Transcript cardiac mri- basics and role in coronary artery disease.

Dr Vivek Pillai.
Senior resident in cardiology,
Kozhikode medical college.
REFERENCES.
 How does MRI work?-Introduction to physics and

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function of MRI-Dominik Weishaupt.
MRI of the heart and vessels-Lombardi and
bartolozi.
MRI , basic principles and applications-Brown and
Semelka.
Mayo clinic guide to CMR-Mcgee and Williamson.
Clinical cardiac MRI-Jan Bogaert,Dymarkowski and
Taylor.
Hurst’s – The Heart 13th edn.
MRI PHYSICS
 All living objects are composed of atoms.
 Atoms- electrons, protons, neutrons.
 Proton is of interest in MRI.
 Positively charged protons in the nucleus continuously
rotate around an axis and create their own magnetic
field.
 SPIN-intrinsic property of all
elementary particles.
i.e the proton rotates about its axis
like a spinning top.
 The proton possesses angular
momentum-acts like a spinning
top that strives to retain the spatial
orientation of its rotation axis.
 As a rotating mass with an
electrical charge, the proton
additionally has magnetic
moment (B) and behaves like a
small magnet.
 PRECESSION-When an
external force (typically the
earth’s gravitational field G)
acts on a spinning top and
tries to alter the orientation of
its rotational axis, the top
begins to wobble.
 LARMOR FREQUENCY-Precession of the nuclei
occurs at a characteristic rate that is proportional to
the strength of the applied magnetic field.
 ω0=γ0 X B0.
ω0 is the Larmor frequency in Megahertz.
γ0 is the gyromagnetic ratio specific to a particular
nucleus.
B0 is the strength of the magnetic field in Tesla.
• Protons have a gyromagnetic ratio of γ=42.58 MHz/T,
resulting in a Larmor frequency of 63.9 MHz at 1.5 T.
Immediately after
excitation, the
magnetization rotates in
the xy-plane and is now
called transverse
magnetization or Mxy.
It is the rotating
transverse
magnetization that gives
rise to the MR signal in
the receiver coil.
TI RELAXATION.
Transverse magnetization
decays and the magnetic
moments gradually realign
with the z-axis of the main
magnetic field B0.
As transverse magnetization
decays, the longitudinal
magnetization, Mz – the
projection of the
magnetization vector onto the
z-axis – is slowly restored.
The time constant for this
recovery is T1 and is
dependent on the strength of
the external magnetic field,
B0, and the internal motion of
the molecules (Brownian
motion.
Spin lattice interaction.
“lattice”- surroundings.
T2: TRANSVERSE
RELAXATION.
Transverse relaxation is the
decay of transverse
magnetization because
spins lose coherence
(dephasing). Transverse
relaxation - spins exchange
energy with each other.
Spin- spin interaction.
“Phase”-refers to the
position of a magnetic
moment on its circular
precessional path and is
expressed as an angle
T2* RELAXATION.
 T2* refers to the effects of additional field
inhomogeneities contributing to dephasing.
 These are intrinsic inhomogeneities that are caused by
the magnetic field generator itself and by the very
person being imaged.
 Most of the inhomogeneities that produce the T2*
effect occur at tissue borders, particularly at air/tissue
interfaces.
 The loss of the MR signal due to T2* effects is called
free induction decay (FID).
 T1 and T2 relaxation are completely independent of
each other but occur more or less simultaneously.
 The decrease in the MR signal due to T2 relaxation
occurs within the first 100–300 msec, i.e before there
has been complete recovery of longitudinal
magnetization Mz due to T1 relaxation (0.5–5 sec).
LONGITUDIN
AL
RECOVERY
AND
TRANSVERSE
DECAY
FOLLOWING
A 90
DEGREES
FLIP
EXCITATION.
IMAGE CONTRAST
 3 intrinsic features of a biological tissue contribute to
its signal intensity or brightness on an MR image and
hence image contrast:
-Proton density -the no. of excitable spins per unit
volume- determines the max. signal that can be
obtained from a given tissue.
-T1 time of a tissue-time it takes for the excited spins
to recover and be available for the next excitation.
-T1-weighted images-Images with contrast that is
mainly determined by T1.
T2-weighted images-Images with contrast that is
mainly determined by T2.
