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Utility of Cardiac Magnetic Resonance Imaging in the Diagnosis of Hypertrophic Cardiomyopathy
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     the Department of Radiology (C.R., N.M.W., M.J.-H., P.P., N.P.), Fairview-University Medical Center, Minneapolis, Minn

    Division of Pediatric Cardiology (J.W.), University of Hamburg-Eppendorf, Hamburg, Germany

    the Hypertrophic Cardiomyopathy Center (S.A.C., A.G.Z., B.J.M.), Minneapolis Heart Institute Foundation, Minneapolis, Minn.

    Abstract

    Background— Two-dimensional echocardiography is currently the standard test for the clinical diagnosis of hypertrophic cardiomyopathy (HCM). The present study was undertaken to determine whether cardiac MRI (CMR) affords greater accuracy than echocardiography in establishing the diagnosis and assessing the magnitude of left ventricular (LV) hypertrophy in HCM.

    Methods and Results— Forty-eight patients (age 34±16 years) suspected of having HCM (or with a confirmed diagnosis) were imaged by both echocardiography and CMR to assess LV wall thickness in 8 anatomic segments (total n=384 segments) and compared in a blinded fashion. Maximum LV thickness was similar by echocardiography (21.7±9.1 mm) and CMR (22.5±9.6 mm; P=0.21). However, in 3 (6%) of the 48 patients, echocardiography did not demonstrate LV hypertrophy, and CMR identified otherwise undetected areas of wall thickening in the anterolateral LV free wall (17 to 20 mm), which resulted in a new diagnosis of HCM. In the overall study group, compared with CMR, echocardiography also underestimated the magnitude of hypertrophy in the basal anterolateral free wall (by 20±6%; P=0.001), as well as the presence of extreme LV wall thickness (30 mm) in 10% of patients (P<0.05).

    Conclusions— CMR is capable of identifying regions of LV hypertrophy not readily recognized by echocardiography and was solely responsible for diagnosis of the HCM phenotype in an important minority of patients. CMR enhances the assessment of LV hypertrophy, particularly in the anterolateral LV free wall, and represents a powerful supplemental imaging test with distinct diagnostic advantages for selected HCM patients.

    Key Words: hypertrophy ; cardiomyopathy ; magnetic resonance imaging ; echocardiography

    Introduction

    Hypertrophic cardiomyopathy (HCM) is a relatively common form of genetic heart disease and the most frequent cause of sudden cardiac death in the young.1–6 The clinical phenotype is characterized by otherwise unexplained left ventricular (LV) hypertrophy and a myriad of patterns of wall thickening.7 Although the distribution of LV hypertrophy has shown little relation to clinical outcome,2 the magnitude of LV wall thickness conveys prognostic significance, showing a direct relationship to the risk for sudden death.3,4 Although 2D echocardiography is the standard imaging modality for clinical identification of the LV hypertrophy, a not inconsequential number of patients apparently affected by HCM fail to have their diagnosis resolved clinically with echocardiography for technical or other reasons, including some who are seemingly free of LV hypertrophy during at least a portion of their clinical course.2,8–16

    Cardiac MRI (CMR) has the capability of acquiring tomographic images of the hypertrophied LV chamber, with tissue contrast and border definition that are often superior to that achievable with echocardiography.17–22 In the present study, we assessed a group of HCM patients with both CMR and 2D echocardiography to determine whether CMR provided diagnostic imaging information with respect to LV hypertrophy not accessible with standard echocardiography.

    Methods

    Patient Selection

    Both 2D echocardiographic and CMR studies were performed prospectively over a 25-month period in 48 consecutively enrolled patients, in whom a diagnosis of HCM was either suspected or had been established. Each patient was first evaluated clinically with echocardiography in the Hypertrophic Cardiomyopathy Center of the Minneapolis Heart Institute Foundation, and subsequently, CMR studies were performed on the same day in the Department of Radiology at the Fairview-University Medical Center (University of Minnesota).

