Volume estimation from multiplanar 2D ultrasound images using a remote electromagnetic position and orientation sensor

Experimental 3-D Ultrasound Imaging with 2-D Sparse Arrays using Focused and Diverging Waves

Three dimensional ultrasound (3-D US) imaging methods based on 2-D array probes are increasingly investigated. However, the experimental test of new 3-D US approaches is contrasted by the need of controlling very large numbers of probe elements. Although this problem may be overcome by the use of 2-D sparse arrays, just a few experimental results have so far corroborated the validity of this approach. In this paper, we experimentally compare the performance of a fully wired 1024-element (32 × 32) array, assumed as reference, to that of a 256-element random and of an “optimized” 2-D sparse array, in both focused and compounded diverging wave (DW) transmission modes. The experimental results in 3-D focused mode show that the resolution and contrast produced by the optimized sparse array are close to those of the full array while using 25% of elements. Furthermore, the experimental results in 3-D DW mode and 3-D focused mode are also compared for the first time and they show that both the contrast and the resolution performance are higher when using the 3-D DW at volume rates up to 90/second which represent a 36x speed up factor compared to the focused mode.

Freehand 3-D Ultrasound Imaging: A Systematic Review

Two-dimensional ultrasound (US) imaging has been successfully used in clinical applications as a low-cost, portable and non-invasive image modality for more than three decades. Recent advances in computer science and technology illustrate the promise of the 3-D US modality as a medical imaging technique that is comparable to other prevalent modalities and that overcomes certain drawbacks of 2-D US. This systematic review covers freehand 3-D US imaging between 1970 and 2017, highlighting the current trends in research fields, the research methods, the main limitations, the leading researchers, standard assessment criteria and clinical applications. Freehand 3-D US systems are more prevalent in the academic environment, whereas in clinical applications and industrial research, most studies have focused on 3-D US transducers and improvement of hardware performance. This topic is still an interesting active area for researchers, and there remain many unsolved problems to be addressed.

Three-dimensional ultrasound scanning

The past two decades have witnessed developments of new imaging techniques that provide three-dimensional images about the interior of the human body in a manner never before available. Ultrasound (US) imaging is an important cost-effective technique used routinely in the management of a number of diseases. However, two-dimensional viewing of three-dimensional anatomy, using conventional two-dimensional US, limits our ability to quantify and visualize the anatomy and guide therapy, because multiple two-dimensional images must be integrated mentally. This practice is inefficient, and may lead to variability and incorrect diagnoses. Investigators and companies have addressed these limitations by developing three-dimensional US techniques. Thus, in this paper, we review the various techniques that are in current use in three-dimensional US imaging systems, with a particular emphasis placed on the geometric accuracy of the generation of three-dimensional images. The principles involved in three-dimensional US imaging are then illustrated with a diagnostic and an interventional application: (i) three-dimensional carotid US imaging for quantification and monitoring of carotid atherosclerosis and (ii) three-dimensional US-guided prostate biopsy.

3-D ultrasound volume reconstruction using the direct frame interpolation method

A new method for 3-D ultrasound volume reconstruction using tracked freehand 3-D ultrasound is proposed. The method is based on solving the forward volume reconstruction problem using direct interpolation of high-resolution ultrasound B-mode image frames. A series of ultrasound B-mode image frames (an image series) is acquired using the freehand scanning technique and position sensing via optical tracking equipment. The proposed algorithm creates additional intermediate image frames by directly interpolating between two or more adjacent image frames of the original image series. The target volume is filled using the original frames in combination with the additionally constructed frames. Compared with conventional volume reconstruction methods, no additional filling of empty voxels or holes within the volume is required, because the whole extent of the volume is defined by the arrangement of the original and the additionally constructed B-mode image frames. The proposed direct frame interpolation (DFI) method was tested on two different data sets acquired while scanning the head and neck region of different patients. The first data set consisted of eight B-mode 2-D frame sets acquired under optimal laboratory conditions. The second data set consisted of 73 image series acquired during a clinical study. Sample volumes were reconstructed for all 81 image series using the proposed DFI method with four different interpolation orders, as well as with the pixel nearest-neighbor method using three different interpolation neighborhoods. In addition, volumes based on a reduced number of image frames were reconstructed for comparison of the different methods' accuracy and robustness in reconstructing image data that lies between the original image frames. The DFI method is based on a forward approach making use of a priori information about the position and shape of the B-mode image frames (e.g., masking information) to optimize the reconstruction procedure and to reduce computation times and memory requirements. The method is straightforward, independent of additional input or parameters, and uses the high-resolution B-mode image frames instead of usually lower-resolution voxel information for interpolation. The DFI method can be considered as a valuable alternative to conventional 3-D ultrasound reconstruction methods based on pixel or voxel nearest-neighbor approaches, offering better quality and competitive reconstruction time.

