AJR 2000; 175:1495-1506
© American Roentgen Ray Society
Role of the Radiologist in Cardiac Diagnostic Imaging
Martin J. Lipton1,
Lawrence M. Boxt2 and
Ziyad M. Hijazi3
1
Department of Radiology, University of Chicago, MC 2026, 5841 S. Maryland
Ave., Chicago, IL 60637.
2
Department of Radiology, Beth Israel Medical Center, First Ave. and 16th St.,
New York, NY 10003.
3
Department of Pediatrics, University of Chicago, MC 4051, 5841 S. Maryland
Ave., Chicago, IL 60637.
Received May 24, 2000;
accepted after revision July 19, 2000.
Honoring Hollis E. Potter, MD and George W. Holmes, MD
This is the 12th in a series of Centennial Dissertations that the
AJR is publishing this year in honor of the former presidents of the
American Roentgen Ray Society, two of whom are pictured above.
Address correspondence to M. J. Lipton.
Introduction
Radiologists have always been associated with new advances in cardiac
imaging and in the application of these new technologies to the clinical
management of patients with heart disease. It is therefore paradoxic that they
now find themselves struggling to maintain their role in this area.
Radiography, echocardiography, angiocardiography, and coronary arteriography
were developed by radiologists either alone or in collaboration with adult or
pediatric cardiologists, physicists, and engineers. These modalities allowed
diagnosis of morphologic changes amenable to traditional surgical or medical
intervention, and, more recently, to percutaneous interventional techniques.
The estrangement of radiologists from their cardiology colleagues occurred for
two reasons: the aggressive pursuit of control of cardiac imaging by the
cardiologists and the consequences of radiologists' walking away from cardiac
imaging itself. Although cardiac imaging was once a primary radiology
subspecialty associated with the imaging of patients with congenital and
acquired heart disease, the number of specialty-trained cardiac radiologists
and radiology departments with cardiac radiology divisions has
dwindled.
,
The role of the radiologist in cardiac diagnosis has changed dramatically
since the 1960s and 1970s. In that era, radiologists were trained in many
centers in the performance and interpretation of coronary arteriography using
Melvin Judkins' new catheter technique
[1]. As designed, the Judkins
catheters lent themselves to adaptation of the Seldinger percutaneous
transfemoral puncture. This avoided brachial arteriotomy, which required
repair after the procedure [2].
Subsequently, cardiologists became competent in performing the Judkins
technique. There were several reasons for this change, but self-referral
played a significant role. An increase in the amount of cardiac surgery drove
an increasing demand for coronary arteriography that radiology departments did
not meet. Cardiologists recognized this demand, and after having initially
been trained by radiologists, began to train other cardiologists to perform
these procedures. Despite the role radiologists played in creating this
technique, advancing the field, and developing new and more accurate imaging
technology, radiologists were excluded from cardiac catheterization
laboratories. Once radiologists were removed from the adult cardiac
catheterization laboratory, the field of radiology withdrew from the
investigation of the heart and the examination of patients with heart
disease.
Remarkable advances have been made during the past three decades in the
understanding, diagnosis, and treatment of heart disease. Traditionally,
treatment of congenital and acquired heart disorders has been based on
identifying morphologic changes or on clinical presentation. Radiography and
cineangiography, the core of radiologic evaluation, were interpreted in terms
of morphologic change and the instant physiologic aberration reflected in
those changes. The rise of other cardiac imaging modalities, including nuclear
medicine imaging and echocardiography, came about as a result of their ability
not only to facilitate morphologic diagnosis, but also to allow quantification
of morphologic changes and the resulting regional and global changes in
cardiac function. Such quantitative information had significant value,
providing the basis for risk stratification and objective assessment of the
effects of medical or surgical treatment.
The development and growth of these quantitative techniques also marked a
divide in the roles of radiologists and cardiologists in the treatment of
patients with cardiac disease. The traditional interest of radiologists in
morphologic diagnosis was no longer adequate for active participation in the
treatment of patients with cardiac disease. Cardiologists were drawn to these
newly available cardiac indexes as part of their routine clinical
investigations and demanded their availability. To the extent that
radiologists working in cardiac disease failed to perform those examinations
that could provide needed data, they were replaced by cardiologists who were
willing to do so. Hence, the contemporary approach of the clinical
cardiologist, after taking the patient's history and performing a physical
examination, is to obtain an ECG and institute an imaging algorithm, usually
beginning with a chest radiograph. Depending on the clinical problem,
echocardiography is the next most frequent study, performed for the purpose of
quantifying regional and global cardiac function. Once the bailiwick of
radiologists, these examinations are now performed by cardiologists. Further
diagnostic testing, including left ventriculography, coronary arteriography,
or nuclear medicine studies, which are performed for quantitative analysis, is
now usually performed by cardiologists as well.
Advances in CT and MR imaging from which quantitative analysis of cardiac
function can be determined allow acquisition of imaging data in radiology
departments. The availability of these devices has brought a new excitement to
the field of cardiac imaging, and radiologists are realizing that this has
created new opportunities as well as concerns and challenges. The success of
radiologists in maintaining a role in the treatment of patients with
cardiovascular disease requires an understanding of the clinical problems of
these patients and the ability to perform the appropriate imaging
procedures.
