This issue of Applied Imaging discusses the various applications
of MRI for diseases of the heart. While MRI has been useful for
many years to characterize masses in the heart, only recently have
the new MR systems with stronger, faster "cardiovascular" gradients
been able to acquire images quickly enough to avoid motion
artifact. MRI can now be used to assess ventricular function,
myocardial perfusion and wall motion, valvular function, and
myocardial viability (using "delayed hyperenhancement" imaging).
With anticipated future improvements in coronary MR angiography,
MRI will truly give us the opportunity for "one-stop shopping" for
cardiac evaluation.William G. Bradley, Jr., MD, PhD, FACR
Cardiac magnetic resonance imaging (CMRI) has been performed for
almost 2 decades, since MRI was first introduced into clinical
Early nongated images were rapidly supplanted by those with cardiac
gating, using either chest EKG leads or finger plethysmography.
Typically, T1-weighted images were acquired gated to every r-wave
and T2-weighted images were acquired gated to every second or third
r-wave. Multislice, multiphase imaging was time-intensive and the
spatial and temporal resolution was fairly limited. The current
excitement in CMRI reflects the relatively recent introduction of
stronger, faster gradients and novel k-space acquisition methods.
As a result of the sophisticated imaging techniques and the
complicated anatomy and pathology in the heart, the most successful
CMRI programs are based on collaboration between MR radiologists
and cardiologists. This issue of
Applied Imaging: Applications in MRI
describes the current status of MRI of the heart.
The major technological advancements that have made CMRI a
clinical reality include stronger and faster gradients, the ability
to more reliably obtain ECG gating, and use of a surface coil.
Current CMRI units have gradients of 40 mT/m, rise times of 200 to
250 msec, and slew rates of 150 to 200 T/m/sec. Such speed is
necessary to provide images of reasonable spatial resolution of a
beating heart. With current technology, it is possible to provide a
complete image of one slice through the heart in 96 msec,
effectively eliminating motion artifact. A complete multislice,
multiphase (4D) data set of the heart can be acquired in a fraction
of a minute. CMRI units can provide 25-msec temporal and 1-mm
spatial resolution or better. Images are further optimized by using
a phased array surface coil.
The images acquired during the multiple phases of the cardiac
cycle can be viewed in a cine loop, thereby displaying the
contracting heart. Chest ECG leads provide optimal timing for image
acquisition, but finger plethysmography can be used if it is not
possible to obtain an adequate ECG signal. ECG lead technology has
improved with use of carbon fiber. Arrhythmias still pose problems
for image acquisition and bradycardia increases study time.
Magnetohydrodynamic (MHD) effects on the ECG elevate the T wave,
which can result in triggering on both the R and T waves thereby
yielding non-diagnostic cine images. MHD effects also reduce ST
segment depression, which can affect identification of myocardial
ischemia. The greater blood velocity induced by stress studies
results in a larger MHD effect, further complicating stress
The three orthogonal cardiac imaging planes used in nuclear
imaging include the short axis, 4-chamber (horizontal) long axis,
and 2-chamber (vertical) long axis images. Using fast gradient echo
imaging, the imaging plane can be adjusted in real time similar to
ultrasound (MR fluoroscopy).
The following imaging techniques are necessary for optimal CMRI:
1) "bright blood" cine imaging (for ventricular function and
stenotic and regurgitant valvular lesions) (Figure 1); 2) "black
blood" imaging (for anatomy) (Figure 2); 3) perfusion imaging (for
stress and rest perfusion to evaluate ischemia) (Figure 3); 4)
delayed hyperenhancement (for precise sizing of myocardial
infarction) (Figure 4); 5) real-time" imaging (for dobutamine
stress imaging and MR fluoroscopy); 6) phase-contrast imaging (for
flow quantification); 7) myocardial "tagging" (for quantification
of regional wall motion, stress and strain); and 8) MR angiography
(for evaluation of the coronary arteries and great vessels). Each
of these will now be discussed in the context of the specific
Ventricular function (ventricular volumes [systolic and
diastolic at stress and at rest], ejection fraction, stroke volume,
and cardiac output) is best assessed with "bright blood techniques"
These images are acquired with ECG gating using a fast gradient
echo pulse sequence, e.g., TrueFISP or FIESTA, with short TR (e.g.,
5 to 10 msec) and TE (e.g., 1 to 2 msec) and a flip angle of 10 to
20. Imaging starts after the R wave and can be prospectively gated
(i.e., traditional EKG triggering that samples end diastole poorly
due to respiratory variations) or retrospectively gated (which
requires rebinning the data after the acquisition to cover the
complete cardiac cycle). A segmented k-space acquisition (aka
"multishot echo planar imaging") reduces the time required to
acquire images by increasing the number of lines of k-space
acquired per cine frame. The cine frame duration equals the TR
interval multiplied by the lines of k-space acquired (typically 8)
and is ideally <100 msec to prevent ghosting. Images are
acquired at a single anatomic location for each breathhold with
multiple (e.g., 12 to 20) cine frames (phases) per RR interval.
