3-Dimensional Obstetric Ultrasound Tips of the Trade

8/12/2019 3-Dimensional Obstetric Ultrasound Tips of the Trade

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3-Dimensional Obstetric Ultrasound:Tips of the TradeMaryam Tarsa, MDa, Dolores H. Pretorius, MDb,*,Deborah DAgostini, RDMSb

& Acquisition of volumes& Mechanical positioning& Display modalities& Scanning Tips& Archiving 3-D US and 4-D US data& Transvaginal 3-D sonography& Usefulness of 3-D and 4-D US in obstetrics

& Good normal face& Cleft lip and palate& Extremities and skeleton& Pitfalls/limitations/artifacts& Summary& References

There is no question that the practice of perinatalmedicine has changed due to advances in radiol-

ogy, particularly ultrasound. Prenatal diagnosis offetal abnormalities has become one of the mostimportant aspects of maternal fetal medicine.

Patients and referring physicians heavily rely oncorrect diagnosis of potential congenital abnormali-ties. With a diagnosis in hand, parents have theoption to meet with the pediatric subspecialtyteam and are prepared for possible events after thebirth of their child.

Three-dimensional ultrasound (3-D US) has revo-lutionized the field of imaging with its ability touse multiple planes in which the area of interest

may be displayed. Given its ability to examinecomplex anatomic structures by using surface analy-sis and volumetric measures, it has become oneof the major modalities used in assessing fetalabnormalities. The role of 3-D US in obstetrics is

evolving rapidly. There are many advantages inusing this modality including more accurate diag-nosis about fetal abnormalities, improved com-

prehension of fetal anatomy by families, and

improved maternal fetal bonding[13]. The impor-tance of reassurance provided by 3-D US in the

absence of any fetal abnormalities especially inhigh-risk families should also be considered [4].This article briefly reviews some applications of

3D-US in obstetrics. A summary of image acquisi-tion and display including some how to tech-niques are provided.

Acquisition of volumes

The key factors in obtaining a 3-D image are USspositioning flexibility and data acquisition speed[5,6]. The quality of images obtained depends on

acquisition speed. Fast speeds are required for mov-ing structures such as a fetal limb. The challengein producing a good quality 3-D image stems fromthe difficulty locating the position of the 2-D USimage within the volume of investigation. The cur-

rent 3-D transducers primarily use a position-sensingdevice. The data-regarding position may be ob-tained using electromagnetic position sensor, rota-

tion device, or stepping motors in the scan head

U L T R A S O U N DC L I N I C S

Ultrasound Clin 1 (2006) 321334

a Department of Reproductive Medicine, University of California, San Diego Medical Center, 200 West ArborDrive, San Diego, CA 92103, USAb Department of Radiology, University of California, San Diego Medical Center, 200 West Arbor Drive,San Diego, CA 92103, USA* Corresponding author. Thornton Hospital, 9300 Campus Point Drive, 7756, La Jolla, CA 92037.E-mail address: [email protected](D.H. Pretorius).

1556-858X/06/$ see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.cult.2006.01.004

ultrasound.theclinics.com

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mailto:[email protected]://ultrasound.theclinics.com/http://ultrasound.theclinics.com/http://ultrasound.theclinics.com/http://dx.doi.org/10.1016/j.cult.2006.01.004http://ultrasound.theclinics.com/mailto:[email protected]://dx.doi.org/10.1016/j.cult.2006.01.004http://ultrasound.theclinics.com/


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[7]. Most commercially available equipment beingused today acquires 3-D US data using a mechani-cally driven transducer where the scan head movesback and forth. This article discusses techniquesusing this type of data acquisition. Volumes canbe acquired using hand-held transducers that are

moved across the abdomen in a smooth sweep, butassumptions must be made regarding the distancebetween images and thus measurements are gener-

ally not accurate.

Mechanical positioning

In general, multiple digitally recorded 2-D USimages are recorded and then reconstructed imme-diately into a 3-D US volume that can be displayedin different orientations. The obvious problem with

reconstructing images is geometric inaccuracies.To avoid this issue, the relative position and angu-lation of each image must be known. Therefore,every time the transducer moves, the images areobtained at a predefined angle and distance inter-

val. Thanks to advances in computer technology,these planar images are stored in the US equip-ment computer system. 3-D US is a display of the

acquired static volumeno motion can be seen,

such as bowel peristalsis or fetal movement. Four-dimensional ultrasound (4-D US) adds time to theinformation acquired and allows for continuouslyupdated volume display allowing for motion of thefetus to be observed. This is also referred to as realtime 3-D US or live 3-D US.

