly increased signal intensity resulting from fatty infiltration.

• In-phase MR images show similar intensity for normal liver parenchyma and fatty changes.

• Opposed-phase T1-weighted MR images are the most useful for detecting low signal intensity caused by fatty change.

SEGMENTAL DISTRIBUTION

Normal Liver

Fatty Liver

Subacute Hepatitis

• Subacute hepatitis is caused primarily by viral infection, such as hepatitis B or C, or by drug use.

• When the liver is severely damaged, parenchymal intensity is reduced on T1-weighted MR images and increased on T2-weighted MR images.

• Segmental atrophy may manifest as abnormal signal intensity (i.e., areas of focal confluent fibrosis with abnormal signal intensity and abnormal enhancement)

T1-weighted MR image reveals diffuse atrophy of liver with irregular surface and decreased signal intensity of left lobe (arrows).

T2-weighted MR image shows segmental increase in signal intensity of left lobe

Post contrast gradient-echo T1-weighted MR image -segmental delayed enhanced area probably corresponding to segmental fibrosis.

Liver Cirrhosis

• Cirrhosis, the chronic response to repeated episodes of hepatocellular injury, is characterized by regeneration and fibrosis.

• Common causes of cirrhosis include alcoholism and viral infections such as hepatitis B and C.

• The role of imaging in cirrhosis is the early detection of HCC and the differentiation of regenerative nodules from dysplastic nodules and HCC.

• Regenerative changes caused by cirrhosis appear throughout the liver as small round masslike structures.

• These lesions appear as hypointense nodules on T2-weighted MR images, are most apparent on gradient-echo images, and are believed to be caused by deposition of hemosiderin in the regenerative nodules.

NODULAR DISTRIBUTION

T2-weighted MR image reveals multiple regenerative nodules with low signal intensity

Wilson's Disease

• In patients with Wilson's disease the biliary excretion of copper is impaired, resulting in the accumulation of toxic levels of copper in the liver, brain, and cornea.

• Copper has no ferromagnetic effect on MR imaging because copper in hepatocytes may combine with proteins.

• The most common finding of Wilson's disease is cirrhotic change.

• Iron in regenerative nodules causes numerous small nodular intensities scattered throughout the liver on T2-weighted MR images.

T2-weighted spin-echo MR image shows numerous small low-signal-intensity nodules scattered throughout liver, probably corresponding to iron in regenerative nodules

Sarcoidosis• Sarcoidosis, a common systemic granulomatous disease,

occasionally involves the liver, spleen, and subdiaphragmatic lymph nodes.

• Noncaseating epithelioid granulomas with surrounding fibrosis are present in the periportal region and portal tracts

• T2-weighted MR images shows liver lesions as numerous areas of low signal intensity and faint patchy high-signal-intensity structures that corresponds to the enhanced areas seen on contrast-enhanced T1-weighted images.

• Such intensities may correspond histologically to granulomas with surrounding fibrosis.

T2-weighted MR image shows faint patchy or geographic high-signal-intensity lesions (large arrows) in liver. Numerous small low-signal-intensity nodules (small arrows) are also seen.

Gadolinium-enhanced gradient-echo T1-weighted MR image reveals patchy or geographically enhanced area

Congested Liver

Congested liver is a common complication of congestive heart failure, constrictive pericarditis, and right-sided heart failure resulting from pulmonary artery obstruction caused by lung cancer. T2-weighted MR images of patients with congested liver show periportal hyperintensity and prominent perivascular zones of diminished attenuation resulting from presumed perivascular lymphedema

MRI in focal liver lesions

Simple hepatic cyst. Axial contrast-enhanced 3D GRE T1-weighted VIBE MR image shows a simple cyst (arrow) with the typical features of nonenhancement and low signal intensity.

Hepatic cysts in a patient with Caroli disease. Axial contrast-enhanced 3D VIBE MR image shows cystic dilatation of the biliary ducts manifesting as multiple small, nonenhancing cysts. Note the presence of the “central dot sign” (arrow).

Regenerating Nodules

The lesions exhibit low signal intensity on both images and no enhancement after contrast material injection.

Hemangioma

• A hepatic haemangioma is a benign hypervascular liver lesion. It is the most common benign tumour of the liver, and the most common liver tumour overall

• Blood supply is predominantly hepatic arterial

MRI• T1 - hypointense relative to liver parenchyma• T2 - intensely hyperintense relative to liver

parenchyma • T1 C + (Gd) - often shows peripheral nodular

enhancement which progresses centripetally (inward) on delayed images. – haemangiomas tend to retain contrast on delayed (>5

minute) contrast-enhanced images.– atypical haemangiomas may demonstrate slightly altered

enhancement patterns.– in general delayed (1 hour) imaging may not be helpful,

since haemangiomas can have a variable appearance that ranges from hypointensity to diffuse and central enhancement.

A hepatic hemangioma with the typical findings of early peripheral nodular enhancement and progressive centripetal filling. These findings helped confirm the diagnosis.

PORTAL VENOUS PHASE

DELAYED PHASE

Dysplastic Nodule

• A dysplastic nodule is defined as a small cluster of hepatocytes with dysplasia indicating the presence of nuclear and cytoplasmic changes but without definite histologic criteria for malignancy.

