RF tumour ablation: computer simulation and mathematical modelling of the effects of electrical and thermal conductivity.

This study determined the effects of thermal conductivity on RF ablation tissue heating using mathematical modelling and computer simulations of RF heating coupled to thermal transport. Computer simulation of the Bio-Heat equation coupled with temperature-dependent solutions for RF electric fields (ETherm) was used to generate temperature profiles 2 cm away from a 3 cm internally-cooled electrode. Multiple conditions of clinically relevant electrical conductivities (0.07-12 S m-1) and 'tumour' radius (5-30 mm) at a given background electrical conductivity (0.12 S m-1) were studied. Temperature response surfaces were plotted for six thermal conductivities, ranging from 0.3-2 W m-1 degrees C (the range of anticipated clinical and experimental systems). A temperature response surface was obtained for each thermal conductivity at 25 electrical conductivities and 17 radii (n=425 temperature data points). The simulated temperature response was fit to a mathematical model derived from prior phantom data. This mathematical model is of the form (T=a+bRc exp(dR) s(f) exp(g)(s)) for RF generator-energy dependent situations and (T=h+k exp(mR)+n?exp(p)(s)) for RF generator-current limited situations, where T is the temperature (degrees C) 2 cm from the electrode and a, b, c, d, f, g, h, k, m, n and p are fitting parameters. For each of the thermal conductivity temperature profiles generated, the mathematical model fit the response surface to an r2 of 0.97-0.99. Parameters a, b, c, d, f, k and m were highly correlated to thermal conductivity (r2=0.96-0.99). The monotonic progression of fitting parameters permitted their mathematical expression using simple functions. Additionally, the effect of thermal conductivity simplified the above equation to the extent that g, h, n and p were found to be invariant. Thus, representation of the temperature response surface could be accurately expressed as a function of electrical conductivity, radius and thermal conductivity. As a result, the non-linear temperature response of RF induced heating can be adequately expressed mathematically as a function of electrical conductivity, radius and thermal conductivity. Hence, thermal conductivity accounts for some of the previously unexplained variance. Furthermore, the addition of this variable into the mathematical model substantially simplifies the equations and, as such, it is expected that this will permit improved prediction of RF ablation induced temperatures in clinical practice.

Special radiofrequency electrode system for computed tomography-guided pain-relieving procedures.

We present a new radiofrequency needle electrode system for use in pain-relieving procedures. The electrode system is used for computed tomography-guided percutaneous cordotomy, extralemniscal myelotomy, and trigeminal tractotomy. These special electrodes make it possible to perform selective cordotomies and selective trigeminal tractotomies under direct computed tomographic image control.

The relationship between ventricular fluid pressure and body position in normal subjects and subjects with shunts: a telemetric study.

Using a chronically implanted telemetric pressure sensor, we have determined the quantitative relationship between changes in body position and ventricular fluid pressure in normal subjects and subjects with shunts. The method allows accurate, reliable measurement of negative as well as positive pressures. We describe the derangement of postural intraventricular pressure regulation caused by placement of a shunt, as well as the influence of various shunt systems and the antisiphon device on this problem. Ventriculoatrial, ventriculoperitoneal, and ventriculopleural shunts all caused similar severely abnormal postural intracranial pressure relationships. The antisiphon device was generally effective in restoring normal pressures in patients in the upright position. We discuss the implications of our findings for understanding the mechanisms of postural intracranial pressure regulation in patients without hydrocephalus.

A new insulated caudalis nucleus DREZ electrode. Technical note.

A new insulated radiofrequency electrode for making nucleus caudalis dorsal root entry zone lesions reduces the incidence of ataxia.

CT-guided aspirations for the body: comparison of hand guidance with stereotaxis.

Forty computed tomography (CT)-assisted aspirations performed with only hand guidance were prospectively compared with 40 performed with a CT body-stereotaxic system. Although there was no statistically significant difference in lesion size and path length between the two groups, use of stereotaxis compared with hand guidance decreased by 75% the number of needle manipulations required to place a needle within a lesion. With the stereotaxic method, only 43 needle manipulations were required to confirm a needle placement in 40 lesions, with no lesion requiring more than two attempts. Use of stereotaxis decreased the number of localization scans by 80% and biopsy time by 50%. It is concluded that CT-guided needle placements with hand guidance are often inaccurate and, unless the lesion is large, require multiple needle manipulations to place a needle within the lesion. Stereotaxis-guided biopsies, on the other hand, decrease radiation exposure, biopsy time, and trauma from multiple needle punctures.

Radiofrequency lesion generation and its effect on tissue impedance.

The method of radiofrequency heat lesion generation is reviewed with specific reference to the dorsal root entry zone. Experimental data on the impedance of electrolytic media as a function of temperature are reported, and their relation to what should be observed during radiofrequency lesioning in the body is commented upon. The future utility of impedance monitoring is discussed as well as possible implications of bipolar lesion electrode systems.

