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Volume acquisition is arguably the most important part of the whole process of using 3DUS clinically.
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Ultrasound volume acquisition methods are divided into 3 main categories:
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The manual or "free-hand" acquisition method acquires volumes using a conventional 2DUS probe. Commonly, 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 in-line (where image acquisition, transfer, storage, and manipulation is done in the same unit) and off-line systems. With off-line systems, volume data are commonly captured using the ultrasound machine's video out port and downloaded to an off-line 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 off-line 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 43-1). 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 43-17 and 43-18).
Mechanically swept 3DUS 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, 2 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, up to 42 times per second (at the time of this writing), 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 Figures 43-17 and 43-18). 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 43-1). 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 gray-scale-only volumes.
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 (4000 to 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.14,26,27, and 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 43-1). 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 will need to be improved (see Figures 43-17 and 43-18). One workaround for this problem for the moment 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.
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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. (4) Real-time biplanar imaging where 2 orthogonal images are acquired and displayed in real time in gray scale 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. This particular mode is extremely helpful and has a central role in the use of 2D matrix arrays.
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Matrix arrays are the next phase in volume acquisition. Their main advantages are related to the fast acquisition speeds, significantly reducing motion artifacts (especially useful in imaging the fetal heart), and focusing achieved at 360 degrees, making possible visualization in any image plane with no loss in resolution. Their narrow apertures and still relatively low-frequency probes are their main drawbacks at the moment.
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Gray-Scale Scanning and Acquisition Guidelines
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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.
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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 43-19).
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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 43-20). Registration artifacts are more prevalent in the views other than the acquisition plane.
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It is important to maintain an even pressure with the transducer over the area of interest. Otherwise, distortions from uneven pressures can be seen on superficial (near-field) structures. Another important aspect is maternal breathing. If politely asked, most patients will have no problem holding still for 5 to 6, or even up to 10 seconds. The operator too should be still (in automated scans) and maintain good contact with the patient's abdomen during acquisition. If scanning with a free-hand transducer, maintain a constant acquisition speed as you move over the area of interest with your hand and try to keep the acquisition planes parallel and equidistant from each other.1
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At times, the acquisition plane of interest is simply not in a favorable position. Most of the time, the spine can be brought down (or up, if the spine is of interest) by reorienting the probe typically from the side where there is some fluid. An effort should always be made to use a pocket of fluid between the probe and the acquisition plane. If the fetal face is right up against the placenta and there is no fluid interface between them, slightly releasing the probe might help bring some fluid in. If not, another way to deal with this is to gently push either the frontal part or the chin of the fetus with your left hand until some fluid comes in. The best rendered volumes will come from good tissue interfaces that were acquired by rigorous scanning.
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At the end of the acquisition, the volumes are explored for the presence of artifacts, and if artifacts are present the volume is discarded and a new one is obtained. If artifact-free, it is saved in the hard drive of the machine as a 3D volume, or if a 4D sequence is acquired, as a 4D sequence, a 3D volume, an .avi video file, or all of the above.
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Acquisition plane matters (see Figures 43-3,43-21,43-22,43-23, and 43-24). Certain diagnostic planes can only be obtained within a volume acquired in a specific acquisition plane that is proven to generate the expected image plane.13,29 A good practice is to always scan in 2D ultrasound over the area where volume acquisition will take place. With practice, the operator will know what volumes will or will not generate the desired views just from scanning in 2D over the area of interest. Within the 3D volume, resolution is highest in the plane of acquisition or close to it (for example, sagittal planes through the uterus in a volume acquired with the probe in the sagittal orientation). Resolution is progressively degraded as the image plane deviates from the acquisition plane (see Figure 43-24).30
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Examples are acquiring the fetal heart volumes at the level of the 4-chamber view with the fetal spine below the 9 or 3 o'clock level yields the highest number of heart views. Fetal spine volumes are generally best acquired when the spine is above 10 or 2 o'clock. The closer the acquisition plane to the anatomic plane desired, the better the chance that it will be generated from the volume. Acquisition planes close to 90 degrees to the desired planes are generally the poorest in resolution. As a result, the coronal reconstructed plane generally has the lowest resolution of the 3 planes in the multiplanar display. Manufacturers commonly interpolate (as in fill in the empty spaces to make the image more understandable) the C plane (the plane 90 degrees to the axis of the beam) to improve the quality to a certain degree.
