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Am J Physiol Heart Circ Physiol 289: H1898 –H1907, 2005;
doi:10.1152/ajpheart.00041.2005.
Regional ventricular wall thickening reflects changes in
cardiac fiber and sheet structure during contraction: quantification with
diffusion tensor MRI
Junjie Chen,1,2 Wei Liu,1,2 Huiying Zhang,1 Liz Lacy,1
Xiaoxia Yang,1 Sheng-Kwei Song,3 Samuel A. Wickline,1,2 and Xin Yu1,2
1
Cardiovascular Magnetic Resonance Laboratories, Cardiovascular Division, Department of Medicine; 2Department of
Biomedical Engineering; and 3Departments of Chemistry and Radiology, Washington University, St. Louis, Missouri
Submitted 14 January 2005; accepted in final form 21 June 2005
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 ⬃8% increase in myocyte diameter,
which cannot fully account for the observed 28 –50% increase
in average wall thickness (4, 28, 36). Apex-to-base (longitudi-
nal) 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 of the technical difficulties in making
such measurements.
The organization of myocardial fibers can be described in
part by individual fiber orientations and by multiple myocyte
“sheet” arrangements separated by extensive “sheet cleavage”
planes. Using histological methods, several investigators documented the helical patterns of the myocardial fibers that
shifted continuously from a right-handed helix at the endocardium to a left-handed helix at the epicardium (1, 23, 36).
Several studies have demonstrated significant transverse shear
along the sheet planes (7, 20), which suggests that the laminar
organization of the myocytes may provide a structural basis for
systolic wall thickening. However, the destructive nature of
conventional histological analysis employed in such meticulous studies complicates direct evaluation of any changes in
myocardial fiber and sheet structure that accompany cardiac
contraction.
Recently, diffusion tensor MRI (DTMRI) has been validated
as an alternative method for rapid and nondestructive analysis
of 3-D myocardial structure in both normal (13, 14, 29) and
diseased (6) hearts. This method was applied previously to
depict fiber tracts in the central nervous system to detect stroke
and other fiber-disrupting pathologies (18, 40, 43). The recent
work of Tseng et al. (41) confirmed that quantitative measurements derived from DTMRI (e.g., the secondary and tertiary
eigenvectors) are highly aligned with myocardial “fiber sheet”
and “sheet-normal” directions, respectively. Accordingly,
DTMRI represents an ideal candidate method for delineating
changes in myocardial fiber and sheet architecture from diastole to systole.
The goal of this study was to measure changes in 3-D
myofiber and sheet structure at selected phases of the cardiac
cycle to elucidate alternative mechanisms of myocardial wall
thickening beyond simple myocyte shortening. DTMRI of
myofiber structure first was performed on isolated, perfused
heart arrested at end diastole with potassium. The same heart
was then induced to undergo either an isometric or isotonic
contraction after BaCl2-induced cardiac contracture. The heart
Address for reprint requests and other correspondence: Xin Yu, Dept. of
Biomedical Engineering, Case Western Reserve Univ., Wickenden 319, 10900
Euclid Ave., Cleveland, OH 44106 (E-mail: [email protected]).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
magnetic resonance imaging; potassium arrest; barium contracture;
myofiber; reorientation; myocyte shortening
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Chen, Junjie, Wei Liu, Huiying Zhang, Liz Lacy, Xiaoxia
Yang, Sheng-Kwei Song, Samuel A. Wickline, and Xin Yu. 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
(BV⫹), which represents isovolumic contraction or early systole; and
3) barium-induced contracture without volume (BV⫺), 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 BV⫹, but endocardial
and epicardial fibers became more longitudinally orientated in the
BV⫺ 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 BV⫹ and BV⫺ 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.
MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
was rapidly fixed either in systole with volume or in systole
without volume to preserve fiber and sheet structures at their
contracting states for subsequent DTMRI. Our results demonstrate that the initial event of cardiac contraction comprises the
reduction of sheet angles without significant changes in fiber
angles, which suggests the reorientation of myocardial sheets
toward a more radial direction accompanied by minimal
changes in myofiber orientation. At end systole, both myofiber
and fiber sheet orientations have changed substantially. These
observations indicate that the geometric alterations in fiber and
sheet structures represent a fundamental mechanism of circumferential shortening and regional wall thickening in systole.