Combined use of a new target-centered arc system with the BRW floor stand and phantom base for a wider range of approaches and applications in stereotaxy of the head and neck.

A unique target-centered arc system has been developed which combines the azimuth and declination angle approach concept of the standard BRW arc with the anteroposterior, lateral and vertical movements of the earlier Todd-Wells Guide for maximal versatility of image-guided stereotaxy. The arc system mounts on the BRW floor stand and provides completely unobstructed approaches to the upper hemisphere, base of the skull, posterior fossa or cervical region. It is ideally adapted for implant arrays and laser surgery. A new transferable phantom pointer affords target confirmation using the BRW phantom base.

The Brown-Roberts-Wells (BRW) arc: its concept as a spatial navigation system.

The Brown-Roberts-Wells (BRW) Arc System can be compared to spatial navigation because both utilize the concept of direction to and spatial location of a point in space by referencing to a horizontal angle (azimuth) and a vertical angle (declination) relative to the horizon. The BRW system also permits the determination of the distance from a reference surface of the arc system to the point (target). The methods of determining these parameters are explained in detail with illustrations.

Angiographic localizer for the BRW stereotactic system.

Preliminary experience with a newly constructed angiographic localizer system for use in stereotactic neurosurgery is reported. This localizer ring, mounted on the BRW head ring, allows for the transformation of target points with known stereotactic coordinates (e.g., visible on computerized tomography scans) onto angiograms, as well as the determination of stereotactic coordinates of a set of points (e.g., arteriovenous malformations) indicated on at least two angiograms.

CT body stereotaxic instrument for percutaneous biopsy and other interventive procedures: phantom studies.

This article describes a new body stereotaxic system that defines a fiducial point by means of a skin localization device placed directly on the patient. The system uses a rectilinear stereotaxic frame to guide the needles along the calculated path. A method for calculating paths that require angulations from one scan slice to another is described, as well. The system was tested in a foam phantom and shown to be intrinsically accurate in vitro to within 2 mm in both the x and y axis.

Theoretical aspects of radiofrequency lesions in the dorsal root entry zone.

The history and physical principles of radiofrequency (rf) lesion making are reviewed. The advantages of the rf lesion method are presented, with emphasis on its importance in small, critical regions such as the dorsal root entry zone (DREZ). The evolution and specifications of a satisfactory DREZ rf electrode are described. DREZ lesion sizes for this electrode at specific electrode tip temperatures were experimentally determined in animals and were used as a guide to determine acceptable clinical lesioning parameters. Emphasis is placed on lesion temperature monitoring and on the stability of electrode penetration into the spinal cord to achieve consistent and safe results.

Modification of the Richmond subarachnoid screw for monitoring intracranial pressure. Technical note.

The authors describe a modification of a subarachnoid screw for monitoring intracranial pressure by hydrostatic coupling of the subarachnoid space to an external transducer. The device can be used in both children and adults and features more assured placement of the distal tip, increased stability, and enhanced safety on insertion.

Stereotactic radiofrequency lesion making.

The physical principles of radiofrequency (RF) lesion making in stereotactic neurosurgery are summarized. Empirical data are given on the relationship between lesion size and lesioning parameters. Currently accepted ranges of lesioning parameters for selected stereotactic procedures are discussed. Advanced RF lesion electrode designs are described which improve the capabilities to reach and confirm targets.

A telemetric pressure sensor for ventricular shunt systems.

A fully implantable telemetric differential pressure sensor has been developed which is incorporated with a shunt valve system. It is a unique single diaphragm device that allows in vivo confirmation of zero-point and pressure calibrations. The telemetry is implemented by a resonant circuit in the sensor and a radio frequency detector outside the body, and the in vivo pressure calibration is derived from a pressure-balancing method. The pressure sensor has been implanted with shunts in several patients, and is shown to be valuable both for measurements of intraventricular pressure and for determining proper function of hydrocephalus shunt valves and catheters.

A pressure-balanced radio-telemetry system for the measurement of intracranial pressure. A preliminary design report.

A fully implantable radio-telemetric differential intracranial pressure sensor is described with a zero-point calibration that can be confirmed repeatedly. Intracranial pressure (ICP) is measured with the device by the principle of applying a known external pressure to the scalp above the sensor and simultaneously detecting by radio-telemetry the zero-point of the sensor corresponding to a balance of pressures across it. The radio-telemetry is implemented by a resonant circuit in the sensor of which the resonant radiofrequency is detected outside the body. The sensor is passive, has built-in barametric compensation, negligible permeability of temperature drift, no calibration ambiguities, and fast dynamic response. The implanted sensor has been used successfully for short-term as well as long-term ICP monitoring. It has been implemented primarily for intermittent ICP measurements, but also adapted to continuous recording. Preliminary clinical experience with the system indicates that it is effective, safe, and simple to operate.