Probe Trajectory Research Articles

Galileo entry probe recovery of fine structure in the zonal wind profile at Jupiter

The algorithm developed to recover the zonal (east‐west) wind profile at Jupiter by Doppler tracking of the Galileo probe is well established in theory and verified by computer simulation. We extend the Doppler wind recovery algorithm to include detection and extraction of microscale wind structure. When the probe encounters regions of high wind shear, turbulence, atmospheric waves, or nonsteady aerodynamics (buffeting) a rapidly varying component is introduced into the probe telemetry Doppler data. These effects can be isolated by their respective Doppler signatures. Analysis of residuals allows fine structure in the wind profile to be located and retrieved. The overall wind recovery is a three‐step process: (1) The large‐scale zonal wind profile is recovered by converting the probe telemetry frequency residuals (as caused by the Doppler effect) to a velocity and back‐projecting it into the local horizontal east‐west direction at the probe location. The zonal winds are derived from the velocity residuals by a least squares algorithm. (2) Regions of high zonal wind shear are recovered numerically relative to the large‐scale wind structure and serve as a correction to the previously derived large‐scale profile. (3) The probe trajectory is updated to reflect both the large‐ and small‐scale wind structure, and the large‐scale wind profile is recalculated. The Galileo Doppler wind experiment described previously and the wind microstructure retrieval algorithm outlined here are the only reasonable means by which the vertical profile of the zonal winds at Jupiter can be directly sampled.

Tracing the planned probe trajectory on axial computed tomography slices: a computation-based support for stereotactic procedures. A computational tool for tracing of probe trajectories on CT slices.

Stereotactic frames are designed to hit point-like intracranial targets. Their routine performance does not include a display of the intended transcerebral probe path. Since the latter can render stereotactic procedures safer, more accurate and versatile, it has increasingly been implemented by potent computer systems. A low-cost alternative is presented in the form of a small extension of the computation program which is usually employed for the conversion of imaging coordinates to frame adjustments. It allows any desired point on the planned probe trajectory, in terms of its distance to the target, to be defined in Cartesian coordinates and thus to be traced on computed tomography. Thus, in biopsy procedures, for instance, instead of sampling tissue at the target only or in a 'blind' serial way, samples can be collected in a previewed and therefore safer and more purposeful way, thereby improving the yield of pathological tissue. The method seems particularly interesting for institutions that have a low to moderate number of stereotactic procedures and for which the investment of sophisticated computer systems for this purpose alone would not be economically feasible.

A frameless method for 3D MRI- and CT guided stereotaxic localisation

MR and CT are well recognized as the best imaging procedures for localisation in stereotaxic neurosurgery. However special difficulties necessitate specific adaptation to: localise targets in the steriotaxic frame. A method using MR and CT images is presented: no special frame must be. used during imaging examinations, but only the four intracranian holders of the neurosurgical stereotaxic frame. The images data are transferred to: a PC-based system for 3D localisation of brain targets. By locating the four markers on the images, the transformation matrices can be computed to: obtain the coordinates of a lesion in the stereotaxic space. The system calculates the arc setting parameters of a probe trajectory to: the target, either for an orthogonal or a double oblique approach if needed. Simulated probe trajectory intersections with the consecutive slices can be viewed in order to: validate the trajectory before the surgical procedure. The systematic geometrical errors have been carefully evaluated. Original procedures and corrective algorithms have been implemented as well as checks to: detect geometrical distortion or movements made by the patient. In this way, millimeter accuracy is achieved in the three dimensions.

A computer-assisted system for 3-D frameless localization in stereotaxic MRI

A low-cost PC-based system for 3-D localization of brain targets in stereotaxic imaging is presented. It relies on a method, using MR images, in which four markers are inserted in the fastenings of a Talairach stereotaxic frame during MRI examination. By locating these markers on the images with this system, the transformation matrixes can be computed to obtain the 3-D coordinates of the center of a tumour in the stereotaxic space or in the MRI space. The system calculates the frame and arc setting parameters of a probe trajectory to the target, either for an orthogonal or a double oblique approach if needed. Simulated probe trajectory intersections with consecutive slices can be viewed in order to validate the trajectory before and during the surgical procedure. The method presents no major constraints in routine examinations. Mathematical details on the calculation of the transformation matrices are given.

