Ventricular MechanicsTechniques and Applications

Maria Clara N. Lorca, MD, Henrik Haraldsson, PhD, Karen G. Ordovas, MD, MAS*

sures such as ejection fraction can therefore beinsensitive to these early regional dysfunctions.2

Thus, the assessment of the regional myocardiumfunction with magnetic resonance imaging (MRI)poses as a novel potentially important tool for earlyidentification of cardiac pathology.Regional function of the myocardium can be

quantified using myocardial strain analysis.Myocardial strain is defined as the relative length-ening of the tissue, thus: a normalized measure ofdeformation.3,4 Cardiac MRI (cMRI) is consideredthe reference standard for measurement ofmyocardial strain. Some of the cardiac MRI tech-niques used for measuring regional myocardialfunctions are myocardial steady-state free preces-sion (SSFP) cine, tagging, displacement encodingwith stimulated echoes (DENSE),1 strain encodingimaging (SENC), and feature tracking techniques.2

fore, the main parameters used to quantify strainon MRI are circumferential, longitudinal, and radialstrain (Fig. 1). Another measure used to describedeformation is strain rate, which describes therate at which the strain is changing over time.5

Finally, it is possible to quantify diastolic functionbased on myocardial strain. The main parameterused for quantification of diastolic function is thestrain relaxation index (SRI), which is calculatedbased on the relationship of the circumferentialstrain with strain rate curves.6

TECHNIQUESSteady-State Free Precession Cine

SSFP has been used routinely in clinical practicefor quantification of regional myocardial function.Qualitatively, this technique allows for visual

Department of Radiology and Biomedical Imaging, University of California San Francisco, 505 Parnassus

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.comMagn Reson Imaging Clin N Am 23 (2015) 713 iAvenue, Box 0628, San Francisco, CA 94143, USA* Corresponding author.E-mail address: Karen.ordovas@ucsf.eduThe authors have nothing to disclose.globally in an early stage.1 Normal global mea-structure of the heart; however, the complex ge-ometry of the heart makes this challenging. There-Many cardiac diseases do not affect the heart

preferable to present the strain given the fiberINTRODUCTION

KEYWORDS

Strain Magnetic resonance Cardiac Imagi

KEY POINTS

Regional function of the myocardium can be qu Myocardial strain is defined as the relative lengtdeformation.

The main parameters used to quantify strain onlongitudinal, and radial strain.

Many cardiac magnetic resonance techniques htion of regional strain abnormalities includingdisplacement encoding with stimulated echoes

Recent clinical studies have shown potential usement guidance in patients with congenital andhttp://dx.doi.org/10.1016/j.mric.2014.08.0051064-9689/15/$ see front matter 2015 Elsevier Inc. AllFrom a biomechanical point of view, it would be

Ventricular mechanics

tified using myocardial strain analysis.

ing of the tissue, thus: a normalized measure of

agnetic resonance imaging are circumferential,

e been developed for detection and quantifica-eady-state free-precession cine, tagging, and

these techniques for risk stratification and treat-uired heart diseases.rights reserved. mr

Lorca et al8inspection of myocardial thickening during the car-diac cycle in all cardiac regions. Common cardiacplanes, named short axis, horizontal long axis, andvertical long axis, provide comprehensive assess-ment of myocardial contractility in the anterior,lateral, inferior, and septal regions, including theapex, midventricular level, and base of the heart.Fig. 1. Parameters for cardiac strainquantification. Diagrams illustratemeasurements as changes in distancebetween 2 points aligned to thetransmural myocardial axis for radialstrain (A), aligned to the circumfer-In addition to visual assessment of cardiaccontractility, postprocessing tools allow for quan-tification of myocardial thickening during the car-diac cycle, which consists of the increase inmyocardial wall thickness from end-diastole toend-systole, and is expressed in millimeters. Mea-surement of wall thickening in the short-axis plane

ence of the heart for circumferentialstrain (B), and aligned to the longaxis of the heart for longitudinalstrain (C).

corresponds to the radial strain measurement ofthe entire transmural myocardium, and can be ob-tained in each of the 16 American Heart Associa-tion (AHA) cardiac segments.

