Clinical Applications of Three-Dimensional Sonography in Obstetrics

Basics of Volume Sonography

Volume data sets are typically obtained with a 3D mechanical probe by performing an automated volume sweep, of a certain size and quality, at a constant speed. This volume information may be displayed on the 2D screen in various types of display ( Fig. 18.1 ). The multiplanar display allows for demonstration in three perpendicular planes—the X , Y , and Z planes. Manipulation of the images in these planes can be performed using a reference dot to localize a particular region of interest. Multiple parallel images from the volume can also be displayed, similar to CT and MRI. In addition, a 3D-rendered image may be displayed from the volume showing information from the entire volume, such as the surface of the face ( Fig. 18.1A and B ) or the bones of the skeleton ( Fig. 18.1D ). New volume-rendered image displays have been introduced over the last few years that allow for curved displays (e.g., OmniView) and thick slices (narrow volumes), in addition to automatically retrieved anatomic and biometric planes out of standardized volumes of the fetal head ( Fig. 18.1F ) and heart. Three-dimensional image volumes can be further manipulated and edited offline for more targeted information and measurements.

As with all new imaging tools, much emphasis is placed on the 3D surface-rendered image. However, the multiplanar data often contribute the most information diagnostically. A significant advantage of 3D over 2D US is that once the data are acquired, the image can be reviewed and manipulated in different planes for diagnostic clarification, even after the patient has left the US examination suite.

Three-dimensional US is routinely used as an adjunct to 2D US of the fetus. For example, if the question of a cleft lip or cleft palate is raised on the initial 2D examination or if a pertinent family history exists, a targeted 3D US study of the face and palate should be considered. ^, Multiplanar images have been introduced in our clinic for routine screening of the vermis and cavum septi pellucidi during the anatomy scan. Three-dimensional US is also often used to obtain a rendered image of the fetal face to give to expectant mothers as a part of their diagnostic examination.

Conventional 2D imaging is sometimes limited by interobserver and intraobserver variation in image interpretation. This limitation is minimized considerably by volume acquisition, as all three planes are available for review by different clinicians and can be manipulated with the 3D software for a comprehensive evaluation. However, it is paramount to utilize standardized volume acquisition from an optimized 2D image.

Three-dimensional US and its clinical application are limited by various factors, including gestational age, maternal obesity, fetal motion, and imaging technique. With increasing experience among sonographers and sonologists as well as more user-friendly 3D and 4D software programs, 3D US is expected to provide important additional information.

Imaging Tools in Volume Sonography

The type of 3D US imaging tool used depends on the specific clinical question ( Table 18.1 ). For example, when evaluating bony abnormalities, use of maximum-intensity projection would be appropriate ( Fig. 18.1D ). Conversely, surface rendering would be most helpful in assessment of soft tissue pathology, such as cleft lip, and minimum mode for the study of vasculature ( Fig. 18.1C ).

Standardized and optimized 2D image acquisition technique is crucial for maximal quality of 3D US imaging ( Table 18.2 ). This is achieved by imaging the fetus in a position that is most favorable for the clinical issue at hand, with sufficient amniotic fluid surrounding the area in question. For example, when evaluating cleft lip and palate, a supine fetal position with the face directed toward the anterior uterine wall is most desirable. However, in cases of fetal spine anomalies, fetal prone position is optimal. Other technical considerations, such as route of scan, probe selection, focal zone, system gain, region of interest, and the use of harmonics, play a significant role in achieving the best results. Factors that can adversely affect image quality, including fetal motion, maternal body habitus, early gestation, and polyhydramnios or oligohydramnios, may be beyond the examiner’s control.

Other 3D tools include the electronic scalpel, which allows for controlled removal of undesired echoes, and stereographic displays of bony structures that have been studied in the research arena and show promising results with the use of enhanced depth perception. Nelson and colleagues found that stereoscopic viewing improved the conspicuity of complex bony structures and added structural detail information in 14% of fetal skull and 26% of fetal spine cases reviewed.

