Native T1 values identify myocardial changes and stratify disease severity in patients with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disorder affecting 1:3500 to 1:6000 male births worldwide, causing severe disability and death due to cardiopulmonary failure associated with dilated cardiomyopathy and restrictive pulmonary disease [1]. The dilated cardiomyopathy onset and progression are variable; however severe congestive heart failure is nearly universal by adulthood [2, 3]. Current efforts to improve both lifespan and quality of life in this disease are targeted towards delaying the onset of cardiac remodeling and subsequent dysfunction [4].

The design and evaluation of early prophylactic therapies to prevent DMD cardiomyopathy require quantitative measures of left ventricular (LV) remodeling to detect early, subclinical myocardial changes and monitor effectiveness [5]. However, current measures of LV remodeling, such as ECG [6], echocardiography evaluation including diastolic indices and myocardial strain imaging [7], and serum biomarkers [8], are limited. Cardiovascular magnetic resonance (CMR) offers additional diagnostic benefits in the assessment of myocardial changes present in cardiomyopathy. CMR using late gadolinium enhancement (LGE) imaging is able to show areas of edema and fibrosis [9, 10], however, the extent of disease can be underestimated in conditions where the myocardium is affected globally, such as DMD cardiomyopathy [11].

T1-mapping by CMR is an emerging technique, which offers the ability to quantify myocardial fibrosis. Widely used protocols for T1-mapping in the heart are based on either inversion or saturation recovery using imaging sequences such as Modified Look-Locker (MOLLI) and Saturation recovery single shot acquisition (SASHA), respectively. Baseline normal values of T1 are different for different methods [12], and there is current debate on the pros and cons for each technique [13]. Pre- and post-contrast T1 can be measured and combined for the assessment of extracellular volume (ECV) for detection and quantification of diffuse myocardial fibrosis [12, 14, 15]. However, with new concerns regarding gadolinium accumulation in the brain [16], pre-contrast, or”native” T1 measurements have been studied and shown to provide useful clinical data without contrast administration [17, 18].

The ability to detect and follow subclinical myocardial fibrosis using noninvasive imaging may be a powerful tool to monitor response to early prophylactic myocardial therapies. Therefore, our aim was to compare native T1 and extracellular volume (ECV) measurements in DMD and controls, and evaluate their ability to stratify myocardial disease.

With IRB approval and informed consent/assent, 20 boys with DMD who were not mechanically ventilated underwent CMR with gadolinium contrast on a Siemens Aera 1.5T MR scanner (Siemens Healthcare, Erlangen, Germany). The CMR study included volumetric analysis to obtain left ventricular ejection fraction (LVEF) and fat-water separation imaging to exclude significant intra-cardiac deposits of fat which would alter T1 values [19]. Native T1 mapping was performed in four short axis slices (excluding apical regions to avoid partial volume effect) and a four chamber slice using both a MOLLI and SASHA sequence. Following intravenous administration of gadobutrol 0.15 mmol/kg, late gadolinium enhancement (LGE) imaging and post-contrast T1 mapping were performed, again using both the MOLLI and SASHA methods in identical slice positions. The order of MOLLI and SASHA acquisition following contrast administration was randomized and all post-contrast T1 maps were acquired between 15 and 22 min after contrast administration to assure that a dynamic equilibrium was achieved.

Additionally, and with IRB approval and informed consent/assent, 16 age-matched boys undergoing CMR with gadolinium for a separate clinical indication with normal results and no evidence of LGE were included in the analysis as a control group. These subjects underwent identical study procedures in terms of T1 mapping technique and gadolinium administration. Subjects with any history of cyanotic congenital heart disease, confirmed cardiomyopathy, history of cardiopulmonary bypass were excluded prior to imaging procedures, and subjects with calculated abnormal ejection fraction or evidence of late gadolinium enhancement were excluded from the control group following the imaging procedure.

The DMD subjects’ (n = 20) average age was 14.4 ± 4 years, average LVEF was 56.3 % ± 7.4 % (range 39–66 %) and 12/20 had evidence of LGE. All LGE was patchy, subepicardial and was confined to the lateral wall of the left ventricle; thus all septal myocardium in the DMD cohort appeared normal (nulled) on LGE imaging. No regions of intramyocardial fat deposition were noted on fat-water separation imaging. The male controls (n = 16) had an average age of 16.1 ± 2.2 years with an LVEF 60.4 ± 5.1 % and no LGE. Study indications for the controls were to rule out arrhythmogenic cardiomyopathy, evaluate the thoracic aorta, and evaluate the right ventricle post pulmonary balloon valvuloplasty (with a normal RV mass, RV volumes and RV function and mild pulmonary regurgitation). None had undergone cardiopulmonary bypass, were cyanotic or have confirmed or suspected myocarditis. Table 2 displays demographic information for the two groups. A summary of the degree of cardiopulmonary disease is listed in Table 3, including current glucocorticoid/cardiac therapies and most recent pulmonary function testing for the DMD cohort.