There are three different mechanisms used toproduce 3-D US data: linear, tilt, and rotationalmotion. In linear scanning the images are arranged

parallel to each other. Tilt scanning, most com-monly used in obstetrics, has images arranged ina fanlike shape. The disadvantage of this method

is loss of resolution with increasing depth. Theresolution will be best in the focal zone and atten-tion must be made to identify the area of interestduring 2-D setup scanning to optimize the focalzone positioning arrows. Rotation scanning is used

in some endovaginal tranducers for first trimesterobstetrics and gynecology, arranging images in pro-peller like geometry.

Display modalities

Multiplanar and volume rendering are the two pri-mary display modalities. In multiplanar or orthogo-

nal mode, the stored volume is displayed in three

Fig. 1. Multiplanar image of 12-week embryo. Marker dot is located on the fetal stomach. Top left image iscoronal, bottom left is sagittal, and top right is transverse through embryo.

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planes perpendicular to each other. Where the threeplanes intersect, a marker dot is placed. Using thisdot, the same structure can be located and dis-played on all three different planes [Fig. 1]. Themarker dot is invaluable in following structuresthat traverse the volume, such as hydrosalpinx

and cerebral ventricles. It is also widely used ingynecology to display the endometrium in a coro-nal projection. When evaluating very fine structures

such as the primary palate, it is important toacquire volumes in the optimal plane of resolution(ie, axial plane for the palate) to examine it in themultiplanar display. The volume can be evaluatedby scrolling through each plane in a parallel fash-ion, similar to scrolling through axial CT images.

We often say this is like going through the volumelike a deck of cards. The volume can be rotated in

the three planes, X, Y, and Z, to optimize visualiza-tion of anatomic structures. It is ideal to displayanatomy in a standard orientation (eg, with thefetal face upright; frontal or coronal), in a profile

view (sagittal), and transversely (axial). We have

found it easiest to rotate the image that is recogniz-able in the Z plane first, as it rotates the volumelike a record player and the action is more read-

ily recognizable.Determining how to rotate a volume to see the

desirable information is clearly the crux of the 3-DUS beginner. When a specific region needs to be

seen, it is acquired in a recognizable plane, themarker dot placed on it, and then rotated to astandard orientation. The volume is then evaluatedby scrolling through the parallel planes to assessanatomy and size, if necessary. It is important torealize that the right and left side of the structureare not identified on the volume. The operatormust label right and left based on 2-D US informa-

tion. By rotating the volume 180, the right and leftwill be reversed.

The rendering display is used to demonstrate in-formation within the entire volume. There are multi-ple display modes emphasizing different structures.In surface rendering, a sharp interface between twostructures must be present, such as the fetal skinsurface and the amniotic fluid or blood in theheart and heart walls. Surface-rendered images ofthe fetal face are impressive and very realistic forboth families and medical care providers [Fig. 2].

The 3-D US stored volume can be rotated in the

three cardinal directions. In learning how to rotatethe image it is important to understand which knobto turn to move the image in the desired direction.

In general, rotation of the rendered image using theX axis moves the face up and down (similar to

saying yes), using the Y axis moves the face sideto side (similar to saying no), and using the Z axismoves the face around (similar to a record player). Itis important to understand how to do these rota-tions on the equipment you have in laboratorybecause different equipment will rotate differently.Sometimes the rendered image is linked to the mul-tiplanar images and sometimes it is not. Sometimes

the X, Y, and Z are only connected to the renderedimage and other times to the multiplanar images. As

you adjust to your own equipment, you will be able

to predict which way the knobs will move the imagealthough at first, it will be necessary to rotate allthree knobs to find the desired rotation.

Different light mode and filtering levels can

change the quality of the image. Each US vendoruses different terms to identify changes in rendering

Fig. 2.Surface-rendered image of a normal face at 31weeks. (Courtesy of Philips Medical Systems, Bothell,WA; with permission.)

