The role of magnetic reson-ance imaging (MRI) in cardiac patients has evolved significantly over the last decade, with MRI now considered most useful in the evaluation of the pericardium, complex congenital heart disease, and the right ventricle. The radiologist's role has changed as well, from one of "hands-off" to that of being an integral team member together with cardiologists in the evaluation of heart disease. The future of cardiac MRI is bright as applications have been developed for nearly every facet of heart disease evaluation.
After receiving an Associate degree from North Central
Technical College in Mansfield, Ohio, Dr. Calendine worked as a
radiologic technologist until graduating with a Bachelor degree
from Ashland University in Ashland, Ohio. He attended medical
school at Wright State University, School of Medicine in Dayton,
Ohio. Dr. Calendine is currently in radiology residency,
post-graduate year four, at Ohio State University.
Magnetic resonance imaging (MRI) of the heart has many potential
applications in the noninvasive evaluation of cardiac disease,
including congenital anomalies, cardiomyopathy, neoplasms, valve
disease, coronary artery disease, and myocardial perfusion,
function, and viability.
Cardiac MRI is unsurpassed in anatomic detail, and superior in
volumetric and motion analysis. Now more widely available thanks to
advances in computer technology, MRI of the heart is beginning to
take a more important role in the work-up of cardiac disease.
The intent of this article is to serve as an overview of current
cardiac MRI techniques and clinical applications.
The heart, an organ nearly abandoned by radiologists, has had a
remarkable resurgence in popularity. This cardiac renaissance has
resulted in an increased demand by consultant cardiologists, who
often lack familiarity with MRI, rejuvenating a symbiotic
relationship with the radiologist similar to that in nuclear
cardiology. Currently, MRI is still often reserved for cases in
which echocardiography (ECHO) has been nondiagnostic or confusing.
ECHO, although less expensive
and more readily available and familiar to clinicians, is far
inferior to cardiac MRI for anatomic structure detail and
volumetric assessment of the atria and ventricles.
Getting started: Technique
Perhaps the greatest obstacle in obtaining cardiac MR images is
the continuous movement of the heart and adjacent great vessels.
Since the early evaluation of electrocardiogram- (ECG) gating MR in
intense work has continued in the field to acquire images with
fewer artifacts. Achieving this requires more precise cardiac
gating, reducing scan time, and increasing signal-to-noise ratio
--In ECG-gated imaging, the R wave is used for acquisition
triggering in order to obtain each portion of an image at the same
time in the cardiac cycle. The phase encoding steps occurring at
the same time interval with respect to the R wave for multiple ECG
waves are then used in creating an image of the heart at that
single point in the cardiac cycle. For instance, when the R wave is
detected, phase encoding then occurs x seconds afterward. At the
next R wave, advancement of the phase encoding is triggered to
occur again x seconds after the R wave, and so on (figure1). With
the appropriate number of phase encoding steps (usually 256 or
512), an image is generated. When interference occurs with the ECG
signal, altering the R wave, the detector switch responsible for
initiation of phase encoding can be activated inappropriately. The
result is randomization of acquisition giving images that are
For optimization of ECG signal, the skin must be cleaned and
abraded to obtain good contact. Care must be taken for appropriate
lead placement on the chest, typically anterior over the heart, all
within a 10-cm radius (figure 2), and the lead wires should exit
the bore over the shoulder parallel to the magnetic field to reduce
A new approach to cardiac gating known as vectorcardiographic (VCG)
gating could potentially negate the dependence on traditional ECG
By depending on the direction of the heart movement as detected by
VCG instead of the electrical conduction, more accurate gating can
be achieved because of avoidance of intrinsic ECG noise from
magnetohydrodynamic effect signal created by flow of blood within
the patient in the magnetic field. In addition, gating for patients
with arrhythmias may be performed with greater accuracy.
A number of different coils and coil combinations can be used.
At our institution, two surface coils are used, one placed along
the anterior chest and the other posterior (figure 3). Four coils
can be used with the addition of coils along the lateral chest wall
--Many cardiac se-quences are in use today, each with specific
strengths and weaknesses. The most popular are the "black blood"
and "white blood" sequences. Black blood refers to fast double or
triple inversion recovery (IR) fast spin echo (FSE) techniques
obtained in a single breath hold.
Double IR is best at discriminating between structures and
evaluation of morphology, as soft-tissue contrast is optimized. A
low echo time (TE) (20 to 60 msec) is used for the double IR-FSE.
