Chapter 47: THREE-DIMENSIONAL VOLUMETRIC SONOGRAPHY IN GYNECOLOGY

Three-dimensional (3D) volume imaging of the female pelvis is one of the most important advances in women's imaging within the last decade.^1,2 Volume imaging is not new to radiology, as it has been used extensively as part of computed tomography (CT) and magnetic resonance (MR) imaging for several decades. Ultrasound, on the other hand, had been relegated for more than 30 years to individual acquisitions of thin two-dimensional (2D) image slices that are very operator dependent and time consuming to obtain.³ These individual 2D images are largely dependent on the expertise of the operator that produces them, and even a careful review of these individual "snapshots" will often fail to demonstrate the anatomy if the operator does not specifically perceive the finding.

More recently, 3D sonography has enabled us to move ultrasound into the new era of rapid, automated, and comprehensive imaging and displays.^{1,2,3, and 4} Images of the entire pelvis can be accomplished with only 3 volume acquisitions: 1 of the uterus and 1 of each ovary.⁴ These volume acquisitions typically contain all of the sonographic anatomic information within the pelvis and can subsequently be rescanned electronically in any plane by a different operator who may be off site.^{1,2,3, and 4}

The current methods of sonographic volume acquisition entail automated transducers, which have a dedicated 3D probe within their housing, mechanized to gather 2D ultrasound information from one side of the probe to the other (Figures 47-1,47-2,47-3,47-4,47-5,47-6, and 47-7). The operator holds the transducer still, while the probe sweeps from one side of the housing to the other and the volume is acquired in a systematic and reproducible fashion. However, the faster the speed of this mechanical acquisition, the lower the resolution of the images. Different types of processing can also modify the quality and appearance of the volume data acquired, such as the use of harmonics or other types of image-modification settings.

Once the 3D volume has been acquired, this large amount of sonographic information can then be displayed in many different ways, as follows: The raw information within the volumes can be displayed in any plane of orientation within the volume—even in planes that cannot be acquired directly and must be completely reconstructed from the volume data.^{1,2,4,5, and 6} Such planes include the coronal view of the uterus and adnexa, much like the appearance of these organs on a Netter anatomic diagram, designed to best demonstrate the pelvic anatomy.⁷ Such a coronal view of the female pelvis may be the most accurate and informative view for diagnosing uterine and tubal pathology. The most common initial display used to evaluate a volume is the multiplanar image set of 3 individual pictures at right angles to each other, representing planes X, Y, and Z. There is a single dot which represents the intersection of these 3 planes, and the operator can navigate easily through the volume using the 4 basic tools of X, Y, and Z rotation and slicing from one end of the volume to the other.⁸ The display of these 3 orthogonal planes is usually the way the volumes are commonly first presented to the operator for manipulation.

Volumes of 3D information can also be displayed using surface rendering techniques, where fluid is used as an interface to show the surface of an organ.⁹ If there is fluid within the volume, a rendering line of sight can be drawn through that volume, close to the interface with the solid area, such that the topographical surface of the solid area can then be displayed. Alternatively, multiple tomographic cuts that are equidistant from each other can also be generated to display the information within the entire volume, much like a CT or MR display.¹⁰

To accomplish the multiple tomographic cuts, a single plane is selected from within the volume, and the entire volume is sliced in that orientation. Finally, the volume can be displayed in an inverse mode, in which the solid areas melt away and all of the cystic areas are made into a cast of the inside of the volume.^11,12 In addition, once the volume is obtained, volumetric measurements can be made within the data set, thus enabling the specific quantitative measurements of certain portions of the anatomy.¹³ Finally, the acquisition of the volume can also be made using Doppler, such that blood flow data can be incorporated into the information obtained within the volume.¹⁴ One can then interpret the Doppler image qualitatively in terms of mapping or quantitatively in terms of blood flow velocity and waveform.^{14,15,16, and 17}