Fig. 3.Maximum intensity-rendered image of the tho-rax and spine at 19 weeks. The posterior ribs and spineare visualized. (Courtesy of Philips Medical Systems,Bothell, WA; with permission.)

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mode: surface, transparency, x-ray, opacity, thresh-old, maximum, minimum, smoothness, and soforth. Each of these modes emphases certain struc-tures, such as surface, bones, and vessels. Themodes can often be used in combination to opti-mize visualization. For example, the skeleton is

often displayed with the maximum intensitymode [Fig. 3] or the x-ray mode. Using this tech-nique, soft tissue structures potentially can beeliminated. The threshold knob is perhaps themost important parameter to optimize in display-ing surface features. When the threshold is turned

Fig. 4.Acquiring 4-D volumes using the rendering line to set up the acquisition. (A) Acquisition plane of fetal face.Notice that no structures are present between the face and the rendering line (dotted line). (B) Rendered image offace resulting from sweep in (A). Notice that the 3-D sweep is made with the face in a profile or sagittal planewhile the rendered image (B) is frontal, or 90 from the acquisition plane.

Fig. 5. Gestational sac in a septate uterus. On 3-D US, the sac was seen in the right horn of a septate uterus(arrow). Top left image is the uterus transverse, and top right uterus is sagittal, and bottom left image is coronalshowing two horns and normal myometrium in the fundus.

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up, the low-level echoes are eliminated from theimage. If, however, the threshold is turned up toohigh, the image disappears before your eyes.

Scanning Tips

First, set up your image parameters as you wouldgenerally for a 2-D US except use a little morecontrast in the 2-D images. A poor 2-D image willresult in a poor 3-D image, whereas good 2-Dimages will likely result in good 3-D images. Afteroptimizing your image quality, scan over the areaof interest as per a standard 2-D US. Second, deter-mine which plane is most important anatomicallyfor the region of interest. For example, the best

views of the palate are obtained in axial plane. It

is in this acquisition plane that you have the best

resolution in the volume. Scan over the region in alinear fashion, as the 3-D transducer will do auto-

matically and position the transducer directly overthe center of the region of interest since the volume

will be swept out equally on either side of the

center of the transducer. Third, the optimal imageis obtained when there is no motion. Therefore,only the motion from the transducer positionchanges is desired. This is best obtained by askingthe patient to hold her breath during the acquisi-tion, even when performing endovaginal scans.

Fourth, hold the transducer steady and pushthe button to acquire a volume. Fifth, review the

volume quickly by scrolling through one of the

planes to make sure the images collected are fromthe desired region of interest and that there wasminimal motion.

If using 4-D US, similar scanning tips are used.The transducer automatically acquires continuousvolumes by moving the acoustic array within theprobe. When setting up the volume (ie, scanning in2-D), it is important to find an area with fluid

adjacent to the fetus. Turn on 4-D and place therendering line adjacent to the structure in the am-niotic fluid so that there are no intervening struc-tures [Fig. 4]. When the desired rendered image isseen then it can be displayed on the monitor as a

Fig. 6. Standard orientation of the fetal face. Upper left image in profile. Upper right image is symmetricalorbits. Lower left image is coronal view. Lower right image is rendered image of fetal face. The marker dot is onthe fetal nose.



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single image but as the fetus moves viewing the 2-Dlocalizing image will again be necessary. Generally

we use only a 2-image display to localize for 4-Dimaging as we find that it is difficult for our brainsto evaluate more than two moving images simul-taneously. However, if we are having difficulty in

figuring out what structure is obstructing our view,we will occasionally turn on a four-image display toassist in understanding the orientation issues. Gen-

erally, the operator holds the transducer still on thematernal abdomen and allows the fetus to moverather than moving the transducer around theabdomen. Occasionally, very minor, slow move-ments of the transducer can be helpful.

One advantage of 4-D US scanning is that thevolume can be steered while scanning the pa-tient. This is not possible with 3-D US because

the volume is stationary and is viewed after theacquisition. But in 4-D US, the volume can be

viewed from any angle while the scanning isbeing performed. This means that when one sideof the face is up against the placenta, the volume

can be rotated (using the X, Y, and Z knobs) to viewthe fetus from a more optimal angle. This is differ-ent from rotating the rendered image 90,180, or

270to see the fetus in a more anatomic position, atechnique that is available on equipment today.

