Three-dimensional ultrasonography (3DUS) can be combined with conventional two-dimensional ultrasonography (2DUS) for the evaluation of normal and abnormal fetal anatomy. Development of cost-effective high-performance computers and sophisticated image analysis software now makes it practical for 3DUS to be conveniently integrated with standard diagnostic imaging equipment. The previous chapter summarized basic concepts regarding the acquisition and fundamental concepts of ultrasound volume data. This overview will describe how this information can be applied to specific problems during pregnancy.
Although 3DUS applications have been described for over 2 decades, one of our initial challenges was to evaluate their clinical value against a backdrop of emerging technology. In 2005, a literature review of more than 500 related articles suggested that additional diagnostic information was particularly useful for facial anomalies, neural tube defects, and skeletal malformations.1 Other potential benefits involved early pregnancy evaluation, nuchal translucency measurements, weight estimation, fetal lung volumetry, growth evaluation, and possibly maternal–fetal bonding. Technical developments, such as spatiotemporal image correlation (STIC) algorithms, were also cited for improving our ability to examine rapid changes in the fetal heart. Chapter 45 will more closely examine the emerging field of four-dimensional ultrasonography.
IMPORTANT CONCEPTS FOR VOLUME SONOGRAPHY
We have witnessed an impressive refinement of image analysis tools and their migration from dedicated image workstations to notebook computers. This technology permits the examiner to choose among many different visualization techniques with a variety of output display modes as summarized in Chapter 43. In most cases, volume sonography can be generally considered as an important diagnostic technique that is complementary to conventional 2D imaging. Before specific examples of how 3DUS can be applied to pregnant women are reviewed, we will first describe a few general concepts for how 3DUS can be best applied during pregnancy.
Use 2D Ultrasonography for an Initial Diagnostic Impression
Health care professionals are classically trained to translate 2D images into volume reconstructions for an improved understanding of spatial relationships and image patterns that suggest congenital abnormalities. Volume sonography provides the examiner with an opportunity to systematically evaluate anatomic structures with less interobserver dependency, sometimes in ways that are not possible through conventional 2D methods. A preliminary 2D sonographic scan for suspected fetal abnormalities can be very useful, and the results can be used to guide the subsequent 3D study. Once an initial differential diagnosis has been generated, the examiner should be able to formulate pending questions about the initial diagnostic impression that may or may not be satisfactorily addressed using 3DUS. If spina bifida is detected, for example, there may be a need to more precisely define the anatomic level of the defect. This information would help the clinician to estimate the risk of problems related to ambulation and bowel or bladder function.
In order to better understand the contribution of conventional imaging to 3DUS, Gonçalves et al2 compared both modalities ...