Hyperechoic liver nodules: characterization with proton fat-water chemical shift MR imaging

Focal fat deposition in the liver: diagnostic challenges on imaging

While focal fat deposition in the liver mostly occurs in typical locations related to non-portal venous supply, unusual patterns of focal fat deposition, including multi-nodular, mass-like, and perivascular patterns, mimic malignancies and cause diagnostic challenges. Patients with unusual focal fat deposition often have potential underlying etiologies such as diabetes, alcohol abuse, metabolic disease, or various medications/chemotherapy. Some cases can be explained by non-portal venous supply or ischemia. Chemical-shift MRI or contrast-enhanced ultrasound (CEUS) is useful for non-invasive diagnosis of focal fat deposition. We illustrate a series of US, CT, and MR imaging features of focal fatty deposition in the liver mimicking other conditions and seek possible causes. Understanding of imaging patterns of focal fat deposition and its potential causes can help a non-invasive diagnosis by performing confirmatory imaging tests and prevent unnecessary invasive procedures.

Comparison of MRI features for the differentiation of hepatic angiomyolipoma from fat-containing hepatocellular carcinoma

OBJECTIVE

The purpose of this study was to evaluate MRI features for the differentiation of hepatic angiomyolipoma (HAML) from fat-containing hepatocellular carcinoma.

METHODS

We retrospectively reviewed the MRI findings of 20 patients with 22 hepatic angiomyolipomas and 25 patients with fat-containing hepatocellular carcinomas before surgery. The MRI features and apparent diffusion coefficient (ADC) for the two types of tumors were compared and analyzed.

RESULTS

Fat was not detected in nine (40.9%) of the angiomyolipomas. An enhancement pattern of the washout area was seen in eight (36.4%) of the angiomyolipomas and 21 of the hepatocellular carcinomas (84%) (p = 0.001). The sensitivity, specificity, and accuracy of the enhancement pattern for HAML were 63.6% (14/22), 84% (21/25), and 74.5% (35/47), respectively. An early draining vein was seen in 16 (72.7%) angiomyolipomas and two hepatocellular carcinomas (8%) (p < 0.001). The sensitivity, specificity, and accuracy of an early draining vein for detecting HAML was 72.7% (16/22), 92% (23/25), and 83.0% (39/47), respectively. Tumor vessels were noted in 18 (81.8%) angiomyolipomas and six hepatocellular carcinomas (24%) (p < 0.001). The sensitivity, specificity, and accuracy of tumor vessels for HAML were 81.8% (18/22), 76% (19/25), and 78.7% (37/47), respectively. Pseudocapsules were absent in 21 (95.5%) angiomyolipomas as compared with 3 (12%) hepatocellular carcinomas (p < 0.001). The sensitivity, specificity, and accuracy of pseudocapsules for HAML were 95.5% (21/22), 88% (22/25), and 91.5% (43/47), respectively. The ADC of the angiomyolipomas (1.92 ± 0.29 × 10(-3 )mm(2)/s) was significantly higher than that for hepatocellular carcinomas (1.33 ± 0.25 × 10(-3 )mm(2)/s) (p < 0.001).

CONCLUSION

The presence of an early draining vein and tumor vessels, the absence of pseudocapsules and a higher ADC in the hypervascular hepatic tumor on the MRI were helpful for the differentiation of hepatic angiomyolipoma from fat-containing hepatocellular carcinoma.

Renal artery embolization: application and success in patients with renal cell carcinoma and angiomyolipoma

Renal artery embolization is a procedure primarily performed by interventional radiologists that can be utilized for treatment of renal tumors, both malignant and benign. It has many applications, including pretreatment of renal cell carcinomas prior to planned resection to decrease hemorrhagic complications intraoperatively, treatment of malignant renal tumor in patients who are not deemed suitable surgical candidates, as well as treatment of benign renal tumors and their potential hemorrhagic complications. There are many different techniques. We describe how the procedure is approached at the University of Florida-Gainesville and provide examples of two cases, a renal cell carcinoma and an angiomyolipoma, treated at our institution with transcatheter embolotherapy.