Real-time vessel segmentation and tracking for ultrasound imaging applications

A method for vessel segmentation and tracking in ultrasound images using Kalman filters is presented. A modified Star-Kalman algorithm is used to determine vessel contours and ellipse parameters using an extended Kalman filter with an elliptical model. The parameters can be used to easily calculate the transverse vessel area which is of clinical use. A temporal Kalman filter is used for tracking the vessel center over several frames, using location measurements from a handheld sensorized ultrasound probe. The segmentation and tracking have been implemented in real-time and validated using simulated ultrasound data with known features and real data, for which expert segmentation was performed. Results indicate that mean errors between segmented contours and expert tracings are on the order of 1%-2% of the maximum feature dimension, and that the transverse cross-sectional vessel area as computed from estimated ellipse parameters a, b as determined by our algorithm is within 10% of that determined by experts. The location of the vessel center was tracked accurately for a range of speeds from 1.4 to 11.2 mm/s.

What does 2-dimensional imaging add to 3- and 4-dimensional obstetric ultrasonography?

OBJECTIVE

The purpose of this study was to determine whether 2-dimensional (2D) ultrasonography adds diagnostic information to that provided by the examination of 3-dimensional/4-dimensional (3D/4D) volume data sets alone.

METHODS

Ninety-nine fetuses were examined by 3D/4D volume ultrasonography. Volume data sets were evaluated by a blinded independent examiner who, after establishing an initial diagnostic impression by 3D/4D ultrasonography, performed a 2D ultrasonographic examination. The frequency of agreement and diagnostic accuracy of each modality to detect congenital anomalies were calculated and compared.

RESULTS

Fifty-four fetuses with no abnormalities and 45 fetuses with 82 anomalies diagnosed by 2D ultrasonography were examined. Agreement between 3D/4D and 2D ultrasonography occurred for 90.4% of the findings (123/136; intraclass correlation coefficient, 0.834; 95% confidence interval, 0.774-0.879). Six anomalies were missed by 3D/4D ultrasonography when compared to 2D ultrasonography (ventricular septal defect [n = 2], interrupted inferior vena cava with azygous continuation [n = 1], tetralogy of Fallot [n = 1], horseshoe kidney [n = 1], and cystic adenomatoid malformation [n = 1]). There were 2 discordant diagnoses: transposition of the great arteries diagnosed as a double-outlet right ventricle and pulmonary atresia misinterpreted as tricuspid atresia on 3D/4D ultrasonography. One case of occult spinal dysraphism was suspected on 3D ultrasonography but not confirmed by 2D ultrasonography. When compared to diagnoses performed after delivery (n = 106), the sensitivity and specificity of 3D/4D ultrasonography (92.2% [47/51] and 76.4% [42/55], respectively) and 2D ultrasonography (96.1% [49/51] and 72.7% [40/55]) were not significantly different (P = .233).

CONCLUSIONS

Information provided by 2D ultrasonography is consistent, in most cases, with information provided by the examination of 3D/4D volume data sets alone.

Three-dimensional ultrasound volume calculations of human embryos and young fetuses: a study on the volumetry of compound structures and its reproducibility

OBJECTIVE

To evaluate volumetry with three-dimensional (3D) ultrasonography in the assessment of the size of human embryos and fetuses.

METHODS

Forty-four healthy embryos/fetuses with crown-rump length (CRL) ranging from 9 mm to 58 mm were studied using a 7.5-MHz annular array transvaginal 3D probe. EchoPAC 3D software was used to calculate the volumes of the head, body and limbs in the same data set by two observers working independently of each other. Regression analysis was used to assess the relationship between estimated volumes and CRL.

RESULTS

The embryonic and fetal volume estimates of both observers ranged from a mean of 93 mm3 at 10 mm CRL to a mean of 11 169 mm3 at 55 mm CRL. The volume of the limbs as a proportion of the mean whole-body volume increased from 4.7% at a CRL of 15 mm to 9.3% at a CRL of 55 mm. Limits of agreement between the observers were calculated to be -0.12 +/- 9.2%.

CONCLUSION

It is possible to reconstruct complex small anatomic structures and calculate the volumes of human embryos and fetuses in vivo by using dedicated 3D ultrasound equipment. The reproducibility of whole-body volume estimates seems to be high. The limbs represent a significant proportion of the size of the embryonic/fetal body.

System for deep venous thrombosis detection using objective compression measures

A system for objective vessel compression assessment for deep venous thrombosis characterization using ultrasound image data and a sensorized ultrasound probe is presented. Two new objective measures calculated from applied force and transverse vessel area are also presented and used to describe vessel compressibility. A modified star-Kalman algorithm is used for feature detection in acquired ultrasound images, and objective measures of vessel compressibility are calculated from the detected features and acquired force and location data from the sensorized probe. A three-dimensional shape model of the examined vessel that includes compressibility measures mapped as colors to its surface is presented on the user interface, as well as a virtual representation of the image plane. The compressibility measures were validated using expert segmentation of healthy and diseased vessels and compared using paired t-tests, which showed a significant difference between healthy and diseased cases for both measures. 100% sensitivity and specificity were obtained for both measures. The system was implemented in real-time (16 Hz) and evaluated using a tissue phantom and on healthy human subjects. Sensitivity was 100% and 60%, while specificity was 97% for both measures when implemented. The initial results for the system and its components are promising.

