Diagnostic clinical cardiac magnetic resonance imaging (MRI) requires an appropriate combination of temporal and spatial resolution. Cardiovascular MRI is making considerable advances toward the fulfillment of these requirements, largely because of continued improvements in hardware and software. Optimal diagnostic quality MRI implies a balance among signal-to-noise ratio, tissue contrast, acquisition time, and spatial and temporal resolution.
is a Radiology Research Fellow, Division of Diagnostic
is a Radiology Resident,
is a Visiting Assistant Professor of Radiology, and
is a Professor of Radiology and Medicine, the Chief of Diagnostic
Cardiovascular Imaging, and the Director of Magnetic Resonance
Research, Department of Radiology, David Geffen School of
Medicine at UCLA, Los Angeles, CA.
is a Staff Radiologist and Cardiologist in the Hoag Hospital,
Newport Beach, CA.
Diagnostic clinical cardiac magnetic resonance imaging (MRI)
requires an appropriate combination of temporal and spatial
resolution. At present, cardiovascular imaging is making
considerable advances toward the fulfillment of these requirements,
largely because of continued improvements in MRI hardware and
software. Optimal diagnostic-quality MRI implies a balance among
signal-to-noise ratio (SNR), tissue contrast, acquisition time, and
spatial and temporal resolution. Magnetic field strength is one of
the major factors affecting image SNR.
In 2002, the U.S. Food and Drug Administration (FDA) approval of
3T clinical whole-body scanners opened the way for multiple
advanced clinical applications, particularly with regard to MR
angiography (MRA). Compared with 1.5T, the higher field strength
results in a doubling of SNR due to increased spin polarization.
Furthermore, imaging at higher field strengths with
gadolinium-based agents can produce further improvements in image
contrast. Cardiac imaging at 3T is, however, noticeably different
from imaging at 1.5T because of a variety of artifacts that result
from susceptibility effects and augmentation of radiofrequency (RF)
In this article, we review the advantages and disadvantages of
cardiac imaging at higher magnetic fields for cardio- vascular
clinical applications, including contrast-enhanced MRA techniques,
functional imaging, flow quantification, assessment of myocardial
viability, perfusion imaging, and coronary arterial imaging at 3T,
providing reference to current clinical practice while highlighting
potential pitfalls and discussing techniques to overcome them.
Additionally, we supplement this technical discussion with a number
of illustrative pathologic cases that are imaged at 3T. We hope to
provide an up-to-date summary of current practices in the field,
addressing technical challenges while encouraging motivated readers
toward appropriate further study of this extensive topic.
Contrast-enhanced MRA techniques
At 3T, SNR increases significantly relative to 1.5T: first,
because background noise is reduced at 3T; and second, because the
longitudinal relaxation time (T1) of blood undergoes more marked
shortening than does background tissue.
The advent of parallel imaging,
with improved RF electronics and coil design, has facilitated the
use of higher accel-eration factors, improving the speed,
resolution, and coverage of contrast bolus imaging. However,
parallel data acquisition invokes a penalty in SNR, which is more
limiting at lower field strengths. Recently, several investigators
have shown positive results using contrast-enhanced MRA (CEMRA)
with parallel imaging at higher field strengths, findings that have
already been integrated into routine clinical practice.
Nael et al
showed an increase in main pulmonary arterial SNR at 3T, however,
with a reduced pulmonary parenchymal SNR on time-resolved MRA
relative to 1.5T. These authors concluded that time-resolved MRA at
3T provides results comparable to 1.5T. However, the relatively
lower parenchymal enhancement seen at 3T was thought to reflect
greater susceptibility effects at a higher magnetic field because
of air-tissue interfaces within the lung. Susceptibility effects
increase linearly with field strength and may limit pulmonary
perfusion imaging when integrated with inherent SNR drops because
of the use of higher acceleration factors.
Nonetheless, while it is important to recognize this phenomenon, it
is usually not limiting in clinical practice (Figures 1 and 2).
Cardiac functional imaging
Until recently, cardiac cine MRI in-volved the use of spoiled
gradient-echo (SGE) imaging,
based largely on apparent T1-weighted shortening due to
through-plane blood to generate contrast between blood and
myocardium. If the repetition time (TR) is too short or the flow is
too slow, blood saturation may result (Figure 3). This is
particularly relevant for long-axis cardiac imaging, where blood
may linger within the slice (Figure 4) but may be encountered
during short axis imaging if myocardial function is poor. As a
result, steady-state free-precession (SSFP) pulse sequences have
replaced spoiled gradient-refocused echo (GRE) cine for almost all
routine clinical MRI at 1.5T.