Stationary tissue is saturated hence it is dark. Blood is bright
because of flow-related enhancement.
Myocardial ischemia imaging
Myocardial ischemia can be evaluated on the basis of perfusion
or wall motion, comparing rest and stress. Stress imaging with
either an intravenous (IV) infusion of a vasodilator (e.g.,
adenosine) or an inotropic agent (e.g., dobutamine) requires the
presence of medical personnel with expertise in evaluation of
ischemia and in advanced cardiac life support.
First-pass myocardial perfusion imaging utilizes a T1-weighted
pulse sequence that results in enhancement of normal myocardium.
These images are a result of perfusion and do not depend on
gadolinium moving into the myocyte. Therefore, on immediate
post-injection images, normal myocardium is bright, and nonperfused
myocardium is dark (Figure 3).
The pulse sequence used for myocardial perfusion is segmented
echo planar imaging in which it is possible to acquire >=7
anatomic slices per RR interval at rest and per 2 RR intervals
during stress. Ideally, these acquisitions should be performed
during a breathhold. The chances of success are increased if oxygen
is administered by mask before starting. Since it may still not be
possible for many patients to hold their breath during a 60-second
acquisition, they should be instructed to take small, even breaths
when they start to breathe so respiratory motion artifacts are
MR perfusion stress imaging is performed with vasodilator stress
(i.e., adenosine or dipyridamole IV).
During MR perfusion studies, it is of tantamount importance that
signs and symptoms of ischemia be monitored rigorously, since only
a rhythm strip and not a full 12-lead ECG is available during
imaging. In addition, the current pulse sequence requires the
patient to hold her/his breath for approximately 1 minute while
machine noise precludes effective verbal communication.
The definition of ischemia on a perfusion stress study is an
area of decreased perfusion on the stress, but not the non-stress
images. The definition of myocardial infarction on a perfusion
stress study is an area of decreased perfusion on both the stress
and the non-stress images. Studies of multislice myocardial
perfusion imaging have yielded sensitivities of 72% to 91% and
specificities of 94% to 98%. Comparable numbers for cine imaging in
the same patients were 85% and 94%.
Dobutamine stress imaging assesses the induction of ischemia by
evaluation of regional wall motion during progressively increased
doses of IV dobutamine. Ischemia results in a new or worsening wall
motion abnormality compared with the baseline study. Real-time
white blood cine imaging and the ability to simultaneously view
wall motion for all the doses of dobutamine are essential to
identify ischemia-induced wall motion abnormality so the test can
be terminated immediately. The temporal course of the ischemic
cascade consists of a reduction of blood flow followed by abnormal
wall motion, symptoms, ECG ischemic changes, hypotension, and
There is a 60-second interval between onset of abnormal wall motion
and chest pain and hypotension. (Unfortunately, ECG is not useful
while the patient is in the magnet due to magnetohydrodynamic
effects). Hundley et al
studied 153 patients who had poor acoustic windows that prevented
adequate second harmonic transthoracic echocardiographic (TTE)
imaging. Using 50% coronary artery stenosis, as defined by
computer-assisted quantitative coronary angiography (QCA), the
sensitivity and specificity were both 83%.
Cardiac wall motion can also be studied with myocardial tagging.
Saturation bands or grids are produced by applying a sequence of
saturation pulses to evaluate myocardial contraction throughout the
cardiac cycle. For each RR interval, the tagging sequence is
applied immediately following the ECG trigger and before data
acquisition. Longitudinal magnetization is altered so that tissue
appears dark in the subsequent images. The tagged images are used
to assess regional myocardial contraction and for sophisticated
analysis of myocardial stress and strain.
Another exciting measure of myocardial ischemia is the coronary
flow reserve (CFR). It is defined as maximal hyperemic flow divided
by resting flow. In normal patients, the coronary blood flow
increases by a factor of 3 to 5 following vasodilator stress. CFR
has been used to assess the functional significance of coronary
artery stenoses involving the left main and left anterior
descending (LAD) coronary arteries. Using a phase-contrast
technique, Hundley and Link
demonstrated that a CFR <1.7 was 100% sensitive and 83% specific
for a >=70% LAD stenosis.
