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Pulmonary artery anatomy & aorto- pulmonary collaterals in tetralogy of Fallot’s physiology.
A comparative study of gadolinium enhanced 3-D magnetic resonance angiography with cardiac catheterisation-X-ray angiography.
Introduction
Tetralogy of Fallot (TOF) is the most common form of cyanotic congental heart
disease according to Baltimore Washington infant study (BWIS), the most recent &
accurate study to assess the prevalence of different subtypes of tetralogy of Fallot.1A
number of studies indicate that the prevalence of TOF ranges from 0.26 – 0.48/1000
live births.2
The four components of TOF are an outlet ventricular septal defect(VSD) ,
obstruction to the right ventricular outflow, overriding of aorta ( <50%) & right
ventricular hypertrophy.3 The central pulmonary arteries may be hypoplastic ,
discontinuous or absent in some cases of tetralogy of Fallot.The pulmonary vascular
bed may be supplied with blood flow from several sources including antegrade flow
through the pulmonary valve, aorto pulmonary collaterals, and surgically placed
shunts. Surgical and trans catheter procedures are often required to augment effective
pulmonary blood flow and alleviate cyanosis or to eliminate sources of excessive
pulmonary blood flow.4
Complete delineation of all sources of pulmonary blood supply and of the size and
morphology of pulmonary arteries is therefore essential to patient management.
Traditionally, cardiac catheterization with X-ray angiography has been used for this
purpose . A non invasive alternative procedure would have advantages of ease of
serial assessment , decreased risk and reduced cost.
Echocardiography is often of limited value in these patients because of poor acoustic
windows.Several studies have previously shown that standard magnetic resonance
imaging(MRI) techniques, such as spin echo MRI, gradient echo cine MRI require
long scan times for complete anatomic coverage, and small vessels ( < 2 mm) may
not be detected. 5
These 2dimensional techniques are not ideally suited for imaging long and tortuous
blood vessels. Gadolinium enhanced 3 dimensional magnetic resonance angiography
is a fast imaging technique that has been shown to accurately evaluate major arteries
& veins. 6
1
REVIEW OF LITERATURE
Tetralogy of Fallot
The malaligned ventricular septal defect (VSD) in tetralogy of fallot(TOF) is located
in the perimembranous septum with extension into the infundibular septum.The crest
of muscular trabecular septum forms the floor of the VSD and roof is formed by the
valve of overriding aorta.7
VSD in TOF is nonrestrictive and it remains nonrestrictive. Malalignment of
infundibular septum is the essential cause of right ventricular outflow obstruction.8
Other causes of obstruction to right ventricular(RV) outflow are hypertrophy of the
septo parietal trabeculations, the trabecula septomarginalis and the infundibular
septum .
Pulmonary valve is frequently stenotic & bicuspid, less commonly unicommissural &
unicuspid. Occasionally the hypoplastic annulus or stenosis of the ostium infundibular
is the main site of obstruction.9
Pulmonary trunk,its bifurcation and right & left pulmonary arteries tend to be
segmentally or diffusely hypoplstic .The severe form of TOF is pulmonary atresia
with a non restrictive malaligned ventricular septal defect.
Due to obstruction to pulmonary blood flow, lungs are frequently supplied by
collateral blood vessels. There are 3 types of collateral vascular blood supply to the
lungs.10
1) Major systemic arterial collaterals
2) Ductus arteriosus
3) Small diffuse pleural arterial plexuses.
Systemic arterial collaterals are classified according to their origin
1) Bronchial arterial collaterals – originate from bronchial arteries and anastamose
with pulmonary arteries with in the lung.
2) Direct systemic arterial collaterals - originate from descending aorta ,enter the
hilum and directly supply the lung parenchyma .
3) Indirect systemic arterial collaterals – originate from major aortic branches other
than bronchial arteries. They arise from internal mammary, innominate,
subclavian arteries and anastomose with proximal pulmonary arteries outside the
lung . 11
2
All 3 major types of systemic arterial collaterals are present in TOF with pulmonary
atresia . Approximately 10 % collaterals originate from coronary arteries .
Direct aorta to pulmonary artery collaterals originate from inter segmental branches
of dorsal aorta during the 3rd and 4th weeks of gestation.
Bronchial arterial collaterals develop in the 9th gestational week after the paired inter
segmental arteries have been resorbed .Indirect collaterals arise later than 9 th week of
gestation.11,12Lung growth & survival depends on the size and patency of collateral
arteries. Bronchial artery collaterals have intra pulmonary anastamoses. Direct
arterial collaterals have hilar anastamoses. Indirect arterial collaterals have extra
pulmonary anastomoses.Systemic arterial collaterals have a strong tendency to
harbour intimal cushions that serve as sites of potential segmental stenosis.13,14
In the absence of segmental stenosis, large collateral arteries transmit systemic arterial
pressure to the pulmonary vascular bed resulting in morphological changes analogous
to pulmonary vascular obstructive disease (PVOD).
Pulmonary artery anatomy evaluation before surgical repair of Tetralogy of Fallot(TOF).
In TOF, definitive repair as a primary procedure should be done if operation can be
done at a low risk, with good result. Primary repair of TOF can be done as early as 4-
6 months of age when there are no significant prohibitive factors. 15
Kirklin etal. studied the incremental risk factors involved in completing a primary
repair at a young age. In both tetralogy of Fallot with pulmonary stenosis( TOF with
PS) and tetralogy of Fallot with pulmonary atresia, small size of the proximal portion
of the right and left pulmonary arteries is a strong risk factor for death after repair. 16,17
It is shown that a post-operative right to left ventricular pressure ratio (PRV/ PLV) of
less than 0.75 will be associated with a good functional result. Conversely, post op
PRV/PLV greater than 1 is associated with steeply high risk of death early after repair 18. The presence of hypoplastic pulmonary arteries will usually require a palliative
shunt procedure to augment pulmonary blood flow which will stimulate the growth of
the pulmonary arteries before undergoing a total repair.
Similar increase in post repair PRV occurs in the presence of small pulmonary valve
annulus. Such patients with a small pulmonary valve annulus will require a
transannular patch to relieve or avoid a postoperative PRV/ PLV of greater than 0.75.
3
In this situation, transannular patch achieves a successful result with low mortality.
To decide regarding the need for transannular patch, intra operative measurements of
the pulmonary annulus by means of Hegar dilators has been used. Transannular patch
is required for successful outcome if a pulmonary valve annulus is less than 50% of
the diameter of the ascending aorta or less than a minimum acceptable diameter from
a table of normal values used. 19,20
TOF with pulmonary atresia requires additional attention before surgery with regards
to confluence of the pulmonary arteries, their connection with the pulmonary trunk,
the presence of arterial duct and the number of aortopulmonary collaterals(APC’S).
About 20-30% of patients with TOF with pulmonary atresia have nonconfluent
pulmonary arteries. One important feature of TOF with pulmonary atresia is the
frequent failure of the pulmonary arteries to distribute to all 20 of the pulmonary
vascular segments. More than 80% of those with nonconfluent right and left
pulmonary arteries have incomplete distribution of one or both pulmonary arteries.
More than 1/3 of this group will have less than 10 pulmonary vascular segments in
continuity with a central pulmonary artery , the segments which are not connected to a
central pulmonary artery usually receive a large aorto – pulmonary collateral (APC).21
APCs typically 2-6 in number, usually arise from the anterior wall of the aorta
opposite to the origin of the intercostals arteries. They most commonly terminate by
joining an interlobar or intra-lobar pulmonary artery that arborises normally. In 40%
of the subjects anastamosis occur between pulmonary arteries and aortopulmonary
collaterals at the hilum or within the lung ,in the remaining 60% of cases, APCs enter
the hilum, travel with bronchi as pulmonary arteries and supply a variable number of
bronchopulmonary segments. Usually there will be multiple sources of blood supply
to a lung, called multifocal blood supply. This will result in fragmented pulmonary
arterial distribution (arborisation abnormality).
60% of large APCs will have stenoses at branching points and at the junction with
pulmonary arteries. These stenoses prevent pulmonary over circulation. When this is
not the case, pulmonary over circulation is present early in life and pulmonary
vascular obstructive disease develops in patients surviving infancy.
