Cardiac gating is used in a variety of magnetic resonance imaging applications. Its role has evolved significantly over the last several years, with advances in imaging hardware and software.
Dr. Daniel Parker
is in his third year of Radiology residency at the Baylor
University Medical Center in Dallas, TX. He graduated from the
University of Utah School of Medicine and will begin a Body Imaging
fellowship at the University of Utah in 2003.
Dr. Dennis Parker
is a Professor of Radiology and Medical Informatics at the
University of Utah School of Medicine.
Cardiac gating is used in a variety of magnetic resonance
imaging applications. Its role has evolved significantly over the
last several years, with advances in imaging hardware and
software. The accuracy of gated sequences depends on an accurate
representation of the cardiac cycle. New techniques, including
modified electrocardiogram (EKG) detection algorithms and vector
electrocardiography, have improved detection of the R wave.
Gating techniques are useful for reducing motion artifact in the
thorax, especially in cardiac imaging, and for optimizing blood
signal in time-of-flight (TOF) and phase-contrast imaging.
Selectively acquiring k-space at discrete phases in the cardiac
cycle may offer increased signal in small vessels in TOF
Cardiac gating can be used in magnetic resonance imaging (MRI)
applications to evaluate the heart and great vessels
and to help maximize inflow signal in time-of-flight (TOF) and
phase-contrast (PC) magnetic resonance angiography (MRA).
Cardiac gating is a valuable tool, especially in cardiac imaging
applications in which the complex motion of the heart would
preclude imaging with traditional methods. The role of MRI in
cardiac imaging is expanding.
Cardiac MRI can provide useful information regarding cardiac and
myocardial function, and myocardial perfusion and viability.
Ultimately, real-time cardiac cine is expected, which could obviate
the need for cardiac gating, though at the expense of
signal-to-noise ratio (SNR). This would likely increase the
clinical utility of cardiac MRI, as gating methods are imperfect
and less time-efficient.
Cardiac gating still has a role in thoracic applications where
it can reduce artifacts caused by great vessel pulsation. It may be
useful in evaluating the anterior mediastinum, which is also
subject to motion artifacts. Contrast-enhanced magnetic resonance
angiography (CEMRA) has in many instances supplanted traditional
TOF and PC applications in the body where cardiac gating has been
Gating techniques have been explored in TOF applications in an
effort to exploit phases of the cardiac cycle with highest
segmental blood velocities. Modified techniques have attempted to
capitalize on increased blood velocity as a function of time in the
cardiac cycle and yet maintain imaging times at acceptable levels.
Given the limitations of intracranial CEMRA, gating applications
may help to provide increased SNR, especially in small vessels. A
brief description of some ongoing research evaluating the pulsatile
effects of blood flow in intracranial MRA is outlined in this
Multiple cardiac gating techniques have been developed to
overcome observed pitfalls. Special algorithms have been developed
that aid in the detection of the "true" R wave even with a limited
Techniques such as vector gating provide a trigger without using
One weakness all cardiac gating sequences share is the requisite
increased acquisition time. Modified MRA gating techniques that
selectively acquire the center of k-space near systole offer a
compromise between temporal resolution and scanning time.
Cardiac gating techniques
Cardiac gating applications depend on an electric trigger
received by the magnetic resonance scanner that corresponds with
the patient's heart cycle. An EKG voltage source, specifically the
R wave, is commonly used as the trigger.
Most detecting algorithms use a set threshold voltage to determine
the cardiac trigger. The R wave is a useful trigger because the
voltage peak is usually much higher than the other points of the
EKG waveform, allowing for easier detection (figure 1). Imaging
sequences allow acquisitions at desired phases in the cardiac cycle
relative to the cardiac trigger.
In cardiac MR, gating is used to depict phases of systole and
diastole. The QRS waveform signals ventricular contraction. This
typically occurs approximately 160 ms after the initial P wave. An
image acquired at the same time as the R wave would depict the
heart in end diastole, because mechanical systole occurs
approximately 200 ms after electrical systole. An image obtained
200 ms after the R wave would depict mechanical systole.
Velocity-dependent sequences (eg, TOF and PC MRA) can be
optimized by acquiring data at intervals in the cardiac cycle where
there is maximum arterial velocity. A TOF MRA sequence of the
abdominal aorta, obtained entirely in early systole, should yield
much higher signal than one obtained in diastole. Magnetic
resonance angiography of the coronary arteries can be optimized by
imaging at select phases in the cardiac cycle with maximum blood
flow (ie, diastole).
For a conventional MRI sequence, imaging time is the product of
the radio frequency repetition time (TR), the number of
phase-encoding steps, and the number of excitations. In
conventional MR, the phase encoding steps (usually 128 to 512)
progress without a delay and the total imaging time is therefore
relatively short. In a gated sequence, phase encoding steps advance
with sequential R waves. Acquiring 256 phase-encoding steps would
require 256 separate R waves, in the case of conventional cine
imaging. In the case of segmented k-space techniques, in which
multiple radiofrequency (RF) pulses and phase-encoding steps are
obtained within a given "segment" of the cardiac cycle, fewer R
waves are required to fill k-space.
