How to Acquire Cardiac Volumes for Sonographic Examination of the Fetal Heart: Part 2

Abstract

The effective performance of fetal cardiac examination using spatiotemporal image correlation (STIC) technology requires 2 essential steps: volume acquisition and postprocessing. An important prerequisite is training sonologists to acquire high-quality volume data sets so that when analyzed, such volumes are informative. This article is part 2 of a series on 4-dimensional sonography with STIC. Part 1 focused on STIC technology and its features, the importance of operator training/experience and acquisition of high-quality STIC volumes, factors that affect STIC volume acquisition rates, and general recommendations on performing 4D sonography with STIC. In part 2, we discuss a detailed and practical stepwise approach for STIC volume acquisition, along with methods to determine whether such volumes are appropriate for analysis.

Keywords: 4-dimensional; STICLoop; fetal echocardiography; fetus; spatiotemporal image correlation; ultrasound.

Figure 1:

Multiplanar display of a STIC volume (normal fetal heart). The ROI box around the acquisition plane (apical four-chamber view) determines the height (y-plane) and width (x-plane) of the volume. Note that the box encompasses the entire fetal chest circumference. The B plane (sagittal image; upper right corner) demonstrates the acquisition angle of the volume, which determines the acquisition depth. Since the reference dot has been placed in the cross-section of the descending aorta in the A plane, the longitudinal descending aorta is visible in both the B and C planes. ROI, region of interest; STIC, spatiotemporal image correlation.

Figure 2:

Apical four-chamber view with the fetal spine positioned at different locations: A) 7 o’clock; B) 6 o’clock; C) 5 o’clock. Such positions reduce the possibility of acoustic shadowing from the fetal ribs or spine.

Figure 3:

“Driving the transducer” technique in a vertex fetus. A) Spine originally located at 8 o’clock in the apical four-chamber view; B) the transducer is driven on the maternal abdomen (toward the fetal right side) in a fixed arc until it lies above the cardiac apex; C) on the monitor screen, the fetal spine has “converted” to a more posterior position (6 o’clock), and the cardiac apex is now “up”; D) spine located at 5 o’clock in the apical four-chamber view; E) the transducer is driven leftward on the maternal abdomen (toward the fetal left side) in a fixed arc until it lies above the cardiac apex; F) on the monitor screen, the fetal spine has “converted” to a more posterior position (6 o’clock), and the cardiac apex is now “up.” L, fetal left; R, fetal right.

Figure 4:

“Driving the transducer” technique in a breech fetus: A) Spine originally located at 8 o’clock in the subcostal four-chamber view. The transducer is driven on the maternal abdomen (to operator’s right, toward the fetal left side) in a fixed arc until it lies above the cardiac apex; B) on the monitor screen, the fetal spine has “converted” to a 6 o’clock position, and the apical four-chamber view is visible.

Figure 5:

STIC volume acquired from the subcostal four-chamber view, in which the spine was originally located at 4 o’clock. The acquisition plane (A plane) has been manually rotated on the z-axis so that the spine location is at 6 o’clock. As a result, the B plane image (ductal arch) becomes more blurred or “waxy” in appearance with diminished image clarity. STIC, spatiotemporal image correlation.

Figure 6:

During the STIC volume acquisition, fetal breathing occurred at the beginning, leading to a motion artifact in the area of the upper mediastinum. As a result, there is distortion of the 3-vessel and trachea view, as evidenced in the acquisition plane (upper left corner). Note that the pulmonary artery and ductus arteriosus (ductal arch view, B plane) appears distorted, and the anatomy cannot be assessed with confidence. STIC, spatiotemporal image correlation.

Figure 7:

Transverse view of the fetal upper mediastinum, demonstrating the transverse aortic arch (“dolphin”), cross-section of the superior vena cava, and cross-section of the trachea. Just immediately before beginning a STIC volume acquisition of the four-chamber view, one should tilt the transducer to ensure that the transverse aortic arch is also clearly visualized. STIC, spatiotemporal image correlation.

Figure 8:

Transverse section of the fetal thorax demonstrating a true axial plane of the apical four-chamber view (vertex presentation). Note that the size of the fetal lungs is symmetrical, and the left atrium and ventricle are not foreshortened in this insonation.

Figure 9:

Staircase spine (downstairs type). In the A plane (transverse view) of the STIC volume, note that the ossification centers of the spine appear “stacked” upon each other like a staircase or a caterpillar, and a coronal view of the curved ribs can be seen because the ossification centers are being imaged obliquely. The staircase spine is confirmed in the B plane, which shows the fetus inclined vertically downward. During a STIC volume acquisition, the ossification centers will appear to be “moving” in a vertical direction on the monitor screen. STIC, spatiotemporal image correlation.

Figure 10:

Staircase spine (upstairs type). With the fetus tilted upward (feet raised higher than the head), this position leads to an upstairs spine. In this image, the fetal head is located on the right side (not in view). The fetal bladder is visualized on the left side.

Figure 11:

Example of motion artifacts in the wall of the longitudinal descending aorta in a STIC volume dataset. The left image shows a straight wall, consistent with absence of fetal breathing during the STIC acquisition. However, the right image shows a “wavy lasagna wall,” indicating regular fetal breathing during the STIC acquisition. Thus, this volume is not appropriate and should be discarded. STIC, spatiotemporal image correlation.

Figure 12:

Overall practical and step-wise approach to performing 4D sonography with spatiotemporal image correlation (STIC). See text for further details.

Figure 13:

Two methods to evaluate STIC volumes and determine their appropriateness for analysis (using multiplanar display and STICLoop). See text for further details. ROI, region of interest; STIC, spatiotemporal image correlation.