28 Fetal Three-Dimensional Echocardiography
I. WHY A FETAL SCAN IS A CHALLENGE
A. Obstacles
1. In the pediatric or adult patient, predetermined windows are used to obtain standardized views of the heart.
2. Obstacles to views in the fetus.
a. The size of the heart varies as a function of gestational age.
b. The fetal position can vary during the examination.
c. Fetal movement during the examination can result in not acquiring all of the diagnostic images for analysis.
d. Oligohydramnios can make imaging more difficult.
B. Evaluation
1. For effective conversation with the parents, interpretation must be done in real time, either during the examination or quickly after reviewing digital records.
2. Given these potential obstacles, evaluation of the fetal heart is one of the most challenging diagnostic tasks the fetal examiner encounters during an early second- or third-trimester ultrasound examination.
II. OVERVIEW
A. Superiority to two-dimensional echocardiography
1. Accurate quantification of:
a. Volumes of the right ventricle (RV) and left ventricle (LV).
c. LV ejection fraction without the need for geometric assumptions.
a. Spatial relations between cardiac structures, which is important when assessing complex congenital heart disease.
III. THREE-DIMENSIONAL TERMINOLOGY AND PHYSICS
B. Physics
a. It is generally stated that the human eye and brain retain a visual impression for about 1/10 to 1/30 of a second, equivalent to a frame or volume rate of 10 to 30 Hz.
b. The exact time depends on the brightness and clarity of the image and pattern recognition.
c. When referring to real-time imaging, we are actually talking about whether an imaging system can achieve the visual continuity that will satisfy the human eye and brain.
2. Built-in, spatial, and temporal realignment of 4-D data.
a. It is hard to consistently find a stationary volume of interest in utero due to unwanted patient movements (e.g., fetal activity, maternal respiration) and/or environmental movements such as abdominal deformation from probe movement when using conventional approaches.
b. Real-time 3-D systems can acquire a volume data set without manual displacement of the transducer.
3. Sufficient temporal resolution for the time scale of interest.
a. For general analysis of dynamic anatomy of the fetal heart, a minimum volume rate of about 15 to 25 Hz is required with fetal heart rates of 150 bpm or more. This is equivalent to a maximum of 40 ms for each imaging volume sampling period.
b. Except for isovolumetric contraction and relaxation, other cardiac phases each rarely last longer than 80 ms (rounded numbers are used in this discussion for mathematical simplicity) and can be fully sampled at least once in one imaging volume per cardiac cycle.
c. To achieve 90% accuracy for the measurement, a 500-Hz volume rate may be necessary.
IV. THREE-DIMENSIONAL ECHOCARDIOGRAPHY SYSTEMS
A. Rapidly oscillating cross-sectional transducer
a. Volumetric scanning is less dependent on the angle of acquisition, and thus less dependent on fetal lie or operator expertise, than 2-D scanning.
b. The most promising aspect is the possibility of storing and compressing a volume of 4-D information for later offline evaluation by an expert.
c. Because the heart is beating, nongated acquisition produces artifacts in the reconstructed volume data.
2. Spatiotemporal image correlation (STIC).