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Hepatobiliary Imaging |
1 Department of Radiology, University of California San Francisco School of
Medicine, 513 Parnassus Ave., San Francisco, CA 94143-0410.
2 Department of Radiology, Abdominal Imaging, University of California San
Francisco, Box 0628, 505 Parnassus Ave., San Francisco, CA 94143-0628.
3 Present address: Department of Radiology, New York Medical College, Valhalla,
NY 10595.
Received October 2, 2003; accepted after revision February 17, 2004.
Address correspondence to B. M. Yeh.
OBJECTIVE. We sought to describe the role of opposed-phase MRI in the diagnosis of nodular fatty sparing in the liver.
CONCLUSION. Nodular fatty sparing of the liver displays characteristic findings on opposed-phase MRI. Because the lesion may otherwise mimic a hepatic tumor, knowledge of these findings may prevent unnecessary biopsy or inappropriate treatment.
Focal fatty infiltration of the liver is a well-recognized entity that usually occurs adjacent to the porta hepatis, fissure for the falciform ligament, fossa of the gallbladder, or subcapsular region [1]. Occasionally, focal fatty infiltration has a nodular configuration and occurs outside these typical locations. Such nodular focal fatty infiltration can mimic a hepatic tumor and may be misinterpreted as hepatic metastasis [2]. We are aware of only one previously described case of the inverse of this phenomenon, in which a hepatic pseudotumor was due to an atypically located nodule of focal fatty sparing in an otherwise diffusely fatty liver [3]. Opposedphase gradient-echo MRI is a recognized technique for the diagnosis of fatty infiltration or sparing of the liver [4]. With the increasing use of MRI, cases of nodular fatty sparing are likely to be more commonly encountered, and recognition of the characteristic opposed-phase MRI findings is crucial to preventing misinterpretation or inappropriate intervention. The aim of our report is to describe the role of opposedphase MRI in the diagnosis of nodular fatty sparing in the liver on the basis of our experience with two cases of this entity.
Materials and Methods
Ours was a retrospective single-institution study. Our institutional review board approved the study, and written informed consent was not required. At routine readout between January 2001 and June 2002, we identified two patients whose sonographic findings were suggestive of one or more hepatic masses but whose opposedphase MRI findings led to the diagnosis of nodular fatty sparing. Clinical and imaging findings were retrieved through a review of all available medical and radiologic records. Neither patient had known risk factors for hepatic steatosis such as diabetes, obesity, hepatitis, alcohol use, other drug or medication use, or known dietary or endocrine disorder.
Abdominal MRI was performed on a 1.5-T whole-body MR scanner (Signa, GE Healthcare). Patients were examined in the supine position, using the body coil for excitation and a torso phased-array surface coil for signal reception. MRI techniques used included in-phase and opposed-phase axial T1-weighted spoiled gradient-echo (TR range/TE, 90–150/4.2 and 2.1; flip angle, 75°; slice thickness, 8 mm; gap, 1 mm; field of view, 25–40 cm; number of acquisitions, 1; matrix, 256 x 128–192); fat-saturated axial T2-weighted fast-recovery fast spin-echo (TR/TEeff, 2,500/100; slice thickness, 8 mm; gap, 1 mm; field of view, 25–40 cm; number of acquisitions, 1; matrix, 256 x 160); coronal T2-weighted single-shot fast spin-echo (TR/TEeff, infinite/100; slice thickness, 6 mm; gap, 1 mm; field of view, 25–40 cm; number of acquisitions, 1; matrix, 256 x 160–192); and dynamically enhanced in-phase axial T1-weighted spoiled gradient-echo (TR range/TE, 90–150/4.2; flip angle, 75°; slice thickness, 8 mm; gap, 1 mm; field of view, 25–40 cm; number of acquisitions, 1; matrix, 256 x 128–192) after bolus IV administration of 0.1 mmol/kg of gadolinium chelate (Magnevist, Schering) sequences. All sequences were performed during patient breath-holding.
CT was performed using a 4-MDCT scanner (LightSpeed, GE Healthcare) in high-quality mode with 5-mm collimation and a tabletop speed of 15 mm/sec after administration of oral diatrizoate meglumine (Hypaque, Nycomed Amersham). No IV contrast material was administered. Transabdominal sonography was performed using Sequoia 512 scanners (Acuson Solutions) with dynamically focused phased-array sector transducers (5-3.5–MHz). Color-flow Doppler sonography (4-MHz transducer) was also performed.
Results
Patient 1
A 13-year-old boy with known familial adenomatous polyposis underwent
abdominal sonography because gastric polyps were detected on an upper
gastrointestinal barium examination. The liver parenchyma was diffusely
hyperechoic, suggesting diffuse fatty infiltration. Several masslike
hypoechoic lesions were seen in hepatic segments VI and VII, the largest
measuring 1.4 cm in maximum diameter. These foci did not show increased
vascularity on power Doppler insonation. The sonographic findings were
considered nonspecific but were thought to possibly be caused by hemangiomas,
and the patient was further evaluated on MRI.
