The superior pericardial sinus: normal appearance on gradient-echo MR images

The purpose of this study was to describe and explain the normal appearance of the superior pericardial sinus (the retroaortic extension of the superior pericardial recess) on gradient-recalled-echo (GRE) MR images. This is important clinically because on some GRE images the superior pericardial sinus may have a signal like that of flowing blood, which could be mistaken for an aortic dissection or an anomalous vascular structure.

Six patients had MR imaging for evaluation of the mediastinum. In two cases, CT scans were also obtained. Both T1- and T2-weighted axial, spin-echo, ECG-gated MR images were obtained in all cases. The superior pericardial sinus was imaged with axial gradient-recalled acquisition in the steady state (GRASS) by using flip angles between 5 degrees and 60 degrees. The nature of the signal of the superior pericardial sinus on GRASS images was further studied in one patient by using spoiled GRASS (SPGR), GRASS with an inferior saturation pulse, GRASS with a slice thickness that encompassed the entire superior pericardial sinus, and cine GRASS sequences.

The superior pericardial sinus was visualized well in all six patients. It had low signal on T1-weighted images and high signal on T2-weighted images. The signal of the sinus on T2-weighted images was heterogeneously high in three of six cases. On GRASS and cine images with flip angles less than 20 degrees, the signal of the superior pericardial sinus was similar to that of flowing blood. The sinus also had high signal on SPGR images with flip angles less than 10 degrees. No decrease in the signal of the sinus was seen on GRASS images obtained with an inferior saturation pulse. On SPGR and GRASS images obtained with the same parameters, the signals of the superior pericardial sinus were similar. When the slice thickness of the GRASS sequence was increased to encompass the entire sinus without an inferior saturation pulse, little or no decrease in signal occurred.

On flow-sensitive GRE MR images obtained with a low flip angle and moderate TE, the superior pericardial sinus has high signal similar to that of flowing blood. This is a reflection of the high proton density and long T2* of pericardial fluid, and not a consequence of flow or steady-state free precession. Radiologists must be aware of this phenomenon in order to avoid misdiagnosis of aortic dissection or confusion with a vascular structure.