• Premalignant.

• Variable MR appearance• Generally hypointense or, more commonly, hyperintense on T1-

weighted images and iso- or hypointense on T2-weighted images, without prominent arterial phase enhancement after contrast material administration.

• Dysplastic nodules are almost never hyperintense on T2-weighted images

Axial unenhanced and contrast-enhanced 3D GRE T1-weighted VIBE MR images

The lesions exhibit high signal intensity on both images and no enhancement after contrast material administration.

Hepatic adenoma• Hepatic adenoma is a focal benign proliferation of

hepatocytes within an otherwise normal liver. • MR imaging appearance of hepatic adenomas ranges from

mildly hypointense to hyperintense on T1-weighted images.

• The high signal intensity is due to the presence of fat or blood products.

• On T2-weighted images, hepatic adenomas have a nonspecific, heterogeneous, slightly hyperintense appearance.

• Immediate enhancement is seen on arterial phase images after intravenous gadolinium chelate administration but rapidly fades to near isointensity on subsequent images

An adenoma (white arrow) with typical immediate enhancement. Note also the focal area of hemorrhage (black arrow).

Focal Nodular Hyperplasia• Common benign tumor• The origin of FNH is thought to be due to hyperplastic

growth of normal hepatocytes with a malformed biliary draining system.

• Macroscopically, typical lesions demonstrate a tumour which is often quite large with well marginated margins.

• A prominent central scar is usually noted with radiating fibrous septae : <50% of cases.

• A large central artery is usually present with spoke-wheel like centrifugal flow(no portal veins).

• Kupffer cells are present

MRI is both sensitive (70%) and specific (98%).

• T1– iso to somewhat hypo intense– hypo intense central scar

• T2– iso to somewhat hyperintense– hyperintense central scar

• T1 C+ (Gd) : – gadolinium enhancement is similar to CT

intense early arterial phase enhancement – iso intense to liver on portal venous phase– central scar retains contrast on delayed scans

• T1 C+ (hepatobilliary contrast) : demonstrates enhancement

High T2 signal intensity with immediate lesion enhancement, poor enhancement of the central scar on the early phase image and increased enhancement of the scar on the delayed phase image

Hepatocellular Carcinoma• T1

– variable– iso or hyperintense compared to liver– hyperintensity may be due to

• intratumoural fat • decreased intensity in surrounding liver

• T1 C+ (Gd) • enhancement is usually arterial and may be brief

– rapid wash out becoming hypointense compared to remainder of the liver (96% specific). This is on account of the supply to HCCs being from the hepatic artery rather than portal vein

– rim enhancement may persist (referred to as a capsule)– the larger the lesion the more heterogeneous the enhancement

• T2 - variable, typically hyperintense

ARTERIAL PC WITH FAT SAT

ARTERIAL PHASE MINUS PRE CONTRAST

• A large HCC may have a number of characteristic features, such as a – mosaic pattern, – a tumor capsule, – extracapsular extension with formation of satellite

nodules, – vascular invasion, and – extrahepatic dissemination, including lymph node

and distant metastases

Hepatic Metastases

• Hepatic metastases can be classified according to their enhancement pattern into two categories: – hypovascular and hypervascular.

• Hypovascular– Colon, lung, prostate, gastric, and transitional cell

carcinomas• Hypervascular– Islet cell tumors, breast cancer, melanoma, thyroid

cancer, and carcinoid tumor

Hypovascular Metastases

• These metastatic lesions usually demonstrate low signal intensity on T1-weighted MR images and are iso-to hyperintense on T2-weighted images

• Delayed enhancement on contrast-enhanced images.

• Occasionally, they show early ring enhancement

Early phase contrast-enhanced 3D GRE T1-weighted VIBE MR image shows hypovascular metastases with low signal intensity and no enhancement (arrow).

Hypervascular metastases are best seen during the arterial phase of enhancement .Most of these lesions have high signal intensity on T2-weighted MR images

Lymphoma• Primary hepatic lymphoma is rare. • Secondary involvement of the liver can occur in up

to 50% of patients with non-Hodgkin lymphoma and 20% of patients with Hodgkin lymphoma.

• Lymphomatous lesions usually have low signal intensity on T1-weighted MR images and variable signal intensity on T2-weighted images.

• They usually enhance minimally just after gadolinium administration and may show faint peripheral rim enhancement on subsequent images.

Axial contrast-enhanced 3D GRE T1-weighted VIBE MR image demonstrates two small, well-defined hepatic lymphomatous deposits with faint marginal enhancement

Recent advances

• Recent advances in MRI have led to a growing interest in optimizing and applying functional MRI methods for assessment of liver disease. These methods include—but are not limited to— diffusion-weighted imaging (DWI), perfusion-weighted MRI, MR elastography (MRE), and MR spectroscopy (MRS).

• DWI- Restricted diffusion observed in patients with cirrhosis - reflects diminished capillary perfusion

• PWI-A prospective study of liver perfusion parameters showed increased arterial flow and distribution volume and decreased portal venous flow in patients with advanced fibrosis and cirrhosis

• MRE-have tested the usefulness of the technique for diagnosing hepatic fibrosis.

• MRS-for measuring hepatic fat content,– with many studies showing strong correlations

between MRS and histologic grade of steatosis

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