    Patients were aged 8 to 69 years (mean 34±16 years); 34 (71%) were male. Twenty-five patients (52%) were asymptomatic (New York Heart Association functional class I), 18 (38%) were mildly symptomatic (class II), and 5 (10%) were severely limited by heart failure symptoms (classes III and IV). Thirteen patients (27%) had obstruction to LV outflow under resting conditions (gradient 30 mm Hg ranging to 150 mm Hg).

    Diagnosis of HCM was based on the presence of LV hypertrophy without cavity dilatation (maximum wall thickness 15 mm in adults, or the equivalent relative to body surface area in children), in the absence of another cardiac or systemic disease capable of producing the magnitude of hypertrophy evident in that patient.1,2,7 The study protocol was approved by an Institutional Review Board (Human Subjects Committee) at the University of Minnesota, and all patients signed an informed consent form before participation.

    Echocardiography

    Two-dimensional echocardiograms were obtained with commercially available instruments (Philips Agilent 5500) with 2.5- and 3.5-MHz transducers equipped with harmonic imaging. The extent and distribution of LV hypertrophy were assessed from 2D echocardiographic images in multiple cross-sectional planes, as described previously.7 Other cardiac dimensions were measured from the derived M-mode echocardiogram consistent with previous recommendations.7 Wall thicknesses were assessed directly from the television monitor with a calibration scale produced by the instrument. Locations of endocardial and epicardial borders were identified by viewing the pertinent portions of 0.5- or 0.75-inch format videotape in slow-motion freeze-frame and real-time modes. The greatest thickness measured at any site within the LV wall represented the maximum wall thickness. Peak instantaneous LV outflow gradient was estimated with continuous-wave Doppler under basal (resting) conditions.23

    Cardiac MRI

    A 1.5-T dedicated CMR scanner (Sonata, Siemens Medical Systems) with a torso phased-array coil for cardiac imaging was used for all studies. Initially, scout images with a 4-chamber view were acquired to prescribe short-axis image planes from base to apex for cine imaging. True fast imaging with steady-state precession (with segmented acquisition of k-space lines) was applied for cine imaging, with breath holding by the patient for 15 cardiac cycle lengths.24–26 The protocol/sequence parameters were as follows: repetition time (TR) for each 11-line k-space segment=35 ms; echo time (TE)=1.6 ms; receiver bandwidth=930 Hz/pixel; 256 readout points with oversampling; 165 phase encodings; 60° flip angle; a field of view with dimensions of 300 mm in the phase-encoding direction by 350 mm in the readout direction; and 15 phases per R-R interval. Contiguous cardiac images of 8-mm-thick slices with no interslice gap were acquired in both the long- and short-axis planes (10 to 15 per patient).

    Analysis of Cine CMR

    LV wall thicknesses at end diastole were calculated by the centerline method from chords that connect the endocardial and epicardial boundaries and are perpendicular to an automatically determined myocardial centerline (located halfway between endocardial and epicardial borders; Figure 1).27,28 The length of each chord was determined with the centerline algorithm in the MASS software, and the longest chord defined absolute wall thickness in each segment of LV (in millimeters).

    LV Wall Thickness Comparisons

    Echocardiography

    The LV chamber was circumferentially divided into 4 anatomic segments in the parasternal short-axis plane (at end diastole): anterior and posterior ventricular septum; and anterolateral and posterior free wall (that portion between the papillary muscles).7 Specifically, the ventricular septum, the portion of LV wall situated between the 2 ventricular cavities (as defined anatomically by the insertion sites of right ventricular wall into septum), was divided into 2 segments, the anterior and posterior septum. In addition, in the parasternal long-axis plane (base to apex), the LV chamber was divided into 2 regions: proximal (basal) portion extending from the inferior margins of aortic valve to the distal extent of mitral leaflets, and the distal (apical) portion visualized beyond the mitral valve leaflet tips. Therefore, as described previously,7 in each patient, LV end-diastolic wall thickness was assessed in 4 proximal and 4 distal segments (n=8), for an overall total of 384 segments in the 48 study patients.