With SSFP, blood signal is dependent upon its relaxation time
(more specifically upon the ratio of T2*/T1) rather than its speed
of flow, so that even with very poor systolic function,
blood-myocardial contrast is excellent. Cine SSFP performs
optimally with the shortest possible TR (on the order of ≤3 msec),
so imaging speed is almost 3 times greater than that allowed by
spoiled GRE cine.
Indeed, a high contrast-to-noise ratio (CNR) cine SSFP may be
acquired in ≤7 seconds when imaging at 1.5T.
Several investigators have confirmed the SNR advantages of
cardiac cine at 3T.
These studies indicate an overall quantitative improvement in SNR
for cardiac imaging at 3T. Similar to angiography, the SNR
improvements achieved have facilitated the use of parallel imaging
with higher acceleration factors and multi-channel RF receiver
coils (Figure 5). As a result, the temporal performance of cine
SSFP may be further enhanced.
However, accelerated acquisition results in the loss of SNR and in
the introduction of noise nonuniformly (Figure 5).
The main challenges specific to 3T as compared with 1.5T
inhomogeneity, increased RF power deposition (which generally
limits the flip angle and the minimum achievable TR), longer T1,
and shortened T2*, all of which combine to offset the advantages
from higher field strengths. While these factors are by no means
absent at 1.5T, they are more troublesome at 3T. With regard to
SSFP imaging, RF inhomogeneity, which oc-curs because of the
shorter wavelength of the RF pulse, makes penetration of deeper
structures more difficult (Figure 5, arrows). Furthermore, resonant
frequency shifts (off-resonance effects) due to magnetic
susceptibility gradients, which oc-cur adjacent to tissue
interfaces, manifest as dark bands in the image. Depending on where
these dark bands occur, this may be of no clinical significance or
may result in severe image artifact (Figures 3, 6, and 7).
If an off-resonance band traverses a major inflow vessel (Figure
8, arrowheads) or a cardiac chamber, this may disrupt the steady
state and degrade the image contrast. The distance between these
bands is inversely related to the TR, thus using the shortest
possible TR is desirable. The homogeneity of the main magnetic
field is a major determinant in the location of these dark bands on
The distance between these bands is indirectly related to the slope
of the frequency gradient, with steeper gradients increasing the
propensity toward disruption of the steady state. Deshpande et al
proposed that a prescan that simulates different imaging
frequencies (frequency scout imaging) may be used to visually
determine the central frequency. This technique often results in
artifact-free SSFP images at 3T but does not necessarily fix all
slices for all cardiac phases, as illustrated in Figure 6 and
Figure 8. Compared with SSFP, SGE se-quences are less demanding at
a higher field. Because of spin saturation, however, SGE cine
imaging may yield poor results unless intravascular contrast agents
(Figures 3 and 4). At the time of this writing, however, no
in-travascular contrast agents are approved by the FDA for cardiac
imaging, and 1.5T has remained the optimal magnetic field for
clinical cardiac cine imaging.
Phase-contrast flow quantification imaging
Velocity-encoded, phase-contrast (PC) cine MRI measures phase
shift due to blood flow, allowing direct noninvasive derivation of
blood velocity and volume flow. Flow rate can be calculated by
integrating this velocity over the measured cross-sectional area.
Since first proposed by Moran,
this technique has been optimized and synchronized to the cardiac
cycle, using either retrospective or prospective
electrocardiographic (ECG)-gating for clinical vascular assessment
A high SNR profile is particularly desirable when imaging with
PC-cine MRI, because the noise of velocity-encoded images has an
inverse relationship to the SNR of magnitude-encoded images.
Generally, an increased SNR ratio can be achieved by optimizing
sequence parameters such as section thickness, flip angle, echo
time, and receiver bandwidth. However, increasing the echo time
renders the sequence increasingly vulnerable to motion artifact,
and an increase in the section thickness may induce significant
partial-volume effects in smaller vessels, such as the coronary
arteries or bypass vessels. Nael et al
and Gutberlet et al
studied the PC imaging of the pulmonary artery and aorta
(respectively) at 3T, comparing their findings with those at 1.5T.
Both authors reported an increase in SNR due to noise reduction at
3T without significant impact upon velocity and flow measurements.
