Robert S. Ledley, Georgetown University Medical Center Department of Physiology and Biophysics 3900 Reservoir Road, NW, Washington, DC 20007
Using 3-D stereo imaging, both the embryo and the fetus can be optimally visualized. Such in-vivo stereo imaging can be done by means of computerized tomography (CT), magnetic resonance (MR), ultrasound (US), or direct endoscopic viewing. CT is considered invasive for both the embryo and the fetus whereas MR and US are considered non-invasive. Endoscopic viewing is invasive only for the mother. MR requires a large complex machine and therefore cannot be used at the bedside. On the other hand, US can be accomplished with relatively small instrument which is generally used at the bedside. But US has less resolution than CT or MR and the automatic segmentation problems of differentiating between various tissues are more difficult. With endoscopic viewing, details of the internal fetal parts are not easily imaged. All four modalities can be used to produce stereo images of the different structures in the embryo or fetus, although the fetal structures, such as bone, muscle, and organs, are, of course, more differentiated than those in the embryo.
We have developed methods for obtaining stereo US 3-D images by (1) scanning consecutive sequences of sector sections on the patient by using a motorized track guide for the probe, (2) reconstructing the 3-D image in the computer, (3) projecting from this computer image stereo pairs of 2-D images, and (4) displaying and viewing the stereo US 3-D images on a special stereo television (TV) monitor. Quantitative measurements in depth are made interactively by using a stereo 3-D cursor (i.e., a 3-D "flying" mouse called a BAT). The stereo US 3-D images are interactively rotated by repeating steps (3) and (4) above at different angles of projection.
The stereo US images are displayed and measured interactively by using a personal computer, a special 3-D reconstruction engine, and a special TV stereo display unit. The stereo US 3-D in-vivo fetus images are viewed from any arbitrary angle, including views not possible while the patient study is being conducted because of the limitations of the fetus location within the uterus. The BAT is used to make linear and circumferential measurements in depth on the image.
The stereo US 3-D fetus image allows the clinician an excellent opportunity to view the fetus in its spatial relationship to the placenta and to the umbilical cord within the amniotic fluid volume. Most important, this newest non-invasive method of viewing the fetus promises to be a significant addition to the clinician's ability to diagnoses structural and soft-tissue fetal deformities and malformations.
In addition, we have built an endoscope that can be used to directly visualize the fetus in stereo. The endoscope produces stereo pairs of images in real-time and no 3-D reconstruction is required. The Image can not be rotated or otherwise manipulated. However, computer processing can be done on the image including enhancement and making measurements. If a light is placed behind the fetus, some of the internal parts can be viewed with the endoscope using our 'cut-plane' technique. An internal part can be examined even more clearly with the 'cut-plane' method, by having the computer remove overlying and underlying image structures in front of and behind that part.