Historically, radiologists have shown considerable interest and expertise
in examining patients with congenital and acquired heart disease. In this
review, we hope to show not only how newer modalities may expand the role of
radiologists in this traditional endeavor, but also how conventional imaging
methods have maintained radiology's involvement. First, we describe advances
in the treatment of patients with congenital cardiac diseases and the role of
cardiac imaging in treating many of these malformations using interventional
procedures. New percutaneous interventional techniques continue to rely on
imaging procedures to provide the basis for determining their indication,
guiding the performance of the intervention, and assessing results. Although
interventional techniques have evolved, the importance of radiologic imaging
and the skill and experience of the radiologist in performing and interpreting
these imaging studies remain crucial.
Next, we describe the resources available in radiology departments for the
examination of patients with heart disease. Unenhanced chest radiography has
generated renewed interest. Universally available, safe, inexpensive, and
reproducible, chest radiography lies at the core of cardiac diagnosis. A
snapshot of the heart, great arteries, and lungs, it is useful for excluding
noncardiac causes of a patient's clinical problem, assessing cardiac chamber
size and mass, and providing a rapid means of assessing the patient's
physiologic status. Acquisition of faster CT images augmented by IV contrast
enhancement and off-line three-dimensional reconstruction provide the means
for visualizing intracardiac abnormalities and the basis for investigation
into quantitative analysis of acquired data. MR imaging of the myocardium and
coronary arteries is an exciting area of great potential for radiologists.
These scanners produce images with high contrast and temporal resolution that
appear to allow differentiation between normally functioning and dysfunctional
myocardium; these scanners are operated in radiology departments.
We next address our understanding of the atherosclerotic process. Although
many radiologists believe that they no longer play a role in the treatment of
patients with heart disease, the ubiquitousness of this disease argues the
opposite view. Our enhanced understanding of atherosclerotic plaque biology
provides new avenues for the diagnosis and treatment of patients with this
disease. In particular, we will discuss atherosclerotic coronary artery plaque
formation and acute rupture, which will provide an understanding of why
symptoms occur and explain the rationale behind the use of newer pharmacologic
agents for therapy. Furthermore, we show the clinical problems that imaging
studies may help to elucidate and describe how these modalities are faring in
addressing these clinical issues.
Finally, this review addresses the means for reinforcing and increasing
radiology's role in cardiac imaging. We emphasize the importance of research
training and funding in addition to clinical training to support and maintain
the viability of the specialty. This refers to the entire radiology community,
not merely the subspecialty of cardiac imaging. Radiologists could learn a
good deal from the cardiology community, which responded rapidly and
effectively in the 1970s in obtaining grant funding from the National
Institutes of Health and the American Heart Association. In that era,
tremendous research funding became available for investigating heart disease;
however, radiology departments failed to recognize the importance of research
training for residents and fellows in this field.
Today few radiology residency training programs have cardiac imaging
sections and even fewer have fellowship-trained faculty to teach this
subspecialty. Cardiac radiology has therefore been allocated the least amount
of supervised time in the residency curriculum. Radiologists have come to
believe that cardiac radiology is not a viable radiology subspecialty and,
consequently, have largely ignored it. Unfortunately, heart disease,
particularly coronary artery occlusive disease, accounts for greater morbidity
and mortality rates in adult patients 35-55 years old in the Western
hemisphere than all other diseases combined, including cancer. Many noncardiac
disorders mimic heart disease, and cardiologists often have limited skills and
knowledge beyond their own specialty. Recognizing this fact, radiology
organizations and leaders in academic radiology departments are now attempting
to develop and expand training programs for radiology residents in this
specialized area of medical imaging. This development is heartening because it
suggests that previous advances and progress in cardiac imaging can be used as
the basis on which the role of radiologists in cardiac imaging can build and
expand to provide faster and more accurate diagnosis for patients with heart
disease.
Congenital Heart Disease
Approximately eight of every 1000 children in the United States are born
with some form of congenital cardiovascular disease. This incidence has
remained stable over the past few decades. Through much of the 1940s and
1950s, cardiac catheterization was used to obtain pressure and oxygen
saturation data for physiologic evaluation. Improvement in contrast media
flow-controlled injectors, rapid radiography film changers, and imaging chain
systems led to the widespread use of angiocardiography for precise definition
of intracardiac and great vessel anatomy. This growth in imaging was
paralleled by the growth in surgical palliation or cure of patients with
congenital heart disease.
Similarly, the growth in the number and range of percutaneous interventions
in children and adults with congenital heart disease is related to accurate
imaging of the lesions. The first therapeutic transcatheter intervention took
place in 1953 when Rubio-Alvarez incised a stenotic pulmonary valve using a
wire [3]. The era of
interventional cardiology was born in 1966 when Rashkind and Miller described
their balloon atrial septostomy
[4]. Increasingly, the
pediatric cardiac catheterization suite is used to perform therapeutic
procedures or to obtain complementary morphologic data not readily available
using noninvasive imaging techniques. Although modalities used by radiologists
for primary morphologic diagnosis of patients with congenital heart disease
are changing, the role of the radiologist in the area of MR imaging of these
patients persists.