Lotz et al,
in their in vitro validation study, showed slight overestimation of
flow measurement at 3T when compared with phantom pump settings,
with an increase in SNR by a factor of 2.5 when compared with 1.5T
for magnitude-encoded images. It is assumed that moving to a higher
field system will be of interest for small-size vessels, such as
cor-onary arteries or bypass arteries, which require higher spatial
resolution acquisition in order to reduce the impact of
partial-volume effects, and for slow diastolic flow-patterns, which
require higher temporal resolution.
Potential sources of error for clinical PC cine
MRI (as Doppler ultrasound) samples discrete sections of
vascular flow and, therefore, it is vulnerable to aliasing if flow
velocities are excessively high. Ve-locity sensitivity with MRI may
be ad-justed so that such aliasing does not oc- cur. By definition,
the velocity-encoding value (VENC) is the velocity, which produces
a phase shift of 180°. However, there is also a linear dependency
between noise and the velocity encoding, with excessively high VENC
levels producing compromises in SNR.
The VENC value can be estimated a priori by using a quick VENC
scout imaging sequence for the vessel of interest. Typical VENC
values employed for various anatomic sites are provided in Table
Once the possibility of velocity aliasing has been addressed, a
number of additional important pitfalls should be borne in mind
when quantifying blood flow using this technique. Through-plane
flow measurements are more accurate if the imaging plane is
orthogonal to the main flow direction. It has been shown that a
deviation of more than ±15º can cause significant deviations from
both the true peak velocity and flow rate.
Furthermore, the use of suboptimal temporal resolution may result
in an underestimation of peak velocity and flow.
In the heart and great vessels, flow quantification may be
performed in the presence or absence of respiratory suspension.
Breath-hold acquisitions are fast and can prevent respiratory
motion artifact but are limited with regard to temporal and spatial
resolution. Additionally, Sakuma et al
studied the effect of different lung-volume respiratory
breath-holds on flow measurements and concluded that a small
lung-volume breath-hold by shallow inspiration can provide a blood
flow quantification that is close to phys-iological blood flow when
compared with an acquisition with a large-volume breath-hold.
Sugano et al
compared 3 different patterns of respiratory suspension: midnormal
respiration, inspiratory, and expiratory breath-holding
acquisitions for portal-vein flow assessment, concluding that
expiratory breath-hold increases blood flow to the greatest degree,
with deep inspiratory breath-holding resulting in the least
portal-venous flow. Generally, at end-inspiration, venous return
can be expected to be in-creased, when compared with cardiac
output, with reversal of this relationship at end-expiration.
Prolonged breath-holds affect cardiovascular hemodynamics and,
thus, may influence flow quantitative measurements. By reducing
turbulence, end-expiratory breath-holds may benefit the
interrogation of intracardiac shunts and bypass grafts.
In current practice, the validity of flow characteristic
assessment and quantification has been widely assessed at 1.5T for
valvular heart disease,
congenital heart disease,
and disease secondary to pulmonary hypertension.
Comparable results for 3T MRI are awaited (Figure 9).
Delayed contrast-enhanced imaging
MRI has been proven to be a reliable means of showing the extent
of myocardial infarction at 1.5T. Using myocardial viability
infarcted myocardium exhibits delayed hyperenhancement after
contrast injection. The improved spatial resolution of MRI compared
with radio-nuclide imaging provides distinct advantages,
particularly so for nontransmural infarction, such that MRI is now
considered to be a preferred alternative to nuclear tomography
where it is available.
After intravenous injection, contrast material accumulates within
non- viable myocardium within minutes of administration, with
relatively slow wash-out. As described by Simonetti et al,
the conspicuity of such enhancement may be increased by using an
appropriate T1-weighted imaging technique, such as
segmented-k-space inversion recovery (IR) GRE (Figure 10).
At the optimal inversion time (TI), which can be confirmed on a
patient-specific basis using TI scout imaging, the signal from
normal myocardium is nulled, while hyperenhanced tissue is bright.
However, this is a dynamic phenomenon, with the T1 of myocardium
increasing as the contrast agent washes out. Thus, in a multislice
acquisition, using a fixed TI might degrade the contrast at the
final slice, which is acquired several minutes after the initiation
of image acquisition. In order to address this issue and decrease
sensitivity to the chosen TI, a phase-sensitive (PS) implementation
of segmented IR GRE has been described by Kellman et al.