The repetition time (TR) is dependent on the patient's heart rate
and is usually equal to the R-R interval on the ECG (i.e., heart
rate of 60 beats per minute will have an R-R interval of 1 second
and the TR would be 1,000 msec). The blood in this case does not
receive the full 90 and then 180 degrees radiofrequency (RF) pulses
given in IR because it is moving, hence diminishing the signal.
White blood refers to ultrafast gradient echo cine images. White
blood images are best at depicting wall motion, and can be used for
They are acquired using FSPGR (fast spoiled gradient-recalled
acquisition in the steady state) with TE minimized, a flip angle of
15 to 20 degrees, and a very short TR (i.e., 8 msec). Circulation
during the sequence moves blood with unsaturated protons into the
imaging volume with resulting high signal of the blood. For cine of
a cardiac cycle, 16 to 30 images are required.
Echo-planar imaging (EPI) may also be instituted in cardiac MR
because of the extraordinarily short acquisition time.
Unlike conventional MR sequences, EPI can fill in the entire
k-space of an MR image with one measurement, like a snapshot.
Images may be taken without the use of ECG gating and even without
a breath hold. The major deficiency with single-shot EPI is
decreased detail compared with IR-FSE techniques. Fat suppression
is required for EPI sequences, and the heterogeneity of the heart
can result in image degradation. Also, IR sequences reduce the
artifact caused by blood flow. Multi-shot EPI demonstrates great
promise, particularly for the evaluation of the coronary arteries.
Multi-shot EPI has the advantage of increased detail compared to
single-shot EPI, but ECG gating is then needed to reduce motion
Another technique, developed in 1997, is termed SMASH
(simultaneous acquisition of spatial harmonics) imaging.
The SMASH technique exploits the geometry of an RF coil array to
encode multiple lines of MR image data at the same time, decreasing
image acquisition time significantly.
There is ongoing research in the evaluation, application and
im-provement of these techniques.
Another useful technique developed about 10 years ago is called
A grid of magnetic saturation is produced by applying a sequence of
RF pulses, either with the magnetic field gradients switched on
while they are applied
or separated by magnetic field gradients.
This creates a crosshatch pattern made of alternating low (initial
RF pulses) and high (conventional sequence) signal (figure 4).
During the cardiac cycle, the boxes of the grid change shape,
allowing for quantification of wall thickening and motion using
mathematical analysis. Though shown to be quite accurate, it is
labor intensive in post-processing time.
Two popular sequences for achieving myocardial tagging are used.
One is termed CSPAMM (complementary spatial modulation of
magnetization), described by Fischer et al
as improvement on the originally described SPAMM (spatial
modulation of magnetization) sequence.
The other sequence is called DANTE (delays alternating with
nutations for tailored excitations). The biggest advantage of the
DANTE tagging sequence is higher resolution of the tags and the
ability to get more closely spaced tag lines.
In DANTE, myocardial velocity mapping, also used for myocardial
motion analysis, uses the phase shifts of the spins to encode the
velocity into the MR signal. Once the myocardial contours have been
segmented, the data can be automatically processed to obtain
Other areas of technique research, include 23Na MRI of the
which can be used to detect myocardial viability and diffusion
imaging are in early investigation.
At the RSNA 2000 meeting, another new method called UNFOLD was
reviewed. UNFOLD exploits the variable movement of the field of
view (FOV) centrally compared with the relatively static periphery,
and can result in significant reduction of image acquisition time.
Radiologists feel comfortable with the classical anatomic
planes: axial, sagittal, and coronal. From computed tomography (CT)
images, we are accustomed to seeing the heart in an axial plane.
Nothing is more confounding in image interpretation than becoming
disoriented to the relation of the anatomic structures. For this
reason, some radiologists in the field of cardiac MRI argue that
looking at images in the three classical anatomic planes will
suffice in many cases.
However, having the long and short axes of the heart obliquely
oriented across the classical planes can result in over estimation
of wall thickness, and functional studies cannot be performed
So it is important, if not imperative, to evaluate the heart with
the views analogous to those used in ECHO. Indeed a tremendous
advantage of MRI is the ability to image in any plane, and some of
these views require double obliquity from the classical planes.
Obtaining these "ECHO-equal" images requires knowledge of the
planes and how to prescribe them.