Methods for assessing intrahepatic fat content and steatosis

PURPOSE OF REVIEW

Intrahepatic fat content is increasingly being recognized as an integral part of metabolic dysfunction. This article reviews available methods for the assessment of hepatic steatosis.

RECENT FINDINGS

Apart from liver biopsy, there are several noninvasive radiologic modalities for evaluating nonalcoholic fatty liver disease. Ultrasonography, computed tomography, and traditional MRI remain largely qualitative methods for detecting mild to severe degrees of steatosis rather than quantitative methods for measuring liver fat content, even though novel attempts to collect objective quantitative information have recently been developed. Still, their sensitivity at mild degrees of steatosis is poor. Undoubtedly, most methodological advances have occurred in the field of MRI and magnetic resonance spectroscopy, which currently enable the accurate quantification of intrahepatic fat even at normal or near normal levels. Xenon computed tomography was also recently shown to offer another objective tool for the quantitative assessment of steatosis, although more validation studies are required.

SUMMARY

Several modalities can be used for measuring intrahepatic fat and assessing steatosis; the choice will ultimately depend on the intended use and available resources.

Fatty hepatocellular carcinoma: radiofrequency ablation--imaging findings

PURPOSE

To describe the imaging features during follow-up after radiofrequency (RF) ablation of fat-containing hepatocellular carcinoma (HCC).

MATERIALS AND METHODS

Institutional review board approval was obtained; informed consent was waived. A retrospective search in an electronic radiologic archive was performed for a 40-month period between February 2004 and May 2007 to identify patients who had undergone RF ablation of fat-containing HCCs. The presence of intratumoral fat was determined at imaging (magnetic resonance or computed tomography) prior to the RF procedure; eight fat-containing HCCs, which had a mean size of 25 mm (range, 20-30 mm), were found. Images during follow-up were reviewed and compared with images prior to RF ablation to determine changes in fat content, complete or partial ablation, and local tumor progression. Tumor response was on the basis of assessment of lesion characteristics and enhancement for a follow-up of at least 6 months.

RESULTS

Persistent fat content was found at imaging in all ablation zones. Six patients were considered to have completely ablated tumors (mean follow-up, 16 months; range, 6-29 months), and two patients had local progression (mean follow-up, 18 months; range, 14-22 months). In the ablation zone of completely ablated tumors, the fat content progressively decreased (n = 4) or was unchanged during follow-up (n = 2). In the two tumors with local progression, the fat portion enlarged (n = 1) or did not change after ablation (n = 1).

CONCLUSION

Persistence of fat in the ablation zone during imaging follow-up after RF ablation of fat-containing HCCs does not necessarily indicate treatment failure. Changes in fat content of the ablation zone during follow-up (increase or decrease in size) could be used as additional criteria to determine success or failure of RF ablation in fat-containing HCC.

MR Imaging of the Adrenal Glands: 1.5T versus 3T

MR imaging at 1.5T is considered the prime cross-sectional imaging modality for characterization of adrenal lesions. This is of utmost clinical importance, because non-functioning adenoma and adrenal metastasis are fairly common. The differentiation of these two tumor entities primarily is based on chemical shift imaging, also known as dual echo in-phase and opposed-phase imaging. At 3.0 T, the echo time pairs for in-phase and opposed-phase MR imaging need to be adjusted because the frequency difference is double that of standard 1.5T MR systems. Unfortunately, the acquisition of the first opposed-phase echo at 1.1 milliseconds and the first in-phase echo at 2.2 milliseconds within the same breath-hold requires unacceptably high receiver bandwidths at 3.0 T. Therefore, alternative data collection schemes have been implemented. This article reviews the current literature regarding adrenal imaging at 3.0 T with a focus on the chemical shift technique.

Enhancement patterns of focal liver masses: discordance between contrast-enhanced sonography and contrast-enhanced CT and MRI

OBJECTIVE

The purpose of this study was to investigate the origin of the infrequent discordance between the contrast enhancement patterns of liver lesions on sonography and those on CT and MRI. Forty-four discordant cases were reviewed retrospectively.