    Measurement of LV wall thickness could be made with confidence in 384 (100%) of the segments by CMR and in 363 (95%) with echocardiography. LV wall thickness measurements by echocardiography have proved to be highly reproducible by prior intraobserver and interobserver variability analyses of patients with HCM and other conditions.3,29,30

    Cardiac MRI

    Similarly, 8 LV segments were identified in each patient from the CMR image designed to correspond directly to those anatomic regions defined and analyzed in the echocardiographic studies. Maximum end-diastolic wall thickness in each LV segment was measured with CMR and echocardiography, independently by 2 investigators (C.R. for CMR and B.J.M. for echocardiography), both of whom were blinded to the wall thickness data obtained for the alternative imaging test.

    CMR Observer Variability

    Intraobserver variability was assessed for the measurement of LV wall thickness in each of the 8 segments in all 48 study patients (total of 384 segments). The initial measurements were those that constitute the data set included in the present report. The second set of measurements were made for the purpose of the reproducibility analysis 11 months later by the same observer (C.R.), without knowledge of the initial measurements.

    Statistical Analysis

    Data were expressed as mean±SD. Noncontinuous variables expressed as proportions were compared by 2 test. Paired Student t tests were used for the comparison of normally distributed data. Intraobserver variability in comparative LV wall thickness measurements by CMR and echocardiography was assessed by calculating the coefficient of variation between the 2 measurements of LV wall thickness (by the same observer). Probability values were considered significant when 0.05.

    Results

    Patients Without HCM Diagnosis by Echocardiography

    In 3 (6%) of the 48 patients (each a HCM family member), 2D echocardiograms showed normal wall thicknesses ( 12 mm) in all LV segments; the 3 patients were asymptomatic and without mitral valve systolic anterior motion or evidence of LV outflow obstruction. However, CMR identified LV hypertrophy confined to the anterolateral free wall in each of these 3 patients. Two were identical male twins, aged 13 years, with maximum LV thicknesses of 10 and 12 mm, respectively, by echocardiography, but 17 and 20 mm, respectively, by CMR (Table 1; Figure 2). The third patient was a 42-year-old woman with normal LV wall thickness of 12 mm by echocardiography but 17 mm by CMR (Table 1). Abnormal ECG patterns were present in each of these 3 patients and preceded recognition of LV hypertrophy by CMR in the twins.

    Comparison of LV Wall Thickness With Echocardiography and CMR

    Overall Study Group

    LV end-diastolic wall thicknesses obtained by echocardiography and CMR were compared among the 48 study patients. Overall, maximum LV wall thickness did not differ significantly between the 2 imaging modalities, ie, 21.7±9.1 mm (range to 55 mm) with echocardiography and 22.5±9.6 mm (range to 53 mm) with CMR (P=0.21).

    CMR imaging showed a broad spectrum of patterns and magnitude of LV hypertrophy, including patients in whom predominant wall thickness was most commonly in the anterior ventricular septum, but also patients with the most substantial hypertrophy in the posterior ventricular septum or the anterolateral free wall (Figure 3). When wall thicknesses measured by echocardiography and CMR were compared with respect to each of the individual LV segments, the 2 imaging tests yielded values that did not differ significantly, except for the anterolateral LV free wall (Table 2).

    Notably, CMR measurements in the basal anterolateral LV free wall significantly exceeded those made with echocardiography (17±8 versus 13±6 mm; P=0.001), a difference of 20±6% (Figures 3A and 3B). The differences between echocardiographic and CMR measurements of the anterolateral LV free wall were most substantial when associated with the greatest magnitude of absolute LV wall thickness (P=0.001).

    With regard to the power for detection of the HCM phenotype, both echocardiography and CMR were diagnostic in 45 patients, CMR alone in 3, and echocardiography alone in none. The site of predominant LV hypertrophy within the wall was most commonly the anterior ventricular septum on both echocardiogram (n=24 patients; 50%) and CMR (n=21; 43%; P=NS).

