Magnetic Resonance Elastography as a Method for the
Assessment of Effective Myocardial Stiffness
Throughout the Cardiac Cycle
Arunark Kolipaka, Philip A. Araoz,* Kiaran P. McGee, Armando Manduca,
and Richard L. Ehman
MR elastography (MRE) is a noninvasive technique in which
images of externally generated waves propagating in tissue
are used to measure stiffness. The first aim is to determine,
from a range of driver configurations, the optimal driver for
the purpose of generating waves within the heart in vivo. The
second aim is to quantify the shear stiffness of normal myo-
cardium throughout the cardiac cycle using MRE and to com-
pare MRE stiffness to left ventricular chamber pressure in an
in vivo pig model. MRE was performed in six pigs with six dif-
ferent driver setups, including no motion, three noninvasive
drivers, and two invasive drivers. MRE wave displacement
amplitudes were calculated for each driver. During the same
MRI examination, left ventricular pressure and MRI-measured
left ventricular volume were obtained, and MRE myocardial
stiffness was calculated for 20 phases of the cardiac cycle. No
discernible waves were imaged when no external motion was
applied, and a single pneumatic drum driver produced higher
amplitude waves than the other noninvasive drivers (P < 0.05).
Pressure–volume loops overlaid onto stiffness–volume loops
showed good visual agreement. Pressure and MRE-measured
effective stiffness showed good correlation (R
2
5 0.84). MRE
shows potential as a noninvasive method for estimating effec-
tive myocardial stiffness throughout the cardiac cycle. Magn
Reson Med 64:862–870, 2010. V
C
2010 Wiley-Liss, Inc.
Key words: myocardial stiffness; MRE; cine-MRE; stiffness–
volume loops
Myocardial stiffness relates myocardial deformation
(strain) to loading (stress) and is thought to affect the
heart’s function. To date, the primary method of evaluat-
ing myocardial stiffness in vivo has been by inferring it
from pressure–volume (P–V) relationships (1,2). For
example, it has been shown that patients with diastolic
heart failure exhibit increased chamber stiffness (dP/dV)
(3), as do patients with myocardial ischemia and patients
with myocardial infarction (4). However, P–V methods
are invasive, require technical precision, assess the left
ventricular (LV) chamber rather than the true intrinsic
properties of the myocardium, and provide only a global
measure of stiffness. Therefore, there is a need for a tech-
nique capable of noninvasively assessing true intrinsic
mechanical properties of the myocardium such as shear
modulus (i.e., shear stiffness or stiffness) (m).
MR elastography (MRE) is a novel imaging technique
that can be used to measure shear stiffness (5–9). In
MRE, cyclic motion is applied to a tissue and a phase-
contrast MR image is acquired in which motion-encod-
ing gradients are synchronized with the external
motion. This produces MRI images of the waves pro-
pagating in the tissue. The wave displacements
obtained from these images can be mathematically con-
verted to stiffness maps.
To date, MRE has been shown to resolve the shear
stiffness of static tissues (10,11). However, there are chal-
lenges to applying this technique to dynamic organs
such as the heart. These include performing faster data
acquisition to capture the different phases of cardiac
cycle and introducing external shear waves into the heart
while avoiding bulk motion artifacts. Previous studies
(12,13) have shown the feasibility of using a cine MRE
acquisition strategy in a simulated, dynamic LV spheri-
cal phantom when the acquisition is appropriately
synchronized with the motion of the phantom. One of
the studies (12) demonstrated a linear correlation
between effective stiffness and pressure, with the stiff-
ness estimates being validated against an established P–
V relationship.
There are two aims of this study. The first aim is to
determine, from a range of driver configurations, the
optimal driver for the purpose of generating waves
within the heart in vivo. The second aim is to quantify
the shear stiffness of normal myocardium throughout
the cardiac cycle using MRE and to compare MRE
stiffness to LV chamber pressure in an in vivo pig
model.
MATERIALS AND METHODS
Six pigs underwent cardiac MRE. In each pig, six driver
configurations were studied to determine the most opti-
mal method of delivering waves to the myocardium.
Shear stiffness measurements obtained from MRE wave
images using the optimized driver were compared to LV
pressure and MRI-measured LV volume obtained during
the same MRI examination.
Evaluating MRE Drivers
Experimental Setup
In vivo cardiac MRE was performed on six pigs (mean
weight: 43.2 kg; female) in compliance with our
Department of Radiology, Mayo Clinic, Rochester, Minnesota, USA.