MATERIALS AND METHODS
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solution was kept at room temperature, and the rest of the solutions
were maintained at 37°C.
Magnetic resonance imaging. Diffusion-weighted magnetic resonance (MR) images of both perfused (potassium-arrested) and fixed
(barium-arrested) hearts were acquired on a 4.7-T Varian INOVA
system (Varian Associates; Palo Alto, CA) using a custom-built
solenoid coil. Long-axis scout images were acquired as previously
described (6). A multislice spin-echo sequence with diffusion-sensitizing bipolar gradient was used to acquire short-axis, diffusionweighted images. Diffusion-sensitizing gradients were applied in six
noncollinear directions (6). Imaging parameters were as follows: echo
time, 36 ms; time interval between gradient pulses, 20 ms; gradient
pulse duration, 6 ms; gradient factor, 948 s/mm2; and in-plane resolution, 156 ⫻ 156 ␮m. Seven slices that covered the heart from base
to apex were acquired from the potassium-arrested heart. The slice
thickness was 1 mm, and gaps between adjacent slices ranged from
0.3 to 0.5 mm. For barium-arrested and fixed hearts, 11 contiguous
slices were acquired. The slice thickness (0.8 – 0.9 mm) was adjusted
according to the longitudinal shortening that occurred during bariuminduced contracture. Repetition time for potassium-arrested heart was
1.3 s, and total image-acquisition time was ⬃1 h. Repetition time for
barium-arrested heart was 1.7 s, and total image-acquisition time was
⬃2 h.
Data analysis. The effective diffusion tensor (Deff) was calculated
from diffusion-weighted images as described previously (6). Subsequently, the primary, secondary, and tertiary eigenvalues (␭1, ␭2, and
␭3, respectively) of the diffusion tensor were calculated. Diffusion
anisotropy was analyzed with fractional anisotropy (FA; 3). The
primary eigenvector of the diffusion tensor is considered to be the
myofiber orientation (11, 13, 14, 29), and the secondary and tertiary
eigenvectors are considered to be the sheet and sheet-normal orientations, respectively (41).
Epicardial and endocardial borders of the heart were traced manually on both short- and long-axis images (6). LV wall thickness was
calculated as the mean distance between epicardial and endocardial
borders. Interstitial edema was assessed by measuring changes in wall
thickness before and after DTMRI of potassium-arrested hearts.
The local wall-bound myocardial coordinates were used to quantify
myofiber structure (34). The LV long axis was determined as the line
that best fit the centers of the epicardial borders. A prolate spheroid
was fit to the epicardial borders as the epicardial surface. Subsequently, the three principal axes of the wall-bound coordinates, i.e.,
the radial, circumferential, and longitudinal axes, were determined.
Specifically, the radial axis was defined as normal to the local
epicardial surface. The circumferential axis was tangent to the local
epicardial surface and perpendicular to the LV long axis. The longitudinal axis was perpendicular to both the radial and circumferential
axes.
The DTMRI-determined myofiber helix angle (␣h), myofiber transverse angle (␣t), and sheet angle (␤s) were calculated to quantitatively
describe the cardiac structure in the local wall-bound coordinates (7,
29). The ␣h was defined as the angle between the circumferential axis
and the projection of the myofiber onto the circumferential-longitudinal plane; ␣h for a right-handed helix at the subendocardial region
was set as positive. The ␣t was defined as the angle between the
circumferential axis and the projection of myofiber orientation onto
the radial-circumferential plane. The ␤s was defined as the angle
between the radial axis and the secondary eigenvector (7); ␤s for a
sheet orientated toward the base from endocardium to epicardium was
set as positive. The ␣h, ␣t, and ␤s were characterized in basal,
midventricular, and apical slices located at 25, 50, and 75% of the
distance from the mitral valve to the apex.
