Biomedical subjects

R W Prager

Publications and source records attributed to R W Prager.

12 recordsLinked to original sources

PubMed2002

Correction of probe pressure artifacts in freehand 3D ultrasound.

We present an algorithm which combines non-rigid image-based registration and conventional position sensing to correct probe-pressure-induced registration errors in freehand three-dimensional (3D) ultrasound volumes. The local accuracy of image-based registration enables the accurate freehand acquisition of high resolution (>15 MHz) 3D ultrasound data, opening the way for 3D musculoskeletal examinations. External position sensor readings guarantee the large-scale positional accuracy of the data. Pressure correction is shown to dramatically increase the perceived quality of extended-field-of-view data sets and reslices through volumetric data sets, while quantitative comparisons of multiple in vivo volumes demonstrate the superior precision of the corrected data.

Algorithms↗

PubMed2001

Fetal head moulding: finite element analysis of a fetal skull subjected to uterine pressures during the first stage of labour.

Fetal head moulding is a phenomenon which may contribute to satisfactory progress during delivery as it allows the fetal head to accommodate to the geometry of the passage. In contrast, excessive head moulding may result in cranial birth injuries and thus affect the infant shortly or even long after birth. One group of researchers in the past investigated the biomechanics of fetal head moulding from an engineering point of view and limited themselves to a static, linear model of the parietal bones. In this paper, we present a non-linear model of the deformation of a complete fetal skull, when subjected to pressures exerted by the cervix, during the first stage of labour. The design of the model involves four main steps: shape recovery of the fetal skull, the generation of a valid and compatible mesh for finite element analysis (FEA), the specification of a physical model and the analysis of deformation. Results of the analysis show good agreement with those obtained from clinical experiments on the quantitative assessment of fetal head moulding. The model also displays shapes after moulding which have been reported in previous studies and which are generally known in the obstetric and paediatric communities.

Biomechanical Phenomena↗

PubMed2000

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.

Abdomen↗

PubMed2000

Neuro-computing versus linear statistical techniques applied to liver transplant monitoring: a comparative study.

This paper explores the potential for the application of neurocomputing in on-line monitoring in the liver transplantation domain. It extends our previously documented work to provide both an assessment of the performance gains achievable by incorporating temporal and dynamical information about the measurements made on a patient as well as presenting a novel computerized clinical decision aid for this domain. A comparison of the performance of linear and nonlinear classification system is made and used to motivate the final selection of the diagnostic inputs.

Biomedical Engineering↗

PubMed2000

Surface interpolation from sparse cross sections using region correspondence.

The ability to estimate a surface from a set of cross sections allows calculation of the enclosed volume and the display of the surface in three-dimensions. This process has increasingly been used to derive useful information from medical data. However, extracting the cross sections (segmenting) can be very difficult, and automatic segmentation methods are not sufficiently robust to handle all situations. Hence, it is an advantage if the surface reconstruction algorithm can work effectively on a small number of cross sections. In addition, cross sections of medical data are often quite complex. Shape-based interpolation is a simple and elegant solution to this problem, although it has known limitations when handling complex shapes. In this paper, the shape-based interpolation paradigm is extended to interpolate a surface through sparse, complex cross sections, providing a significant improvement over our previously published maximal disc-guided interpolation. The performance of this algorithm is demonstrated on various types of medical data (X-ray computed tomography, magnetic resonance imaging and three-dimensional ultrasound). Although the correspondence problem in general remains unsolved, it is demonstrated that correct surfaces can be estimated from a limited amount of real data, through the use of region rather than object correspondence.

Computer Simulation↗

PubMed1999

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.

Anatomy, Cross-Sectional↗

PubMed1999

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.

Anatomy, Cross-Sectional↗

PubMed1998

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.

Calibration↗

PubMed1997

Bayesian registration of models using finite element eigenmodes.

This paper is concerned with registering three-dimensional wire-frame organ models. This involves finding correspondences between points on the models of two different examples of the same organ. Such registration is widely used in the processing of medical data; for example in segmentation, or to superimpose functional information on a more detailed structural map. The algorithm described in this paper is based on matching the modes of deformation of organ shapes. Modes with lower spatial frequency characterise large scale organ features whereas small scale variations determine the high frequency modes. First, the organ sizes are normalised using a generalised version of the centroid size metric. The axes of the fundamental frequency modes are then aligned to provide initial rigid-body registration. The registration is refined by matching increasingly high frequency modes using the 'Highest confidence first' algorithm. The matches are evaluated using a Bayesian combination of local prior and likelihood functions. The prior is derived from the Gompertz metric of biological growth and ensures that physically impossible matches are not accepted. The likelihood function is a measure of the similarity between local modal deformation components. The registration algorithm has been applied by the authors in the analysis of three dimensional ultrasound data. Results are presented showing the registration of two liver models derived from 3D ultrasound.

Algorithms↗

PubMed1997

Design, construction and evaluation of systems to predict risk in obstetrics.

We present a systematic, practical approach to developing risk prediction systems, suitable for use with large databases of medical information. An important part of this approach is a novel feature selection algorithm which uses the area under the receiver operating characteristic (ROC) curve to measure the expected discriminative power of different sets of predictor variables. We describe this algorithm and use it to select variables to predict risk of a specific adverse pregnancy outcome: failure to progress in labour. Neural network, logistic regression and hierarchical Bayesian risk prediction models are constructed, all of which achieve close to the limit of performance attainable on this prediction task. We show that better prediction performance requires more discriminative clinical information rather than improved modelling techniques. It is also shown that better diagnostic criteria in clinical records would greatly assist the development of systems to predict risk in pregnancy.

Algorithms↗

PubMed1995

Application of neural networks to the ranking of perinatal variables influencing birthweight.

In this paper we compare Multi-Layer Perceptrons (a neural network type) with Multivariate Linear Regression in predicting birthweight from nine perinatal variables which are thought to be related. Results show, that seven of the nine variables, i.e., gestational age, mother's body-mass index (BMI), sex of the baby, mother's height, smoking, parity and gravidity, are related to birthweight. We found no significant relationship between birthweight and each of the two variables, i.e., maternal age and social class.

Age Factors↗

PubMedDate not supplied

3D shape recovery of a newborn skull using thin-plate splines.

The objective of this paper is to construct a mesh-model of a newborn skull for finite element analysis to study its deformation when subjected to the forces present during labour. The current state of medical imaging technology has reached a level which allows accurate visualisation and shape recovery of biological organs and body-parts. However, a sufficiently large set of medical images cannot always be obtained, often because of practical or ethical reasons, and the requirement to recover the shape of the biological object of interest has to be met by other means. Such is the case for a newborn skull. A method to recover the three-dimensional (3D) shape from (minimum) two orthogonal atlas images of the object of interest and a homologous object is described. This method is based on matching landmarks and curves on the orthogonal images of the object of interest with corresponding landmarks and curves on the homologous or 'master'-object which is fully defined in 3D space. On the basis of this set of corresponding landmarks, a thin-plate spline function can be derived to warp from the 'master'-object space to the 'slave'-object space. This method is applied to recover the 3D shape of a newborn skull. Images from orthogonal view-planes are obtained from an atlas. The homologous object is an adult skull, obtained from CT-images made available by the Visible Human Project. After shape recovery, a mesh-model of the newborn skull is generated.

Adult↗