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The concept of acquisition speed is often misunderstood. The misconception has been that the faster the acquisition speed, the better that particular system should perform overall. Add to this the marketing hype of new systems across the industry with terms such as 3D live, 3D real time, 4D, 4D live, and 4D real time and you can see why the casual user may get the impression that going from live to real time and/or from 3D to 4D (adding more Ds) is what it takes to improve image quality. There are quite a few 3DUS users that routinely use 4D acquisition as the default acquisition method whenever a volume is contemplated. With mechanized volume probes, which are the main probes used for 3DUS today, you mostly aim to acquire one volume at a time, with the slowest speed for best quality. If the target is moving, either get a faster 3D volume or use 4D mode, which acquires several volumes one after another, and on the fly it displays them on the screen at the selected display mode (see Figure 43-19). In short, you either get one 3D volume or many 3D volumes in faster speeds. How fast should you go in 4D mode? The slowest possible (with no artifacts) the better, is the answer!
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What about real-time 4D ultrasonography (4DUS)? The real-time concept needs to be explained. By film and graphics industry criteria, 30-frames-per-second update of any image in a TV monitor (NTSC video in North America) is generally considered real time. That does not mean below the 30 frames per second the video speed is slow. Film has been traditionally shot and displayed at 24 frames per second, and PAL-SECAM video in other parts of the world is also updated 24 frames per second with equal clarity to NTSC. But as a general rule, over 30-volumes-per-second acquisition and over 30-frames-per-second display are considered real time. If you keep the volume box really thin, most equipment today can achieve acquisitions over 30 volumes per second. In clinical practice, this concept takes a different value. What we consider important is sampling motion fast enough to see its different points in time (temporal resolution) by maintaining acceptable anatomical detail (spatial resolution). These variables differ among structures. Motion of the extremities can be abrupt, but usually they are of slow speed. So the sampling rate does not have to be high, and usually 5 to 10 volumes per second are sufficient to visualize them adequately. On the other hand, fetal heart chambers and valves move quite fast and a higher sampling rate would be required, something only 2D matrix arrays can really offer. For the moment, we use a dedicated program for the fetal heart called spatio-temporal image correlation (STIC). Commonly, we scan fetal extremities with up to 5 to 10 volumes per second (sometimes a bit faster or slower depending on volume box size) and do not think much about the concept of "real-time imaging" rather than opening up the volume box enough to include the whole area and visualize motion. Volume depth directly affects acquisition speed, and in order to get over 24 volumes per second we would have to make the volume width thin and reduce depth. That would be impractical with current equipment, as most fetal structures would need a reasonable volume box width and depth.
