Biomedical subjects

A H Gee

Publications and source records attributed to A H Gee.

8 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↗

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

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

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

Automatic registration of 3-D ultrasound images.

One of the most promising applications of 3-D ultrasound (US) lies in the visualisation and volume estimation of internal 3-D structures. Unfortunately, artifacts and speckle make automatic analysis of the 3-D data sets difficult. In this study, we investigated the use of 3-D spatial compounding to improve data quality, and found that precise registration is the key. A correlation-based registration technique was applied to 3-D ultrasound data sets acquired from in vivo examinations of a human gall bladder. We found that the registration technique performed well, and visualisation and segmentation of the compounded data were clearly improved. We also demonstrated that an automatic volume estimate made from the compounded data (13.0 mL) was comparable to a labour-intensive manual estimate (12.5 mL). In comparison, automatic estimates of uncompounded data are less accurate (ranging from 13.5 mL to 16.7 mL). The registration technique also has applications in intra- and interpatient comparative studies.

Gallbladder↗

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↗

PubMed1987

Prediction of outcome in depressed patients by weekly monitoring with the dexamethasone suppression test.

Forty-three depressed patients in hospital were studied with weekly dexamethasone suppression tests (DSTs) and were followed as out-patients for at least three months after discharge. The detection rate of patients with LHPA axis dysfunction increased from 41% with a single DST to 59% with serial DSTs. There was a poor correlation between weekly post-dexamethasone cortisol levels and Hamilton depression rating scores. In patients with evidence of LHPA axis dysfunction, a DST at discharge discriminated effectively between a good and a poor outcome group; persistent non-suppression was strongly linked with a relapse of depression in the first three months after discharge. In general, our results support previous claims that the DST is a state marker for depressive illness.

Depressive Disorder↗

PubMed1986

The effects of weight change on the dexamethasone suppression test in depressed and anorexic patients.

Prior studies on weight change and hypothalamic-pituitary-adrenal (HPA) axis functioning are reviewed. Data on 58 depressed and eight anorexic patients is presented. No significant difference in the frequency of cortisol non-suppression in the dexamethasone suppression test (DST) was found between depressed patients with a history of weight loss and those without, nor between depressed patients who lost weight during their first week in hospital and those who did not. Mean weight loss of suppressors did not significantly differ from that of non-suppressors. Of 12 patients whose DST normalised during their stay in hospital, only four gained weight. Five anorexics who were non-suppressors were less than 70% of their ideal body weight (IBW), while three suppressor anorexics were greater than or equal to 70% IBW. These results indicate that mild to moderate weight change is not a significant influence on DST response in depression.

Anorexia Nervosa↗