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Two methods are used to acquire volumes of ultrasound data for 3-D US. In the so-called freehand technique, a position sensor is mounted on a conventional 2-D US probe. As the probe is swept through the region of interest, the position sensor allows accurate capture of multiple 2-D US images. These are used to create multiplanar images and/or 3-D surface reconstruction.

The freehand technique has been used in both in-line and off-line systems. In-line units are capable of image acquisition, transfer, storage, and manipulation. With off-line systems, the volume ultrasound data are downloaded to an off-line computer workstation equipped with appropriate software for subsequent 3-D US image reconstruction and analysis.

Although the freehand technique can be incorporated into existing systems and uses conventional 2-D US probes, the volumes acquired generally cannot be used for accurate measurement of structures and are more likely to have acquisition-related artifacts. In the off-line systems, the external probe-holding mechanisms are typically bulky and the image transfer process to an off-line workstation is time-consuming.1,2 Three-D ultrasound images can also be acquired using dedicated 3-D US probes, which have a mechanized drive contained within the probe case itself. These are commonly referred to as volume probes. When volume probes are activated, the transducer elements automatically sweep through the operator-selected region of interest (volume box), while the probe is held stationary.

Both methods rely on specialized 3-D software that processes the acquired volumes either online through a built-in computer or off-line on a workstation. Images obtained with the automated approach, using either dedicated 3-D US probes or an external probe-holding mechanism, provide relatively more accurate measurements and demonstrate more precise spatial relationship.

Volume data can be displayed and manipulated in several ways. Digitally stored volume data can be displayed in a multiplanar array that simultaneously shows three perpendicular planes through the volume: axial, sagittal, and reconstructed coronal views. In the multiplanar display, the volume can be explored by scrolling through parallel planes in any of the three views and by rotating the volume to obtain an optimal view of the structures of interest. The operator can manipulate the volume data to obtain any desired plane of section after the patient is discharged.

Digitally stored volume data can also be displayed as true 3-D images using various rendering algorithms, including maximum intensity projections and transparent and surface renderings, whenever a tissue interface is present. This type of rendering provides realistic, lifelike images that patients and practitioners easily understand. Depending on the rendering parameters chosen, images can be created to highlight features of interest, such as the bony skeleton or superficial soft tissues.

The multiplanar display can also be studied simultaneously with a rendered image, allowing correlation of the planar images with the 3-D image. A useful feature of 3-D display is the cine loop, in which the rendered 3-D US volumes are viewed as they rotate. This capability enhances depth perception and gives a true 3-D perspective of both normal and abnormal structures.3,4

APPLICATIONS IN OBSTETRICS

High-resolution transvaginal sonography is widely considered the standard approach to imaging the first trimester of pregnancy. Anatomic factors limit the mobility of the vaginal probe, however, and thereby limit the scan planes. Three-D ultrasound overcomes this limitation because any plane in the acquired volume can be obtained. In addition, volume measurements of the gestational sac are possible.

Fetal nuchal translucency measurement between 10 and 14 weeks' gestation is an effective screening method for chromosomal anomalies. The exact midsagittal plane for assessing the nuchal region may be difficult to obtain with 2-D US, depending on fetal position. Three-D ultrasound assists in achieving the required diagnostic plane and may also allow detailed morphologic characterization of the increased nuchal translucency. In the first trimester, 3-D ultrasound can demonstrate normal and abnormal embryonic and early fetal anatomy with exquisite detail, especially with the transvaginal approach. Detailed imaging of the yolk sac, omphalomesenteric duct, trunk and umbilical cord, head and face, spine, extremities, and even genitalia is possible (Figures 1 and 2). This provides opportunities for earlier and improved diagnosis in the first trimester and may limit the need for more invasive imaging techniques.

By displaying soft-tissue information in surface rendering mode, 3-D US provides unique and useful images of facial features (Figures 3, 4, and 5). The anatomy of cranial sutures and fontanelles in the developing fetus is difficult to display with 2-D US, as are other curved bony structures such as the ribs.5 Pretorius et al have demonstrated that 3-D US offers clearer visualization of cranial structures and bony plates, permitting improved understanding of cranial anatomy, craniosynostosis, and abnormal cranial contours such as cloverleaf skull.6

Several authors have reported improved visualization of fetal face in pregnancies at high risk for dysmorphology due to chromosomal abnormalities, exposure to teratogens such as Dilantin, or fetal alcohol syndrome. Cleft lips, micrognathia, malformed ears, and frontal bossing have all been better displayed and analyzed by 3-D US rendering methods.7

Studies have demonstrated that 3-D US aids evaluation of the fetal skeleton (Figure 6). Obtaining detailed structural information about the fetal spine and thorax is important in evaluating fetuses at risk for skeletal dysplasia, abnormalities leading to small thorax, and subsequent pulmonary hypoplasia and neural tube defects.8 Other types of skeletal abnormalities affecting the thorax and spine, such as scoliosis, spinal disruption, hemivertebrae, butterfly vertebrae, and rib abnormalities, are more easily assessed by 3-D US.

