Cardiac Magnetic Resonance Imaging in Appraising Myocardial Strain and Biomechanics: A Current Overview

Subclinical alterations in myocardial structure and function occur early during the natural disease course. In contrast, clinically overt signs and symptoms occur during late phases, being associated with worse outcomes. Identification of such subclinical changes is critical for timely diagnosis and accurate management. Hence, implementing cost-effective imaging techniques with accuracy and reproducibility may improve long-term prognosis. A growing body of evidence supports using cardiac magnetic resonance (CMR) to quantify deformation parameters. Tissue-tagging (TT-CMR) and feature-tracking CMR (FT-CMR) can measure longitudinal, circumferential, and radial strains and recent research emphasize their diagnostic and prognostic roles in ischemic heart disease and primary myocardial illnesses. Additionally, these methods can accurately determine LV wringing and functional dynamic geometry parameters, such as LV torsion, twist/untwist, LV sphericity index, and long-axis strain, and several studies have proved their utility in prognostic prediction in various cardiovascular patients. More recently, few yet important studies have suggested the superiority of fast strain-encoded imaging CMR-derived myocardial strain in terms of accuracy and significantly reduced acquisition time, however, more studies need to be carried out to establish its clinical impact. Herein, the current review aims to provide an overview of currently available data regarding the role of CMR in evaluating myocardial strain and biomechanics.

Quantitative mechanical dyssynchrony in dilated cardiomyopathy measured by deformable registration algorithm

OBJECTIVES

To investigate the diagnostic value and reproducibility of deformable registration algorithm (DRA)-derived mechanical dyssynchrony parameters in dilated cardiomyopathy (DCM) patients.

METHODS

The present study included 80 DCM patients (40 with normal QRS duration (NQRS-DCM); 40 with left bundle branch block (LBBB-DCM)) and 20 healthy volunteers. The balanced steady-state free-precession (bSSFP) cine images were acquired using a 3.0T scanner. Mechanical dyssynchrony parameters were calculated based on DRA-derived segmental strain, including uniformity ratio estimate (URE) and standard derivation of time-to-peak (T2Psd) parameters in circumferential, radial, and longitudinal orientations.

RESULTS

DCM patients showed significant mechanical dyssynchrony reflected by both URE and T2Psd parameters compared with controls. Among DCM patients, LBBB-DCM showed decreased CURE (0.78 ± 0.21 vs. 0.93 ± 0.05, p < 0.001) and RURE (0.69 ± 0.14 vs. 0.83 ± 0.15, p = 0.001), and increased T2Psd-Ecc (median with interquartile range, 94.1 (54.4-123.2) ms vs. 63.7 (44.9-80.4) ms, p = 0.003) and T2Psd-Err (91.1 (61.1-103.2) ms vs. 62.3 (46.3-104.5) ms, p = 0.041) compared with NQRS-DCM patients. CURE showed a strong correlation with QRS duration (r = - 0.54, p < 0.001), with maximum AUC (0.791) to differentiate LBBB-DCM from NQRS-DCM patients. Improved intra- and inter-observer reproducibility was found using URE indices (coefficient of variation (CoV), 1.20-3.17%) than T2Psd parameters (CoV, 15.28-41.18%).

CONCLUSIONS

The DRA-based CURE showed significant correlation with QRS duration and the highest discriminatory value between LBBB-DCM and NQRS-DCM patients. URE indices showed greater reproducibility compared with T2Psd parameters for assessing myocardial dyssynchrony in DCM patients.

KEY POINTS

• The strain analyses based on DRA suggested that DCM patients have varying degrees of mechanical dyssynchrony and there is a significant difference from normal controls. • CURE showed the strongest correlation with QRS duration and was the best parameter for differentiating DCM patients with normal QRS duration from patients with LBBB, and with normal controls. • URE indices showed improved reproducibility compared with T2Psd parameters in all three orientations (circumferential, radial, and longitudinal).

Echocardiographic Strain Analysis for the Early Detection of Left Ventricular Systolic/Diastolic Dysfunction and Dyssynchrony in a Mouse Model of Physiological Aging

Heart disease is the leading cause of hospitalization and death worldwide, severely affecting health care costs. Aging is a significant risk factor for heart disease, and the senescent heart is characterized by structural and functional changes including diastolic and systolic dysfunction as well as left ventricular (LV) dyssynchrony. Speckle tracking-based strain echocardiography (STE) has been shown as a noninvasive, reproducible, and highly sensitive methodology to evaluate LV function in both animal models and humans. Herein, we describe the efficiency of this technique as a comprehensive and sensitive method for the detection of age-related cardiac dysfunction in mice. Compared with conventional echocardiographic measurements, radial and longitudinal strain, and reverse longitudinal strain were able to detect subtle changes in systolic and diastolic cardiac function in mice at an earlier time point during aging. Additionally, the data show a gradual and consistent decrease with age in regional contractility throughout the entire LV, in both radial and longitudinal axes. Furthermore, we observed that LV segmental dyssynchrony in longitudinal axis reliably differentiated between aged and young mice. Therefore, we propose the use of echocardiographic strain as a highly sensitive and accurate technology enabling and evaluating the effect of new treatments to fight age-induced cardiac disease.