This study used CMR to demonstrate significant differences in the native T1and ECV measurements in the myocardium of DMD subjects compared to age-matched controls. Most importantly, this is the first study to demonstrate that native T1 measurements in LGE-negative myocardium are proportional to the degree of cardiovascular disease in the DMD cohort using regression analyses. Thus, native T1 values can be measured with CMR and used to assess myocardial changes, including fibrosis and inflammation, and disease state among myocardial segments that all appear “normal on LGE”, making this a viable, quantitative measure of subclinical disease without the use of contrast that could be used to monitor early cardiac therapies.

Initial studies focused on the presence of LGE in DMD patients using contrast-enhanced imaging techniques [21‐23]. More recently, clinical implications of LGE in DMD were demonstrated when Florian et al. [24] showed that “transmural” LGE was associated with more adverse events in muscular dystrophy subjects and Tandon et al. [25] found correlations between the presence of LGE, LVEF and genotype. Soslow et al. also looked at post-contrast T1 ratios and found lower ratios in DMD subjects compared to controls, even in LGE negative subjects [26].

ECV is the most recent technique used to measure myocardial extracellular matrix expansion. Soslow et al. recently showed significantly increased global ECV values in DMD compared to controls, but no difference in global ECV in DMD subjects with and without LGE [27]. This current study confirms significantly elevated ECV values in DMD vs. controls, and in addition, LGE-free myocardium has a significantly higher ECV in DMD than in normals. While ECV can discriminate between DMD and controls, it performs poorly in the ability to distinguish disease severity within the DMD cohort. ECV demonstrated only the ability to predict presence of LGE, but couldn’t distinguish between controls and DMD without fibrosis.

With the recent concerns regarding frequent use of gadolinium contrast [16], a marker of subclinical myocardial disease without contrast is optimal. As with Soslow et al. (26), we demonstrated increased native T1 measures in DMD subjects compared to controls. Due to variability in T1 mapping sequences, this study also included SASHA sequences to further validate T1 mapping. We also studied septal native T1 measures within the DMD cohort based on age, LVEF and presence of LGE, but the average native T1 values did not differ significantly within these groups (Table 3). However, when we controlled for age and LVEF, native T1 measures using the SASHA protocol demonstrated an improved ability to stratify DMD subjects by disease severity (normal, DMD without fibrosis and DMD with fibrosis; Fig. 4). Native T1 determined by MOLLI demonstrated ability to only stratify the presence of fibrosis. Therefore, this study suggests the utility of native T1 measures, and not ECV, to stratify disease severity in DMD subjects without the use of contrast in a small cohort of subjects.

This study incorporated both MOLLI and SASHA imaging strategies to increase confidence of any observed differences in T1 values between the DMD and control groups, as such differences, if present, were expected to be subtle. This is based on a recent study that showed how measurement precision and accuracy can be affected by the T1 mapping technique chosen and MOLLI and SASHA were among the strongest techniques [28]. In terms of accuracy, SASHA values are known to be approximately 10–15 % higher than MOLLI and our data is consistent with this observation. Also, SASHA is known to be a less precise technique [13] and our SASHA measures have larger standard deviations compared to MOLLI. However, in the current study, SASHA showed a better ability in distinguishing clinical disease states. This may be because MOLLI is more susceptible to measurement imperfections due to high and/or variable heart rates which may have diminished the precision in a pediatric cohort. Based on these results, further studies are needed to determine if SASHA is a better technique to measure early, diffuse myocardial changes in DMD.

Limitations of this work include general limitations of T1 mapping techniques, which can yield variable results at higher heart rates, and are prone to partial-volume errors with hearts that are thin and small. Although T1 mapping sequences were generally tailored to the heart rate, heart rates above 100 bpm may affect T1 values. Additionally, although no overt intra-cardiac fat was noted on fat-water separation imaging; theoretically, microscopic fat deposition may be part of the DMD cardiomyopathy, which would affect the ECV and T1 mapping values in addition to cardiac fibrosis [29]. While prior studies have linked elevated T1 values with cardiac fibrosis, DMD myocardial disease may include an inflammatory component which may be altering T1 values in addition to fibrosis. Since no endomyocardial biopsies were obtained in this study, it is unclear how much of a role fibrosis vs. inflammation played in the T1 values of this study population. Another limitation is that the extent of LGE in the study population is more difficult to capture by T1 mapping compared to the severity of LGE, as the validity of T1 relies on exclusion of any questionable pixels from the ROI, thus extent will always be underestimated. The control population is a population of convenience, as acquiring contrast-enhanced imaging from known normal children is not practical or ethical due to need for IV placement and exposure to gadolinium without a clinical indication. Another limitation is the small sample size, which is typical for rare diseases [25] and can limit conclusions drawn from these results as the study was not powered to detect small differences amongst this heterogeneous disease. Finally, all T1 mapping was carried out on a single magnet, from a single vendor, and therefore has limited utility to the overall community of cardiologists involved in the care of all patients with DMD. Ideally, future work would focus on creating reference values for normal children at different magnet strengths and vendors, which would require multi-center pooling of data.

This study is the first to demonstrate the novel utility of native T1 measures to stratify DMD patients by extent of myocardial disease. ECV measures, while increased in DMD compared to control, could not distinguish disease severity within the DMD cohort. Native T1 imaging provides a novel imaging biomarker for monitoring myocardial changes related to early subclinical disease in DMD and possibly other cardiomyopathies.