After reading this article the learner will be able to:

• Evaluate clinical situations in gynecology where 3D scanning is helpful and where it is not useful beyond the 2D examination.
• Discuss the multiplanar reconstruction technique in scanning the pelvis, including the usefulness of looking at the coronal view of the uterus to evaluate the endometrium and uterine shape.
• Evaluate the use of 3D sonohysterography in the detection of endometrial abnormalities such as polyps as well as submucous fibroids.
• Determine in which patients 3D of the adnexa is of benefit beyond traditional 2D ultrasound.

Two-dimensional ultrasound of the female pelvis has been established for several decades as the most reliable and effective way to image the uterus and ovaries and the surrounding structures. Many years ago a full bladder was necessary to visualize the pelvic organs effectively since sound transmission was enhanced through the urine in the bladder and bowel gas was displaced above the uterus. Women are no longer being subjected to a fully distended bladder for pelvic ultrasound in most facilities, thanks to the advent of improved sonographic equipment, and most importantly, the endovaginal probe. (1)

Although the endovaginal approach has revolutionized our ability to image the female pelvis at high frequencies, it is limited by the port of entry, which is narrow (the vagina), only allowing certain views of the pelvis. In order to formulate an opinion of the anatomy, therefore, we (as ultrasound professionals) are dependent upon our own mental ability to reconstruct the two-dimensional image into three dimensions in order to make a diagnosis. The recent advent of three-dimensional volume acquisition scans of the female pelvis, specifically using a transvaginal approach, has been one of the most important advances in ultrasound of the last few years. This allows us to display an actual image that, so far, we have only been able to conjure in our mind’s eye. Ultimately the reconstructed images, particularly the coronal planes, which allow visualization of both the cornua and cervix in the same image slice, are among the most valuable views for the diagnosis of uterine abnormalities.(2) In addition to producing transverse and longitudinal images of the pelvis (for example, the uterus), the coronal view can be displayed for the first time, showing the exact shape of the uterine cavity with the relationship of the cornua to the cervix. This results in a new and enhanced capability of ultrasound to display uterine anatomy, heretofore only available using MRI.

Figure 1. Reconstructed coronal view of a normal uterus showing the intrauterine cavity. Note that the display demonstrates both cornua and the cervix in the same plane, an orientation which cannot be obtained with direct scanning but can only be reconstructed using a volume.

Transvaginal 3-D ultrasound is performed by acquiring a volume of 2-dimensional scans, which forms a data set to be stored in the machine. Three data sets are sufficient to image the uterus and the ovaries, using one for the uterus and one for each of the ovaries. The data set can then be manipulated and evaluated after the patient leaves the ultrasound office, by displaying images cuts from the original volume in any orientation desired. This enables us to go over the images, not only in the original scan plane but also in any other scan plane desired, including planes which were not obtainable on standard 2D imaging. The ability to display the anatomy in orientations other than the original scan plane, is among the most important features of three-dimensional reconstruction in the pelvis. (2) This technique provides the ability to “rescan” the entire saved acquisition volume in any plane, and to navigate through that volume at will.

Once the volume has been stored, a single point in space can be selected in the volume, and this point can be visualized in all three perpendicular planes. One can navigate through the volume, keeping track of a single point in space in all three planes. This is valuable not only for imaging the uterus and adnexa, but also for measuring distances and even volumes of organs. One can then “rotate” the organ by spinning the image in any of the planes. One can also navigate through a single plane while watching the corresponding effect on the other two planes. This gives us complete control of the volume to view it or perform measurements in any display desired.

By displaying the volume in a plane that cannot be acquired by scanning directly (reconstructed plane), we are able to display planes that, prior to 3-D, could only be imagined and reconstructed mentally. One can also cut away certain portions of the image to view the inside of a cavity. This “cut-open” view enables the operator to carve out part of the organ on the entire volume and examine, for example, a portion of the endometrium as well as the myometrium directly below it.(2) This is particularly useful when evaluating possible invasion of a uterine cancer from the endometrium into the myometrium.

Three-dimensional ultrasound is also necessary to reconstruct the surface views of organs or interfaces. This is particularly useful when evaluating the surface of the endometrium. For example, in a sonohysterogram (see later), when fluid has been introduced into the intrauterine cavity, a partial rendering of the surface of the intrauterine cavity can be used to display a possible polyp or submucous fibroid. This technique is described as virtual hysteroscopy showing the surface of the endometrium; much like what is seen using a hysteroscope.