Atleast half of the segments of both lungs must be supplied by true pulmonary arteries
otherwise the predicted postop PRV/ PLV may be high ,which is associated with high
surgical mortality.
Aorto pulmonary collaterals (APC’s) pose special problem in management and
4
deserve special consideration. Ideally, they should be unifocalised. Otherwise they
have to be surgically ligated or embolised by transcatheter techniques before total
repair .15
These variables relating to pulmonary arterial morphology are the strongest risk
factors for death after repair. Kirklin etal. have found that small size of the central
pulmonary arteries, their non confluence, number of large APCs and post op PRV/
PLV are the incremental risk factors for post operative death .17
From the above discussion, it is clear that a preoperative prediction of PRV/PLV
which is dependent on the size, confluence of PAs, and delineation of APCs will be
of considerable importance. A number of methods have been developed to predict
the postoperative PRV/PLV on the basis of preoperative cine angiographic
measurements. McGoon’s ratio is calculated as the ratio of the combined diameter of
pre branching right and left pulmonary arteries to that of the descending aorta at the
diaphragm. The normal vlue is ≥1.5:1.0 22. Another method was adopted by Nakata
et al. 23 They calculated the pulmonary artery area index(Nakata index) . The normal
value is > 250mm2.
Z value is calculated as : observed diameter of pulmonary – mean normal diameter
/standard deviation around mean normal diameter. Pulmonary arteries are considered
small if Z < − 3.5.
McGoon’s ratio calculated from cineangiogram is commonly employed to decide the
surgical strategy. But, the catheterization, being an invasive procedure, has inherent
risk of mortality, especially in sick children with cyanotic congenital heart disease.
A co-operative study on cardiac catheterisation gathered data during 12,367 cardiac
catheterisations. A total of 55 deaths(4%) occurred with in 24hrs of the catheterisation
and 40 of these were in children. 29 of the paediatric deaths were in children less than
2 years of age, who had a total mortality of 6%. Of the 55 patients who died, 27 were
in NYHA class IV, 11 in class III and 5 in class II. 24
Several later studies have shown death related to cardiac catheterisation to be in the
range of 0.26 – 1.6% Mortality was highest in children less than 2 years age. In
infants , mortality rate was 1.6% - 5%. Patients with highest mortality were those who
were acidemic, hypoxemic and those with poor peripheral perfusion. These high risk
patients had a mortality upto 30-36%.
The major risks of cardiac catheterisation include cardiac perforation with tamponade,
blood loss, failure to improve from progressive hypoxemia, contrast induced
5
hypotension, acidosis, depressed myocardial function and thrombotic occlusion of
ileofemoral vessels.
In addition, the constraints of contrast usage will be a limiting factor in sick children.
To avoid the attendant risk of large amount of contrast, the study may have to be
staged, which means repeat catheterisation.
Hence, a non invasive alternative that provides most of the required information will
minimize the risk of catheterisation by enabling a limited cath study or on occasion, it
may obviate the need for catheterisation.
Two dimensionl echocardiography has the limitation of poor acoustic window. In a
comparative study of X-ray angiography, cine MRI and echocardiography, Wesely
vick etal, have shown gradient echo cine MRI to compare favourably with x-ray
angiography in delineating APCs. Gradient cine MRI was superior to spin echo MRI
and echocardiography for this purpose.25
These 2D MRI techniques have shown good agreement with X-ray angiography in
measuring central pulmonary arteries.26 But, these traditional MRI techniques have
several limitations. Although fast spin echo with double inversion recovery and
segmented k-space fast gradient echo techniques significantly shorten image
acquisition time, total anatomic coverage of entire thorax may still be lengthy and
require multiple breathholds which may be difficult for cyanotic patients. Because
APCs may arise from subclavian arteries and from the abdominal aorta, required
anatomic coverage is large which will require more scan time.
Other limitation of traditional MR techniques include their difficulty in imaging very
small blood vessels, vessels with slow flow, and intra pulmonary segments of APCs
and pulmonary arteries.
Gadolinium (Gd) enhanced 3D MR angiography (MRA) is a fast imaging technique
which is capable of imaging large anatomic volumes in a single breathhold. 27 Its high
signal to noise ratio allows for depiction of blood vessels as small as 0.5 mm, vessels
with slow blood flow and intraparenchymal pulmonary vessels. It is accurate in
evaluating pulmonary and systemic venous anomalies.28
MRI in postoperative TOF
6
Residual ventricular septal defect(VSD)Residual ventricular septal defect(VSD), although rare, should be sought and
excluded. Although most residual VSDs can be accurately diagnosed with 2D
Doppler echocardiography, they can also be identified with steady-state free
precession (SSFP) imaging, and the magnitude of the shunt can be quantified with
phase-contrast imaging. Prior to the advent of cardiac magnetic resonance(MR)
imaging, reliable shunt quantification was possible only with invasive catheterization.
High-velocity shunting through a small VSD creates a “jet” of turbulent flow, which
manifests as local loss of signal intensity or a signal void on steady–state free
precession(SSFP) images.29
Pulmonary valve regurgitation (PR),Right ventricular(RV) enlargement and
dysfunction
Pulmonary valve regurgitation(PR), the most common sequela of transannular or RV
outflow tract patch repair, occurs in nearly all TOF patients and can be accurately
quantified with phase-contrast imaging. In addition, the effects of PR on RV size and
function can be serially measured. Chronic PR is generally well tolerated; however,
the evidence is beginning to point to PR as an important contributing cause of
longterm morbidities, including atrial and ventricular arrhythmia, RV dilatation, and,
possibly, sudden death. 30,31,32
The combination of limited outcome data and suboptimal therapy (pulmonary valve
replacement) leaves clinicians with no clear guidelines as to when chronic PR should
be treated. 33 Because cardiac MR imaging can accurately help to quantify severity of
PR and RV ejection fraction, it will play a critical role in establishing better clinical
management guidelines .
Cardiac MR imaging is the standard of reference for measuring RV size and function .
Although RV ejection fraction, like most ejection-phase indices, is load dependent, it
is presently the most accurate and reproducible criterion for assessing RV systolic
function. Depressed RV ejection fraction is linked to adverse outcomes , and early
detection, with intervention to preserve RV function, will likely contribute to better
long-term outcome in TOF patients.34
Residual Pulmonary Stenosis
7
Approximately 10% to 15% of patients will have residual or recurrent branch
pulmonary stenosis . A significant number of these patients will require additional
surgery or catheter-directed angioplasty for residual or recurrent pulmonary stenosis.
The level of stenosis varies from the proximal RV outflow tract to the distal branch
pulmonary arteries, including surgically placed RV–pulmonary artery conduits .31
Tricuspid Regurgitation
The prevalence of moderate or greater TR is estimated to be approximately 10% . TR
is usually a consequence of progressive RV dilatation with subsequent annular
dilatation of the tricuspid valve. The magnitude of the regurgitation can be quantified
with phase-contrast imaging data either alone or in combination with RV volumetric
data. Quantification of TR has yet to be vigorously validated, in part due to the lack of
a robust standard of reference. Incorporating RV volumetric data with phase-contrast
imaging data may have inherent error greater than that for semilunar
(aortopulmonary) regurgitation. As with PR, the effects of TR on RV size can be
assessed with RV volumetric data.31
RV Outflow Tract Aneurysm
RV outflow tract aneurysms are often present and are related in part to transannular or
RV outflow tract patching . Other possible contributing factors include extensive
infundibular muscle resection and ischemic insult 35. The size of the RV outflow tract
is easily and accurately evaluated with SSFP imaging, and reduced RV ejection
fraction has been associated with the presence of RV outflow tract aneurysm 36.The
significance of this finding is unclear in that, by definition, dyskinesia results in a
reduction of global ejection fraction; how this relates to contractile dysfunction at the
myocardial level is less clear. In the measurement of the RV volumes, inclusion of the
dyskinetic segment necessarily “contaminates” global ejection fraction, potentially
exaggerating the degree of myocardial contractile dysfunction when the RV sinus
actually has normal regional shortening 37. The inefficient transfer of stroke volume to
the pulmonary arteries may play a part in the presence and progression of reduced
cardiac output and exercise intolerance.
