Ultrasound imaging in obstetrics and gynecology is commonly used for numerous indications. The most commonly used technology in everyday practice is the conventional high-resolution two-dimensional ultrasound (2DUS), and is widely considered clinically useful. Using 2DUS imaging technology, the operator scans the anatomy with a flat 2D ultrasound beam to obtain cross-sectional planes. These 2D imaging planes are typically quite thin, and the corresponding images obtained represent thin cross-sectional anatomical information along the path of the 2D beam. As they scan through the anatomy, ultrasound professionals typically orient themselves by mentally developing a spatial three-dimensional (3D) concept of the region of interest, accounting for depth, and perspective. Depending on fetal position, using 2DUS at times some image planes can be very difficult, or even inherently impossible to be obtained with conventional 2DUS techniques, especially if they are perpendicular (or close to) to the axis of the beam.

Research work on 3D volume acquisition and display methods started in 1970.¹ Sophisticated early prototypes had static arms, later equipped with position sensors to more reliably register the acquired 2D data in time and space. Early to mid-1990s saw this technology starting to leave the industry’s research and development labs and become a more appealing clinical tool.^2-6 Significant developments in transducer technologies, especially manually swept mechanical probes, electronic 2D arrays, miniaturization, graphic boards, active cooling, as well as advances in computer signal processing and image display finally made possible reliable techniques to acquire and visualize ultrasound volumes.⁷

Volume ultrasound imaging can depict the anatomy of the region of interest as static, or dynamic, in motion. The volume derived displays have become fully interactive environments where the operator has full control over selecting any scan planes, and evaluate the anatomy in user-selectable uniplanar, biplanar, orthogonal, multiplanar displays, as well as rendered 3D views, alone or in a combination with any of the aforementioned. We know that volume imaging techniques are not new, as computed tomography (CT) and magnetic resonance (MR) imaging has acquired and displayed volume-derived image slices in a volume for years. What sets apart 3DUS volume acquisition and display technologies is that they provide classic volume imaging methods to be used in a much smaller anatomical scale, even intracavitary, with the known efficiency and economics of ultrasound.

Terminology can be at times an issue for some when describing 3D and 4D ultrasound imaging concepts. As the technology is being developed by several different competing manufacturers, often we see quite different terms, or acronyms, used for the same processes or technologies. Industry’s use of marketing catchphrases assigned to technological processes also can be confusing, especially for newcomers. In this chapter, we will describe and explain the most commonly used terms today, and when possible, provide also some of the more common alternatives. In the published literature, the terms “3D ultrasound” and “volume ultrasonography” are commonly used, interchangeably. They both will also be used here, understanding that they represent the same concept. In 3DUS, a volume (a number of slices rather than a single slice) of ultrasound data is acquired and stored. The stored data can be later visualized, reformatted, postprocessed, and analyzed in a number of ways. Uniquely, these volume data can also be reliably quantified, both in 2D and 3D. For the first time, we can reliably measure volumes of any organ system, provided they were included and could fit in the volume acquisition box, and provided that the organ’s borders have decent interfaces. All of the acquired volume data, along with any preprocessing information, can be digitally saved and retrieved at any later time. In addition, small movie clips of these volumes, rotated at different projections to enhance depth perception, can be created and stored as such.

The clinical applications of 3DUS in obstetrics and gynecology have been explored in numerous published works.^8-19 Although there has been much more extensive research into the obstetrical applications of 3DUS, there is evidence in the literature that 3DUS does have very substantial applications in gynecology as well.^10,20-24.