Left ventricular regional myocardial motion and twist function in repaired tetralogy of Fallot evaluated by magnetic resonance tissue phase mapping

OBJECTIVES

We aimed to characterise regional myocardial motion and twist function in the left ventricles (LV) in patients with repaired tetralogy of Fallot (rTOF) and preserved LV global function.

METHODS

We recruited 47 rTOF patients and 38 age-matched normal volunteers. Tissue phase mapping (TPM) was performed for evaluating the LV myocardial velocity in longitudinal, radial, and circumferential (Vz, Vr, and VØ) directions in basal, middle, and apical slices. The VØ peak-to-peak (PTP) during systolic phases, the rotation angle of each slice, and VØ inconsistency were computed for evaluating LV twist function and VØ dyssynchrony.

RESULTS

As compared to the controls, the rTOF patients presented decreased RV ejection fraction (RVEF) (p = 0.002) and preserved global LV ejection fraction (LVEF). They also demonstrated decreased systolic and diastolic Vz in several LV segments and higher diastolic Vr in the septum (all p < 0.05). A lower VØ PTP, higher VØ inconsistency, and reduced peak net rotation angle (all p < 0.05) were observed. The aforementioned indices demonstrated an altered LV twist function in rTOF patients in an early disease stage.

CONCLUSIONS

MR TPM could provide information about early abnormalities of LV regional motion and twist function in rTOF patients with preserved LV global function.

KEY POINTS

• Patients with rTOF presented significantly reduced systolic and diastolic Vz in the LV. • rTOF patients demonstrated significantly increased diastolic Vr in the septum. • Abnormal characteristics of the segmental dynamic velocity evolution were shown in rTOF. • rTOF patients presented altered circumferential rotation and velocity inconsistency in early stage.

On the influence of respiratory motion in radial tissue phase mapping cardiac MRI

PURPOSE

To investigate the impact of respiratory motion on radial tissue phase mapping (TPM) measurements, and to improve image quality and scan efficiency without compromising velocity fidelity by increasing the respiratory acceptance window with and without motion correction.

MATERIALS AND METHODS

A radial golden angle TPM sequence was measured in 10 healthy volunteers in three short axis slices at 3T. Ungated ( CFREE), self-gated with a single acceptance window ( CREF), motion-corrected averaging using all ( CMCall), or selected ( CMC) data reconstructions were compared by means of various image quality measures and resulting velocities.

RESULTS

Using all data ( CFREE) resulted in significantly higher perceived signal-to-noise ratio (SNR) (P < 0.001), but significantly reduced sharpness (P < 0.001) and contrast (P = 0.02), when compared to CREF. Coefficient of variation (CV) and perceived sharpness were not significantly different (P > 0.05). With motion-correction, perceived sharpness could be significantly improved ( CMC: P = 0.002; CMCall: P = 0.002) in comparison to CFREE. Velocity peaks of CFREE were significantly reduced compared to CREF (all peaks: P < 0.001; except the longitudinal "E" peak: P = 0.03). The peak velocities in CMC and CMCall were not significantly different from CREF (all peaks: P > 0.08; except longitudinal "E"/"A" peaks: P > 0.01).

CONCLUSION

Free-breathing reconstruction results in good perceived image sharpness and velocity information with slightly, but significantly, reduced peak velocities. For achieving velocities and image quality comparable to data from a single acceptance window, but higher gating efficiency, selected motion-corrected TPM (CMC) can be applied. J. Magn. Reson. Imaging 2016;44:1218-1228.