For each slice, data were analyzed in four 20°-wide sectors at
anterior, lateral (between the papillary muscles), inferior, and septal
regions, respectively (Fig. 1A). The through-wall difference of ␣h,
defined as the difference in endocardial and epicardial helix angles,
i.e., ⌬␣h ⫽ ␣h(endocardial) ⫺ ␣h(epicardial), was used to quantify
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Isolated heart model. All procedures conformed to the guidelines
set forth by Washington University. Sprague-Dawley rats (2– 4 mo
old, 240 –360 g, n ⫽ 21) were heparinized (100 U) and anesthetized
with 5% isoflurane. The heart was excised and cannulated for retrograde perfusion at 100 cm hydrostatic pressure with a modified
Krebs-Henseleit buffer equilibrated with 95% O2-5% CO2 (6). The
left atrium was opened, and a water-filled latex balloon was inserted
into the left ventricle (LV) through the mitral valve to record the left
ventricular developed pressure (LVDP) and the heart rate (HR). The
rate-pressure product (HR ⫻ LVDP) was calculated as an index of
cardiac work. The perfused heart was placed within a sample tube in
a 2-cm radio-frequency coil for MRI data collection.
Experimental protocol. Cardiac structure was characterized with
DTMRI at three different states that are representative of different
phases of a cardiac cycle including 1) potassium arrested (PA), which
represents end diastole; 2) barium-induced contracture with volume
(BV⫹), which represents isovolumic contraction or early systole; and
3) barium-induced contracture without volume (BV⫺), which represents end systole. Diastolic arrest was induced by perfusing the heart
with a modified St. Thomas’ cardioplegic solution that contained
excessive potassium [(in mmol/l) 118 NaCl, 16 KCl, 16 MgCl2, 1.2
CaCl2, 10 NaHCO3, and 10 glucose]. LV pressure was maintained at
5–10 mmHg by adjusting the volume of the intraventricular balloon.
Diffusion-weighted images of the arrested viable heart were acquired.
Once image acquisition of the potassium-arrested heart was completed, the perfusate was switched to regular Krebs-Henseleit buffer
for the heart to resume normal cardiac work. After cardiac work was
stabilized (⬃10 min), BaCl2 was introduced to achieve cardiac contracture (22). Specifically, the heart was first perfused with a modified
Tyrode solution that contained (in mmol/l) 140 NaCl, 5.4 KCl, 1
MgCl2, 0.078 CaCl2, and 10 HEPES for 1.5 min to reduce the calcium
content in the myocardium and was then perfused with the modified
Tyrode solution that contained 2.5 mmol/l BaCl2. Both solutions
contained adenosine (1 mg/min) to dilate the coronary vessels. Perfusion with BaCl2 lasted for 5 min to achieve maximal contracture.
The heart was then rapidly fixed with 5% formalin.
The contracture-arrested hearts comprised two groups at different
phases of ventricular contraction. In the first group (BV⫹; n ⫽ 10),
the volume of the intraventricular balloon was kept constant during
perfusion with BaCl2. Hence, LV volume change and wall thickening
were small. This state was representative of the heart at isovolumic
contraction or early systole. In the second group (BV⫺; n ⫽ 11), the
balloon was replaced by a polyethylene-50 tubing inserted from the
left atrium before BaCl2 perfusion. Perfusate containing BaCl2 was
then introduced to the heart to induce contracture. The tubing allowed
the residual fluid to be drained from the left ventricle. Significant wall
thickening and volume change occurred in this group. This state was
representative of the heart at end systole.
All of the solutions were equilibrated with 95% O2-5% CO2.
Bovine serum albumin (BSA; 0.4% wt/wt) was also added to the
perfusate to reduce interstitial edema (10, 42). The cardioplegic
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MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
transmural changes of fiber orientation. The magnitude of the sheet
angle, ␤s , was used as a measure of sheet slope from base to apex.
Finally, changes of the 3-D fiber and sheet structure in a cardiac cycle
were reconstructed and visualized using measured ␣h, ␣t, and ␤s
values from the PA, BV⫹, and BV⫺ groups.
Histological analysis. Three additional hearts were arrested and
fixed in diastole for histological evaluation of myocardial structure.
All hearts were sliced along the LV long axis that bisected both the
right and left ventricles. Each slice was embedded in paraffin and
sectioned at 4 ␮m. The tissue sections were stained with hematoxylin
and eosin to evaluate the transmural sheet structure. In each section,
two lateral regions corresponding to the size and location of basal and
apical MR slices were selected. The orientation of the sheet cleavage
plane in each region was automatically measured using an intensitygradient technique proposed by Karlon et al. (17). Subsequently, sheet
cleavage angle (␤⬘s) was determined as the angle between the sheet
cleavage plane and the direction normal to the local epicardial border.