Three- and 4-dimensional ultrasound in obstetric practice: does it help?

OBJECTIVE

The purpose of this article was to review the published literature on 3-dimensional ultrasound (3DUS) and 4-dimensional ultrasound (4DUS) in obstetrics and determine whether 3DUS adds diagnostic information to what is currently provided by 2-dimensional ultrasound (2DUS) and, if so, in what areas.

METHODS

A PubMed search was conducted for articles reporting on the use of 3DUS or 4DUS in obstetrics. Seven-hundred six articles were identified, and among those, 525 were actually related to the subject of this review. Articles describing technical developments, clinical studies, reviews, editorials, and studies on fetal behavior or maternal-fetal bonding were reviewed.

RESULTS

Three-dimensional ultrasound provides additional diagnostic information for the diagnosis of facial anomalies, especially facial clefts. There is also evidence that 3DUS provides additional diagnostic information in neural tube defects and skeletal malformations. Large studies comparing 2DUS and 3DUS for the diagnosis of congenital anomalies have not provided conclusive results. Preliminary evidence suggests that sonographic tomography may decrease the examination time of the obstetric ultrasound examination, with minimal impact on the visualization rates of anatomic structures.

CONCLUSIONS

Three-dimensional ultrasound provides additional diagnostic information for the diagnosis of facial anomalies, evaluation of neural tube defects, and skeletal malformations. Additional research is needed to determine the clinical role of 3DUS and 4DUS for the diagnosis of congenital heart disease and central nervous system anomalies. Future studies should determine whether the information contained in the volume data set, by itself, is sufficient to evaluate fetal biometric measurements and diagnose congenital anomalies.

A review of calibration techniques for freehand 3-D ultrasound systems

Three-dimensional (3-D) ultrasound (US) is an emerging new technology with numerous clinical applications. Ultrasound probe calibration is an obligatory step to build 3-D volumes from 2-D images acquired in a freehand US system. The role of calibration is to find the mathematical transformation that converts the 2-D coordinates of pixels in the US image into 3-D coordinates in the frame of reference of a position sensor attached to the US probe. This article is a comprehensive review of what has been published in the field of US probe calibration for 3-D US. The article covers the topics of tracking technologies, US image acquisition, phantom design, speed of sound issues, feature extraction, least-squares minimization, temporal calibration, calibration evaluation techniques and phantom comparisons. The calibration phantoms and methods have also been classified in tables to give a better overview of the existing methods.

A review of calibration techniques for freehand 3-D ultrasound systems

Three-dimensional (3-D) ultrasound (US) is an emerging new technology with numerous clinical applications. Ultrasound probe calibration is an obligatory step to build 3-D volumes from 2-D images acquired in a freehand US system. The role of calibration is to find the mathematical transformation that converts the 2-D coordinates of pixels in the US image into 3-D coordinates in the frame of reference of a position sensor attached to the US probe. This article is a comprehensive review of what has been published in the field of US probe calibration for 3-D US. The article covers the topics of tracking technologies, US image acquisition, phantom design, speed of sound issues, feature extraction, least-squares minimization, temporal calibration, calibration evaluation techniques and phantom comparisons. The calibration phantoms and methods have also been classified in tables to give a better overview of the existing methods.

Redundant system of passive markers for ultrasound scanhead tracking

Scanhead tracking by opto-electronic (OE) systems allows high accuracy in three-dimensional (3-D) freehand ultrasound imaging. In this paper, a new set of methods is proposed and compared with the standard approach [Gram-Schmidt method (GS)]. Three redundancy-based algorithms are introduced to compensate for possible loss of markers during data acquisition: regression plane (RP), multiple Gram-Schmidt (MGS), and center of mass least square (CMLS). When combined with the ultrasound instrument, the root-mean-squared (RMS) uncertainty in locating target points, over a working volume of 420 mm x 490 mm x 100 mm, improved by 7% and 24% using MGS and CMLS method respectively, compared to GS. A lower improvement was obtained with RP methods (5%), using the best marker configuration. In conclusion, CMLS method provides a robust and accurate procedure for 3-D freehand ultrasound scanhead tracking, able to manage possible loss of markers, with interesting perspectives for image fusion and body referenced 3-D ultrasound.