At our institution, a gradient-recalled sagittal localizer is
obtained first (figure 5A). From this, images are prescribed
parallel to the long axis of the heart, from the middle of the
mitral valve to the ventricular apex. Images then taken at a right
angle (a coronal oblique) to the long axis images, but still along
the long axis, will be the 4-chamber views (figure 5B). Using
long-axis views, a short-axis FSE cine series can be prescribed,
perpendicular to the long axis (figure 5C). Utilizing the
short-axis images, an outflow tract view is prescribed (figure 5D).
The result is an off-coronal plane, which parallels the aortic
Long-axis 4-chamber views are best for evaluating ventricular
walls and chambers. Aortic and mitral valves may also be analyzed
in this view. Short-axis views demonstrate concentric rings of
myocardium from base to apex, and can be used for quantification of
myocardial thickening, analogous to cardiac single photon emission
computed tomography (SPECT) imaging.
Cardiac MRI applications
As cardiac MR has evolved, a wide array of applications have
been proposed. Currently, MR is most often utilized when high
anatomic detail is needed (as for the pericardium), for further
characterization of abnormalities discovered on ECHO, for diagnosis
and post-surgical follow-up of complex congenital heart
malformations, and for ventricular morphological and functional
evaluation. Other applications include coronary MR angiography,
myocardial perfusion and viability, valvular heart disease, and
--A perplexing question not uncommonly raised is the
differentiation of restrictive cardiomyopathy versus constrictive
pericarditis. MRI offers far superior pericardial evaluation than
ECHO, and can, at the same time, evaluate physiologic changes
occurring with heart function.
Axial and coronal planes can completely image the pericardium,
whereas the pericardium is very difficult to evaluate with ECHO,
particularly posteriorly (figure 6).
MRI is also very sensitive for detection of pericardial
Tumor localization and characterization
--ECHO is the initial modality of choice for the work-up of cardiac
tumors. The spatial resolution and real-time evaluation are beyond
that of other modalities. The greatest advantage with MRI, however,
is the unsurpassed soft-tissue contrast it provides. Abnormalities
seen on ECHO may be misleading as to where they are located or what
they are. Ectopic or displaced normal structures may even mimic
Paracardiac, pericardial, and cardiac tumors can be depicted on
MRI, and may be characterized by their signal on different
sequences (figures 7 and 8). For instance, angiosarcoma, the most
common primary malignancy of the heart, often hemorrhages,
resulting in high signal intensity on T1-weighted images. Cardiac
lipoma or lipomatous hypertrophy of the atrial septum both show
increased T-1 signal which decreases after fat saturation.
Benign and malignant tumors about the heart increase morbidity and
as lesions that are histologically benign may be malignant by
location. Tumor extent when not well displayed on ECHO can be of
particular importance in prognostication, treatment op-tions, and
Congenital heart disease
--MRI made a place for itself in studying patients with congenital
heart disease (CHD) before and after surgical intervention (figure
Visualization of posterior mediastinal structures and su-pracardiac
anatomy is possible with MR, both of which are blind spots on ECHO.
Functional MRI can limit the need for repeated angiography to
follow patients along their course. Because of these strengths,
many clinicians now turn to MRI for evaluation of complex
cardiovascular anomalies (figure 10). In addition to the anatomic
depiction of CHD, MRI is capable of functional imaging, including
measurement of intracardiac shunts, differential pulmonary blood
flow, pressure gradients across valvular and vascular stenoses, and
valvular regurgitant fraction.
When used in children, these techniques often require conscious
sedation with close nursing and anesthesiologist supervision.
Sedated patients cannot hold their breath on demand, requiring
gating with the respiratory cycle. A technique called
respiratory-ordered phase encoding (ROPE)
orders phase-encoding steps in such a way as to reduce
respiration-induced artifacts by selecting a phase angle
proportional to the current phase of the respiratory cycle.
ROPE can greatly improve image quality and requires no additional
Evaluation of the cardiac images should be done using a
segmental approach, which is beyond the scope of this article, as
this approach has been established as a reliable way to accurately
diagnose and describe CHD.
Evaluation of ventricles and cardiomyopath
y--Cardiomyopathies can be placed into five categories;
restrictive, hypertrophic (figure 11), dilated, obliterative, and
dysplastic. Dilated cardiomyopathy is the most common in the United
States, usually secondary to coronary artery disease (CAD). Cine
MRI or hybrid EPI is important for assessment of ventricular
function in that it gives excellent visualization of the
morphologic changes that occur. Blood flow and wall thickening can
be seen, and parameters, including velocities and ejection
fraction, can be calculated.
