Over the past 50 years, imagers have witnessed dramatic improvements in ultrasound image quality, resolution, availability, and range of indications. Most of these advances occurred within the confines of 2D planar imaging.
Dr. Lazebnik is a resident physician and Dr. Desser is an associate professor and residency program director, both in the radiology department at the Stanford University School of Medicine.
Dr. Desser is a shareholder of SonoSite. Dr. Lazebnik has no significant financial arrangement or affiliation with any manufacturer of any pharmaceutical or medical device and is not affiliated in any manner with any provider of any commercial medical or healthcare professional service.
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Over the past 50 years, imagers have witnessed dramatic improvements in ultrasound image quality, resolution, availability, and range of indications. Most of these advances occurred within the confines of 2D planar imaging. Meanwhile, productivity gains and the explosion of applications for volumetric CT and MRI have whetted sonographers' appetite for comparable volumetric technologies because several fundamental limitations persist in 2D ultrasound imaging.
First, the radiologist is dependent on a series of noncontinuous, presumably representative sections of the imaged organ to depict its entire complex anatomy. Inadequate sampling of pathology, assuming this is even apparent, requires repeat examination. Moreover, the imaging plane is fixed during the examination, precluding reconstruction of other cross sections.
Second, the images submitted for review show no quantitatively documented spatial relationship. The radiologist must rely on image labels and trust that the acquisition technique conformed to a given protocol of orientations with minimal sonographer variability. Comparison of serial exams is thus difficult, as exact corresponding planes are rarely acquired.
Third, volume measurements typically rely on the assumption that a simple geometric model, such as an ellipsoid defined by length and width, is an accurate proxy of the true 3D shape.
Finally, extrapolation from 2D to 3D anatomy is entirely mental and does not exploit modern 3D surface or volume rendering techniques.
Volumetric (3D) ultrasound overcomes these limitations. Numerous approaches allow acquisition of a sonographic volume. The simplest is offline workstation postprocessing of 2D cine clips acquired by freehand scanning into 3D volumes. While easy to implement, these are less accurate than volumetric acquisition. To obtain a true 3D sonographic volume, options include 2D transducer arrays, mechanical localizers (continuous linear, tilt, and rotational motion devices), and freehand scanning with automated localization (articulated arms, magnetic field sensors, and others).
In general, these methods either acquire volume information directly or assign 3D spatial coordinates to a series of continuous or noncontinuous 2D images. An extension of these techniques is 4D ultrasound, the continuous acquisition of 3D volume data so as to represent the volume dynamically through the imaged time interval (1).
FIGURE 1. Tomographic sections of neonatal brain generated using a single sweep of the volumetric transducer. (Provided by Siemens Medical Solutions)
Independent of the imaged organ system, 3D sonography features several universally applicable advantages. In a typical 3D acquisition, the sonographer scans the region of interest with a single sweep of a volumetric transducer. He or she no longer needs to acquire multiple image series through the organ of interest, so scan times can be greatly reduced. For applications such as bedside neonatal 3D neurosonography, scanning is performed quickly, with a single sweep through the brain and without the sedation required for modalities such as MRI (2) (Figure 1).
Similar to volumetric CT data, 3D ultrasound volumes may be processed subsequent to acquisition to obtain different views or to troubleshoot interpretation questions. Evaluation of multinodular thyroid glands or fibroid uteri, for example, may become much easier when the organ is viewed in multiple planes simultaneously on tomographic display workstations. Postprocessing allows remote interpretation and teleradiology for sonography, as the reader has all necessary information in the scanned volume (3). Comparison of serial imaging studies over time is facilitated, as multiple corresponding anatomic landmarks are present in all data sets.
Measurement of organ or lesion volumes with 3D ultrasound does not require assumption of any specific geometry but can be directly computed by manual or automatic segmentation of continuous slice data. Similarly, 3D surface visualization is accomplished by segmentation of the data set by manual or automated methods.
