Regional ventricular wall thickening reflects changes in cardiac fiber and sheet structure during contraction: quantification with diffusion tensor MRI

Abstract

. Regional ventricular wall thickening reflects changes in cardiac fiber and sheet structure during contraction: quantification with diffusion tensor MRI. Am J Physiol Heart Circ Physiol 289: H1898 -H1907, 2005. doi:10.1152/ajpheart.00041.2005.-Dynamic changes of myocardial fiber and sheet structure are key determinants of regional ventricular function. However, quantitative characterization of the contractionrelated changes in fiber and sheet structure has not been reported. The objective of this study was to quantify cardiac fiber and sheet structure at selected phases of the cardiac cycle. Diffusion tensor MRI was performed on isolated, perfused Sprague-Dawley rat hearts arrested or fixed in three states as follows: 1) potassium arrested (PA), which represents end diastole; 2) barium-induced contracture with volume (BVϩ), which represents isovolumic contraction or early systole; and 3) barium-induced contracture without volume (BVϪ), which represents end systole. Myocardial fiber orientations at the base, midventricle, and apex were determined from the primary eigenvectors of the diffusion tensor. Sheet structure was determined from the secondary and tertiary eigenvectors at the same locations. We observed that the transmural distribution of the myofiber helix angle remained unchanged as contraction proceeded from PA to BVϩ, but endocardial and epicardial fibers became more longitudinally orientated in the BVϪ group. Although sheet structure exhibited significant regional variations, changes in sheet structure during myocardial contraction were relatively uniform across regions. The magnitude of the sheet angle, which is an index of local sheet slope, decreased by 23 and 44% in BVϩ and BVϪ groups, respectively, which suggests more radial orientation of the sheet. In summary, we have shown for the first time that geometric changes in both sheet and fiber orientation provide a substantial mechanism for radial wall thickening independent of active components due to myofiber shortening. Our results provide direct evidence that sheet reorientation is a primary determinant of myocardial wall thickening. magnetic resonance imaging; potassium arrest; barium contracture; myofiber; reorientation; myocyte shortening THE VENTRICULAR EXCITATION-contraction process elicits transmural wall thickening that serves as the mechanism for ejection of blood. Although myocyte contraction provides the cellular basis for regional myocardial wall thickening, previous studies of dog hearts (27, 32) suggest that the typical 14% myocyte shortening leads to only an ϳ8% increase in myocyte diameter, which cannot fully account for the observed 28 -50% increase in average wall thickness (4, 28, 36). Apex-to-base (longitudinal) shortening represents another important mechanism that has recently been recognized as a significant contributor to the ejection fraction that necessitates complex myofiber rearrangements during systole. However, the microscopic structural changes that occur during myocardial contraction remain poorly defined because...

Reconstruction and Quantification of Diffusion Tensor Imaging-Derived Cardiac Fibre and Sheet Structure in Ventricular Regions used in Studies of Excitation Propagation

“…Our mean value for the three combined hearts of FA = 0.25 ± 0.09 is similar to measurements from other studies, with values of 0.36 in canine [29], 0.35 in goat [23], 0.27 in mouse [41], and 0.36, 0.32 and 0.3 in diastolic, early and late systolic rat respectively [11] having been reported. Thus, fibre direction in the three hearts was well determined.…”

“…Secondary eigenvectors in the heart point principally in the radial direction [20,41,53,54] and so we calculate the sheet angle as the angle between the transverse plane and the projection of the secondary eigenvector onto the radial plane (see Figure 2b). This also allows comparisons to data from histological studies [11,45,46] which are generally taken from slices made in the radial plane.…”

The impact of endurance exercise training on left ventricular systolic mechanics

“…The strong inverse relationship between septal circumferential strain reduction and both RV dilation and enhanced systolic function may reflect a tethering effect of RV cross-over fibers on the intervening septum. Why the RV appears to impact septal strain only in the circumferential vector is uncertain but may be due to the fact that we performed strain analysis at the mid-LV level where the majority of fibers are oriented along the circumferential plane (5). The importance of this potential ventricular interdependence on exercise capacity and on long-term cardiac function is uncertain.…”

Left Ventricular Form and Function Revisited: Applied Translational Science to Cardiovascular Ultrasound Imaging

“…Indeed, several studies in experimental mechanics and computational modeling have described the LV wall as a chiral structure where right-handed helical myofiber geometry in the subendocardial region transitions gradually into a left-handed geometry in the subepicardial region ( Fig. 1E) (7,(25)(26)(27).…”

“…Indeed, several studies in experimental mechanics and computational modeling have described the LV wall as a chiral structure where right-handed helical myofiber geometry in the subendocardial region transitions gradually into a left-handed geometry in the subepicardial region ( Fig. 1E) (7,(25)(26)(27).Therefore, the long axis of myofibers when viewed from outside the LV rotates clockwise from the endocardium towards the epicardium, with reported net difference in myofiber angulation ranging from +60° to −60° (15). In the short axis views, the myofibers are clustered within layers (myofiber sheets) that are separated by cleft spaces (cleavage planes).…”