CHAPTER 3 • Technical Aspects of the First Trimester Ultrasound Examination. First Trimester Ultr

 CHAPTER 3 • 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

Table 3.1 • Image Optimization for Two-Dimensional Ultrasound in Gray

Scale in the First Trimester

Choose a high frequency transducer when possible

Consider using linear and transvaginal high-resolution transducers

Use the other hand to gently manipulate the uterus when performing a

transvaginal ultrasound

Combine harmonic imaging, compound imaging, and speckle reduction

when possible

Narrow the image sector

Reduce the image depth

Magnify the region of interest in order to fill one-third to half of the

ultrasound image

Use one focal zone positioned at the level of the region of interest

Adapt the dynamic range to have a high or low contrast image

Adjust image resolution• Use cine loop to return back to images stored in the recorded loop for

review

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).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 crownrump 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.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 imagequality 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.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 heartdisease. 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).

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 falsepositive 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.4Figure 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 Mmode 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).

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

Fetal Heart Peripheral Vasculature

Velocity scale High LowColor gain Low High

Color filter High Low

Color persistence Middle High

Color resolution Middle High

Heart and Great Vessels

The use of color Doppler is, in our opinion, essential for the reliable assessment of the

heart and great vessels in the first trimester. The four-chamber and the three-vessel

trachea views are relatively easy to obtain on color Doppler and provide for adequate

screening for cardiac malformations in early gestation (Fig. 3.8).4,7 In pregnancies at

increased risk for congenital heart disease, obtaining additional planes such as the fivechamber view, the short-axis and aortic arch views provides for a comprehensive

evaluation of the fetal heart in the first trimester. In selective cases, the demonstration of

the pulmonary veins draining into the left atrium can be of importance as well as the

demonstration of the course of the right subclavian artery. Detailed evaluation of first

trimester normal and abnormal fetal cardiac anatomy is presented in Chapter 11.

Abdominal Vessels

The axial plane in the lower abdomen allows for the demonstration of the two umbilical

arteries surrounding the bladder, thus confirming a three-vessel umbilical cord (Figs.

3.9A and 3.10A). In the presence of a single umbilical artery, the site of the missing

artery can be documented (Fig. 3.10B). An axial plane at the level of the mid-abdomen

allows for the demonstration of the normal abdominal wall and its umbilical cord

insertion, thus ruling out abdominal wall defects (Fig. 3.9B). In the midsagittal plane of

the fetus (NT plane), color Doppler can be applied over the abdomen to visualize the

course of the umbilical vein and DV toward the heart (Figs. 3.7B and 3.11). The narrow

size and high blood flow velocities of the DV differentiate it from the umbilical vein.

This midsagittal plane can also be used to rule out agenesis of the DV or abnormal

connections of the DV. In the same midsagittal view, two arteries appear to arise from

the abdominal aorta, namely the hepatic artery superiorly and the superior mesenteric

artery inferiorly (Figs. 3.7B and 3.11). In a slightly more angulated view, the inferior

vena cava can be visualized ascending from the middle abdomen and draining into the

right atrium8,9 (Fig. 3.12). Interrupted inferior vena cava can be confirmed in this view

when suspected in left atrial isomerism. Color Doppler applied to a coronal view of the

posterior part of the abdomen demonstrates both renal arteries arising orthogonally from

the abdominal aorta toward the renal pelves (Fig. 3.13). Detailed evaluation of first

trimester normal and abnormal fetal gastrointestinal and urogenital anatomy is presented

in Chapters 12 and 13, respectively.Figure 3.9: Transverse views in color Doppler of the fetal pelvis (A) and midabdomen (B) at 13 weeks of gestation using a linear high-resolution transducer.

Note the two umbilical arteries (arrows) surrounding the bladder (Bl.) in A and

the intact abdominal wall (open arrow) in B. Compare with Figure 3.10.Figure 3.10: Transverse views in color Doppler of the fetal pelvis in two

fetuses (A and B) at 12 weeks of gestation using a curvilinear transducer. Note

in A the presence of two umbilical arteries (arrows) surrounding the bladder

(asterisk). The absence of the right umbilical artery (?) to the right of the

bladder (asterisk) is demonstrated in fetus B. L, left; R, right.