Pulmonary valve stenosis accounts for approximately 10% of the cases of
congenital heart disease. Echocardiography is most commonly used to show the
abnormal pulmonary valve and the morphologic sequelae of the lesion. Ballon
pulmonary valvuloplasty [5] is
indicated for any patient with a peak gradient greater than 40 mm Hg. Doppler
examination may give a spuriously elevated valve gradient; the decision to
dilate is often deferred until after direct measurement of the valve gradient
is obtained in the catheterization laboratory. The procedure, performed with
fluoroscopic guidance, is safe and effective and is the treatment of choice.
Similarly, in patients with aortic valve stenosis, diagnosis is based on
echocardiography findings. However, a valvular gradient obtained using the
Doppler echo technique tends to overestimate values obtained at cardiac
catheterization, so diagnostic catheterization is also often performed in
these patients before an intervention. If the peak-to-peak systolic pressure
gradient at catheterization is in excess of 65-70 mm Hg, then intervention is
performed [6].
Peripheral pulmonic stenosis, which is usually associated with Alagille and
Williams syndromes and congenital rubella infection, occurs in 30% of patients
with tetralogy of Fallot and can develop after placement of
systemic-to-pulmonary shunts or after the arterial switch procedure. The
initial diagnosis may be made using echocardiography or may be inferred from
the results of lung perfusion scanning if the obstruction is unilateral.
Catheter pulmonary arteriography and newer MR imaging techniques are used to
diagnose these conditions. Once the obstruction is localized, catheter-based
balloon angioplasty or stent placement is indicated
[7,
8]. In patients with
coarctation of the aorta, MR imaging is used to characterize the narrow aortic
segment and to reveal aortic collateral circulation. Although surgical repair
is the primary therapy in these patients, balloon angioplasty has become
accepted for treatment of children beyond the neonatal period, especially in
cases of surgical failure. Many cardiac centers use stent implantation in
older patients in whom percutaneous intervention is indicated
[9].
In much the same way, diagnostic visualization of superior vena cava
stenosis or other systemic venous obstruction, followed by percutaneous
balloon angioplasty, has become an accepted method of treatment in these
patients.
Atrial septal defects account for approximately 10% of all congenital heart
disease. Transthoracic and transesophageal echocardiography have a high
diagnostic value (Fig.
1A,1B,1C,1D)
for such defects. The secundum type of atrial septal defect is amenable to
catheter closure using umbrella-type devices. Since the first report by King
and Mills [10], many devices
have been evaluated in clinical trials. Perhaps the most widely used device is
the Amplatzer septal occluder (AGA Medical, Golden Valley, MN)
[11], which offers many
advantages, including the ability to be retrieved and repositioned before
release. This technique has a high rate of complete closure. Atrial septal
defect closure is routinely performed in the catheterization laboratory with
echocardiographic and fluoroscopic guidance. Surgical closure of muscular
ventricular septal defects have a high risk of mortality and morbidity.
Therefore, percutaneous catheter closure using a device is welcomed by
surgeons and cardiologists. The procedure is usually performed with
fluoroscopic guidance, although transesophageal echocardiographic guidance has
been reported for anchoring the device across the ventricular septum
[12].
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Fig. 1A. —47-year-old man with 33-mm secundum atrial septal defect.
Four-chamber view of transesophageal echocardiogram without color Doppler
sonography shows atrial septal defect (arrow) before insertion of
device. RA = right atrium, RV = right ventricle, LA = left atrium, LV = left
ventricle.
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Fig. 1B. —47-year-old man with 33-mm secundum atrial septal defect.
Same image as A with color Doppler sonography shows atrial septal
defect (arrow) and left-to-right shunt before device closure.
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Fig. 1D. —47-year-old man with 33-mm secundum atrial septal defect.
Same type of image as A with color Doppler sonography after closure
with 38-mm Amplatzer septal occluder (AGA Medical, Golden Valley, MN) shows
good device position and no residual shunt.
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Percutaneous catheter closure of a persistent ductus arteriosus has become
one of the most commonly performed procedures in the catheterization
laboratory. For a small- to moderatesized ductus, the technique is easy and
effective regardless of the device used. However, few devices can achieve
complete closure in a large ductus. The Amplatzer duct occluder (AGA Medical)
has recently been introduced with excellent clinical results
[13]. The technique is not
difficult, and most patients achieve complete resolution immediately after
closure (Fig.
2A,2B)
or on follow-up examination. In many patients with various forms of complex
congenital heart disease, aortopulmonary collaterals augment pulmonary blood
flow in right heart obstruction. Although these systemic-to-pulmonary artery
collaterals increase pulmonary blood flow, they also increase the risk of
pulmonary vascular occlusive disease and pulmonary hypertension. Surgical
ligation of these collaterals can have some risk. Therefore, percutaneous
catheter closure immediately before or after surgical correction of the
underlying cardiac condition can be achieved with very little morbidity and a
high success rate.