All of these types of displays, and more, are used to tease-out the valuable raw sonographic information within a volume, regardless of how the volume was acquired. This makes 3D volume imaging of the pelvis far more comprehensive compared to the traditional spotty acquisition of individual 2D ultrasound images.^1,2 Once the volume has been acquired, all of the information resides within that volume, as though the patient were still present. The volume can then be sent to a facility distant from the scanning site and reviewed by one or several different practitioners, as though the patient herself had been sent to that location.^1,2,4 The raw data captured within these volumes permit ultrasound to become far less operator dependent and more automated, making it possible to make diagnoses that were not necessarily noticed by the original operator obtaining the images. With traditional 2D ultrasound, the operator has complete control over which images are photographed and documented, perhaps occasionally overlooking and not recording important anatomy. Volume imaging, on the other hand, permits the acquisition of all of the information, regardless of whether the operator perceived it at the time of the scan. Even if the operator is not as experienced as the person ultimately reading the images, information that may not have been apparent to the original operator can be retrieved from the volume. This finally allows ultrasound to emerge from the operator dependency with which it was plagued in the past.

Four-dimensional sonography (4D) represents 3D in motion, or real-time 3D imaging.¹ Most current instruments still provide mechanical 3D transducers, so that any attempt at showing a 4D display will be choppy. In such cases, the resolution of the image will be traded-off with the rapidity of the frame rate, making the image both blurry and jerky. The emergence of matrix array transducers that can acquire volumes instantaneously without having to sweep mechanically from one end of the transducer housing to the other will likely improve the resolution and provide a smooth real-time capability. It is important for future instruments, therefore, to eliminate the mechanical aspect of volume imaging and replace it with the ability to achieve instant volume acquisitions that can be accomplished far more rapidly, at full resolution.¹

The Uterus

One of the most important benefits of 3D ultrasound is the ability to display planes that cannot be acquired by direct transducer application to the patient (see Figures 47-1 and 47-2). Consequently, these planes are reconstructed from within the volume and include the most important aspect of the female pelvis: the coronal view.^5,6 When looking at the images of the uterus tubes and ovaries diagrammed in anatomy books such as Netter's, the coronal view of the pelvis is generally the plane used to display the anatomy.⁷ Unfortunately, traditional 2D ultrasound technology has not been able to provide this orientation to view the female pelvis, because the port of entry of the probe (whether it be the vagina or the surface of the abdomen) does not provide a direct path to this view. Now, however, using 3D ultrasound we can reconstruct this coronal view and show the shape of the uterine cavity as well as the outline of the serosal surface of the uterus, information which neither 2D ultrasound alone or hysterosalpingography can provide. Abuhamad et al showed how easy it is to obtain the coronal image of the uterus from within a volume.⁸ They demonstrated the z-technique, which is a simple method by which practitioners can be taught to manipulate the volume so as to obtain the coronal view of the uterus in less than 30 seconds.

Uterine Anomalies

The ability to detect uterine shape abnormalities that may impact fertility is one of the most important contributions of the coronal display of the uterus (Figures 47-8,47-9,47-10,47-11,47-12,47-13,47-14,47-15,47-16,47-17,47-18,47-19, and 47-20). It is well known that uterine anomalies can be associated with a significantly higher proportion of infertility and first- and second-trimester pregnancy loss compared with women who have a normal uterus.¹⁸ Therefore, the correct diagnosis of uterine abnormalities in those women is paramount to rendering the proper treatment. Until recently, an MR was needed to map out uterine shape abnormalities. Currently, 3D reconstruction of the Z plane is a quick, easy, and accurate method of displaying these clinically important uterine shape defects.^5,6,8,19,20 Wu et al undertook a prospective study of 40 patients with histories of infertility or habitual miscarriages and showed that 3D ultrasound had a sensitivity and specificity of 100% each for detecting all congenital uterine abnormalities.¹⁹ Raga et al also studied 30 normal patients and 12 with müllerian abnormalities, and showed that 3D ultrasound was accurate in detecting the anomaly in all cases.²⁰ Although these investigations did not have large numbers, the data are extremely promising.

