In the last decade 3D and live 3D ultrasound or the so called 4D (3D/4D) in examination of the fetal heart evolved very rapidly with the development of the new technique called Spatiotemporal Image Corelation – STIC, which enables the aquisition of a volume data concomitent with the beating heart. It appears that 3D/4D ultrasound in fetal echocardiography may make an important contribution to the diagnosis of congenital heart disease, to interdisciplinary management, to parental counseling and to medical personal training.
INTRODUCTION
Three dimensional (3D) and four dimensional (4D) applications in the scanning of fetal heart has largely developed in the last ten years. No other organ or system has this progress so evident that as in the fetal cardiovascular system. Congenital heart disease (CHD) is the most common group of malformations in neonates, occuring in 8 per 1000 live births (1). In 2006 the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) published practice guidelines for the screening of CHD during the second trimester of pregnancy and outlined two levels for the screening of low risk fetuses for heart anomalies. The first level is the basic 4-chamber view and the second level is the extended basic scan which includes examination of the arterial outflow tracts (2). Also the term “fetal echocardiogram” refers to a detailed sonographic evaluation of the fetal heart which is performed by a specialist in prenatal diagnosis of CHD.
There are several imaging modalities that can be used to evaluate the fetal heart anomalies, from M-mode techniques, to color Doppler and to the use of the new 3D/4D ultrasonography. The new technological development allows a real-time 3D/4D of the examination of the fetal heart (3). These ultrasound techniques can have a major contribution to the understanding of normal and abnormal fetal heart and can also extend the benefits of the prenatal cardiac screening. With the introduction of the so called “virtual planes” to fetal cardiac examination, we are able to obtain views of the fetal heart that are not generally accesible with the use of standard 2D examination (4). ❑
3D/4D TECHNIQUES IN FETAL CARDIAC EXAMINATION
There are three steps that must be followed when we are using 3D/4D fetal echocardiography: volume aquisition, volume display and volume manipulation.
Volume acquisition
The volume acquisition can be done either static 3D, either on-line 4D (direct volume scan), either spatiotemporal image correlation-STIC- (off-line 4D which is an indirect volume scan).
Spatio-temporal image correlation acquisition (STIC) is an automated volume acquisition with the array of the transducer by performing a slow single sweep, recording a single 3D data consisting of many 2D frames one behind the other. The volume of interest (VOI) is set for a period of time between 7.5 sec to 30 sec (usualy 10 sec), and a sweep angle between 20 to 40° (usually 25°). After this, the system processes the volume data and detect systolic peak which are used to calculate fetal heart rate. The systolic peaks define the heart cycle. The resultant 40 consecutive volumes represents a reconstructed complete heart cycle. One can extract from that volume acquisition any convenient plane we want, which means that from a single volume we can get the image of 4-chamber view, or 5-chamber view, or three vessels etc (5). In other words, the acquisition of a volume is realized in 2 steps: in the first step data are aquired by a single automatic volume sweep and in the second step the system analyzes the data in their spatial and temporal domains and processes a 4D sequence (Figure (Figure1).1). STIC can be used with gray scale, or color Doppler, or power Doppler, or B-flow fetal echocardiography (6).