Evaluation of the hands and feet is also facilitated with 3-D US, as the examiner can standardize the orientation of the hand and examine the relationships of distal forearm bones, metacarpals, fingers, and thumb in the same plane (Figures 7 and 8). Several papers have shown the usefulness of 3-D US in detecting isolated cases of abnormal extremities such as clubfoot, polydactyly, claw-hand, and phocomelia.

Fetal gender can be readily identified in the late first and early second trimester because the diagnostic midsagittal plane is consistently obtained. Prenatal diagnosis of ambiguous genitalia with 3-D US using surface rendered images has been reported. The umbilical cord and its insertion sites are readily visualized; abdominal wall defects can be thoroughly studied.

An advantage of 3-D US is the ability to obtain different views from one stored volume. One technically adequate volume acquired in the four-chamber plane of the heart yields not only the standard four-chamber view but also additional views of the heart, including the outflow and inflow tracts and ductal/aortic arches. Because the cardiac volumes are stored digitally, thorough examination of the fetal cardiac connections can be accomplished after the patient is discharged. Views that may not be readily obtainable with 2-D US due to fetal position can be extracted from the stored cardiac volume data.9

Volumes sent digitally over networks allow for consultation with experts in fetal echocardiography. Thus 3-D US holds the potential to improve detection of cardiac defects in routine fetal screening. Problems with this technique remain, however. The fetal heart is imaged in static images and not in real time. Several reports have demonstrated the feasibility of gating 3-D US volumes of the fetal heart to produce a real-time study, but this technique is still under development.

We have found 3-D US useful for assessing the pregnant cervix and have compared the cervical measurements obtained with those from 2-D US in a high-risk population. We discovered that the true midsagittal plane of the cervix was not obtained in 27% of the 2-D US exams. Cerclage location and completeness of visualization was assessed on both 2-D US and 3-D US; the precise relationship of the suture with respect to the cervical canal was best assessed in the 3-D axial plane. The 3-D US features of funneling were studied, but the significance of the findings must be correlated with outcomes data.

Organ volume measurements have not been widely used for assessment of fetal growth and organ abnormalities because of limitations of 2-D US in estimating volumes of irregular structures. With the advent of 3-D US, it has become possible to accurately measure the volume of fetal organs. Feasibility has been reported for calculating volumes of the gestational sac, fetal lungs and heart from second trimester to term, placental volume, liver volume, and fetal arm and thigh volume for estimation of fetal weight. These capabilities offer new possibilities in assessing growth and development.

In addition, fetal limb volumetry is being studied as a means to improve prediction of fetal weight.10,11 Several authors have reported the possibility of accurately measuring lung volume and growth. Lung volume monograms have been published showing that lung volume increases with gestational age and fetal weight.12,13 Additional studies should be conducted to evaluate the usefulness of this quantitative technique in the qualitative prenatal assessment of lung maturity and growth.

PSYCHOLOGICAL ISSUES

In a study involving 20 high-risk women at 24 to 32 weeks of pregnancy, 3-D US had a positive influence on patients' perception of the fetus. The mothers reported more incentive to endure pregnancy-related difficulties, reduced anxiety, and improved capacity to cope.14 Improved bonding between the mother and fetus could motivate mothers to refrain from smoking and other harmful behaviors during pregnancy.15 The ability to show patients and their families comprehensible images of the fetus can also assist counseling in cases of abnormal fetuses or provide greater reassurance of normalcy.

APPLICATIONS IN GYNECOLOGY

For gynecologic applications, we have found multiplanar imaging more useful than surface rendering techniques. Nonetheless, some practitioners use rendering techniques to demonstrate the internal contour of cystic masses to look, for example, for papillary projections in ovarian lesions.

Multiplanar imaging provides axial, sagittal, and coronal perpendicular planes through the uterus. The coronal view of the uterus, which is rarely obtainable with 2-D US, is particularly valuable for assessing the shape of the uterus.16 It clearly distinguishes a variety of uterine anomalies for which MR imaging might otherwise be required. The coronal view helps in making clinically important distinctions; for example, between a bicornuate and septate uterus (Figure 9). The coronal plane also optimizes imaging of the cervical region and permits detection of cervical anomalies (Figure10).

The following applications of 3-D US in gynecology have been found to be clinically useful: uterine anomalies, assessment of the endometrial cavity with 3-D sonohysterography (Figure 11), and characterization of tubo-ovarian masses, including hydrosalpinx, cystic ovarian masses, and small intraovarian masses (Figure 12). In addition, 3-D multiplanar imaging has been found useful in demonstrating the complex anatomy of the pelvic floor musculature, including the urethral and anal sphincters.

The value of volume measurements in gynecology is being explored. Among other possibilities, measuring endometrial volume rather than just endometrial thickness may be more valuable in discriminating between benign and malignant conditions. Other investigators have measured the volume of the urethral sphincter in assessing women with urinary incontinence, as women with smaller sphincters may be more at risk for this condition.