The specific diagnosis of a septate versus a bicornuate uterus has been particularly confusing when viewing standard 2D ultrasound images, because the internal uterine anatomy looks similar, but the external (fundal) uterine shape distinguishes the 2 entities. The incidence of the septate uterus is considerably higher than the bicornuate uterus,¹⁸ and has important prognostic and treatment ramifications. Traditionally, any uterus where 2 endometrial islands were separated by myometrium in 2D transverse section was often erroneously called a bicornuate uterus. This is due to the sonographer having to try to recall the relative shape of the uterine fundal shape by memory while scanning, rather than being able to see the entire uterus on image. Now, 3D ultrasound can differentiate between these 2 very different entities accurately and easily using the coronal view of the endometrial cavity.^{18,19, and 20} It is crucial to make the correct diagnosis of a septate uterus, since this type of uterine abnormality has a much higher incidence of infertility and spontaneous first-trimester miscarriage when compared with the bicornuate uterus.¹⁸ The correct diagnosis can lead to important treatment options as the septate uterus can be treated surgically with good success, whereas the bicornuate uterus often does not require surgery. The arcuate uterus, while not as severely abnormal as the complete septate or subseptate uterus, is still associated with a significantly higher incidence of second trimester losses and preterm labor compared to women with normal uteri.¹⁸ Other types of uterine shape abnormalities are complex, and may include a combination of bicornuate and septate uteri as well as duplications of the cervix, rudimentary horns which may or may not connect with the main uterine cavity and which may be a potential site for ectopic gestations. This important uterine anatomy can easily be mapped out with a 3D coronal reconstruction of the image of the uterus, thus providing a correct diagnosis without having to go to other cross-sectional imaging techniques such as MR.^{18,19, and 20}

Uterine Polyps and Fibroids

The coronal view is also very valuable for detecting the presence of endometrial polyps and fibroids within the uterine cavity (Figures 47-21,47-22,47-23,47-24,47-25,47-26,47-27,47-28, and 47-29).^2,4,5,21 Endometrial polyps are typically seen as rounded masses with a sharp demarcation or interface between the polyp and the rest of the endometrium. Frequently, these polyps are identified more accurately with the coronal view than with the traditional 2D imaging of the uterus, owing to a better display of the uterine cavity showing both cornua and cervix in the same plane.^4,5 This type of display allows for a better grasp of the position of the polyp within the cavity and its landmarks. Doppler can also demonstrate blood flow to the polyp in these 3D reconstructed views and may be of additional benefit in diagnosing polyps.

The location of fibroids within the uterus is a very important use for 3D ultrasound in general and the coronal view in particular.^1,4,5 The coronal view of the uterus is instrumental in displaying the correct location of fibroids with respect to the endometrium, including detecting the proportion of submucosal component accurately and the distance between the outside of the fibroid and the serosal surface. These important measurements and determinations can best be examined by navigating through the volume and demonstrating tangentially the correct plane to best display these relationships. Volume sonography has also been helpful in monitoring fibroids before and after invasive or noninvasive procedures designed to shrink fibroids. Fleischer et al have shown that 3D Doppler sonography can assess fibroid vascularity before and after uterine embolization, providing a method of monitoring such patients.^22,23 Three-dimensional color Doppler sonography is accurate in depicting fibroid vascularity and can occasionally reveal collateral flow not detected with arteriography.²³

Detection of ovarian collaterals to the uterus by 3D color Doppler is also helpful prior to attempts at uterine artery embolizations, because embolization may occasionally put patients' ovarian function at risk. Three-dimensional color Doppler sonography performed after uterine artery embolization may also indicate the completeness of the embolization.^22,23 This study reported patients who have become more symptomatic after the embolization and have persistent blood flow. Similarly, the finding of a single vessel within the pedicle of a fibroid may be a contraindication for embolization. It is possible that coexistent adenomyosis may be better seen with 3D color Doppler sonography.²²

The Uterus and IUD Location

The coronal view of the uterus is also extremely useful for evaluating the presence of foreign bodies within the uterus (Figures 47-29,47-30,47-31,47-32,47-33,47-34,47-35,47-36, and 47-37).^24,25 The position of intrauterine devices (IUDs) in particular has been especially difficult to evaluate accurately with traditional 2D ultrasound, because the IUD is a 3D device as is the endometrial cavity. Three-dimensional coronal displays have made IUD localization very simple.²⁵ It is remarkable how many IUDs are actually embedded within the myometrium—a fact that has remained unrecognized with traditional 2D ultrasound. Whether an embedded IUD is symptomatic, and actually causes the pelvic pain that brings patients to ultrasound, remains an unanswered question. In our experience, many IUDs which appeared to be positioned correctly or slightly low using 2D ultrasound alone were actually imbedded, at least in part, within the myometrium.