Conduit Obstruction
Because of the lack of durable long-term RV–pulmonary artery conduits, obstruction
eventually develops in nearly all patients who require a conduit as a part of initial
repair. Conduit obstruction can be identified with multiple imaging sequences.
However, the ability to determine the degree of obstruction is somewhat limited
8
compared with the accuracy of diagnosing branch pulmonary stenosis. Artifacts
related to metal within the conduit and to turbulent flow often interfere with this
assessment. 2D fast spin echo with double inversion recovery( FSE-DIR) imaging is
less affected by these artifacts and can often provide diagnostic information.
Fortunately, Doppler gradients can be obtained in almost all patients, even those with
poor acoustic windows, and correlate well with catheter-derived gradients. This
approach is a good example of recognizing and using the individual strengths of both
echocardiography and cardiac MR imaging to obtain complementary diagnostic
data.34
Left ventricular (LV) Dysfunction
The accuracy and reliability of cardiac MR imaging in measuring LV systolic
function is well established . In addition to its previously mentioned advantages over
echocardiography, cardiacMR imaging derived LV ejection fraction retains its
accuracy in the presence of RV volume overload(diastolic septal flattening) and
abnormal septal motion, unlike echocardiography derived shortening fraction.38. LV
dysfunction is an uncommon complication but has been associated with a number of
predictors, including time elapsed since palliative arterial shunt creation, aortic
regurgitation, and, most significantly, RV ejection fraction.39,40 Recently, LV ejection
fraction was found to be the strongest predictor of poor clinical status 34. Proposed
mechanisms include akinesia resulting from the VSD patch, septal fibrosis, chronic
volume loading from early palliative shunt creation, abnormal septal motion, and
myocardial injury at the time of repair.
9
Cardiac Magnetic Resonance Imaging Techniques
Contrast material enhanced magnetic resonance angiography(MRA) is a particularly
useful technique for the assessment of deep anatomic structures such as the
pulmonary arteries, which are difficult to access at selective angiography. Cine
magnetic resonance(MR) images can provide additional information about cardiac
function, valve patency, and the hemodynamic significance & valvular stenosis.41
Imaging planesThe main cardiac imaging planes are oblique to one another. As the cardiac imaging
planes are also at arbitrary angles with respect to the scanner, they are called “double
oblique” planes. The three main cardiac imaging planes are short axis, horizontal long
axis, and vertical long axis. The horizontal long axis view is also known as the four-
chamber view, and the vertical long axis view is also known as the two-chamber
view. Note that the initial vertical long axis view that is prescribed from an axial
image is only approximate; a true vertical long axis view should be prescribed from
the horizontal long axis view. Methods to determine the correct location and
orientation of the standard cardiac imaging planes are well-described.42
Other imaging planes that may be useful include a left ventricular outflow tract view
for ascending aortic pathology and a three-chamber view. The three-chamber view can
be prescribed from the left ventricular outflow tract view of a short axis view. This
view displays the aortic and mitral valves immediately adjacent to one another.
Unlike the pulmonary and tricuspid valves, which are separated by a muscular crista
supraventricularis, the aortic and mitral valves are in close proximity and are often
both affected by pathologic processes.42
Electrocardiographic(ECG) gating ECG gating can be performed prospectively or retrospectively. Prospective gating is
most common. In prospective gating, the MR acquisition is triggered by the R wave.
Within an R-R interval, there may be a trigger delay, acquisition window, and trigger
window.43 As discussed in the “black blood” imaging section, diastolic imaging may
be desirable with fast spin echo sequences, and a trigger delay can be used to delay
image acquisition after the R wave trigger. A trigger window is an interval between
the end of data acquisition and the next R wave. With a trigger window, heart beats
earlier than expected will still trigger acquisitions. The trigger window is typically 10-
15% of the R-R window. The acquisition window is the duration of data acquisition.
10
With a standard trigger window and no trigger delay, this would be 85-90% of the R-
Rwindow. Because of the trigger window,prospectively gating sequences will exclude
late diastole.43
Common problems with ECG-triggered acquisitions include poor or inaccurate R
wave detection (eg, triggering off a prominent T wave) and cardiac arrhythmias. R
wave–detection problems can often be resolved by adjusting electrode position or
toggling the lead polarity. Arrhythmias can result in inaccuracies in evaluation of
cardiac function. Acquisition time can also be increased, as some heartbeats may not
trigger data acquisition. The effect of arrhythmias can be mitigated with very fast
sequences (eg, single-shot fast spin echo) or real-time sequences. Retrospective gating
is also useful in patients with arrhythmias, because data from irregular heartbeats can
be rejected.43
In retrospective gating, the data are acquired continuously along with an ECG
tracing. The data are retrospectively sorted using the ECG tracing after the
acquisition. This is more computationally intensive. Retrospective gating is helpful in
patients with arrhythmias. In retrospective gating, there is no trigger window and the
full cardiac cycle is imaged. Imaging of the full cardiac cycle may result in more
accurate assessment of cardiac function. Retrospective gating is particularly helpful if
peripheral pulse gating is used. Peripheral pulse gating is an option if central gating
cannot be performed. Prospectively gated peripheral pulse triggered sequences will
start after the onset of systole, as the systolic pulse must propagate to the finger before
being detected.43
Morphologic assessment
For adequate morphologic assessment of cardiac structures & thoracic vessels high
spatial resolution and image contrast is required and this can be obtained by using
ECG gated T1 weighted spin-echo (SE) MR imaging sequences. ECG gated T1
weighted spin echo MR images are affected by respiratory motion artifacts. Fast spin
echo sequences are typically used, often known as “black blood” sequences. Multiple
options are available, but half-Fourier single-shot fast-spin echo (SS-FSE) sequences
are the fastest. Fast imaging sequences that are more robust with regard to motion
artifacts are half-fourier rapid acquisition with dark blood preparation.These are much
less vulnerable to motion related artifacts because of the short acquisition time.41Black
blood" MR images are produced with sequences designed to null the signal of
11
flowing blood. These images allow for anatomic assessment of the heart and vascular
structures without interference from a bright blood signal. While black blood
sequences are standard in most imaging protocols, they are particularly important for
assessment of cardiac masses, the myocardium (eg, in suspected arrhythmogenic right
ventricular dysplasia), and the pericardium. but in clinical practice, there are 3 general
options for black blood imaging.44 :
1. Half-Fourier single-shot fast spin echo with double inversion recovery
2. Breath-hold single-slice fast spin echo with double inversion recovery
3. Multislice fast spin echo
The first 2 options are the most commonly used. Half-Fourier single-shot fast-spin
echo (SS-FSE) sequences are the fastest.
In ECG-gated spin echo cardiac imaging repetition time(TR) depends upon heart rate
or R-R interval. Thus, the acquisition time can be calculated by substituting the R-R
interval for repetition time (TR) in the standard equation:Acquisition time = R-R
interval × number of phase encoding steps × number of acquisitions/echo train length
If the heart rate is 70 beats per minute, the R-R interval is 857 msec, which may not
be adequate for T2-weighted imaging. In this case, triggering can be performed after
every other R wave, and (2 x R-R interval) should be used in place of R-R interval in
the above equation.
In many cases, the purpose of black blood imaging is to assess anatomy and
weighting is not important. In such cases, repetition time(TR) should be as short as
possible to minimize imaging time; thus, black blood MR images are often T1-
weighted. For certain applications, such as cardiac mass evaluation, specific T2-
weighted sequences may be performed. Protons must experience both the 90°
excitation pulse and the 180° refocusing pulse to generate a spin echo. If protons in
flowing blood are not present in the slice long enough to experience both pulses, no
spin echo is generated.Thus, a way to minimize the signal from flowing blood is to
decrease the chance that flowing blood will experience both the 90° and 180° pulses.
