Three-dimensional ultrasound imaging

Accuracy of an electromagnetic three-dimensional ultrasound system for carotid artery imaging

Freehand, three-dimensional (3-D) ultrasound (US) systems, which incorporate an electromagnetic tracking device to register the 3-D spatial location of images acquired using a standard linear array scan-probe, are a flexible and cost-effective solution for many clinical applications. The reconstruction accuracy of one such system was investigated by using a precision-made phantom. The error in 3-D distance measurements, under conditions appropriate to US investigations of the carotid arteries, was found to be -0.45 +/- 1.30 mm, equivalent to -0.53 +/- 3.39% (mean +/- SD). The results are relevant to data acquired using a single sweep scan and for distances in the range 25.00 to 79.06 mm. Both the overall accuracy and precision in point-target location were found to be relatively unaffected by scan depth, and the precision of point-target location was found to be poorest in the elevational direction. In conclusion, the system tested in our laboratory performed with high accuracy, adopting a setup and scan-sweep identical to that used for imaging of the carotid arteries in 3-D.

Development of three-dimensional power Doppler ultrasound imaging of fetoplacental vasculature

To develop an off-line system for three-dimensional (3-D) ultrasound (US) reconstruction of fetoplacental vasculature using colour segmentation and reconstruction software and to determine sources of error in fully freehand ultrasound image acquisition. US images were acquired freehand with the Acuson Sequoia (5C 2-MHz transducer) using power Doppler. After digital transfer to a personal computer, CQ Analysis software (Kinetic Imaging Ltd, Liverpool, UK) was used to segment the colour information from these images, and the resulting 8-bit grey-scale images were used for 3-D rendering using commercial software (VoxBlast, Vaytek Inc., Fairfield, IA, USA). 2-D scanning, software and freehand acquisition accuracy were assessed using a linear test rig and distance and volume phantoms (Dansk Phantom Service Ltd); 2-D scanning accuracy was within 1.3%, and software reconstruction accuracy within 1% for x and y planes and up to 3% for the z plane. Fully freehand acquisition was associated with a 12% to 18% mean percentage error in distance measurement in the plane of acquisition. Volumetric reconstruction inaccuracy was between 1.5% and 19.7% for precisely separated images and between 16.2% and 39.2% for fully freehand image acquisition. Rendered 3-D US vascular images clearly delineated vascular anatomy within the placenta and cord. Fully freehand 3-D US does have a role in off-line reconstruction of vascular anatomy, although variability in the z plane precludes its use for volumetric measurement. (E-mail: a.welsh@ic.ac.uk)

Optimisation and evaluation of an electromagnetic tracking device for high-accuracy three-dimensional ultrasound imaging of the carotid arteries

Electromagnetic tracking devices provide a flexible, low cost solution for three-dimensional ultrasound (3-D US) imaging. They are, however, susceptible to interference. A commercial device (Ascension pcBIRD) was evaluated to assess the accuracy in locating the scan probe as part of a digital, freehand 3-D US imaging system aimed at vascular applications. The device was optimised by selecting a measurement rate and filter setting that minimised the mean deviation in repeated position and orientation measurements. Experimental evaluation of accuracy indicated that, overall, absolute errors were small: the RMS absolute error was 0.2 mm (range: -0.7 to 0.5 mm) for positional measurements over translations up to 90 mm, and 0.2 degrees (range: -0.8 to 0.9 degrees ) for rotational measurements up to 30 degrees. In the case of position measurements, the absolute errors were influenced by the location of the scanner relative to the scan volume. We conclude that the device tested provides an accuracy sufficient for use within a freehand 3-D US system for carotid artery imaging.

Rapid three-dimensional echocardiography : clinically feasible alternative for precise and accurate measurement of left ventricular volumes

BACKGROUND

Clinical applicability of conventional ultrasonographic systems using mechanical adapters for 3D echocardiographic imaging has been limited by long acquisition and processing times. We developed a rapid (6-s) acquisition technique that collects apical tomograms using a continuously internally rotating transthoracic transducer. This study was performed to examine the clinical feasibility of rapid-acquisition 3D echocardiography to estimate left ventricular end-diastolic and end-systolic volumes using electron-beam computed tomography as the reference standard. Methods and Results-We collected a series of 6 to 11 apical echocardiographic tomograms, depending on heart rate, in 11 patients. There was good correlation, low variability, and low bias between rapid 3D echocardiography and electron-beam computed tomography for measuring left ventricular end-diastolic volume (r=0.96; standard error of the estimate, 21.34 mL; bias, -4.93 mL) and left ventricular end-systolic volume (r=0.96; standard error of the estimate, 14.78 mL; bias, -6.97 mL).

CONCLUSIONS

The rapid-acquisition 3D echocardiography extends the use of a multiplane, internally rotating handheld transducer so that it becomes a precise and clinically feasible tool for assessing left ventricular volumes and function. A rapid-image acquisition time of 6 s would allow repeated image collection during the course of a clinical echocardiographic examination. Additional work must address rapid and automated data processing.