Statistical analysis. All data are expressed as means ⫾ SD. The ␣h
and ␤s values were characterized from 5% (endocardial surface) to
95% (epicardial surface) of transmural depth at 10% increments.
Paired Student’s t-tests were used to compare changes in fiber and
sheet angles between PA and BV⫹ groups as well as between PA and
BV⫺ groups. A two-tailed value of P ⬍ 0.05 was considered
significant. Two-way ANOVA and subsequent Freeman-Tukey tests
were performed for comparisons of ⌬␣h and ␤s at the base, midventricle, and apex among PA, BV⫹, and BV⫺ groups. A value of P ⬍
0.05 was considered significant.
RESULTS
Myocardial function and morphology. Cardiac function of
the isolated, perfused heart was preserved during the experiment. Rate-pressure product values before and after potassium
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arrest were 43,711 ⫾ 10,890 and 41,368 ⫾ 10,496 beats/min ⫻
mmHg, respectively [P ⫽ not significant (NS)]. LVDP in
barium-perfused hearts was 80 ⫾ 15 mmHg (BV⫹ group),
whereas LVDP before barium perfusion was 146 ⫾ 25 mmHg.
Average LV wall thickness at midventricle in the PA group
was the same before and after DTMRI of potassium-arrested
hearts (2.3 ⫾ 0.2 mm), which suggests that interstitial edema
was not significant. No significant wall thickening was observed in the BV⫹ group (2.7 ⫾ 0.4 mm; P ⫽ NS compared
with PA). Wall thickness increased by 46% in the BV⫺ group
(3.5 ⫾ 0.1 mm; P ⬍ 0.001 compared with PA). Longitudinal
shortening occurred in both the BV⫹ and BV⫺ groups. LV
length (in cm) was 1.49 ⫾ 0.04 in the PA group, 1.38 ⫾ 0.08
in the BV⫹ group (P ⬍ 0.001 compared with PA), and 1.28 ⫾
0.03 in the BV⫺ group (P ⬍ 0.001 compared with PA).
Diffusion characteristics. FA in all three groups was well
above that of the surrounding media (0.07 ⫾ 0.02), which
indicates significant diffusion anisotropy in all three stages of
ventricular contraction. FA was 0.36 ⫾ 0.03 and 0.32 ⫾ 0.05
in the PA and BV⫹ groups, respectively, and was slightly
decreased in the BV⫺ group (0.30 ⫾ 0.04; P ⬍ 0.001 compared with PA). The ␭1/␭2 values were 1.56 ⫾ 0.07, 1.50 ⫾
0.10, and 1.47 ⫾ 0.10 in the PA, BV⫹, and BV⫺ groups,
respectively. The ␭2/␭3 values were 1.40 ⫾ 0.10, 1.33 ⫾ 0.16,
and 1.30 ⫾ 0.06 in the PA, BV⫹, and BV⫺ groups, respectively.
Myocardial fiber orientation. Representative helix angle
(␣h) maps of a rat heart in diastole and systole are shown in
Fig. 1, B and C, respectively. Transmural courses of ␣h were
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Fig. 1. A: selected left ventricular (LV) sectors (shaded areas) for data analysis. Anterior and inferior fusion sites of the right ventricle (RVA and RVI,
respectively) are shown. B and C: helix-angle (␣h) maps of a basal slice in potassium-arrested (PA; B) and barium-induced contracture without volume (BV⫺;
C) hearts, which represent end diastole and end systole, respectively. D and E: sheet-angle (␤s) maps of the same basal slice in PA (D) and BV⫺ (E) hearts.
MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
The transverse angle, ␣t, ranged between ⫺20 and 20°. No
significant differences in ␣t were observed for PA, BV⫹, and
BV⫺ hearts.
Laminar fiber sheet orientation. Representative color maps
of the sheet angle, ␤s, in diastole and systole are shown in Fig.
