Technical Aspects of the First Trimester Ultrasound Examination
INTRODUCTION
Over the past two decades the detailed ultrasound examination of a fetus before the 16th week of gestation was made possible by two important events: the widespread adoption of first trimester risk assessment with nuchal translucency (NT) and the improvement in ultrasound imaging with enhanced resolution and image processing. High-resolution transabdominal and transvaginal transducers provide images of the fetus in the first trimester with such quality that allows for detailed anatomic evaluation. In addition, the use of sensitive color and high-definition power Doppler improved the visualization of the fetal cardiovascular system, including small peripheral vessels. The widespread use of three-dimensional (3D) ultrasound technology added a new approach to fetal imaging through the acquisition, display, and post-processing of 3D volumes. The embryo can now be imaged on ultrasound from about the sixth week of gestation and detailed anatomic evaluation of the fetus can be performed from about the 12 weeks of gestation onward. This chapter provides an overview of the technical aspects of ultrasound examination in the first trimester.
TWO-DIMENSIONAL GRAY SCALE ULTRASOUND
The quality of the two-dimensional (2D) ultrasound image is dependent on several factors including the choice of transducers, system settings (image presets), access to the anatomic region of interest, and magnification of the target region of interest (see Table 3.1).
Ultrasound Transducers
Ultrasound manufacturers offer a wide range of transducers to choose from. Only a few transducers are optimally suited for imaging the first trimester pregnancy however. Most obstetric transducers have a frequency range between 2 and 12 MHz. Transabdominal and transvaginal transducers that are used in the first trimester of pregnancy are discussed in detail in the following sections.
Transabdominal Transducers
Two groups of transabdominal transducers are used in obstetric scanning: transducers with low frequency range (2 to 5 MHz), which allow for good tissue penetration of sound and acceptable image resolution, and transducers with high frequency range (5 to 9 MHz), which allow for improved resolution but with limited tissue penetration of sound. The authors recommend the use of high frequency range transducers in the first
trimester when available and technically feasible, as this enables a detailed anatomic evaluation of the fetus in keeping with existing guidelines1,2 (see Chapter 1). In the first trimester, the use of high frequency transducers provides adequate imaging, thus allowing for optimal nuchal and intracranial translucency evaluation along with clear visualization of fetal organs such as brain, heart, lungs, stomach, kidneys, and bladder. The general contour of the fetus with the surrounding amniotic fluid can be imaged (Fig. 3.1A), in addition to the skeletal system to include the skull, nasal bone, ribs, spine, and limbs (Fig. 3.1A-E). Limitations of transabdominal high frequency transducers are encountered when the fetus is deep in the pelvis. Recently, linear transducers, that are commonly used for soft tissue imaging in radiology, have been adapted to obstetric imaging.3 These linear transducers are desirable because of their high resolution with good tissue penetration of sound. Unlike the curved array transducers, the linear transducers have ultrasound beams that are uniform throughout all tissue levels and do not diverge in deeper tissue. We have found linear transducers to be well adapted for first trimester ultrasound imaging and can provide detailed anatomic evaluation of the fetus (Fig. 3.2) with comparable resolution to that of transvaginal transducers.4
trimester when available and technically feasible, as this enables a detailed anatomic evaluation of the fetus in keeping with existing guidelines1,2 (see Chapter 1). In the first trimester, the use of high frequency transducers provides adequate imaging, thus allowing for optimal nuchal and intracranial translucency evaluation along with clear visualization of fetal organs such as brain, heart, lungs, stomach, kidneys, and bladder. The general contour of the fetus with the surrounding amniotic fluid can be imaged (Fig. 3.1A), in addition to the skeletal system to include the skull, nasal bone, ribs, spine, and limbs (Fig. 3.1A-E). Limitations of transabdominal high frequency transducers are encountered when the fetus is deep in the pelvis. Recently, linear transducers, that are commonly used for soft tissue imaging in radiology, have been adapted to obstetric imaging.3 These linear transducers are desirable because of their high resolution with good tissue penetration of sound. Unlike the curved array transducers, the linear transducers have ultrasound beams that are uniform throughout all tissue levels and do not diverge in deeper tissue. We have found linear transducers to be well adapted for first trimester ultrasound imaging and can provide detailed anatomic evaluation of the fetus (Fig. 3.2) with comparable resolution to that of transvaginal transducers.4
Table 3.1 • Image Optimization for Two-Dimensional Ultrasound in Gray Scale in the First Trimester | ||||||||||||||||||||||