Three-dimensional realisation of muscle morphology and architecture using ultrasound

Two-dimensional B-mode ultrasound imaging and motion tracking were combined to generate three-dimensional reconstructions of the medial gastrocnemius. Architectural and morphological features of this muscle could be visualised. The length of the gastrocnemius belly was measured in normally (ND) developing children and in children with spastic diplegic cerebral palsy (SDCP) who had plantarflexion contractures. Using a random effects linear model we demonstrated that the gastrocnemius muscle bellies of children with SDCP were shorter than those of ND children (P = 0.001) even when corrected for ankle position. The technique described could be used to evaluate muscular deformity before and after an intervention.

Volume estimation of small phantoms and rat kidneys using three-dimensional ultrasonography and a position sensor

To evaluate the accuracy of small volume estimation, both in vivo and in vitro, measurements with a three-dimensional (3D) ultrasound (US) system were carried out. A position sensor was used and the transmitting frequency was 10 MHz. Balloons with known volumes were scanned while rat kidneys were scanned in vivo and in vitro. The Archimedes' principle was used to estimate the true volume. For balloons, the 3D US system gave very good agreement with true volumes in the volume range 0.1 to 10.0 mL (r = 0.999, n = 45, mean difference +/- 2SD = 0.245 +/- 0.370 mL). For rat kidneys in vivo (volume range 0.6 to 2.7 mL) the method was less accurate (r = 0.800, n = 10, mean difference +/- 2SD = -0.288 +/- 0.676 mL). For rat kidneys in vitro (volume range 0.3 to 2.7 mL) the results showed good agreement (r = 0.981, n = 23, mean difference +/- 2SD = 0.039 +/- 0.254 mL). For balloons, kidneys in vivo and in vitro, the mean percentage error was 9.3 +/- 4.8%, -17.1 +/- 17.4%, and 4.6 +/- 11.5%, respectively. This method can estimate the volume of small phantoms and rat kidneys and opens new possibilities for volume measurements of small objects and the study of organ function in small animals. (E-mail ).

VOLUS--a visualization system for 3D ultrasound data

The main goal of this paper is the description of a computer based system for medical applications to estimate the half-ellipsoid model that fits the left ventricle on its two phases of cardiac cycle: diastole and systole. Techniques for registration and rendering of ultrasound images will be presented using 2D freehand spatial calibrated echocardiography images in order to represent the 3D reconstructed data equivalent to the scanned volume. The 3D reconstruction will be used to estimate and measure parameters of the half-ellipsoid model fitted to the left ventricle.

Automatic virtual transducer locating system to assist in interpreting ultrasound imaging

Bodymarkers are used to label the location and orientation of the transducer during ultrasound examination. We attempt to evaluate the usefulness of a new system that indicates transducer location over that of the conventional bodymarker. The proposed system uses an electromagnetic tracking device to track the three-dimensional (3-D) position and orientation of a small electromagnetic receiver attached to the ultrasound transducer relative to a transmitter placed under the bed. The new bodymarker is displayed as a 3-D graphic model. The physique of the examinee is calibrated by representing five locations on the body on the original bodymarker. To evaluate the accuracy of the system visually, we compared the transducer position indicated in the new bodymarker and the actual transducer position in four abdominal sections. Actual and displayed position and orientation closely agreed in all cases, and the transducer position indicator in the bodymarker display moved smoothly. Automatic transducer locator on the virtual 3-D bodymarker accurately indicated its position and orientation. This system is useful and convenient in clinical examinations.

Three-dimensional ultrasound imaging

Two-dimensional viewing of three-dimensional anatomy by conventional ultrasound limits our ability to quantify and visualize a number of diseases and is partly responsible for the reported variability in diagnosis. Over the past two decades, many investigators have addressed this limitation by developing three-dimensional imaging techniques, including three-dimensional ultrasound imaging. In this paper we describe the development of a number of three-dimensional ultrasound imaging systems that make use of B mode, color Doppler, and power Doppler. In these systems, the conventional ultrasound transducer is scanned mechanically or by a freehand technique. The ultrasound images are digitized and then reconstructed into a three-dimensional volume, which can be viewed and manipulated interactively by the diagnostician with a variety of image-rendering techniques. These developments as well as future trends are discussed with regard to their applications and limitations.

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.

Ultrasound imaging in three and four dimensions

Three-dimensional (3D) reconstruction of ultrasound images was first demonstrated nearly 15 years ago, but only now is becoming a clinical reality. In the meantime, methods for 3D reconstruction of CT and MRI images have achieved an advanced state of development, and 3D imaging with these modalities has been applied widely in clinical practice. 3D applications in ultrasound have lagged behind CT and MRI, because ultrasound data is much more difficult to render in 3D, for a variety of technical reasons, than either CT or MRI data. Only in the past few years has the computing power of ultrasound equipment reached a level adequate enough for the complex signal processing tasks needed to render ultrasound data in three dimensions. At this point in time, the clinical application of 3D ultrasound is likely to advance rapidly, as improved 3D rendering technology becomes more widely available. This article is a review of the present status of 3D ultrasound imaging. It begins by comparing the characteristics of CT, MRI, and ultrasound image data that either make these data amenable or not amenable to 3D reconstruction. The article then considers the technical features involved with acquiring an ultrasound 3D data set and the mechanisms for reconstructing the images. Finally, the article reviews the literature that is available regarding clinical application of 3D ultrasound in obstetrics, ultrasound, the abdomen, and blood vessels.