This can be done by minimizing the time the blood is in the slice, such as by
decreasing the volume of the slice (thinner slices), creating the shortest path (slice
positioning orthogonal to flowing blood), or increasing the speed of flowing blood
(imaging during systole). Another method is to increase the time interval between the
90° and 180° pulses (increase TE, or echo time).44
12
In standard spin echo imaging, acquisition during systole will result in more nulling of
blood signal. However, as will be discussed, in fast spin echo imaging, diastolic
imaging is usually more optimal.
Fast spin echo imaging
Standard spin echo black blood imaging has little utility in clinical practice because
acquisition times exceed patient breath-holding times. Although the resulting
respiratory artifacts can be remedied to some extent with signal averaging (which
further increases acquisition time), acquisition during free breathing is better
performed with multislice fast spin echo imaging.
The fastest fast spin echo sequences can be performed during a breath-hold.
A basic disadvantage of fast spin echo imaging relative to spin echo imaging is the
image blurring that results from acquiring data at different effective echo times during
the echo train. In cardiac imaging, this image blurring is exacerbated by the increased
motion at systole. Thus, to minimize artifact, fast spin echo cardiac MR imaging is
best performed in diastole. However, as previously discussed, blood signal is
optimally nulled at systole where blood flows fastest. Diastolic imaging may result in
more blood signal than optimal. Fast spin echo cardiac MR sequences are therefore
typically performed with the addition of double inversion recovery pulses to achieve
optimal nulling of blood signal.44
Double inversion recovery
Double inversion recovery sequences are designed specifically to null the signal from
flowing blood. There are 2 prepulses. A nonselective 180° RF (radiofrequency) pulse
inverts all protons. This is followed by a slice-selective 180° pulse that reverts all
protons in the imaging slice back to the original alignment. There is no effect on
stationary protons in the imaging slice. However, the flowing blood in the imaging
slice will have experienced only the nonselective pulse (the blood that experienced
both pulses will no longer be in the slice at the time of imaging). 44
Double inversion recovery sequences begin imaging when the magnetization vectors
of the flowing blood crosses the null point – the inversion time.
Typical inversion times for double inversion recovery sequences are between 400 and
13
600 msec and depend upon heart rate. Inversion time is a substantial portion of a
typical R-R window, which limits the amount of time available to acquire the echo
train. Also note that images performed 400-600 msec after the R wave will
conveniently be in diastole.
Type of black blood sequence to be used
The fastest sequences are half-fourier single-shot fast spin echo with double inversion
recovery in which the data needed to generate an image can be acquired during one
heartbeat. Different trade names for these half-Fourier single shot sequences are
HASTE(high speed turbo spin echo T2 weighted image sequences) (Siemens) and SS-
FSE (single shot fast spin echo) (GE, Phillips). However, while these images have the
least cardiac and respiratory motion artifact, the half-Fourier single-shot acquisition
decreases spatial resolution and signal-to-noise. For applications where optimal
resolution and signal are useful (eg, evaluation of the right ventricular wall in
suspected arrhythmogenic right ventricular dysplasia), breath-hold single-shot fast
spin echo with double inversion recovery (one slice per breath-hold) may be more
useful.44
Another option is to use multislice fast spin echo imaging during free breathing. This
technique is similar to basic spin echo imaging with the addition of a short echo train
to decrease the imaging time. As in spin echo sequences, multiple signal averaging is
used to decrease respiratory motion artifact. As blurring is minimal with a short echo
train, systolic imaging is possible, and blood nulling is similar to spin echo sequences.
Inversion recovery pulses may not be necessary with this technique.
It is also possible to use bright blood sequences to evaluate cardiac morphology.
Functional Assessment
Functional evaluation of cardiac wall motion is performed by using ECG gated
gradient – echo techniques or segmented (fast) gradient echo techniques. This gives
multiphase bright blood images and reveals cardiac motion in multiple frames
through the cardiac cycle.41
The recently developed “Ultrafast” sequences allow high temporal resolution & very
rapid acquisition of dynamic ECG-gated images of the heart & great vessels. Cardiac
function is evaluated using cine gradient echo sequences, often known as “bright
14
blood” sequences . Steady-state free precession (SSFP) gradient echo sequences have
largely replaced spoiled gradient echo sequences for this purpose. Different trade
names for these SSFP sequences are TrueFISP (True Fast Imaging with Steady-state
Precession; Siemens), FIESTA (Fast Imaging Employing Steady-state Acquisition;
GE), and b-FFE (Balanced Fast-Field Echo; Phillips). These sequences are typically
used in conjunction with segmented k-space acquisition
Steady-state versus spoiled gradient echo imaging
In gradient echo (GRE) imaging, the TR (repetition time) is often shorter than the T2
of most tissues, and the transverse magnetization will not have fully decayed before
the next RF pulse. Thus, there will be residual transverse magnetization that adds T2
contrast (in addition to T1 contrast) to the image. This additional T2 contrast is
undesirable for many applications, as the T1 and T2 contrast may be competitive; for
example, a liver lesion that is hypointense on T1 and hyperintense on T2 may be
isointense with both T1 and T2 weighting. To achieve T1 weighting with a short TR
GRE sequence, spoiling the residual transverse magnetization is necessary. This
spoiling can be accomplished with an RF pulse or gradients.
The majority of fast GRE sequences used in noncardiac clinical MRI are spoiled.45
In steady-state GRE sequences, spoiling is not performed, and residual transverse
magnetization is retained. The retained residual transverse magnetization increases the
signal-to-noise ratio (SNR) of steady-state sequences relative to spoiled sequences.
The image contrast will depend on the T2-to-T1 ratio. As stated previously, this is
undesirable for many applications. In steady-state sequences, only fluid and fat will
have a high signal (fluid and fat have comparable T1 and T2 times, while in most
other tissues, T2 time is much shorter than T1 time). However, in bright blood cardiac
MRI, hyperintense blood relative to other tissues is exactly what is needed; thus,
steady-state GRE sequences are optimal for cine cardiac imaging (cMRI).45
The sequences used in cardiac imaging are balanced steady-state free precession
(SSFP) sequences. Different trade names for these sequences are TrueFISP (Siemens),
FIESTA (GE), and balanced FFE (Phillips) These sequences are very fast and have a
high SNR , but the T2-to-T1 image contrast limits the role of these sequences to
noncardiac applications.
SSFP cine MRI has largely replaced spoiled-GRE cine MRI for evaluation of cardiac
15
function. SSFP sequences do not depend on flow; they have a higher SNR; and they
are faster. Spoiled-GRE sequences are T1 weighted and depend on through plane flow
enhancement (similar to time-of-flight MR angiography) to generate contrast. The
blood may become saturated if the flow is slow or the TR is short.
Thus, spoiled GRE cine MRI does not allow for the use of very low TRs, because
there is not enough time for saturated blood to be replaced by unsaturated blood
between excitation pulses. With SSFP sequences, blood signal is dependent on
intrinsic contrast rather than inflow effects, and TR can be as short as possible. SSFP
cine MRI can be almost 3 times as fast as spoiled-GRE cine MRI. In addition, the
SSFP sequence has a higher SNR due to the residual transverse magnetization. This is
particularly true at low TRs. With spoiled-GRE sequences, SNR decreases with
decreasing TR. With SSFP sequences, SNR is high even at low TRs, because residual
transverse magnetization increases with shorter TRs.45
Requirements for SSFP Imaging
High-quality SSFP imaging depends on a low TR, a high flip angle, and a uniform
magnetic field. In SSFP imaging, residual transverse magnetization must be
preserved. Field inhomogeneity and unbalanced gradients can disrupt the steady-state
transverse magnetization. The sequences are implemented with balanced gradients to
minimize gradient-induced dephasing. SSFP sequences are very sensitive to field
inhomogeneities. In regions of high local magnetic-field variations, SSFP images
often suffer from characteristic bands of signal loss (off-resonance banding artifact),
which can disrupt the steady-state.45
As TR is increased, any off-resonance banding artifact will become more pronounced
because of the increased off-resonance precession per TR. Thus, the lowest TR
possible is desirable for SSFP imaging. Typical TRs are less than 4 msec, with time of
echo (TE) being less than 2 msec. Banding artifact is a particular limitation for 3D
MRI, as the banding artifact becomes more pronounced as the main magnetic-field
strength (and any associated inhomogeneity) is increased.45
In spoiled-GRE sequences, optimal SNR is dependent on matching the flip angle to
the TR (the lower the TR, the lower the flip angle). In SSFP sequences, the SNR does
not change substantially with different flip angles, but the T2/T1 weighting will
increase with an increasing flip angle. SSFP sequences, therefore, should use the
16
largest flip angle achievable, because this will maximize the contrast-to-noise ratio.