Figure 3.11: Sagittal plane of the fetal chest and abdomen in color Doppler in afetus at 11 weeks of gestation demonstrating an intact anterior abdominal wall

(open arrow) and the umbilical artery (UA) and umbilical vein (UV) at the

insertion of the umbilical cord into the abdomen. This plane also demonstrates

the normal course of the UV in the abdomen and shows the narrow ductus

venosus (DV), connecting into the heart. This is the ideal plane for Doppler

sampling of the DV in early gestation (see Fig. 3.17). From the descending

aorta (Ao) posteriorly, the hepatic artery (Hep.A) and the superior mesenteric

artery (SMA) are seen to emerge in perpendicular orientation to the aorta (see

Fig. 3.7). The inferior vena cava is not seen in this plane as its anatomic course

runs in the right abdomen.

Placenta and Umbilical Cord

The assessment of the placental attachment and course of the umbilical artery is best

demonstrated in the first trimester on color Doppler (Fig. 3.14). The presence of

marginal or velamentous cord insertion can be easily suspected in the first trimester

given that the full length of the placenta can be imaged in one view. When umbilical

cord abnormalities are suspected in the first trimester, follow-up ultrasound

examination in the second trimester is recommended to confirm such findings.

Figure 3.12: Sagittal planes (A and B) in color Doppler of the fetal abdomen in

the same fetus at 13 weeks of gestation. Note in A, the ductus venosus (DV) in

the mid-abdomen along with the descending aorta (Ao), seen posteriorly. Theinferior vena cava (IVC) is not seen due to its anatomic course in the right

abdomen. When the probe was reoriented in B, the IVC connecting to the heart

along with the descending Ao is seen and the DV is not seen. Note the blue

color in the aorta in B (flow away from transducer) due to transducer

reorientation.

Figure 3.13: Coronal plane in color Doppler of the posterior abdomen and

pelvis at 13 weeks of gestation showing the descending aorta (Ao) with the left

and right renal arteries arising from the Ao and coursing into the kidneys

(arrows).

Intracerebral Vessels

Several articles have reported on the course of the cerebral arteries and veins in thefirst trimester in normal and abnormal conditions.10–13 Figure 3.15 shows the cerebral

arteries and veins from an axial view at the base of the skull (Fig . 3.15A),

demonstrating the circle of Willis and from the midsagittal view of the fetal head (Fig.

3.15B), demonstrating the anterior cerebral and pericallosal arteries. The application of

color Doppler of the fetal head should be reserved for pregnancies at increased risk for

central nervous system abnormalities.

Regions of Interest for Pulsed Doppler

The application of pulsed (spectral) Doppler ultrasound in the first trimester is limited

to the assessment of the maternal uterine arteries for evaluation of pregnancy risk and

fetal vessels for aneuploidy risk assessment or for fetal malformations. It is important to

note that pulsed Doppler application in the first trimester is associated with a potential

risk to the fetus and should be performed when the benefit outweighs the risk (see

Chapter 2).

Uterine Arteries

Pulsed Doppler examination of the uterine arteries in the first trimester has been used to

assess uteroplacental impedance and to integrate the results in the risk profiling for

preeclampsia14 (Fig. 3.16). This examination can be performed as part of general

screening or targeted to women with a prior history of fetal growth restriction or

preeclampsia. Given that the uterine arteries are lateral to the gestational sac, pulsed

Doppler can be performed without concern of risk to the fetus.15

The Umbilical Cord

Pulsed Doppler interrogation of the umbilical cord is rarely obtained before the 15th

week of gestation. Pulsed Doppler should not be used for confirmation of cardiac

activity, as the use of M-mode or a motion clip is preferred for that purpose.