Myocardial infarction imaging
A new technique known as "delayed hyperenhancement" is proving
to be extremely sensitive and specific for the diagnosis and
quantitation of myocardial infarction. Normal myocardium is dark 20
to 30 minutes following injection of Gd because the Gd has washed
out and infarcted myocardium is bright.
The pulse sequence is a segmented k-space, inversion recovery (IR),
i.e., magnetization-prepared, gated fast gradient-recalled echo
(GRE) technique. The T1 relaxation time of the normal myocardium
determines the inversion time of approximately 180 msec so normal
myocardium is dark. Delayed hyperenhancement is more accurate in
estimating infarct volume than enzymes, which tend to have a narrow
temporal window and wash out quickly with the increased use of
angioplasty and stenting.
Myocardium that exhibits abnormal wall motion can still be
viable. The ability to identify viable myocardium accurately is
extremely important. "Stunned" myocardium occurs when the nutrient
coronary artery is occluded but reopens. Wall motion is abnormal
but normalizes with small doses of dobutamine. "Hibernating"
myocardium does not contract normally but is still metabolically
active and will improve after revascularizaton.
Right ventricular dysplasia
Right ventricular dysplasia (RVD) is a genetic cardiomyopathy
characterized pathologically by fibrous/adipose replacement of the
right (and rarely left) ventricular myocardium and is associated
with ventricular arrhythmias in young patients. CMRI is able to not
only assess the anatomic and functional abnormalities of the RV
(e.g., wall thinning and wall motion abnormality) but can also
image the fibrous/adipose replacement of myocardium.
This is best accomplished with black blood or double IR techniques
that null signal from moving blood, decreasing partial volume
averaging between the otherwise bright blood and the bright, fatty
myocardium being sought.
[Two 180 pulses applied prior to the fast spin echo (FSE)
acquisition optimizes reduction of the blood signal and related
artifacts. After the ECG trigger, a non-slice selective 180 pulse
inverts all spins including blood. This is followed by a second,
slice-selective 180 pulse that re-inverts only the spins in the
slice. During the inversion time TI between 180 pulses, inverted
spins in the blood that is outside the imaging slice (with negative
magnetization) recover to zero magnetization making them black. For
a heart rate of 60 beats per minute, the TI is approximately 650
msec. A third 180 pulse can be applied before the FSE acquisition
to null fat based on its short T1 (similar to the STIR technique).
This is known as "black blood with fat suppression" or "triple IR."
At 1.5T the interval before the FSE readout should be 150 msec to
null fat.] The diagnosis of fatty replacement is optimally made by
finding high signal on the double IR images that is nulled on the
triple IR images.
Cardiac valve function
Black blood images also provide excellent anatomic detail of
cardiac valves. White blood imaging provides information about
turbulence produced by stenotic or regurgitant lesions because of
the dephasing of the spins.
Phase-contrast imaging can provide quantification of right
ventricular (RV) and left ventricular (LV) stroke volume (SV) by
interrogating the flow measurements from the proximal pulmonary
artery and aorta, respectively.
If there is only one regurgitant valve, then the regurgitant volume
can be calculated by LV SV minus RV SV.
Black and white blood imaging can provide excellent anatomic
detail in patients with congenital cardiovascular abnormalities.
Phase-contrast imaging is helpful in several congenital
It is possible to accurately measure flow in the main pulmonary
artery (Qp) and ascending aorta (Qs)
and thereby calculate the Qp/Qs ratio in simple lesions such as
atrial and ventricular septal defects, patent dutus arteriosus, and
partially anomalous pulmonary venous connection. Follow-up of
patients after repair of Tetralogy of Fallot using CMRI provides
information on residual anatomic problems, the extent of pulmonary
stenosis, the amount of pulmonary regurgitation, and ventricular
size and function.
CMRI is excellent at evaluating the 3 major categories of
cardiomyopathy: dilated, hypertrophic, and restrictive. Anatomy,
function, and valvular disease of cardiomyopathies can be assessed
using black and white blood imaging.
In addition, the effect of therapy on ventricular size and function
can be monitored.
In thalassemia, myocardial iron concentration can be assessed using
a T2*- weighted technique. In sarcoidosis, the myocardium appears
inhomogeneous with black blood imaging and enhancement occurs with
CMRI with Gd visualizes the site, activity, and extent of
inflammation in acute myocarditis.
CMRI can image vegetations, as well as other anatomic complications
CMRI is excellent for imaging the pericardium.
