DOI:10.2214/AJR.07.2286
AJR 2007; 189:1335-1343
© American Roentgen Ray Society
MRI of Hypertrophic Cardiomyopathy: Part I, MRI Appearances
Mark W. Hansen1,2 and
Naeem Merchant2
1 Department of Medical Imaging, The Prince Charles Hospital, Rode Rd.,
Chermside, Brisbane, Queensland 4032, Australia.
2 Department of Medical Imaging, Toronto General Hospital, University Health
Network and Mt. Sinai Hospital, Toronto, ON, Canada.
Received March 15, 2007;
accepted after revision June 29, 2007.
Address correspondence to M. W. Hansen
(mark.hansen{at}qscan.com.au).
Abstract
OBJECTIVE. We present a two-part review about the use of MRI in
patients with hypertrophic cardiomyopathy (HCM). This article, Part 1, focuses
on the MRI appearances of HCM.
CONCLUSION. MRI has proven to be an important tool for the
evaluation of patients suspected of having HCM because it can readily diagnose
those with phenotypic expression of the disorder and can potentially identify
the subset of patients at risk of sudden cardiac death.
Keywords: cardiac imaging • cardiomyopathy • cardiovascular imaging • heart disease • hypertrophic cardiomyopathy • mitral valve • MRI
Introduction
Cardiovascular MRI is now accepted as a valuable tool in the initial
assessment and follow-up of many acquired and congenital disorders. In
particular, assessment of cardiomyopathies with MRI has been successful owing
to its unique ability to depict different enhancement patterns in diseased
myocardium on inversion recovery delayed gadolinium-enhanced images. We
present a two-part review about the use of MRI in hypertrophic cardiomyopathy
(HCM). Part 1 of the review focuses on the MRI appearances of HCM. Part 2 of
the review covers the differential diagnosis, risk stratification, and
posttreatment MRI follow-up appearances in these patients
[1].
HCM is defined as a primary disease of the cardiac sarcomere that leads to
cardiac hypertrophy. It is the most common inheritable cardiac disorder, with
an estimated prevalence of 1:500
[2–5].
HCM is characterized by left ventricular hypertrophy (LVH) with occasional
involvement of the right ventricle. Familial disease with autosomal dominant
inheritance predominates [6].
There is a broad range of phenotypic expressions with asymmetric involvement
of the interventricular septum being the most common pattern. MRI has proven
to be an important tool for the evaluation of patients suspected of having HCM
because it can readily diagnose those with phenotypic expression of the
disorder and can potentially identify the subset of patients at risk of sudden
cardiac death.
Pathology
HCM is the most common inheritable cardiac disorder with an estimated
prevalence of 1:500. The mode of inheritance is autosomal dominant in
approximately 50–60% of the cases. Currently, 10 genes have been
implicated and more than 150 mutations have been characterized, all relating
to the cardiac sarcomere
[2–5].
These mutations typically cause an increase in myocyte stresses and impaired
function that eventually lead to hypertrophy and fibrosis
[4]. Not surprisingly, the
disease has a wide variability in penetrance and in phenotypic expression
leading to a great variability in imaging appearances.
Most cases of HCM are phenotypically expressed in adolescence or early
adulthood; however, research has shown age-related penetrance with certain
phenotypes in which the delayed and de novo emergence of LVH occurs in midlife
or even later [7,
8].
Most of the clinical manifestations of the disease involve either diastolic
or systolic dysfunction, left ventricular outflow tract (LVOT) obstruction,
arrhythmias, and sudden cardiac death.
Morphology
The heterogeneity of HCM is well appreciated by reviewing its wide
variation in morphologic expression. Asymmetric involvement of the
interventricular septum is the most common form of the disease. Other variants
include symmetric, apical, and masslike LVH and an end-stage form, known as
the "burned out phase," that is characterized by progressive wall
thinning and systolic dysfunction. Right ventricular involvement occurs in
approximately 17.6% of all cases of HCM, most commonly involving the mid to
apical portion of the right ventricle
[4,
9]. A more in-depth discussion
of the various gross appearances of HCM is included later in this article.
Microscopic Appearances
In HCM, myocytes show hypertrophy and disarray, with bizarre enlarged
nuclei, hyperchromasia, and pleomorphism. The histologic criteria for the
diagnosis of HCM require disarray of at least 5–10% of the myocytes
within the interventricular septum
[4]. Diagnostic pitfalls
include patchy myocardial involvement that leads to false-negative biopsies
(sampling errors).