Repetition Time
(TR) and T1
Weighting
 Repetition time
(TR)- is the interval
between two
successive excitations
of the same slice.
-is the length of the
relaxation period
between two
excitation pulses and
is therefore crucial for
T1 contrast.
 Short TR → strong T1 weighting.
 Long TR → low T1 weighting.
 Tissues with a short T1 appear bright because they
regain most of their longitudinal magnetization
during the TR interval and thus produce a stronger
MR signal.
 Tissues with a long T1 appear dark because they do not
regain much of their longitudinal magnetization
during the TR interval and thus produce a weaker MR
signal.
ECHO- the signal
induced in the
receiver coil after
phase coherence has
been restored
ECHO TIME (TE) is the
interval between application
of the excitation pulse and
collection of the MR signal.
Short TE → low T2
weighting.
Long TE → strong T2
weighting.
Short T2 → dark on T2weighted images.
Long T2 → bright on T2weighted images.
Presaturation
 This technique employs an initial 90° or




180° inverting pulse that is delivered
before the data for image generation is
acquired.
For enhancement of T1 contrast.
A more pronounced T1 effect is achieved
with a 180° inverting pulse than with a
90° pulse because a 180° pulse inverts all
longitudinal magnetization.
As a result, T1 relaxation begins at –1
rather than 0 and twice as much
longitudinal magnetization is available.
The operator can modulate the T1 effect
by varying the time interval between the
180° inversion pulse and the excitation
pulse (= inversion time, TI).
Saturation at Short Repetition
Times
 SATURATION -When a
series of excitation
pulses is applied, the
MR signal becomes
weaker and weaker after
each repeat pulse.
 Saturation is an
important issue when
fast or ultrafast MR
techniques are used.
Here the MR signal may
become very weak due
to the very short
repetition times.
T1 WEIGHTED( LEFT) T2
WEIGHTED ( RIGHT)
COMPONENTS OF
THE MRI SCANNER.
-Coil that generates a static
magnetic field (B0) to align the
protons to the axis of the field
– a RF pulse transmitter
consisting in a coil that
generates RF pulses for
disturbing proton alignment
along B0
– a RF signal receiver consisting
in a coil that receives the energy
emitted by the protons.
–3 coils for generating magnetic
field gradients.
– a computerized system for the
amplification, digitization, and
processing of the MR signals to
reconstuct for composing the
final MR image..
GRADIENTS-Gradients are
additional magnetic fields that
are generated by gradient
coils and add to or subtract
from the main magnetic field.
A.
B.
C.
A.Orthogonal magnetic
field gradients are used to
localize the MR signal.
B. Slice selection
C. Frequency encoding.
The MR signal from any
given position has a unique
frequency. In the example
shown,
THE K- SPACE.
It is a graphic matrix
of digitized MR data
that represents the
MR image before
Fourier
transformation is
performed.
Each line in k-space
corresponds to one
measurement and a
line is acquired for
each phaseencoding step.
FOURIER
TRANSFORM
Transforming the
frequency-encoded raw
MR data recorded at the
system can
be done with Fourier
Transformation.
Fourier Transformation is a
mathematical model
transforming the
frequency-encoded data
from k-space to MR image
domain.
ECG Gating.
 In many types of cardiac MRI, such as morphological
imaging (e.g. coronary MR angiography) or tissue
characterization (e.g. late Gd) a static image of the
heart is required.
 Traditionally imaged during diastasis, when the
myocardium is most at rest.
 Diastasis occurs during mid to-late diastole .
 -How long after the R-wave should imaging start?
Weissler formula-trigger delay = [(R-R interval 350)x0.3]+350.
Easier approach
 Perform a cine MRI scan with very high temporal
resolution and find the start of diastasis.
 This approach reveals situations when diastasis is not
the most quiescent period in the cardiac cycle.
 In children end- systole is often a better period to
perform imaging, as diastole is short and filling is
continuous.
RESPIRATORY GATING- BREATH
HOLD MRI.
 Breath Hold Imaging-one of the main issues with
breath hold scanning is patient specific optimization.
 ↑ing either spatial or temporal resolution will lead to
prolonged breath hold times.
NAVIGATOR
GATING.