Grant sponsor: National Institutes of Health; Grant number: EB001981;
Grant sponsor: Mayo CR 20.
*Correspondence to: Philip A. Araoz, M.D., Department of Radiology, Mayo
Clinic 200 First St.SW, Rochester, MN 55905. E-mail: paraoz@mayo.edu
Received 15 June 2009; revised 28 January 2010; accepted 10 March
2010.
DOI 10.1002/mrm.22467
Published online 23 June 2010 in Wiley Online Library (wileyonlinelibrary.com).
Magnetic Resonance in Medicine 64:862–870 (2010)
VC 2010 Wiley-Liss, Inc. 862

institutional animal care and use committee. The ani-
mals were anesthetized by intramuscular injections of a
cocktail containing telazol (5 mg/kg), xylazine (2 mg/kg),
and glycopyrrolate (0.06 mg/kg) and were maintained
using an isoflurane inhalation anesthesia (1–3%) and
mechanical ventilation.
Mechanical Wave Generation
To study wave generation, mechanical waves were intro-
duced into the heart, using six different driver configura-
tions (Fig. 1). In the first case, no driver was used and
therefore provided a control method. In the second case
(Fig. 1a, 1-driver), one large noninvasive pneumatic
drum of 13.7-cm diameter was placed on the chest wall.
All the pneumatic drums in our experiments were made
up of acrylic, and the diaphragm, i.e., drumhead, was
made up of polycarbonate with 0.02-inch thickness. In
the third case (Fig. 1b, 2-driver), two small noninvasive
pneumatic drums of 8-cm diameter each were placed ad-
jacent to each other on the chest wall and were driven in
phase. In the fourth case (also Fig. 1b, 2-driver), two
small noninvasive pneumatic drums were placed adja-
cent to each other on the chest wall and were driven out
of phase. In the fifth case (Fig. 1c, Suture), the chest wall
was opened and a thread was sutured directly to the an-
terior wall of the LV, whereas the other end of the thread
was attached to a pneumatic driver. In the sixth case
(Fig. 1d, Direct Contact), with the chest open, a small
pneumatic drum was inserted into the chest cavity and
placed directly on the heart. In the first four cases, the
pneumatic drums were placed on the chest wall with
straps and no coupling gel was used. The amplitude
vibrations experienced by all the drumheads were in the
range of 100–200 mm by providing equal amounts of
power in all cases.
Image Acquisition
All imaging was performed on a 1.5-T MRI scanner
(Signa Excite; GE Health Care, Milwaukee, WI). The ani-
mals were positioned in the supine position and placed
feet first into the scanner. A cine gradient-echo retro-
spective-gated MRE sequence (13) was used to measure
the external motion in the myocardium in two-chamber
long-axis and a single short-axis slice. The short-axis
slice was immediately basal to the papillary muscles.
Mechanical waves were introduced into the heart by the
six different methods, as described above. In the first
case, no driver was used and no mechanical waves were
applied. This was necessary to determine the contribu-
tion from bulk intrinsic, physiologic motion of the heart
in the MRE data. In all other experiments, the driving
methods described above were implemented to deliver
the motion to the heart. A phased-array receive-only coil
was used for all acquisitions. When the chest was open,
the anterior and posterior coil elements were reposi-
tioned to be on either side of the chest. Imaging parame-
ters included pulse repetition time ¼ 25 msec, echo time
¼ 11.7 msec, field of view ¼ 27 cm, flip angle ¼ 30


,
slice thickness ¼ 5 mm, acquisition matrix ¼ 256  64,
receiver bandwidth ¼ 616 kHz, excitation frequency ¼
80 Hz (12.5 msec) and were applied continuously, with
multiple cycles of motion matching the pulse repetition
time, heart rate ¼ 63–100 beats/min, views per segment
¼ 4, four MRE phase offsets, and one 6.25-msec duration
FIG. 1. Examples of the four experimental
setups studied indicating the location of
the pneumatic drivers: (a) one-driver sys-
tem, (b) two-driver system, (c) suture sys-
tem, and (d) direct-contact system. In all
of the figures, the dotted white arrow
shows the location of the drivers and the
bold white arrow shows the pigtail pres-
sure sensor catheter inserted into the left
ventricle through the femoral artery. All the
figures also show inset cartoons demon-
strating all different driver setups. [Color
figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Myocardial Stiffness Using MRE 863