The Diagnostic Value Of Fast Imaging Employing Steady State Acquisition And Single Shot Fast Spin Echo Sequences In Diagnosis Of Normal Fetal Thymuses:Comparative Study

Objective To compare the diagnostic values of MR examination with fast imaging employing steady state acquisition(FIESTA)and single shot fast spin echo(SSFSE)sequences for normal fetal thymuses.Methods Totally 366 normal fetuses were examined by MRI with FIESTA and SSFSE sequences,and the anatomy and MR manifestations of thymuses were observed.The results of the evaluation underwent kappa analysis and x2 test.The transverse area(TA)and transverse diameter(TD)were measured on the "three vessel" level,and the superoinferior diameter(SID)was measured on the sagittal level.The mean values of these parameters of different gestational were calculated,and the regression equations of each parameter and gestational were fitted.Results The fetal thymus showed a moderate or slightly high signal in the FIESTA sequence,and the boundary with the surrounding large vessels and lung tissues was clear.Scattered dots and stripe like hyperintense signals were seen in the fetal thymus.The fetal thymus was homogeneous,medium or slightly low signal on the SSFSE sequence,and the boundaries were not clear with the surrounding large vessels and lung tissues.The thymus is a rectangle with a protruding posterior concave on the axis,located at the front of the "three vessels",between the posterior part of the sternum and the lungs,and in the sagittal plane,the thymus is like a triangle which is wider upper,and located above the pericardium.Two doctors had better agreements with the images of FIESTA and SSFSE sequences(K1=0.745,P1<0.01;K2=0.802,P2<0.01).The average rate of clarity in FIESTA and SSFSE sequences were 86.34%and 37.70%respectively,and the differences were significant(?2=367.520,P<0.001).The display rate of fetal thymus on each sequence is 100%,The thymus size increased with the gestational,and the regression equations and correlation coefficients were:TA=-5.80 + 0.35 × gestational,r= 0.820;TD=-14.59 + 1.63 × gestational,r= 0.817;SID=-9.63 + 1.44 × gestational,r= 0.778,P<0.01.Conclusions The size of fetal thymus increases gradually with the increase of gestational age.We can evaluate the development states of fetal thymus by MRI.The subtle structure and contour of the fetal thymus showed by the FIESTA sequence were more clearly than the SSFSE sequence.The MRI can clearly show the fetal thymus after 22 weeks of pregnancy,and can help in diagnosing the various pathological condition of fetal thymus or the fetal malformation.