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 Figure 3.1: Various planes (A-E) obtained by a transabdominal curved array high-resolution transducer in fetuses at 12 to 13 weeks of gestation. Plane A represents a midsagittal view of the fetus obtained for measurement of crown-rump length, nuchal and intracranial translucency, and for visualization of the nasal bone. Plane B is an axial view of the head. Plane C is a frontal facial view. Plane D shows two lower limbs and plane E represents the four-chamber view. |
Transvaginal Transducers
When ultrasound imaging is suboptimal by the transabdominal approach due to an increased distance between the transducer and target anatomic region (Fig. 3.3A), or when a suspected abnormality is noted, the transvaginal approach is recommended (Fig. 3.3B and C). The main advantage of the transvaginal approach is the short distance of the ultrasound beam to the region of interest, thus allowing for the use of higher frequency transducers with better resolution (Fig. 3.4). Transvaginal transducers have in general a range between 5 and 12 MHz. In the experience of the authors, fetuses with crown-rump length (CRL) greater than 65 mm are often well imaged by transabdominal transducers, whereas fetuses
between 10 and 12 weeks of gestation and embryos before 10 weeks of gestation are better imaged by the transvaginal approach. It has also been our experience that the NT and nasal bones are imaged easily with transabdominal transducers. Figures 3.5 and 3.6 display the fetal abdomen and face respectively with the transabdominal curvilinear, transabdominal linear, and transvaginal transducers. Note that the three transducers provide adequate imaging of upper abdominal structures (Fig. 3.5), whereas the linear and transvaginal transducers provide superior imaging for complex anatomic regions such as the facial profile (Fig. 3.6).
between 10 and 12 weeks of gestation and embryos before 10 weeks of gestation are better imaged by the transvaginal approach. It has also been our experience that the NT and nasal bones are imaged easily with transabdominal transducers. Figures 3.5 and 3.6 display the fetal abdomen and face respectively with the transabdominal curvilinear, transabdominal linear, and transvaginal transducers. Note that the three transducers provide adequate imaging of upper abdominal structures (Fig. 3.5), whereas the linear and transvaginal transducers provide superior imaging for complex anatomic regions such as the facial profile (Fig. 3.6).
 Figure 3.2: Various planes (A-F) obtained by a transabdominal high-resolution linear transducer in fetuses at 12 to 13 weeks of gestation. Compare with Figure 3.1. Plane A represents a midsagittal view of the fetal head. Plane B is a frontal facial view. Plane C shows the intracerebral structures. Plane D shows a hand with digits. Planes E and F show a sagittal and coronal view of the fetal spine respectively with fetal kidneys noted in plane F. Note the high resolution of these images as compared to images in Figure 3.1. |
 Figure 3.3: A: A fetus at 12 weeks of gestation scanned transabdominally with color Doppler at the three-vessel trachea view. Note that the image displays decreased resolution, primarily due to the long distance between the transducer and the region of interest; upper fetal chest in this case (yellow arrow). Note also that the fetus is deep in the pelvis and near the cervix (white arrow). B: A transvaginal view showing that the fetus is in a transverse lie, an ideal fetal position for a transvaginal ultrasound examination. C: A transvaginal ultrasound in color Doppler at the three-vessel trachea view showing improved resolution over the transabdominal approach in A. Planes B and C are obtained in the same fetus as plane A. |
 Figure 3.4: Various planes (A-F) obtained by a transvaginal high-resolution transducer in fetuses at 11 to 13 weeks of gestation. Compare with Figures 3.1 and 3.2. Plane A represents a midsagittal view of the fetal head. Plane B shows the intracerebral structures. Plane C shows a midsagittal view of the spine. Plane D is a four-chamber view of the fetal heart. Plane E shows a fetal hand with digits and plane F is a coronal view of the chest and abdomen showing the fetal kidneys. Note the high resolution of these images as compared to images in Figures 3.1 and 3.2. |
Image Presets
Image presets influence the quality of the displayed image on the monitor of the ultrasound system. The gray scale image presets should be adapted according to the selection of the transducer. For imaging in the first trimester, we generally recommend a high-resolution image with high line density, in combination with harmonic imaging. Despite recommendations to the contrary for NT measurements, we recommend compound imaging as well as speckle reduction, for imaging of fetal anatomy in the first trimester. A wide image angle is recommended at the initial part of the ultrasound examination in order to measure the CRL and to assess for any gross abnormalities. The image angle however should be narrowed in order to examine selective anatomic regions of the fetus, such as the brain or heart. A narrow angle provides a higher image quality with good frame rate.