More recently, variants of SSFP have been successfully used in
the assessment of myocardial hyper-enhancement imaging for
viability at 1.5T. Inversion recovery SSFP can be run as a
single-shot technique within a single heartbeat. When used in this
way, the bandwidth for SSFP is several-fold wider than that used
for GRE, allowing the acquisition of more lines of data within the
same time, and resulting in significantly faster imaging. This
technique may be of value in patients in whom breath-holding
techniques are inappropriate or in those with cardiac arrhythmia.
However, the use of this rapid-imaging technique incurs a penalty
in the form of limited spatial resolution, with a reduction in T1
contrast effects. Huber et al
compared segmented IR GRE at 1.5T with PS IR SSFP at both 1.5T and
3T for 10 patients, using a double dose of a gadolinium contrast
agent (0.2 mmol/kg). An IR SSFP technique allows for multislice
imaging during a single breath-hold without adapting the inversion
time, at a cost of lower CNR when compared with single-slice
segmented IR GRE at 1.5T. However, CNR was higher (with magnitude
imaging) at 3T. Huber et al
showed a very slight overestimation of the infarct volume using PS
IR SSFP at both 1.5 and 3T, a finding previously described during
other comparative studies.
This observation has been related to the inherent contrast
characteristics of the SSFP sequence, which is dependent on the
ratio of T1/T2 and the presence of edema around the infarcted
While delayed myocardial hyper-enhancement represents a key
feature in the assessment of myocardial viability, this technique
should be reviewed in conjunction with cardiac cine images obtained
to provide a corresponding evaluation of the left and right
ventricular morphology and contractile function. This facilitates
the detection of wall motion abnormality in the region of suspected
myocardial infarction. Suggested cine imaging for the current
routine clinical assessment at 1.5T are:
- Short-axis cine covering the entire ventricular length;
- Horizontal-axis cine (4-chamber views); and
- Left ventricle inflow-outflow tract cine imaging.
If the patient also requires chest MRA, viability imaging may be
performed 10 minutes after chest angiography, obviating the
requirement for a separate contrast medium injection of 0.1 mmol/kg
of gadolinium. In our experience and in that of others,
however, the use of an appropriate imaging technique is more
important than the timing of image acquisition after gadolinium
Viability imaging (segmented IR GRE) is usually run as a
breath-hold technique where 1 slice is acquired in approximately 12
seconds, and the whole heart is covered in 10 to 12 slices. In this
implementation, it is necessary to set the acquisition window
during mid-diastole (to eliminate blurring from cardiac motion),
with the acquisition time for a single slice (be-tween 8 to 10
seconds) determined by the patient's heart rate and magnetization
re-laxation of normal myocardium (usually approximately 2 heart
Finally, to minimize partial-volume effects, one can balance
between SNR and spatial resolution.
Myocardial perfusion deficits may not be apparent in studies
performed at rest. However, stress-induced increases in myocardium
contractility (to roughly 3 to 4 times higher)
and coronary vasodilatation enhance the flow rate in normal
coronary vessels, but not in those with flow-limiting stenosis.
Several clinical reports have endorsed the accuracy of dynamic
myocardial perfusion imaging at 1.5T in combination with adenosine
Ingkanisorn et al
ex-amined and followed-up 135 patients for CAD risk factors, who
presented to the emergency room with chest pain, but had a negative
ECG, normal troponin level, and normal average ejection fraction.
In their experience, stress perfusion MR, with 100% sensitivity and
93% specificity, was superior to total cardiac risk factors >3
(sensitivity of 65% and a specificity of 76%) as a predictor of
For MRI evaluation of perfusion deficit, one would want to use
T1-weighted imaging (gradient-echo, echoplanar imaging, or SSFP)
with the use of at least one of the magnetization preparation
schemes (inversion or saturation recovery pulse).
Regardless of pulse sequence design and magnetic field strength,
optimal first-pass perfusion imaging needs the following
requirements to be fulfilled:
- Temporal resolution of every or every other heart beat.
- In-plane spatial resolution better than 3 mm, to enable
subendocardial assessment, through the whole heart by means of
acquiring a minimum of 4 slices (3 short-axis-apex, mid, and
basal-and one 4-chamber view).