There are important artifacts to consider when looking for an IUD using reconstructed planes.²⁶ The most important artifact involves the shadow of the IUD, which can easily be mistaken for the actual IUD itself. In a standard 2D view, the source of the shadow artifact is obvious because the IUD itself is in the same picture and is easily implicated as the cause of the shadow. A reconstructed plane from a 3D volume that contains a shadow may be confusing because the shadow is isolated from its origin and the IUD is in a different plane not displayed. There is often a shadow in a single reconstructed image where the source of the shadow is not recognizable unless the entire volume is available for review. Because it is easy to misinterpret the IUD for its shadow and miss the correct localization of the device, it is crucial to obtain the entire volume so as to navigate through the volume and follow the shadow to the IUD.

There are 2 major studies demonstrating the overall outstanding effectiveness of the coronal view of the uterus in clinical practice.^5,6 There is strong evidence that a coronal image of the uterus and endometrium should be a part of routine pelvic sonography. Andreotti et al looked at 90 patients and found that the 3D reconstructed view of the endometrium and adjacent myometrium was a valuable adjunct to conventional transvaginal sonography, providing additional information in 53% of patients who had abnormal findings by conventional 2D sonography.⁵ Our study also showed that 3D coronal view of the uterus added value to 2D images in 24% of all gynecologic cases, although it was particularly useful among patients with abnormal initial 2D sonography as well as those with endometrium thicker than 5 mm.⁶ The thickness of the endometrium plays an important role when evaluating the coronal view of the uterus, as the normally echogenic endometrium is used as a visual contrast to the darker myometrium for displaying uterine fibroids and polyps. The thicker the endometrium, the easier it is to outline the shape of the uterine cavity and the presence of submucosal fibroids. Both studies (Andreotti et al and Benacerraf et al) show that 3D coronal views of the uterus are particularly helpful in patients who have an abnormality seen on initial 2D imaging of the uterus or in patients with infertility.^5,6 Both studies showed that the 3D coronal view was able to better characterize the abnormality, and in some cases detect an anomaly not even suspected by the traditional 2D views. When the traditional 2D imaging of the uterus is normal, the 3D coronal view is less likely to be contributory.

The coronal view has not only been useful for imaging the uterus, but it also contributes importantly to imaging the adnexa (Figures 47-38,47-39, and 47-40). In patients with hydrosalpinges, the fallopian tube is dilated with fluid-containing loops of tube appearing in many different planes.^2,3,11 It is often difficult to recognize that multiple adnexal cysts seen with conventional 2D imaging may actually represent different cross sections of a dilated tube. Three-dimensional volume imaging (in particular, coronal displays) can help to demonstrate the connection between these multiple cystic areas depicting a dilated tube rather than multiple cysts. It is remarkable how often multiple adnexal (such as paraovarian) cysts seen with standard 2D imaging may be misdiagnosed as ovarian. When these cysts are viewed with 3D volume imaging reconstruction in a coronal plane, they often appear connected into tubular fluid collections rather than cysts, enabling the correct diagnosis of hydrosalpinx. There may not be a single plane within the volume, which will demonstrate the entire fluid-filled dilated tube. In such a case, the inverse mode (see later in chapter) can be used to produce an echogenic cast of the entire tube and display all the permutations of the tube within the volume, even if they are in different planes.^11,12

The use of 3D volume displays can greatly enhance the efficiency and accuracy of sonohysterography when evaluating the uterine cavity (See Figures 47-21,47-22, and 47-23).^{27,28, and 29} Technically, performing 3D volume acquisitions during sonohysterography allows the procedure to be done far more quickly with less discomfort to the patient. Several 3D volumes of the uterus are rapidly accomplished as the saline is injected, and are saved for later review of the entire uterine cavity. This enables the practitioner to concentrate on performing the procedure at the time the patient is present, rather than on photographing and measuring each lesion while the patient is still on the table and instrumented.