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Fig. 2B. —5-year-old boy with continuous heart murmur. Cinearteriogram
after closure with 8- to 6-mm Amplatzer duct occluder (AGA Medical, Golden
Valley, MN) shows complete immediate closure of ductus (arrow) in
this patient.
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Chest Radiography
Interest has increased in the value of unenhanced chest radiography in
diagnosing congenital and acquired diseases of the heart. The chest radiograph
provides perhaps the most rapid, safe, and cost-effective screening for
patients with heart disease. One of the most valuable contributions of chest
radiography is its ability to facilitate the exclusion of noncardiac disease,
which may cause similar symptoms or may coexist with heart disease.
Information obtained from chest radiographic images is provided in a
convenient format with which all clinicians feel comfortable. Indeed, patients
with known or suspected heart disease undergo routine frontal and lateral
chest radiographs at most hospital and clinic visits to evaluate their
clinical status. Cardiac chamber and great vessel size, shape, and position
provide a wealth of information that facilitates the diagnosis of congenital
and acquired heart disease. Before and after therapy, calcifications in the
pericardium, myocardium, and coronary arteries help to confirm and elucidate
the nature and severity of heart disease (Fig.
3A,3B).
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Fig. 3A. —Chest radiographic examination in 54-year-old man with
history of previous myocardial infarction. Posteroanterior radiograph shows
increased curvature of dilated left ventricle. Although left atrial appendage
segment (arrow) is concave, pulmonary vessels in lower lobes are less
sharp than those in upper lobes, indicating left atrial hypertension. Faint
rimlike calcification of left ventricular aneurysm (arrowheads) may
be seen through ventricular contour.
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Fig. 3B. —Chest radiographic examination in 54-year-old man with
history of previous myocardial infarction. Lateral image shows dilated left
ventricle (LV) and calcification of anteroseptal LV aneurysm
(arrowheads).
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In addition, the chest radiograph is an excellent method for evaluating
pulmonary hemodynamics, including the signs of pulmonary edema and heart
failure, both of which may appear before physical signs can be detected.
Furthermore, increased pulmonary blood flow caused by intra-and extracardiac
shunts, venous anomalies such as arteriovenous fistulas, and evidence of
pulmonary arterial hypertension can be diagnosed by a well-trained,
experienced observer. Radiologists appreciate the significance of an
observation that varies from normal findings. Radiology training in the
physics of chest radiography provides a prism through which physiologic change
can be differentiated from the effects of image acquisition and radiographic
technique. In fact, the most significant limitation of chest radiography as a
diagnostic tool is the experience and expertise of the individual interpreting
the examination.
It seems inevitable that digital chest imaging using direct area detectors
or computed radiography with phosphor plate methods will replace conventional
X-ray analogue radiography and fluoroscopy systems. Temporal or energy
subtraction should, therefore, become routine options in most modern radiology
centers. In addition, computer-aided diagnosis will become commonplace for
identifying disorders and for quantifying lesion shape, extent, and mass
(thereby improving disease staging) and for measuring therapeutic
effectiveness. Quantitative computer-aided diagnostic methods are available
for evaluating heart disease and myocardial function.
CT of the Heart
The invention and introduction of CT in the early 1970s not only changed
the field of radiology but also revolutionized the practice of medicine. This
technology was the first to integrate a medical X-ray machine with a computer;
CT pioneered the era of digital as opposed to analogue diagnostic imaging.
Algorithms developed for CT back-projection solutions subsequently made MR
imaging more feasible and acceptable to radiologists, the public, and
government regulators.
Single-slice CT was refined during the past two decades to provide faster
acquisition and reconstruction times and to improve temporal and spatial
resolution. The remarkably broad gray-scale density range of CT opened new
vistas in diagnostic medicine. Blood pool contrast enhancement using
IV-injected contrast media made the first CT images of the cardiac chambers
possible and allowed differentiation of normal from infarcted myocardium
[14]. CT can reveal left
ventricular aneurysms (Fig. 4)
and patency of coronary artery bypass grafts
[15,
16]. CT also became a reliable
technique for evaluating chronic aortic dissection
[17]. Feasibility studies in
patients with pulmonary embolism and myocardial infarction were conducted
[18,19,20].
Pericardial disease can also be well shown using CT
(Fig. 5). Prototype scanners
explored ECG gating and showed the potential of CT for cardiac diagnosis
[21,
22]. Millisecond electron beam
CT extended these applications by measuring coronary artery calcification for
risk stratification of disease. The feasibility and validation of myocardial
and renal blood flow measurements were also performed
[23,24,25].
However, studies in the United States, Europe, and Asia in patients with
coronary artery lesions using electron beam CT indicate that stenoses can be
seen and evaluated before and after angioplasty and surgery
[26]. Contrast-enhanced CT
reveals calcifications in the coronary arteries and arterial lumina
(Fig. 6). The limited number
of electron beam CT scanners and the lengthy period necessary for clinical
trials have delayed the large-scale validation studies necessary for
widespread acceptance of this technique.
