Study Of CT Features Of Pericardial Recesses Differentiating Mediastinal Lymph Nodes

Objective: To evaluate the role of computed tomography(CT)in differentiating pericardial recesses appearing similar to mediastinal lymph nodes.Materials and Methods: In this study from January 2014 to December 2015,we included 240 chest CT scans,of which 112 were male and 128 were female,the age between 23~101 with mean average of 67.15± 14.09 years old.Among them 103 were pericardial recesses and 137 were identified to have mediastinal lymph nodes.Two experienced radiologists retrospectively assessed the CT features.CT machine used in our study was Siemens Somatom definition flash with 100 kVp,mAs depending on BMI,pitch of 1.2 and slice thickness of 7 mm.CT physical parameters such as mediastinal window width of 400 HU and window level of 40 HU.The scans were obtained at end inspiration with the patient lying supine.All statistical analyses were performed using SPSS version 22.0.Mann-whitney U test and Student t-test was used to perform statistical analysis using a two-sided level of 0.05.Results: The retrospective analysis of 240 CT scans revealed 103 pericardial recesses and 137 lymph nodes.We found 12 high riding superior pericardial recesses(12%),89 posterior portion of superior aortic recesses(86%),and 2 right lateral portion of superior aortic recesses(2%).In 3 %,both HRSPR and PSAR were seen in the same patient.Among the pericardial recesses,39 males(38%)and 64 females(62%)with mean age of 61.60±15.65,age range 23-101 were observed.The lymph nodes were seen in 73 males(53.31%)and 64 females(46.7%)with mean age of 71.33±11.15,age range 38-95 and show significant association(p<0.001)compared to pericardial recesses.The mean size for the pericardial recesses(short axis 9.54±2.88 and long axis 16.9±4)was shorter compared to lymph nodes(short axis 11.37±3.56 and long axis 17.10±5.14).The short axis(p<0.001)was more significant compared to the long axis(p>0.05)when compared among both the groups.The density of pericardial recesses was mostly that of water density and for lymph nodes it was similar to that of soft tissue density.The mean density of pericardial recesses 9.21±4.78,range from 0-23 Hounsfield Units(HU)was lesser than that of lymph nodes 57.05±15.56,range from 34-108 HU with significant association(p<0.001)between them.The attenuation of all the pericardial recesses(100%)was homogeneous and for lymph nodes,heterogeneous attenuation was visualised for 62(45.3%)and homogeneous for 75(54.7%).In our study we found that the most of the CT characteristic features like location,shape,margin,density,calcification,changes on follow up CT are important predicting factors for differentiation and were significant(p<0.05).The pericardial recesses were commonly seen in the region of posterior to ascending aorta 88(85.5%)or right paratracheal region 15(14.5%)and lymph nodes in paratracheal 84(61.3%)and right paratracheal 53(38.7%)and the association between them was significant(p<0.001).Oval 42(40.8%),crescentic 20(19.4%)and triangle18(17.5%)shapes were more commonly found for pericardial recesses compared to the lymph nodes,in which oval 86(62.8%),triangle16(11.7%),irregular 16(11.7%)were found to be predominantly with a significant association between the two groups(p<0.05).The margins for all pericardial recesses were exclusively smooth(100%)and for lymph nodes they varied from smooth 111(81%),ill-defined 24(17.5%)or lobulated 2(1.5%).Calcification was not seen in all pericardial recesses(100%)whereas,it was seen in 31(22.6%)and not seen in 106(77.4%)of the lymph nodes.Pericardial recesses had no follow up change for 101(98%),size of recess increased for 1(1%)and decreased for 1(1%).There was size increase 19(13.9%)or size decrease 12(18.8%)and even complete disappearance of the lymph nodes for 3(2.2%).There was no change of lymph node size in 65(47.4%).Some of the cases did not undergo follow up 38(27.7%).The role of other features like mass effect and enhancement was not found to be significant.Conclusions: The superior aortic recess is the most commonly confused recess with that of a mediastinal lymph node.Presence of oval shape,smooth margin,fluid density,absence of calcification,homogeneous attenuation and no follow up change are the specific features for the differentiation of pericardial recesses from mediastinal lymph nodes.For proper assessment and further management,it is important to have knowledge of multiple CT features as well as their anatomical locations.