Cardiac MRI
Phase contrast imaging
David M. Chaky, MD
Cardiac MR primer

The 2 primary sequences used in cardiac imaging are the
spin-echo and gradient-echo sequences.
 The spin-echo, or dark blood, sequence provides
excellent spatial resolution and is used to study anatomy
and morphology.
 The gradient-echo, or bright blood, sequence samples
multiple data points during the cardiac cycle.
 Because there is intrinsic contrast between flowing blood
and static tissue, no contrast agents are required. The
resulting gradient-echo images can be displayed in a
continuous loop of ventricular contraction and relaxation,
so-called cine-MRI, to assess regional and global
ventricular function.

Common orientations used in MRI of the heart include the
vertical long axis (so-called 2-chamber view), horizontal axis (socalled 4-chamber view), and short axis (see Figure 1). From
these standard views, cardiac chamber sizes and volumes can
be measured and calculated. Wall thickness can be measured to
determine myocardial mass.
Cardiac MR primer



MRI is invaluable in visualizing complex congenital heart
disease. Venoarterial, atrioventricular, and
ventriculoarterial connections are readily identified, as are
chamber morphology, position, and relation to other
visceral organs and great vessels.
MRI can demonstrate the location, extent, and attachment
of intracardiac and paracardiac masses.
Generalities regarding tissue composition can be made
based on the signal intensity and homogeneity, because
there are different relaxation properties between normal
and diseased tissue - MRI can help elucidate infiltrative
processes such as hemochromatosis, amyloidosis, and
fatty replacement of myocardium in right ventricular
dysplasia.



Ventricular function
Global and regional ventricular function can be measured from cine-MRI. The
ejection fraction is calculated from tomographically determined end-systolic
and end-diastolic volumes.
Ventricular remodeling and myocardial thinning after myocardial infarction
can be appreciated easily



Valvular assessment
In cine-MRI, turbulent blood flow creates an area of
low signal (“signal void”) due to dephasing
(asynchronous precession) of the hydrogen nuclei.
A qualitative assessment of the degree of valvular disease
can be made based on the size of the jet, the duration of
the jet in the cardiac cycle, and the presence of a
proximal convergence zone (an area of flow acceleration)
on the opposite side of the valve.
Cardiac MR primer

Pericardial evaluation

Pericardial effusions can be defined, and some
generalities about fluid composition can be made,
based on signal intensity. The thickness of the
pericardium can be measured and should be
<3 to 4 mm.
Cardiac MR primer

Magnetic resonance angiography
 Most MRAs can be performed without
administration of a contrast, although contrast
agents may improve image quality in certain
circumstances.
 Magnetic resonance angiography depicts normal
laminar flow as bright signal, whereas turbulent,
diminished, or absent flow is depicted as signal
voids.


Intimal flips, true and false lumen identification, thrombus
formation, origin of the tear, involvement of the
brachiocephalic vessels, and the presence of aortic
regurgitation or hemopericardium all can be seen.
Aortic aneurysms are well delineated with MRA, and 3-Dl
reconstruction is helpful in planning surgical repair.
Cardiac MR – other common adult indications

Myocardial perfusion imaging

Magnetic resonance spectroscopy

Coronary MRA
Case

History -

Sinus venosus septal defect with partial in
almost pulmonary venous
return of the right pulmonary veins.
Evaluate pulmonary venous
drainage.

TECHNIQUE- Axial gated FIESTA, two chamber,
four chamber, and short axis FIESTA images
were performed.
 Three dimensional gadolinium
enhanced magnetic resonance angiography of
the chest was obtained with multiplanar
reformatted images.
 Cine phase contrast imaging of the aorta and
main pulmonary artery was obtained with
calculation of Qp/Qs.
Moderately to severely
dilated right atrium and
ventricle.

LV FIESTA 2 chamber
Moderately to
severely dilated
right atrium and
ventricle.

LV FIESTA axial nongated
Sinus venosus septal defect
associated with partial
anomalous pulmonary venous
return of the right upper and
middle lobe pulmonary veins
to the superior vena cava.

MIP

PC LVOT

PC RVOT
Cardiac MR report

Sinus venosus septal defect associated with partial
anomalous pulmonary venous return of the right upper
and middle lobe pulmonary veins to the superior vena
cava.
Qp:Qs = 4:1

The upper pulmonary vein drains to the superior vena
cava approximately 2.3 cm above the expected location of
the caval atrial junction.
Moderately to severely dilated right atrium and ventricle.
Preserved right and left ventricular systolic function.
Enlarged main, right, left pulmonary arteries.
Ventricular function

Right ventricle- The right ventricle is moderately to
severely enlarged with preserved systolic function.
- RV end diastolic volume- 138 ml
- RV end diastolic volume indexed to body surface area199 ml/m2
- RV ejection fraction- 61%
Left ventricle- The left ventricle appears structurally
normal, with good systolic function.
- LV end diastolic volume- 33 ml
- LV end diastolic volume indexed to body surface area48 ml/m2
- LV ejection fraction- 65%
Phase contrast MR




Dark blood (spatial saturation) – air, bone,
& other substances also have low signal
Bright blood (TOF or CE) – other tissues
may have high signal
PC – changes from motion itself are used
to depict blood flow
PC – better background suppresion & flow
measurement
Phase contrast MR - 2D
2D PC – useful for getting images in suspended
respiration or differing phases of cardiac cycle
 Background tissue is gray
 Flow in the positive direction along the flow-encoded axis
is light (white)
 Flow in opposite direction is dark (black)
 Can even be color coded similar to Doppler US