CONCLUSION

Four categories of discordance were identified, one of which is unexplained. Contrast agent diffusion caused portal venous phase discordance in malignant tumors (n = 6) whereby CT and MRI contrast material diffused through the vascular endothelium into the tumor interstitium, concealing washout. Sonographic microbubbles were purely intravascular and showed washout. Arterial phase timing discordance occurred in metastatic lesions (n = 10) with hypervascularity and rapid washout on contrast-enhanced sonography. CT arterial imaging performed later showed hypovascularity. Rapidly enhancing hemangiomas (n = 7) exhibited hypervascularity on CT when contrast-enhanced sonography also showed peripheral nodules and fast centripetal progression. Discordance caused by fat in lesions (n = 4) or liver (n = 10) reflected the inherent echogenicity of fat on sonography compared with its low attenuation on CT and low signal intensity on MRI. Infrequent cases of discordance remain unexplained. Recognition of the cause of the infrequent disagreement in enhancement patterns on contrast-enhanced sonography with those on CT and MRI improves diagnostic interpretation.

Fat-containing nodules in the cirrhotic liver: chemical shift MRI features and clinical implications

OBJECTIVE

The purpose of this study was to determine the ability of MRI to predict malignancy in fat-containing nodules in the cirrhotic liver.

MATERIALS AND METHODS

In 38 patients with cirrhotic livers, focal lesions > or = 5 mm containing fatty components were identified on chemical shift gradient-echo MRI. Positive predictive values (PPVs) for benignity and malignancy were calculated on the basis of lesion size, T1-weighted hypointensity, T2-weighted hyperintensity, and arterial hypervascularity on the initial MR images. The number of the fatty nodules (group A, up to 4; group B, numerous) in individual patients was also correlated with the malignant potential of the lesions that were verified pathologically or by follow-up imaging studies.

RESULTS

In 31 group A patients, 21 (47%) of the 45 lesions showed a malignant course, and their mean diameter (18.8 mm) was larger (p = 0.007) than that (10.5 mm) of benign lesions. In seven group B patients, all 35 lesions (the five largest lesions in each patient; mean diameter, 7.8 mm) proved to be benign. The PPV of larger (> or = 15 mm) fat-containing nodules for malignancy was 85% (11/13 lesions). Six (55%) of 11 immediately diagnosed hepatocellular carcinomas were entirely hypointense on unenhanced in-phase T1-weighted images. The PPV of T2-weighted hyperintensity and arterial hypervascularity for the diagnosis of malignancy was 100% in group A patients.

CONCLUSION

In the cirrhotic liver, large size (> or = 15 mm) and T1-weighted hypointensity on in-phase images strongly suggest malignancy of the fat-containing nodules. The presence of numerous nodules < 1 cm suggests that the lesions are benign.

Fatty Liver: Imaging Patterns and Pitfalls

Fat accumulation is one of the most common abnormalities of the liver depicted on cross-sectional images. Common patterns include diffuse fat accumulation, diffuse fat accumulation with focal sparing, and focal fat accumulation in an otherwise normal liver. Unusual patterns that may cause diagnostic confusion by mimicking neoplastic, inflammatory, or vascular conditions include multinodular and perivascular accumulation. All of these patterns involve the heterogeneous or nonuniform distribution of fat. To help prevent diagnostic errors and guide appropriate work-up and management, radiologists should be aware of the different patterns of fat accumulation in the liver, especially as they are depicted at ultrasonography, computed tomography, and magnetic resonance imaging. In addition, knowledge of the risk factors and the pathophysiologic, histologic, and epidemiologic features of fat accumulation may be useful for avoiding diagnostic pitfalls and planning an appropriate work-up in difficult cases.

