Speckle tracking for the assessment of fetal cardiac function

Two papers in this month's issue of the Journal report the application of advanced ‘speckle tracking’ techniques in the assessment of fetal cardiac function1, 2. Even experienced practitioners in the field of fetal cardiology may lack familiarity with the techniques described. Most will be more used to methods that interrogate the blood pool, such as pulsed, continuous wave and color flow Doppler, or assessment of endocardial border motion by M-mode echocardiography. None of these techniques, however, measures deformation of the myocardium itself. Echocardiographic interrogation of myocardial motion has advanced dramatically due to newer imaging modalities, notably tissue Doppler and speckle tracking, coupled with computerized postprocessing. With regard to tissue Doppler, normal fetal values have been reported for both pulsed wave and color encoded techniques3-6. However, tissue Doppler techniques are significantly limited in that they are one-dimensional and highly dependent on the angle of insonation. Given that fetal lie is unpredictable, angle dependency remains a significant limitation. This is one reason why such techniques are restricted largely to research, with the possible exception of measuring time intervals in the assessment of fetal arrhythmias7, 8. The use of speckle tracking for the assessment of fetal myocardial velocity and deformation is reported in this issue. Speckle tracking depends on the automated identification of unique patterns of ultrasound speckle patterns, often termed ‘kernels’, which can be tracked by the ultrasound system through the cardiac cycle9. Thus, myocardial deformation (strain), the rate of deformation (strain rate), and the velocity and vector of motion can be measured. Major attractions of this technique for the fetal heart are its lack of angle dependence, rapidity of acquisition (because it analyzes conventional gray-scale images) and relatively rapid analysis. Furthermore, speckle tracking can assess regional as well as global myocardial velocities and deformation. Both papers1, 2 report a decrease in right-ventricular (RV) peak systolic longitudinal strain with advancing gestational age, which is matched by a fall in RV longitudinal strain rate. Willruth et al.2 report a gestational age-related increase in global RV myocardial longitudinal peak systolic velocity, with higher velocities recorded at the base of the heart compared with at the apex, consistent with postnatal data. The fact that the analysis technique utilizes conventional gray-scale imaging, which is familiar and readily acquired, represents a major advantage over other techniques. Both papers confirm that the application of speckle tracking to the fetus is feasible but has a number of challenges. During intrauterine life, the fetal electrocardiogram (ECG) cannot be recorded easily so either mechanical events2 or dummy ECG signals1 are required to determine specific points, such as end-diastole, in the cardiac cycle. The frame rates achieved by cross-sectional ECG are also of paramount importance and impact directly on results, particularly with respect to myocardial velocity, diastolic filling patterns and strain rate1. This is exemplified by comparison of high frame rate and low frame rate images1. Other limitations of the speckle tracking technique include the much lower frame rate compared with tissue Doppler imaging and the fact that, for most ultrasound systems, only speckles which remain within the imaging plane can be tracked; speckles which pass through the plane of insonation during the cardiac cycle are not included. Some 3D echocardiography systems now provide the means with which to track speckles through the plane via acquisition of a 3D volumetric dataset10, 11, but there are currently no published data from the fetus using this technique. Perhaps one of the most important differences between speckle tracking techniques and more ‘traditional’ forms of analysis, such as M-mode or Doppler, is the extent to which the newer techniques are dependent on computer analysis. Using M-mode as an example, the operator dictates the image points and the formulae used to derive results, which themselves are not conceptually complex. In contrast, speckle tracking is more of a ‘black box’ and the user may not be aware of the precise impact of factors such as computational differences (natural versus Lagrangian strain) or the spatial and temporal resolution of the technique1. Recent work in children has demonstrated the impact of the use of different proprietary software on restudy of the same patient with respect to deformation imaging12, 13. This limits the ease of comparison of results between and within centers. A key question when applying such techniques is the potential impact on assessment and management of fetuses. In their Discussion, Willruth et al.2 appropriately differentiate between fetuses who may have global cardiac dysfunction related to systemic disease and those with regional cardiac disease, for example aortic valve or pulmonary valve obstruction. This may be particularly important in the context of fetal intervention, providing new insights into the response to such interventions and aiding prognostication. However, isolated valvar obstruction is rare and likely to be studied on a multicenter basis, emphasizing the importance of methodology in comparison of results. The acid test for such techniques is whether they prove to be robust and impact on patient management, or provide novel insights into mechanisms of disease.