Endogenous T1ρ cardiovascular magnetic resonance in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM), characterized by an unexplained increase in left ventricular (LV) wall thickness, is the most common genetic cardiac disorder, with a prevalence of approximately 1 in 500; this prevalence may be as high as 1 in 200 when accounting for both genotype-positive/phenotype-positive and genotype-negative/phenotype-positive individuals [1]. Typical pathologic findings of HCM include cardiomyocyte hypertrophy and disarray, as well as focal or diffuse interstitial fibrosis [2]. In recent years, cardiovascular magnetic resonance (CMR) has been used to characterize and quantify myocardial fibrosis. Increased fibrosis, seen as late gadolinium enhancement (LGE), has been identified as a risk factor for sudden cardiac death and heart failure in this population [3]. T1 mapping and extracellular volume (ECV) quantification through CMR have also been correlated with increased risk of cardiovascular events [4, 5]. However, not all HCM patients will go on to have an event; LGE has a high prevalence (as high as 70%) in this population [6, 7] but a low specificity for the prediction of future cardiovascular events, limiting its negative predictive value [8]. Additionally, gadolinium-based contrast agents (GBCAs) confer a risk of nephrogenic systemic fibrosis in patients with renal disease, and additionally are deposited in brain tissue [9, 10]. Accordingly, there is interest in the development and validation of more specific and non-contrast methods for myocardial characterization in HCM patients.

T1ρ CMR is an endogenous contrast method for tissue characterization that does not require GBCAs and is distinct from both T1 and T2 contrast. It utilizes a low power radiofrequency pulse, also called a spin-lock pulse, to enable measurement of longitudinal relaxation in the rotating frame (T1ρ). The spin lock pulse mitigates the loss of transverse magnetization, suppressing contributions to relaxation from chemical exchange and water diffusion through magnetic field gradients [11]. Its ability to detect myocardial fibrosis has been validated in animal models of ischemia and reperfusion [12‐14] as well as in explanted hearts from patients with dilated cardiomyopathy [15]. Despite its mechanistic relevance to HCM pathophysiology, few studies have investigated the value of T1ρ in this population. Thus, we sought to evaluate and characterize the role of T1ρ in HCM patients by comparing it to conventional LGE and native T1.

In our study characterizing the role of endogenous T1ρ imaging in the assessment of patients with HCM, we found that (1) percent area enhancement as measured by T1 and T1ρ at FWHM were moderately correlated with LGE area enhancement, (2) HCM short-axis slices categorized as LGE + +, LGE + −, and LGE −− each demonstrated elevated pre-contrast T1, T1ρ, and ECV compared to controls, and (3) ECV was significantly different between images rated LGE + + compared to LGE −−.

Both T1- and T2-weighted imaging have been used to demonstrate elevations in HCM patient myocardial relaxation times relative to normal patients [5, 25‐29]. Cardiac T2 mapping may be sensitive to several different mechanisms of relaxation in vivo. Some of these mechanisms may be considered ‘undesired’ because they suppress ∆T2 between diseased and healthy myocardium. Since each mechanism of relaxation is additive to the overall relaxation rate (i.e., \({R}_{2}={R}_{2,a}+{R}_{2,b}+\dots\), where \(a\), \(b\), and so on refer to a different relaxation mechanism), eliminating these ‘undesired’ sources of relaxation could increase the difference in the net transverse relaxation. While the ‘unwanted’ contributions to T2 in myocardium are not fully elucidated at present, their effect is to dephase magnetization irreversibly. Potential ‘undesired’ mechanisms of relaxation may include diffusion through background magnetic fields, chemical exchange, among others. By using a sufficiently strong SLk pulse, it is possible to prevent these unwanted mechanisms of relaxation [11]. Using a moderate amplitude (> 400 Hz) SL pulse, we have found that there is a significantly larger ∆T1ρ than ∆T2 in these regions [30]. The net effect of this is an increase in the contrast between normal and diseased myocardium.

Patchy fibrosis occurs in the majority of HCM patients. This is observed primarily as replacement fibrosis, but may also take the form of interstitial fibrosis, which can be imaged and quantified by T1 mapping and subsequent ECV calculation [31, 32]. Most studies of fibrosis in HCM patients have focused on LGE imaging, which allows visualization of replacement fibrosis and has demonstrated associations with adverse outcomes [3]. However, fibrosis accumulates throughout the course of HCM, and additionally, LGE has limited specificity for the prediction of events such as sudden cardiac death and heart failure [8]. It is therefore of both clinical and research interest to investigate new contrast mechanisms such as T1ρ in the HCM population.

To date, only one study has measured T1ρ in human patients with HCM; Wang et al. compared visually-assessed LGE area with 2–6 standard deviation-thresholding of T1ρ in 18 HCM patients, finding high correlation (Pearson’s r ranging from 0.81 to 0.88) of percent fibrosis between these modalities [33]. In our cohort, we found a lower correlation of T1ρ with LGE-assessed enhancement area using Spearman’s rho, which may be due to several reasons. Our cohort is larger with 40 HCM patients and is more heterogenous with both genotype-positive and -negative patients. Additionally, our group applied FWHM thresholding to LGE images, rather than manual measurement of enhancement area, decreasing observer bias. The use of FWHM thresholding therefore increases the robustness of our measurements, allowing for direct comparison in future studies. An additional study of T1ρ in a mouse model of cardiac hypertrophy [34] examined T1ρ at several timepoints after transverse aortic constriction and verified fibrosis ex vivo using Masson’s trichrome staining [34]. Similarly, their findings showed that T1ρ increased over time and was highly correlated with fibrotic areas [34].

Our study brings to light several interesting findings. We show moderate correlations between LGE and T1 and T1ρ-assessed percent enhancement area, and mild correlation between T1 and T1ρ. Variations in the enhancement areas calculated by each method may reflect a physiologic difference in the way that LGE, T1, and T1ρ assess healthy and abnormal tissue. Our results indicate that LGE, T1, and T1ρ may each give different and additive information that one method alone cannot provide, a finding that warrants further study. Additionally, we demonstrate that HCM patients showed elevations in non-contrast quantitative MR measurements (pre-contrast T1 and T1ρ) regardless of LGE status. The significance of T1ρ imaging and its added value will need to be prospectively evaluated.

T1 and T1ρ relaxation time moderately correlate with LGE percent enhancement area using FWHM thresholding. Additionally, T1, T1ρ, and ECV distinguish HCM patients from healthy controls, irrespective of whether the patient’s myocardium demonstrated positive LGE, showing potential value as a noninvasive biomarker. Further study is needed to elucidate the role of T1ρ in risk prediction for HCM patients.