Although sonohysterography of the uterine cavity is well established as being extremely useful to detect many uterine abnormalities, polyps, and fibroids, evaluating tubal patency still remains difficult and controversial, even using 3D reconstruction. Tubal abnormalities are visualized particularly well when the tube is abnormal and hydrosalpinges are present. Until better contrast agents are clinically available, however, the evaluation of tubal patency with ultrasound, even volume imaging, remains limited and controversial.^30,31

To validate the accuracy of 3D multiplanar sonohysterography, Lev-Toaff et al compared findings on 3D with those of 2D sonohysterography as well as x-ray hysterosalpingography.²⁷ These investigators showed that the coronal plane was the most useful image for displaying the relationship between lesions such as polyps and fibroids, and the uterine cavity. The 3D sonohysterography provided additional anatomic information compared with standard techniques in the vast majority of cases. de Kroon et al also compared 3D sonohysterography with conventional sonohysterography among patients with abnormal uterine bleeding.²⁸ Their study showed that 3D saline infusion imaging was reliable in patients suspected of having uterine abnormalities and also demonstrated additional benefits not detectable with conventional 2D sonohysterography in 6.7% of cases. Another study, by Sylvestre et al, also showed that 3D sonohysterography allowed precise and reliable localization of polyps and fibroids within the uterine cavity.²⁹

The ability to reconstruct any plane within a 3D ultrasound volume can be used to image many aspects of the female pelvis other than the uterus and adnexa, such as the pelvic floor. Dietz et al have pioneered the imaging of normal and abnormal anatomy of the pelvic floor using reconstructed 3D ultrasound instead of the more traditional and cumbersome MR imaging.^{32,33,34, and 35} Using a transperineal approach, the transducer is placed on the labia and a 3D volume is acquired that includes the urethra, vagina, and rectum.^{32,33,34,35, and 36} This volume can be used to generate the reconstructed view of the pelvic floor that includes the paraurethral and paravaginal structures, mobility of the bladder floor, and attachments of the pelvic floor muscles. Dietz et al have described the sonographic appearance of major abnormalities of the levator ani and pubovisceral muscles, and have made important progress in the ultrasound evaluation of patients with bladder dysfunction.^34,35 MR imaging of the pelvic floor may ultimately be largely replaced by these simple 3D ultrasound techniques. Imaging of perineal masses such as urethral diverticula and other paraurethral, vaginal, or pararectal masses are also easily accomplished using 3D ultrasound of the pelvic floor.³⁷

When there is sufficient fluid within a volume acquisition, an interface is formed between the fluid and a solid surface, permitting a surface-rendering display (See Figures 47-21,47-22,47-24, and 47-25,47-41,47-42,47-43,47-44,47-45,47-46,47-47, and 47-48). For example, in patients having sonohysterography, one can display the surface of the polyps or fibroids, much like a virtual hysteroscopy. This technique allows us to "enter" fluid-filled cavities within the volumes, much like using a scope to visualize the surface of these lesions.⁹ We can also use the same technique when visualizing the surface of the bladder in patients with bladder tumors or other mucosal abnormalities.^37,38 The internal aspect of an ovarian cyst can also be evaluated with surface rendering, to show areas of nodularity or polypoid lesions in cases of complex cystic masses such as ovarian neoplasms.

When using the surface-rendering technique, it is often helpful to utilize the electronic scalpel and carve away portions of the images that are superfluous to the anatomy of interest. The ability to cut away these unwanted echoes from in front of the target area is best accomplished by rotating the rendered image in different directions to establish the best cutting plane between the anatomy of interest and the unwanted echoes. One can even take an entire volume that contains a cyst and slice it in half with the electronic scalpel to then pry the 2 parts apart to see the rendered inner surface of the cyst inside.

Finally, using the surface-rendering mode, the practitioner can render just a thin portion of the volume, thus discarding the rest. This thick-slice technique results in a reconstructed image that comprises a thicker slice of anatomy than standard 2D imaging. This technique emphasizes the edge enhancement of structures and provides a contrast-rich image.