Although the left ventricle can usually be well demonstrated on
ECHO, the right ventricle is often not. This is due to the complex
geometry of the right ventricle and its location behind the
sternum. For this reason, right ventricular dysfunction often goes
undiagnosed until late in disease, after it becomes clinically
apparent. MRI can be useful in early detection of poor ventricular
function, as in cor pulmonale, and thereby result in earlier
MRI of the right ventricle has also proven beneficial in the
diagnosis of arrhythmogenic right ventricular dysplasia
(ARVD)--also called right ventricular cardiomyopathy--in which the
degeneration of right ventricular myocytes is followed by
infiltration of lipoproteinaceous or fibrous material.
The first clinical sign of ARVD is often ventricular tachycardia
(VT), frequently elicited by exercise, but cardiac arrest may be
the presenting manifestation.
In those who survive, conventional work-up with ECHO, angiography,
and SPECT often reveal a normal heart. MRI may show increased
signal within the right ventricular wall (in 50% to 60% of cases),
right ventricular wall thinning or other nonspecific findings of
systolic free wall bulging, right ventricular outflow tract
bulging, or regional wall abnormalities.
Two-dimensional (2D) volumetric analysis of ventricular volume
and ejection fraction using geometric estimation by modified
Simpson rule has been verified and widely applied.
Three-dimensional (3D) volume rendering has proved laborious and
not significantly more precise.
A new development has been made in the evaluation of left
ventricular function building on the conventional 2D methods of
ventricular wall contour mapping in the short axis
to make 3D volumetric assessment possible. Adding information of
contour mapping from long axis views appears to increase accuracy
of left ventricular volume by better defining the level of the
Valvular heart disease
Although the role for MRI is currently limited in the realm of
valve disease, it has been shown efficacious in the ability to
diagnose and quantify valvular abnormalities.
MRI of the heart valves can be used for identification of valvular
stenosis with or without regurgitation (figure 12). Pressure
gradients across a stenotic valve can be calculated. The role of
MRI has primarily been investigational to date, however it often
proves superior in the determination of ventricular mass and
volumes and is effective in monitoring ventricular changes with
disease progression. In the same way, MRI can be utilized for
evaluation of treatment efficacy. Additionally, MRI using velocity
encoded cine imaging is the only noninvasive modality for objective
quantification of valve regurgitation even with multivalvular
Ischemic heart disease
Coronary artery disease is the number one cause of death in the
United States today in both men and women. Because of the high
prevalence of the disease and the catastrophic outcome, early
detection and treatment are imperative. A tremendous amount of work
has been done and is in progress in the application of MRI to the
detection of myocardial ischemia, perfusion, and viability.
Magnetic resonance angiography (MRA) is also under development, and
new techniques for coronary MRA show promise in CAD detection.
Drawing from methods developed for nuclear cardiology and ECHO,
an MRI cardiac stress test has been developed.
The MRI stress test has some advantages over both. Using the MRI
stress tests allows higher spatial resolution than cardiac SPECT,
and on MRI, the full ventricular wall thickness and individual
regions can be analyzed, whereas ECHO is limited to evaluation of
transmural thickness abnormalities alone. MRI stress test
sensitivity and specificity have been shown to equal stress ECHO,
but the detailed MR imaging greatly facilitates wall motion
analysis and ejection fraction calculation, which can be determined
objectively rather than subjectively.
Cardiac MRI in the twenty-first century will be an exciting and
remarkably dynamic field, with applications encompassing all
aspects of heart disease evaluation. MRI has become more practical
as computer speed has increased, and innovative sequencing has been
developed. Its usefulness can no longer be disputed in many cases
and it is of particular benefit in evaluating the pericardium and
right ventricle. Complex congenital heart anomalies are better
depicted with MRI, significantly aiding both preoperative planning
and postoperative follow-up. Valvular disease is often well
depicted, and gradients across stenotic valves can be noninvasively
and accurately measured. Stress cardiac MR imaging is being
performed with excellent correlation to ECHO, although work is
needed to make it more practical. Newer techniques including
coronary angiography, spectroscopy, and 3-D 23Na imaging show
promise for future applications. The role of the radiologist is now
quite different than in the recent past.
As demand for cardiac MRI increases and availability becomes
widespread, we will again be called upon for our expertise.