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Institute of Clinical Medicine, University of Tsukuba, Tsukuba, 305-8575 Japan
I read the article entitled "Targetlike Appearance of Pseudotumors in Segment IV of the Liver on Sonography" [1] with great interest. I agree with the authors' interpretation that this central hyperechogenicity surrounded by peripheral hypoechogenicity is essentially focal spared area of varied degrees in the fatty liver, and I wonder what pathophysiologic mechanism or vascular variant induces such change.
As the authors pointed out, solitary metastatic liver tumor should be differentiated from this pseudotumor on sonography. In addition to the specific site of occurrence, two sonographic findings, if present, are useful in making a correct diagnosis: wide contact with the hepatic capsule without deformity and a wedged or rhomboid shape of the abnormal area with a zigzagged border [2].
The borders of the lesions shown were well-defined, but—to my eyes—they were not smooth. In figure 3 of their article, contradictory to the general rule, an unenhanced CT scan represented the typical wedged shape, but the corresponding sonogram revealed a more roundish shape [1].
When color Doppler sonography is performed to visualize such lesions, one should carefully observe whether any hepatopetal vein comes into the abnormal area from the hepatic hilum. In the case of focal spared area at this site, a direct supply of nonportal splanchnic vein or independent splanchnic vein not via the portal trunk is present [2].
Moreover, I would like to point out that a tumor in the fatty liver may be associated with peritumoral sparing (spared area of various width circumscribing the tumor) [3]. When an echogenic tumor occurs in the fatty liver, this tumor appears on sonography as a similar targetlike lesion associated with a hypoechoic peritumoral rim. This peritumoral sparing is well and characteristically depicted by the combined use of in-phase and opposed-phase T1-weighted gradient-echo MR imaging [4], and, less sensitively, by unenhanced CT [3]. The targetlike focal spared area must represent the characteristic appearance on such MR images, because the central area of the targetlike lesion and the surrounding liver parenchyma lose signal intensity on an opposed-phase image (Fig. 1A,1B,1C).
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Therefore, when a targetlike lesion lacking typical morphology is noted in an oncology patient, color Doppler sonography, highly enhanced thin-slice CT or MR imaging, or a combination of these studies should be performed to identify a nonportal splanchnic venous supply in the lesion from the hepatic hilum. T1-weighted gradient-echo MR imaging may be performed using combined inphase and opposed-phase images to reveal marked high-intensity rim. Biopsy seems to be unnecessary for a definite diagnosis except for the rare fat-containing tumor, although I have not personally observed such a case on MR imaging.
References
University of Padua 35128 Padua, Italy
Cittadella Hospital 35013 Cittadella (PD), Italy
We appreciate Dr. Itai's comments regarding our article [1]. We agree that it is possible to diagnose a hepatic pseudotumor, even one presenting a targetlike appearance, when a wedged shape or a large area of contact with the hepatic capsule is detected on sonography. The other two signs cited by Itai can be questionable for ruling out malignancy because they are difficult to detect.
A targetlike pseudotumor can rarely force a differential diagnosis of malignancy, but in such cases a thorough investigation is mandatory. We believe that MR imaging is the best technique at present for characterizing a pseudotumor.
While our article was in press, we identified on sonography targetlike pseudotumors in the dorsal aspect of segment IV of the liver in two oncology patients. Comparison of in-phase and out-of-phase T1-weighted gradient-echo MR imaging clearly indicated a focal steatosis, although we were unable to identify a target-like appearance on MR images obtained with 7-mm slice thickness (Fig. 2A,2B,2C). Dynamic MR imaging did not reveal pathologic enhancement in either patient.
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In our opinion, MR imaging can correctly identify a pseudotumor not characterized on sonography. Multiphasic CT or biopsy should be reserved only for patients with a contraindication for MR imaging.
References
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  O. W. Hamer, D. A. Aguirre, G. Casola, and C. B. Sirlin Imaging Features of Perivascular Fatty Infiltration of the Liver: Initial Observations Radiology, October 1, 2005; 237(1): 159 - 169. [Abstract] [Full Text] [PDF]
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