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One example of a useful display technique is volume contrast imaging (VCI; GE Healthcare, Milwaukee, WI, USA) (see Figure 43-8). The VCI technique can be a static, 3DUS-based, post-processing function, or a 4D ultrasound–based function consisting of several layers of 2D slices acquired fast in continuity and displayed as a volume-rendered view. Volume-rendered view collects tissue signals from adjacent deeper layers and project them as a conventional 2D view either in the same view as the acquisition plane (VCI-A) or in the plane perpendicular to it (VCI-C). In both those planes it allows standard 2D measurements. The slice thickness is adjustable by the user, ranging from 2 to 15 mm. The surface-rendering mode uses a proprietary algorithm.31
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VCI-C displays a coronal slice as a 2D plane with the extra advantage of better tissue contrast compared to conventional 2D. The proprietary surface algorithm separates the noise from the voxel data and subtracts it. As a result, contrast resolution is improved. Gray-scale information is projected into a 2D plane provided by the rendering of all data within the thickness selected. A mixture of 2 render modes is applied to the data: (1) surface mode, which retains both the original gray levels of the tissue as well as image sharpness, and (2) transparent maximum mode, which contributes tissue contrast derived from deeper layers of voxels. The effect of removing noise and enclosing data from adjacent layers results in a smoother image combined with enhanced contrast resolution. The reason this algorithm was developed was to improve the detection of tissue interfaces that normally are not appreciated with 2D ultrasound. In everyday practice, it is a very straightforward method and can be used similarly to a 2D beam with the added benefit of better contrast resolution and less noise.32
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3D/4D Color/Power Doppler Sonography, B-Flow, and STIC Acquisition Guidelines
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All of the described methods involve volume acquisitions that incorporate color and power Doppler ultrasonography, as well as B-flow (Figures 43-25,43-26, and 43-27). It is important to remember that 4D acquisitions with mechanically steered probes cannot acquire color Doppler, power Doppler, and B-flow. With these probes, STIC can display what appears to be a 4D ultrasound display, but it really is not a 4D acquisition in itself. Matrix arrays can obtain biplanar, multiplanar, and rendered real-time acquisitions with color and power Doppler and B-flow of very high quality. Knowledge and optimization of the settings are the most common problems ultrasound operators encounter in their learning curve of acquiring 3D color and power Doppler volumes. There are no fixed recipes for every occasion, but there is a reliable set of principles and requisites that, if understood, should be helpful in obtaining adequate information. Normal 2DUS principles of visualizing vessels with color and power Doppler assist in a 3D volume acquisition as well. Here is a listing of the most important concepts:
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The scale or pulse repetition frequency (PRF) should be adjusted according to the particular blood flow speed and vessel size. A higher PRF is typically needed for higher velocity vessels (ie, intracardiac flow), and a lower PRF is needed for smaller and/or branching vessels (ie, brain) especially placental or retroplacental (venous) vessels.
The operator should orient the 2D beam to be as parallel to the vessel or vessels of interest as possible.
The overall gain should be adjusted to avoid "bleeding artifacts" in the vessel wall by decreasing its value at the minimally acceptable level, typically no more than 50% to 70%.
The color or power box should fit tightly around the area of interest. The color or power mode 2D frame size does count in relation to frame rate.
When scanning in 2D, try to go over the area of interest with color or power Doppler and see whether you can get any signal. If not, it is likely you do not have the right angle to the vessels of interest (parallel or close to it). As a result, prior to acquiring a volume with color and power Doppler, scan in real time and adjust the angle first and make sure you get signal.
The wall filter should also be adjusted. The medium settings appear to work better most of the time.
Color priority set on high generally displays better quality, and it makes sense to use it whenever color or power Doppler ultrasonography is contemplated. If available in your equipment, set the dynamic motion differentiation to avoid background noise signals.
Pay attention to the sensitivity settings. They should not always be put at high, unless you are looking at the retroplacental vessels. The medium levels usually work best in most scenarios.
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A good practice is to start in 2D gray-scale imaging with the highest frame rate achievable for that particular frequency and penetration. To do so, the user needs to understand the variables to be controlled that influence the frame rate. A high frame rate translates into a volume with higher quality, as there will be more frames in a volume. The most important factors affecting the frame rate are as follows:
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The aperture (or angle) opening in the 2D beam (use the narrowest angle you can).
The number of focal zones, and the amount of depth (the less the better for both).
Harmonic imaging helps significantly in imaging in the near and mid-fields but not much in the far field. It is important to know that it may slow down the frame rate.
Try to put the area to be acquired in the near field of the transducer. If not possible, at least see if you could put a good pocket of fluid in between. This will ensure a low attenuation acoustic window and improve penetration and resolution.