Serial fetal lung volume measurement using three-dimensional ultrasound

OBJECTIVE

To establish reference intervals for fetal lung growth.

DESIGN

Longitudinal observational study.

SUBJECTS

Fifty-eight women with initially uncomplicated singleton pregnancies were recruited from the antenatal population of a teaching hospital. Four women were excluded from the final analysis because of complications arising in their pregnancy.

METHODS

Each subject was serially scanned at monthly intervals. At each visit lung volume was measured using an ultrasound-based computerized three-dimensional imaging system. Multilevel models were used to determine conditional and unconditional reference intervals.

RESULTS

Reference intervals for fetal lung growth were derived. Fetal lung volume increases in a non-linear way with gestation.

CONCLUSIONS

Our computerized system has the capacity to be used in conjunction with any standard two-dimensional ultrasound scanner in order to measure volume. Lung volume measurement may be useful in predicting pulmonary hypoplasia.

Body-centered visualisation for freehand 3-D ultrasound

Three-dimensional (3-D) ultrasound (US) data is typically visualised by any-plane slicing, volume rendering or surface rendering. Typical implementations of these techniques do not readily convey the spatial relationship between the visualised data and the patient's body, something that is particularly important when the data are reviewed after the scan has taken place, perhaps by a remote expert who did not even perform the scan. This paper describes a facility to register the 3-D US data to the patient's body and then display the data correctly superimposed on a rendered mannequin (rigid computer model). This way, the user can appreciate the position and orientation of any visualisation with respect to the patient's body. The facility relies on efficient implementation of progressive meshes to manage the level of detail of the mannequin model.

Accuracy of in-vitro volume estimation of small structures using three-dimensional ultrasound

We describe an ultrasound probe for three-dimensional transvaginal imaging. The transducer was an annular array with a center frequency of 7.5 MHz which was rotated with an internal stepper motor. The probe had no external moving parts, and the total volume covered by a full rotation defined a half sphere. The raw digital data from the scanner were transferred to an external PC for three-dimensional reconstruction. We evaluated the three-dimensional imaging system by measuring the volumes of phantoms (range 24.8-3362.5 mm(3)) in a water tank, and found good correlation with true volumes (two observers' measurements gave a linear regression with a slope of 1. 010 and R(2) = 0.993, and a slope of 0.956 and R(2) = 0.993, respectively). The size of the point-spread function was used in the calculations to eliminate the effect of under- or overestimation due to the limited ultrasound beam resolution. An example of data acquisition, volume estimation and imaging of an embryo less than 8 weeks old in vivo with the brain cavities and body is given. We conclude that the three-dimensional reconstruction and volume estimation were accurate and repeatable.

Reconstruction and visualization of irregularly sampled three- and four-dimensional ultrasound data for cerebrovascular applications

Although recent studies have demonstrated the potential value of compounded data for improvement in signal-to-noise ratio and speckle contrast for three-dimensional (3-D) ultrasonography, clinical applications are lacking. We investigated the potential of six degrees-of-freedom (6-DOF) scanhead position and orientation measurement (POM) devices for registration of in vivo multiplanar, irregularly sampled ultrasound (US) images to a regular 3-D volume space. The results demonstrate that accurate spatial and temporal registration of four-dimensional (4-D) US data can be achieved using a 6-DOF scanhead tracking system. For reconstruction of arbitrary, irregularly sampled US data, we introduce a technique based upon a weighted, ellipsoid Gaussian convolution kernel. Volume renderings of 3-D and 4-D compounded in vivo US data are presented. The results, although restricted to the field of cerebrovascular disease, will be of value to other applications of 3-D sonography, particularly those in which compounding of data through irregular sampling may provide superior information on tissue or vessel structure.

Four-dimensional ultrasonographic characterization of plaque surface motion in patients with symptomatic and asymptomatic carotid artery stenosis

BACKGROUND AND PURPOSE

In vitro studies of atherosclerotic plaque fracture mechanics suggest that analysis of local variations in surface deformability may provide information on relative vulnerability to plaque fissuring or rupture. We investigated plaque surface deformations in patients with symptomatic and asymptomatic carotid artery disease using 4-dimensional ultrasonography and techniques for measuring optical flow.