The volume steering technique is a true advantageof 4-D US scanning, and we believe a major assis-tance to evaluating movement of fetus arms andlegs. In the past, this has been called beam steer-

ing but truthfully, the beam angle is stayingthe same and the volume is being rotated to viewit optimally.

Archiving 3-D US and 4-D US data

Volume data are different than 2-D image data. It isimportant for the sonographer/physician to realizethat saving data for review at a future time requiresan understanding of the types of archiving possi-bilities. When a single image is archived then we

save it as any 2-D image. However, this new tech-nology allows us many new options. If the entire

volume is needed for future manipulation, thenthe volume must be saved. If a cine clip of the3-D volume is needed, then a video clip must be

saved. If a cine loop from the 2-D image data isdesired then a different video clip mechanism is inplay to save it. Multiple volumes from a 4-D acqui-

Fig. 7.Display of the upper lip in the multiplanar view at 29 weeks. Face is tipped posteriorly slightly to show thefetal lip (arrow) optimally in the coronal plane. Marker dot is on the upper lip in all 3D multiplanar images.

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sition can be saved or a single volume from the 4-Dacquisition can be saved. Data can be archived onthe equipment itself and often downloaded onto aCD-rom or other storage device for review on a

workstation with 3-D capabilities. Additionally,jpg images and video clips can be downloaded

onto a CD-rom, which can be given to the patientfor their review with family and friends. Each man-ufacturer uses their own terminology to identify

these different archiving capabilities, and the opera-tor must learn what is available on the machinethey are using. The size of the images, volumes, and

video clips will vary significantly depending on thetransducer used, the size of the region of interestbox, and the equipment vendor.

Transvaginal 3-D sonography

3-D transvaginal scanning has been used not onlyfor assessment of uterine anatomy and detectionof congenital anomalies but also for location of ec-topic pregnancies, particularly cornual (interstitial)and cervical ectopic. The coronal plane of the

uterus can help differentiate a cornual ectopic preg-nancy from a septate uterus with a gestational sac inone horn of the uterus [Fig. 5]. Detailed surfaceand multiplanar images of the embryo [8,9] andthe fetal face can be obtained as early as 9 weeksgestation using transvaginal 3-D US [10]. Fetal

abnormalities are now being detected with trans-vaginal 3-D US [10,11] including the brain [12],spine[13], face, and skeleton.

More recently, evaluation of cervical volumesin pregnant patients has been undertaken [14].

This may prove useful in the evaluation of cervi-cal incompetence.

Usefulness of 3-D and 4-D US in obstetrics

Multiple studies have compared the performanceof 3-D and 2-D US in detecting fetal anomaliesprenatally. A review of the literature has shown a51% to 64% diagnostic advantage of 3-D US[15,16]. 3-D US has been shown to be helpful inassessing anatomy and pathology, evaluating theextent of disease, detecting a variety of abnormali-

Fig. 8.Narrowed region of interest to display fetal face. The boundaries of the region of interest box are narrowedto eliminate echoes in the volume that do not contribute to the surface of the face.



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ties (a few not seen on 2-D US), and counselingpatients. A few of these will be mentioned later.

Good normal face

3-D US is a valuable tool in examining the fetal

face. Multiple abnormalities including cleft lip/palate, micrognathia, dysplastic ear, and midfacehypoplaisa have been examined using this tech-nique [17]. Enhanced images of fetal face may beobtained if there is some amniotic fluid adjacent tothe fetal face. This is achieved by changing maternalposition or tapping on the maternal abdomen nearand distant from the transducer. A gentle pushingof the fetus or fluid by the palm of the hand can behelpful. The best surface-rendered face images areoften obtained sagittally or just obliquely. Using

2-D US technique, the facial profile should beobtained first. The region of interest boundariesare then placed around the entire face and the

3-D US volume swept. Both planar and renderedimages are useful in assessing the fetal facial fea-tures [18]. Rendered images are best obtainedbetween 22 and 30 weeks.

Cleft lip and palate

Cleft lip and palate is one of the most common

congenital abnormalities and is highly associatedwith other congenital abnormalities[19]. 3-D US isa useful tool in correct identification and determi-

nation of the extent of the facial clefting. Accuracyof diagnosis of facial clefting is higher using 3-D US

when compared with 2-D US[20,21]. A clear viewof a fetal face with anomalies has a tremendous rolein making decisions regarding continuation ofpregnancy, bonding, and conceptualization of thepostnatal surgical care of the newborn.

