The Importance of Two-dimensional Echocardiographic Image Quality in Managing Heart Failure

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Citation
European Cardiovascular Disease 2006 - Issue 1;2006:2(1):1-5
DOI
http://dx.doi.org/10.15420/ecr.2006.1.1aa
Background

There are five million patients with a diagnosis of heart failure (HF) in the US, and 550,000 new cases are diagnosed each year. Heart failure hospitalisations have increased 150% over the last 20 years,1 and the lifetime risk of developing HF has been estimated at 20%.2 Transthoracic echo-cardiography is, according to the recently released American College of Cardiology/American Heart Association (ACC/AHA) guidelines for the diagnosis and management of HF, the ÔÇ£single most useful diagnostic test in the evaluation of patients with HF...ÔÇØ3 Echocardiography provides key information for the major aspects of HF management: diagnosis, prognosis and parameters for therapeutic intervention. Compared with other non-invasive imaging modalities, echocardiography is relatively inexpensive and easy to use, but its role in HF management relies crucially on two-dimensional (2-D) image quality. Recent advances in transducer element design using monocrystal PureWave elements have paved the way for a new generation of high resolution, broadband transducers that will offer positive benefits both in qualitative interpretation and diagnostic measurement accuracy. These developments create the possibility for more robust semi-automatic analysis tools that, along with imaging modalities such as tissue Doppler imaging (TDI), promise to play an expanding role in the assessment of HF.

Diagnosis

Transthoracic echocardiography is the most commonly performed imaging test for the detection of impaired ventricular function, but it can also establish the aetiology of HF. HF patients have a long list of causes for their signs and symptoms. Left ventricular (LV) dysfunction leading to pulmonary congestion may result from coronary ischaemia, systemic hypertension, infection and inflammation, toxic exposures, peripartum state, infiltrative disorders, valvular disease, intracardiac shunts, long-standing tachycardia, congenital myocardial lesions or a combination of these disorders. In some cases, no aetiology can be found. Morphologic assessment by echocardiography can provide important diagnostic clues and, often, the definitive diagnosis of the underlying aetiology.

Figures 1-3 demonstrate the importance of image quality in the HF differential diagnosis. Figure 1 shows increased LV wall thickness with a dilated LV cavity, suggestive of the end stage of hypertensive, hypertrophic or infiltrative cardiomyopathy. Like Figure 1, Figure 2 shows apparent increased wall thickness on non-contrast imaging, but contrast infusion separates the true wall thickness from trabeculation. The actual myocardial wall is quite thin, consistent with an aetiology of dilated cardiomyopathy. Figure 3 demonstrates a more profound degree of hypertrabeculation, seen even more clearly on contrast imaging, with deep crypts between the trabeculae and a very thin portion of compacted myocardium. These crypts measure twice the thickness of the myocardium at their base, meeting criteria for a diagnosis of non-compaction of the LV myocardium.

The aetiologic differences between these three cases have important clinical consequences. Accurate measurement of wall thickness is an important component in the determination of LV mass, and image quality is important for determining the true wall thickness by exclusion of trabeculation. Increased LV mass is an independent predictor of poor outcome in patients with reduced ejection fraction (EF).4 Non-compaction is the congenital persistence of embryologic spongiform myocardium, which predisposes to congestive HF from aggressive remodeling and need for heart transplant, thrombus formation within the crypts and sudden cardiac death from ventricular arrhythmias. Thus, the rather subtle morphologic differences in trabeculation illustrated in Figures 1-3 are highly dependant on image quality and establish three quite different diagnoses.

This type of morphologic differentiation illustrates the importance of both detail and contrast resolution. These are components of image quality that are significantly enhanced by the use of PureWave monocrystal technology, a new method of transducer element manufacture using extremely pure, homogenous piezoceramics. New transducers designed with PureWave monocrystal elements are able to yield 80% gains in electromechanical efficiency. This results in extended bandwidth, superior 2-D imaging resolution, more sensitive contrast harmonics, improved colour flow and tissue Doppler sensitivity and superior low frequency tissue harmonics. All of these can significantly improve the discrimination of subtle details and tissue, particularly when addressing technically difficult patients.

Echocardiography also provides important diagnostic information about the presence of intracardiac thrombi in patients with HF. All patients with reduced LV systolic function are at risk for thrombus formation, but the ACC/AHA guidelines recommend anticoagulant therapy for the prevention of thrombus only for patients with large and/or anterior myocardial infarctions.5 The diagnosis of established LV thrombi in HF of any aetiology is an indication for anticoagulation in nearly all patients to prevent peripheral embolisation. However, many of these thrombi are small and easily missed, especially in cases of suboptimal image quality. Echocardiographic contrast for LV opacification and perfusion substantially improves the detection rate for thrombi, as well as the differentiation of thrombi from tumours and normal structures.6,7

Prognosis

Morbidity and mortality in HF patients are closely linked with both EF and LV volumes.8 Therefore, the accurate assessment of these parameters is of paramount importance in prognostication for HF patients. However, echocardiography has traditionally demonstrated considerable inter- and intra-observer variability in the assessment of both EF and LV volumes, as well as reduced accuracy compared with the gold standard of magnetic resonance imaging (MRI).9 Variations in acquisition, geometric assumptions used in the calculation of volumes and subjective assessment on the part of readers play a role, but image quality may account for many of the discrepancies and inaccuracies.