1, D and E, respectively. The hematoxylin and eosin staining of
potassium- and barium-arrested (without volume) hearts are
shown in Fig. 4, A and B, respectively, where sheet structure
can be identified as fine separations of myofiber bundles that
transit from positive sheet angles at the base to negative sheet
angles at the apex. DTMRI-determined sheet structures (lateral
region) in PA, BV⫹, and BV⫺ hearts were also reconstructed
(Fig. 4). Qualitative agreement was evident between DTMRImeasured sheet structure and that revealed by histological
staining, which is similar to the corroboration provided in a
recent study by Tseng et al. (41). Visual inspection of the
reconstructed sheets suggested that reorientation of the sheet
toward the radial direction occurred in both the BV⫹ and BV⫺
groups.
Transmural courses of ␤s are shown in Fig. 5. In accordance
with histology, ␤s changed from predominantly positive at the
base to predominantly negative at the apex. The magnitude of
␤s (兩␤s兩), which is an index of local sheet slope, showed a trend
to decrease from diastole (PA hearts) to early systole (BV⫹
hearts), and was significantly decreased by end systole (BV⫺
hearts). Quantitative analysis of DTMRI data revealed 23 and
44% decreases in 兩␤s兩 in all sectors in BV⫹ and BV⫺ hearts,
respectively (Fig. 6). The 兩␤⬘s兩 values measured from histological analysis exhibited similar changes, as follows: from 34 ⫾
4° in diastole to 18 ⫾ 7° in systole (P ⬍ 0.05) at the base and
Fig. 2. Transmural distribution of myofiber helix angle (␣h) in PA, BV⫹, and BV⫺ groups. Changes in ␣h were evaluated at base, midventricle (Mid), and apex.
A: transmural distribution of ␣h in PA and BV⫹ groups. B: transmural distribution of ␣h in PA and BV⫺ groups. TD, transmural depth; Endo, endocardial surface
(TD ⫽ 0%); Epi, epicardial surface (TD ⫽ 100%). *P ⬍ 0.05; †P ⬍ 0.001 compared with PA group.
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quantified on the whole short-axis slice at the base, midventricle, and apex to characterize changes of myofiber orientation
from diastole (PA) to early systole (BV⫹; Fig. 2A) and from
diastole (PA) to end systole (BV⫺; Fig. 2B). The transmural
distribution of ␣h in BV⫹ hearts was similar to that in PA
hearts with only a slight decrease at the epicardial region. In
the BV⫺ group, ␣h increased from the endocardial to midwall
region (5– 65% transmural depth; P ⬍ 0.05) and decreased at
the subepicardial region (85–95% transmural depth; P ⬍ 0.05).
The lateral region demonstrated the greatest changes in fiber
angles. At the midventricular level, lateral ␣h changed from
47 ⫾ 9° (endocardium) to ⫺50 ⫾ 10° (epicardium) in the PA
group, from 48 ⫾ 9° (endocardium) to ⫺60 ⫾ 7° (epicardium)
in the BV⫹ group, and from 65 ⫾ 10° (endocardium) to
⫺66 ⫾ 7° (epicardium) in the BV⫺ group. Changes in ␣h at
the anterior, inferior, and septal regions were less pronounced
but similar to the changes in the lateral region.
The through-wall difference of the helix angle, ⌬␣h, increased as a result of the changes in ␣h during cardiac contraction (Fig. 3). Maximal increase in ⌬␣h was observed in the
lateral region. At the midventricular level, ⌬␣h was 96 ⫾ 10°
in the PA group, 108 ⫾ 8° in the BV⫹ group (P ⬍ 0.05
compared with PA), and 131 ⫾ 10° in the BV⫺ group (P ⬍
0.001 compared with PA). When averaged over the whole
slice, ⌬␣h values at the base, midventricle, and apex were
102 ⫾ 10, 105 ⫾ 6, and 102 ⫾ 7°, respectively, in PA hearts;
109 ⫾ 7, 111 ⫾ 8, and 101 ⫾ 12°, respectively (P ⬍ 0.05 at
midventricle), in BV⫹ hearts; and 121 ⫾ 6, 134 ⫾ 4, and
113 ⫾ 6°, respectively (P ⬍ 0.001 at all three levels), in BV⫺
hearts.
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MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
from 35 ⫾ 6° in diastole to 21 ⫾ 3° in systole (P ⬍ 0.05) at
the apex. These data provide direct evidence of reorientation of
the sheet toward the radial direction during cardiac contraction.