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Acquiring Volumes with B-Flow
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B-flow or B-mode (brightness mode) is a relatively recent technology that displays the reflected echo intensities as bright dots. A subtraction mode is used to differentiate and visualize the "bright" amplitude of scattered particles. The unique feature of B-flow is that it can visualize the cardiovascular system with a technology that does not depend on the frequency shift. B-flow is based in gray scale, and that it does not depend on the frequency shift means that there is no angle dependency. It also does not display any bleeding artifacts or aliasing commonly seen with color and power Doppler sonography. This algorithm can display the direction of flow but not as well as color Doppler technique (Table 43-2). The resulting image in a rendered volume is a view that displays the cardiovascular system like an angiography but in a volume with depth, so it can be rotated 360 degrees. These B-flow volumes can be acquired as a single 3D volume, as a 4D sequence, and also as STIC volumes.33 They are best rendered with maximum intensity projection, but a combination with surface or light modes can be useful as well (see Figure 43-26B).
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Spatio-temporal Image Correlation
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In the past it was believed that increases in acquisition speeds will have a great impact in 3D fetal echocardiography. Although mechanized volume probes can achieve speeds of acquisition of over 40 frames per second, acquiring fetal heart volumes at these high speeds does not necessarily translate into higher quality volumes. To solve these issues, a new volume acquisition technology was introduced called spatio-temporal image correlation (STIC). With STIC a very slow (always by 3DUS standards) acquisition from 7.5 to 15 seconds is performed over a preselected area of the fetal heart, typically at the level of the 4-chamber view.11,34 The acquisition angle varies from 15 to 40 degrees and is user selectable.35 The same image optimization criteria apply as the ones mentioned above. After the acquisition, post-processing of spatial and temporal data is performed so the 2D-acquired images are correlated in time and space. The acquired data are displayed in a classic multiplanar view and/or in a cine sequence depicting heart motion with total control on interactive re-slicing and/or rendering the same as in a static 3D volume.11,16,34,35,36,37,38, and 39 Typically, with gray scale, this acquisition has a very high B-mode frame rate (approximately 150 frames per second) due to the relatively small region of interest.
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STIC was initially introduced only in gray scale, but later the acquisition was achieved using color and/or power Doppler technology as well. This development opened up a whole new area in evaluating the fetal heart with 3DUS. STIC represents a significant development in the field of volume fetal heart scanning having the potential to perform a full fetal echo exam out of a single fetal heart volume at the level of the 4-chamber view. Studies done prior to and after the introduction of STIC were able to demonstrate that a volume of the fetal heart can provide all the standard imaging views for a fetal echocardiography (see Figures 43-27 and 43-28).16,39,40, and 41
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Although the potential of this technique is clear, there are a number of pitfalls worth mentioning. The acquisition is still relatively slow, from 7.5 to 15 seconds, and as such fetal breathing and gross body movement can occur. Careful evaluation of windows B and C in multiplanar display after the acquisition is crucial to ensure that no artifacts are present. Adjusting the volume box tightly around the region of interest helps as it generally increases the frame rate. The best frame rates recommended are the ones above 15 Hz, typically with higher rates in smaller hearts and slower rates in bigger hearts. An important adjustment is the acquisition angle, as the default settings tend to be very narrow. Optimal angles vary from 15 to 35 degrees and are related to the gestational age. As a rule, the earlier the gestational age the narrower the angle. STIC acquisitions perform well in spine-down fetal positions, but not so well, if at all, when the fetal spine is up. Ideally the fetal spine should be maintained below 9 o'clock to 3 o'clock with the apex of the heart pointing up. As a rule, apical 4-chamber views are the preferred acquisition planes, while the lateral and basal 4-chamber views are inferior or even unusable at times. While axial views of the chest at the 4-chamber view are the best overall acquisition planes with STIC, ductal/aortic arches and inferior vena cava (IVC)/superior vena cava (SVC) can be best extracted from sagittal acquisition planes with the currently used mechanically swept transducers.42 With 2D matrix array probes, this will not be the case as they focus in both directions. It is important to understand that while STIC is a useful technology for the moment, it will eventually be replaced by 2D matrix arrays, which improve precisely where STIC suffers most: elevational focusing, slow frame acquisition times, and motion artifacts. Last but not least, STIC remains a great tool to learn fetal echocardiography.