Measurement of a zonal wind profile on Titan by Doppler tracking of the Cassini entry probe

As the Cassini probe enters and descends through the Titan atmosphere, the winds will cause perturbations to the probe local horizontal velocity resulting in an anomalous drift in the probe location and a measurable shift in the frequency of the probe telemetry due to the Doppler effect. By using an iterative algorithm in which the time variation of the probe telemetry frequency is monitored throughout the descent and the probe trajectory is updated to reflect the effect of wind on the probe location, a highly accurate relative wind profile can be recovered. Coupled with a single wind velocity measured at a known altitude by independent means, an absolute profile results. The accuracy with which the wind profile can be recovered is primarily limited by the a posteriori uncertainties in the probe and orbiter trajectories and oscillator frequencies. An unmodeled drift in oscillator frequency will cause a time‐varying frequency that is inseparable from Doppler effects. Similarly unmodeled changes in the probe‐orbiter relative geometry will lead to Doppler frequency contributions that are indistinguishable from the wind signature.

Autonomy and on-board mission management aspects for the cassini titan probe

The Cassini mission is a joint ESA NASA mission for the exploration of the Saturnian System. It consists of two components: an Orbiter designed to tour Saturn for 4 years (developed by NASA) and an atmospheric Probe to Titan, Saturn's largest moon (developed by ESA). The Probe's objective is to explore Titan's atmospheric composition and dynamics as well as its surface properties. The Probe will be carried by the Orbiter on a 6.5 year journey from Earth to Saturn with gravity assists at Earth and Jupiter. From a Saturn-centred orbit, the Probe is targeted for a 2.75 h atmospheric descent into Titan's atmosphere. The prediction of the Probe's trajectory and mission characteristics is complicated by the uncertainties included in the available data about Titan's atmosphere. As the mission success is dependent upon critical factors like the radio relay link geometry to the Orbiter (serving as relay for data transmission towards Earth), the limited power resources and the atmospheric descent profile, there is a strong need for adaptive control measures. Due to the transmission delay towards Saturn, ground control during the Probe's atmospheric entry and descent is excluded. Thus, on-board data analysis should lead to autonomous updates of the mission operations profile. Concepts to approach these problems will be discussed.

Integration of Stereoscopic DSA with Three-Dimensional Image Reconstruction for Stereotactic Planning

Following 4 years of experience using a microcomputer-based system for the planning of stereotactic neurosurgery, we have now developed a workstation with the capability of displaying and analyzing three-dimensional images for this purpose. In addition to viewing volumetrically rendered three-dimensional computer tomograms and magnetic resonance images, we may directly view and analyze stereoscopic digital angiograms. In addition to each set of images being viewed in isolation, we may also combine the three-dimensional anatomical images with the stereoscopic angiograms. This new system is based on a computer equipped with a light polarization switched screen capable of displaying stereoscopic images directly to the observer, thus permitting him to interact with the three-dimensional volume directly, determining coordinates and positioning probe trajectories.

A Prototype Medical Workstation for Computer-Assisted Stereotactic Neurosurgery

We have developed a prototype display workstation for use in stereotactic neurosurgery. Patient image data from computed tomography, magnetic resonance imaging, and digital subtraction angiography are acquired with the stereotactic frame in place and subsequently transferred to the workstation for further processing. Target points may be identified on any image type and probe trajectories defined. Any point or line indicated on one set of images may be transferred immediately to other images, to determine, for instance, safe avascular probe paths. We present some general outlines for the use of computers for stereotactic neurosurgery and discuss the different components of the current system. Finally, we make some suggestions as to further developments.

Microphysiological recordings at CT gantry site during stereotactic thalamotomy.

Neural unit recording was performed during CT-guided stereotactic thalamotomy in 2 patients referred for severe cerebellar ataxia and dysmetric movements. Periodic CT scanning was performed during the recording in order to verify probe location and trajectory. Our experience with electrophysiological recordings at the CT gantry site demonstrates the feasibility of acquiring satisfactory neural unit recordings in spite of the high-voltage environment.

Three-dimensional reconstruction of neuroradiological data within a stereotactic frame of reference for surgery of visible targets.

Three-dimensional information obtained with neuroradiological exams performed under stereotactic conditions is displayed on a surgical console within the graphic reconstruction of the geometry of stereotactic frames. Planning of convenient probe trajectories can be carried out taking into account all data derived from different diagnostic techniques.

Digital image processing to handle neuroanatomical information and neurophysiological data.

An application of computer graphics technology to functional stereotactic neurosurgery is presented. Neuroanatomical images derived from a stereotactic atlas and information retrieved from a neurophysiological data base are drawn on a graphic monitor. The pictures are oriented and scaled according to appropriate landmarks. The aim of this work is to add confidence in the surgeon's selection of the probe trajectory and to improve the knowledge of sensory and motor organization within the thalamus.

Tissue impedance along probe trajectories in thalamotomy.

Tissue impedance along probe trajectories in thalamotomy.