Processing of volumes of fetal face using 3-D UShas been extensively studied yet we continue tolearn new ways to obtain optimal pictures. It is

important to standardize the orientation so that

the face is viewed symmetrically. This is done byacquiring, in a standard plane if possible, either

sagittal, axial, or coronal. After the volume hasbeen acquired the face should be rotated so thatthe orbits are symmetrical. This is performed easilyif the marker dot is positioned on the center of thenose and the volume is rotated; this often requires a

Fig. 9.Multiplanar view of normal primary palate (anterior alveolar ridge). The face is in profile in upper left. Themarker dot is on the palate. The green reference line overlying the upper lip on the rendered image localizes thepalate on the upper right-hand axial image.

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Fig. 10.Multiplanar and rendered view of cleft in primary palate and lip (anterior alveolar ridge). The left upper isan image of the profile; the right upper is cleft of primary palate (short arrow) in an axial view; bottom left imageis coronal view of cleft lip (long arrow), and bottom right is a rendered image of cleft lip. The green line representsthe level of cleft palate on the coronal view.

Fig. 11. Rendered view of normal primary palate (anterior alveolar ridge) and cleft palate. (A) Normal primarypalate. (B) Cleft of primary palate (arrow). These images were obtained by increasing the threshold on the surface-rendered image.



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slight rotation in X, Y, and Z planes. The profileshould be lined up so that the face is pointingupward in the upper-left image [Fig. 6]. The bestmultiplanar views of the lip are obtained in coronalplane with face tipped slightly posteriorly [Fig. 7].

For optimal-rendered images of the face, theregion of interest boundaries can be narrowed,eliminating much of the echoes in the volumethat do not contribute to the surface information[Fig. 8]. This technique can be used with both 3-D

and 4-D imaging and are quite helpful as the gesta-tion progresses, often allowing quite good images

in the late third trimester when only a sliver of fluidis present in front of the face.

The palate can be evaluated in both multiplanarand rendered views. The multiplanar method ismore familiar to most sonographers because theaxial view is routinely obtained during 2-D scan-ning. The face should be acquired in the axial planeso that there is minimal shadowing from adjacentstructures (the upper half of the palate or extremi-ties) and that there is optimal resolution in theaxial plane. 3-D offers additional information in

some cases because the face position can be stan-dardized. On 2-D imaging, it can be difficult todifferentiate the palate from the mandible and

although we have rules for making the distinction,in clinical practice we have found that sometimes

we incorrectly identify the maxilla. For this reason,

3-D provides additional clues from the perpen-dicular views and from then rendered view to con-firm that the maxilla or anterior alveolar ridge arebeing evaluated. A reference line can be seen cross-ing the rendered view at the level of the upper lip,identifying the appropriate axial image of the palate

[Figs. 9 and 10]. This can also be confirmed usingthe marker dot on the multiplanar images. After theface is standardized, the axial plane is scrolled

through mm by mm to assess each tooth bud inthe anterior alveolar ridge (primary palate or hardpalate). The palate can also be examined on the

rendered view. This can be done by looking at theface directly and increasing the threshold until onlythe bones of the face are visualized [Fig. 11]. If thethreshold is increased too high, then pseudo cleftscan be seen. Interestingly, when skeletal parameters

are used, we do not obtain optimal images to dis-play the cleft. An additional pitfall occurs whenshadowing from other structures lead to a breakin the maxilla. It is, therefore, very important toconfirm any suspected cleft palate seen on therendered views with the multiplanar view, particu-larly the plane of acquisition where shadows aremost obvious. Although preliminary data suggest

that the soft palate or secondary palate can beevaluated with 3-D US and 4-D US [2224], wehave not been able to consistently evaluate this

Fig. 12. Rendered view of scoliosis in 22-week fetus.Scoliosis is at T5 level.

Fig. 13. Rendered image of club foot. The leg isupright (short arrows) and the foot is turned inward(short arrows).