Three-dimensional ultrasound imaging

Ultrasound is an inexpensive and widely used imaging modality for the diagnosis and staging of a number of diseases. In the past two decades, it has benefited from major advances in technology and has become an indispensable imaging modality, due to its flexibility and non-invasive character. In the last decade, research investigators and commercial companies have further advanced ultrasound imaging with the development of 3D ultrasound. This new imaging approach is rapidly achieving widespread use with numerous applications. The major reason for the increase in the use of 3D ultrasound is related to the limitations of 2D viewing of 3D anatomy, using conventional ultrasound. This occurs because: (a) Conventional ultrasound images are 2D, yet the anatomy is 3D, hence the diagnostician must integrate multiple images in his mind. This practice is inefficient, and may lead to variability and incorrect diagnoses. (b) The 2D ultrasound image represents a thin plane at some arbitrary angle in the body. It is difficult to localize the image plane and reproduce it at a later time for follow-up studies. In this review article we describe how 3D ultrasound imaging overcomes these limitations. Specifically, we describe the developments of a number of 3D ultrasound imaging systems using mechanical, free-hand and 2D array scanning techniques. Reconstruction and viewing methods of the 3D images are described with specific examples. Since 3D ultrasound is used to quantify the volume of organs and pathology, the sources of errors in the reconstruction techniques as well as formulae relating design specification to geometric errors are provided. Finally, methods to measure organ volume from the 3D ultrasound images and sources of errors are described.

Three-dimensional ultrasound in obstetrics and gynecology

Three-dimensional ultrasound is a new modality finding its way into clinical practice. Most of the major ultrasound vendors are now developing three-dimensional ultrasound capabilities. We expect that although three-dimensional ultrasound will not replace two-dimensional ultrasound, many additional benefits will be identified and its use will continue to grow. The ability to evaluate anatomy and pathology with multiplanar and surface-rendered images provides physicians additional valuable clinical information. Volume data allows for a specific point in space to be evaluated from many different orienta tions by rotating, slicing, and referencing the slice to other orthogonal slices. It also allows for new volume-rendering displays that show depth, curvature, and surface images not available with conventional methods. The current limitations of image resolution, intuitive interfaces for obtaining and displaying optimal images, and technologic limitations for data storage and manipulation (including real-time three-dimensional ultrasound) will surely be overcome in the near future.

The role of three-dimensional ultrasound in obstetrics

Three-dimensional ultrasound is a relatively new imaging modality with several potential advantages over conventional two-dimensional sonography. There is now increasing evidence that three-dimensional ultrasound can in many ways assist in the examination of the fetus. The enthusiasm generated by some groups, however, is not universally shared. It is the aim of this review to summarize the contemporary role of three-dimensional ultrasound in obstetric sonography by giving a critical appraisal of the relevant literature published recently, with emphasis on first and second trimester anatomy and fetal organ and placenta volumetry.

Measurement of abdominal aortic aneurysms with three-dimensional ultrasound imaging: preliminary report

OBJECTIVE

Accurate measurements of abdominal aortic aneurysms (AAAs) are required for surgical planning and monitoring over time. We have examined the feasibility of using a three-dimensional (3-D) ultrasound imaging system to derive quantitative measurements of interest from AAAs.

METHODS

A normal aorta, a small AAA, and an AAA repaired with an endovascular stent graft were scanned with a 3-D ultrasound imaging system. For each case, a 3-D surface reconstruction was generated from manual outlines of a sequence of two-dimensional ultrasound images, registered in 3-D space with a magnetic tracking system. The surfaces were resampled in planes perpendicular to the vessel center axis to calculate cross-sectional area and maximum diameter as a function of distance along the length of the aorta.

RESULTS

Cross-sectional area and maximum diameter were plotted along the length of the aneurysmal aortas from the renal arteries to the aortic bifurcation. The overall maximum diameter was found for both aneurysms. For the small AAA, the distances of the aneurysm from the renal arteries and the bifurcation were measured. For the repaired AAA, the location of the stent graft relative to the renal arteries was measured.

CONCLUSIONS

3-D surface reconstructions from ultrasound images show promise for quantitatively characterizing the geometry of AAAs both before surgery and after endovascular repair.

Three-dimensional ultrasonography in obstetrics: the clinical value

To investigate the clinical value of three-dimensional ultrasonography (3DUS) in obstetrics, various 3DUS rendering methods including surface mode, transparent mode and multiplanar mode were employed to scan 30 fetuses in second and third trimester by using the transabdominal volume transducer. The results showed that surface mode could vividly demonstrate the surface morphologic features of the fetuses, as well as the stereo-shape and the spatial relationship among the surface structures. The face, limbs, umbilical cord and outer genitalia of the fetus could be well displayed by surface mode. Transparent mode could reveal the bony structures under the surface, such as ribs, vertebrae, crania, etc. The result was not affected by the sophisticated curvature of these bony structures and the success rate was up to 100%. When rendered by multiplanar mode, the region of interest (ROI) could be viewed from different directions. It should be concluded that 3DUS could serve as a supplement to two-dimensional ultrasonography (2DUS). 3DUS might play an important role in prenatal diagnosis and enhance the diagnostic confidence level of the physicians.