Figure 3.14: Color Doppler showing in A the placental cord insertion in a fetus

at 13 weeks of gestation, in B a free loop of umbilical cord in a fetus at 13

weeks and in C nuchal cord in a fetus at 12 weeks of gestation resulting in a

slight increase in nuchal translucency (arrow).Figure 3.15: A: An axial plane of the fetal skull base at 13 weeks of gestation

in color Doppler showing the circle of Willis with the middle cerebral artery

(MCA). B: A midsagittal plane of the fetal head in color Doppler at 12 weeks of

gestation showing the anterior cerebral artery (ACA), the proximal portion of

the pericallosal artery (Peric.A.), and the internal cerebral vein (ICV) shown

coursing posteriorly along the borders of the thalamus. Blood flow in the

sagittal sinus (SS) is also demonstrated anteriorly.Figure 3.16: Color and spectral Doppler of the uterine artery in a pregnancy at

12 weeks of gestation (GA). Uterine artery Doppler waveforms have been used

for pregnancy risk assessment in some settings. See text for details.Figure 3.17: Color and pulsed Doppler of the ductus venosus (DV) in two

fetuses A and B. Note that the Doppler sample gate is small and is placed

within the DV, with an insonation angle of less than 20 to 30 degrees. Normal

DV Doppler waveforms show the characteristic biphasic pattern with antegrade

flow during the atrial (A) contraction phase as shown in fetus A. Note the

presence of abnormal reverse flow during the atrial contraction (A) in fetus B.

The Ductus Venosus

The most common use of pulsed Doppler in the first trimester is probably related to the

examination of the DV flow velocity waveform. In normal conditions, the DV

waveforms are biphasic with low pulsatility and with antegrade flow in the diastolic

components (a-wave) throughout the cardiac cycle (Fig. 3.17A). The presence of high

pulsatility or a reverse flow of the a-wave in the first trimester (Fig. 3.17B) increases

the risk for chromosomal anomalies, cardiac defects, and the occurrence of twin-twintransfusion syndrome in monochorionic twins.16–18 There is currently no consensus on

whether DV Doppler assessment should be a screening test performed on every fetus or

reserved as a second-line assessment in mid- and high-risk fetuses.

Tricuspid Valve

Color and pulsed Doppler examination across the tricuspid valve is commonly used inthe first trimester to assess for the presence of tricuspid valve regurgitation (TR) (Fig.

3.18). The presence of TR in the first trimester (Fig. 3.18B) has been associated with

chromosomal abnormalities.19,20 In the first trimester, TR is found in less than 5% of

chromosomally normal fetuses, in more than 65% of fetuses with trisomy 21, and in

more than 30% of fetuses with trisomy 18.19 Interrogation of other cardiac valves with

color or pulsed Doppler is reserved for fetuses at risk for valve obstruction or when a

cardiac malformation is suspected.

Other Vessels

In rare situations, clinical indications arise in early gestation for the pulsed Doppler

assessment of other fetal vessels such as the hepatic (Fig. 3.19) and middle cerebral

arteries. It has been reported that high peak velocities in the hepatic artery are present in

the first trimester in fetuses at risk for trisomy 21 (Fig. 3.19B).21 Furthermore, in rare

conditions of suspected fetal anemia in early gestation, such as in pregnancies with

serologically confirmed Parvovirus B19 infection, middle cerebral artery Doppler can

be of help in assessing for the presence of anemia.

THREE-DIMENSIONAL ULTRASOUND

3D ultrasound has been associated with a view of the fetal face or the body in surface

mode, primarily for keepsake purposes. Beyond its keepsake properties, 3D ultrasound

in the first trimester can be accurately used to reconstruct planes and to visualize

structures not seen on conventional ultrasound examination. The ability of multiplanar

reconstruction of a 3D volume is important, especially during transvaginal ultrasound,

where transducer manipulation is limited and the fetus is not in an appropriate position

to directly visualize target anatomic regions. For further information on the value of 3D

ultrasound, the reader is referred to dedicated monographs and articles on this

subject.22,23Figure 3.18: Doppler velocity waveforms across the tricuspid valve at 12

