Diffusion-weighted MRI for distinguishing non-neoplastic cysts from solid masses in the mediastinum: problem-solving in mediastinal masses of indeterminate internal characteristics on CT

Objectives To evaluate the usefulness of diffusion-weighted (DW) magnetic resonance images for distinguishing nonneoplastic cysts from solid masses of indeterminate internal characteristics on computed tomography (CT) in the mediastinum. Methods We enrolled 25 patients with pathologically proved mediastinal masses who underwent both thoracic CT and magnetic resonance imaging (MRI) including diffusion-weighted imaging (DWI). MRI was performed in patients with mediastinal masses of indeterminate internal characteristics on CT. Two thoracic radiologists evaluated the morphological features and quantitatively measured the net enhancement of the masses at CT. They also reviewed MR images including unenhanced T1- and T2-weighted images, gadolinium-enhanced images and DW images. Results The enrolled patients had 15 solid masses and ten nonneoplastic cysts. Although the morphological features and the extent of enhancement on CT did not differ significantly between solid and cystic masses in the mediastinum (P > 0.05), non-neoplastic cysts were distinguishable from solid masses by showing signal suppression on high-b-value DW images or high apparent diffusion coefficient (ADC) values of more than 2.5 x 10(-3) mm(2)/s (P < 0.001). ADC values of non-neoplastic cysts (3.67+/-0.87x10(-3) mm(2)/s) were significantly higher than that of solid masses (1.46+/-0.50x10(-3) mm(2)/s) (P < 0.001). Conclusions DWI can help differentiate solid and cystic masses in the mediastinum, even when CT findings are questionable. Key Points Non-invasive diagnosis of non-neoplastic cysts can save surgical biopsy or excision. Conventional CT or MRI findings cannot always provide a confident diagnosis. Mediastinal masses can be well-characterised with DWI. Non-neoplastic mediastinal cysts show significantly higher ADC values than cystic tumours. DWI is useful to determine treatment strategy.