METHODS

Four-dimensional ultrasound examinations of carotid artery plaques were performed in 23 asymptomatic and 22 symptomatic patients with 50% to 90% stenosis of the internal carotid artery. Plaque surface motion during 1 cardiac cycle was computed with a hierarchical model-based motion estimator. Results were compared with plaque echogenicity and surface structure.

RESULTS

Of the 45 patients examined, plaque surface motion estimates were obtained for 18 asymptomatic and 13 symptomatic patients. There were no significant differences in echogenicity or surface structure of asymptomatic and symptomatic plaques (P>0.05). Results of motion estimation showed that asymptomatic plaques had surface motion vectors of equal orientation and magnitude to those of the internal carotid artery, whereas symptomatic plaques demonstrated evidence of inherent plaque movement. There was no significant difference in maximal plaque velocity between symptomatic and asymptomatic plaques (P<0.14). Maximal discrepant surface velocity (MDSV) in symptomatic plaques was 3.85+/-1.26 mm/s (mean+/-SD), which was significantly higher (P<0.001) than MDSV of asymptomatic plaques with 0.58+/-0.42 mm/s (mean+/-SD).

CONCLUSIONS

++MDSV of carotid artery plaques is significantly different in asymptomatic and symptomatic disease. Further studies are warranted to determine whether plaque surface motion patterns can identify vulnerable plaques in patients with carotid artery stenosis.

In vitro volume estimation of kidneys using three-dimensional ultrasonography and a position sensor

OBJECTIVE

A new 3D ultrasound system using a position sensor based on magnetic scanhead tracking and new software utilising automatic contour tracing between manually traced contours was tested for volume estimation of kidneys in vitro.

METHODS

Kidneys from piglets and pigs were fixed in formaldehyde. A reservoir with 0.9% saline kept at 37 degrees C was used. The kidneys were scanned either by a linear translational movement along the organ or by a tilting movement. The outer contour of the kidneys was traced manually, by two independent investigators. The volume of each kidney was also measured using the Archimedes principle (true volumes).

RESULTS

Good agreement between 3D ultrasound volume estimates and true volumes was found for both probe movements. For translational movement of the transducer, the mean errors between the methods were 4.17 and 4.31 ml for the two independent investigators, and the volume range was 96-203 ml. The corresponding error values for tilting movement were 1.10 and 0.19 ml. The interobserver variation was also small, there was no difference in the volumes obtained by the two investigators, or by the two scanning movements.

CONCLUSION

Volume estimates using this 3D ultrasound method showed very good agreement with true volumes, both mean errors and interobserver variation were low.

Three-dimensional morphometry in ultrasound

The clinical use of three-dimensional (3D) ultrasound has rapidly spread to many specialities over the last ten years. The reason is easy to see, namely that single two-dimensional (2D) scans are often difficult to interpret and the mental correlation of multiple 2D scans to form a 3D image of anatomical morphology is taxing and uncertain. The rapid development of techniques for the realtime tracking of the spatial position and orientation of ultrasound probes and the development of computer graphics techniques for the presentation of anatomical images have made 3D ultrasound a realistic diagnostic tool. The authors describe the range of methods of data acquisition and display and provide illustrations of some current clinical applications.

Dynamic three-dimensional freehand echocardiography using raw digital ultrasound data

In this paper, we present a new method for simple acquisition of dynamic three-dimensional (3-D) ultrasound data. We used a magnetic position sensor device attached to the ultrasound probe for spatial location of the probe, which was slowly tilted in the transthoracic scanning position. The 3-D data were recorded in 10-20 s, and the analysis was performed on an external PC within 2 min after transferring the raw digital ultrasound data directly from the scanner. The spatial and temporal resolutions of the reconstruction were evaluated, and were superior to video-based 3-D systems. Examples of volume reconstructions with better than 7 ms temporal resolution are given. The raw data with Doppler measurements were used to reconstruct both blood and tissue velocity volumes. The velocity estimates were available for optimal visualization and for quantitative analysis. The freehand data reconstruction accuracy was tested by volume estimation of balloon phantoms, giving high correlation with true volumes. Results show in vivo 3-D reconstruction and visualization of mitral and aortic valve morphology and blood flow, and myocardial tissue velocity. We conclude that it was possible to construct multimodality 3-D data in a limited region of the human heart within one respiration cycle, with reconstruction errors smaller than the resolution of the original ultrasound beam, and with a temporal resolution of up to 150 frames per second.

