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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