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Fig. 15.Artifact from an intrauterine device (IUD) simulating an IUD within the sac. The white echo arising fromthe IUD is a comet tail from reverberations off the IUD and then a shadow as it goes through the uterus behindthe gestational sac. The marker dot is on the comet tail in all three planes. Upper left image is the acquisitionplane, and the comet tail is well recognized emanating from the echogenic IUD ( arrows), which is outside of thegestational sac. In both the upper right image and lower left image, the comet tail is seen as a curvilinear structurewhich could be misinterpreted as IUD. In reality, it is only an artifact.

Fig. 14.Normal hand (A) and a picture of bilateral polydactyly (B). Arrows point to extra digits.



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region, and we believe that further experience is

needed to evaluate it confidently.

Extremities and skeleton

Using surface rendering, maximum intensity andx-ray mode, fetal bony structures and extremities

can be visualized clearly. The surface-rendered imagesare often helpful in assessing motion of the ankles,opening and closing of the hands, and movementof the arms and legs. They are also helpful incounting digits of the hands. The maximum inten-sity mode or increasing the threshold on the surface-rendering mode (described above to evaluate thepalate) are useful in evaluating the bones (ribs,long bones, clavicles, spine), which can be seen

with reliability and reproducibility [25,26]. Using

these techniques is it possible to evaluate deformi-ties of fetal spine [Fig. 12], thorax, or long bones.

We have found 3-D to be particularly helpful in

evaluating for club foot, both because the ankle canbe displayed and evaluated in a standard orienta-tion (without movement on 3-D US) [Fig. 13] andmovement of the ankle can be assessed using 4-D

US. Some investigators have suggested that 3-D USis the tool of choice in evaluating the hands andfeet due to capability of rotating the volumes[Fig. 14] [26]. However, fast movement of handsand feet, positioning of the extremities adjacent tothe uterine wall, or some other suboptimal positioncan potentially be an obstacle in their evaluation.

Although 4-D US may prove to be more useful, theprimary stumbling blocks remain the same.

Fig. 17.Motion artifact presented as conjoined twinsat 24 weeks (A). Actual image of face is shown in (B).

Fig. 16. Rendering artifact creasting the false image of hole in the head. This occurs due to the boundaries

of the rendering box passing through the cranium.

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Pitfalls/limitations/artifacts

Despite advantages of 3-D US mentioned pre-viously, there are still some problems in the 3-Dtechnique that should be taken into account:

1. Similar to learning any new imaging modality,the learning curve is variable in different indi-viduals. The examiner must adjust to examiningthe stored volume to obtain the desired view.

Also, common means of orientation such asleft, right, cranial, and caudal are not necessarilyrelated to the position on the screen. The sided-ness should be directly correlated with theimage of the fetus because the volume can berotated in all directions.

2. During acquisition of the volume, the probe aswell as the fetus should remain stationary to

avoid movement artifacts. This may take as longas 4 seconds depending on the size of vol-ume acquired.

3. Quality of surface-rendered images is as goodas the quality of original data. The examiner

needs to have knowledge on how to obtainthe best image by rotating and eliminatingstructures that disturb the surface views (for

example, umbilical cord or extremities obstruct-ing the facial views). For this reason, surfacereconstruction in oligohydramnios cases is

nearly impossible.4. Artifacts that simulate pathology are commonin 3-D US and 4-D US as they are in 2-D USimaging. The shadows that we routinely seein 2-D US emanating from calcified structures(bones) or other dense objects are easily recog-nized. The shadows seen in 3-D US are easilyrecognized in the plane of acquisition but arenot obvious in the reconstructed planes. Thesecan lead to misinterpretation of the study[Fig. 15]. Region of interest boundaries can

lead to apparent defects such as a hole in

the head or a missing limb [Fig. 16]. Motioncan lead to an apparent image of conjoined

twins [Fig. 17].

Summary

Clinical data on the application of 3-D US and itsrole as a screening modality in obstetrics and as anadjunct to 2-D US in the diagnosis of fetal anoma-lies are limited. Although investigators have been

using 3-D US for more than 12 years in theirclinical practices, our knowledge base is still atthe fundamentals level. As this new technology is

dispersed widely and is used by a vast number ofclinical practitioners, we are sure that advance-ments will be forthcoming. In summary, 3-D US

is a promising tool for the evaluation of the fetus.Well-designed studies are needed to show its clini-cal effectiveness is daily practice.

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