Accurate gating depends on an accurate gating signal. As
depicted in figure 2, if the trigger threshold is set too low, or
there is an inadequate R wave, the cardiac cycle will be
misrepresented. An amplified T wave, arrhythmias, or premature
ventricular contractions can cause inaccurate gating. Newer gating
algorithms analyze the slope of the waveform as well to determine
the R wave.
Quality control can greatly aid in producing a reliable and
accurate EKG waveform. EKG leads should be placed either on the
front or the back of the thorax following Enthoven's Law.
The lead with the highest R wave peak voltage is selected. When
using an EKG trigger, the skin lead interface should be maximized.
Oils and dead dermis can cause increased conduction impedance,
which can be resolved with an abrasive gel. Sometimes it is
necessary to shave excessive hair to provide better lead-skin
EKG leads in MRI have a few unique potential problems. Radio
frequency pulses used in signal generation can cause increased lead
impedance and a noisy waveform. The large magnetic fields inherent
in MRI may generate induction currents within EKG leads that could
potentially burn a patient. The lead wires should exit the bore of
the magnet parallel to the magnetic field to reduce artifacts in
the lead signal.
Vector triggering methods in cardiac MRI have demonstrated the
potential of depicting the cardiac cycle more accurately. In vector
cardiac gating a trigger is produced that depends on heart motion.
Many of the deficiencies of traditional EKG gating are overcome
with this technique, even in problematic patient populations (ie,
patients with cardiac arrhythmias).
Less precise cardiac gating can be done with a peripheral pulse
oximeter. A pulse oximeter detects blood flow with an optical
trigger. Blood flow is monitored on a finger, toe, or ear. In
contrast to EKG gating, in which cardiac systole happens shortly
after the R wave, in peripheral gating the signal is a delayed
representation of systole.
Cardiac gating in thoracic imaging
Magnetic resonance imaging is establishing itself as a key
resource in cardiac imaging, although development of cardiac MRI
applications has been limited by the heart's complex motion.
Cardiac gating is still a valuable tool in the majority of cardiac
imaging sequences. Magnetic resonance sequences are being developed
and used to evaluate heart morphology, myocardial wall motion,
perfusion and viability,
and coronary artery caliber.
Both acquired and congenital heart disease can be evaluated.
Magnetic resonance imaging has the potential to provide superior
pericardial and mediastinal imaging.
The ability of MR to acquire a three-dimensional volume data set,
its superior intrinsic soft tissue contrast, and ability to
quantify blood velocity are indicators of its great potential.
Advances in imaging software and hardware have made MRI of the
Basic cardiovascular imaging sequences can be classified as
"white blood" and "black blood." White-blood techniques use fast
gradients to obtain cine type images and are useful for wall motion
evaluation. Key white-blood cardiac MR sequences include
steady-state free precession (SSFP) or true fast imaging with
steady-state precession (true FISP)
and fast gradient echo. Advances in SSFP in recent years have
markedly improved the ability of MRI to evaluate cardiac anatomy
and function. Steady-state free precession sequences offer much
more uniform blood signal even in slow flow regions. The increased
temporal and spatial resolution of SSFP is becoming a mainstay in
Black-blood sequences include traditional spin-echo, and more
recently double and triple inversion recovery fast spin-echo
The double inversion recovery fast spin-echo sequence removes the
signal of flowing blood by applying an RF inversion pulse. This
technique is limited in regions with slower blood flow, such as in
aneurysms and dissections, and long vessel segments. In triple
inversion recovery sequences, an additional inversion RF pulse is
added to null fat signal. This may lead to problems when trying to
differentiate hematoma from fat as methemoglobin may also be
suppressed by this sequence.
Cardiac wall motion can be evaluated with "tagging" sequences.
These techniques evaluate wall motion and wall thickening during
contraction. A grid is formed in the imaging plane by applying RF
pulses. This grid allows accurate quantification of myocardial
characteristics. Potentially, MRI can differentiate viable from
nonviable myocardium by comparing enhancement characteristics.
Healthy myocardium enhances as contrast extends from the capillary
bed into the interstitial space. Nonviable myocardium demonstrates
decreased early phase enhancement, but shows increased delayed
enhancement as infarcted tissue allows more gadolinium to
Magnetic resonance imaging is becoming a useful tool for
evaluating the coronary arteries. New techniques in coronary MRA
include FISP and double-inversion recovery black-blood methods.
True FISP often suffers from heterogeneous signal and requires
careful shimming before an examination. Coronary MRA is currently
useful in evaluating the proximal coronary arteries, including
variant anatomy (eg, origins of the right and left coronary
arteries), aneurysms and pseudoaneurysms, and in the evaluation of
Cardiac gating in intracranial MRA
In current intracranial TOF techniques, large blood vessels with
high flow rates are visualized routinely. The smallest blood
vessels, which are routinely visualized on X-ray angiography, are
not visualized routinely by MRA. There is evidence that this loss
in visibility is due to signal saturation experienced by the
slow-moving blood in the smallest vessels.
There may be blurring and signal loss due to the pulsatile nature
of blood flow in the brain.