Fast surface and volume estimation from non-parallel cross-sections, for freehand three-dimensional ultrasound

Volume measurements from ultrasound B-scans are useful in many clinical areas. It has been demonstrated previously that using three-dimensional (3-D) ultrasound can greatly increase the accuracy of these measurements. Freehand 3-D ultrasound allows freedom of movement in scanning, but the processing is complicated by having non-parallel scan planes. Two techniques are proposed for volume measurement from such data, which also improve surface and volume estimation from data acquired on parallel planes. Cubic planimetry is a more accurate extension of a volume measurement technique involving vector areas and centroids of cross-sections. Maximal-disc shape-based interpolation is an extension of shape-based interpolation which uses maximal disc representations to adjust the interpolation direction locally and hence improve the quality of the surface generated. Both methods are tested in simulation and in vivo. Volumes estimated using cubic planimetry are more accurate than step-section planimetry, and require fewer cross-sections, even for complex objects. Maximal-disc shape-based interpolation provides a reliable means of reconstructing surfaces from a handful of cross-sections, and can therefore be used to give confidence in the segmentation and hence also the cubic planimetry volume.

Stradx: real-time acquisition and visualization of freehand three-dimensional ultrasound

Conventional freehand three-dimensional (3-D) ultrasound is a multi-stage process. First, the clinician scans the area of interest. Next, the ultrasound data is used to construct a 3-D voxel array, which can then be visualized by, for example, any-plane slicing. The strict separation of data acquisition and visualization disturbs the interactive nature of the ultrasound examination. Furthermore, some systems require the clinician to wait for an unacceptable amount of time while the voxel array is constructed. In this paper, we describe a novel freehand 3-D ultrasound system which allows accurate acquisition of the raw data and immediate visualization of arbitrary slices through the data. Minimal processing separates the acquisition and visualization processes: in particular, at no stage is a voxel array constructed. Instead, the standard graphics hardware found inside most desktop computers is exploited to synthesize arbitrary slices directly from the raw B-scans.

Three-dimensional ultrasound imaging

The objective of this article is to provide scientists, engineers and clinicians with an up-to-date overview on the current state of development in the area of three-dimensional ultrasound (3-DUS) and to serve as a reference for individuals who wish to learn more about 3-DUS imaging. The sections will review the state of the art with respect to 3-DUS imaging, methods of data acquisition, analysis and display approaches. Clinical sections summarize patient research study results to date with discussion of applications by organ system. The basic algorithms and approaches to visualization of 3-D and 4-D ultrasound data are reviewed, including issues related to interactivity and user interfaces. The implications of recent developments for future ultrasound imaging/visualization systems are considered. Ultimately, an improved understanding of ultrasound data offered by 3-DUS may make it easier for primary care physicians to understand complex patient anatomy. Tertiary care physicians specializing in ultrasound can further enhance the quality of patient care by using high-speed networks to review volume ultrasound data at specialization centers. Access to volume data and expertise at specialization centers affords more sophisticated analysis and review, further augmenting patient diagnosis and treatment.

3-D sonographic volume measurement of the cerebral ventricular system: in vitro validation

This in vitro study evaluates the accuracy and observer dependency of a freehand three-dimensional (3-D) ultrasound system for measuring cerebral ventricular volume in infants. A sphere, a cylinder, and three cerebral ventricle phantoms were made of agarose and embedded in an echogenic matrix after measuring true volumes by water displacement. Volumes of the models were calculated by 3-D software after manual contour marking on ultrasound cross-sections. Mean +/- SD sonographic volume was 94.6%+/-7.3% (n = 130) of true volumes. Intraobserver variation (n = 10) was 2.3%-5.3% for complete investigation (scanning and marking), and 1.8%-4.1% for marking alone. Difference of means (n = 10) between two observers was 7.6% to 10.8% for complete investigation, and 0.6% to 1.6% for the marking process. We conclude that 3-D freehand ultrasonography may be useful for examining ventricle dilatation in infancy.

Rapid calibration for 3-D freehand ultrasound

3-D freehand ultrasound is a new imaging technique that is rapidly finding clinical applications. A position-sensing device is attached to a conventional ultrasound probe so that, as B-scans are acquired, they can be labelled with their relative positions and orientations. This allows a 3-D data set to be constructed from the B-scans. A key requirement of all freehand imaging systems is calibration; that is, determining the position and orientation of the B-scan with respect to the position sensor. This is typically a lengthy and tedious process that may need repeating every time a sensor is mounted on a probe. This paper describes a new calibration technique that takes only a few minutes to perform and produces results that compare favourably (in terms of both accuracy and precision) with previously published alternatives.

In vitro estimation of foetal liver volume using ultrasound, x-ray computed tomography and magnetic resonance imaging

Sixteen formalin-fixed foetal livers were scanned in vitro using a new system for estimating volume from a sequence of multiplanar 2D ultrasound images. Three different scan techniques were used (radial, parallel and slanted) and four volume estimation algorithms (ellipsoid, planimetry, tetrahedral and ray tracing). Actual liver volumes were measured by water displacement. Twelve of the sixteen livers also received x-ray computed tomography (CT) and magnetic resonance (MR) scans and the volumes were calculated using voxel counting and planimetry. The percentage accuracy (mean +/- SD) was 5.3 +/- 4.7%, -3.1 +/- 9.6% and -0.03 +/- 9.7% for ultrasound (radial scans, ray volumes), MR and CT (voxel counting) respectively. The new system may be useful for accurately estimating foetal liver volume in utero.