DISCUSSION
The present study was designed to quantify the changes of
cardiac fiber and sheet structure from diastole to systole with
respect to their independent contributions to local wall thickening. Despite previous detailed investigations of the fiber and
laminar structure of the left ventricle, quantitative descriptions
of fiber structural changes during ventricular contraction are
lacking. Early studies that compared diastolic and systolic
myocyte orientations were conducted by Streeter et al. (36)
with the use of histological methods. Because of large intersubject variations, histological analyses of hearts fixed separately in diastole and systole have not succeeded in demonstrating statistically significant changes in myofiber angles with
contraction. However, calculations based on diastolic fiber
structure and ventricular geometry in both diastole and systole
have predicted a small increase in the endocardial fiber angles
in systole (35). Takayama et al. (39) have also calculated a
small decrease in fiber angle (⬍5°) with increased end-diastolic pressure based on measured myocardial deformation.
Compared with histology, DTMRI allows direct quantification of structural changes during cardiac contraction in the
same heart. In a recent in vivo study on a normal human
subject, Dou et al. (8) observed the broadening of the histogram of myofiber helix angles at end systole, which suggests
that fibers become more longitudinally oriented in systole.
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However, spatial changes in the helix angle could not be
evaluated because of low spatial imaging resolution. The
present study provides the first quantitative measurements of
the myofiber and sheet structural changes during active cardiac
contraction. These observations both confirm and quantify the
alteration in myofiber helix angles at end systole.
We deduce from these data that the contraction-induced
changes in myofiber orientation per se will not contribute
directly to the wall thickening for the following reason. The
cross-section of an obliquely oriented myocyte is typically an
ellipse on the short-axis plane, and the diameter of the ellipse
is determined by the diameter of the myocyte and the helix
angle of the myofiber. Changes in myofiber orientation alone
do not change the diameter of such ellipses in the radial
direction, which is the direction of wall thickening.
In contrast, circumferential shortening could result in part
from systolic changes in fiber orientation. The projection of the
myocyte diameter in the circumferential direction can be significantly reduced when the myocytes become more longitudinally orientated (Fig. 7). A simple calculation suggests that
even with the 8% increase in myocyte diameter by end systole,
the observed increase in helix angle can lead to up to 12%
decrease in circumferential diameter at 30% transmural depth.
Therefore, changes in fiber orientation may provide a significant mechanism for circumferential shortening.
The mechanical role of the laminar sheet structures of heart
with respect to myocardial wall thickening was investigated in
a series of studies by analyzing diastolic sheet structure and
systolic wall strain. LeGrice et al. (20) previously reported that
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Fig. 3. Through-wall difference of myofiber
helix angle (⌬␣h) in PA (open bars), BV⫹
(gray bars), and BV⫺ (solid bars) groups. A:
lateral. B: septum. C: anterior. D: inferior.
*P ⬍ 0.05; †P ⬍ 0.001 for intergroup comparisons.
MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
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for the inner third of the myocardium, maximum sheet sliding
was coplanar with the orientation of local myocardial sheets.
Costa et al. (7) also reported large sheet-normal shear strain in
the inner half of the myocardial wall. In addition, they also
observed negative sheet-normal strain and positive sheet strain,
indicating sheet thinning and extension. Furthermore, it was
demonstrated that both sheet extension and sheet-normal shear
contributed significantly to the radial wall thickening. Based on
these observations, it was proposed that the shearing of adjacent muscle layers along the cleavage planes between them
might provide a mechanism of systolic wall thickening. However, direct measurements of the changes of sheet structure
have not been reported.
Dou et al. (9) recently combined diffusion and strain rate
measurements in an MRI study involving human subjects. By
expressing the strain tensor in the local fiber and sheet coordinates, the fractional contribution of each strain component to
the overall radial thickening was calculated. It was demonstrated that a major contribution to radial thickening was
associated with sheet-related strains, but that the three components of fiber-related strains contributed less to radial thickening. Their observation of sheet-normal thickening (positive
sheet-normal strain) deviates from Costa’s observation of sheet
thinning. Nevertheless, the significant contribution from sheetnormal shear strain provides additional support that transverse
sheet shear contributes to systolic wall thickening. However,
quantitative evaluation of the geometric changes in the sheet
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structure induced by sheet-shear strain could not be evaluated
because of low imaging resolution.