Determination of which myocardial segments are involved before biopsy may
provide the interventional cardiologist with the ability to perform a targeted
biopsy, thus potentially improving sampling. Unfortunately, myocyte disarray
is not specific for HCM and may be seen in many common cardiovascular diseases
and even in the normal heart, where it is usually found at the points at which
the right ventricle interdigitates with the left ventricular septum. In these
locations, the normal disarray is accompanied by increased interstitial
adiposity [3,
4].
HCM also affects small intramural coronary arteries. Typical appearances
include reduced arteriolar density and intimal and medial smooth-muscle
hyperplasia, with many vessels showing dense perivascular collagen deposition
[3,
4].
How to Image
Although echocardiography is an excellent technique to use for the
assessment of patients with HCM, it is occasionally limited by poor acoustic
windows, incomplete visualization of the left ventricular wall, and inaccurate
evaluation of left ventricular mass
[10,
11].
MRI has the ability to evaluate wall thickness and the distribution of
disease better than echocardiography, especially in the anterolateral wall of
the left ventricular myocardium
[11,
12]. MRI's ability to more
accurately assess left ventricular wall thickness is important in evaluating
myocardium greater than 30 mm
[12], a key prognostic
indicator that we discuss later in this article.
MRI also has the ability to more accurately evaluate left ventricular mass,
volumes, and function than echocardiography and to assess for areas of
regional wall motion abnormalities, aneurysms, and foci of delayed enhancement
[10,
11].
Protocol
At our institution, imaging begins with acquisition of an abbreviated set
of ungated localizer images in all three planes. From these images, the
imaging planes for the left ventricle are determined. Vector ECG-gated
steady-state free precession (SSFP) pulse sequences are used to assess the
heart as a stack of contiguous images in the short-axis oblique, four-chamber,
vertical long-axis, and LVOT imaging planes. Fast cine phase-contrast imaging
is then performed in the LVOT plane. After that sequence, gadolinium is
administered and delayed imaging of the myocardium is performed in the
short-axis oblique, four-chamber, and vertical long-axis planes.
Postprocessing and Image Analysis
Left ventricular wall thickness should be calculated during end-diastole,
preferably in the short axis for assessment of the mid to basal thirds of the
left ventricular myocardium. The vertical and horizontal long-axis planes are
useful in evaluating wall thickness at the left ventricular apex. Measurements
of thickness in these planes, however, should be made with care. Images chosen
for measurement should be cross-referenced with another view to avoid
obliquity because only a slice that perpendicularly transects the left
ventricular wall will yield accurate measurements
[13]. Care should also be
taken to exclude free muscle bundles when measuring wall thickness.
Measurements of left ventricular volumes, function, and mass are derived
from an SSFP pulse sequence in the left ventricular short-axis oblique plane.
Techniques for performing these measurements have been well described
previously
[14–16].
The decision to either include or exclude papillary muscles in measurements of
left ventricular end-diastolic volume, end-systolic volume, or mass differs
among institutions.
Assessment of regional wall motion abnormalities is generally made by
visual inspection alone. In some centers, research protocols may include
complementary spatial modulation of magnetization (CSPAMM), which is otherwise
known as "myocardial tagging," to assess for regional wall motion.
Although fractional shortening can be formally measured, these measurements
are not performed routinely in most clinical practices
[10].
HCM Variants
Asymmetric HCM
Asymmetric disease is the most common morphologic presentation of HCM with
the anteroseptal myocardium being the most commonly hypertrophied segment
[10,
12]. Clinically, patients may
be asymptomatic or may present with dyspnea, presyncope or syncope, or angina
[5].
Asymmetric septal wall hypertrophy causes obstruction of the LVOT in
20–30% of the cases [3,
4,
17]. This obstruction may be
present when the patient is at rest or may be latent (provocable) or labile
(variable) [5]. The severity of
the obstruction can be affected by ventricular afterload (systolic blood
pressure), preload, and contractility
[5]. Sudden standing, eating a
large meal, or drinking alcohol, for example, may cause a drop in afterload
[5].
