three-dimensional ultrasound: techniques and applications in obstetrics

Lindsay Meyer(Dianna Zosche)

Three-dimensional Ultrasound:

Techniques and Applications in Obstetrics

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Abstract

Ultrasound has been used in medicine for over half a century, and is recognized as a non-

invasive, non-radiative, and inexpensive imaging modality. Three-dimensional (“3D”) medical

imaging is now being widely employed in the clinical setting. This report reviews the

development of ultrasound, its method of function, and its practical applications of 3D

ultrasound in fetal embryology and obstetrics.


Introduction

“Ultrasound” is the vernacular term for medical sonography, a non-invasive imaging

technique used in diagnosing disease and developmental defects. In physics, “ultrasound” refers

to acoustic energy outside the range of human hearing. Medical sonographic scanners typically

operate between two and 18 megahertz, with a unique relationship between resolution and depth.

Lower frequencies penetrate body tissues deeper than higher frequencies, but produce lower

image resolution (and vice-versa). The improving economics of 3D ultrasound technology

coupled with advances in 3D image visualization have made the technique increasingly routine

for pregnant women in the past decade.

This paper is meant to achieve two goals. First, an exploration of the origin of ultrasound

will provide the basis for building a compelling case for the use of 3D ultrasound. Second, a

discussion of common congenital anomalies will illustrate the efficacy of the technology.

A Brief History of Ultrasound

In 1950, the first commercial “ultrasonic locator” became available by General Precision

Laboratories (Woo, 2002). George Ludwig, a Naval Officer in Bethesda, Maryland first began

experimenting with the conduction of pulse-echo techniques several years prior. His

methodology was similar to the radar utilized by the military to detect the presence of foreign

boats or flying objects. Ludwig collaborated with physicists and engineers to study gallstones in

muscle tissues in the human body. Using a transducer to send and receive high-frequency sound

waves at a rate of 60 pulses per second, Ludwig recorded reflections with an oscilloscope to

detect the presence and position of foreign bodies. Much of this early work with ultrasound was

clandestine until 1949 because the information was considered classified naval information.

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From 2D to 3D – Imaging Techniques

Conventional, 2D ultrasound relies on the reflection of high frequency sound waves by

bones and muscles. Soft tissue and hollow structures do not reflect the waves and appear dark.

Anywhere there are changes in density in the body, sound waves are generally reflected. This

technology was prevalent around the world for nearly half a century.

In 1994, 3D ultrasound was popularized in the European Journal of Ultrasound and

detailed three discrete steps – scanning, reconstruction, and visualization. Four scanning

techniques (mechanical, free-hand with position sensing, free-hand without position sensing, 2D

array) exist. Among these methods, mechanical scanning is most relevant to medicine because

the relative position and orientation of each image can be known precisely. This approach uses a

scanning apparatus (transducer) to acquire 2D ultrasound images over the area of interest

(Fenster, Downey & Cardinal, 2000).

In the reconstruction step, 2D images are placed in their correct relative positions and

orientations in the 3D image volume (Fenster et al., 2002). Feature-based reconstruction uses

anatomical structures to determine boundary surfaces, offering efficient manipulation by

computer. In contrast, the more popular voxel method of reconstruction uses a Cartesian grid to

build elements in three dimensions. Each 2D coordinate (x, y) is interpolated to determine a 3D

coordinate (x, y, z). Automated reference tables stored in computers help accelerate this process.

The voxel approach preserves all original information and enables the generation of new views

not in the original set of 2D images. This method is also superior because it allows the operator

to use different segmentation and classification schemes to segment boundaries, measure volume

or perform various volume-based rendering (Fenster et al., 2000).

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The third and final step to 3D ultrasound is visualization. For optimal image perception,

depth shading and color and texture mapping is used. Volume rendering helps to approximate

the passage of light through soft tissues. Computer algorithms have vastly simplified this

process of manipulating and isolating images. Dynamic 3D imaging is used to show fluid

motion within the fetus and is a consequence of continued technological improvements.

Building a Case for 3D Ultrasound

In conventional, two-dimensional (2D) ultrasounds, physicians were left to mentally

integrate multiple 2D images to understand 3D structures. This guesswork introduced a high

margin of error in addition to prohibiting the imaging of certain anatomical features. As 2D

ultrasounds capture images at randomized planes, generating the same picture more than once is

nearly impossible (Fenster et al., 2000). This impedes effective therapeutic monitoring. It also

increases the risk of incorrect diagnosis. Systematically assessing fetal development is

compromised in 2D ultrasound because accurate volume estimations are difficult to make. Each

of these obstacles was overcome with the development of 3D ultrasound.