The cardiovascular system is dynamic, and a wide range of
velocities is experienced throughout the system as a function of
time in the cardiac cycle. Pulsatile blood flow is hypothesized to
cause variation in signal intensity as a function of time in the
Cerebral MRA imaging was performed at 1.5 T with actively
shielded gradients. A commercially available cardiac gated sequence
(2D Fastcard, General Electric Medical Systems, Milwaukee, WI) was
modified to perform EKG-synchronized high-resolution
three-dimensional (3D) acquisition. Cardiac gating was performed
using peripheral pulse oximetry. Images were obtained using a
four-element bilateral phased-array coil.
For all studies, imaging parameters were: TR/TE = 25 ms/4.0 ms; tip
angle = 30š; imaging field-of-view (FOV) = 220 mm (x), 165 mm (y),
and 32 mm (z) with nx, ny, nz = 512, 192, 32, giving voxel
dimensions of 0.43 (x), 0.86 (y) and 1.0 (z). The phase-encoding
technique of the modified Fastcard pulse sequence is illustrated in
In this technique, the cardiac cycle is divided into multiple
time intervals or "phases." Phase 1 (*1) is the first interval
after the cardiac trigger. By sharing views, images at intermediate
time points are also obtained (ie, the first n/2 samples of the
second phase (*2) are combined with the last n/2 samples of phase 1
(*1) assuming n ky phase-encodings). Thus, images for seven cardiac
phases are obtained from the four time intervals shown in figure 1.
After the same set of ky phase-encodings has been obtained for each
cardiac phase, and for each kz phase-encoding, a subsequent set of
ky phase encodings is used and the process is repeated for
subsequent cardiac cycles.
Six volunteers were imaged. Images were acquired using: 1) at
least one cardiac gated seven-phase acquisition, 2) one ungated 3D
TOF acquisition, and 3) two or three cardiac gated single-phase
acquisitions where the ky phase encodings were obtained with a
centric order in each phase (see below). For the cardiac gated
acquisitions, a user-specified delay from the trigger was chosen in
terms of a number of repetitions (TR times) from the cardiac
trigger (pressure pulse). For all seven-phase protocols, the
cardiac cycle was divided into four intervals. After reconstruction
and combining of overlapped phases, images are obtained in seven
overlapping intervals, with temporal spacing of T/2 ms.
The 3D Fastcard sequence was modified to allow centric encoding
of the ky phase-encodings as a function of time during the cardiac
cycle. The lowest ky encodings are acquired early in each cardiac
cycle progressing to the highest ky phase-encodings at the end of
the cardiac cycle. To obtain a single phase, approximately 32 views
per segment were acquired. The time for the centric acquisition is
only slightly higher (about 5%) than that of the nongated 3D TOF
Blood signal was measured using a computer program, which
estimates the signal-difference-to-noise ratio (SDNR) along a
In a given volunteer, the blood signal in the same vessel segments
was measured for each acquisition.
The third, fifth, and seventh images from a seven-phase study
obtained from Volunteer A are shown in figure 4A through 4D. The
seventh image in the sequence appears to have significantly more
vascular detail than any of the previous images in this sequence
(figure 4C). The seventh image represents flow acquired in a 200-ms
window centered at 1200 ms after the initial pressure wave. The
first image was acquired in a 200-ms window beginning after the
initial pressure wave.
Images obtained using the centric technique for Volunteer A are
shown in figure 5. In this case, offsets of 500, 750, and 1000 ms
(20, 30, and 40 X TR) were used.
Figure 6 demonstrates the average peak SDNR averaged over 94
vessel segments for Volunteer A for two seven-phase studies (the
first initiated immediately and the second 500 ms after the
pressure pulse), a 3D TOF study, and three single-phase centric
acquisitions (at various delays). The measurements are plotted as a
function of effective time in the cardiac cycle where the center of
k-space was sampled.
The percent difference of the average SDNR measurement for the
seven-phase and centric acquisitions from each volunteer compared
with a nongated 3D TOF sequence was computed. The maximum (most
positive) percent difference results for all six volunteers are
summarized in Table 1.
This work shows that a significant increase in intracranial
small vessel visibility and SDNR can be achieved by selectively
acquiring images in early systole. The multiple-phase image studies
clearly demonstrate that blood signal intensity fluctuates as a
function of time in the cardiac cycle (figure 6). This fluctuation
is likely caused by a fluctuation in the blood velocity and in the
number of excitation pulses experienced by the blood between the
time that it enters the slab and the time of arrival in the
A technique that acquires the central lines of k-space in early
systole was developed and tested. This cardiac centric technique
shows the potential of significantly increasing small vessel
visibility and SDNR with very little change in acquisition time.
Although these new techniques appear promising, further studies
will be necessary before their full clinical utility can be
Cardiac gating is becoming a key tool in cardiovascular
applications. Such applications are currently used to suppress
artifacts caused by the heart's complex motion and to exploit
signal in velocity sensitive sequences. This recent research
illustrates how cardiac gating and modified k-space data collection
may be used in intracranial TOF imaging. As MRI hardware and
software evolve, cardiac gating is establishing a major role in