NEW YORK — Three-dimensional ultrasound represents an emerging advance in imaging with important applications in obstetrics, Alfred Z. Abuhamad, M.D., said at an obstetrics symposium that was sponsored by Columbia University and New York Presbyterian Hospital.
The ability to rotate images, change planes, and manipulate displays according to signal strength makes it possible to visualize skeletal and vascular structures and fluid spaces, in addition to providing detailed views of fetal appearance, according to Dr. Abuhamad, who is professor and chair of obstetrics and gynecology and the director of the division of maternal-fetal medicine at Eastern Virginia Medical School, Norfolk.
The term “3-D” is something of a misnomer in that the image is displayed on a 2-D monitor.
“[Instead of '3-D'] the term should be volume sonography that gives the appearance of depth,” Dr. Abuhamad said.
The volume image is created by the summation of 2-D slices from multiple planes, as the probe is steered from side to side.
The size of the acquired image is determined by the angle across which the probe is moved. Since fetal movement makes the speed of volume acquisition highly important, “we use the smallest angle [needed for evaluation of the structure of interest] in the smallest box,” Dr. Abuhamad said.
With a multiplanar display, an image constructed from sagittal, coronal, and transverse planes can be rotated along the x-, y-, and z-axis to visualize the same structure from different angles. To maintain orientation, it is helpful to determine a reference point, and it may be necessary to use 2-D ultrasound to locate key structures.
The surface display shows the external aspects of the fetus, allowing the same views as in 2-D ultrasound, to review in “tremendous detail” such fetal abnormalities as clefting of the lip and palate, he said during the meeting.
With “maximum mode,” which manipulates the signal to enhance light (i.e. echoic) objects and dim dark (anechoic) ones, skeletal structures can be visualized, affording a look at the cranium and its fontanelles and sutures.
It also facilitates assessment of bone quality and detection of fractures and permits close examination of the vertebral column.
“It's like an x-ray of the fetus,” Dr. Abuhamad commented.
“Minimum mode” reveals vasculature; while “inversion mode,” which dims light structures and highlights dim ones, brings out fluid cavities and makes it possible to visualize such structures as the chambers of the heart and determine the number of gestational sacs, he said.
Other image manipulations permit the clinician to see the back of structures and to remove from the image, as with an “electronic scalpel,” structures that may obscure features of interest.
The 3-D procedure does not use more power, increase fetal exposure, or magnify the thermal effect, compared with 2-D ultrasound, he said.
Limitations of the technique include a steep learning curve and the need to dedicate sufficient time for training.
The quality of the volume image is limited by 2-D resolution. Artifacts such as motion of the woman or fetus, surface rendering, and shadowing can interfere with interpretation.
The technique is highly operator dependent, and the lack of standardization magnifies the possibility of human error, Dr. Abuhamad said.
An estimated 10% of ultrasound units currently have this technology, he said.
This 3-D image shows an absent T12 rib on one side in a fetus with balanced translocation.
Swelling involving the dorsal aspect of both feet in a fetus with Turner syndrome is shown here on 3D ultrasound. Photos courtesy Dr. Alfred Z. Abuhamad