Tagging

Tagging is a cardiac magnetic resonance tech-nique introduced by Zerhouni and colleagues7

that noninvasively creates markers in the moving

tissue in order to observe and assess myocardialmotion. Tagging creates visible orthogonal satura-tion lines (by perturbing the magnetization) thatcan be imaged and tracked (Fig. 2).2 Becausemagnetization is an intrinsic property of the under-lying tissue, the tagged lines follow tissue move-ment, therefore allowing for measurement of thedeformation. The main techniques used for strainmeasurement with tagged lines are based onspatial modulation of magnetization (SPAMM).2,8

the(rigir oaft

Ventricular Mechanics 9Fig. 2. Tagging magnetic resonance images acquired attom) levels, during end-diastole (left) and end-systoletags during the cardiac cycle in this patient after repaLease KE, et al. Impaired regional left ventricular strain

2012;35(1):7985; with permission.)basilar (top), midventricular (middle), and apical (bot-ht). Note the distortion of the vertical and horizontalf tetralogy of Fallot. (From Ordovas KG, Carlsson M,er repair of tetralogy of Fallot. J Magn Reson Imaging

This technique has been further developed toreduce the fading of the tag lines fading at end-diastole using complementary SPAMM(CSPAMM). The main advantage of this methodis its extensive validation studies, including animaland clinical studies.912 In addition, a range ofnormal values has already been determined. Dis-advantages of the method are mainly related tothe fact that the tagged lines fade over time andmay not be clearly detected at the end of the car-diac cycle. In addition, postprocessing tools areavailable, but the process is very time consuming.In addition to conventional tag analysis tech-niques, a method of extracting myocardial strainmeasurement from tagging sequences utilizingthe tagging frequency called harmonic phase iswidely used, and is currently considered themost reliable and reproducible method for post-processing of tagged images.1315

analysis. Disadvantages of the method are relatedto the lack of extensive clinical research validatingits applications.

Strain-encoding Imaging

Strain encoding imaging (SENC) is another tech-nique that can be used for measuring myocardialstrain. SENC does this by applying out-of-planetags to modulate the magnetization in the throughplane direction with a predefined frequency.Through-plane contraction would cause this fre-quency to increase, whereas stretch would causeit to decrease. To measure the change of fre-quency, SENC uses readouts with differentthrough-plane phase encodings, and the fre-quency is obtained by straightforward calculationsof the signal of these different phase encodings.16

Feature Tracking

(DEormht.ion

Lorca et al10Displacement Encoding with StimulatedEchoes

Displacement encoding with stimulated echoes(DENSE) is an alternative method to assess car-diac motion with MRI. Tissue displacement ismeasured at the pixel level, providing both in-plane and through-planemotion, and it can be per-formed in a 2-dimensional or 3-dimensionalfashion. In this technique, there are no visualtagged lines generated, but instead the trackingis based on the differences in phase of the MRIsignal between the stationary and the movingspins (Fig. 3). The main advantages of this tech-nique are in its high spatial resolution, its inherentblack blood, which simplifies the delineation ofthe myocardium, and easy postprocessing

Fig. 3. Displacement encoding with stimulated echoesomy, as seen to the left, whereas the phase provides infthe phase is encoded to show displacement left-to-rigment to the right, while the darker inferiolateral reg

have been masked for illustrative purpose.Cardiovascular magnetic resonance featuretracking (FT) is a postprocessing method that de-tects quantitative wall motion derived from SSFPcine imaging.FT measures tissue voxel motion and derivation

of wall mechanics and strain without the need foracquisition of additional sequences.1719

This feature showed reasonable agreement withtagging and acceptable interobserverreproducibility.20,21

CLINICAL APPLICATIONS

The main area of clinical research on strain param-eters measured by cardiac MRI has been ischemicheart disease. It has been shown that systolicstrain magnetic resonance parameters are

NSE). The magnitude in DENSE MRI depicts the anat-ation of the displacement, as seen to the right, whereThe brighter anterioseptal region indicates displace-indicates displacement to the left. The phase data

with congenital heart diseases, particularly tetral-

Ventricular Mechanics 11strongly associated with known risk factors of cor-onary artery disease.22,23 The most importantstudy that evaluated the role of MRI strain imagingas a predictor of heart disease is the CARDIA (Cor-onary Artery Risk Development in Young Adults)arm of the MESA (Multi-Ethnic Study of Athero-sclerosis) trial.5