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Fig. 1 —Line diagram of the heart in the left ventricular outflow
tract (LVOT) projection shows typical findings of systolic anterior motion of
the mitral valve. Note displacement of the anterior mitral valve leaflet tip
(red leaflet) into the LVOT. This displacement has resulted in
failure of the valve leaflets to coapt, a turbulent high-velocity jet within
the LVOT, and a posteriorly directed jet of mitral valve regurgitation into
the left atrium.
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Although previously subject to periodic controversy, a gradient of 30 mm Hg
or more is now generally recognized to be of pathophysiologic and prognostic
importance [18,
19]. Peak outflow tract
gradients can be estimated using phase-contrast MRI; however, these gradients
are currently better assessed using echocardiography because of its higher
temporal resolution when measuring peak instantaneous velocities. Efforts to
improve flow measurements on MRI are focused on both improving the temporal
resolution of phase-contrast imaging and developing new techniques, such as
real-time Fourier velocity encoding
[20].
Evaluation of left ventricular functional parameters shows that mass
measurements may not necessarily be greater in patients with asymmetric HCM
than in those with a normal heart. Although the left ventricular ejection
fraction (LVEF) is typically preserved in asymmetric HCM, subtle evidence of
regional wall hypocontractility that can be quite variable may be present
[10]. The results of a study
by Sipola et al. [10] suggest
that impaired contractility may precede findings of hypertrophy, with a
progressive decline in systolic function accompanying more marked LVH.
Delayed enhancement of the left ventricular myocardium occurs in up to 80%
of all patients with HCM and is usually patchy and mid wall in location
[21,
22]. We discuss this topic
separately later in this article.
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Fig. 2A —41-year-old man with asymmetric septal hypertrophic
cardiomyopathy (HCM) and systolic anterior motion of the mitral valve. A
high-velocity jet can be detected as an area of high signal intensity within
the left ventricular outflow tract (LVOT) on the four-chamber view (A)
and as an area of spin dephasing on the LVOT projection (B).
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Fig. 2B —41-year-old man with asymmetric septal hypertrophic
cardiomyopathy (HCM) and systolic anterior motion of the mitral valve. A
high-velocity jet can be detected as an area of high signal intensity within
the left ventricular outflow tract (LVOT) on the four-chamber view (A)
and as an area of spin dephasing on the LVOT projection (B).
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Fig. 2C —41-year-old man with asymmetric septal hypertrophic
cardiomyopathy (HCM) and systolic anterior motion of the mitral valve.
Short-axis oblique steady-state free precession image shows enlarged septal
perforators arising from the left anterior descending artery, a finding
commonly seen in cases of HCM.
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Fig. 2D —41-year-old man with asymmetric septal hypertrophic
cardiomyopathy (HCM) and systolic anterior motion of the mitral valve. Note
delayed enhancement within the anterior and posterior right ventricular
insertion points on the short-axis oblique delayed enhancement image.
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Abnormalities of the mitral valve may occur because of a primary
abnormality of the valve itself or as a result of an LVOT obstruction.
Systolic anterior motion of the mitral valve is due to the portion of the
anterior mitral leaflet distal to the coaptation point being displaced into
the LVOT by venturi or drag forces (Figs.
1,
2A,
2B,
2C,
2D,
3A,
3B,
3C,
3D). Over time, the systolic
anterior motion of the mitral valve leads to a subaortic mitral impact lesion
on the septum, which undergoes fibrosis; thickening of the anterior mitral
leaflet and chordae from the resultant trauma; a posteriorly directed mitral
regurgitant jet into the left atrium; and a systolic gradient along the LVOT
[3–5].
If the mitral regurgitant jet is either anteriorly or centrally placed, other
structural abnormalities such as prolapse or leaflet restriction should be
excluded [3,
5]. These other abnormalities
of the mitral valve occur in approximately 20% of patients with subaortic
obstruction and include abnormal insertion of the papillary muscles, mitral
valve prolapse, or excessive fibrotic thickening of the anterior mitral valve
leaflet [5].
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Fig. 3A —36-year-old man with asymmetric septal hypertrophic
cardiomyopathy and systolic anterior motion of the mitral valve. Steady-state
free precession images show displacement of the anterior mitral valve leaflet
into the left ventricular outflow tract (LVOT) accompanied by a high-velocity
spin dephasing jet into the LVOT. There is also a jet of mitral regurgitation
into a moderately enlarged left atrium. Note the area of high signal within
the LVOT on the four-chamber projection. This high signal is a result of high
velocities generated in the outflow tracts of patients with systolic anterior
motion.