Use of free-hand three-dimensional ultrasound software in the study of the fetal heart

OBJECTIVE

To propose a new approach to the study of the fetal heart using free-hand three-dimensional (3D) ultrasound software.

METHODS

We studied a total of 28 fetuses, of which 26 were normal and two were known to have heart pathology. In all of them a B-mode scan was performed. After obtaining a four-chamber view, and keeping the transducer in the same position, the free-hand 3D-View software was activated. A sequence of frames was stored. Additional information was recorded looking from the four-chamber view towards the outflow tract, and also adding color Doppler.

RESULTS

We obtained a multiplanar display of one B-mode and two perpendicular M-modes in all cases studied. In this multiplanar mode, Y and X axes represent distance and Z axis represents time. We were able to obtain M-modes and a variation of color-M-mode in any desired position. Moving the information along the Z axis, a frame sequence of the B-mode was obtained. Using this approach we observed one case of fetal supraventricular extrasystoles and other having an interventricular septum defect.

CONCLUSIONS

This modality gives a new perspective of fetal heart scanning using the free-hand 3D-View software, in which there is benefit from many of the advantages of the 3D software, although the power of this procedure must be improved.

Obstetric US imaging: the past 40 years

Although diagnostic ultrasonography (US) was developing in the late 1940s and early 1950s, it was not until the 1960s, with the availability of commercial equipment, that its usefulness in obstetrics began to be realized fully by radiologists and obstetricians around the world. Advances from A-mode to bistable and then to gray-scale static imaging were followed by the introduction of automated compound imaging and real-time US. Also, the development and initial use of Doppler US for the detection of fetal heart motion and the eventual use of pulsed and color Doppler US for the evaluation of such fetal structures as the major vessels and heart chambers contributed to increasing the usefulness of US in obstetrics. The development of specialized transducers--in particular, endovaginal probes--resulted in images of the early fetus. At the present time, the development of multiplanar, three-dimensional imaging shows great promise for more complete imaging of the fetus. The importance of US in the examination of the pregnant patient and, in particular, of the fetus has led to its worldwide dominance as the imaging modality of choice. The contributions of obstetric US to improving maternal well-being and fetal health have been recognized as a key component in all countries around the world.

Analysis of geometrical distortion and statistical variance in length, area, and volume in a linearly scanned 3-D ultrasound image

A linearly scanned three-dimensional (3-D) ultrasound imaging system is considered. The transducer array is initially oriented along the x axis and aimed in the y direction. After being tilted by an angle theta about the x axis, and then swiveled by an angle phi about the y axis, it is translated in the z direction, in steps of size d, to acquire a series of parallel two-dimendional (2-D) images. From these, the 3-D image is reconstructed, using the nominal values of the parameters (phi, theta, d). Thus, any systematic or random errors in these, relative to their actual values (phi0, theta0, d0), will respectively cause distortions or variances in length, area, and volume in the reconstructed 3-D image, relative to the 3-D object. Here, we analyze these effects. Compact linear approximations are derived for the relative distortions as functions of the parameter errors, and hence, for the relative variances as functions of the parameter variances. Also, exact matrix formulas for the relative distortions are derived for arbitrary values of (phi, theta, d) and (phi0, theta0, d0). These were numerically compared to the linear approximations and to measurements from simulated 3-D images of a cubical object and real 3-D images of a wire phantom. In every case tested, the theory was confirmed within experimental error (0.5%).

Clinical utility of three-dimensional US

Three-dimensional (3D) ultrasonography (US) is rapidly gaining popularity as it moves out of the research environment and into the clinical setting. This modality offers several distinct advantages over conventional US, including 3D image reconstruction with a single pass of the US beam, virtually unlimited viewing perspectives; accurate assessment of long-term effects of treatment; and more accurate, repeatable evaluation of anatomic structures and disease entities. In obstetric imaging, 3D US provides a novel perspective on the fetal anatomy, makes anomalies easier to recognize, facilitates maternal-fetal bonding, and helps families better understand fetal abnormalities. Three-dimensional pelvic US allows volume data sets to be acquired with both transvaginal and transabdominal probes. Viewing multiple 3D power Doppler US images in a fast cine loop has proved useful in angiographic applications. Three-dimensional prostate US can help make accurate volume assessments for dosimetry planning or for estimating prostate-specific antigen levels. In breast imaging, 3D US has the capacity to demonstrate lesion margins and topography, thereby helping differentiate benign from malignant masses. Three-dimensional US can also help determine the need for biopsy and help facilitate needle localization and guidance during biopsy. With recent advances in computer technology and display techniques, 3D US will likely play an increasingly important role in medicine.

Three-dimensional fetal echocardiography

The complex anatomy and dynamics of the heart make it a challenging organ to image. The fetal heart is particularly difficult because it is located deep within the mother's abdomen and direct access to electrocardiographic information is difficult. Thus more complex imaging and analysis methods are necessary to obtain information regarding fetal cardiac anatomy and function. This information can be used for medical diagnosis, model development and theoretical validation. The objective of this article is to provide scientists and engineers with an overview of three-dimensional fetal echocardiography.