Advances in Noninvasive Fetal Imaging
Wesley Lee, MD and Anil Shetty, PhD
Department of Obstetrics and Gynecology, Baylor College of Medicine/Texas Children's Pavilion for Women, Houston, Texas, USA
Birth defects have been the leading cause of infant mortality in the United States for the past 20 years – accounting for 1 in 5 infant deaths. According to the March of Dimes, at least 120,000 babies are born each year with major structural birth defects in the United States. Our presentation will review and demonstrate state-of-the-art medical imaging techniques that permit noninvasive assessment of the human fetus for prenatal diagnosis.
2D ultrasonography has been the primary means for detecting fetal structural anomalies – typically during a mid-trimester scan at 20 weeks gestation. Improved ultrasound resolution from the use of higher frequency ultrasound transducers, innovative engineering developments, and better signal processing have allowed physicians to more accurately characterize fetal structural anomalies. For example, high frequency endovaginal transducers can provide excellent images of the embryo even during the first trimester of pregnancy. Doppler ultrasonography can also be used to detect the direction and velocity of fetal blood flow.
3D ultrasound requires rapid ultrasound volume acquisition techniques that are now possible through advances in image processing software algorithms, micro-fabrication techniques and nanotechnology that are constantly improving over time. This 3D technology adds another dimension to 2D image pixel elements for creating volume pixels or "voxels". These ultrasound voxels are then placed into volume datasets that are processed, examined, segmented, manipulated, and visualized using off-line image analysis software. Volume imaging provides qualitative and quantitative information not possible by conventional 2D ultrasound. Good quality voxel data can be analyzed with several visualization techniques that include multi-planar imaging with arbitrary cutting planes, threshold and transparency tools, surface and volume rendering, tomographic slicing, and thick slice display techniques.
4D ultrasonography (4DUS) adds the element of time to ultrasound volume datasets for dynamically moving organs such as the fetal heart. Dynamically acquired ultrasound volumes are possible through spatiotemporal image correlation that is based on a fast Fourier transformation of timing information extracted from fetal cardiac motion. Newer matrix array ultrasound transducers also allow improvements with electronic beam steering for arbitrary cutting planes and faster real-time volume acquisition. However, ultrasound may not provide sufficient details for physicians to satisfactorily characterize fetal anatomy due to limitations from acoustic shadowing by surrounding dense structures such as bone.
As a complementary imaging modality, magnetic resonance imaging (MRI) uses the body's natural magnetic properties to produce detailed images of the fetus that are not possible by ultrasonic imaging. Many different MRI settings (e.g. conventional T2 and T1 weighted images) can be used to obtain these fetal images. Advanced MR techniques permit assessment of water diffusion through biological tissues (diffusion weighted imaging), white matter tract development of the brain (diffusion tensor imaging), and even clues about their metabolic environment by using magnetic resonance spectroscopy (MRS) that is based on similar principles of nuclear magnetic resonance.
All of these non-invasive imaging methods have provided important ways for clinicians to evaluate the development, structure, and growth of fetuses. Further novel advances in our ability to evaluate ultrasound and MR based images of the fetus may benefit from translating currently existing or well established post-processing image analysis techniques that are currently being used in the petroleum industry.
AAPG Search and Discovery Article #90206 © AAPG Hedberg Conference, Interpretation Visualization in the Petroleum Industry, Houston, Texas, June 1-4, 2014