text: The main disadvantage of 3D US appears to be the extra time required for image reconstruction and rendering. Regarding real-time 3D images, Dr. Marx notes that "right now, the biggest limitation is that it does take additional time to analyze and reconstruct the pictures-maybe a couple of minutes to 15 to 20 minutes." Currently, a similar analysis done in an MRI suite or a CT suite is considered to be part of the study. But this is not the case for ultrasonography. "In our laboratory, we do anywhere from 50 to 70 studies a day, and it's very hard to have that additional time to do the offline analysis." Dr. Marx hopes that medical insurance payers will see the importance of volume echocardiography and reimburse for these analyses.
  There is a learning curve involved for both physician and sonographer who begin to perform 3D US imaging. Because there are many options for viewing the region of interest, there are more controls. Instead of acquiring a plane of data, for example, real-time systems acquire a volume of data. This imaging method requires vast computational power and data storage. In addition, motion artifacts can be a problem. According to Dr. Benacerraf, there is also some loss of image quality with any reconstruction, but this is improving with new advances in imaging technology.
  The medical community is attempting to create some guidelines for standardization of 3D views and patient positions. Dr. Marx has been working on a position paper in echocardiography on generalized 3D imaging. "This is something that is realized and recog-nized-that we have to start developing a nomenclature or language as to how this should be best performed, best understood, and best communicated between people doing the procedures."
  The advantages of 3D US have been exploited in a number of clinical applications, from image-guided therapy, field placement verification in radiotherapy, and intraoperative imaging to measurement and quantification of organ function, tumor vascularity, and anatomic volumes. Just a few of the many and diverse clinical trials in which 3D US imaging has shown promise are in the fields of cardiology and vascular medicine, obstetrics and gynecology, and ophthalmology.
  Three-dimensional US excels at diagnostic imaging of the heart, according to Dr. Roberto Lang: "The heart is a 3-dimensional object, so it makes sense to image it as a 3-dimensional object." Dr. Lang uses real-time 3D US to obtain dynamic volumetric information on cardiac function. "When you image the heart in 2 dimensions, you make assumptions about the cardiac volumes based on a geometric model, rather than take actual measurements." He and his colleagues recently tested the feasibility of volumetric quantification of global and regional ventricular function by using 3D echocardiography. 9 In this study, global and regional left ventricular volume time and wall motion curves were obtained for 30 patients and analyzed according to 3 protocols that compared the results with 2D US and MRI findings. They concluded that 3D US findings correlated well with MRI data and could differentiate populations by left ventricular function. Automated detection of regional wall motion abnormalities agreed with 2D US findings in 86% of segments.
  Dynamic 3D US imaging of the fetal heart has been done by using tissue Doppler data to calculate a gating signal. 10 In this experimental study, B-mode cineloops were synchronized with tissue Doppler gating signals. The dynamic cardiac reconstructions thus produced were of high quality, and these, along with classical 2D views, were judged to be adequate for clinical use.
  Three-dimensional power Doppler US has been used to quantify arterial stenosis and may become an alternative to X-ray angiography. 11,12 A study comparing digital subtraction angiography (DSA), 2D color Doppler sonography (CDS), and 3D reconstruction for assessment of internal carotid artery stenosis in 49 patients found good agreement between DSA and 3D CDS (mean sensitivity of CDS, 93%; specificity, 82.5%; positive predictive value, 82%; negative predicative value, 98%). 12
  In addition to the well-known application of imaging fetal development, 3D US can be used for imaging female reproductive anatomy. After commenting that looking at the fallopian tubes is a lot easier with reconstructed views, Dr. Benacerraf continued, "With 2D images, all of the views of the pelvis are acquired through the transvaginal portal of entry, whereas if volume data are acquired, you can reconstruct the pelvis in whatever orientation you want, making the portal of entry irrelevant."
  Infertility specialists need to evaluate the shape of the uterus for any abnormality that might lead to reproductive failure, but uterine shape abnormalities are undetectable by regular 2D US. In the past, an MRI study was required, but now abnormalities in uterine shape can be diagnosed with 3D US (Figure 7).
  The reproducibility of 3D US findings in gynecology have been confirmed by several recent studies. Yaman and colleagues 13 found that measurement of endometrial volume was more reproducible by 3D US than by 2D US in women with postmenopausal bleeding. Salim and coworkers 14 found that interobserver and intraobserver variabilities were within satisfactory limits of agreement in the diagnosis of congenital uterine anomalies.
  The structure and vascularity of the ovaries can be easily assessed with 3D US. Kurjack and colleagues 15 compared 3D sonography and power Doppler imaging with standard 2D transvaginal gray-scale and color/power Doppler modalities to assess suspected ovarian stage I cancer in 43 patients in a retrospective analysis. They found that combined 3D sonography and power Doppler achieved a diagnostic accuracy of 97.7% compared with only 69.8% and 86.1% for 2D gray-scale and combined 2D gray-scale and color Doppler, respectively.
  Three-dimensional US reconstruction of eyes with retinoblastoma were analyzed as to provide unique planar views that were previously unavailable with 2D instruments (Figure 8). Oblique and coronal images have been shown to be indispensable in evaluation and follow-up after treatment. In an initial experience reported by Finger and coworkers, 16 volume US imaging of 5 eyes of 3 children allowed the discovery of new oblique and coronal views that could be used to determine relative distances between tumors; simultaneous viewing of these tumors was not possible with ophthalmoscopy. In addition, interactive sectioning of the ocular volume, which is not possible with 2D US, allowed differentiation of the optic nerve from orbital shadowing caused by intratumor calcification.
  In another report by Finger and colleagues, 17 3D US was used to measure tumor volumes of choroidal melanomas (Figure 9). They evaluated 50 3D US images and found that derived measurements of tumor volume correlated well with standard tumor measurement techniques. They believed 3D US volume measurements better accounted for tumor geometry than did calculated estimates of tumor volume based on basal area and height, and they concluded that volume US could be used for measurement of choroidal melanomas.
  Recently, this same group explored the use of coronal C-scan images to measure optic nerve sheath diameter in healthy volunteers, patients with optic nerve sheath meningiomas, and in 1 patient with retinoblastoma. 18-20 The automated measurements were similar to those obtained with MRI and CT, and the authors conclude that 3D US may have a role as a screening tool for this application.
  Although 3D US imaging is gaining popularity, the extent to which it will become integrated into routine US examinations may be related to the time required for volume calculations and various data set reconstructions. Dr. Marx believes that automation is the key to overcoming these problems. He would also like to see miniaturization of the matrix array transducer to allow 3D imaging by use of the transesophageal echocardiography probe, or in intravascular probes that could image inside the heart itself. In Dr. Marx's ideal future, wireless technology would accompany this new intraorgan scanning. "Certainly, having a miniaturized transducer that could both send and receive electronically and communicate with the US machine via wireless technology would be terrific."