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Fig. 4. —62-year-old man with history of recent acute myocardial
infarction. Contrast-enhanced CT scan shows apical left ventricular (LV)
aneurysm. Note calcification (arrow) on anterior LV wall, behind
which lies unenhanced thrombus, sharply cutting off LV cavity
(arrowheads). Thinned anteroseptal myocardium contrasts with
compensated and hypertrophied posterior LV wall.
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Fig. 5. —59-year-old man with shortness of breath. Contrast-enhanced
CT scan shows calcified and thickened pericardium (arrows) along
right and left heart borders. Right ventricle (RV) is compressed by thickened
pericardium and is tubular in appearance. Right atrium is enlarged and
atrioventricular groove is prominent, both hallmarks of pericardial
constriction.
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Fig. 6. —Contrast-enhanced helical CT scan of 63-year-old man with
chronic aortic regurgitation. Dilated aortic root (Ao) and normal left atrium
(LA) are opacified. In addition, contrast material increases visualization of
left main artery (arrowhead) and calcified anterior descending
(short arrow) and circumflex (long arrow) coronary
arteries.
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The advent of helical and, more recently, multidetector CT has changed
several traditional clinical approaches to cardiovascular diagnosis
[27]. However, CT angiography
still has enormous potential in the new millennium. The widespread
availability of rapid image acquisition has provided the basis for
investigating direct imaging in real time of cardiac structures using CT.
Application of thin-section image acquisitions to computerized
three-dimensional reconstruction algorithms produces three-dimensional data
sets from which images of the heart in any anatomic plane may be obtained.
This enables radiologists with CT scanners to evaluate morphologic changes
previously revealed only on echocardiography and, to some degree, on MR
imaging. Naturally, cardiologists are interested in these new applications
[28]; however, cardiologists
tend to embrace new techniques only after careful validation. For example,
there was a long period before angiocardiography, echocardiography, and
nuclear medicine techniques were sufficiently refined, proven, and accepted by
cardiologists for routine application. Nevertheless, the introduction and
growth in availability and use of fast CT scanners should help stimulate
radiologists to refocus their attention on heart disease.
Cardiac MR Imaging
Since MR imaging equipment became commercially available in the 1980s, an
enormous number of radiologists and physicists have become involved in the
assessment of this technique for examining patients with cardiac disease. The
high sensitivity of MR imaging in depicting soft-tissue morphologic
abnormalities and showing blood flow is the basis for MR imaging applications
in examining patients with congenital heart disease. In face of the prevalence
of atherosclerotic heart disease in our society, major effort has been focused
on methods for direct visualization of the coronary arteries, quantification
of regional myocardial perfusion, and evaluation of myocardial viability
(Fig. 7). Driven by basic
research in contrast media and newer, more rapid MR acquisition techniques, an
increasing spectrum of new paramagnetic contrast-enhancing agents has become
available for clinical use (Fig.
8). Materials for contrast enhancement of the vascular tree and
solid organs, including the ventricular myocardium, have been developed and
tested in patients with acute and chronic ischemic heart disease and other
forms of atherosclerotic cardiovascular disease.
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Fig. 7. —Short-axis spin-echo MR image of 63-year-old man with chronic
stable angina pectoris shows increased signal in papillary muscle (long
arrow) and posterior left ventricular myocardium (short arrows)
ischemia in circumflex coronary artery territory.
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Fig. 8. —Contrast-enhanced spin-echo MR image obtained during acute
myocardial infarction in 54-year-old man. Apical left ventricular myocardial
infarct (arrow) is seen to enhance. (Courtesy of Duerinckx A, Dallas,
TX)
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Early studies reported that edema, typically present in acute myocardial
injury, appeared as a locus of increased signal intensity on ECG-gated
spin-echo images [29,
30]. It was further shown that
IV administration of gadopentetate dimeglumine and other chelates
[31] would enhance the
difference between infarcted and normal myocardium. Fat-saturated,
contrast-enhanced, or unenhanced breath-hold gradient-reversal acquisition
produces remarkable but limited images of the coronary tree. MR coronary
arteriography is sensitive to motion artifacts resulting from patient
breathing. Gating image acquisition to diaphragm position (representing phase
of inspiration), termed the "navigator technique," improves image
quality and sharpness [32].
This technique has been helpful in imaging the coronary arteries when applied
to contrast-enhanced K-space—segmented two- and three-dimensional
gradient-reversal acquisition because it increases the signal-to-noise ratio
[33]
(Fig. 9). Studies of the
sensitivity and specificity of MR coronary arteriography reveal its
limitations for clinical application
[34]. However, MR imaging is
an evolving technology. Intense investigation in areas of acquisition pulse
sequences, high-gradient-strength imaging devices, and contrast enhancement
techniques continues. Contrast enhancement may provide methods for
differentiating infarcted from adjacent ischemic but viable, noninfarcted
myocardium at risk. Normal myocardium exhibits signal enhancement on the first
pass as a result of T1 shortening effects of the contrast agent, and reduced
signal has been shown to correlate with regions of reduced perfusion
[35]. Vasodilator (dobutamine
or adenosine) stress cardiac MR imaging has the potential to evaluate
subclinical disease and myocardial viability. Identification of ischemic as
opposed to infarcted myocardium is crucial for making decisions concerning
surgical or percutaneous interventional revascularization. Cine MR or
gradient-echo imaging techniques as well as myocardial tagging
(Fig. 10) continue to be
studied.