 These are simple MR
measurements of diaphragmatic
position that enable data
acquisition to be restricted to
certain points in the respiratory
cycle.
 A navigator usually consists of a
2D RF pulse that excites a cylinder
of tissue (a so-called pencil beam
excitation) and a single readout
along the length of the cylinder.
 The navigator is usually placed
on the dome of the right hemidiaphragm with the position of
the diaphragm being the same as
liver-lung interface
PRACTICAL SET UP- SAFETY ISSUES
 Attraction of ferromagnetic objects due to the static
magnetic field is the most important consideration.
 Also RF-induced heating and peripheral nerve
stimulation in the MR environment.
 Permanent pacemakers and ICD’s-heating and
current induction in leads, significant image
degradation.
 Stents , coils and filters- recommended waiting
period of 6-8 wks, but shown to be safe on day of
implantation.
 Pulm. Artery catheters-potential for excessive
heating , hence unsafe.
 Heart valve prostheses-safe during MRI, but
compromise on image quality.
 Metallic cardiac occluders-safe for non
ferromagnetic devices immediately after implant.
 Aneurysm clips,carotid artery vascular
clamps,insulin or infusion pumps,bone
growth/fusion stimulators,cochlear, otologic or
ear implant- all contraindicated.
MR CONTRAST AGENTS.
 MR contrast media fundamentally alter the intrinsic
contrast properties of biological tissues in two ways:
 –directly by changing the proton density of a tissue .
 – indirectly by changing the local magnetic field or
the resonance properties of a tissue and hence its T1
and/or T2 values.
 The local magnetic field strength is altered because
the unpaired electron spins of the contrast
medium (CM) interact with the surrounding
hydrogen nuclei of the water, fat, or protein
molecules in the tissue.
 PARAMAGNETIC SUBSTANCES-have magnetic
moment (resulting from individual spins) because
they consist of atoms or molecules that have magnetic
moment due to unpaired electron orbits in their outer
electron shells.
 eg.s include Co2+, Co3+, Fe2+, Fe3+, Gd3+, Mn2+,
Mn3+, and Ni3+.
 Most of the clinically available MR contrast media are
paramagnetic metal ion compounds (gadolinium
chelates, manganese, iron).
EXTRACELLULAR CONTRAST
AGENTS.
 Low-molecular-weight, water-soluble compounds that
distribute in the vascular and interstitial spaces following
IV administration.
Most MR contrast media used today belong to this gp of
gadolinium(III) complexes.
 Gd-DTPA (gadopentetate dimeglumine =
MagnevistR/linear ioniccomplex).
 Gd-DOTA (gadoterate meglumine =
DotaremR/macrocyclic ionic complex),
 Gd-DTPA-BMA (gadodiamide = OmniscanR/linear
nonionic complex),
 IV administration of a standard dose of an
extracellular contrast medium shortens T1, producing
an increase in signal intensity in the vessels -first pass,
and in the tissues due to tissue perfusion or disruption
of the capillary barrier.
 Eliminated renally by glomerular filtration.
 Extracellular contrast media are administered
intravenously as a bolus or drip infusion at a dose of
0.1–0.3 mmol/kg body weight.
ADVERSE EFFECTS
 Headache, nausea, or mild allergic reactions of the
skin and mucosa occur in 1-2% of cases.
 Extravasated contrast medium can cause local pain
and inflammatory reactions including tissue necrosis.
 Anaphylactic shock induced by an MR contrast agent
is extremely rare (about 1:50,000 cases).
 Nephrogenic systemic fibrosis.
NEPHROGENIC SYSTEMIC FIBROSIS
 characterised by↑ tissue deposition of collagen
 Thickening & tightening of skin - distal > proximal.
 Fibrosis- sk.muscles, lungs , pul.vasculature, heart, diaphragm
 Warrants cautious use in-
CKD (GFR≤ 30mL/min/1.73 m2)
Peritoneal dialysis
Hemodialysis
ARF
Hepatorenal synd
Peritransplant period
MRI FOR HEART MORPHOLOGY
 Why MRI?
-No window limitations, has a large field of view.
-can provide 3D images.
-accurate non-invasive assessment of Right
ventricular (RV) mass and function (quantitative
measurements of volumes).