As RF pulses are continuously applied to maintain the steady state, specific
absorption rate limits are often a factor in SSFP sequences and limit the use of very
high flip angles. Flip angles in SSFP sequences are typically 40 to70°.
Limitations of SSFP imaging
SSFP sequences are prone to off-resonance banding artifacts. As these artifacts are
caused by local field inhomogeneities, a very uniform magnetic field is required to
avoid artifacts. Because SSFP sequences are typically performed with very low TRs
and TEs, the low TE time may result in a chemical shift artifact of the second kind
(India ink artifact).SSFP sequences may be less sensitive to turbulent flow (eg; in
regurgitant valves) than spoiled-GRE sequences , because SSFP sequences do not
depend on time-of-flight effects.45
Temporal resolution, Spatial resolution and Imaging time. 45
Multiple images at the same slice position, corresponding to different time points in
the cardiac cycle, are obtained during cine GRE imaging. Each image is called a
frame. Typically, 12-18 frames are obtained during a cardiac cycle. The temporal
resolution is the duration of the cardiac cycle that each frame represents. High
temporal resolution is necessary to accurately assess cardiac motion, particularly
during systole. The ideal temporal resolution should be 50-60 msec or less. With
faster heart rates, greater temporal resolution is needed. The temporal resolution and
the number of frames are directly related, but in general, the temporal resolution is
more important than the number of frames obtained .
Segmented k-space cine GRE
In cine MRI, the echoes are partitioned into k-spaces, with each k-space
corresponding to a frame. If there are 12 frames, the echoes would be partitioned into
12 k-spaces. The amount of data (number of phase-encoding steps) needed to fill each
of the k-spaces corresponds to the spatial resolution. In conventional cine MRI, each
of the 12 k-spaces is filled with only one phase-encoding step of the necessary data
during a single heartbeat.45The total acquisition time is therefore the number of
heartbeats necessary to fill a k-space.
A standard study with 128 phase-encoding steps will take 128 heartbeats to complete,
which does not allow for breath-hold imaging. With segmented k-space cine MRI,
17
multiple phase-encoding steps of data (per frame) are acquired after a single
heartbeat. The number of lines of k-space per frame acquired per heartbeat is referred
to as the views per segment or lines per segment. For a study with 128 phase-
encoding steps, 8 views per segment would reduce the imaging time from 128
heartbeats to 16 heartbeats. This allows for breath-hold cardiac cine imaging.45
Relationship between temporal resolution, spatial resolution, and
imaging time
It is important to understand the relationship between temporal resolution, spatial
resolution, and imaging time in cine cardiac MRI.The temporal resolution is directly
related to the views per segment:
Temporal resolution = repetition time(TR) × views per segment
In this case, TR is used in the standard sense to refer to the time between consecutive
RF pulses. Lee refers to this as “true TR,” because TR is also used to refer to the
temporal resolution in cine MRI.45There is a direct trade-off between imaging time
(views per segment) and temporal resolution. Decreasing the imaging time by
increasing the views per segment will decrease the temporal resolution. For example,
if the number of views per segment is doubled, the overall imaging time will decrease
by half, because twice as much data are acquired during every heartbeat. However,
acquiring twice as much data per heartbeat takes twice as long per frame, which will
halve the number of attainable frames per cardiac cycle and worsen the temporal
resolution by a factor of 2. 45
Another way to decrease the imaging time is to decrease the resolution by decreasing
the number of phase-encoding steps. An in-plane spatial resolution of 2-2.5 mm is
adequate for most cardiac function studies, although higher spatial resolution can be
helpful for evaluating structures such as cardiac valves.Heart rate can be helpful in
determining the number of views per segment. With slow heart rates, more views per
segment can be used. Because the R-R interval is longer, more views per segment can
be added while maintaining an adequate number of frames, but temporal resolution
will still be decreased. This will decrease the number of heartbeats necessary to
complete the study, which is especially helpful if the heart rate is slow.45
18
Perfusion
Magnetization-prepared gradient echo sequences are used to assess myocardial
perfusion.The magnetization preparation prepulse can be a saturation or inversion
recovery pulse and is used to improve T1-weighted contrast. Different trade names for
these sequences are TurboFLASH (Fast Imaging using Low Angle Shot; Siemens),
Fast SPGR (Spoiled Grass [Gradient Recall Acquisition using Steady States]; GE),
and TFE (Turbo Field Echo; Phillips). Echoplanar sequences can also be used. 46
Viability/infarction
Contrast-enhanced MR evaluation of myocardial viability utilizes inversion recovery
gradient echo sequences, with the inversion time set to null viable myocardium. Either
spoiled gradient echo or SSFP sequences can be used in conjunction with the
inversion recovery prepulse. These sequences typically utilize segmented k-space
acquisition. 46
Contrast enhanced magnetic resonance angiography
With progress in MR imager development & the ability to acquire 3D MR images
with in a single breath hold, contrast enhanced MR angiography has become the
method of choice for visualization of the great vessels of chest & abdomen. Contrast-
enhanced magnetic resonance angiography (ceMRA) has proven to be highly
accurate, especially when compared with noncontrast techniques . ceMRA is a robust,
reproducible technique that can be performed in seconds rather than minutes with few
flow-related artifacts, unlike the noncontrast techniques.47 Gadolinium chelate agents
are typically used for ceMRA because they are paramagnetic. This means that they
cause shortening of the T1 relaxation of blood compared with background tissue
leading to the high signal intensity of blood on T1-weighted sequences. Unlike, time-
of-flight (TOF) or phase contrast (PC) imaging, the signal of the blood in ceMRA is
based on the intrinsic T1 signal of blood and rather less on flow effects; therefore, this
technique is less flow sensitive.
To obtain high-quality images, it is important to have specialized coils overlying the
patients to ensure a high signal to noise ratio.48 Initially, a noncontrast data set is
acquired to act as a mask and eliminate background signal. Imaging of the chest
should be performed in a breathhold so that respiratory motion artifact is limited.
19
Postcontrast images are obtained, and the unenhanced data set is subtracted from the
contrast-enhanced data set. Images can be further postprocessed with maximum and
minimum intensity projections and volume rendering to generate more visually
appealing images .
Several important issues must be taken into consideration for image optimization,
including the timing, amount and rate of the injection of contrast agent. The goal is to
record the central region of k-space during the maximum enhancement of the artery.49
The center of k-space contains the lowest (spatial) frequency wave data, so it
represents the major structures of the image and thus most of the gross image form
and contrast; therefore, the center of k-space should be acquired during the time of
highest contrast agent concentration. Also, a high rate of change of the contrast agent
concentration during the acquisition of central k-space must be avoided to prevent
ringing artifacts, arising in the Fourier transform. When agent administration and
imaging are timed properly, ringing artifacts can be reduced or even eliminated.50 This
timing can be coordinated using several methods. Special considerations with respect
to timing must be undertaken with certain vascular problems such as aneurysms: since
the flow can be much slower through an aneurysm, more time must be allowed
between the injection of contrast agent and the image acquisition. 49
The amount and rate of contrast agent injection have been an extensively studied
topic. Gadolinium chelate agents do have minor side effects and can even cause
severe anaphylaxis and renal dysfunction. Although they are generally considered
safe in patients even with abnormal renal function, the dose and potential
complications, such as nephrogenic systemic fibrosis, need to be considered. There
are US Food and Drug Administration (FDA) limits as to how much gadolinium can
be injected; thus, the injection duration must be short, and this, in turn, requires care
to ensure proper timing of central k-space acquisition . 47One of the largest problems in
using ceMRA is venous enhancement, especially in peripheral ceMRA. Timing may
be determined by a test bolus and timing formula, fluoroscopic triggering, or time-
resolved imaging. A variety of techniques are employed to limit venous
contamination, including shortening acquisition time, using a moving table, altering
the method of filling k-space, using venous compression devices, and altering the
sequence of imaging.49 Furthermore, a variety of elegant postprocessing approaches
have been proposed for resolving ambiguity between arteries and veins in
20
MRA.Imaging parameters & partition dimensions should be carefully adjusted to
achieve the smallest possible voxel size while allowing sufficient spatial coverage of
the target vessels with in a single breathhold acquisition. 51
Imaging time can be decreased by using partial fourier imaging, fewer partitions,
fewer phase encoding steps or a rectangular field of view.