FIGURE 2. Coronal reconstructed images of uterus demonstrate fundal contour. Visualization of this contour permits differentiation of this arcuate uterus from a bicornuate uterus. (Provided by GE Healthcare Technologies)
Almost all organ systems that are feasibly evaluated by 2D ultrasound benefit from the general advantages of 3D ultrasound outlined above. Abdominal imaging, for example, may involve estimating volumes of liver masses, gallbladder, or gallstones. Other size measurements such as the kidney long axis, which has traditionally relied on the sonographer's accurate imaging of this plane, are made easier and are more readily reproducible. As 3D ultrasound technology continues to mature, many more novel and specific applications will be investigated. Several already demonstrate clinical utility.
Given the unmatched benefits of 2D ultrasound for obstetric imaging, it is not surprising that much of the initial 3D ultrasound literature focused on obstetric applications (4,5). Many gynecological applications have also been explored, and these demonstrate great promise. They include evaluation of congenital uterine anomalies, where postprocessing into the coronal plane permits visualization of the uterine fundal contour (Figure 2).
FIGURE 3. Axial, sagittal, reconstructed coronal, and volumetric images of sonohysterogram demonstrate endometrial polyps. Volumetric image displays both polyps in one image and shows their orientation relative to each other and uterine cavity. (Provided by Siemens Medical Solutions)
Volumetric images of the fibroid uterus can facilitate measurement and comparison studies. Three-D imaging has also proven useful for endometrial polyps (Figure 3), cornual ectopic pregnancies, intrauterine devices, and adnexal lesions. Interventions, including abscess drainage of the pelvis and abdomen as well as fertility procedure guidance, are made easier by viewing multiple planes at once (6).
Urologic sonography is well established for both urodynamic imaging and anatomic survey. In these contexts, 3D imaging offers several benefits. For pediatric applications, 3D ultrasound is intrinsically superior to 2D in documenting congenital renal anomalies and ureter configuration (in context of reflux), as these 3D structures cannot be completely visualized in a single plane.
Transabdominal 4D ultrasound accurately depicts voiding and dynamic bladder volume when validated by uroflowmetry measurements (7). In comparison with 2D sonography, 3D ultrasound is superior for evaluation of hematuria with regard to identifying bladder cancer, bladder wall hypertrophy, bladder diverticula, mucosal bladder folds, and regrowth of the prostate, as validated by cystoscopy and/or bladder biopsy (8).
In the scrotum, 3D ultrasound may provide better depiction of the complex geometry of the epididymis and other extratesticular structures, potentially improving diagnostic confidence and speed workflow.
For patients with suspected prostate cancer, 3D power Doppler sonography improves the diagnostic and staging accuracy of anatomic imaging through improved depiction of prostate vascular structures. Optimization of biopsy site selection is also aided by identification of areas of abnormal blood flow. Finally, extracapsular involvement is evaluated by detecting the presence of vessels perforating the capsule (9). Use of microbubble-based contrast agents further increases the sensitivity of 3D ultrasound for prostate malignancy (10).
The role of volumetric scanning continues to evolve in sonography of breast masses. The multiplanar capability of 3D has introduced to the imaging palette the coronal plane, which some investigators suggest improves depiction of tumor margins and of the orientation of tumors relative to ductal structures (Figure 4). One study, however, found no significant benefit for 3D ultrasound compared with 2D techniques for distinguishing benign from malignant breast masses (11).
FIGURE 4. A. Axial breast ultrasound image demonstrates lobulated mass with refractive edge shadowing. (Provided by GE Healthcare Technologies)
FIGURE 4. B. Coronal reconstructed image demonstrates intraductal location of mass. (Provided by GE Healthcare Technologies)
Ultrasound is a proven modality for vascular imaging. In addition to anatomic detail, color and spectral Doppler imaging allow visualization and quantification of blood flow. But 2D ultrasound estimation of flow volume is inherently inaccurate because vessel cross-sectional area is unknown. Recent work suggests that 3D techniques provide true volumetric flow estimates that are angle-independent (12). While 2D ultrasound allows for detection of arterial atherosclerosis primarily through flow velocity measurements, 3D ultrasound allows for direct visualization and quantification of plaque volume.