-MRI offers additional diagnostic information on
characteristics of tissue and gives images with a high
contrast between stationary tissues and circulating
blood.
Study of heart morphology.
 The best morphological images are those obtained
with fast techniques, requiring acquisitions in breathhold,breath held in mid expiration.
 One of the prerequisites for an accurate morphological
study of the heart by MRI is an efficient
synchronization with ECG and the presence of sinus
rhythm or, at least, the absence of uncontrolled
arrhythmias.
As recommended by the
American Heart Association
(AHA), a correct evaluation
of heart morphology
requires that the images be
obtained at oblique planes
along the main axes of the
heart. This involves the use
of planes passing through
the short axis of the left
ventricle, and the long
vertical and horizontal axes
(Fig. 7.2), and oriented in
space 90° one to the other. In
Echo these planes
correspond respectively to
the short parasternal axis, to
the apical projection in 2
chambers and 4 chambers.
The cardiac muscle
can be visualised by
using the 17
segment model.
The slice thickness
used for evaluating
myocardial
segments (for
example,
for locating
ischemic areas) is
generally between
6-8mm,.Slices less
than 3 mm do not
present particular
advantages.
STRATEGY OF IMAGE AQUISITION.
 Take scout images – to locate the approx position of
the heart and great vessels.
 Scout images are obtained in Gradient Echo, which
has the advantage of being acquired in a few seconds
(10-30 depending on the number of images and on
heart rate).
 Classically, the study of the morphology is based on
“black blood” sequences (Spin Echo, SE) that generate
static images with an excellent spatial resolution.
IMAGING STRATEGIES FOR OBTAINING QUANT. DATA ON
VENTRICULAR CHAMBERS.
Fig. 7.9 a-f. (a) Coronal
scout; (b) end-diastolic
image in axial projection;
(c) end-diastolic image
on long vertical axis; (d)
end-diastolic image on
long horizontal axis; (e)
end-diastolic image on
long vertical axis that is used
in alternative for obtaining
images on the short axis; (f)
end-diastolic
image on short axis at
medium ventricular level
that is used in alternative for
obtaining images on
the long horizontal axis
Application of
Simpson’s rule is apt
here where the volume
of a complex structure –
the ventricle in this case
– is obtained by dividing
the structure into
subvolumes – to yield
the total volume.
Endocardial border of
each slice – 2d area.
Area× distance b/w 2
slices( slice thickness +
interslice distance)= 3d
volume.
CALCULATION OF MYOCARDIAL
MASS.
 The endocardial and epicardial edges of each ventricle
should be recognised, manually or automatically.
 The papillary muscles and the endocardial trabeculae
should be included in the calculation of the mass.
 The mass is given by the volume of the myocardium X
specific weight(i.e 1.05 g/cm3).
Quantitative data.
 SV= EDV- ESV.
 EF= SV/EDV.
 CO= SV X HR.
TIMELINE FOR CMR
IMAGING STRESS TEST.
-Adenosine (140micro g/ kg–1
min–1)
Infused continously for 2 mts prior
to initaiation of perfusion imaging..
-Gadolinium
contrast (0.075 -o
0.10 mmol/kg body weight) is
then administered followed by a
saline flush (50 mL) at a rate of
at least 3 mL/s by means of an
antecubital vein.
-Breath-holding starting from
the appearance of contrast in the
RV cavity.
Once the contrast bolus has
transited the LV myocardium,
adenosine is stopped, and
imaging is completed 5 to 10
seconds later. Typically, the total
imaging time is 40 to 50
seconds, and the total time of
adenosine infusion is 3 to 3.5
minutes.
Prior to the rest perfusion scan, a
waiting period of approximately 15
minutes is required for gadolinium
to sufficiently clear from the blood
pool.
For the rest perfusion scan an
additional dose of 0.075 to 0.10
mmol/kg gadolinium is given, and
the imaging parameters are
identical to the stress scan.
Approximately 5 minutes after rest
perfusion, delayed enhancement
imaging can be performed.
MYOCARDIAL MR PERFUSION
STUDIES.
 Myocardial MR perfusion imaging has several
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advantagesHigher spatial resolution.
No radiation exposure.
No attenuation problem related to overlying breast
shadow, elevated diaphragm, or obesity.