Phase contrast MR - 2D


Measurement of pulsatile flow most
accurate w/ cardiac gating (aka cine phase
contrast)
2D useful for blood velocity and volume
Phase contrast MR - 3D


3D PC are useful for depicting flow w/ high
SNR, high spatial resolution, and high
background suppression
Major disadvantage is long acquisition time
Phase contrast MR




Flow encoded phase shifts result from the application of bipolar
magnetic field gradients, which are composed of two lobes w/
opposite signals.
When the 1st lobe is applied, the spins of the stationary and moving
tissues begin to accumulate phase.
Immediately after, the 2nd lobe is applied, and the stationary protons
lose their phase and accumulate a net phase of 0.
Blood that moves in
between the 2 gradients
experiences unequal positive
and negative gradients and
therefore accumulates a net
phase shift

Phase-difference methods
are based on the principle
that hydrogen nuclei moving
through a magnetic field
gradient accumulate a phase
shift proportional to their
velocity.

Magnitude and phase
images. Blood in the
ascending aorta appears
bright on the phase image,
indicating flow in a cranial
direction through this
transverse image plane.
Caudally directed flow in the
descending aorta appears
dark.
Phase contrast MR

The accurate measurement of the ratio of
pulmonic to systemic flow (Qp/Qs) is important in
the evaluation of the patient with left-to-right
intracardiac shunting, since it is often used to
determine subsequent therapy.
 The asymptomatic patient with a Qp/Qs of <1.5
usually is managed conservatively, whereas the
patient with a Qp/Qs of >1.5 whose pulmonary
vascular resistance is not markedly elevated
usually is referred for surgical correction.
Phase contrast MR


The available methods of determining the magnitude of
left-to-right intracardiac shunting include oximetry,
indicator dilution, radionuclide scintigraphy, and
Doppler echocardiography. The oximetric and indicator
dilution techniques, although relatively easy to perform,
are invasive. Radionuclide scintigraphy and Doppler
echocardiography are noninvasive, but the former
requires the injection of a radionuclide, and the latter may
be technically difficult or impossible to perform in some
patients.
PC MR is a rapid, accurate, noninvasive, and widely
applicable method of estimating the magnitude of
intracardiac left-to-right shunting is desirable.
Circulation.
1995;91:29552960.)

Pulmonary-to-systemic flow ratios (Qp/Qs) by catheterization (horizontal
axis) and by magnetic resonance imaging (MRI) (vertical axis) for 21
subjects. Each symbol represents the data from 1 patient. The regression
line is shown. All points fall within the two shaded areas, indicating that
compared with oximetry and indocyanine green, MRI correctly identified all
subjects with a Qp/Qs < or 1.5.
Phase contrast MR



With MRI, the cardiovascular system can be imaged with
high spatial resolution.
Velocity-encoded, phase-difference MR images have
been used to measure flow in the carotid artery, proximal
aorta and pulmonary artery, and abdominal aorta.
Flow is calculated by multiplying blood velocity by the
cross-sectional area of the vascular structure of interest.
With this technique, flow can be measured quickly; scans
can be performed in less than 2 minutes.

Magnitude (top) and corresponding velocity maps (bottom) acquired for measuring flow
in the proximal aorta. Frames 4 and 6 from the 9 frames acquired during the cardiac
cycle are displayed. The letter A indicates the lumen of the proximal aorta. On the
velocity map, the gray scale intensity for each pixel encodes for velocity. For each
frame of the cardiac cycle, velocity within the vessel is calculated as the average
velocity for all the pixels within the lumen.

Magnitude (top) and corresponding velocity maps (bottom) of the pulmonary artery in
the same two frames. The letter P indicates the lumen of the main pulmonary artery. In
frame 4, the area of the vessel and velocity of the blood flowing through it are larger
than in frame 6, thus indicating substantially higher flows for earlier frames of the
cardiac cycle. This patient had substantial left-to-right intracardiac shunting
(pulmonary-to-systemic flow ratio of 2.1 by magnetic resonance imaging).
Phase contrast MR


Mean blood flow = spatial mean velocity x
cross sectional area of vessel
Pressure gradients are calculated by the
modified Bernoulli equation: Delta P = 4v^2
(p is in mm of mercury and v is in m/sec)
Phase contrast MR



Flow measurements can be plotted on a
graph (throughout the cardiac cycle) and
the area under the curve can be used to
derive flow volume
Aliasing is possible is the max velocity
value is lower than the actual velocity
Small vessels are poorly evaluated

Each point on the
curve represents the
mean blood flow within
the artery at a phase of
the cardiac cycle.

Each value is derived
from a separate VEC
MR image.

The blood flow is a
product of the spatial
mean velocity and the
cross-sectional areas
of the vessel.
Effect of different VENC values on 2d PC images
sensitive to flow in the superior/inferior axis
References

Cardiovascular MRI & MRA by Charles B. Higgins, Albert De Roos

MRI Principles by Donald Mitchell

Cardiovascular Magnetic Resonance by Warren Manning

Assessment of Left-to-Right Intracardiac Shunting by VelocityEncoded, Phase-Difference Magnetic Resonance Imaging
Circulation. 1995;91:2955-2960.