The lesions were poorly visualized on T2-weighted single-shot fast spin-echo, fat-saturated T2-weighted fast-recovery fast spin-echo, and dynamic gadolinium-enhanced in-phase T1-weighted spoiled gradient-echo images. On opposed-phase T1-weighted spoiled gradient-echo images, the liver displayed a diffuse reduction in signal, and the lesions were well visualized as areas of relative hyperintensity (Figs. 1A, 1B, and 1C). A diagnosis of nodular fatty sparing was made. Endoscopic removal of the gastric polyps revealed benign fundic gland polyps, and colonoscopic removal of colonic polyps revealed benign polyps with adenomatous change. The patient is doing well after 2 years of follow-up.
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Patient 2
An asymptomatic 20-year-old man underwent abdominal sonography because of a
strong family history of cancer. The liver parenchyma was diffusely
hyperechoic, suggesting diffuse fatty infiltration. Masslike hypoechoic
lesions were seen at the junction of segments IVa and VIII and in segment VI,
measuring 4.0 and 1.8 cm, respectively, in maximum diameter. The sonographic
findings were considered suggestive of malignancy, and the patient was further
evaluated on CT and MRI.
The lesions were relatively hyperdense compared with the surrounding liver parenchyma on both unenhanced and contrast-enhanced CT. The lesions were poorly visualized on T2-weighted single-shot fast spin-echo, fat-saturated T2-weighted fast-recovery fast spin-echo, and dynamic gadolinium-enhanced in-phase T1-weighted spoiled gradient-echo images. On opposed-phase T1-weighted spoiled gradient-echo images, the liver displayed a diffuse reduction in signal, and the lesions were well visualized as areas of relative hyperintensity (Figs. 2A, 2B, and 2C). The diagnosis of nodular fatty sparing was made. The patient is doing well after 13 months of follow-up.
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Discussion
Diffuse fatty infiltration of the liver, or hepatic steatosis, may be idiopathic or secondary to obesity, starvation, parenteral nutrition, steroid therapy, diabetes mellitus, alcohol use, or hepatitis. Focal fatty sparing on a background of diffuse fatty infiltration typically occurs in segments IV and V near the gallbladder fossa and porta hepatis [1]. Focal fatty sparing is usually recognized on imaging when areas of decreased echogenicity on sonography or hyperdensity on CT are seen in the typical locations for this entity in patients with evidence of diffuse fatty infiltration. Our study highlights the importance of considering nodular fatty sparing as a potential diagnosis when focal lesions are seen in a fatty liver outside these typical locations. The sonographic assessment of focal lesions in a fatty liver is known to be problematic [5, 6], and the sonographic appearances of the lesions were nondiagnostic in both of our patients.
Biopsy is usually considered the definitive test for the diagnosis of focal hepatic lesions but is limited by sampling error and interpretative variation [7, 8]. We believe that the constellation of MRI findings seen in our two patients is sufficiently diagnostic to obviate tissue sampling. Focal lesions in a diffusely fatty liver that are clearly seen only on opposed-phase MRI during an examination that includes T2-weighted and dynamic gadolinium-enhanced sequences are likely to represent nodular fatty sparing. Although it is conceivable that a true primary or secondary tumor in a diffusely fatty liver could also be better visualized on opposed-phase imaging than on other MRI techniques, such a lesion would almost certainly show increased T2 signal intensity or altered vascularity relative to the liver parenchyma on dynamic gadolinium-enhanced MRI. Failure to recognize the characteristic MRI findings of nodular fatty sparing could lead to a diagnosis of hepatic tumor and unnecessary biopsy. Because of sampling error, a negative finding at biopsy could be further misinterpreted as a false-negative result, compounding the error.
In our study, neither patient had known or suspected malignancy, and the benign nature of the focal nodular fatty sparing was confirmed at follow-up. Nonetheless, the diagnosis of benign focal fatty sparing of the liver must be made with caution in patients with known or suspected malignancy because primary liver tumors or metastases in patients with hepatic steatosis may present as nonfatty liver lesions [9, 10]. On MRI, metastases may have increased T2 signal intensity, a rim of peritumoral fatty sparing, rim enhancement with IV contrast administration [11], or a wedge shape resulting from the obstruction of an associated portal vein [12]. None of these findings was seen in either patient in our series.
In conclusion, masslike lesions in a diffusely fatty liver that are well seen only on opposed-phase MRI during an examination that includes T2-weighted and dynamic gadolinium-enhanced sequences may represent nodular fatty sparing in the appropriate clinical setting. Recognition and accurate diagnosis of this pseudotumor may prevent unnecessary biopsy or other inappropriate intervention.
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