In the CARDIA study, 1768 patients randomlyselected from the MESA trial database, whichincluded 6814 men and women without heart dis-ease representing 4 racial/ethnic groups, under-went a tagged MRI at baseline and after 5.5 yearsof follow-up. Systolic function as measured bycircumferential strain was shown to predict inci-dent heart failure and cardiovascular events witha hazard ratio of 1.15 (confidence interval [CI]:1.011.31) after adjustment for known cardiovas-cular risk factors.10 An additional study was per-formed with a subset of the CARDIA studypopulation (743 patients followed for 8 years) tointerrogate the role of diastolic dysfunction,measured by SRI calculated from tagged MRI im-ages, as a predictor of incident heart failure andatrial fibrillation.24 The hazard ratio for SRI wascalculated as 2.54 (CI: 1.763.66), and remainedsignificant for the combined heart failure and atrialfibrillation end points as well as for heart failurealone after adjustment for common heart failurepredictors.Clinical applications of myocardial strain mea-

surements on MRI in patients with nonischemiccardiac diseases have also been interrogated toa lesser extent. Circumferential left ventricularstrain has been shown to be abnormal in patientswith arrythmogenic right ventricular dysplasiacompared with normal controls.25 Patients withdilated cardiomyopathy have also been investi-gated with MR strain analysis. A recent study hasshown abnormal peak systolic torsion in 26 pa-tients with dilated cardiomyopathy patientscompared with controls.26 It has also been shownthat circumferential strain improves after partialleft ventriculotomy in patients with dilated cardio-myopathy, despite small improvement in globalleft ventricular (LV) function.27 Therefore, magneticresonance strain analysis has the potential toassess efficacy of dilated cardiomyopathy treat-ments and identify best responders.Cardiac MRI with myocardial tagging can also

help identify early systolic and diastolic dysfunc-tion in hypertrophic cardiomyopathy (HCM)disease, despite normal global functional parame-ters. Ennis and colleagues23 showed circumferen-tial septal strain and diastolic strain in all regionssignificantly reduced in patients with HCM andnormal LV ejection fraction (LVEF). In addition,

tagging was able to identify regional strainogy of Fallot, in an effort to characterize earlymyocardial dysfunction that could inform theneed for pulmonary valve replacement or be aprognostic indicator. Ordovas and colleagues16

have previously demonstrated decreased circum-ferential strain in patients with tetralogy of Fallotand preserved global contractility (normal LVEF)compared with normal controls using taggingMRI (Fig. 4). LV dysfunction has been identifiedas a poor prognostic parameter in patients afterrepair of tetralogy of Fallot.21 Therefore, the au-thors suggested that identification of early subclin-ical LV dysfunction may be an important clinicalparameter to guide early interventions in these pa-tients.16 In addition, wall thickening is particularlyabnormal in the ventricular septum in patientswith abnormal septal excursion during the cardiaccycle, an indication that interventricular interactionmay play an important role in LV dysfunction afterrepair of tetralogy of Fallot.30

In addition, Wald and colleagues31 have quanti-fied strain abnormalities of the right ventricularoutflow tract using tissue tracking MRI, and haveshown that regional abnormalities in the right ven-tricular outflow tract (RVOT) adversely affectglobal right ventricular function and exercise ca-abnormalities in areas of nonhypertrophiedmyocardium of patients with HCM.26,28 Finally,myocardial tagging can aid in the differentiationbetween an athletes heart and asymptomaticnonobstructive hypertrophic cardiomyopathy orhypertensive hypertrophy.27

Another potential important application of mag-netic strain imaging is for depicting and mappingdyssynchrony in patients with decreased LV func-tion. It has been shown in multiple small single-center trials that the presence of dyssynchronypredicts better response to cardiac resynchroniza-tion therapy (CRT), with higher ejection fractionand improvement of symptoms.14 However, alarge multicenter trial using echocardiography fordyssynchrony evaluation (Predictors of Responseto CRT [PROSPECT]) failed to demonstrateimproved survival rates in patients who had CRTbased on documented dyssynchrony, comparedwith patients without evidence of dyssynchronyprior to treatment.5 Tagging has the ability togenerate precise and reproducible dyssynchronymaps for guidance of CRT.29 Future clinical trialsare needed to assess the role of magnetic reso-nance tagging for mortality reduction after CRT.

Congenital Heart Disease

Strain imaging has been investigated in patientspacity after tetralogy of Fallot repair. The authors

Lorca et al12concluded that these regional measures may haveimportant implications for patient management,including RVOT reconstruction, at the time of pul-monary valve replacement.

SUMMARY

Multiple magnetic resonance methods are avail-able for detection and quantification of myocardialstrain abnormalities. Recent clinical studies haveshown potential use of these techniques for riskstratification and treatment guidance in patientswith congenital and acquired heart diseases,particularly for early detection of heart failure.

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Ventricular MechanicsKey pointsIntroductionTechniquesSteady-State Free Precession CineTaggingDisplacement Encoding with Stimulated EchoesStrain-encoding ImagingFeature Tracking

Clinical applicationsCongenital Heart Disease

SummaryReferences