Myocardial MR perfusion imaging approaches are
currently mainly based on the changes in myocardial
signal intensity (SI) during the first pass of an
intravenously injected contrast agent (first-pass
imaging)
GENERAL REQUIREMENTS NEEDED
FOR QUANTIFICATION
 A nondiffusable tracer.
 A complete washout of the tracer from the
myocardium.
 Linear correlation between the tracer and the SI.
- However these are not fulfilled by Gd-DTPA.
 To circumvent the problems associated with
quantitative analysis of myocardial perfusion,
semiquantitative parameters have been used such as
the
- upslope.
-mean transit time.
-maximal SI.
-time to 50% maximal SI.
 The early part of the SI-time curve is mainly
influenced by perfusion.
 The later parts are influenced by diffusion.
 A linear fit of the upslope has been shown to be the
most reliable parameter for evaluating myocardial
perfusion.
 The upslope is easy to determine, highly reproducible,
with low inter- and intraobserver variability.
MPR INDEX.( MYOCARDIAL
PERFUSION RESERVE INDEX.)
 MPR index is calculated as the relative difference of
perfusion before and after vasodilatation with
dipyridamole or adenosine.
 All semiquantitative parameters and the calculated
MPR indices show an underestimation of perfusion
estimates that seems to be less when evaluating the
upslope.
A cut-off value of 1.5
(mean - 2 SD of normal
segments) allowed
discrimination of normal
from ischemic segments
with good sensitivity and
specificity (90 and 83%
respectively).
MYOCARDIAL MR PERFUSION
SEQUENCE DESIGN.
 Assessment of myocardial perfusion can be obtained
by means of fast techniques such as spoiled gradientecho (GE), echoplanar imaging (EPI) techniques, and
balanced steady-state free precession (b-SSFP).
GRADIENT ECHO
SEQUENCE.
GRE sequences employ
the gradient coils for
producing an echo rather
than pairs of RF pulses.
Utilise a smaller flip angle
< 90 degrees, to optimize
T1 weighting.
This is done by first
applying a frequencyencoding gradient with
negative polarity to destroy
the phase coherence of the
precessing spins
(dephasing).
Subsequently, the gradient
is reversed and the spins
rephase to form a gradient
echo.
SPOILED GRADIENT ECHO.
 In order to obtain images in gradient Echo without artifacts, the
transverse magnetization Mxy along the plane
perpendicular to B0 needs to be completely null at the end
of the TR interval so that only the longitudinal component Mz
is left when the next RF pulse occurs.
 The amplitude of the signal that is obtained depends uniquely
on the longitudinal relaxation..
 “Spoiling” operation – from which the name of Spoiled Gradient
Echo. This method spoils the phase coherence in transverse
magnetization between successive TR intervals
ECHO PLANAR
IMAGING.
ADVANTAGE- ultrafast
data acquisition,excellent
for dynamic and
functional MR imaging.
DISADVANTAGE- poor
image contrast, noisy and
infield homogeneities.
SSFP- STEADY STATE FREE
PRECESSION.
 Non zero steady state develops for both transverse and
longitudinal components of magnetization.
 The MR signal will never completely decay- the spins
in the transverse plane will never completely dephase.
 results in a higher SNR and hence better image
contrast.
COVERAGE OF THE ENTIRE
VENTRICLE.
 3 short-axis slices covering the basal, mid and apical
part of the LV are a strict minimum to appropriately
evaluate regional myocardial perfusion.
 It is appropriate to acquire all image slices during one
phase of the cardiac cycle-resting MBF is independent
of the cardiac phase, adenosine-induced hyperemia
yields significantly higher MBF and MPR in diastole
than in systole.
MYOCARDIAL PERFUSION ANALYSIS
 Visual analysis-A true stress-induced myocardial
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perfusion defect has typical features that help to
distinguish fromdark-rim artifacts
diffuse microvascular ischemia
fixed defects
true ischemia at an infarct border.
IMPORTANT FEATURES OF A
PERFUSION DEFECT.
 Onset of a myocardial perfusion defect coincides with the
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start of myocardial enhancement.
Later than the occurrence of a dark-rim artifact.
Most pronounced in the subendocardium (as is the darkrim artifact) and the transmural extent is variable.
