Four-dimensional flow imaging is an evolving cardiovascular imaging technique allowing for revolutionary flow imaging visualization and characterization whose main fields of application are cardiac magnetic resonance imaging (cMRI) and echocardiography ^{[1][2][3][4][5]}[1,2,3,4,5]. Four-dimensional flow echocardiography was introduced in the late 2000s/beginning of the 2010s ^[4][5][6][7][4,5,6,7]. The first 4D echocardiographic techniques were contrast-enhanced ultrasound echocardiographic particle image velocimetry (EPIV) (e.g., the speckle tracking of injected microbubbles using contrast echocardiography) [6] and color-Doppler-based vector flow mapping (e.g., a technique combining information on speckles emerging naturally from the blood and color Doppler information on unidirectional flow along the axial axis of ultrasound beam in an angle dependent way) [7]. Potentialities of these preliminary 4D flow echocardiographic techniques were first tested for the study of the direction of septal shunts [7], for the evaluation of vortices within systemic ventricles [4], and for the understanding of complex flow dynamics in stenotic pulmonary valve [5].

More recently (2019), a new echocardiographic technique was introduced ^{[8][9][10][11][12][13][14][15][16][17][18][19]}[8,9,10,11,12,13,14,15,16,17,18,19]: high-frame-rate blood speckle tracking (BST), using ultrafast ultrasound imaging for blood visualization. BST uses a new non-focal plane wave ultrafast (e.g., 2500–5000 frames per second, reduced on the display to 400–600 frames per second) ultrasound technology ^{[12][13][14][15]}[12,13,14,15]. BST offers the advantages of being fast, angle-independent, non-invasive, and very easy to use ^{[12][13][14][15]}[12,13,14,15]. BST echocardiography may be helpful in the evaluation of complex flow patterns in congenital heart diseases (CHDs) ^{[12][13][14][15]}[12,13,14,15]. BST acquisition requires just a few seconds, like common color Doppler, and imaging re-elaboration is also extremely fast ^{[12][13][14][15]}[12,13,14,15]. Compared to conventional color Doppler, BST allows for a more direct and intuitive visualization of complex flow dynamics and for visualization of vortices that are not identified by conventional Doppler techniques ^{[12][13][14][15]}[12,13,14,15]. Thus, in complex CHD, the use of BST in conjunction with color Doppler may allow a deeper understanding of the physiology of the cardiac defect, without a significant loss of time ^{[12][13][14][15]}[12,13,14,15].

2. Feasibility of BST in Children

2.1. BST Imaging Acquisition Technique

BST acquisition is like common color Doppler, very fast and totally non-invasive ^{[12][13][14][15]}[12,13,14,15]. To acquire a BST movie, it is sufficient going on the color Doppler function, select the BST icon on the screen, and save the image. Re-elaboration of BST saved frames is also very fast and easy. One just needs to press the bottom “show the particles” and the software automatically generates the vortex movie ^{[12][13][14][15]}[12,13,14,15]. The whole process (e.g., imaging acquisition and re-elaboration) will take just a few seconds ^{[12][13][14][15]}[12,13,14,15]. Good-quality BST images may be acquired in normal conditions, without the need for sedation ^{[12][13][14][15]}[12,13,14,15].

2.2. Feasibility in Different Conditions

Feasibility of BST in healthy children and in those with different congenital heart diseases has been proven in studies with good sample sizes ^[11][13][17][11,13,17]. Extremely high (e.g., >99%) feasibility of BST [11] in visualizing flow patterns in the area of interest was firstly proved in 2019 in a study over a mixed population of healthy subjects and fetuses and children with cardiac disease (e.g., 102 subjects, 21 weeks to 11.5 years of age, 4 fetuses, 51 healthy children and 47 children with CHD) [11]. Blood speckle tracking echocardiography furthermore provided accurate for velocity measurements down to 8 cm/s, but compared with pulsed-wave Doppler, BST displayed lower velocities [11]. In another series of 20 infants with CHD [13], BST echocardiography showed its potentialities for a better visualization and deeper understanding of flow dynamics in complex CHD [13] in adjunction with conventional color Doppler. Furthermore [13], it was remarked that BST was highly feasible, reproducible, fast, and easy to use. Other studies ^[15][18][15,18] have proved how feasibility of BST for LV vortex analysis was also very high, varying from 95.6% to 98% ^[15][18][15,18] in healthy children to 93.7% for children with CHD [18]. BST, furthermore, was highly feasible in the evaluation and characterization of aortic flow patterns ^[8][19][8,19], as well as for the quantification of 2D of the left ventricle ^[15][18][15,18] and aortic [19] vortex dimensions. BST offered the advantage of accurate and reproducible quantification of complex and new parameters, such as vorticity [8] and energy loss ^[8][10][8,10], but only with dedicated research software ^[8][10][8,10]. Research software has recently employed (2023) [14] for BST evaluation of left ventricular intraventricular pressure difference (IVPD) in healthy children and in those with different cardiomyopathies with a good feasibility (e.g., feasibility of 88.3% in controls, 80% in children with dilated cardiomyopathy (DCM), and 90.4% in hypertrophic cardiomyopathy) [14].

2.3. Summary of Current Evidence

BST analysis is very easy, fast, reproducible, and accurate for blood flow visualization across the heart chambers and main vessels ^{[8][11][13][17][19]}[8,11,13,17,19]. Analysis of left ventricular vortex ^[15][18][15,18] and aortic flow patterns ^[8][19][8,19] is very feasible in both healthy neonates and children ^{[8][15][18][19]}[8,15,18,19] and in those with CHD. Two-dimensional quantification of vortexes is feasible with current technology ^[18][19][18,19], while more complex analysis (e.g., energy loss, intraventricular pressure difference, vorticity) ^[8][14][8,14] are feasible, but only with dedicated research software.

3. BST for the Evaluation of Vortex in Ventricular Chambers

3.1. Vortex in the Left Ventricle

Vortices naturally form in all cardiac chambers, but have been studied most extensively in the left ventricle (LV) ^[12][15][18][12,15,18], where they have supposed to have the function of a reservoir of kinetic energy facilitating systolic ejection of blood flow into the left ventricular outflow tract. The geometry and anatomical locations of vortices are different in healthy adult subjects and in those with cardiac disease ^[2][3][2,3]. Preliminary observations by contrast-enhanced ultrasound echocardiographic particle image velocimetry (EPIV) in a small sample of 9 adults with a Fontan circulation (mean age 31.5 ± 12 years) [4] showed how height and sphericity index of the vortex in the systemic ventricle were significantly smaller and vortex width larger when compared to 15 age-matched controls. The limited data available in children by BST also demonstrate that LV vortex may differ in children with CHD compared to healthy counterparts [19]. A study in 50 preterm infants (weight 500–2020 g) [15] showed how LV vortex area positively correlated with cardiac dimensions including LV diameters (p < 0.01), and mitral annulus (p < 0.01). In a study [19] of over 60 children with different congenital heart diseases (median age 1.28 years, interquartile range 0.2–6.82 years) and 193 age-matched healthy children, limited differences were noted in vortex distance to apex, distance to interventricular septum, height, width, and sphericity index among CHD and healthy children. Vortex area indexed by body surface area (Vai), however, was significantly higher in children with CHD than healthy subjects (p < 0.0001) [18]. Differences in vortex position among different CHDs were furthermore noted in CHD characterized by left ventricle volume or pressure overload associated with vortices localized closer to the interventricular septum [18] Table 12.