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Common Congenital Anomalies

Embryology texts suggest the incidence of birth defects to be approximately 5%. For the

purposes of this paper, common congenital anomalies have been classified into four groups –

those of the body surface, extremities, spine, and cranium/face. The development of 3D

ultrasound has improved the detection of several anomalies in each of these four groups (Xu et

al, 2002).

Conjoined twins, tumors, edema, and placental defects are

abnormalities of the body surface. Difficult to detect and diagnose,

these conditions vary in severity. Teratomas result from the

unregulated division of pluripotent (stem) cells. Sacrococcogeal

teratomas appear at the base of the tailbone and are the most common

type of tumor in newborns. These teratomas can obstruct the normal

passage of fluids from surrounding organs. Placental defects include

placental previa (placenta impedes the cervix) and placental abruption (placental lining separates

from the uterus). Often fatal to infant and mother, these conditions are associated with heavy

bleeding late in pregnancy and require careful monitoring.

Poldactyly, a condition in which too few or too many fingers or

toes develop is a common defect of the extremities caused by genetic

mutation. It may also occur as part of a cluster of defects related to

teratogen-induced syndromes (i.e.: fetal alcohol syndrome, Accutane-

exposure). Other defects to the extremities include stunted growth

(osteogenesis imperfecta) and club foot. This class of defects is not

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considered life-threatening and occurs in 1-2 live births per 1000, with a higher incidence in

males.

Spina bifida is a condition in which the neural tube does not

close. While surgery can correct the opening, affected individuals will

experience reduced quality of life due to nerve and spinal cord

dysfunction. Some fetuses with the condition will spontaneously abort.

Research by Kurjak, Pooh, Merce, Carrera, Salihagic-Kadic &

Andonotopo (2005) suggested that the condition could be diagnosed

with 3D ultrasound at 9 weeks gestation. In addition to neural tube defects, scoliosis can also be

diagnosed with 3D ultrasound.

Environmental influences such as maternal retinoid intake and

cigarette smoking may interact with genetics to cause cleft lip and cleft

palate. Hydrocephalus is caused by an accumulation of cerebrospinal

fluid and manifests in an abnormally large head. The defect occurs in

approximately one of every 500 live births and is more common than

Down syndrome. Anencephaly is a disorder in which infants are born

without a forebrain. If spontaneous abortion or stillbirth does not occur,

death occurs shortly after delivery.

Applications

Study data released by Xu, Zhang, Lu & Ziao in 2002 compared the efficacy of 2D and

3D ultrasounds on the basis of accurate diagnosis of prenatal malformations. For the four

categories of congenital anomalies outlined above, the study showed diagnostic superiority with

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3D ultrasound. The study made a definitive diagnosis of 79% of defects with 2D ultrasound (n =

49). The use of 3D ultrasound improved the percentage of correct diagnoses to 94% (n = 58). In

60% of cases (n = 35), malformations were correctly diagnosed by both 2D and 3D ultrasound

but the use of 3D ultrasound provided better qualitative diagnostic information. Extrapolating

this 15% improvement in correct diagnoses to larger populations suggests a cascade of public

health benefits associated with early detection of defects and improved prenatal care.

Aggregating data from the Xu et al. (2002) study demonstrated that detection of

craniofacial anomalies was 19% greater with 3D ultrasound (n = 30). No apparent difference

between ultrasound modalities for the body surface (n = 26) reflects the reality that diagnosing

these defects in utero can be extremely challenging. This study also left out conjoined twins, a

rare abnormality, but one that is easily detected with 3D ultrasound. All spinal defects were

detected with 3D ultrasound, but the low sample size (n =4) may overstate the comparative

diagnostic usefulness of 3D technology. Low sample size (n = 2) was also encountered with

defects to the extremities, reducing the integrity of the suggested 50% improvement in diagnosis

with 3D ultrasound.