In daily use, volume ultrasound data represent the acquisition of a number of 2DUS data and an extension of conventional technique. With the introduction of matrix arrays, the thinking process becomes a matter of how thin, or thick, do you want the beam to be, in your region of interest, depending on what you want to see—thin slices for sectional straight cuts, or thick slices for depth perception, fluid filled or bony structures, surface anatomy, displayed in straight or curved planes. As such, volume imaging does not necessarily replace sectional 2D imaging, but rather extends its capabilities. Using 3DUS volume imaging for clinical applications initially may require an additional investment of time (typically 3-5 minutes). With some applications, this time depends on the complexity of the case and experience of the examiner. The additional time is not as much related to scanning and volume acquisition, as to exploring the saved volume data in different scan planes. As a result, although the total examination time may be longer due to the amount of information to be processed, the scanning time, or the time the patient is exposed to the ultrasound beam, with experience should be reduced. During the examination itself, the acquired volume is only briefly scanned to ensure that the region of interest has been adequately included, and there are no artifacts present. If so, the volume is saved in the hard drive of the machine or is sent to an archive server or picture archive and communication system (PACS). Exploration of the saved volumes can be performed after the patient leaves the room or is discharged. A typical sonographic examination workflow using a 3DUS system would involve: (1) scanning the anatomy using conventional real-time 2DUS, mapping the position and orientation of the organs to be visualized relative to the axis of the beam and obtaining standard image planes for volume acquisition; (2) acquiring volumes either slow or fast (depending on whether it is a moving target, or not) in specific acquisition planes known to yield the required planes; (3) volume exploration and inspection for the artifact-free presence of the expected image planes; (4) reformatting, or the quantitative and qualitative analysis of stored volume data such as multiplanar analysis, rendering, calculating volumes, and creating cine loop clips if need be; and (5) storage of volume data in the hard drive (Figure 45-1).

Ultrasound operators often ask why 3DUS should be used in a particular clinical scenario. Experienced 2DUS operators especially have to convince themselves why they would need a 3DUS scan in addition to 2DUS, which has been successfully used for many years. If 3DUS proves to be useful, we all have to put additional amount of effort into learning about and keeping up with the latest developments and purchasing and using 3D equipment in everyday clinical practice.

There are two main reasons why a clinician would need to consider the use of 3DUS (Figure 45-2):

1. 
   

   Volume ultrasound can obtain image planes that are either impossible or very difficult to obtain.

   
   
2. 
   

   Volume ultrasound can provide depth, which is impossible with 2D ultrasound.

In everyday clinical work, an ultrasound operator is required to obtain the so-called standard ultrasound views and extract clinically useful information based on them. These standard views have been studied in the past for their ability to be acquired by most operators with adequate training. They also have been found to be useful in ruling out major abnormalities. Examples are the biparietal diameter (BPD) plane of the fetal head, the posterior fossa, the atria of the lateral ventricles, the 4-chamber view, and the outflow tracts, umbilical insertion, sagittal, and transverse views of the uterus and endometrial cavity in gynecology, etc. Although these standard image planes can be successfully obtained in the great majority of patients, factors such as fetal position makes their visualization quite difficult if not impossible at times. The patient might have to be brought back for a follow-up scan at a later time because of nonvisualization. The use of volume ultrasound imaging significantly reduces the number of these unsuccessful instances. It also reduces the time it takes to obtain certain anatomical views that might otherwise require quite a bit of scanning and probe manipulation. These views can be generated from an acquired volume even though the probe in 2D mode could not have had direct access to those planes. In addition, volume ultrasound imaging provides access to certain image planes that are not considered standard. Typically, these nonconventional image planes are either difficult (but not impossible) to obtain, such as the midline sagittal view of the fetal brain to visualize the corpus callosum, long axis of the cerebellar vermis, and/or the brainstem (Figure 45-3), or image planes that are simply impossible to obtain. The impossible image planes with 2DUS are typically those that are either perpendicular or close to perpendicular to the ultrasound beam (Figure 45-4). As a result, they are physically impossible and therefore not a requirement in any 2D ultrasound protocol. These impossible image planes can be generated from an ultrasound volume, giving the operator access to an unlimited amount of scan planes at virtually any angle. Of course, some of the main requirements and conditions of ultrasound should be met, such as acquisition plane, resolution, and depth.