Electro-mechanical dysfunction in long QT syndrome: Role for arrhythmogenic risk prediction and modulation by sex and sex hormones

Long QT syndrome (LQTS) is a congenital arrhythmogenic channelopathy characterized by impaired cardiac repolarization. Increasing evidence supports the notion that LQTS is not purely an "electrical" disease but rather an "electro-mechanical" disease with regionally heterogeneously impaired electrical and mechanical cardiac function. In the first part, this article reviews current knowledge on electro-mechanical (dys)function in LQTS, clinical consequences of the observed electro-mechanical dysfunction, and potential underlying mechanisms. Since several novel imaging techniques - Strain Echocardiography (SE) and Magnetic Resonance Tissue Phase Mapping (TPM) - are applied in clinical and experimental settings to assess the (regional) mechanical function, advantages of these non-invasive techniques and their feasibility in the clinical routine are particularly highlighted. The second part provides novel insights into sex differences and sex hormone effects on electro-mechanical cardiac function in a transgenic LQT2 rabbit model. Here we demonstrate that female LQT2 rabbits exhibit a prolonged time to diastolic peak - as marker for contraction duration and early relaxation - compared to males. Chronic estradiol-treatment enhances these differences in time to diastolic peak even more and additionally increases the risk for ventricular arrhythmia. Importantly, time to diastolic peak is particularly prolonged in rabbits exhibiting ventricular arrhythmia - regardless of hormone treatment - contrasting with a lack of differences in QT duration between symptomatic and asymptomatic LQT2 rabbits. This indicates the potential added value of the assessment of mechanical dysfunction in future risk stratification of LQTS patients.

Increased myocardial dysfunction, dyssynchrony, and epicardial fat across the lifespan in healthy males

Background

Evaluation of sensitive myocardial mechanics with speckle tracking echocardiography (STE) across the lifespan may reveal early indicators of cardiovascular disease (CVD) risk. Epicardial adipose tissue (EAT) and left ventricular (LV) myocardial dyssynchrony; subclinical risk-factors of CVD, are of particular clinical interest. However, the evolution of EAT and LV-dyssynchrony across the lifespan, and their influence on myocardial dysfunction remains unclear. We aimed to establish a profile of the healthy aging-heart using conventional, tissue-Doppler imaging (TDI) and speckle-tracking echocardiography (STE), while also exploring underlying contributions from EAT and LV-dyssynchrony towards LV myocardial mechanics, independent of blood biology.

Methods

Healthy males aged 19–94 years were recruited through University-wide advertisements in Victoria and New-South Wales, Australia. Following strict exclusion criteria, basic clinical and comprehensive echocardiographic profiles (conventional, TDI and STE) were established. LV-dyssynchrony was calculated from the maximum-delay of time-to-peak velocity/strain in the four LV-annulus sites (TDI), and six LV-segments (STE longitudinal and circumferential axes). Epicardial fat diameter was obtained from two-dimensional grey-scale images in the parasternal long-axis. Blood biological measures included glycemia, hsCRP, triglycerides, total cholesterol, high-density and low-density lipoprotein levels.

Results

Three groups of 15 were assigned to young (<40 years), middle (40–65 years), and older (>65) aged categories. Five participants were excluded from STE analyses due to inadequate image quality. Decreased longitudinal strain, increased circumferential apical strain and LV twist were age-related. Moreover, independent of blood biology, significant increases were observed across age categories for EAT (young: 2.5 ± 0.9 mm, middle: 3.9 ± 1.0 mm, older 5.7 ± 2.4 mm; p < 0.01), longitudinal STE-dyssynchrony (young: 42 ± 7.7 ms, middle: 58.8 ± 18.9 ms, older 88.6 ± 18.2 ms; p < 0.05), and circumferential-basal STE-dyssynchrony (young: 50.2 ± 20.5 ms, middle: 75.9 ± 20.6 ms, older 97.9 ± 20.2 ms; p < 0.05). These variables collectively explained 37% and 31% (p < 0.01) of longitudinal strain and LV twist, respectively.

Conclusions

This study enabled comprehensive profiling of LV mechanics at different stages of aging using sensitive echocardiographic technology. Novel findings included increased epicardial fat, and both longitudinal and circumferential LV-dyssynchrony across the healthy age groups. These factors may be key underlying contributors to myocardial dysfunction during aging, and their recognition may promote an advanced understanding of early signs of cardiovascular disease.

Cold ischaemic time and time after transplantation alter segmental myocardial velocities after heart transplantation

OBJECTIVES

The aim of this study was to investigate changes in segmental, three-directional left ventricular (LV) velocities in patients after heart transplantation (Tx).

METHODS

Magnetic resonance tissue phase mapping was used to assess myocardial velocities in patients after Tx (n = 27) with normal LV ejection fraction (63 ± 5%) and those without signs of rejection. Regional wall motion and dyssynchrony were analysed in relation to cold ischaemic time (150 ± 57 min, median = 154 min), age of the donor heart (35 ± 13 years, median = 29 years), time after transplantation (32 ± 26 months, median = 31 months) and global LV morphology and function.