Dual Gradient-Echo In-Phase and Opposed-Phase Hepatic MR Imaging: A Useful Tool for Evaluating More Than Fatty Infiltration or Fatty Sparing

A T1-weighted gradient-echo in-phase and opposed-phase sequence has become a routine part of every hepatic magnetic resonance (MR) imaging protocol. Although this sequence is primarily used to identify common pathologic conditions, such as diffuse or focal steatosis and focal fatty sparing, it is also helpful in detection of pathologic entities associated with T2* effects owing to the double-echo approach. Thus, pathologic conditions such as hemochromatosis or hemosiderosis can be identified and characterized with a high level of confidence. In cases of iron storage disease, the hepatic parenchymal signal intensity decreases on the image with the longer echo time due to the continued decay of the transverse magnetization. In addition, susceptibility artifacts can be easily detected and characterized with in-phase and opposed-phase MR imaging. Metallic objects demonstrate a larger susceptibility artifact on the image with the second or longer echo time, which is usually the in-phase image. Finally, intrahepatic pneumobilia can be identified with the T1-weighted gradient-echo in-phase and opposed-phase sequence because gas also causes a susceptibility artifact, which is more pronounced on the image with the longer echo time. A complete understanding of both the chemical shift cancellation artifact and the T2* effects of the in-phase and opposed-phase sequence is important for correct interpretation of hepatic MR images.

Benign and malignant processes: normal values and differentiation with chemical shift MR imaging in vertebral marrow

PURPOSE

To establish retrospectively a range of values for signal intensity change in normal vertebral marrow by using chemical shift magnetic resonance (MR) imaging and to assess the use of this technique in differentiating benign from malignant marrow abnormalities.

MATERIALS AND METHODS

Institutional Review Board approval for this retrospective, HIPAA-compliant study was obtained; informed consent was waived. A total of 569 normal vertebrae in 75 patients (42 women, 33 men; mean age, 57.5 years; age range, 26-84 years) (control group) and 221 lesions in 92 patients (50 women, 42 men; mean age, 59.0 years; age range, 27-85 years) (study group) who had focal vertebral marrow abnormalities were studied by using 1.5-T chemical shift MR imaging. Imaging time was less than 1 minute. The proportional change in signal intensity on in-phase compared with out-of-phase images was calculated by using 1 x 1-cm regions of interest (ROIs) in the control group and ROIs as large as possible for focal lesions in the study group. This change in signal intensity (expressed as a percentage) was compared with that of normal levels and benign and malignant lesions. For statistical analysis, a random effect model was used that was adjusted for multiple comparisons.

RESULTS

A substantial decrease in signal intensity was noted for all normal vertebrae (mean, 58.5%) and for benign lesions, including endplate degeneration (mean, 52.2%), Schmorl nodes with edema (mean, 58.0%), hemangiomas (mean, 49.4%), and benign fractures (mean, 49.3%). Metastases exhibited either a minimal decrease or an increase in signal intensity (mean, 2.8%). Although there was some overlap in the range of signal intensity values among malignant lesions, benign lesions, and normal marrow, the differences in signal intensity loss for normal marrow and benign and malignant lesions were significant (P < .01 for all pairwise comparisons after adjusting for multiplicity).

CONCLUSION

Bone marrow in the vertebral bodies displays somewhat variable behavior at chemical shift MR imaging. Results suggest that a decrease in signal intensity greater than 20% on out-of-phase images compared with in-phase images should be used as a cutoff threshold for normalcy to allow distinction between benign and malignant causes of vertebral marrow abnormalities.

Imaging Features of Perivascular Fatty Infiltration of the Liver: Initial Observations

PURPOSE

To retrospectively identify and describe the imaging features that represent perivascular fatty infiltration of the liver.

MATERIALS AND METHODS

The institutional review board approved the study and waived informed consent. The study complied with the Health Insurance Portability and Accountability Act. Ten patients (seven women, three men; mean age, 78 years; range, 31-78 years) with fatty infiltration surrounding hepatic veins and/or portal tracts were retrospectively identified by searching the abdominal imaging teaching file of an academic hospital. The patients' medical records were reviewed by one author. Computed tomographic (CT), magnetic resonance (MR), and ultrasonographic (US) imaging studies were reviewed by three radiologists in consensus. Fatty infiltration of the liver on CT images was defined as absolute attenuation less than 40 HU without mass effect and, if unenhanced images were available, as relative attenuation at least 10 HU less than that of the spleen; on gradient-echo MR images, it was defined as signal loss on opposed-phase images compared with in-phase images; and on US images, it was defined as hyperechogenicity of liver relative to kidney, ultrasound beam attenuation, and poor visualization of intrahepatic structures. Perivascular fatty infiltration of the liver was defined as a clear predisposition to fat accumulation around hepatic veins and/or portal tracts. For multiphase CT images, the contrast-to-noise ratio was calculated for comparison of spared liver with fatty liver in each imaging phase.