Technical Skills
The technical skills of the operator performing the first trimester ultrasound examination play a critical role in the quality of images. In general, the operator performing the first trimester ultrasound should be well versed in the second trimester examination and should adapt its approach to early gestation. A systematic approach to the first trimester ultrasound, as shown in Chapter 5, standardizes the examination approach and provides consistency in image display. In contrast to ultrasound imaging in the second trimester, the small size of the fetus and the relatively flat maternal abdomen limits the insonation angles in early gestation. Increased mobility of the fetus in the first trimester however commonly overcomes this
obstacle as it provides various approaches to imaging within a relatively short time frame. Asking the mother to cough or to walk around for few minutes can often lead the fetus to move and change position. Furthermore, applying gentle pressure with the transducer during the transabdominal ultrasound examination may shorten the distance to the fetus and improves imaging. With the transvaginal approach, the transducer should be inserted gently into the vaginal canal, thus making the examination well tolerated by most women.5 Following the introduction of the transvaginal transducer, the operator should visualize the entire uterine cavity, including the fetus, without magnification. Following this overview, the region of interest can be magnified to optimize imaging and to get detailed anatomic assessment. Occasionally, a gentle manipulation of the uterus with the other hand placed on the maternal abdomen can lead to a change in the position of the fetus and brings the region of interest into the focus region.
obstacle as it provides various approaches to imaging within a relatively short time frame. Asking the mother to cough or to walk around for few minutes can often lead the fetus to move and change position. Furthermore, applying gentle pressure with the transducer during the transabdominal ultrasound examination may shorten the distance to the fetus and improves imaging. With the transvaginal approach, the transducer should be inserted gently into the vaginal canal, thus making the examination well tolerated by most women.5 Following the introduction of the transvaginal transducer, the operator should visualize the entire uterine cavity, including the fetus, without magnification. Following this overview, the region of interest can be magnified to optimize imaging and to get detailed anatomic assessment. Occasionally, a gentle manipulation of the uterus with the other hand placed on the maternal abdomen can lead to a change in the position of the fetus and brings the region of interest into the focus region.
 Figure 3.5: Axial views of the fetal abdomen at 12 weeks of gestation in three fetuses (A-C) using three different high-resolution transducers: A—transabdominal curved array, B—transabdominal linear, and C—transvaginal. Note the increased resolution in planes B and C. St, stomach. |
 Figure 3.6: Midsagittal views in three fetuses at 12 to 13 weeks of gestation, imaged with three different high-resolution transducers: A—transabdominal curved array, B—transabdominal linear, and C—transvaginal. Note the increase in resolution and tissue characterization in C as compared to A and B. Also note that the nasal bone (arrows) has sharp borders in B and C, as compared to blurred borders in A. When fetal malformations are suspected, the transvaginal approach provides more detailed assessment of fetal anatomy in early gestation. |
COLOR AND PULSED DOPPLER
Color and pulsed Doppler ultrasound has been useful in the evaluation of the first trimester pregnancy. The application of color Doppler has been shown to be helpful in the assessment of the fetal cardiovascular system (Fig. 3.7) and in guiding placement of pulsed Doppler for the study of fetal vasculature. It is important to note that color and pulsed Doppler application involves higher energy than conventional gray scale imaging and its prudent application in early gestation is recommended. Respecting the ALARA (as low as reasonably achievable) principle (described in Chapter 2), and using color Doppler when indicated and in a standardized fashion, allow its safe application in the first trimester. Pulsed Doppler application across the tricuspid valve and ductus venosus (DV) has been used to assess aneuploidy risk and to screen for congenital heart disease. The authors however recommend the limited use of pulsed Doppler in the first trimester to specific indications, given its increased focused energy. In our experience, the prudent application of color Doppler selectively on few anatomic planes in the first trimester helps to complete the assessment of fetal anatomy. Color Doppler is especially important for the assessment of fetal cardiac anatomy in the first trimester (Fig. 3.8).