RESULTS

Segmental myocardial velocities were significantly altered in patients with cold ischaemic times >155 min resulting in an increase in peak systolic radial velocities (2 of 16 segments, P = 0.03-0.04) and reduced segmental diastolic long-axis velocities (5 of 16 segments, P = 0.01-0.04). Time after transplantation (n = 8 patients <12 months after Tx vs n = 19 >12 months) had a significant influence on systolic radial velocities (increased in 2 of 16 segments, P = 0.01-0.04) and diastolic long-axis velocities (reduced in 5 of 16 segments, P = 0.02-0.04). Correlation analysis and multiple regression revealed significant relationships of cold ischaemic time (R = -0.384, P = 0.048), the donor heart's age (β= 0.9, P = 0.01) and time from transplantation (β= -0.36, P = 0.03) with long-axis diastolic dyssynchrony.

CONCLUSIONS

Time after transplantation and cold ischaemic time strongly affect segmental systolic and diastolic motion in patients after Tx. The understanding of alterations in regional LV motion in the transplanted heart under stable conditions is essential in order to utilize this methodology in the future as a potentially non-invasive means of diagnosing transplant rejection.

Self-navigated tissue phase mapping using a golden-angle spiral acquisition-proof of concept in patients with pulmonary hypertension

PURPOSE

To create a high temporal- and spatial-resolution retrospectively cardiac-gated, tissue phase mapping (TPM) sequence, using an image-based respiratory navigator calculated from the data itself.

METHODS

The sequence was based on a golden-angle spiral acquisition. Reconstruction of real-time images allowed creation of an image-based navigator. The expiratory spiral interleaves were then retrospectively cardiac-gated using data binning. TPM data were acquired in 20 healthy volunteers and 10 patients with pulmonary hypertension. Longitudinal and radial myocardial velocities were calculated in the left ventricle and right ventricle.

RESULTS

The image-based navigator was shown to correlate well with simultaneously acquired airflow data in 10 volunteers(r=0.93±0.04). The TPM navigated images had a significantly higher subjective image quality and edge sharpness (P<0.0001) than averaged spiral TPM. No significant differences in myocardial velocities were seen between conventional Cartesian TPM with navigator respiratory-gating and the proposed self-navigated TPM technique, in 10 volunteers. Significant differences in the velocities were seen between the volunteers and patients in the left ventricle at systole and end diastole and in the right ventricle at end diastole.

CONCLUSION

The feasibility of measuring myocardial motion using a golden-angle spiral TPM sequence was demonstrated, with an image-based respiratory navigator calculated from the TPM data itself.

Volumetric motion quantification by 3D tissue phase mapped CMR

BACKGROUND

The objective of this study was the quantification of myocardial motion from 3D tissue phase mapped (TPM) CMR. Recent work on myocardial motion quantification by TPM has been focussed on multi-slice 2D acquisitions thus excluding motion information from large regions of the left ventricle. Volumetric motion assessment appears an important next step towards the understanding of the volumetric myocardial motion and hence may further improve diagnosis and treatments in patients with myocardial motion abnormalities.

METHODS

Volumetric motion quantification of the complete left ventricle was performed in 12 healthy volunteers and two patients applying a black-blood 3D TPM sequence. The resulting motion field was analysed regarding motion pattern differences between apical and basal locations as well as for asynchronous motion pattern between different myocardial segments in one or more slices. Motion quantification included velocity, torsion, rotation angle and strain derived parameters.

RESULTS

All investigated motion quantification parameters could be calculated from the 3D-TPM data. Parameters quantifying hypokinetic or asynchronous motion demonstrated differences between motion impaired and healthy myocardium.

CONCLUSIONS

3D-TPM enables the gapless volumetric quantification of motion abnormalities of the left ventricle, which can be applied in future application as additional information to provide a more detailed analysis of the left ventricular function.

Combination of tagging and tissue phase mapping to accelerate myocardial motion measurements in three directions

OBJECT

Until now, a three-directional velocity field has mostly been obtained by velocity encoding in three directions, which is very time-consuming and hence not usually used in clinical routine. We show the feasibility of combining in-plane tagging with through-plane tissue phase mapping (TPM) to encode a three-directional velocity field at 3 T with reduced overall acquisition time.

MATERIALS AND METHODS

Assessment of a three-directional velocity field was performed for 10 healthy volunteers. The motion patterns obtained by use of five different sequences including three-directional TPM, TPM in the through-plane direction, TPM in the through-plane direction with horizontal or vertical tagging lines, and TPM in the through-plane direction combined with a tagging grid were evaluated and compared.

RESULTS

A three-dimensional velocity field can be obtained in approximately half the acquisition time by combining through-plane TPM with in-plane tagging. Although the velocity information is derived by different means, differences between the information obtained by three-directional TPM encoding and the suggested technique are only minor.

CONCLUSION

The combination of tagging and TPM enables assessment of the three-directional velocity field in nearly half the time taken when the conventional three-directional TPM sequence is used.