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Fig. 3B —36-year-old man with asymmetric septal hypertrophic
cardiomyopathy and systolic anterior motion of the mitral valve. Steady-state
free precession images show displacement of the anterior mitral valve leaflet
into the left ventricular outflow tract (LVOT) accompanied by a high-velocity
spin dephasing jet into the LVOT. There is also a jet of mitral regurgitation
into a moderately enlarged left atrium. Note the area of high signal within
the LVOT on the four-chamber projection. This high signal is a result of high
velocities generated in the outflow tracts of patients with systolic anterior
motion.
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Fig. 3C —36-year-old man with asymmetric septal hypertrophic
cardiomyopathy and systolic anterior motion of the mitral valve. Steady-state
free precession images show displacement of the anterior mitral valve leaflet
into the left ventricular outflow tract (LVOT) accompanied by a high-velocity
spin dephasing jet into the LVOT. There is also a jet of mitral regurgitation
into a moderately enlarged left atrium. Note the area of high signal within
the LVOT on the four-chamber projection. This high signal is a result of high
velocities generated in the outflow tracts of patients with systolic anterior
motion.
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Fig. 3D —36-year-old man with asymmetric septal hypertrophic
cardiomyopathy and systolic anterior motion of the mitral valve. Steady-state
free precession images show displacement of the anterior mitral valve leaflet
into the left ventricular outflow tract (LVOT) accompanied by a high-velocity
spin dephasing jet into the LVOT. There is also a jet of mitral regurgitation
into a moderately enlarged left atrium. Note the area of high signal within
the LVOT on the four-chamber projection. This high signal is a result of high
velocities generated in the outflow tracts of patients with systolic anterior
motion.
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Diastolic Dysfunction and the Left Atrium
Diastolic dysfunction is thought to occur secondary to abnormal
dissociation of actin and myosin filaments during the active phase of
relaxation in early diastolic filling and to left ventricular myocardial
properties that affect passive characteristics of late diastole, such as the
widespread increased volume of the intercellular collagen network
[23]. This dysfunction may
also result in a restrictive picture. Mitral regurgitation from systolic
anterior motion or from morphologically abnormal mitral valve leaflets may
also contribute to left atrial dilatation
[24]. Patients with left
atrial enlargement may then subsequently develop atrial fibrillation
[5]. Stretch and distention of
the atrial appendage have been shown to perhaps be related to the onset of
atrial fibrillation due to an increase in the dispersion of the atrial
effective refractory period
[25].
Tani et al. [24] found that
an increase in left atrial volume and, to a lesser extent, in left atrial
diameter correlated well with the presence of paroxysmal atrial fibrillation.
They concluded that left atrial enlargement may be used to help predict which
patients with HCM will likely develop atrial fibrillation. This distinction
among HCM patients is important because the presence of atrial fibrillation
reduces left ventricular pre-load and may lead to cardiac failure and systemic
thromboembolism [24].
Symmetric HCM
Symmetric HCM is said to occur in up to 42% of HCM cases
[4]. This entity should be
evaluated closely to differentiate it from other causes of symmetric left
ventricular thickening, which we discuss in more detail in Part 2.
Apical HCM
Apical HCM was originally described in individuals of Asian descent but is
now being increasingly diagnosed in Western populations
[4,
5]. Reports of the rate of
occurrence of apical HCM vary in the literature, ranging from 25% of all
patients with HCM in Japan to fewer than 2% of HCM patients in Western
countries [26]. Wigle
[5] reported an incidence of 9%
in an HCM population in a clinic in Toronto.
ECG findings of giant negative T waves in the precordial leads raise
initial suspicion of apical HCM in up to 50% of cases in some studies
[5,
26]
(Fig. 4). The diagnosis of
apical HCM can occasionally be difficult on echocardiography. MRI plays an
important role in the diagnosis and exclusion of other diseases and
abnormalities that mimic apical HCM on echocardiography
[5].
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Fig. 5A —Apical hypertrophic cardiomyopathy in a 43-year-old man. Left
anterior oblique (A) and four-chamber (B) views. Note apical
hypertrophy and obliteration of the left ventricular apical cavity on the
four-chamber view.
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Fig. 5B —Apical hypertrophic cardiomyopathy in a 43-year-old man. Left
anterior oblique (A) and four-chamber (B) views. Note apical
hypertrophy and obliteration of the left ventricular apical cavity on the
four-chamber view.