- Clinical considerations for administrating adenosine
- No caffeine or theophylline intake within 24 hours of
- Two IV lines are needed to infuse the contrast and
- To monitor any possible side effects of adenosine
infusion, the patient is out of the magnet during the first 2
minutes of infusion and then placed back into the bore, and
contrast is infused, preferably, after 3 minutes of adenosine
- Acquisitions are breath-hold; however, if too long
(usually 45 to 60 seconds), patients are told to take shallow
breaths during imaging.
Studies at 3T are not different for adenosine; however, there
must be some optimization for pulse sequence properties. Some data
regarding 3T myocardial perfusion imaging are currently available,
evaluating perfusion imaging at 3T compared with 1.5T and showing
higher SNR and CNR and higher peak enhancement at 3T.
inhomogeneity is higher at 3T and may com- promise suppression of
background tissues. In addition, higher T2* effects may require a
reduction of gadolinium dose. Because of these issues, pulse
sequences and protocols utilized at 1.5T need modification to
provide diagnostic images at 3T. For example, to reduce the effects
of field inhomogeneity, shim gradients may be helpful.
During first-pass perfusion studies, a dark rim in the left
ventricular subendocardium is sometimes seen. This artifact has
been attributed to susceptibility artifacts because of the bolus of
contrast, motion, spatial resolution, or a combination of these
Di Bella et al
showed in ex vivo hearts, studied at 3T using GRE sequence that by
increasing the spatial resolution of both data acquisitions, the
size and intensity of the artifact decreased. In practical terms,
this artifact could mimic ischemia where none was present. For the
same reason, it is recommended that perfusion images be assessed in
conjunction with corresponding cardiac cine and delayed enhancement
images. Therefore, a complete clinical perfusion assessment is
recommended to be ac-quired at 1.5T.
Quantitative myocardial perfusion using arterial input function
(AIF), which is the concentration of contrast in the left ventricle
as a function of time during its first pass, has not been
adequately re-solved to date, because a high gadolinium dose is
required for high T1 sensitivity and high myocardial SNR. However,
a high gadolinium concentration causes a nonlinear relationship
between signal intensity and contrast agent concentration, and
leads to underestimation of the AIF.
Kim et al
used a hybrid echoplanar imag-ing pulse sequence at 3T that was
modified to acquire data for a less distorted and higher calculated
AIF and the myocardium by using 2 different saturation-recovery
time delays. However, more studies are needed to evaluate the
clinical effect of these findings.
Coronary artery imaging
Routine clinical imaging of coronary arteries is still
challenging because of relatively low spatial resolution, cardiac
and respiratory motion, and the diminutive size and tortuosity of
the vessels of interest. The caliber of the proximal coronary
arteries rarely exceeds 5 mm in normal
subjects. Their spatial displacement during the heart cycle is
typically on the order of 1 to 2 cm, with peak velocities of more
than 20 cm/sec during rapid cardiac contraction and relaxation.
One preferred current MRI method for imaging the coronary
arteries at 1.5T uses noncontrast 3D SSFP imaging.
Steady-state free-preces-sion imaging holds a number of advantages
relative to SGE when imaging at 1.5T; however, for the same reasons
discussed above, gradient-echo sequences are more desirable when
imaging coronary angiography at 3T,
particularly contrast-enhanced 3D SGE (Figure 11). Longer
myocardial T1 values at 3T warrant a longer IR time for myocardium
suppression, when compared with those used at 1.5T. However, the
arterial half-life of extravascular contrast agents is very short,
resulting in a compromise in the spatial resolution achievable
during a single breath-hold acquisition, with the data being
acquired during the mid-diastolic phase in order to minimize
cardiac motion-related artifacts. Bi et al
have shown the feasibility of using a cardiac motion-resolved 3D
coronary MRA technique at 3T, using an ex-travascular agent, in a
When compared with their extravascular counterparts,
intravascular contrast agents remain in the blood pool for a longer
time, thus permitting the use of respiratory gating. Early results
using a 1.5T scanner indicate that intravascular agents hold
promise for coronary MRA
; however, further evaluation is required. For further review of
cardiac pulse sequences, the reader is referred to a recent
publication on this topic.
Currently, routine cardiac imaging at 1.5T has been proven as
highly accurate in the evaluation of myocardial function,
perfusion, and viability (Figure 12). However, functional imaging
at 3T using SSFP cine imaging may be quite time-consuming because
of the necessity for sequence fine-tuning to avoid artifacts. There
is little doubt, however, that ongoing developments in machine
pulse sequence design,
and image reconstruction methods will address these shortcomings in