The duration of the defect ranges from brief to prolonged
(persistent till the second pass), while it resolves from the
edge to the center of the perfusion defect, thus from subepito subendo-cardium.
The defect obeys anatomic borders as well as the
boundaries of the CA perfusion territories, whereas the
dark-rim artifact does not.
 Microvascular disease, presents as a circular
subendocardial defect not respecting perfusion
territories- hence difficult to differentiate from
artifacts.
 Perfusion defects, caused by hemodynamically
significant stenoses, are usually only visible during
stress perfusion imaging.
 Perfusion-like defects may occur in chronic, scarred
infarcts in the absence of a coronary stenosis.
 This is due to the low capillary density in the scar
compared to normal myocardium, simulating a
perfusion defect on MPI.
 These defects are present also at rest ( ‘‘fixed defects’’),
and the extent matches well with the findings on late
gadolinium imaging.
DIAGNOSTIC PERFORMANCE OF
PERFUSION MRI.
 On average, the sensitivity and specificity of perfusion
MRI for detecting obstructive CAD were 83 percent
(range, 44–93 percent) and 82 percent (range, 60–100
percent.
 The consensus report on CMR imaging classified
perfusion imaging as a Class II indication for the
assessment of CAD (provides clinically relevant
information and is frequently useful).
NO REFLOW.
 In patients with a
successfully reperfused
acute myocardial
infarction, MPI shows in
50% of patients a
perfusion-like defect in
the core of the infarct
territory. This is caused
by severe microvascular
damage in the infarct
core, a phenomenon also
called no-reflow.
DARK RIM
ARTIFACTS
 They typically occur at the
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interface between bloodpool and myocardium.
Appear as soon as the
contrast arrives in the LV
cavity.
More likely to occur with a
higher contrast
dose,balanced SSFP
sequence.
Occur on the basal half of
the left ventricle, along the
septal border and around
the papillary muscles.
Darker than a true
perfusion defect.
EDEMA IMAGING.
 Increased free water in the infarcted myocardium prolongs T1- and T2-
relaxation, and this prolongation is related to the duration of ischemia.
 T2-relaxation time linearly correlates with the % of free water, and
infarcted myocardium is visible on T2-weighted MR sequences
(T2w-imaging) as areas of increased signal intensity.
 In acute ischemia, the amount of free water increases not only in the
irreversible but also in the reversible injured myocardium, leading to
an overestimation of the true extent of myocardial necrosis.
-reversible cell swelling.
- increased capillary permeability in the surrounding ischemic rim.
- myocardial edema
 Healed infarcts, because of the lower water content in
the fibrotic scar have decreased signal intensity
compared with adjacent normal myocardium.
 Thus, T2w-imaging distinguishes b/w a recent and a
healed MI.
 Acute myocarditis or transplant rejection may present
equally focal or diffuse myocardial edema.
 A no.of acute MI patients do not show homogeneous
bright signal on T2w-imaging in the jeopardized
myocardium-show a central hypo-intense zone- the
breakdown of hemoglobin into deoxyhemoglobinshortening of T2-relaxation times.
 Useful to detect post reperfusion myocardial
hemorrhage.
CONTRAST ENHANCED MRI.
 Paramagnetic gadolinium chelated contrast agents,
mainly Gd-diethylenetriamine pentaacetic acid
(DTPA) have been used for MI imaging in both the
acute and chronic setting.
 Infarcted/ scarred myocardium appear bright.
SALVAGEABLE
MYOCARDIUM.
Area of increased T2
signal- area of LGE.
Contrast Enhanced MRI in Acute
Myocardial Infarction
 Early after reperfusion, the hyperemic response in the
reperfused myocardium results in an ↑ed delivery and
higher gadolinium concentrations in the jeopardized
myocardium as compared to normal myocardium.
 The supply of contrast agent to the infarcted region is
dependent
-on the patency of the infarct related vessel.
-collaterals to jeopardised myocardium.
-patency of microcirculation of the infarcted
myocardium. .
 The optimal time window for infarct imaging should
be somewhere between 10 and 25 min post-contrast
administration. Because of the delayed or late period
of imaging, this kind of imaging is called (gadolinium) (DE/DGE) or late (gadolinium)
enhancement (LE/LGE).
