Sequential Change in the Findings of Fat Necrosis on US, MRI and Tissue Specimen Radiogram Correlating with Histopathologic Findings: Experimental Study in Pigs

Kim, Hyeon Sook; Kim, Hak Hee; Park, Seog Hee

Abstract

PURPOSE
This study compared the sequential findings of ultrasound, MRI and tissue specimen radiograms of experimentally induced fat necrosis in pigs with the histopathology findings.
MATERIALS AND METHODS
Fat necrosis was induced in five experimental groups containing three pigs per group (3-day, 2-week, 4-week, 6-week and 8-week groups), which were divided according to the amount of time after inducing fat necrosis until when the tissue specimen was obtained. Ultrasound (US) and MRI (T1 weighted images, T2 weighted images, STIR sequence, T1 fat-suppressed contrast enhanced images) were obtained. X-ray tissue specimen radiograms were obtained after sacrificing the pigs. The changes in the findings of these imaging modalities were compared with the histopathology findings.
RESULTS
Ultrasound showed the lipase-induced masses of fat necrosis to be isoechoic (47%) and mixed echoic (42%) masses in 3-day group, mixed (56%, 41%) and hypoechoic (42% and 59%) masses in the 2-week and 4-week groups respectively, and hypoechoic (67% and 56%) masses in the 6-week and 8-week groups. MRI showed the T1 weighted images to have significantly high rate of isosignal intensity in the 3-day, 2, 4 and 6-week groups, but the isosignal intensity lesions decreased gradually while the low signal intensity lesions increased with time. The T2 weighted images showed that the high signal intensity lesions were the main type of lesion in the early stage but the isosignal intensity lesions increased markedly in the late stage. The STIR (short tau inversion recovery) sequence showed that all the masses of fat necrosis were observed to have high signal intensity in the 3-day group, and the high signal intensity lesions decreased while the isosignal intensity lesions increased significantly during the follow-up period between 2 weeks to 8 weeks. All the masses of fat necrosis were enhanced on the fat-suppressed contrast-enhanced T1 weighted images. Among the total 45 cases X-ray tissue specimen radiograms, fat necrosis was observed as a radiopaque mass in 28 cases (62%). Among these 28 cases, calcifications within the masses were observed in 4 cases from the 8-week group and 1 case from 6-week group. Two cases from 2-week group were observed as spiculated masses. The histopathology findings of fat necrosis were acute inflammatory cell infiltration, edema and hemorrhage in the 3-day group, the proliferation of fibroblasts, connective tissue and capillaries around the fat necrosis in the 2-week group. However, the number of vessels decreased gradually and the fibrosis had progressed further in the 4, 6 and 8-week groups.
CONCLUSION
The sequential findings of fat necrosis on US, MRI and tissue specimen radiogram were correlated with the histopathology findings. These results may be helpful for the differentiating fat necrosis from more ominous breast masses.