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Comparitive Diagnosis of Prenatal Malformations

69%

0%

50%

97%

88%

100%

100%

97%

0% 20% 40% 60% 80% 100%

Cranium/face

Spine

Extremities

Body surface

% of correct diagnoses (2D) % of correct diagnoses (3D)

Kurjak et al. (2005) showed that structural and functional developments in the first 12

weeks of gestation could be assessed more objectively and reliably with 3D ultrasound. Because

the first trimester presents the greatest risk of developmental abnormalities, accurate fetal

monitoring during this period is critical. The anatomy and physiology of embryonic

development is a field where medicine exerts its greatest impact on early pregnancy and is a

foray into fascinating aspects of embryonic differentiation (Kurjak et al., 2005).

Dyson, Pretorius, Budorick, Johnson, Skylansky, Cantrell, et al. (2000) suggested that 3D

images were useful in counseling patients about the severity of fetal abnormalities. Dyson et al.

also determined that the level of diagnostic confidence was heightened and used to support

diagnoses made on the basis of 2D ultrasound images.

Clinical Correlates

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In addition to improved detection of prenatal malformations, 3D ultrasound has provided

an unintended benefit by strengthening the maternal-fetal bonding process. Ji, Pretorius,

Newton, Uyans, Hull, Hollenbach et al. (2004) found that mothers who received 3D ultrasound

showed their ultrasound images to more people than mothers receiving 2D ultrasounds alone.

Seventy percent of mothers who had 3D ultrasounds felt that they “knew” their baby

immediately after birth versus 56% of mothers that had 2D ultrasounds, reflecting the fact that

82% of mothers who had 3D ultrasounds had a tendency to form a mental picture of their child,

post-examination. This contrasts with the 39% of subjects who began to picture their infant after

having a 2D ultrasound. Image quality of 2D ultrasounds has improved but most laypersons are

not equipped to understand even the highest resolution 2D images. Three-dimensional

ultrasounds produce more recognizable images, improving the maternal-fetal bonding process.

Conclusion

As medicine continues to evolve, imaging techniques will concurrently improve to

address the challenges of modern science. The advent of 3D ultrasound technology represents

one giant leap for medical imaging. Three-dimensional ultrasound provides accurate

representation of internal structures and improved visualization capacity. By integrating

traditional 2D imaging techniques with 3D ultrasound, clinicians improve the statistical

probability of accurate diagnoses. Detection of congenital anomalies affords parents and

physicians substantial latitude in formulating and implementing prenatal care regiments, thereby

improving public health.

The prevalence of 3D ultrasound in obstetric exams is increasing as the technology

becomes more cost-effective. As this trend continues, it offers the potential for safe, non-

invasive monitoring of fetal development. Drawing attention to 3D ultrasound and its medical

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applications helps perpetuate awareness of its inherent diagnostic value. Assessing the value of

3D ultrasound encourages economic investment in improved medical technologies and propels

continued innovation, raising standards of care.

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References

Dyson, R., Pretorius, D., Budorick, N., Johnson, D., Sklansky, M., Cantrell, C., et al. (2000).

Three-dimensional ultrasound in the evaluation of fetal anomalies. Ultrasound in

Obstetrics and Gynecology, 16, 321-28.

Fenster, A., Downey, D., Cardinal, H. (2001). Three-dimensional ultrasound imaging. Physics

in Medicine and Biology, 46, R67-99.

Ji, E., Pretorius, D., Newton, R., Uyan, K., Hull, A., Hollenbach, K. et al. (2005). Effects of

ultrasound on maternal-fetal bonding: a comparison of two- and three-dimensional

imaging. Ultrasound in Obstetrics and Gynecology, 25, 473-77.

Kurjak, A., Pooh, R., Merce, L., Carrera, J., Salihagic-Kadic, A. & Andonotopo, W. (2005).

Structural and functional early human development assessed by three-dimensional and

four-dimensional sonography. Fertility and Sterility, 84(5), 1285-1299.

Medical ultrasonography. (2007, December 7). In Wikipedia, The Free Encyclopedia.

Retrieved December 9, 2007, from http://en.wikipedia.org/w/index.php?

title=Medical_ultrasonography&oldid=176296551

Woo, Joseph (2002). A short history of the development of ultrasound in obstetrics and

gynecology. http://www.ob-ultrasound.net/history1.html

Xu, H., Zhang, Q., Lu, M., Xiao, X. (2002). Comparison of two-dimensional and three-

dimensional sonography in evaluating fetal malformations. Journal of Clinical

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http://www.ob-ultrasound.net/history1.html

http://en.wikipedia.org/w/index.php?title=Medical_ultrasonography&oldid=176296551

http://en.wikipedia.org/w/index.php?title=Medical_ultrasonography&oldid=176296551

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