DEMRI- DELAYED ENHANCEMENT
MRI
 Simultaneous study of viability and infarction.
 The goal of DEMRI is to create images with high
contrast between abnormal myocardial tissue, which
generally accumulates excess gadolinium and normal
tissue in which gadolinium concentration is low.
 Imaging -approx 5 mts after rest perfusion-imaging
OR 10 - 15 mts after a one-time intravenous
gadolinium dose of 0.15 to 0.20 mmol/kg if stress-rest
perfusion imaging is not performed.
 Following an iv bolus, gadolinium distributes
throughout the intravascular and interstitial space,
while simultaneously being cleared by the kidneys.
 In normal myocardium- myocytes are densely packed,
tissue volume is mostly intracellular (~75–80 percent
of the water space)- gadolinium is unable to penetrate
intact sarcolemmal membranes- the volume of
distribution is small- viable myocytes exclude
gadolinium media.
 In acute MI, myocyte membranes are ruptured,
allowing gadolinium to passively diffuse into the
intracellular space -↑ed gadolinium vol of distn. And
↑ed tissue conc. Compared to normal myocardium.
 Chronic infarction, as necrotic tissue is replaced by
collagenous scar, the interstitial space is
expanded, and gadolinium tissue concentration is
increased.
PHYSIOLOGIC
INSIGHTS OF DEMRI
VIABLE MYOCARDIUM- black.
INFARCTED MYOCARDIUMwhite.
REMOTE ZONE- max. viability.
DE-MRI – shows only a 1.5 mm
subendocardial infarction.
Direct method – shows
predominant viability.
Compared to indirect method(
nuclear scintigraphy) of assessing
viability.
EXTENT OF HYPERENHANCEMENT
AND SUCCESSFUL
REVASCULARISATION.
 Transmural extent of hyperenhancement reflects the
transmural extent of scar- irreversible loss of
contractile fn.
 Post revasc. studies have indicated that > 75%
transmural enhancement predicts nonreversibility of
contractile fn.
 < 25 % enhancement predictive of post
revascularisation improvement.
PATTERNS OF ENHANCEMENT ON
LATE GD IMAGING.
 Patterns 1 (subendocardial infarct)
 Pattern 2 (transmural infarct)
 Pattern 3 the presence of microvascular obstruction
(no-reflow)
 Pattern 4 epicardial coronary artery
obstruction(occlusive infarct).
HYPERENHANCEME
ENT PATTERNS
IHD- always involves the
subendocardium.
Isolated midwall or epicardial
hyperenhancement strongly
suggests a non ischemic
pathology.
Also globally present
subendocardial
hyperenhancement is unlikely
even in diffuse CAD, to
consider amyloidosis, systemic
sclerosis.
INFARCT RELATED
COMPLICATIONS
ANEURYSM
FORMATION.
-VALVULAR
COMPLICATIONS.
-THROMBUS.
-- ASSOCIATED
PERICARDIAL
EFFUSION
-
CHRONIC ISCHEMIC
CARDIOMYOPATHY.
 The dysfunctional ventricle or myocardium in CAD
often contains a mixture of different ischemic
substrates (i.e. stunned, ischemic, hibernating,
necrotic, scarred myocardium) within the same
perfusion territory ,characterization of these ischemic
substrates, especially viability- important to
determine success of revascularisation.
 MRI has the unique capability to visualize even subtle
amts of scar formation.
MYOCARDIAL VIABILITY
ASSESMENT
 First step is to measure segmental end-diastolic wall
thickness(EDWT).
 Infarct healing with scar formation leads to a wall
thinning of the infarcted region.
 Thinned myocardium <6 mm has a low likelihood of
functional recovery after revascularization and
accurately reflects scar tissue.
 A substantial % of segments with preserved wall thickness,
however,do not improve in function following
revascularization.
 The most likely explanation is the presence of
subendocardial infarction with preserved wall thickness.
 Thinned myocardial segments may undergo a process of
‘‘reverse remodeling’’ after successful myocardial
revascularization with recovery of function and regain in
regional wall thickness .(John AS, et al2005 Images in
cardiovascular medicine. Reversible wall thinning in
hibernation predicted by cardiovascular magnetic
resonance. Circulation 111:e24–e25)
 In patients with chronic Lv dysfunction, dysfunctional
but viable (hibernating) segments improve
contractility during low-dose (5–10 microgm/kg body
weight) dobutamine infusion (i.e. increased systolic
wall thickening> 2mm).
