A QUANTITATIVE DESCRIPTION OF DYNAMIC LEFT-VENTRICULAR GEOMETRY IN ANESTHETIZED RATS USING MAGNETIC-RESONANCE-IMAGING

We report a functional application of magnetic resonance imaging (MRI) for the quantitative description of left ventricular geometry through systole and diastole in normal anaesthetized Wistar rats that might b e applicable for the analysis of chronic changes resulting from pathol ogical conditions. Images of cardiac anatomy were acquired through pla nes both parallel and perpendicular to the principal cardiac axis at t imes that were synchronized to the R wave of the electrocardiogram. Th e images of the transverse sections were assembled into three-dimensio nal representations of left ventricular geometry at consecutive time p oints through the cardiac cycle. This confirmed the geometrical cohere nce of the data sets, that each slice showed circular symmetry, and th at the images were correctly aligned with the appropriate anatomical a xes. Different models for the three-dimensional geometry of the left v entricle were then tested against the epi-and endocardial surfaces rec onstructed from images of the transverse sections of the left ventricl e in both systole and diastole using least-squares minimizations in th ree dimensions, In agreement with previous reports in the human heart, an elliptical figure of revolution offered an optimal fit to the epic ardial and endocardial geometry for the rat heart in diastole. This wa s in preference to models that used spherical, quartic or parabolic ge ometries. However, in contrast to contraction in the human heart, all these geometrical representations broke down during systolic ejection in the rat heart, We therefore introduced a more general hybrid model which described left ventricular geometry in terms of the variation of the radii r(z), independently determined for each slice, with its pos ition z along the principal cardiac axis. The resulting function r(z) could then be described by a simple ellipsoid of revolution not only d uring diastole, bur also throughout ventricular ejection. The findings also ruled out alternative geometrical representations. It was then p ossible additionally to reconstruct the luminal and total left ventric ular volumes, wall thicknesses and ejection fractions through the card iac cycle and to confirm that the predicted total ventricular wall vol ume was conserved throughout the cardiac cycle. Our hybrid model of ca rdiac geometry may thus be useful for non-invasive serial studies of c hronic pathological changes that use the rat as a model experimental s ystem.