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Fig. 9. —Contrast-enhanced K-space-segmented gradient-reversal MR
image reconstructed in coronal section in 54-year-old man shows origin of left
main coronary artery (arrow) from posterior left sinus of Valsalva
(L).
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Fig. 10. —Systolic short-axis tagged MR image of 42-year-old man with
hypertrophic cardiomyopathy. Note asymmetric posterior left ventricular
thickening (arrow). Furthermore, changes in tagging intersections are
less pronounced in this region of myocardial disarray.
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Manufacturers of radiographic equipment recognize the potential of MR
imaging for cardiac applications. Future advances in clinical applications of
32P MR spectroscopy and the development of dedicated cardiac
imaging systems with higher gradient strength and faster gradient switching
will continue to be seen. Radiologists must have the ability to image the
peripheral vascular tree, heart, and coronary arteries and to understand the
relevance of abnormal findings so that they are prepared to provide the
technical expertise in what may become one of the primary technologies for
examining patients with cardiovascular disease
[36].
Pathophysiology of Atherosclerosis
Atherosclerosis is the major cause of coronary artery disease, stroke,
aneurysms, and peripheral vascular disease. Atherosclerosis has been
extensively investigated [37]
because these disorders represent frequent causes of morbidity and mortality.
Although several risk factors have been recognized, the mechanisms that cause
atherosclerotic lesions remain largely unknown. It is now known that an intact
vascular endothelium (intima) is essential for normal vascular physiology. In
health, intact endothelium does not interact with platelets; therefore,
thrombus formation does not occur in normal arteries. Damage to the vascular
endothelium, which is only one cell thick, triggers platelet receptors. In
response, these receptors typically produce homeostasis with the formation of
a platelet plug, which is composed of fibrin and fibrinogen. Endothelial
damage is associated with two types of processes. The first mechanism,
biochemical in nature, is associated with the local effects of inflammatory
cytokinins, growth factors, and circulating hormones. The second mechanism,
associated with local hemodynamic alterations, is more familiar to the
radiologist. The latter mechanism includes alterations in pulsatile blood
flow, which modulates arterial wall tone, as well as abnormalities in stress
and shear forces that are directly borne by the vascular endothelium.
Atherosclerosis has been recognized for more than a century, but its
relevant pathophysiology has only recently been identified by means of modern
cellular and molecular biology techniques
[38,39,40,41,42,43,44].
Endothelial damage has been attributed to bacterial infection, notably
Chlamydia pneumoniae, which is common in the general population and
is responsible for 10% of community-acquired pneumonia. Infection is typically
mild or asymptomatic [45].
Serologic testing has shown a relationship with heart disease, but various
studies show conflicting data, and the discussion continues. Nevertheless, the
organism has been cultured from atherosclerotic arteries
[46,
47].
Plaque Formation and Progression
Most acute thrombotic episodes occur in atherosclerotic arteries
[48]. Normally, circulating
lipoproteins and cholesterol esters pass through the arterial intima into the
arterial media. Arterial injury results in the uneven focal deposition of
these materials, leading to the formation of plaque. Modification of plasma
low-density lipoprotein, which enters the intima, produces an allergic
reaction that results in monocytic infiltration. Monocytes ingest the
cholesterol esters and low-density lipoprotein and become lipid-filled foam
cells. This early stage of atherosclerosis is manifested by a series of yellow
dots or streaks along the luminal surface of blood vessels visible to the
naked eye. Until this point no endothelial denudation has occurred; the
intimal layer of the blood vessel is intact. Platelet adhesion plays no part
in the initiation of plaques. Driven by growth factors released by macrophages
as well as endothelial and other cells, smooth muscle cells proliferate and
the plaque thickens. Platelet deposition becomes a factor in plaque growth
only after the intima is damaged. Figure
11 illustrates endothelial erosion and plaque formation, and
Figure 12 illustrates plaque
disruption, which may result in acute or chronic symptoms.
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Fig. 11. —Diagrammatic representation shows atherosclerotic plaque with
homogeneous lipid core and platelet-rich thrombus overlying thin but intact
cap. This is early unruptured plaque in its early developmental phase of
endothelial erosion. (Reprinted with permission from
[64])
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Fig. 12. —Diagram illustrates plaque disruption in classic
"shoulder" region. Lipid-rich core contents are seen escaping into
arterial lumen. Mixture of blood, platelets, thrombin fibrinogen, and lipids
is extremely volatile and may cause sudden vessel occlusion, embolism, or
spasm. (Reprinted with permission from
[64])
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Arterial Thrombosis and Myocardial Infarction
Thrombus may arise on the surface of an intact arterial plaque. This
thrombus is termed "superficial." Alternatively, plaque may
spontaneously rupture, exposing its lipid core to circulating blood and
resulting in thrombus formation in the ruptured plaque. A third type of
injury, commonly seen after angioplasty, is associated with iatrogenic tears
involving the intima and arterial media.