These planes can be very important clinically, as they might be the best views to rule out some difficult conditions. Examples of impossible planes with clinical importance are the coronal plane of the uterus and endometrium (Figure 45-5), coronal plane of the cervical canal, coronal planes of the rectum (Figure 45-6), or the coronal plane of the perineum with short-axis views of the urethra, vaginal canal, and rectum, as well as the long axis of the puborectalis portion of the levator plate seen in one single view, to name just a few (Figure 45-7).

The second major reason to use volume sonography is that it can provide depth, which is also impossible to obtain with 2D ultrasound. The reason is simple: with 3DUS a volume of 2D ultrasound data is acquired. As a result, the beam is “thickened,” and the anatomy does not have to be viewed only by cross sections via a “thin” 2D beam (Figures 45-7I, J, and 45-8). The anatomy can be analyzed in user-selectable transparent and nontransparent (surface) modes through this “thick” beam. Depth is provided for both the possible conventional planes (ie, fetal profile or frontal face view; Figures 45-9, 45-10, 45-11), and for the impossible ultrasound views previously mentioned (Figures 45-12 and 45-13). In addition, there are some impossible image views that can only be adequately visualized using depth, as their shape is not straight and visualization of different depth levels is needed. Coronal views of the perineum, fetal secondary palate, and optic chiasm are typical examples (see Figures 45-7I, and J, 45-12, and 45-13).

In everyday practice, either one or both of these reasons (impossible planes and/or depth) are used to gain access to certain anatomic planes that are deemed clinically useful. As a rule, we use the mutiplanar views for cross-sectional analysis, using our knowledge gained over the years with 2DUS, and correlate that with the many choices offered by the rendered view, which offers depth. We do this analysis sometimes in the same display, and at other times in individual displays (Figures 45-14, 45-15, 45-16).

Volume acquisition is arguably the most important part of the whole process of using 3DUS clinically.

Ultrasound volume acquisition methods are divided into three main categories:

1. 
   

   The initial, and the oldest, way is the manual or “free-hand” acquisition method, which acquires volumes using a conventional 2DUS probe. In some of the better configurations, a position sensor is mounted externally to the probe assembly to provide position-sensing data to accurately register the acquired 2DUS data in space and time. The free-hand technique can be used in both inline (where image acquisition, transfer, storage, and manipulation is done in the same unit) and offline systems. With offline systems, volume data are commonly captured using the ultrasound machine’s video-out port and downloaded to an offline computer workstation equipped with a video frame grabber PCI card. Volume imaging software is used for subsequent 3DUS image reconstruction and analysis. Although the free-hand technique can be economically implemented into existing systems as it uses conventional 2DUS probes, its main drawback is that it cannot be used for accurate measurements, and its volumes are likely to suffer from acquisition-related artifacts.^7,25 With offline systems, the external probe-holding mechanisms are typically bulky, and the image transfer process to an off-line workstation is labor intensive and time-consuming (Table 45-1). The free-hand acquisition techniques are traditionally used to image parts of the anatomy that do not move (the nonpregnant uterus and adnexa) and generally are avoided when imaging moving targets such as the fetus (Figures 45-17 and 45-18).

   
   
2. 
   

   Mechanically swept 3DUS probes. These probes (ie, probes with a mechanized drive contained within the probe case itself) are commonly referred to as dedicated volume probes. When these probes are activated, the transducer elements are swept by a motor through the region of interest within the limits of a volume box, with an operator-selected speed and angle, while the probe is held stationary. The most common design of these probes has an electronic array achieving imaging in one direction (the 2D beam) that is almost simultaneously swept mechanically in the orthogonal direction (volume acquisition in the Z plane). So, two orthogonal planes are acquired: one is achieved electronically, and the other mechanically by sweeping the beam. The main advantage of this method is that images obtained with the automated approach permit accurate 2D and volume measurements and demonstrate more precise spatial relationships. The dedicated volume acquisition methods are considered more reliable, especially in imaging moving targets. As the operator’s hand stands stationary and the transducer elements move through a motor, the volumes can be acquired and displayed fast, at over 60 times per second, allowing the depiction of moving targets as they change position (fetus, fetal heart). This fast acquisition and display method is commonly called four-dimensional ultrasound (4DUS) with the element of time considered the temporal or the fourth dimension (see Figure 45-17). One major drawback of this method is that it does not focus in the elevational plane, or the plane along the depth or volume acquisition axis (see Table 45-1). While the volumes can be also acquired with color or power Doppler, as well as with B-flow, these acquisitions cannot be obtained fast and in continuity in a 4D sequence. They can only be obtained as one single volume, and the volume acquisition time is typically longer compared with grayscale-only volumes.