Finally, ultrasound experts or offsite practitioners often receive images from remote sites for review and diagnosis. Standard 2D ultrasound such as video clips or still images provide the central site with user dependent images that must be read as acquired originally. Sending a 3D volume set enables the central reader to “rescan” the volume in a different orientation than originally acquired, and provides far more opportunities to display and interpret the anatomy.

The most important application of three-dimensional ultrasound of the uterus involves the visualization of the anatomy in planes not available for direct scanning (reconstruction planes). This enables us to display coronal views of the uterus and, for example, show the precise position of an intrauterine contraceptive device within the uterine cavity. In a study done by Lee et al., complete simultaneous imaging of all parts an IUD was possible in 95% of patients using three-dimensional ultrasound.(3) The same coronal view can also be used to evaluate abnormalities in uterine shape, such as congenital uterine anomalies.(4) Comparing the shape of the endometrial cavity with the uterine surface contour can clearly display the differences between a bicornuate and septate uterus . It is also very difficult to discern the difference between an arcuate and a septate uterus using two-dimensional ultrasound images only, as the exact shape of the uterine cavity can only be displayed in a non-scanning plane.(4)

Figure 2. Bicornuate uterus: Coronal reconstructed view of a bicornuate uterus, showing that the endometrial cavity is divided into two horns and the surface of the uterus is also divided or indented in its superior portion, making this different from a septate uterus.

Patients with septate uteri have a much higher incidence of first-trimester pregnancy loss compared with those with normal uteri, and women with arcuate uteri have a higher proportion of second-trimester losses and pre-term labor. There is, therefore, great value in the accurate determination of uterine shape, particularly in patients who are of reproductive age. (4) Woelfer, et al, have shown that three-dimensional ultrasound is accurate in depicting these abnormal uterine shapes.(4) Uterine surgery to correct septi, for example, can be planned based on three-dimensional ultrasound imaging without the need for MRI, which until now was the only modality available to demonstrate uterine shape anomalies accurately.

Figure 3. Septate uterus:Coronal reconstructed view of a septate uterus, showing that the uterine cavity is divided in two by a substantial septum; however, the outer surface of the uterus is not indented, indicating that this is not a bicornuate uterus. This is a reconstructed view which cannot be obtained directly in most cases.

Fig. 4a. Standard transverse view of the uterus, showing two separate portions of the endometrium divided by a tongue of myometrium or septation. It is unclear using standard 2-dimensional imaging whether this represents a septate or arcuate uterus.

Fig. 4b. Arcuate uterus: Reconstructed coronal view of the same uterus, showing a shallow broad septum or arcuate uterus. This image could only be displayed through reconstruction of the volume set.

The reconstructed coronal view is also important for the evaluation of uterine masses and endometrial abnormalities. Often it is possible to assess the endometrium for polyps and submucous fibroids without even having to do a sonohysterogram. In many cases, the coronal views of the uterus and the ability to navigate through them permit us to visualize the cavity and material within more precisely and accurately than using traditional two-dimensional slices. In a pilot study, we evaluated the endometrium of 43 patients with abnormal uterine bleeding. These patients first underwent a standard 2D scan, followed by a 3D volume acquisition with multiplanar reconstruction. In 15/43 (35%) the addition of the 3D coronal reconstructed view was helpful to detect and /or localize polyps or fibroids. While these abnormalities were suggested on standard 2 D imaging, the exact nature of the anatomic finding was ill-defined. Coronal views obtained by 3D reconstruction, clarified the findings and resulted in a more definitive diagnosis. Patients with normal initial 2D scans of the uterus and endometrium did not have additional benefit from the 3D imaging. The diagnostic gain of 3D Sonography in our study occurred among patients with suspected abnormalities on 2D.

Fig. 5. Coronal view of the uterus, showing a large endometrial polyp occupying the left fundus, distending the region of the left cornu. It was not necessary to introduce contrast or fluid into the uterine cavity to demonstrate this polyp due to the ability to reconstruct non-scan planes such as this one..