Post processing and volume rendering.
The acquisition of 3D data in contiguous slabs allow the reformating of images in an
oblique orientation. From the 3D data set from MR angiography, 2D images can be
obtained in any oblique plane across the volume of data .
With the high target – to – background contrast on MR angiographic images it is often
desirable to vary the section thickness to obtain 2D reformatted images from 3D data
sets that contain the required anatomic structure. By increasing the thickness of the
reconstructed orthogonal or oblique plane, we can obtain a better view of multiple
vascular branches & their course.
The simplest and most widely used technique for visualization of 3D MR
angiographic data is the maximum intensity projection technique.
This technique is based on single algorithm of projecting all the data on to one plane
by selecting the highest intensity data element (voxel) in the data set along the
projection lines. The resulting image is similar in appearance to images obtained
from traditional X ray angiography. 51
Maximum intensity projection does not differentiate the front from the back. this
overlap between adjacent structures makes it difficult to visually appreciate the exact
spatial location of a given structure.
An alternative rendering technique called surface rendering introduces a degree of
opacity and thus allows better perception of the anatomic structures proximal to the
viewing point and obscuring of structures that are located behind them.51
The surface rendering techniques requires a selection of a surface, or threshold,
between the object of interest & the surrounding structure. More advanced rendering
techniques rely on sophisticated combinations of transparency & opacity of different
anatomic structures. Volume rendering techniques use variety of sophisticated
algorithms that assign different degrees of opacity and even different colours and
textures to objects in the volume of interest.
21
MR imaging appearance
Pulmonary Artery Anomalies
A vast spectrum of pulmonary artery abnormalities are seen in patients with tetralogy
of Fallot. In mild cases, there is a VSD and mild pulmonary valve stenosis, known as
“pink” tetralgoy which may be asymptomatic.
The opposite end of the spectrum consists of complete pulmonary artery atresia with
absence of the main pulmonary arteries, also known as pseudo truncus arteriosus. In
patients with this condition, systemic- pulmonary collateral vessels and a right – to-
left shunt are essential for survival.52
Anomalies in the size and morphologic configuration of pulmonary arteries and the
presence and location of aortopulmonary collateral vessels, as well as pulmonary
artery pruning can be known by MRI.
Pulmonary artery hypoplasia is associated with unilateral or segmental hypoplasia of
the lungs and both abnormalities are well depicted with MR imaging.52,53
The size and morphologic structure of the lungs may be assessed directly from the
MR image, which complement MR angiograms . MRI is helpful in the diagnosis of
post operative complications in follow up after corrective or palliative surgery. In
patients with severe pulmonary stenosis, aneurysmal dilatation of the pulmonary
arteries may result from a post obstructive jet lesions. These conditions are well
depicted with MRI.
22
Figure 1 Anterior 3D volume-rendered MR image in the coronal plane shows pulmonary
atresia and nonconfluence of the left pulmonary artery(LPA) with a blind right
ventricular outflow tract.
Figure 2. Left lateral and posterior coronal 3D volume-rendered MR images show
aneurysmal dilatation of the left (LPA) and right (RPA) pulmonary arteries in an adult
patient with uncorrected tetralogy of Fallot and severe pulmonary valve stenosis.
23
Figure 3. Posterior 3D volume-rendered MR image in the coronal
plane shows an enlarged aortopulmonary collateral vessel (AP col.)
supplying distal branches of the right pulmonary artery (RPA) in a
patient with tetralogy of Fallot. Ao - aorta, LPA- left pulmonary
artery.
24
Studies comparing MRI with echocardiography and cardiac
catheterization angiography
A distinct advantage of cardiac MRI over echocardiography stems from its ability to
depict distal pulmonary branches and delineate systemic – pulmonary collateral
vessels, which are most visible on 3D volume – rendered images and reformatted
images in oblique planes along the course of each vessel.52,53
Echocardiography is limited by its poor ability to dipict distal pulmonary artery
segments. This limitation of echocardiography is due to lack of an acoustic window
that would allow the transmission of ultrasound waves through the air filled lungs .
A study done by Beekman et.al concluded that MR imaging was superior to
echocardiography for the evaluation of RV hypertrophy and overriding aorta.52
Greenberg etal. have done a study comparing echocardiography and MR imaging in
the evaluation of pulmonary abnormalities post operatively in children with tetralogy
of Fallot. In this study MR imaging has greater sensitivity than echocardiography.
Echocardiography was inadequate for depiction of the right and left pulmonary
arteries in 8 of 20 and 10 of 20 children, respectively.54
Inadequate depiction and lack of recognition of stenosis, aneurysm, non confluence,
and patency of hypoplastic pulmonary arteries are difficulties encountered with echo
cardiography.
Geva etal compared MR imaging with conventional angiography in the evaluation of
pulmonary arteries and collateral aorto pulmonary vessels in 32 patients and found
complete agreement between the features depicted on conventional angiograms and
those depicted on MR angiograms with regard to diagnosis of hypoplasia or stenosis
of a pulmonary artery branch.55
Geva etal study was conducted in thirtytwo patients with pulmonary stenosis or
atresia. The study patients had Tetralogy of Fallot with pulmonary atresia (n = 13),
TOF with pulmonary stenosis (n=4), post fontan palliation (n=5) and other complex
congenital heart diseases (n = 10).51 Gadolinium enhanced 3D MRA and cardiac
catheterization were done in all patients and compared. MRA had a 100 % sensitivity
and specificity for the diagnois of main (n = 10) and branch pulmonary stenosis of
hypoplasia ( n = 38) as well as absent ( n = 5) or discontinuous (n = 4) branch
pulmonary arteries.
All 48 major aorto pulmonary collaterals diagnosed by catheterization were correctly
25
diagnosed by MRA. Three additional APCs were diagnosed by MRA but not by
catheterization. The mean difference between MRA and catheterization measurements
of 33 pulmonary vessel diameters was 0.5 ± 1.5 mm with a mean inter observer
difference of 0.4±1.5 mm.
Choe et al . evaluated whether MR imaging could depict pulmonary arterial anatomy
in greater detail than routine angiography in patients with congenital or acquired
occlusion of the left pulmonary artery or with pulmonary atresia.56
Patients in whom the pulmonary artery anatomy could not be completely identified at
angiography were selected for this study. In the study group, angiography with an
injection via the right ventricle or main pulmonary artery or aortography could not be
used to assess the pulmonary artery segments, as often happens in the presence of
severe pulmonary artery stenosis.
In seven patients, the main pulmonary artery was not seen, in two patients with a
preexistent left sided Blalock – Taussig (B-T shunt), the hilar portion of the left
pulmonary artery was not visualized and in one patient with a prior modified right
sided B-T shunt, the entire pulmonary arterial tree was not depicted at angiography.
In addition, the distal segments of pulmonary arteries in nine patients with unilateral
pulmonary artery occlusion or discontinuity could not be identified at X ray
angiography. Evaluation with MR imaging was accurate in 67% ( n = 10) of patients
in whom pulmonary artery obstruction was proved at surgery.
26
Aims of the study
1. To compare 3D MRA and X-ray angiography measurements of pulmonary arteries
in Tetralogy of fallot physiology.
2. To determine whether gadolinium – enhanced 3D magnetic resonance angiography
( MRA) can provide a noninvasive alternative to diagnostic catheterization in
evaluation of pulmonary artery anatomy in tetralogy of fallot.