Use of Three-Dimensional Ultrasonography in the First Trimester

Advances in 3D US have enabled sonologists to diagnose a spectrum of fetal anomalies, once difficult to detect, at an early stage during pregnancy. The question is not whether 3D US is better than 2D US but rather in which situations 3D US can be used as an adjunct to 2D US to yield maximum information. Traditional 2D US is sufficient with respect to many diagnoses; however, in cases in which findings are equivocal or may change management, adjunctive 3D techniques have proved to be valuable and can often reduce the duration of exposure to the US beam.

Bhaduri and colleagues evaluated the fetal anatomic survey using 3D US in conjunction with first-trimester nuchal translucency (NT) screening and found that 3D US was adequate for assessment of the head, abdominal wall, stomach, limbs, and vertebral alignment, but it was less effective in assessing the kidneys, the heart, and the intactness of the skin over the spine. Although these investigators thought that acquiring multiple volumes could assist in excluding gross abnormalities, they also acknowledged the disadvantage of the slightly inferior resolution of 3D images compared with 2D images in this gestational age group.

Key advantages of 3D imaging are primarily related to the size of the fetus and the ability to obtain a single volume of the entire fetus that contains within it all of the anatomic and biometric planes needed for a complete assessment ( Fig. 18.1B ). ^, The various clinical applications for volume sonography in the first trimester are detailed below.

Ectopic Pregnancy

An important potential application of 3D US is in the transvaginal diagnosis of an ectopic pregnancy. Scanning in different spatial planes allows for thorough evaluation of the adnexae, assisting in the diagnosis of tubal ectopic pregnancies and aiding in the evaluation for possible interstitial ectopic pregnancy. In patients with a uterine malformation, the gestational sac may be located laterally within the uterus. This is of diagnostic importance because the sonologist must decide whether the pregnancy is a laterally located intrauterine pregnancy or an interstitial ectopic pregnancy.

Tanaka and colleagues directly addressed the differential diagnosis of intrauterine pregnancy in a septate uterus, interstitial ectopic pregnancy, and angular pregnancy. These three entities may appear similar on 2D US, as the gestational sac will appear within a very short distance from the uterine serosa with only minimal myometrial tissue between the sac and serosa. Showcasing the comparison between 2D and 3D images of each of these diagnoses, the authors demonstrated that 3D US can clearly depict each, which is critical, as each is managed in a different manner. The key to these diagnoses on 3D US is to determine whether the sac is contained within the uterine cavity.

Interstitial pregnancies are surrounded by a thin myometrial layer that is difficult to visualize by traditional 2D US but may be seen with 3D. The so-called interstitial line ( Fig. 18.2 ), a sonographic sign for interstitial ectopic pregnancies, was described by Ackerman and coworkers. Using data based on a retrospective study of 12 interstitial ectopic pregnancies, they determined that visualization of a hyperechoic line extending from the lateral aspect of the uterine cavity into the midsection of the gestational sac corresponds to the interstitial portion of the fallopian tube. This finding, along with a mantle of myometrium surrounding the ectopic gestational sac, is highly specific for diagnosing an interstitial ectopic pregnancy, and visualization of both is facilitated by 3D US. Jurkovic and Mavrelos also used these two signs to diagnose interstitial ectopic pregnancies in their patient population.

Harika and colleagues suggested that 3D US may be useful in early diagnosis of tubal ectopic pregnancies in asymptomatic patients. In a study of 12 patients, all of whom were asymptomatic and were at less than 6 weeks’ gestation, 2D and 3D US were performed. The 2D images showed no feature of intrauterine or extrauterine pregnancy. In four patients, however, 3D transvaginal US demonstrated tubal ectopic pregnancies with small gestational sacs in the fallopian tube. Visualization of the fallopian tube was possible because of a feature noted on 3D US not previously described: the fallopian tube was surrounded by a fine hypoechoic border.