RESULTS

Fatty infiltration surrounded hepatic veins in three, portal tracts in five, and both hepatic veins and portal tracts in two patients. Six of the 10 patients had alcoholic cirrhosis, two reported regular alcohol consumption (one of whom had acquired immunodeficiency syndrome and hepatitis B), one was positive for human immunodeficiency virus, and one had no risk factors for fatty infiltration of the liver. In three of the 10 patients, fatty infiltration was misdiagnosed as vascular or neoplastic disease on initial CT images but was correctly diagnosed on MR images.

CONCLUSION

Perivascular fatty infiltration of the liver has imaging features that allow its recognition.

Fat-containing lesions of the liver: cross-sectional imaging findings with emphasis on MRI

OBJECTIVE

The purpose of this pictorial essay is to identify different types of liver lesions that contain fat. Cross-sectional imaging findings of fat- or lipid-containing lesions can help in characterizing focal liver lesions. We searched our archive retrospectively and reviewed the literature for fat-containing liver lesions and identified 16 different types.

CONCLUSION

These lesions can contain macroscopic fat (i.e., angiomyolipoma, lipoma, liposarcoma, hydatid cyst, lipopeliosis, adrenal rest tumor, pseudolipoma, hepatic teratoma, pericaval fat, extramedullary hematopoiesis, and metastases) or intracellular lipid (i.e., focal steatosis, adenoma, focal nodular hyperplasia, regenerative nodules, and hepatocellular carcinoma). CT, MRI, and sonographic findings of these lesions can help in characterization by allowing specific diagnosis or narrowing the differential diagnosis of liver lesions.

Magnetic resonance imaging techniques

In this article we describe state-of-the art techniques for magnetic resonance imaging of the liver. T1-weighted, T2-weighted, and heavily T2-weighted pulse sequences are discussed. Gadolinium-enhanced hepatic parenchymal imaging and magnetic resonance angiography are also described. A comprehensive MR imaging examination of the liver affords evaluation of focal and diffuse hepatic parenchymal disease, biliary disease, and vascular pathology.

Imaging the fatty liver

Even though various imaging techniques are available to detect fat, chemical-shift imaging is the most accurate for both qualitative and quantitative measurement of fat. It can be very useful in liver studies, not only because it definitively demonstrates fatty liver but also because it improves lesion detection and characterization in some cases.

Nondiffuse fatty change of the liver: discerning pseudotumor on MR images enhanced with ferumoxides-initial observations

PURPOSE

To clarify the findings of nondiffuse fatty change of the liver on ferumoxides-enhanced magnetic resonance (MR) images.

MATERIALS AND METHODS

Of 202 patients who underwent ferumoxides-enhanced MR imaging, eight who had nondiffuse fatty change of the liver at computed tomography (CT) were examined as study subjects. MR imaging findings before and 1 hour after ferumoxides administration were compared with CT findings.

RESULTS

Focal fatty areas of the liver showing low attenuation on CT images were depicted as areas of relatively high intensity on the ferumoxides-enhanced T1-weighted images in all patients. On enhanced T2-weighted images, focal fatty change showed relatively high intensity in three and isointensity in one of the four patients. Focal spared areas appearing as areas of relatively high attenuation on CT images were depicted as areas of relatively low intensity on the ferumoxides-enhanced T1- and T2-weighted images in all patients.

CONCLUSION

Although prior reports of hepatic MR imaging with ferumoxides indicated that there is accumulation of ferumoxides within focal fatty areas that are no longer seen after the administration of contrast medium, this study revealed that focal fatty change and focal spared areas of fatty liver may be pseudotumors because of the relatively high intensity of fatty areas of the liver. Radiologists can distinguish these conditions from hepatic tumors by using the opposed-phase gradient-echo sequence or the fat-saturation technique.