Morphology of an atherosclerotic plaque is characterized by a fibrous cap
covered by endothelium, shielding the plaque from the circulation. As long as
the fibrous cap remains intact, the disease progresses slowly and results in
progressive narrowing of the coronary arteries and angina pectoris. However,
some plaques develop progressive thinning followed by rupture of the fibrous
cap, which exposes tissue factor and other extracellular matrix components to
circulating platelets and coagulation factors, with subsequent thrombus
formation. Events responsible for progressive thinning of the fibrous cap are
not completely understood but appear to be the result of an intense local
inflammatory process. Autopsy specimens have shown the presence of
inflammation, especially at the "shoulder" region of
atherosclerotic plaques, which is the most frequent site of rupture, composed
of a large number of macrophages, T lymphocytes, and mast cells. Macrophages
produce metalloproteinases, which degrade collagen. Peripheral
metalloproteinase levels are elevated in patients with unstable coronary
syndromes. Mast cells are also found in large numbers in atherosclerotic
plaques and are in an activated state when the plaques rupture. Mast cells
contain a variety of mediators, including proteases (chymase and tryptase),
that can cleave metalloproteinases to their active forms that are capable of
digesting collagen [48].
Plaque rupture is confined to plaques with a large lipid core.
Vulnerability to plaque rupture is associated with a thin cap. Erosion and
rupture produce thrombi, which vary widely in size. Variably sized clumps of
platelets, which may be up to several hundred microns in diameter, are
periodically swept downstream from these plaques and may occlude small
arteries and arterioles in the myocardium.
The pathology of unstable angina is that of a nonoccluding thrombus. Angina
at rest may be caused by bursts of platelet emboli or spasm at the site of
injury in an epicardial artery. Intermittent growth of the thrombus can
occlude the vessel, which may reopen by natural lysis. Blood flow is also
important in determining the fate of the thrombus. As a result of these
causes, unstable angina can last for hours or days. Occlusive thrombosis may
develop rapidly in a major coronary artery, or it could evolve over days. Once
a thrombus partially or completely occludes an artery, it may propagate
downstream into its branches. The time infarction occurs after thrombosis will
depend on hemodynamic and other factors. Radiolabeled fibrinogen has been
given to patients presenting with angina at hospital admission who
subsequently died from infarction. These studies revealed that the isotope was
found only on new thrombus in the distal tail region after infarction had
occurred; the thrombus that formed over the plaque rupture site was not
radiolabeled [48].
Sudden and complete occlusion of a coronary artery results in transmural
myocardial infarction. Myocardial necrosis is uniform as a result of sudden
and complete vessel occlusion for at least a few hours. In contrast, in
patients with unstable angina, residual antegrade blood flow or collateral
flow around the arterial obstruction preserves the subepicardium. In the
latter group of patients, histologic examination reveals numerous small areas
of necrosis, which vary greatly in age.
Atherosclerotic changes in the heart appear identical to those found in
other vascular beds; however, plaques in the carotid, femoral, and iliac
arteries and those in the aorta are much larger. They may be 2 cm in length or
more and may undergo both erosion and rupture. Because these vessels are much
larger, occlusion is rare. After rupture, the contents of the plaque wash away
and an ulcer crater may form. This crater is covered by a layer of platelets
and may be the source of emboli (e.g., to the brain from the carotid
artery).
Clinical Manifestations of Acute Coronary Syndromes
The term "acute coronary syndrome" embraces a spectrum of
clinical circumstances ranging in severity from sudden death to unstable
angina. Acute coronary syndrome accounts for 2 million hospitalizations per
year in the United States alone. Mortality is approximately 25%, including
those patients who never reach a hospital. The cause of death in these
individuals is cardiac arrhythmia. Pathologic findings are the same in all
forms of acute coronary syndrome. Most individuals have multivessel disease
with at least one ruptured plaque with suppurated acute thrombus. Most
patients report having had vague symptoms, including fatigue and dyspnea, for
some months before the event. Many others report atypical chest discomfort or
frank pain that was previously denied. Interestingly, most patients see a
physician during this premonitory phase; these individuals frequently have
experienced unusual stress for weeks or months. A high degree of adrenergic
stimulation may be present, which may play a role in plaque rupture, enhanced
thrombogenesis, and the increased likelihood of ventricular fibrillation,
which is known to be associated with an acute thrombotic event
[48,
49].
Other Types of Acquired Heart Disease
Coronary artery occlusive disease accounts for most cases of adult cardiac
disease; however, valvular heart disease and cardiomyopathy are significant
disorders for which diagnostic imaging plays an increasingly important role
[50]. The availability of MR
imaging has led to the recognition of more patients with arrythmogenic right
ventricular dysplasia
[51].
