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Assessment of Regional Myocardial Velocities by Tissue Phase Mapping and Feature Tracking in Healthy Children and Pediatric Patients with Hypertrophic Cardiomyopathy: A Comparison Study
Alexander Ruh1, Arleen Li2, Joshua D Robinson1,3,4, Cynthia K Rigsby1,4,5, and Michael Markl1,6

1Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, 2Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, 3Department of Pediatrics, Division of Pediatric Cardiology, Ann & Robert H. Lurie Children's Hospital of Chicago, Chicago, IL, United States, 4Department of Pediatrics, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, 5Department of Medical Imaging, Ann & Robert H. Lurie Children's Hospital of Chicago, Chicago, IL, United States, 6Department of Biomedical Engineering, McCormick School of Engineering, Northwestern University, Chicago, IL, United States

Synopsis

In this study, we compare tissue phase mapping (TPM) and feature tracking (FT) of standard cine SSFP images for the assessment of regional myocardial velocities in 15 pediatric patients with hypertrophic cardiomyopathy (HCM) and 20 age-matched healthy controls. Data analysis included the calculation of segmental (AHA 16-segment model) left ventricular radial and long-axis peak velocities in systole and diastole. Both techniques detected significantly decreased diastolic velocities in HCM patients compared to controls, suggesting reduced myocardial relaxation despite normal ejection fraction. Lower temporal resolution of FT derived velocities resulted in systematically lower peak velocities compared to directly measured TPM velocities.

Introduction

Tissue Phase Mapping (TPM) is a well-established technique to quantify myocardial velocities1-4 and has been successfully applied for the detection of regional left ventricular (LV) abnormalities in cardiac pathologies such as hypertensive heart disease, cardiomyopathy, or after heart transplantation.5-9 Recently, the development of feature tracking (FT) algorithms has enabled the retrospective quantification of myocardial strain and velocities from standard 2D cine SSFP images.10-15 An advantage of FT is that it does not require additional MRI sequences, but it can derive functional information based on standard-of-care images. Hypertrophic cardiomyopathy (HCM) is a common genetic condition characterized by otherwise unexplained increased LV wall thickness and associated with increased risk of heart failure, atrial fibrillation, and sudden death.16 MR techniques (tagging, FT) have demonstrated reduced LV myocardial function in patients with HCM despite normal LV ejection fraction (LVEF),11-13,17,18 but to the best of our knowledge no comparison between different MRI techniques has been performed. In this study, we systematically evaluated the performance of TPM and FT for the assessment of regional LV myocardial velocities in pediatric patients with HCM compared to healthy controls.

Methods

Fifteen pediatric HCM patients (15±4y, 9f) and 20 age-matched controls (16±4y [p=0.6], 10f) underwent standard CMR (1.5T Siemens Aera) including k-t accelerated (R=5) TPM using a prospectively gated, black-blood prepared phase-contrast sequence with 3-directional velocity encoding1,6,19 in short-axis orientation (base, mid, apex; each in one breath-hold; in-plane resolution = (1.6-2.5 mm)2; temporal resolution = 21-25 ms). FT analysis was performed on standard short- (base, mid, apex) and long-axis (2, 3 and 4 chamber views) retrospectively gated cine SSFP images (in-plane resolution = (0.7-1.1 mm)2; true temporal resolution = 30±5 ms) using commercially available software (TomTec ImageArena). For TPM data analysis an in-house developed MATLAB tool was used to manually segment LV endo-/epicardial contours and to transform the measured Cartesian velocities (vx,vy,vz) to radial (vr), circumferential (vΦ) and long-axis (vz) velocity components more closely resembling LV contraction, rotation, and shortening. For both FT and TPM the AHA 16-segment model was used to obtain systolic and diastolic, radial and long-axis peak velocities for each segment (Fig. 1). For 5 controls and 3 patients not all cine long-axis views have been acquired resulting in incomplete FT segmental information. Thus, these subjects had to be excluded for long-axis analysis.

Results

Table 1 lists global LV peak velocities (averaged over all segments) showing significantly reduced diastolic radial and long-axis peak velocities for HCM patients using both TPM (p<0.01) and FT (only vz significant with p<0.05), while LVEF in HCM patients was normal (65±8%). These global differences in diastolic peak velocities were also present on the segmental level as depicted in Fig. 2. Although both TPM and FT showed significant differences between HCM patients and controls on the global and regional level, peak velocities obtained from FT were generally lower compared to TPM. Bland-Altman analysis comparing FT with TPM regional velocities (Fig. 3) demonstrated a systematic underestimation for FT compared to TPM with an increasing difference for higher peak velocities. Nevertheless, there were significant correlations between the segmental radial and long-axis peak velocities (systole and diastole) obtained from both techniques, 0.44<r<0.65 with p<0.001.

Discussion

While two previous FT studies on children only investigated systolic strain / strain rate,11,13 the present study is the first to assess diastolic function in pediatric HCM patients via radial and long-axis peak velocities. Here, both TPM and FT demonstrate reduced diastolic function in patients, which is in agreement with previous studies on adults.12,18 However, the derived FT velocities are systematically lower than the directly measured TPM velocities. This underestimation might be explained by enhanced temporal averaging for FT. First, the temporal resolution of the cine images is lower (30±5ms vs. 23.5±0.6ms for TPM), and second, more importantly, the FT algorithm needs at least two time points to derive one velocity value, whereas TPM directly measures the velocity for each time point. Thus, the effective temporal resolution for FT velocities is more than two times lower than for TPM. For future studies it might be beneficial to use high temporal resolution cine images20 for FT applications

Conclusion

Both TPM and FT show decreased diastolic myocardial velocities in HCM patients, suggesting reduced myocardial relaxation despite normal LVEF. This may be an early indicator of disease and helpful in multiparametric HCM patient analysis. This study further shows that FT systematically underestimates myocardial velocities compared to directly measured TPM velocities. This implies that a quantitative comparison of velocities obtained from different techniques needs to be treated carefully.

Acknowledgements

Grant support by the National Institute of Heart, Lung and Blood Disorders (NHLBI) R01 HL 117888.

References

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Figures

Figure 1: Tissue phase mapping and feature tracking analysis. Both tools provide regional velocity time courses (16-segment AHA model; myocardial values were used from TomTec), from which systolic and diastolic peak velocities are obtained.

Table 1: Global LV peak velocities as well as diastolic basal peak velocities, averaged over all segments and all basal segments, respectively. For diastolic radial peak velocities a significant difference is only present in the basal slice, whereas all other components show significant differences for the global diastolic values. Five controls and 3 patients had to be excluded for long-axis analysis since not all cine long-axis views have been acquired for these subjects.

Figure 2: Bullseye plots for regional diastolic peak velocities. Asterisks mark significant differences between controls and HCM patients (* p<0.05, ** p<0.01). For TPM almost all segments for both radial and long-axis velocities are significantly different, whereas for FT there are only two significant segments for each component. This is also reflected in the global diastolic values, which are more significant for TPM and less or not significant for FT (Tab. 1). However, considering the average of all segments within one slice, the basal peak velocities from FT are both significantly reduced in patients (Tab. 1).

Figure 3: Bland-Altman analysis for the segmental peak velocities confirms the underestimation of velocities from FT compared to TPM. Dashed lines mark 95% confidence intervals and the oblique lines are least-square fits indicating an increased difference for higher velocities

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
0366