NETWORKING

Expertise in difficult ultrasound examinations is not always readily available at primary scanning facilities. A potential benefit of 3-D US lies in ultrasound documentation, storage, and networking. Digitally saved volumes of patient data can be readily transferred to a remote site for interpretation or second-opinion consultation. Further research is needed to define the indications, potential advantages, and limitations of volume data transfer across networks.

The role of 3-D US off-line interpretation is not firmly established, but there is no doubt it will have a substantial impact on the way clinical ultrasound is performed. Using this technology, primary clinical sites in remote areas would have access to expert consultation and off-line interpretation, enabling high-quality, cost-effective medical care. The digitally stored patient volumes can also be accessed and analyzed by physicians and sonographers in training.

LIMITATIONS

Because 3-D US information is based on compilation and manipulation of conventional 2-D US data, it is still prone to problems that affect 2-D US, such as unfavorable maternal body habitus, motion artifacts due to fetal movement, and decreased amniotic fluid. There are also technical problems and artifacts that are specific to 3-D ultrasound. As with 2-D US, factors affecting resolution must be considered. Within the 3-D volume, resolution is lower in the planes that are near parallel to the acquisition plane, as well as in the coronal reconstructed plane. Recent improvements have resulted in faster volume acquisition, less bulky and lighter probes, and better segmentation software, but a need remains for higher resolution, improved software, and automatization in volume measurements.

Although 3-D US might improve overall comprehension of anatomy, it does not make up for poor scanning technique. Poor resolution in 2-D US will most likely produce suboptimal image quality with 3-D US as well. Additional training and experience is required to acquire clinically useful information and reliably analyze volume ultrasound data. Further research is needed to develop standardized techniques and methods for optimal acquisition, display, and rendering of 3-D US data.

DR. BEGA is a research assistant and DR. LEV-TOAFF is a professor of radiology at Thomas Jefferson University in Philadelphia.

References

1. Nelson TR, Downey D, Pretorius DH, Fenster A. Three-dimensional ultrasound. Philadelphia: Lippincott Williams and Willkins, 1999.

2. Riccabona M, Pretorius DH, Nelson TR, et al. Three-dimensional ultrasound: display modalities in obstetrics. J Clin Ultrasound 1997;25(4):157-167.

3. Merz E, Bahlmann F, Weber G. Volume scanning in the evaluation of fetal malformations: a new dimension in prenatal diagnosis. Ultrasound Obstet Gynecol 1995;5(4):222-227.

4. Steiner H, Staudach A, Spitzer D, Schaffer H. Three-dimensional ultrasound in obstetrics and gynaecology: technique, possibilities and limitations. Hum Reprod 1994;9(9):1773-1778.

5. Pretorius DH, Nelson TR. Prenatal visualization of cranial sutures and fontanelles with three-dimensional ultrasonography. J Ultrasound Med 1994;13:871-876.

6. Nelson TR, Pretorius DH. Three-dimensional ultrasound of fetal surface features. Ultrasound Obstet Gynecol 1992;2:166-174.

7. Merz E, Weber G, Bahlmann F, Miric-Tesanic D. Application of transvaginal and abdominal three-dimensional ultrasound for the detection or exclusion of malformations of the fetal face. Ultrasound Obstet Gynecol 1997; 9(4):237-243.

8. Nelson TR, Pretorius DH. Visualization of the fetal thoracic skeleton with three- dimensional sonography. AJR 1995;164:1485-1488.

9. Zosmer N, Jurkovic D, Jauniaux E, et al. Selection and identification of standard cardiac views from three-dimensional volume scans of the fetal thorax. J Ultrasound Med 1996;15:25-32.

10. Liang RI, Chang FM, Yao BL, et al. Predicting birth weight by upper-arm volume with use of three-dimensional ultrasonography. Obstet Gynecol 1997;177(3):632-638.

11. Lee W, Comstock CH, Kirk J, et al. Birthweight prediction by three-dimensional ultrasonographic volumes of the fetal thigh and abdomen. J Ultrasound Med 1997;16:799-805.

12. Pohls UG, Rempen A. Fetal lung volumetry by three-dimensional ultrasound. Ultrasound Obstet Gynecol 1998;11(1):6-12.

13. Laudy J, Janssen M, Struyk P, et al. Three-dimensional ultrasonography of normal fetal lung volume: a preliminary study. Ultrasound Obstet Gynecol 1998;11:13-16.

14. Maier B, Steiner H, Wieneroither H, Staudach A. The psychological impact of three-dimensional fetal imaging on the fetomaternal relationship. In: Baba K, Jurkovic D, eds. Three-dimensional ultrasound in obstetrics and gynecology. New York: Parthenon, 1997: 67-74.

15. Pretorius DH. Maternal smoking habit modification via fetal visualization. University of California tobacco related disease research program. Annual report to the California State Legislature, 1996:76.

16. Jurkovic D, Gruboeck K. Three-dimensional ultrasound of the uterus. In: Baba K, Jurkovic D, eds. Three-dimensional ultrasound in obstetrics and gynecology. New York: Parthenon, 1997:67-74.