The observed sheet-normal shear will lead to the reorientation of myocardial sheets toward the radial direction during
systole (7). An earlier study by Spotnitz et al. (33) also
indicated that an increase in wall thickness was associated with
a decreased sheet slope and an increased number of fibers
across the wall, rather than with an increase in fiber size. The
progressive decrease of the sheet slope from diastole to systole
observed in the present study for the first time provides direct
evidence of the changes in sheet orientation during ventricular
contraction. These observed changes in sheet angle were uniform in the whole left ventricle despite the considerable regional variations in sheet structure. On average, sheet angle
changed from 36° at end diastole to 20° at end systole. Thus
sheet reorientation alone may increase wall thickness by 16%.
Accordingly, for an average wall thickening of 40%, reorientation of the sheet toward the radial direction may contribute up
to 40% of the overall wall thickening. This observation is in
agreement with the estimation from strain measurements provided by Costa et al. (7). However, our present study cannot
quantify the contribution from sheet extension, i.e., positive
sheet strain measured by Costa et al. (7). Given the small
increase in myofiber diameter and the fact that changes in fiber
orientation do not directly contribute to radial wall thickening,
it is very likely that sheet extension is another important
mechanism for wall thickening.
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Fig. 4. A and B: hematoxylin and eosin-stained, long-axis slices of rat hearts fixed at diastole (A) and systole (B), respectively. C: quantification of diastolic sheet
angle of the boxed areas in A. D: quantification of systolic sheet angle of the boxed areas in B. Measured orientation of sheet cleavage plane was overlaid. E:
reconstructed sheet structure from diffusion tensor MRI (DTMRI) data at base, midventricle, and apex. Sheet surfaces are color coded with local myofiber helix
angle.
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MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
Longitudinal shortening is another important component of
ventricular contraction. It is directly associated with the contraction of longitudinally and obliquely oriented fibers. Previous studies have suggested that longitudinal shortening may
occur before radial shortening, so that the LV cavity initially
becomes more spherical (15). Longitudinal shortening has
been observed during the isovolumic phase of contraction and
can reach 10 –12% at end systole in dog heart (12). In the
Fig. 6. Average magnitude of the sheet angle (兩␤s兩) across the wall at base, midventricle, and apex in PA (open bars), BV⫹
(gray bars), and BV⫺ (solid bars) groups. A:
lateral. B: septum. C: anterior. D: inferior.
*P ⬍ 0.05; †P ⬍ 0.001 for intergroup comparisons.
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Fig. 5. Transmural distribution of lateral sheet angle (␤s) at base, midventricle, and apex. A: from PA to BV⫹. B: from PA to BV⫺. *P ⬍ 0.05; †P ⬍ 0.001
compared with PA group.
MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
present study, the measured longitudinal shortening in contracture-arrested hearts was 14%, which is similar to that observed
in vivo. Interestingly, longitudinal shortening and reorientation
of the sheet toward the radial direction occurred progressively
from PA to BV⫹ to BV⫺, which suggests that sheet reorientation may be directly related to the longitudinal contraction of
myocardial fibers.
The parallel relationship between the axes of myofiber sheet
structure and the eigenvectors of the diffusion tensor is supported by the results of previous studies and the present study.
The work of Scollan et al. (30) showed a qualitative agreement
between the helix angle of the sheet normal and that of the
tertiary eigenvector. Recently, Tseng et al. (41) validated this
relationship in a quantitative study. In the present study,
DTMRI-determined changes in sheet angle from diastole (PA)
to systole (BV⫺) were comparable to those measured from
histology. Our data provide additional evidence that DTMRI
can be used as a nondestructive method to characterize alterations in myocardial fiber and sheet structure.
Several factors may have contributed to the large standard
deviation of the DTMRI-determined sheet angle. First, the
sheet structure of the myocardium demonstrates significant
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heterogeneity. Our present imaging resolution was not sufficient to depict the finer structure of myocardial laminae. The
slice thickness of the MR images ranged from 0.8 to 1.0 mm,
whereas the average thickness of each individual sheet is ⬃50
␮m (19). Thus the measured sheet angle in each image voxel
represents the average value from ⬃20 sheets. Second, sheet
branching was observed in the wall (2, 19), which will contribute to the measurement error for the sheet angle. Finally,
although the three eigenvalues are statistically distinct from
one another, the measured ␭2/␭3 value was ⬃17% lower than
␭1/␭2. Thus somewhat higher sorting errors for the secondary
(sheet direction) and tertiary (sheet-normal direction) eigenvectors are anticipated.