weeks of gestation in a normal fetus (A) and in a fetus (B) with trisomy 21

(T21) with severe tricuspid regurgitation (arrows).Figure 3.19: Doppler velocity waveforms of the hepatic artery at 12 weeks of

gestation in a normal fetus (A) and in a fetus (B) with trisomy 21 (T21). Note

the presence of low peak systolic velocities (18 cm per second) in the normal

fetus A, as compared to high velocities (35 cm per second) in the fetus with

trisomy 21 (T21) (B).Figure 3.20: Transvaginal 3D volume of the fetal face, obtained from an

oblique plane of the face (solid arrow in B), given that a midsagittal plane of the

fetal face could not be imaged on 2D ultrasound (open arrow in A). The volume

data are displayed in the multiplanar orthogonal mode showing A, B, and C

planes. Compare with Figure 3.21, produced after manipulation of this volume.

Multiplanar Reconstruction

Given that embryos and fetuses rarely present in the first trimester in an ideal position to

visualize all of the anatomic structures on 2D ultrasound, the acquisition of static 3D

volumes with multiplanar display of reconstructed planes can be of significant help.

Using tomographic mode display of a 3D volume, the examiner is able to show in one

image several anatomic regions of the fetus. Figures 3.20 and 3.21 show examples of

reconstructed fetal profile and NT respectively out of a 3D volume. Figures 3.22 and

3.23 show an example of the fetal head in tomographic mode. Examples of tomographic

display of the fetal chest and abdomen are shown in their respective chapters (Chapters

10 and 12). The fetal spine, limbs, profile, and internal organs such as lungs, diaphragm,

and kidneys can be reconstructed in sectional planes from a 3D volume. The brain is

probably the best organ to examine starting at 7 weeks of gestation using multiplanar

mode. Brain development can be followed from early gestation and into the earlysecond trimester. Careful rotation of the volume along the X, Y and Z-axes, in

multiplanar mode, helps to display midline planes of the face (Fig. 3.22), head (Fig.

3.23), lungs, and kidneys. The use of 3D ultrasound in the assessment of fetal anatomy in

the first trimester is presented in more detail in Chapters 8 to 15 of this book.

Three-Dimensional Volume Rendering

The use of surface mode is the most commonly used 3D rendering mode in the first

trimester as it allows for optimal visualization of the developing embryo and fetus (Fig.

3.24). Images acquired using 3D surface mode of the embryo and fetus (Fig. 3.24) are

similar to images shown in embryology textbooks. As early as the 11th week of

gestation, the head, trunk, extremities, and other fetal anatomic details can be reliably

demonstrated (Figs. 3.24 to 3.27). On occasions, 3D ultrasound can better display

normal internal anatomy of the fetus in the first trimester (Fig. 3.28). Major anomalies

affecting the external surface and internal organs of the body can be well recognized in

the first trimester in 3D surface mode (Figs. 3.29 and 3.30). In the first trimester fetal

anatomy survey, the authors caution about relying on 3D ultrasound only before a

detailed evaluation of fetal anatomy is performed on 2D imaging. In addition to 2D

ultrasound examination, 3D ultrasound plays an important role in ruling out major fetal

malformations in the first trimester in pregnant women with a prior history of severe

fetal malformations. In multiple pregnancies, fetuses can be well visualized on 3D

ultrasound along with surrounding structures. The diagnosis of chorionicity in multiple

pregnancies is best performed on 2D ultrasound. (See Chapter 7 for more details on this

subject.)Figure 3.21: The result of post-processing of the volume data set displayed in

Figure 3.20. Post-processing allowed for the retrieval of the midsagittal plane in

the left upper plane (open arrow in A), with the display of the nasal bone (NB)

and nuchal translucency (NT).Figure 3.22: Transvaginal 3D volume of the fetal head at 12 weeks of

gestation displayed in the multiplanar orthogonal mode showing A, B, and C

planes. This volume was obtained from an oblique orientation of the fetal head

as shown in the upper right image with oblique falx cerebri (dashed line). Postprocessing of this volume to display important anatomic brain landmarks is

shown in Figurve 3.23 and in the surface mode display in Figure 3.28.Figure 3.23: Post-processing of the 3D volume data set shown in Figure 3.22.

