Bright pleural effusion and ascites on gradient-echo MR images: a potential source of confusion in vascular MR studies

Motion of fluids other than blood can cause flow-related signal enhancement on MR images, including MR angiograms. In order to study this problem, the appearance of ascites (20 patients) and pleural effusions (five patients) was assessed on MR images made during suspended respiration with flow-compensated gradient-echo sequences as well as T1- and T2-weighted sequences. Signal intensities of vessels, fluid collections, and muscle were measured and vessel/muscle and vessel/fluid contrast were calculated. Fluid motion was measured with a bolus tracking technique that tags a selected volume of fluid with an RF presaturation. Fluid collections had a bright signal in four of five patients with pleural effusion and in 15 of 20 patients with ascites. The average contrast ratio between bright components of the fluid collections and vessels was only 0.03 +/- 0.09. Bright fluid collections were seen on MR angiograms and could obscure blood vessels. Bolus tracking measurements of ascites revealed multidirectional flow, suggesting that its bright signal is related to motion that continues during suspended respiration. Fluid collections appeared dark on T1-weighted images in all patients, indicating that a short T1 relaxation time was not a cause of the high signal intensity. The results indicate that, despite breath-holding, ascites and pleural effusions can show bright signal intensity on gradient-echo images. Awareness of this phenomenon will avoid confusion between moving fluid collections and flowing blood and identify a source of image degradation on both gradient-echo and T2-weighted spin-echo MR acquisitions.