Three-dimensional inversion display is a method by which the volume is rendered by forming a cast of all of the cystic areas within the volume, and having the solid areas become lucent such that only the cast is visible (Figures 47-49,47-50, and 47-51).^11,12 This provides a view of the entirety of the cystic areas that are within the volume all at one time. The volume can be rotated in any way necessary to view all of the aspects of these cystic portions. An example of how this can be useful is in the evaluation of follicles within an ovary. All of the follicles can be made opaque within an ovary and counted and measured simultaneously. One can image an ovary that has polycystic ovarian syndrome, showing the characteristic appearance of the cluster of small follicles along the periphery of the ovary. The evaluation of hydrosalpinges lends itself particularly well to the inverse mode, since the fluid-containing tube may enter multiple different planes. The inverse mode is sometimes the only way to display the entire structure, because there is often not a single plane that will demonstrate all of permutations of the ectatic tube. One can also do an inverse-mode image of the endometrial cavity containing fluid, such as during a sonohysterogram. This technique gives the appearance of positive contrast within the uterus, and the image produced is similar to a hysterosalpingogram.

Three-dimensional volume acquisitions provide the opportunity to make accurate volume measurements of regions within the volume. Investigators have demonstrated the interobserver reliability and validity of in vitro volume calculations done within these ultrasound data sets.³⁹ In one study, volume measurements performed on a 3D ultrasound data set were highly reliable to within 4% of the true volumes.³⁹ Studies in vivo indicate that 3D volume measurements are also accurate when comparing, for example, the measured volume within a urinary bladder with the voided urine volume.⁴⁰ Studies indicate that 3D ultrasound provided more accurate volume measurements than traditional 2D ultrasound images, particularly in irregularly shaped organs where the shape of the organ must be traced in multiple planes.^40,41 Both the volume and distance measurements are typically done offline within a stored volume.⁴¹

Virtual Organ Computer-Aided Analysis (VOCAL) is a software program that allows the practitioner to trace the contour of a region of interest within an acquired volume, stepwise over the course of 180 degrees, thus enabling the calculation of the approximate volume of a selected region.^42,43 This permits the practitioner to highlight even irregularly shaped portions of the organ of interest so that the volume is measured accurately. VOCAL provides a reproducible method of estimating, for example, the endometrial volume as a measure of the dimensions of the endometrium.⁴² The volume of the endometrium may be a more accurate method of assessing the endometrium than the standard single-width measurement.⁴²

Doppler evaluation of blood flow using 3D displays can provide 2 important pieces of information (See Figures 47-6,47-26, and 47-27; Figures 47-51 and 47-52). First, the subjective evaluation of the morphology of the vessel tree provided by 3D power Doppler ultrasound volumes aid in identifying malignancy within ovarian masses.⁴⁴ In particular, 3D sonography color Doppler can display the characteristic appearances of tumor vessels, such as the areas of focal stenosis, irregularity, and aneurysmal dilatation of blind-ending vessels seen in neoplasms.⁴⁴ The actual color mapping of the vascular architecture revealing the location of vessels within the lesion is of particular importance, such as displaying the clustering of irregular tumoral vessels in the center of a malignant ovarian tumor. There are several studies showing that 3D sonography with color Doppler can help to depict the overall vessel density and branching patterns within ovarian tumors.^14,44,45 These types of vessels are more likely distributed centrally in malignant tumors rather than peripherally. The presence of central intratumoral vascularity had a 90% positive predictive value in a study by Fleischer et al for predicting malignancy.¹⁴ Conversely, the absence of such central vascularity had a high negative predictive value of 96%. In another study, 3D power Doppler imaging was more accurate than 2D sonography at defining the morphologic and vascular characteristics of ovarian tumors.⁴⁵ Although all malignancies in this group of 71 women were correctly identified, both with 2D and 3D standard imaging, the specificity increased by adding 3D power Doppler, thus improving the diagnostic accuracy. In another study of 77 benign tumors, 6 borderline tumors, and 21 invasive malignancies, the vascular features differed significantly between benign and malignant tumors.⁴⁴

The second major contribution of Doppler imaging within 3D volumes is the ability to quantify the number of vessels and volume of flow within the area of interest. Jokubkiene et al studied the vascularity of the normal ovary, looking at the changes in volume and vascularization of normal ovaries during typical menstrual cycles using 3D power Doppler.⁴⁶ They showed that vascular indices in the dominant ovary and the dominant follicle/corpus luteum increased during the early part of the cycle, with the vascular flow index being 1.7 times higher on the day before ovulation than 4 days before ovulation. The vascular flow index in the corpus luteum was subsequently 3.1 times higher 7 days after ovulation than in the follicle on the day before ovulation.⁴⁶ This study demonstrates that substantial changes in the vascularization of the dominant ovary take place during the course of the menstrual cycle—a phenomenon potentially helpful when evaluating patients with infertility as well as those with ovarian masses.