 Figure 3.7: A: An axial plane of the fetal chest at 13 weeks with the application of pulsed-wave Doppler on the heart to demonstrate and document cardiac activity. The authors do not recommend this practice given the increased energy associated with pulsed-wave Doppler. It is recommended to use M-mode or to save a gray scale movie clip for this purpose (see Chapter 2). When color Doppler is indicated, an application of the color box over the fetus (B) can document cardiac activity and demonstrate an intact anterior abdominal wall (arrow) and a normal course of the ductus venosus (DV). |
Color Doppler Presets
The most common use of color Doppler in the first trimester is for the examination of the fetal heart and occasionally for the visualization of umbilical arteries, the umbilical vein, and the DV. Ideally the examiner has to be familiar with the optimization of the ultrasound equipment in order to properly examine the heart in early gestation.6 Improper use of color Doppler of the fetal heart bears the risk of false-negative or false-positive diagnoses. The optimum color Doppler image is a compromise between image quality and frame rate. Optimizing the gray scale image is essential before the application of color Doppler. Choosing the smallest color box needed for your target anatomic region will ensure the highest frame rate possible for
the ultrasound examination. Velocity scale or pulse repetition frequency is used to determine the range of mean velocities within the color box. For color Doppler interrogation of the cardiac chambers and the great vessels, a high velocity range (>30 cm per second) should be selected. For the examination of the umbilical arteries and veins, renal arteries, or other fetal peripheral vasculature, lower velocity ranges should be selected (5 to 20 cm per second). Table 3.2 summarizes the presets that we commonly use for color Doppler application in the first trimester. For a more comprehensive presentation on this subject, the readers are referred to our previous work on the optimization of the color Doppler ultrasound examination of the fetal heart.4
the ultrasound examination. Velocity scale or pulse repetition frequency is used to determine the range of mean velocities within the color box. For color Doppler interrogation of the cardiac chambers and the great vessels, a high velocity range (>30 cm per second) should be selected. For the examination of the umbilical arteries and veins, renal arteries, or other fetal peripheral vasculature, lower velocity ranges should be selected (5 to 20 cm per second). Table 3.2 summarizes the presets that we commonly use for color Doppler application in the first trimester. For a more comprehensive presentation on this subject, the readers are referred to our previous work on the optimization of the color Doppler ultrasound examination of the fetal heart.4
 Figure 3.8: Images of the fetal heart at 11 to 13 weeks of gestation, examined with color Doppler ultrasound. A: Diastolic flow from both right (RA) and left (LA) atrium into the right (RV) and left (LV) ventricle, respectively. B: A normal three-vessel trachea view with aorta (Ao) and pulmonary artery (PA). C: An oblique view showing both left and right ventricular outflow tract in systole with the crossing of Ao and PA. |
Regions of Interest for Color Doppler Application
The same anatomic regions of interest examined in the second trimester can also be applied in the first trimester. It is important to note that not all second trimester anatomic regions have the same clinical importance or are easy to image on color Doppler in the first trimester. We hereby present important anatomic regions for the first trimester color Doppler application.
Table 3.2 • Image Optimization for Color Doppler Ultrasound in the First Trimester | ||||||||||||||||||
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