Three-dimensional volume ultrasound is therefore superior to two-dimensional views in identifying the exact location of endometrial polyps, the degree of protrusion of a submucous fibroid into the cavity, as well as the amount of myometrium remaining outside of the submucous fibroid, all information which is needed before hysteroscopic resection of these lesions can be undertaken. One can also measure the volume of such a fibroid or polyp or even the volume of the entire endometrium, quickly and easily using a 3D volume set.(5)

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Fig. 6a. Longitudinal acquisition view of the uterus, showing a small endometrial polyp		Fig. 6b. The polyp is seen best in this coronal reconstructed view of the endo-metrial cavity, showing that the polyp resides along the left lateral border of the endometrium, just below the left cornu.
Click on image for larger view.
Fig. 7a. Longitudinal view of a post-menopausal patient, showing a thin endometrium. The endometrial stripe appears to be interrupted, suggesting there may be a small polyp.		Fig. 7b. Reconstructed coronal view of the same uterus, showing a small endo-metrial polyp, almost completely residing in the intrauterine cavity but based along the right side of the cavity, mid-body. This is an easily resectable fibroid, hysteroscopically.
Click on image for larger view.
Fig. 8a. Acquisition transverse view of a uterus with a submucus fibroid, which normally would require the performance of a sonohysterogram to identify its exact location.		Fig. 8b. Coronal reconstructive view of the same submucosal fibroid, showing that it is almost completely intra-cavitary and based along the left side of the cavity. The echogenicity of the endometrium acts as a contrast medium.

In early pregnant patients, the location of the gestational sac, particularly if it is near a cornu, can be assessed most accurately using the reconstructed cornual view of the uterus.(2) It is far easier to determine whether or not an early pregnancy is cornual versus eccentrically located within the uterus using a reconstructed coronal image of the cavity which demonstrates the relationship between the cornua, the endometrial cavity and the cervix all in the same plane. Reconstruction of the coronal plane is also helpful in assessing sac location within a bicornuate or septate uterus thus not mistaking a sac within a horn as a cornual pregnancy.

It may be necessary to distend the uterine cavity with saline or other contrast material in order to evaluate the endometrial lining effectively. By filling the cavity, the endometrial contour can be displayed showing the shape of the cavity, as well as any mucosal or submucosal defects that may be present. Although sonohysterography is widely used with two-dimensional ultrasound to display endometrial abnormalities, it is the 3D volume acquisitions and the display of any reconstructed plane that permits the optimal assessment of the endometrium and uterine cavity.(6) This technique allows the detection and accurate location and volume of endometrial polyps and submucous fibroids. Sonohysterography with three-dimensional reconstruction gives the gynecologist all of the needed information for planning route of surgery for fibroids. The decisions regarding whether a fibroid can be resected at hysteroscopy or whether an abdominal procedure is necessary requires accurate information such as the exact location of the fibroids, degree of protrusion into the cavity and width of remaining myometrium around the fibroid. One can also perform surface rendering of the endometrial lesion, such as a virtual hysteroscopy, in preparation for the surgery. Sonohysterography can also be done in real time (4-D), where the distention of uterine cavity, cornua and tubes with saline or other contrast media can be displayed as it takes place.

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Figure 9. 3-D Sonohysterography
a) Standard acquisition longitud-inal view of an endometrial cavitgy distended with fluid, showing a polyp.		b) The location of the polyps is clearly seen on this reconstructed coronal view of the uterus, manipulated such that the base of the polyp is well displayed.

Another advantage of 3-D hysterography over 2-D is the rapidity with which the images can be obtained. A couple of quick volumes of the uterus can be generated in seconds and the patient may leave the ultrasound suite while the practitioner reviews the images in any orientation desired. This may preclude the need for using a balloon catheter to keep the fluid in the uterine cavity while multiple 2D images are taken. A simple and relatively inexpensive insemination catheter is usually sufficient to distend the cavity with fluid for only a few seconds to obtain a volume. Balloon catheters are far more expensive and may be uncomfortable for the patient if the fluid is to remain in the cavity under pressure during imaging.

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Fig.10. Sonohysterography using 3-D
a.)Single acquisition image of fluid in the endometrial cavity, showing a normal endometrium. The region of the left cornu was difficult to demonstrate in 2-D although there was the question of an abnormality in that area (not shown).		b) 3-D reconstructed coronal view of the uterus shows a polyp in the region of the left fundus/cornu, which is far easier to demonstrate using this display but which was only suspected with 2-D

Ultrasound has clearly been the most important and accurate imaging modality for the evaluation of the adnexae. The use of two-dimensional ultrasound has been well studied and the characteristics of malignant ovarian masses as compared to benign ones are well established using two-dimensional ultrasound, as well as Doppler imaging. Three-dimensional imaging of adnexal masses adds an additional tool to our ability to visualize the pelvis. It enables us to take a single sweep of each of the adnexae and save it as a volume to be displayed later in any orientation desired, similarly to the methods of imaging the uterus described earlier. One can evaluate the location and volume of any solid or nodular area, and then display the vascular component using 3D power and color Doppler. (7, 8, 9) The branching of tumor microvasculature can be demonstrated and the pattern and location of these branches can be studied.