There are several novel methods for quantifying the vascularity within a sonographic volume.^{15,17,47,48,49, and 50} Vascular quantification is done using a 3D histogram, where the percentage of color and flow amplitudes can be measured within the volume. When a 3D volume is obtained with color-flow information, the area of interest can be delineated so that the vascular parameters can be calculated inside the box of interest.¹⁷ The vascularization index (VI) measures the percentage of color voxels within the box and basically provides a vessel count. The flow index (FI) is a measure of the flow traversing the region at the moment the volume was acquired. The vascular flow index (VFI) combines the information from the previous 2 measurements and provides a measurement of the blood flow and vessel count within the selected volume box.

Alcázar et al used Doppler vascular sampling with these quantitative methods to predict the presence or absence of ovarian cancer within vascularized complex adnexal masses.¹⁵ They displayed the vascularization of highly suggestive areas with gross papillary projections or solid areas using the vascular indices (vascularization index, flow index, and vascular flow index), which were automatically calculated for this area within the volume. The median vascularization index (MVI), FI, and VFI were significantly higher among the malignant tumors compared with nonmalignant lesions.¹⁵ There were no differences, however, in the resistive index, pulsatility index, and peak systolic velocity.¹⁵ Several other studies investigated the MVI, FI, and VFI in benign versus malignant cancerous tumors and found that malignancies had significantly higher values.^{15,17,47,48,50} On the other hand, another study by Alcázar et al suggested that 3D power Doppler sonography did not significantly add specificity or sensitivity; therefore, the concept remains somewhat controversial for use in clinical practice.⁴⁹

To validate the accuracy of 3D power Doppler sonography to evaluate the ovary, Järvelä et al studied the intra- and interobserver variability in determining ovarian volume using gray-scale and color-flow index measurements with 3D power Doppler ultrasound.⁵¹ The results indicated that measurement of gray-scale and color-flow indices was sufficiently reproducible to be used in clinical practice and research.

Three-dimensional color Doppler is also used to assess the vascularity of the uterus and the endometrium.^22,52 Fleischer et al report that 3D color Doppler provides valuable information before and after fibroid embolization.²² Among 20 patients who had a total of 31 fibroids studied, the maximum reduction in vascularity occurred 1 day after the procedure, and the volume change was observed at the 3-month follow-up examination. The depiction of fibroid vascularity by color Doppler improved the ability to determine size, location, and extent of myometrial involvement. Following treatment, the hypervascular fibroids tended to shrink more readily than those that were isovascular and hypovascular.

Investigators have also used 3D power Doppler as a tool to study the changes in endometrial and subendometrial volume and vascularity during the normal menstrual cycle.⁵² The endometrial and subendometrial vascularity appears to increase throughout the follicular phase, decreasing after follicular rupture, only to increase again during the luteal phase. Volume 3D color sonography accurately depicts the vascularity of fibroids and can occasionally reveal collateral flow not seen during arteriography.²³ Color Doppler using 3D ultrasound can also be used to evaluate the uterus for cervical carcinoma, with a significantly higher color score and lower resistive index associated with a tumor diameter 4 cm or greater.^{53,54, and 55}

Similar to work on the ovary, VOCAL is useful to assess endometrial and subendometrial blood flow semiquantitatively. VOCAL is a valuable tool to delineate the areas of interest and generate the histogram that provides indices of vascularity such as the VI, VFI, and FI. Intratumoral vascularization is detectable not only in the ovary, but also in the endometrium and cervix to identify cancerous foci.^{53,54, and 55} Three-dimensional endometrial volume and power Doppler can predict endometrial carcinoma and hyperplasia, particularly in patients with post- and perimenopausal bleeding. Studies show excellent intra- and interobserver reliability of these measurements, suggesting that 3D power Doppler angiography is an important method for assessing the ovary, endometrium, and subendometrial blood flow.^51,52,56

There are a number of important artifacts and limitations of 3D sonography that should be mentioned (Figure 47-53).²⁶ Conventional 2D ultrasound is known to have many potential artifacts that are easily recognizable in clinical practice, such as shadowing behind calcified objects. Solid masses can also cast an acoustic shadow or attenuation of the beam, and similarly, cystic areas produce through-transmission that is easily recognized. There are also important color and power Doppler artifacts to consider, such as aliasing and motion.