    Patients With Extreme LV Hypertrophy

    Of the 48 patients, 41 had maximum LV wall thickness <30 mm by echocardiography. However, in 6 of these 41 patients, CMR showed a more extreme degree of hypertrophy, with LV wall thickness 30 mm identifiable in anterolateral free wall (n=4), posterior septum (n=1), and anterior septum (n=1). In these 6 patients, maximum LV wall thickness was 32±1 mm with CMR compared with 24±3 mm on echocardiography (P<0.001). Conversely, of the 35 patients with maximum LV wall thickness <30 mm by CMR, only 1 had extreme hypertrophy identified with echocardiography (wall thickness 30 mm).

    CMR Observer Variability

    Intraobserver variability for LV wall thickness measured by CMR averaged 4.9% overall for the combined 8 LV segments. Variability for individual segments were as follows: anterior ventricular septum, basal 4.9%, apical 4.2%; posterior septum, basal 4.2%, apical 4.5%; anterolateral LV free wall, basal 4.9%, apical 5.0%; and posterior free wall, basal 7.2%, apical 4.1%.

    Discussion

    HCM is a heterogeneous cardiac disease in which clinical diagnosis is predicated on the demonstration of otherwise unexplained LV hypertrophy, which appears in a myriad of diverse patterns, and with a wide range in the magnitude of wall thickening, as documented in the present analysis.1–3,7 Although the distribution of hypertrophy is often diffuse, it is also frequently segmental and confined to relatively small regions of the LV chamber.1,2,7 Although the anterior portion of ventricular septum is the area of the wall most commonly involved in the hypertrophic process, localized wall thickening may preferentially involve posterior septum, apex, anterolateral, or even posterior free wall.2,7,31,32

    Two-dimensional echocardiography is generally regarded as the "gold standard" noninvasive diagnostic test for HCM.1,2,7 However, echocardiography has certain technical limitations. Reliable quantitative delineation of LV wall thickness is dependent on adequate acoustic windows. Also, because the echocardiographic transducer is situated at a fixed point on the anterior chest wall, cross-sectional images are often unavoidably obtained with obliquity.7,31,33

    On the other hand, CMR provides the power of high-resolution, nonoblique images with excellent and uniform contrast at the endocardial borders, encompassing all levels and regions of the LV and permitting virtually complete reconstruction of the chamber.17–21,27,34 Therefore, CMR imaging has the potential to detect segmental wall thickening in any area of the LV wall, even when these regions are quite limited in size, and therefore can provide critical supplemental morphological information beyond that obtained from conventional and clinically adequate echocardiographic studies.

    In this regard, the findings of the present study define a new subgroup of HCM patients and a novel diagnostic role for CMR imaging within the heterogeneous clinical and morphological spectrum of this disease.1,2 This has not been delineated clearly in prior comparative CMR and echocardiographic studies in HCM.18–20 Indeed, a relatively small (5%) but important patient subset had a HCM diagnosis confirmed solely by the morphological identification of segmental areas of hypertrophy confined to the anterolateral LV free wall that was provided only by CMR. Two of these patients were asymptomatic identical twins aged 14 years whose father had survived a sudden and unexpected HCM-related cardiac arrest triggered by vigorous physical activity at age 33 years (with residual cerebral damage and neurological deficit).

    Both children had 5 serial 2D echocardiograms each (over 4 years) that were within normal limits and without evidence of wall thickening in any LV segment; however, over this time period, 12-lead ECGs consistently showed distinctly abnormal patterns involving right-axis deviation, as well as RSr' pattern in V1, narrow deep Q waves, and deep S waves in these 2 patients, which raised a heightened suspicion that they harbored an HCM mutant gene and are affected family members,13 albeit with a phenotype that could not be confirmed with echocardiography. Indeed, as previously emphasized,2,13 ECG abnormalities can be the initial and sole clinical clue to HCM, particularly in young family members.2,13

    However, in the present study, it was CMR that had a direct impact on diagnosis and clinical management, specifically in the twins. Given the appearance of the HCM phenotype (documented only by CMR) with maximum LV wall thicknesses of 17 and 20 mm, in conjunction with the high-risk profile created by their father’s cardiac arrest,1,2,5 a cardioverter-defibrillator was implanted prophylactically in both children.2,35 Finally, a 17-year-old symptomatic sibling of the twins with an abnormal ECG has recently been identified by CMR with similar anterolateral LV free wall hypertrophy associated with a normal echocardiogram.