 Contractility in non-viable scarred segments remain
unchanged or worsen during stress imaging
 Contrast enhanced MRI using late Gd imaging, the
third approach, detects scar tissue but not viability.
MR OF CORONARIESANGIOGRAPHIC APPROACH
 The diagnostic capacity of MR is based on the
administration of a contrast agent and the acquisition
of 3D images (CEMRA).
 Coronary imaging is difficult-small dimensions of the structure < 5mm.
-variable coursing.
-rotational and translational activity caused by
cardiac and respiratory activity.
TECHNIQUES
 “Black blood” or “white blood”.
 2 or 3 D.
 Breath hold or free breathing.
 With or without contrast medium.
BLACK BLOOD
TECHNIQUES
Completely cancel-out the signal
from blood through the
application of a double inversion
pulse (double inversion recovery).
The first pulse inverts the
magnetization field throughout
the body, blood included. The
second re-inverts the
magnetization of the tissues on
the scanning plane.
The final result in the image is the
annulment of the signal emitted
from blood, whereby the lumen
of the vessel is black in contrast to
the more or less intense signal of
the wall.
Significant limitation is that this
is a 2D method.
WHITE BLOOD
TECHNIQUES.
Can be obtained by several
acquisition techniques(
Gradient echo, echo planar
imaging) that are faster, with
excellent contrast b/w vessel
lumen and surrounding fat
which can be easily
cancelled.
ADVANTAGE- 3D technique
, easy for operator to centre
the coronary vessel more
easily.
DISADVANTAGE- cannot
visualize arterial wall, only
lumen.
PLAQUE CHARACTERISATION BY
MRI.
 Better validated in carotid arteries.
 The lipid plaques have both a short T1 and a short T2,
therefore they are hyperintense in T1-weighted images
and hypointense in T2.
 Fibrous plaques have a quite similar signal intensity in
T1- and T2-weighted images; the signal intensity is
lower compared to lipidic plaques in T1-weighted
images.
STUDY OF CORONARY RESERVE
 The coronary reserve expresses the vasodilatation
capacity of coronary vasculature in response to cardiac
metabolic demand.
 Coronary flow can increase up to 5 times the baseline
value.
 Coronary flow decreases only in the presence of a
stenosis over 85%,while the coronary reserve begins to
decrease, in presence of a stenosis of about 35-40%.
 When stenosis is over 90% the reserve goes down to
zero.
 It is possible to measure coronary flow using a Phase
Contrast sequence(principle- signal from moving
blood will undergo a phase shift relative to stationary
tissue if a magnetic field is applied in that direction.)
 The values of coronary reserve as measured by PC
images (ratio of flow in maximum vasodilatation
obtained with adenosine over baseline flow) correlate
well with severity of stenosis and CFR measured
invasively.
 The sensitivity and specificity for the diagnosis of
coronary stenoses >70%,are respectively 100 and 83%.
VENC- MRI ( Velocity
encoded MRI)
ECG-gated velocity-encoding MRI.
Analogous to cine imaging, each
velocity-encoded image (top row of
images) corresponds to a cardiac
phase, and gating to the
electrocardiogram is necessary. On
the images, white represents
maximal velocity (in this case
across the aortic valve, red arrow).
Black represents flow in the
opposite direction (in the
descending thoracic aorta, blue
arrow). Grey represents no flow.
The bottom row demonstrates the
corresponding cineMRI images.
BYPASS AND STENTS
 The application either of SE or GRE sequences, and


•
•

especially CEMRA, allows an accurate evaluation of
the morphology of both arterial and venous grafts.
It is possible to visualize 98% of bypasses and give
their accurate description with an 88% specificity.
The measurement of blood flow by means of the
Phase Contrast sequence can provide further
information on the hemodynamic effect of a stenosis
of the graft.
The flow in the bypasses can also be measured after
adenosine for the evaluation of the flow reserve.
Reliable in identifying coronary artery aneurysm in
Kawasaki disease.
Imaging of anomalous coronary arteries.
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