Material and Methods
Thirtyfive consecutive patients with cyanotic congenital heart disease with tetralogy
of Fallot (VSD + PS) physiology attending cardiology OPD during 2008 january to
2009 dec ember period were included in the study.
Inclusion criteria : All patients of cyanotic congenital heart disease with tetralogy
of Fallot physiology(VSD+PS) were included
Exclusion criteria: Patients other than tetralogy or pentology of Fallot or not
having VSD + PS physiology were excluded. Critically ill patient’s were excluded.
All patients underwent both cardiac catheterization with X-Ray angiography and 3D
MR angiography within one month interval.
.
27
MRI PROTOCOL
3D MR angiography was done with commercially available 1.5 T scanner (Magnetom
symphony, maestro class. Seimens.). MRI studies were done with a torso or cardiac
phased array radiofquency coils.
MRA sequence was taken in a coronal and sagital view with the help of axial
localizing image and centred at the level of the mid thorax.
The MRA sequence consisting of a non ECG- triggered 3D spoiled gradient echo
pulse sequence. Contrast used is meglumine gadoterate (0.5 mmol/ml).
General anesthesia was used in children who were not cooperative. Patients were
instructed to take several deep breaths before image acquisition.
Two sequential breath hold 3D MRA acquisitions were performed 10-15 seconds
apart. In patients under anesthesia, ventilation was suspended during imaging.
Images taken before contrast will be HASTE and TRUE FISP sequences. In HASTE
sequences, blood appears black. In TRUEFISP sequences, blood appears white. Post
contrast images are flash 3D sequences.
MRA image analysis
MRA images were reviewed by using a combination of sub volume maximal intensity
projections (MIPs), multi planar reformatting and 3D shaded surface displays. The
following anatomic variables were recorded in each examination.
The presence of main pulmonary artery atresia, hypoplasia or stenosis, continuity of
pulmonary arteries, presence of branch pulmonary atresia, hypoplasia or stenosis,
identification of aorto pulmonary collaterals and their course. Pulmonary atresia was
defined as luminal discontinuity, hypoplasia was defined as long segment narrowing
and stenosis was defined as discrete narrowing.
The criteria for classification of pulmonary artery morphology.
Normal – lumen of the pulmonary artery at the position described is 40-50% of the
aortic lumen,
Small – pulmonary artery lumen is 20-40 % of the aortic lumen,
Hypoplastic – pulmonary artery lumen is less than 20% of the aortic lumen
Narrow – focal decrease in the caliber of the pulmonary artery lumen.
Atretic – no patent pulmonary artery lumen.
28
Cardiac catheterization
Both right heart & left heart catheterization were done. Contrast angiography was
used for right ventricle (RV) angiography, pulmonary artery angiography(In feasible
cases), left ventricle(LV) angiography, aortic root angiography and descending
aortography . Pulmonary valve, main pulmonary artery, right and left pulmonary
artery branches,lobar pulmonary arteries , and aorto pulmonary collaterals were
studied.
Statistical Analysis
The MRA and cardiac catheterization findings regarding the anatomic variables
detailed above were recorded on a spread sheet ( Microsoft excel, version 5.0
Microsoft) and analyzed for discrepencies. When discrepencies were noted, a
consensus was arrived assuming catheterization findings as a reference standard.
The agreement between MRA and catheterization measurement were analysed by
calculating the mean difference (bias) and the standard deviation of the difference as
described by Bland and Altman. 57 All statistical analyses were done with SPSS
software.
29
Study case sheet proforma
Name : Date :
Age : Hospital No :
Sex :
Clinical examination and diagnosis :
Chest Xray
ECG
Echocardiography :
Cardiac catheterization with X ray Angiography data
RV Angiography
Pulmonary Artery Angiography
1) Main pulmon artery
2) Right pulmonary Artery
3) Left pulmonary Artery
4) Confluence
5) Lobar pulmonary Arteries
Aortic root angiogram
Descending aortic angiogram
3D MR angiogram
Main pulmonary artery
Right pulmonary artery
Left pulmonary artery
Confluence
30
Lobar pulmonary arteries
Aorto pulmonary collaterals
Ascending aorta
Brachiocephilac
Right subclavian
Right internal mammary
Left subclavian
Left internal mammary
Descending thoracic aorta
Abdominal aorta
31
RESULTS
In our study total number of patients studied were thirtyfive. Among them
twentyone were males, fourteen were females. Age of the studied patients ranged
from three years to twentyone years, with mean age of 9 ± 4.15 years. Among the
thirtyfive patients, thirtytwo patients had tetralogy of fallot with varying
severities of valvular and infundibular stenosis. Three patients had tetralogy of
fallot with pulmonary atresia. There was total agreement between the two
modalities (MRA and catheterization) of investigations in the diagnosis of
confluent and hypoplastic or normal sized pulmonary arteries in TOF cases.
Measurements of branch pulmonary arteries by both methods were analysed.
Diameter of RPA ranged from 6mm to 19mm by MRA and 7 to 24mm by cine,
diameter of LPA ranged from 7 to 20 mm by MRA and 6 to 22mm by cine.
The number of APC’s were 18, all of which were imaged by MRA and
catheterization equally .
There was good correlation between MRA and catheterization cine measurements
of branch pulmonary arteries . Pearson’s correlation coefficient values for right
pulmonary artery (RPA) r =0.8828(P value < 0.0001) and for left pulmonary
artery(LPA) r=0.9046( Pvalue < 0.0001) were in the range of good correlation.
Pearson’s correlation coefficient values for MC goon’s ratio and Nakata index
r=0.6064(P value =0.0002) , r = 0.8688( P value= < 0.0001) respectively were in
the range of good correlation.
Bland Altman analysis of branch pulmonary artery measurements by both
methods(MRA vs X -ray cine angio) showed good agreement beween MRA and
X-ray cine angio with mean bias values for RPA measurements and LPA
measurements were − 0.2813(95% of limits of agreement −3.4 to +2.9) and
0.09375 ( 95% of limits of agreement −2.649 to2.837) respectively.
Bland Altman analysis of MCgoon’s ratio and Nakata index showed good
agreement between MRA and X-ray cine angiography . Mean bias value for
MCgoon’s ratio was 0.02813(95% of limits of agreement −0.5961 to 0.6524) and
mean bias value for Nakata index was −5.125(95% limits of agreement − 106.7 to
32
+96.47) .
Table :1
Table RPA(mm) LPA(mm) Desc.Aorta(mm) APC’S(number) Patient Cine MRA Cine MRA Cine MRA Cine MRA
1 11 11 11 10 10 11 0 0
2 13 13 13 14 10 11 0 0
3 11 10 12 10 16 12 0 0
4 8 10 8 11 9 9 1 1
5 24 19 22 18 15 18 0 0
6 11 9 12 11 12 9 0 0
7 9 11 11 11 12 10 0 0
8 10 10 11 10 10 11 0 0
9 10 9 9 9 11 12 0 0
10 12 12 11 11 10 11 0 0
11 12 10 11 10 14 13 0 0
12 11 11 11 11 12 12 0 0
13 14 13 13 13 14 16 0 0
14 9 6 7 7 12 12 2 2
15 10 11 11 12 12 10 1 1
16 9 9 10 9 12 11 0 0
17 10 9 10 10 11 10 0 0
18 7 9 8 7 10 9 1 1
19 13 14 16 18 18 20 0 0
20 15 16 18 20 18 18 0 0
21 8 9 9 11 13 14 1 1
22 12 10 14 13 13 12 0 0
23 11 10 10 9 14 13 0 0
24 12 10 10 10 12 12 0 0
25 16 15 14 15 17 16 0 0
26 9 10 9 11 12 13 0 0
27 14 13 13 13 16 15 0 0
28 8 9 8 9 12 11 1 1
29 11 10 9 10 14 13 0 0
30 6 8 7 8 11 10 2 2
31 12 12 11 10 14 13 0 0
32 8 9 6 7 10 9 1 1
33 - - - - 13 13 3 3
33
34 - - - - 19 20 1 1
35 - - - - 17 16 4 4
Note: RPA- hilar right pulmonary artery,LPA- hilar left pulmonary artery,
APC’s – aorto pulmonary collaterals, desc.aorta- descending thoracic aortaTable:2
Mc goon’s ratio Nakata index Patient S. No Cine MRA Cine MRA
1 2.2 1.9 146 134
2 2.6 2.4 332 358
3 1.4 1.6 173 118
4 1.8 2.3 132 229
5 3.0 2.0 460 363
6 1.9 2.2 220 172
7 1.6 2.2 236 258
8 2.1 1.8 162 156
9 1.7 1.5 354 317
10 2.3 2.0 346 346
11 1.6 1.5 461 349
12 1.8 1.8 422 422
13 1.9 1.6 318 293
14 1.3 1.0 115 75
15 1.8 2.3 182 219
16 1.6 1.6 179 161
17 1.8 1.9 216 196
18 1.5 1.7 164 189
19 1.6 1.6 296 362
20 1.8 2.0 313 374
21 1.3 1.4 80 111
22 2.0 1.9 254 126
23 1.5 1.4 216 177
24 1.8 1.6 239 196
25 1.7 1.8 322 321
26 1.5 1.6 195 266
27 1.6 1.7 238 221
28 1.3 1.6 133 169
29 1.4 1.5 158 157
30 1.1 1.6 121 182
31 1.6 1.7 218 201
32 1.4 1.7 62 81
34
Pearson’s correlation between MRA and Cine measurements of
hilar right pulmonary artery (RPA).