Usefulness of gated three-dimensional fetal echocardiography to reconstruct and display structures not visualized with two-dimensional imaging

Cardiac-gated 3-dimensional fetal echocardiography can reconstruct and display cardiac structures and views not visualized with conventional 2-dimensional ultrasonography. This new technique may become an integral part of screening ultrasonography, complementing 2-dimensional fetal echocardiography when real-time imaging is incomplete.

Three-dimensional spatial compounding of ultrasound images

One of the most promising applications of 3-D ultrasound lies in the visualization and volume estimation of internal 3-D structures. Unfortunately, the quality of the ultrasound data can be severely degraded by artefacts and speckle, making automatic analysis of the 3-D data sets very difficult. In this paper we investigate the use of 3-D spatial compounding to reduce speckle. We develop a new statistical theory to predict the improvement in signal-to-noise ratio with increased levels of compounding, and verify the predictions empirically. We also investigate how registration errors can affect automatic volume estimation of structures within the compounded 3-D data set. Having established the need to correct these errors, we present a novel reconstruction algorithm which uses landmarks to register each B-scan accurately as it is inserted into the voxel array. In a series of in vitro and in vivo trials, we demonstrate that 3-D spatial compounding is very effective for improving the signal-to-noise ratio, but correction of registration errors is essential.

The accuracy of a new system for estimating organ volume using ultrasound

A new system is described for estimating volume from a series of multiplanar 2D ultrasound images. Ultrasound images are captured using a personal computer video digitizing card and an electromagnetic localization system is used to record the pose of the ultrasound images. The accuracy of the system was assessed by scanning four groups of ten cadaveric kidneys on four different ultrasound machines. Scan image planes were oriented either radially, in parallel or slanted at 30 degrees to the vertical. The cross-sectional images of the kidneys were traced using a mouse and the outline points transformed to 3D space using the Fastrak position and orientation data. Points on adjacent region of interest outlines were connected to form a triangle mesh and the volume of the kidneys estimated using the ellipsoid, planimetry, tetrahedral and ray tracing methods. There was little difference between the results for the different scan techniques or volume estimation alogorithms, although, perhaps as expected, the ellipsoid results were the least precise. For radial scanning and ray tracing, the mean and standard deviation of the percentage errors for the four different machines were as follows: Hitachi EUB-240, -3.0 +/- 2.7%; Tosbee RM3, -0.1 +/- 2.3%; Hitachi EUB-415, 0.2 +/- 2.3%; Acuson, 2.7 +/- 2.3%.

Three-dimensional freehand ultrasound: image reconstruction and volume analysis

A system is described that rapidly produces a regular 3-dimensional (3-D) data block suitable for processing by conventional image analysis and volume measurement software. The system uses electromagnetic spatial location of 2-dimensional (2-D) freehand-scanned ultrasound B-mode images, custom-built signal-conditioning hardware, UNIX-based computer processing and an efficient 3-D reconstruction algorithm. Utilisation of images from multiple angles of insonation, "compounding," reduces speckle contrast, improves structure coherence within the reconstructed grey-scale image and enhances the ability to detect structure boundaries and to segment and quantify features. Volume measurements using a series of water-filled latex and cylindrical foam rubber phantoms with volumes down to 0.7 mL show that a high degree of accuracy, precision and reproducibility can be obtained. Extension of the technique to handle in vivo data sets by allowing physiological criteria to be taken into account in selecting the images used for construction is also illustrated.

Application of a new discreet form of Gauss' theorem for measuring volume

Volume measurements are useful in many branches of science and medicine. They are usually accomplished by acquiring a sequence of cross sectional images through the object using an appropriate scanning modality, for example x-ray computed tomography (CT), magnetic resonance (MR) or ultrasound (US). In the cases of CT and MR, a dividing cubes algorithm can be used to describe the surface as a triangle mesh. However, such algorithms are not suitable for US data, especially when the image sequence is multiplanar (as it usually is). This problem may be overcome by manually tracing regions of interest (ROIs) on the registered multiplanar images and connecting the points into a triangular mesh. In this paper we describe and evaluate a new discreet form of Gauss' theorem which enables the calculation of the volume of any enclosed surface described by a triangular mesh. The volume is calculated by summing the vector product of the centroid, area and normal of each surface triangle. The algorithm was tested on computer-generated objects, US-scanned balloons, livers and kidneys and CT-scanned clay rocks. The results, expressed as the mean percentage difference +/- one standard deviation were 1.2 +/- 2.3, 5.5 +/- 4.7, 3.0 +/- 3.2 and -1.2 +/- 3.2% for balloons, livers, kidneys and rocks respectively. The results compare favourably with other volume estimation methods such as planimetry and tetrahedral decomposition.