    Of particular note, in each of these 3 patients in whom CMR identified the HCM phenotype, the region of LV wall thickening did not involve the ventricular septum but was confined to a small portion of the LV chamber, specifically in the anterolateral LV free wall. This is a region of the LV, spatially removed from the center of the echocardiographic sector in the parasternal short-axis cross-sectional plane, in which it is frequently difficult to reliably assess the magnitude of wall thickening, ie, due to poor lateral resolution and often substantial image distortion that result in indistinct recognition of the epicardial border situated at the lateral margins of the sector.33 Therefore, the detection by echocardiography of small segmental areas of hypertrophy by echocardiography in the anterolateral LV free wall is generally fraught with more substantial technical difficulties than is imaging of LV wall thickening in the anterior portion of ventricular septum (where superior axial resolution is operative). However, by utilizing high-resolution tomographic CMR imaging in patients in the present study, such technical limitations in epicardial (and endocardial) border recognition were largely resolved. The inherent capacity of CMR to image hypertrophied areas of the LV not readily identifiable by echocardiography has already been established in HCM patients with respect to apical wall thickening.36

    This principal was substantiated by 2 other analyses performed in this patient cohort. First, in the overall study group, we found that measurements of LV anterolateral free wall thickness were generally underestimated by 2D echocardiography (relative to CMR). Second, in an analysis of that subset of patients with the most extreme degrees of LV hypertrophy, a phenotypic expression of HCM to which increased risk for sudden death has been attributed,1–5 echocardiography appeared to underestimate the absolute magnitude of LV wall thickening. Indeed, in 6 of the present study patients, echocardiography failed to completely identify massive degrees of hypertrophy (usually in the anterolateral free wall) recognized by CMR, which may have placed these patients in a higher-risk subgroup2–5,37 as possible candidates for primary prevention of sudden death with an implantable defibrillator.35

    The latter observation creates a new clinical dilemma, because the available data in HCM relating risk of sudden death to extreme degrees of LV hypertrophy are based entirely on echocardiographic assessments of wall thickness. To somehow extrapolate and translate the available echocardiographic data to CMR-derived wall thicknesses will require future prospective clinical studies with substantial follow-up spanning many years (using CMR). Therefore, at present, the most prudent strategy for such patients (with LV wall thickness 30 mm only by CMR) will require a measure of individual clinical judgment that relies on an integrated assessment of the overall sudden death risk profile to determine the advisability of recommending an implantable defibrillator.

    In addition to the value of CMR for the diagnosis of HCM described in this report and elsewhere,36 other potential applications are likely. These include identification of the HCM phenotype when echocardiography is not of adequate diagnostic technical quality, as well as the recognition of delayed hyperenhancement after gadolinium infusion indicative of myocardial fibrosis.37–39 It is also expected that the indications for CMR in HCM will expand as more data become available, thereby selectively justifying the additional healthcare costs involved.

    In conclusion, the present data define distinctive advantages afforded by high-resolution CMR for the clinical diagnosis of HCM in selected patients. CMR imaging does not offer general superiority over echocardiography in all aspects regarding the morphological identification of the HCM disease phenotype, and at present, its use is not required in all HCM patients. Nevertheless, importantly, CMR is capable of establishing the diagnosis of HCM in some patients not reliably identifiable by conventional echocardiography.

    Footnotes

    Dr Rickers is presently at Pediatric Heart Center, Universit;tsklinikum Schleswig-Holstein, Kiel, Germany; Drs Wilke, Prasad Panse, and Neeta Panse are currently at Health Science Center, University of Florida, Jacksonville, Fla; and Dr Jerosch-Herold is currently at Advanced Imaging Research Center, Oregon Health and Science University, Portland, Ore.

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