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Fig. 6A —Apical hypertrophic cardiomyopathy with marked left and right
ventricular involvement in a 51-year-old man. Steady-state free precession
images show near obliteration of the apical cavities of both ventricles.
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Fig. 6B —Apical hypertrophic cardiomyopathy with marked left and right
ventricular involvement in a 51-year-old man. Steady-state free precession
images show near obliteration of the apical cavities of both ventricles.
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Diagnostic criteria for apical HCM reported in the literature include an
absolute apical wall thickness of > 15 mm or a ratio comparing apical left
ventricular and basal left ventricular wall thicknesses of 
1.3–1.5
[27,
28]. More subjective criteria
for the diagnosis of apical HCM include obliteration of the left ventricle
apical cavity in systole and failure to identify a normal progressive
reduction in left ventricular wall thickness toward the apex (Figs.
5A,
5B and
6A,
6B).
Moon et al. [27] found that
10 patients with a history of ECG abnormalities (i.e., antero-lateral T wave
inversion) and negative findings for apical HCM on echocardiograms had
positive MRI findings for apical HCM, with one patient having a left
ventricular wall thickness of 28 mm on MRI. They found no evidence of a
correlation between the extent of repolarization abnormality and the degree of
morphologic severity [27].
The characteristic spadelike configuration of the left ventricular cavity
initially described in angiographic studies is well appreciated on left
ventricular vertical long-axis views on MRI
[26,
28] (Fig.
5A,
5B).
Concomitant apical involvement of the right ventricle is also commonly seen
(Fig. 6A,
6B).
Other common findings include apical aneurysm formation with delayed
enhancement, which is sometimes referred to as a "burned out apex"
[4] (Fig.
7A,
7B,
7C,
7D). This process is thought
to be due to ischemia that results from reduced capillary density, hyperplasia
of the arterial media, increased perivascular fibrosis, and myocardial
bridging [23]. This process
usually occurs in the presence of normal epicardial coronary arteries.
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Fig. 7A —Mid to apical hypertrophic cardiomyopathy with a
"burned out apex" in a 58-year-old man. Diastole (A) and
systole (B) steady-state free precession (SSFP) images in the vertical
long-axis plane. Note hypertrophy of the middle third left ventricle and
formation of an apical aneurysm.
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Fig. 7B —Mid to apical hypertrophic cardiomyopathy with a
"burned out apex" in a 58-year-old man. Diastole (A) and
systole (B) steady-state free precession (SSFP) images in the vertical
long-axis plane. Note hypertrophy of the middle third left ventricle and
formation of an apical aneurysm.
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Fig. 7C —Mid to apical hypertrophic cardiomyopathy with a
"burned out apex" in a 58-year-old man. Small mural thrombus, best
seen on the left anterior oblique delayed enhanced image (C), can also
more subtly be identified on the SSFP projections (A, B, and
D). This patient had normal epicardial coronary arteries on coronary
angiography (not shown).
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Fig. 7D —Mid to apical hypertrophic cardiomyopathy with a
"burned out apex" in a 58-year-old man. Small mural thrombus, best
seen on the left anterior oblique delayed enhanced image (C), can also
more subtly be identified on the SSFP projections (A, B, and
D). This patient had normal epicardial coronary arteries on coronary
angiography (not shown).
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HCM with Midventricular Obstruction
This variant of asymmetric HCM, predominantly involving the middle third of
the left ventricle, may result in severe midventricular narrowing and
obstruction. HCM with midventricular obstruction may be associated with the
formation of an apical aneurysm, which is thought to result from the
generation of increased systolic pressures within the cardiac apex from the
midventricular obstruction. A similar appearance may be due to a burned out
apex, as is seen in the apical variant
[4,
5] (Fig.
7A,
7B,
7C,
7D).
Masslike HCM
Patients with HCM occasionally present with a masslike thickening of the
left ventricle. This scenario may cause difficulties in diagnosis. In theory,
a true mass will have no contractile portion to it, whereas HCM will show
variable degrees of contractility. MR tagging by spatial modulation of
magnetization has been used to assess for tag displacement and deformation.
Tag deformation should be identified in hearts with HCM but should not be
noted in neoplastic masses
[29]. This technique has yet
to be thoroughly investigated, however, and care should be made to distinguish
minimal contraction from scant residual normal myocardium in the setting of
infiltrative processes such as lymphoma, amyloid, or sarcoid.