Black blood imaging at multiple cardiac levels enables anatomic
characterization of the pericardium. Diseases that can be assessed
include congenital absence of the pericardium, as well as
pericardial effusion, thickening, cysts, diverticula, and tumors.
It is possible to differentiate constrictive pericarditis from
restrictive pericardial disease, which is an important distinction
since constrictive pericarditis is treated by surgical removal of
CVMR is not only able to identify the anatomy of cardiac tumors
but can also provide tissue characterization (Figure 5).
The most common primary cardiac malignancy is angiosarcoma, which
often occurs in the right atrium and involves the pericardium. If
hemorrhage is present, T1-weighted images can demonstrate increased
signal intensity. In children, rhabdomyosarcoma is the most common
primary cardiac malignancy and may involve the valves.
Undifferentiated sarcoma tends to occur in the left atrium. Primary
osteogenic sarcoma commonly occurs in the left atrium and may
demonstrate calcification. Leiomyosarcoma not only tends to occur
in the left atrium, but may also invade pulmonary veins and the
mitral valve. Fibrosarcoma often occurs in the left atrium and may
be necrotic. Liposarcoma is rare and is usually a large,
infiltrating mass that may have foci of fat. Primary cardiac
lymphoma often involves the pericardium. Atrial myxomas tend to be
attached to the interatrial septum and have low MR signal.
Myxomas may demonstrate Gd enhancement in the core, which at
pathology has dense neovascular channels.
The MR image characteristics of cardiac thrombus differ
depending on the age of the clot.
Subacute clots exhibit homogeneously low MR signal, do not enhance
with Gd, and demonstrate magnetic susceptibility effects.
Histopathology has demonstrated that these clots are avascular and
contain dense iron deposition. Chronic clots exhibit intermediate
and heterogeneous MR signal as well as multiple areas of Gd
enhancement. Using black blood imaging, it may be difficult to
differentiate areas of slow blood flow from thrombus. In these
cases, Gd imaging is recommended to delineate blood pool from
myocardium more clearly.
Using CVMR, it is possible to identify true
of the left ventricle, atrial septal aneurysm,
and sinus of Valsalva aneurysm.
A cardiac pseudoaneurysm is a rupture of the ventricular myocardium
that is contained by the pericardium. If the pericardium also
ruptures, this condition can be fatal, hence the importance of its
With CMRI, it is possible to pursue many exciting avenues of
investigation. These include coronary artery imaging, plaque
characterization, and tissue characterization using spectroscopy.
It is currently possible to image anomalous coronary arteries.
Routine, accurate imaging of coronary arterial stenosis is an area
of active development and research.
The ability to assess atherosclerotic plaque accurately, with the
goal of identifying "vulnerable" plaque at high risk of rupture and
causing acute coronary artery closure, is of great importance.
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1. Which of the following cardiac MR imaging techniques cannot
be used to detect ischemia prior to infarction?
A. Myocardial tagging
B. Ventricular wall
C. Delayed hyperenhancement motion
D. Perfusion imaging
E. Coronary flow reserve
2. Which of the following is not characteristic of right
A. Fatty replacement
D. Wall thinning
3. Delayed hyperenhancement is seen in:
E. All of the above
4. Optimal cardiac MRI requires:
A. Strong gradients
B. Fast gradients
C. Segmented coverage of
D. Phased-array surface k-spacecoils
E. All of the above.
5. Which of the following is NOT true regarding evaluation of
coronary flow reserve by MRI?
A. It requires an injection of dobutamine.
B. It can be used to assess blood flow in the left main and LAD
C. It is based on phased contrast MRA to quantitate blood
D. It has 100% sensitivity for predicting a >=70%
E. It can preselect which patients will require angioplasty or
1. C. Delayed hyperenhancement only demonstrates infarcted
myocardium, not ischemia;. 2. E.; 3. A. Delayed hyperenhancement is
only positive with infarction.; 4. E; 5. A. Coronary flow reserve
is performed during vasodilator stress using adenosine or
dipyridamole, not inotropic stress (dobutamine).
Note: No contrast agents are approved by the U.S. Food and
Drug Administration for use in imaging of the heart.
William G. Bradley, Jr., MD, PhD, FACR * Editor-in-Chief
O. Oliver Anderson * Publisher
Elizabeth A. McDonald * Editor
Beverly Harris Assistant * Editor
Felice Ponger-Shaloum * Art Director/Production Manager
Applied Imaging is published by Anderson Publishing, Ltd. It is
supported by a grant from Amersham Health. The views and opinions
expressed in this publication are those of the authors and do not
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