Post-processing of the 3D volume included rotations and display in tomographic

mode. Five planes are shown at 2.5 mm spacing. Anatomic details of the fetal

brain that are shown in these five planes include skull ossifications (arrows), the

falx cerebri (dashed line), the choroid plexuses (CP), the lateral ventricles (LV),

the thalami (Th), the developing cerebellum (Cer), and the fourth ventricle (4V).Figure 3.24: Surface mode display of 3D volumes of three normal embryos A,

B, and C at 8, 9 and 10 weeks, respectively. Note at 8 weeks, the relatively

large size of the embryo’s head as compared to the body.Figure 3.25: Surface mode display of 3D volumes of four normal fetuses

between 11 (B) and 13 weeks of gestation (A,C,D). Note the clear display of

surface anatomy and of extremities.

Other volume rendering modes used in 3D ultrasound include the maximum mode,

which is infrequently applied in the first trimester due to the reduced level of

ossification in the fetal skeleton, the inversion mode, which is used to visualize

intracerebral ventricular system in early gestation, and the silhouette mode (Fig. 3.26C),

which has potential for more clinical applications in the future. Combining 3D with

color Doppler in glass-body mode highlights internal vasculature. This can be used in

the first trimester to visualize the fetal heart, and the arteries and veins inside the

abdomen and thorax (Fig. 3.31).Figure 3.26: Three-dimensional volume of a normal fetus at 11 weeks of

gestation displayed in surface mode and showing the effect of various postprocessing tools. The upper panel (A–C) shows the effect of augmenting the

transparency effect, with the display of fetal internal anechoic structures. In the

lower panel (D and E), adjusting light effects in D and digitally erasing

surrounding structures in E shows the fetus without a background.Figure 3.27: Three-dimensional volume in surface mode at 12 weeks of

gestation in a normal fetus (A) and in a fetus with facial dysmorphism with

abnormal ears and micrognathia (B).Figure 3.28: Transvaginal 3D volume of a fetal head at 12 weeks of gestation

displayed in the orthogonal (A,B,C) and surface display mode (3D). This is the

same volume shown in Figures 3.22 and 3.23. Rendering of the volume shows

in the right lower panel the large choroid plexuses (CP), the falx cerebri (Falx),

and the lateral ventricles (LV). Compare with Figure 3.29.Figure 3.29: Transvaginal 3D volume of the fetal head at 12 weeks of

gestation with holoprosencephaly displayed in the orthogonal (A, B, C) and

surface display mode (3D). Rendering of the volume shows in the right lower

panel fused choroid plexuses (CP), single ventricle (double arrow in A), and

absent falx cerebri. Compare with normal brain anatomy shown in Figure 3.28.Figure 3.30: Three-dimensional (3D) ultrasound images in surface mode in a

fetus at 13 weeks of gestation with a body stalk anomaly visualized from the

front (A) and back (B). The abdominal wall defect is recognized (asterisk) and

fetal deformities of body and spine are shown in panels A and B. 3D ultrasound

is optimal imaging modality in such cases as it clearly displays the extent of

fetal deformities. Compare with normal anatomy in Figure 3.25.

Figure 3.31: The left upper panel (A) shows a 3D volume of the fetal heart in

color Doppler at 12 weeks of gestation. The left lower panel (B) shows the

same volume as in A displayed in glass-body mode with transparency. The

right upper panel (C) shows a 3D volume of the fetal abdomen in high-definition

color Doppler in glass-body mode at 12 weeks of gestation. The right lower

panel (D) displays the same volume in unidirectional Doppler flow. The right

upper (C) and lower (D) panels show the spatial anatomic relationship of the

descending aorta (D.Ao), the umbilical vein (UV), the umbilical artery (UA), the

inferior vena cava (IVC), and the ductus venosus (DV). RV, right ventricle; LV