Perhaps more useful than this tag deformation is signal characterization of
the myocardium with double inversion recovery or spin-echo imaging using
varying tissue weighting and observing perfusion of the area with first-pass
perfusion and delayed enhancement techniques. Masslike HCM more precisely
parallels the homogeneous signal characteristics and perfusion of adjacent
normal myocardium (Fig. 8A,
8B,
8C,
8D,
8E,
8F), whereas tumors often show
varying degrees of signal heterogeneity before and after gadolinium
administration and show perfusion characteristics that differ from those of
the remainder of the left ventricle.
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Fig. 8A —61-year-old woman with masslike hypertrophic cardiomyopathy.
Delayed gadolinium-enhanced images (A and B), steady-state free
precession images (C and D), and first-pass perfusion images
(E and F) reveal a focal area of thickening involving anterior
left ventricular myocardium, which shows a small amount of mid wall
enhancement but perfuses in a uniform manner similar to adjacent normal
myocardium (seen in E and F).
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Fig. 8B —61-year-old woman with masslike hypertrophic cardiomyopathy.
Delayed gadolinium-enhanced images (A and B), steady-state free
precession images (C and D), and first-pass perfusion images
(E and F) reveal a focal area of thickening involving anterior
left ventricular myocardium, which shows a small amount of mid wall
enhancement but perfuses in a uniform manner similar to adjacent normal
myocardium (seen in E and F).
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Fig. 8C —61-year-old woman with masslike hypertrophic cardiomyopathy.
Delayed gadolinium-enhanced images (A and B), steady-state free
precession images (C and D), and first-pass perfusion images
(E and F) reveal a focal area of thickening involving anterior
left ventricular myocardium, which shows a small amount of mid wall
enhancement but perfuses in a uniform manner similar to adjacent normal
myocardium (seen in E and F).
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Fig. 8D —61-year-old woman with masslike hypertrophic cardiomyopathy.
Delayed gadolinium-enhanced images (A and B), steady-state free
precession images (C and D), and first-pass perfusion images
(E and F) reveal a focal area of thickening involving anterior
left ventricular myocardium, which shows a small amount of mid wall
enhancement but perfuses in a uniform manner similar to adjacent normal
myocardium (seen in E and F).
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Fig. 8E —61-year-old woman with masslike hypertrophic cardiomyopathy.
Delayed gadolinium-enhanced images (A and B), steady-state free
precession images (C and D), and first-pass perfusion images
(E and F) reveal a focal area of thickening involving anterior
left ventricular myocardium, which shows a small amount of mid wall
enhancement but perfuses in a uniform manner similar to adjacent normal
myocardium (seen in E and F).
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Fig. 8F —61-year-old woman with masslike hypertrophic cardiomyopathy.
Delayed gadolinium-enhanced images (A and B), steady-state free
precession images (C and D), and first-pass perfusion images
(E and F) reveal a focal area of thickening involving anterior
left ventricular myocardium, which shows a small amount of mid wall
enhancement but perfuses in a uniform manner similar to adjacent normal
myocardium (seen in E and F).
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Burned Out Phase
The burned out phase occurs in up to 10% of patients with HCM and severe
cardiac symptoms. It is characterized by left ventricular dilatation and by
loss of myocardium and replacement fibrosis, which are thought to be due to
small-vessel ischemia [4,
30,
31].
The burned out phase may present as a localized process with an appearance
similar to that of a myocardial infarct but with normal epicardial coronary
arteries, as seen in Figure
7A,
7B,
7C,
7D, or as a more diffuse
end-stage process with an appearance similar to that of a dilated
cardiomyopathy. Little has been written in the literature regarding the
cardiac MRI appearances of diffusely burned out end-stage HCM.
Enhancement
It has been well established that delayed enhancement of the myocardium
using a segmented inversion recovery pulse sequence can differentiate between
normal and infarcted myocardium or fibrosis
[32]. Delayed enhancement is
thought to occur in areas of abnormal myocardium secondary to expansion of the
extracellular space, alterations in the extracellular matrix composition, and
altered distribution kinetics
[32–34].
Delayed enhancement occurs in up to 80% of patients with HCM
[21,
22]. The most common form of
hyperenhancement in HCM is patchy and mid wall in location. Teraoka et al.