The evaluation of septate cystic masses that are suspicious for hydrosalpinges lends itself particularly well to the 3D volume acquisition and displaying of reconstructed planes. Hydrosalpinges can have a confusing sonographic appearance, often mimicking other complex adnexal masses such as ovarian tumors. Due to the small portal of entry of the vaginal probe, the 2D acquisition views of these masses are limited, sometimes making the precise diagnosis of hydrosalpinx uncertain. With the ability to display many more reconstructed planes, a 3D volume can be manipulated to demonstrate the tubal appearance of a hydrosalpinx much more clearly.

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Fig 11a ) 3-D acquisition view of the adnexa of a patient with a septate cystic mass. The mass can be viewed in any orientation desired.		Fig 11b) 3-D reconstructed coronal view of the cystic mass showing that the shape is consistent with a hydrosalpinx. This view was not possible with 3D reconstruction.

Not enough information is available to date to determine whether 3-dimensional imaging of the adnexae adds significant information not available with standard 2-dimensional scanning, although some practitioners suggest that it does. It certainly increases the speed and ease by which the adnexa can be imaged since volumes of information can be stored in seconds, to be evaluated later. This can markedly decrease the time of the examination of the patient in the ultrasound suite and therefore increase the turnover of the room. The interpretation time of the scan, on the other hand, can be lengthened due to the time needed by the practitioner to manipulate the volume into readable images. There is a definitely a learning curve with more experienced personnel being able to achieve faster and better 3D acquisitions and reconstructed displays than novices.

One of the largest and most important applications for pelvic ultrasound is infertility and three-dimensional ultrasound has been useful for evaluating and following infertile patients. Certainly the evaluation of the shape of the uterine cavity, as well as any lesions within the cavity and the endometrium, are particularly important in these patients. These techniques were described above. There is also information that the receptivity of the endometrium can be evaluated using transvaginal color Doppler and three-dimensional power Doppler.(10) Patients with a low resistive index of the subendometrial vessels reportedly have a higher incidence of pregnancy. Further investigation is needed to evaluate the reliability of these findings as well as the role of 3D ultrasound in the assessment of endometrial receptivity. Three-dimensional ultrasound is also used for more effective embryo transfer and other invasive procedures. Three-dimensional guidance of embryo transfer procedures can be helpful to seek an optimal transfer area in the uterine cavity in order to increase pregnancy success rates.(11)

Finally, the assessment of the ovaries using three-dimensional ultrasound can be helpful for measuring and evaluating follicles. In situations where there are many developing follicles evaluated from day to day, it may be helpful to obtain an ovarian volume which can then be oriented in the same way time after time, to facilitate tracking and measurement of individual follicles.

In conclusion, following the advent of the transvaginal probe, 3-D ultrasound for the female pelvis is one of the most important recent advances in ultrasound. It enables the ultrasound specialist to obtain an entire volume of the area of interest, such as the uterus, and display it in orientations where scan acquisition was not possible. Pelvic sonography is particularly hampered by the narrow portal through which the entire transvaginal pelvic scan must be performed. Additional reconstructed plane of imaging, not directly obtainable in 2D, allow far better evaluation of the uterine cavity and its lining than ever before. The techniques described here have applications for a wide variety of patients and many different indications, spanning from infertility to postmenopausal bleeding. In the future, we look forward to even better and higher speed real-time capabilities in 3-D (4D), with higher frame rates similar to what we have currently in real-time 2-D. We also anticipate more research into the application of Doppler, both color and pulsed, as well as volume measurements that could only be estimated previously. The use of contrast in the pelvis will most likely be enhanced by 3-D imaging as well. While 2D ultrasound remains the basis for the sonographic examination of the female pelvis today, much research is needed as the 3D technology is further developed, to determine where the adjunct of 3-D ultrasound will contribute in the evaluation of the pelvis.

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