   
   
3. 
   

   Two-dimensional matrix arrays. These transducers represent the future of volume acquisition. They are composed of a transducer mosaic arranged in rows and columns with a very high number of 2D piezoelectric elements (8000 elements and even more projected in the near future). These probes scan in both orthogonal planes electronically and can acquire a pyramidal-shaped volume.^14,26-28 Because these transducers allow volume acquisitions in all planes, they allow focusing not only in one plane, but in the elevational plane as well, significantly increasing the quality of the acquired volumes. In addition, these transducers provide very fast frame rates without significant deterioration in image quality because all the transducer elements are fired at the same time (or in sequential groups). To increase the area of interest, they may employ 3D beam steering (see Table 45-1). The information is typically displayed in a biplanar display or typical orthogonal multiplanar displays, as well as in rendered displays. Due to the huge amount of data these transducers collect, the hardware and software needed to process this information is still in its early days, and in constant evolution (see Figures 45-17 and 45-18). One workaround for this problem is the reduction in the number of available channels, otherwise called sparse arrays, which results in inferior image quality, but the data can be processed faster.

The 2D matrix arrays can acquire the following: (1) a single 3D volume, which is typically acquired very fast, in roughly 1 to 2 seconds; but again, the speed of acquisition depends on volume size; (2) a 4D volume sequence, where the volumes are acquired one after another, but they are not that fast because all of the elements are sequentially fired at full aperture, so they do not yet acquire in real time; these volumes can also be acquired with color or power Doppler and this is one of the strongest points for this mode; (3) fast 4D real-time imaging of up to 24 volumes per second; the trade-off here is that the transducer only uses roughly one-third of the aperture; and (4) real-time biplanar imaging where two orthogonal images are acquired and displayed in high-resolution grayscale as well as with color or power Doppler.²⁸ The biplanar image is fully focused and can rotate as well as tilt laterally in real time (Figures 45-19, 45-20, 45-21). This particular mode is extremely helpful and has a central role in the use of 2D matrix arrays.

In order to get a better appreciation and understanding of electronic 4D imaging, it would be informative to briefly present the evolution of matrix array technology. For many years, the main ultrasound manufacturers have used matrix array technology to enhance image quality. The first matrix probes were primarily developed for 2D imaging. Matrix arrays introduce multiple element rows, which allow focusing in the near, mid, and far field—providing a uniformly thin beam throughout the field of view. With standard volume probes, the transducer is designed with a single row of elements aligned parallel to one another across the array. This standard array design has a fixed elevation focus, with the resulting slice thickness to increase in both near and far field, as you move away from the focus area. Increased slice thickness, outside of the focus area, results in volume averaging and decreased contrast resolution in near and far field. To create a volume, mechanical 3D probes utilize a mechanical motor to sweep the beam throughout the field of view. A series of 2D images are acquired and translated into a volume.

Mechanical matrix probes utilize a more sophisticated transducer array compared to standard volume probes. With mechanical matrix volume probes, elements are diced in two directions for improved elevation plane focusing. The end result is a uniform slice thickness from near field to far field with less volume averaging. The resultant image has improved image quality and uniformity compared to a standard transducer array. To create a volume, the array is once again swept through the field of view by a mechanical motor, and 2D images are acquired and translated into a volume. While the addition of multiple element rows provides significant improvements in image quality, we still experience the same limitations associated with mechanical probes—volume rates and orthogonal plane resolution.

