MRI of fat necrosis of the breast : the “black hole” sign at short tau inversion recovery

Objective: To describe MRI features of fat necrosis of the breast. Materials and methods: Twenty-five lesions in 16 patients were retrospectively analyzed. MRI was performed due to equivocal findings at conventional imaging after surgical treatment of cancer (n = 14) or during anticoagulant therapy (n = 1), after focal mastitis treated with ductal resection (n = 1). In the 15 patients with previous surgery MRI was performed after a median interval of 24 months, using short tau inversion recovery (STIR) and contrast-enhanced dynamic T1-weighted sequences. Signal-to-noise ratio (SNR) inside the lesion and surrounding healthy fat was calculated on both STIR and unenhanced T1-weighted images. Maximal lesion diameter was measured on STIR images. All lesions had final clinical and imaging assessment in favor of fat necrosis and negative clinical and imaging follow-up (21-40 months; median 24 months). Results: At STIR sequence, fat necrosis appeared as a "black hole", being markedly hypointense (median SNR = 29) compared with surrounding fat (median SNR = 95) (P < 0.001), while no significant difference was found at unenhanced T1-weighted sequence. No significant correlation with time from treatment was found. Of 25 lesions, 15 showed ring enhancement, with continuous increase (n = 10), plateau (n = 2), or wash-out curve (n = 3). The 11 enhancing lesions in the 8 patients with previous radiation therapy showed an initial enhancement higher than that of the 4 enhancing lesions in the 2 patients who did not, although the difference was not significant (P = 0.104). Conclusion: Fat necrosis of the breast exhibits a "black hole" sign on STIR images, allowing for an easier diagnosis in clinical practice.