Furthermore, the accurate assessment of EF and LV volumes is hampered by the need for time-consuming off-line measurements. The development of tissue harmonic imaging has improved image quality, but many studies with poor acoustic windows require the application of contrast for LV cavity opacification to improve the accuracy and reproducibility of EF assessments.10,11

By improving the sensitivity and resolution of low frequency tissue harmonic imaging, PureWave transducers provide improved yield for border detection in technically difficult patients and should reduce the number of studies needing left ventricular opacification (LVO) contrast.

Realtime 3-D transthoracic echocardiography may provide the most accurate measurements compared with MRI, as it addresses the important issue of geometric assumptions needed for volume estimation from 2-D image measurements. Second-generation matrix transducers allow realtime acquisition of high-resolution 3-D volumes for live anatomic interrogation of cardiac anatomy and near realtime acquisition of fully sampled, LV datasets for analysis of global and regional volume data.

Until this technology is more widely available, realtime tracking of the endocardial border via acoustic quantification (AQ) algorithms holds promise for automating EF and volumetric measurements, leading to improved reproducibility and rapidity of EF and LV volume determination. Advances in image quality, as well as improvement of the original algorithms for border tracking, promise to increase the sensitivity and specificity of AQ (see Figure 4).13,14 The 2DQ plug-in for QLAB software, which is available both on-cart and as an off-cart tool, allows selection of a new smooth border for endocardial tracking. This uses a sophisticated, multi-factorial algorithm for discriminating tissue from blood pool, with the possibility for selective enhancement and tracking of tissue regions for correction of image non-uniformities. It also offers tissue-tracking tools that interrogate the mitral or tricuspid valve annulus for qualitative assessment of atrioventricular (AV) plane motion and long axis shortening.

Therapeutic Assessment

Echocardiography not only provides diagnostic and prognostic assessments in patients with HF, but also provides critical information to guide the application of HF therapies, again underscoring the importance of high-quality imaging to reduce the errors mentioned above. New HF guidelines recommend that key medication decisions be based on echocardiographic parameters rather than only on clinical signs and symptoms; for example, patients with asymptomatic LV systolic dysfunction should receive ace-inhibitor and beta-blocker medications to prevent or delay the progression to overt clinical HF.3 Improvement in EF and LV volumes as determined by echocardiography is a commonly used endpoint in trials of HF therapies.15,16

Guidelines and reimbursement strategies also rely on echocardiographically determined EF Ôëñ30% as the most common reason for placement of an implantable cardioverter defibrillator for primary prevention of sudden cardiac death.17,18 While current recommendations advise that only patients with EF Ôëñ30%, moderate to severe HF symptoms and a widened QRS on electrocardiagram (ECG) should undergo cardiac resynchronisation therapy (CRT) with bi-ventricular pacemaker implantation, there is recognition that not all patients meeting these criteria will respond to CRT, and that many patients who lack these criteria may benefit from CRT.19 Though definitive conclusions await the results of on-going studies, initial work has shown echocardiographic measurements, using both m-mode and tissue Doppler techniques, can accurately predict beneficial response to CRT (see Figure 5).20-22

Ventricular reconstruction surgery is an option for patients with apical dyskinesis and for patients with significant functional mitral regurgitation (see Figures 6 and 7, respectively). These mechanical techniques, designed to address ventricular and valvular remodelling, have improved both morbidity and mortality in HF patients.23-25 Decision-making for these surgical procedures (and, in the future, for percutaneous valve procedures) relies heavily on accurate echocardiographic determination of apical dyskinesis and mitral valve morphology and regurgitation severity.

Tissue Doppler Imaging Criteria for Selection of CRT Patients

Evidence of intraventricular dyssynchrony can be readily obtained using regional volume curves acquired from 3-D datasets or regional tissue Doppler velocity curves to derive an index of dyssynchrony. Time to peak systolic velocity (Tpsv) for up to 12 segments can be measured using on-cart or off-cart analysis according to the methodology of Yu et al.27 Intraventricular dyssynchrony may be inferred from the maximum difference (MaxDiff) between two regions (e.g. basal septum and basal lateral wall) or the standard deviation of 12 segments (basal and mid segments in both walls in each of three apical views).

The SQ plug-in for QLAB software provides the possibility of defining a multi-region, curved m-line (using user-definable sub-regions) for fast comparison of Tpsv between four or more segments per apical view to provide 12 or more segments in total (see Figure 6 - a pre-CRT TDI case).

Other Uses of Tissue Doppler Imaging in HF Assessment

TDI may also be used to evaluate several other important parameters in HF. For example, the mitral E/EÔÇÖ ratio (mitral inflow E velocity divided by mitral annulus pulsed wave tissue Doppler velocity) can provide an estimate of left ventricular filling pressures to assist in the diagnosis and monitoring of diastolic heart failure.31

Analysis of strain and strain rate, which can be derived from TDI velocity data using the QLAB SQ plug-in, allows identification of localised postsystolic shortening or other markers of ischaemia (such as reduced peak negative strain) for detection of regional contraction abnormalities.32 Unlike tissue Doppler velocity measurements, strain and strain rate data are not prone to the effects of tethering from adjacent segments or overall translational movement and so may be useful in tandem with stress echo in helping to establish an ischaemic aetiology for HF.

Conclusion

Transthoracic 2-D echocardiography is the single most important test in heart failure because it yields information critical to diagnosis, prognosis and therapeutic decision-making. All of this information relies on accurate and reproducible echocardiographic assessments, which, in turn, rely on high-quality images. Recent advances in ultrasound technology, such as PureWave transducers, have resulted in significant gains in the area of image quality.

These advances, along with a new generation of semi-automated quantification algorithms, will be essential components in the provision of quality care to the growing population of HF patients. Ôûá

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