With electronic 4D probes, a matrix array contains a multitude of element rows and thousands of elements. With more than 8000 elements, they provide so much control that the ultrasound beam can be actively focused in any direction, creating a narrow symmetric beam, which leads to excellent resolution in all imaging planes. This ability to actively steer and focus the ultrasound beam in any direction, in any sequence, provides an incomparable level of control, efficiency, and image quality. These features provide greater flexibility in imaging planes to enable advanced functionality such as real time biplane imaging, or real time VCI-A (Figures 45-22, 45-23, 45-24).

Heat is an inherent side effect of generating ultrasound energy, whether it is from the advanced electronics used to control the thousands of elements in the probe or from the excitation of the piezoelectric material that generates the ultrasound beam. To maximize performance, General Electric’s (GE Healthcare) eM6C probe incorporates active cooling, an innovative solution to dissipate heat from the transducer. Active cooling utilizes a thermally conductive liquid that is cycled through the probe handle and probe connector. Warm fluid travels from the probe handle to the connector where it is cooled and cycled back up to the probe (see Figure 45-20). Actively cooling the transducer allows the eM6C to realize its full potential and achieve greater penetration, frame rates, and image quality compared to passive cooling techniques.

With standard probes, the control and timing of the individual elements for transmit and receive beam-formation lies within the ultrasound system. The transducer cable links each individual element to the ultrasound system. While this is a proven and robust architecture, it is not feasible with electronic 4D probes that contain thousands of elements. GE Healthcare’s Voluson e4D is a synergy of probe and system design that utilizes a two stage beam-formation (see Figure 45-21) to extract the maximum amount of information from the eM6C probe. The first stage of beam-formation occurs within the probe handle, which itself is equivalent to a mini ultrasound system that translates the information from the less than 8000 elements in the transducer to the system. The sophisticated electronics within the probe provide the beam agility to steer and focus the ultrasound beam in any direction and any sequence. This beam agility is the key to unlocking new modes in which multiple planes are imaged simultaneously. The Voluson E10 provides the second stage of beam-formation. During the second stage, the system rapidly processes information while applying advanced image processing algorithms to create the final image.

Grayscale Scanning and Acquisition Guidelines

Before a volume is acquired, certain important principles should be considered. Considering that the most widely available and used probes currently in the market are the mechanically swept ones, these guidelines deal mainly with issues pertaining them, even though many of these principles apply to matrix arrays as well.

Prior to volume acquisition, optimize the 2D image to maximize the quality of the volume by adjusting depth (improving the refresh rate and making the acquisition faster), aperture (or beam angle, improving lateral resolution), number of focal zones (one is usually enough), and harmonics. If imaging the fetal heart, lower or even completely remove the persistence value, as frame averaging does not adequately display the valve motion. Pay close attention to the refresh rate value displayed in the screen, as a lot of the preprocessing options such as compound resolution imaging, speckle reduction, or coded excitation may negatively influence it (Figure 45-25).

Acquisition of volumes can be accomplished via both transabdominal and transvaginal transducers. In gynecology and first-trimester acquisitions, although the quality of the transabdominal acquisitions and rendering is adequate, transvaginally the quality of the volumes is generally very high. Otherwise, second- and third-trimester acquisitions are performed transabdominally. The examination is started in conventional 2D mode to target the region of interest (ie, the fetus), preferably in a long-axis view. The aperture is kept at the narrowest possible but still including the longest axis of the fetus. Acquisition is started with the lowest speed that translates into the highest quality setting. As a rule, the faster the acquisition speed, the lower the number of 2D slices acquired in the volume. Therefore, the multiplanar analysis in the planes other than the acquisition plane is likely to display low-resolution quality. If the fetus is moving, a fast 4D acquisition is needed to keep up with the fetal motion and avoid artifacts. The slower the acquisition speed, the greater the number of 2D slices in the acquired volume. Consequently, slower acquisition speeds yield higher resolution volumes. On the downside, lower speeds typically are more likely to suffer from motion artifacts (especially when imaging a moving target) (Figure 45-26). Registration artifacts are more prevalent in the views other than the acquisition plane.