Although these artifacts are fairly standard and easily recognized using 2D standard ultrasound, they are often confusing and can even be compounded within a volume. A shadow or an area of sound enhancement is recognized as such because the source of the artifact is usually seen in the same 2D image. Once an image is reconstructed from a volume, the artifact will be there, but often without the origin of that artifact, making it much harder to recognize. For example, a shadow may be confusing on an image where the calcification that caused that shadow may only be seen in a different plane. The shadow may be mistaken for pathology if it is not properly recognized and if the source of the artifact is not seen. Doppler artifacts such as those relating to gain, aliasing, and flash may be even more confusing when seen within volume images that are sliced and diced differently from the acquisition view. It is therefore important to go back and navigate through the original volume using the orientation of acquisition to determine the source of shadows and unusual findings on an image. There are also artifacts that are unique to 3D volume acquisition and reconstruction, which include motion artifacts and shadows from objects located in front of the area of interest but not visible in the volume obtained.

Tomographic ultrasound imaging (TUI) is another method by which the information within a volume can be displayed (Figure 47-54).⁴ This brings ultrasound closer to other forms of cross-sectional imaging, such as CT and MR. The pelvis can be imaged by acquiring 3 or 4 volumes (at least 1 of the uterus and 1 of each ovary), then displaying them using TUI so that they can be read as still images offline or at a site distant from the patient. One can also navigate through the volume and do a virtual rescan of the patient if necessary.⁴ We studied the feasibility of performing pelvic ultrasounds using this technique and found that pelvic imaging could be done very accurately and more efficiently using volume imaging displayed in this way.⁴ The mean time needed to perform a 2D transvaginal scan was more than twice as long as the time needed to perform the 3D volume acquisitions that were stored and contained all the anatomic information necessary. The accuracy of the scan read offline was similar to the original reading of the standard 2D ultrasound, done at the time of the patient's visit. The use of TUI and reading these images offline could help us make great progress toward standardizing and optimizing the efficiency of pelvic sonography in the future.

Are the planes that are the most favorable to image the female pelvis those directly available vaginally or transabdominally? Or have we made do with the only options that technology has offered ultrasound for the last several decades because 2D ultrasound could only produce images from fixed portals like the vagina or abdominal wall? Now, 3D volume imaging has allowed us to generate any plane of section within a volume from any orientation, many of which are inaccessible through the standard 2D portals. This provides many more imaging options within ultrasound, and we may even find that the pelvis is better scanned in a plane other than those traditionally used.

Three-dimensional imaging is not new. CT and MR have used it for decades, with reconstruction of volume in many planes, using many different types of displays. Now that ultrasound has acquired similar capabilities, the benefits of using ultrasound as a first-imaging test are infinite. Three-dimensional volume imaging is therefore one of the most important advances in modern sonography, as 3D ultrasound will challenge MR's imaging capabilities in the future. It is now up to the ultrasound community to discover areas that have become accessible to ultrasound, due to its ability to reconstruct any plane and scan it even in real time. This process will allow us to maintain and cement ultrasound's role in cross-sectional imaging applications.

Volume sonography is a technology still in its infancy. Many of our current transducers are still mechanical, resulting in choppy 4D ultrasound, and the current trade-off between resolution and speed remains frustrating. Matrix technology, which is based on the electronic transducer that can acquire an instantaneous volume, will likely change 3D ultrasound technology dramatically and provide the much needed speed and improved image resolution in all planes. Four-dimensional imaging in true real time will likely provide adequate guidance for pelvic procedures such as follicular aspirations, abscess drainages, and 3D/4D-guided dilatation and curettage.

Thus far, however, it is apparent that the reconstruction of additional imaging planes such as the coronal plane should be a part of standard pelvic ultrasound examinations, as much important information can be gathered from these additional images. Measurements of volume, and evaluation of blood flow and vascularity as well as morphology are all important parts of volume imaging that are likely to become increasingly used in the future.