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produce a 3D volume (Video 6.8) There are several unique maturational features of the normal early human fetal heart On external examination at 11 weeks’ gestation (Fig 6.6), the atrial appendages are large compared with the relatively small atrial chambers, and the coronary arteries are prominent Internally the normal off-set of the mitral and tricuspid valves is infrequently appreciated before 13 gestational weeks, and the perimembranous region and muscular part of the ventricular septum are thickened The semilunar valves are gelatinous and poorly delaminated from the arterial walls The arterial wall-to-lumen ratio in the first trimester fetus is threefold that of the mid-second trimester specimens Histologic staining of the 3-µm resolution slices is feasible and provides more information about gestational changes in the normal and malformed heart A comparison of the information available from 4D high-resolution transvaginal sonography of the first-trimester fetal heart and HREM confirms the relatively poor ultrasound detail available to us clinically at this early gestational age.47 FIG 6.6 In this normal 11-week heart (modeled with an isotropic resolution of 3 µm), several specific features of the human fetal heart can clearly be observed, including large atrial appendages, prominent coronary arteries, and the relatively small size of the atrial chambers (B and F) Detailed structures can be seen more clearly in the high-resolution episcopic microscopy reconstruction (B) than in the original photograph (A) Volume rendering, which allows internal morphology of the heart to be examined at high resolution, shows thickened great arterial walls (C–E) and details of valve and septal architecture (C and D) Multiplanar reconstructed images demonstrate detailed structures of the inside of cardiac chambers (E and F) Ao, Aorta; AV, aortic valve; EV, eustacian valve; FF, foramen flap; LA, left atrium; Lapp, left atrial appendage; LV, left ventricle; MV, mitral valve; PT, pulmonary trunk; PV, pulmonary valve; RA, right atrium; Rapp, right atrial appendage (From Matsui H, Mohun T, Gardiner HM Three-dimensional reconstruction imaging of the human foetal heart in the first trimester Eur Heart J 2010;31[4]:415.) These observations suggest we continue to exercise caution in the interpretation of first trimester echocardiographic studies as 2D echo and color Doppler imaging may erroneously give the impression of valvar stenosis and narrowed outflow tracts, or of an atrioventricular septal defect Micro-Computed Tomography Micro-CT provides near-histologic levels of detail in the early gestation human heart It has exposure times between 500 and 1000 ms and produces isotropic voxel sizes between 19 and 31 µm, depending on specimen size.48 Similar to HREM, postprocessing techniques allow the digital removal of supporting material, and multiplanar reconstructions, to create virtual dissections of the fetal heart (Fig 6.7) It has the advantage over HREM of being nondestructive so the images can be reexamined by other specialists to provide additional diagnostic opinions (while reducing the need for transportation of tissues), and it can image larger sized specimens than HREM It may be superior to traditional autopsy in assessment of the myocardium, and its resolution is superior to 1.5 Tesla MRI49,50 and uses far shorter imaging times, usually less than 1 hour Current challenges include the effects of tissue coloration and distortion secondary to fixation and the use of contrast In addition, the file sizes are very large (10 to 30 GB), and this may provide a practical barrier to its routine implementation for fetal postmortem