BaCl2 was used to induce myocardial contracture. Although
barium is paramagnetic, the work of Litt and Brody (21)
suggests that a low concentration of barium (⬍25 mM) in
agarose gel does not substantially alter the relaxation time of
the gel. Nevertheless, this approach has several restrictions.
First, the heart must be fixed in the systolic phases to avoid
degradation of barium-induced contracture during image acquisition (31). Although formalin fixation did not substantially
change fiber structure in physiological tissues (13, 29, 37), the
irreversible fixation process prevented us from analyzing fiber
and sheet structure at multiple phases of ventricular contraction
from the same heart. Second, a small amount of perfusate can
be observed between the balloon and the endocardial surface
from MR images, which indicates that the intraventricular
balloon did not occupy the whole LV cavity. As a result, the
wall thickness was slightly increased in the BV⫹ group compared with the PA group. However, these changes were very
small, and the BV group still should be directly informative of
events that accompany isovolumic contraction.
Diffusion-encoding gradients were applied only in six noncollinear directions to minimize image-acquisition time. Although the accuracy of DTMRI-determined fiber architecture
could be improved by increasing the directions of the applied
diffusion-encoding gradients and using the optimized diffusion-encoding scheme (16), the present diffusion-encoding
scheme, which required minimal image-acquisition time, was
optimized to maximally preserve myocardial contractility of
the isolated perfused heart. The DTMRI-determined myofiber
and sheet orientations were consistent in each group of hearts.
Direct evaluation of myocardial fiber and sheet structure
may have important clinical implications. Present clinical approaches to evaluate regional ventricular function are based
mainly on wall thickening. However, recent studies indicate
that developed ventricular wall stress and strain are very
sensitive to changes in fiber and sheet structure over the cardiac
cycle (5, 24 –26, 38). Thus it is conceivable that evaluation of
preclinical abnormalities of ventricular function can be improved by incorporating quantitative data on fiber and sheet
structure in systole (44). Furthermore, characterization of such
structural changes in diseased hearts may facilitate the investigation of the mechanisms of structural and functional adaptations in the ventricular remodeling process.
In summary, we have shown for the first time that geometric
changes in both fiber and sheet orientations provide a substantial mechanism for radial wall thickening independent of active
components due to myofiber shortening. In other words, myocyte contraction contributes to radial wall thickening and
ventricular ejection both by myocyte shortening and by the
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Fig. 7. Contribution of myofiber reorientation to circumferential shortening
during cardiac contraction. Value of dc, the cross-sectional length of myocyte
in the circumferential direction (Xc), is determined by the myocyte diameter
(d) and the helix angle (␣h). Myofiber becomes more longitudinally oriented
(increased ␣h) from diastole to systole, which leads to a decrease in dc.
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MYOCARDIAL STRUCTURAL CHANGES DURING CONTRACTION
related secondary induction of changes in fiber and sheet
organization. This mechanism depends critically on the interaction of the myocytes and the extracellular matrix throughout
the ventricular wall during myocardial contraction. The complexity of this mechanism of wall thickening suggests that
abnormalities in either the contractile apparatus itself (myocyte) or the infrastructure (extracellular matrix and cellular
syncytium) can dramatically affect wall thickening, and as
such, both require concordant assessment for comprehensive
and accurate analysis of contractile abnormalities.
15.
16.
17.
18.
19.
ACKNOWLEDGMENTS
20.
21.
GRANTS
22.
The authors acknowledge the support of the Washington University Small
Animal Imaging Resource, which is funded in part through National Cancer
Institute Small Animal Imaging Research Program Grant R24 CA-83060. This
work was also supported by National Institutes of Health Grants R01 HL73315 (to X. Yu) and R01 HL-42950 (to S. A. Wickline). J. Chen was
supported by Predoctoral Fellowship 0315249Z from the Heartland Affiliate of
the American Heart Association.
23.
24.
25.
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