Pearson’s correlation coefficient (r ) = 0.8828
95% confidence interval is 0.7716 to 0.9416
P value (two tailed) = < 0.0001
35
Pearson’s correlation between MRA and Cine measurements of
hilar left pulmonary artery (LPA).
Pearson’s correlation coefficient ( r) = 0.9046
95% confidence interval is 0.8120 to 0.9528
P value (two tailed)= < 0.0001
36
Pearson’s correlation between MRA and Cine measurements of
Mc Goon’s ratio.
Pearson’s correlation coefficient (r ) = 0.6064
95% confidence interval is 0.3267 to 0.7884
P value (two tailed) = 0.0002
37
Pearson’s correlation between MRA and Cine measurements of
Nakata index.
Pearson’s correlation coefficient ( r ) = 0.8688
95% confidence interval is 0.7462 to 0.9345
P value ( two tailed) = < 0.0001
38
Bland altman plot for hilar right pulmonary Artery (RPA) measurements(MRAvsCine):difference(MRA−Cine)Vs average(MRA+Cine)/2.
Mean bias = − 0.2813standard deviation of bias = 1.59195% of limits of agreement(mean bias± 2 SD) = − 3.4 to +2.9
39
Bland Altman plot for hilar left pulmonary artery(LPA)
measurements ( MRA Vs Cine ): difference(MRA – Cine) Vs
average (MRA+Cine)/2.
Mean bias = 0.09375
Standard deviation of bias = 1.4
95 % of limits of agreement is (mean bias ±2D) – 2.649 to 2.837
40
Bland altman plot for Mc Goon’s ratio measurements (MRA vs
Cine): difference( MRA- Cine) Vs average(MRA+Cine)/2
Mean bias = 0.02813
Standard deviation of bias = 0.3185
95% of limits of agreement(mean bias ± 2D) is – 0.5961 to 0.6524
41
Bland-Altman plot for Nakata index
measurements :difference(MRA - Cine) Vs average(MRA+Cine)/2
Mean bias = − 5.125
Standard deviation of bias = 51.84
95% of limits of agreement( mean ± 2SD) is – 106.7 to + 96.47
42
Discussion
The main objective of our study was to see whether Gadolinium enhanced 3D MRA
can give accurate information about pulmonary artery anatomy, i.e the size of MPA,
RPA, LPA, confluence of branch pulmonary arteries and aortopulmonary collaterals
in patients with Tetralogy of fallot with pulmonary stenosis or pulmonary atresia.
In a study done by Tal Geva et.al comparing 3D MRA with X- ray cine angiography,
there was excellent agreement between MRA and X-ray cine angiography in
pulmonary artery branch measurements and anatomy. In that study, X-ray cine
angiography missed 3 APCS among a total of 51 APCS but MRA diagnosed all 51
APCS.55
In a study done by MA Fogel etal , thirtysix patients with functional single ventricle
were studied to compare the efficacy of non-invasive measures in determining
pulmonary artery size. They have analysed T1 weighted spin echo magnetic
resonance and echocardiographic images throughout stages of fontan reconstruction,
and compared them with angiography images at cardiac catheterization. Magnetic
resonance imaging had high degree of agreement with angiography, with Mc goon’s
index agreeing better than the Nakata index and absolute right and left pulmonary
artery diameters. MRI was superior to echo in determining pulmonary artery size and
in determining branch pulmonary artery discontinuity and stenosis.58
Andrew j. powell et.al have studied thirteen patients with TOF with pulmonary
stenosis, TOF with pulmonary atresia and single left ventricle with pulmonary
stenosis to compare MRI and cardiac catheterization. MRI sequences used in the
study were ECG gated Spin Echo and gradient echo sequences. Compared to
catheterization, MRI had 100% sensitivity & specificity for the diagnosis of main
pulmonary artery size and branch pulmonary artery hypoplasia or stenosis as well as
their confluence and there was complete agreement between catheterization and MRI
in identifying18 APCS.59
Kritvikrom durongpisitkul etal studied fourtythree patients with pulmonary atresia
with VSD to compare MRA with cardiac catheterization 60 . There was an agreement
among measurements of both LPA & RPA of more than 0.8(kappa statistics). All
major MAPCAS that were diagnosed by catheterization were correctly diagnosed by
MRA. 3 additional MAPCAS were diagnosed by MRA but not by catheterization .
In our study total of thirtyfive patients were studied . Age of patients ranged from
43
3years to 21 years. Mean age of patients was 9 ± 4.155 [mean ± SD ] years. Among
them twentyone were male, fourteen were female patients.
Among them thirtytwo patients had Tetralogy of fallot with pulmonary stenosis with
varying degrees of valvular and infundibular stenosis. Three patients had TOF with
pulmonary atresia .
Pulmonary artery measurements by both methods correlated well with significant
values of Pearson’s correlation coefficient for RPA, LPA, McGoon’s ratio, Nakata
index.
There was good agreement between 3D MRA & Catheterization measurements of
pulmonary artery sizes. Highest agreement was for Mc goon’s ratios followed by LPA
measurements , RPA measurements and Nakata index .In our study a total of 18 major
aortopulmonary collaterals were found. All 18 MAPCA’s were identified with equal
efficacy by MRI when compared to catheterization. All the measurements of
pulmonary arteries were in good correlation [3D MRA versus catheterization] as
determined by pearson’s Correlation .
No complications were reported during MRA or cardiac catheterization studies.
Our study results are in conformity with the previous published studies, suggesting
3D MRA as a non invasive alternative to cardiac catheterization.
Limitations of the study.
1. As both pulmonary arteries and veins are enhanced with contrast, one should be
careful to distinguish them. This can be achieved by using sub volume MIP
method.
2. MRA cannot accurately delineate more peripheral branches of pulmonary arteries
beyond 3rd or 4th generation
44
Conclusions
1.Gadolinium enhanced 3D MRA is fast and accurate technique for delineation of
pulmonary arterial anatomy.
2.It can be used as a reliable non-invasive alternative to to x-ray cine angiography.
3. 3D MRA can provide required information to plan surgical strategy in sick
infants and young children obviating the need for catheterization.
4.3D MRA can delineate all sources of pulmonary blood supply in pulmonary
atresia to plan surgical and transcatheter therapies.
45
TOF with severe valvular and infundibular PS,hypoplastic MPA and normal
sized LPA and RPA in a 8 yr male child (3D MRA and x-ray cine images in
the same patient)
3D MRA image
X-ray cine angio image
46
Pulmonary atresia with VSD in 16 yr female. Two large MAPCA’S arising
from descending thoracic aorta . (3D MRA and X-ray cine images in the
same patient)
3D MRA image
X-ray cine angio image(desc. thoracic aortogram)
47
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