[35] found delayed enhancement
in 75% of patients with HCM, 89% of whom had patchy mid-wall-type enhancement
(Figs. 2A,
2B,
2C,
2D and
9).
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Fig. 9 —Short-axis oblique projection image in a 52-year-old man with
hypertrophic cardiomyopathy shows typical delayed enhancement involving the
left ventricular septum at both the anteroseptal and posteroseptal right
ventricular insertion points. Note enhancement is patchy and mid wall and
involves areas of grossly thickened myocardium.
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The various proposed histologic correlates of enhancement in HCM include
plexiform fibrosis (fibrosis seen in HCM in areas of myocyte disarray),
expanded interstitial spaces, and microscopic replacement fibrosis due to
ischemia [22,
27–39].
However, Mahrholdt et al. [40]
have suggested that areas of visually detected delayed enhancement are most
likely explained by macroscopic replacement fibrosis and that enhancement
secondary to diffuse interstitial processes, such as plexiform fibrosis, is
less likely to cause recognizable differences in regional signal
intensity.
In a case study, Moon et al.
[37] compared the MRI findings
and histology results of an explanted heart with HCM and found that an
increased percentage of histologic collagen correlated directly with increased
delayed enhancement of the myocardium. Those researchers also found that
increased enhancement was associated with an increased incidence of regional
wall motion abnormalities
[37]. Other studies have also
shown a significant decrease in global and regional cardiac function in
patients with increasing volumes of delayed enhancement
[21,
35,
41,
42].
Delayed enhancement tends to involve the interventricular septum,
particularly the anteroseptal mid to basal segments. These segments are also
the most commonly thickened segments in patients with asymmetric HCM
[35,
42,
43]. If abnormal enhancement
occurs elsewhere outside the interventricular septum, it also tends to occur
in areas of maximal left ventricular thickness.
As we mentioned earlier, delayed enhancement that occurs in areas of
hypertrophy tends to be mid wall and patchy in distribution. Another
interesting feature of delayed enhancement in HCM is a predilection for
enhancement to occur at the anterior and posterior right ventricular insertion
points (Figs. 2A,
2B,
2C,
2D and
9). An exception to this is in
areas of burned out myocardium where the left ventricular wall is typically
thinned and enhancement is full thickness
[41] (Fig.
7A,
7B,
7C,
7D).
Delayed enhancement in areas of burned out myocardium bares striking
similarity to delayed enhancement in patients with chronic myocardial
infarction secondary to coronary artery disease. This type of enhancement
however may not necessarily conform to a particular vascular territory. The
postulated causes of ischemia in patients with HCM and normal epicardial
coronary arteries include reduced capillary density; hyperplasia of the
arterial media; increased perivascular fibrosis, leading to reduced coronary
vasodilator reserve; and myocardial bridging
[23].
Investigators have noted that in patients without LVH there is usually no
enhancement. Lack of enhancement in this population suggests that regions of
delayed enhancement occur only after the development of LVH, although this
theory has yet to be proven across all the many genotypic variants of LVH
[21].
Despite the low yield for discovering delayed enhancement in patients with
only minimally thickened left ventricular myocardium, administration of
gadolinium remains useful in differentiating other causes of thickened left
ventricular myocardium that show characteristic patterns of enhancement.
Examples include cardiac involvement by amyloid and Fabry's disease, both of
which are discussed in Part 2
[1].
The presence of delayed enhancement and its relationship to patients with
an increased risk of sudden cardiac death as established by traditional risk
factors [21,
22,
34,
44] are elaborated on in Part
2 [1] also.
Conclusion
This section completes Part 1 of this two-part review on MRI of HCM. This
article focuses on the abnormalities and imaging characteristics of HCM, along
with a discussion about its various morphologic subtypes and the all-important
topic of delayed enhancement. The significance of delayed enhancement imaging
in HCM is emphasized in Part 2
[1], which addresses the topics
of sudden cardiac death and risk stratification. Part 2 also deals with the
differential diagnosis and posttreatment follow-up of this disorder.
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M. W. Hansen and N. Merchant
MRI of Hypertrophic Cardiomyopathy: Part 2, Differential Diagnosis, Risk Stratification, and Posttreatment MRI Appearances
Am. J. Roentgenol.,
December 1, 2007;
189(6):
1344 - 1352.
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Abstract
Introduction