Molecular Biology
Cardiovascular diagnosis and treatment are undergoing a revolution that
began just over a decade ago. The first battles in this revolution did not
begin in the cardiac catheterization laboratory or in the operating room.
Rather, a new breed of cardiovascular scientist has emerged from the molecular
biology research laboratory to address fundamental issues concerning the basic
biology of cardiac development and the molecular basis of inherited and
acquired heart disease.
The cell was once considered primarily a metabolic unit, whereas now it is
more appropriate to regard it as a signal processing unit. Accordingly,
molecular cardiologists have developed methods to introduce genes directly
into cells in the heart and vascular tree. For example, restenosis can be
reduced by preventing the uncontrolled accumulation of smooth muscle cells
after angioplasty. Modified adenoviruses can be used as vectors to deliver
genes engineered to inhibit cell division of smooth muscle cells. Such
molecular "cytostatins" may be used to suppress one or more
molecular signals required for cells to divide. Other gene therapy techniques
stimulate angiogenesis and arteriogenesis in ischemic zones.
The human genome will soon be sequenced so that the primary encoding
protein structure of every cell will become known and available. The field of
molecular cardiology is still in its infancy, but the funding opportunities
and potential for making important discoveries are profound. Radiologists with
a few notable exceptions are, at best, only spectators in this panorama, yet
they are heavily involved in vascular imaging, which will certainly play an
important role in the molecular biology revolution
[52]. Radiologists must
recognize the crucial need to train our residents and fellows in this field
and other new areas of imaging science
[53].
New technologies continue to become available for imaging, including MR
microscopy, light source imaging, and imaging using infrared and monochromatic
radiation. The brightness of synchrotron radiation from the accumulation ring
of accelerated positrons or electrons is much greater than that from a
conventional X-ray source. High-resolution contrast-enhanced images can be
obtained with fluorescent screens using a high-definition television camera.
Studies are in progress at synchrotron sites in Germany, Japan, and the United
States [54].
Summary
Remarkable technologic advances have occurred in diagnostic imaging during
the past decade, but even more astounding is their accelerated pace between
1972 and 1990 (Fig. 13).
Although there is evidence that heart disease has been on the decrease in the
United States, it remains ubiquitous. Health care costs and patient access are
two major forces currently driving reform in the delivery of health care to
the aging population.
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Fig. 13. —Graph shows accelerating pace of introduction of new
diagnostic imaging technologies and applications since 1891. Note how many
tools and applications have appeared between 1972 and 1990. PET = positron
emission tomography.
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Selecting the most appropriate imaging modality, avoiding duplicate
studies, and showing improved patient outcome based on cost-effective
diagnostic and therapeutic algorithms are being demanded of radiologists.
However, it is considerably more difficult to show the value of diagnostic
studies in patient outcome research than it is for therapeutic procedures.
Nevertheless, this is an important area for radiology.
It has been estimated that more than $100 billion is spent in the United
States annually on heart disease, much of which is for reimbursing diagnostic
imaging studies. Indeed, a group of the 10 most common CPT-4 (Current
Procedural Terminology
[55]) codes are for cardiac
procedures and account for 32% of all Medicare part B imaging costs
[56]. Reimbursement from
echocardiography alone, which is the most common procedure (15%), was twice
that of all MR imaging studies (7.4%), including neuroradiology procedures
[56]. These values place the
frequency and dollar value of cardiac imaging and its importance to society as
a whole into perspective. Furthermore, there continues to be increased
utilization and self-referrals of patients by cardiologists.
Nuclear medicine is an important clinical and research area that does not
attract sufficient new radiology specialists. This is unfortunate, because the
field offers outstanding opportunities as new methods continue to emerge
[57,
58]. Targeting contrast media
to cellular elements to explore the pathophysiology of disease is only one
example of the types of applications and potential of the field
[59].
Considerably more funding is available today for imaging research than
there was in the past. Furthermore, many millions of new research dollars are
available for molecular biology research studies of vascular disease. These
are compelling arguments for radiologists to become more interested and
involved in this area and more active in the cardiac imaging field
[60]. The radiologist's window
of opportunity lies in the potential of CT and MR imaging to replace more
invasive diagnostic methods and to improve the management of patients with
cardiovascular disease [61,
62].
Because radiologists are responsible for the whole installed base of CT and
MR imaging scanners, they are well positioned to reenter the exciting and
rewarding field of cardiac imaging
[63].
The next 3 years will determine if the major radiology societies and the
American Board of Radiology are successful in persuading the radiology
community to respond to this challenge, which requires that radiologists
recognize the implications and become personally involved. Training sufficient
numbers of new radiologists in cardiac imaging will be a critical challenge,
but the rewards will be worthwhile. Radiology leadership, however, must show
their commitment to this initiative. The tragic alternative for radiologists
may be that most vascular radiology intervention—not merely cardiac
imaging—will be performed exclusively by members of other
specialties.
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Introduction
Congenital Heart Disease