The addition of microbubble contrast agents allows for up to 30 dB of signal enhancement and depiction of minute detail (13). Several studies have demonstrated the utility of 3D ultrasound for accurate carotid and aortic plaque characterization (14-16). Correlation of 3D ultrasound with gold standard digital subtraction angiography (DSA) demonstrates excellent agreement in describing plaque morphology (17,18).
The addition of 3D color Doppler information may provide benefits in the grading of stenotic lesions with respect to flow dynamics (19). Given the ability to reformat volumetric data to visualize flow in a plane parallel to the interrogated vessel, very low interobserver variability is achievable (20,21). Rather than searching for a single plane that contains related vasculature, such as the hepatic arterial system, the viewer can observe the entire vascular tree in 3D.
Evaluation of cardiac anatomy and functional is commonly performed using echocardiography techniques. Given that the heart's contraction motion is intrinsically three-dimensional, 3D imaging presents several advantages. The most intuitive is more accurate measurement of chamber volume and ejection fraction. While 2D echocardiography requires assumption of a simplified geometric model of ventricular shape, a 3D approach allows for direct ventricle segmentation (22).
Another dramatic advantage is visualization of an entire valve at a given time point or cinematically throughout the cardiac cycle (23). Other advantages include improved visualization of septal defects and assessment of anatomic relationships in congenital heart disease (24).
Ultrasound's real-time visualization capability has well-established benefits in guidance of both minimally invasive and operative interventional procedures. While many surgical specialties may benefit from 3D ultrasound technology, its application to neurosurgical procedures has prompted particularly rapid adoption and intensive research. Continuous intraoperative monitoring of 3D brain shift is very useful during resection of intracranial tumors. This information allows a surgeon to modify preoperative planning maps to account for warping and tissue removal (25).
Doppler imaging allows for improved visualization of vasculature of interest (26). This is beneficial in a variety of procedures, including biopsy guidance, resection guidance, arteriovenous malformation localization (and involved vessel identification), localization of peripheral aneurysms, and delineation of cavernous hemangiomas in both brain parenchyma and the brain stem (27,28).
For glioma resection, 3D ultrasound volumes provide delineation of metastases and solid component at least as reliably as navigated 3D MRI (29). Unlike MRI-based navigation, no preoperative data set is required, and thus craniotomy does not affect navigational accuracy.
While 3D ultrasound demonstrates much promise, several limitations and pitfalls remain that must be considered to successfully integrate the technology in clinical practice.
First, 3D image acquisition is logistically but not fundamentally different from 2D ultrasound. The source of contrast -- acoustic impedance differences -- is identical, so current contrast limitations still apply. In obstetrics, where amniotic fluid provides high contrast between the background and the fetal surface, 3D renderings provide impressive verisimilitude, but in other organ systems, the effects have been less dramatic.
Second, user interfaces are complex and challenging to master. No standardized display convention has emerged to date for the reconstructed images, so image orientation can be difficult to determine. Common artifacts used for characterization, such as posterior enhancement, are more difficult to interpret in the context of multiple simultaneous transducer orientations.
Third, while most 2D studies are standardized on a per-organ or per-indication basis, currently no standard methodology for 3D data set acquisition or interpretation exists.
Fourth, rendering of data in 3D using volume or surface rendering techniques introduces an additional layer of potential artifacts and lack of standardized review.
Finally, while 3D images are qualitatively more intuitive than traditional 2D sections, data as to the quantitative clinical benefits of 3D ultrasound imaging are limited, particularly compared with the vast volume of literature and expertise available for 2D imaging. Thus, more research is required to identify the benefits and indications for 3D imaging.
Three-D ultrasound is an emerging modality that extends the scope and potentially improves the clinical utility of 2D ultrasound for a variety of organ systems and indications. Unlike 2D ultrasound, 3D technology is only in its infancy. Undoubtedly, the next decade will see both technological refinement and increased standardization of 3D ultrasound evaluation. Thus, 3D ultrasound is a frontier that has been discovered but is yet to be settled.
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