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TECHNICAL PAPERS: Fluids/Heat/Transport

Thermally Induced Convective Movements in a Standard Experimental Model for Characterization of Lesions Prior to Radiofrequency Functional Neurosurgery

[+] Author and Article Information
Joakim Wren1

Dan Loyd

Vattenfall Utveckling AB, Älvkarleby, Sweden

Rolf Karlsson

Vattenfall Utveckling AB, Älvkarleby, Sweden and Chalmers University of Technology, Gothenburg, Sweden

1

Corresponding author.

J Biomech Eng 129(1), 26-32 (Jul 24, 2006) (7 pages) doi:10.1115/1.2401180 History: Received June 28, 2004; Revised July 24, 2006

Abstract

Experimental exploration of equipment for stereotactic functional neurosurgery based on heating induced by radio-frequency current is most often carried out prior to surgery in order to secure a correct function of the equipment. The experiments are normally conducted in an experimental model including an albumin solution in which the treatment electrode is submerged, followed by a heating session during which a protein clot is generated around the electrode tip. The clot is believed to reflect the lesion generated in the brain during treatment. It is thereby presupposed that both the thermal and electric properties of the model are similar to brain tissue. This study investigates the presence of convective movements in the albumin solution using laser Doppler velocimetry. The result clearly shows that convective movements that depend on the time dependent heating characteristics of the equipment arise in the solution upon heating. The convective movements detected show a clear discrepancy compared with the in vivo situation that the experimental model tries to mimic; both the velocity (maximum velocity of about $5mm∕s$) and mass flux are greater in this experimental setting. Furthermore the flow geometry is completely different since only a small fraction of the tissue surrounding the electrode in vivo consists of moving blood, whereas the entire surrounding given by the albumin solution in the experimental model is moving. Earlier investigations by our group (Eriksson, 1999, Med. Biol. Eng. Comput.37, pp. 737–741; Wren, 2001, Ph.D. thesis; and Wren, 2001, Med. Biol. Eng. Comput.39, pp. 255–262) indicate that the heat flux is an essential parameter for the lesion growth and final size, and that presence of convective movements in the model might substantially increase the heat flux. Thus, convective movements of the magnitude presented here will very likely underestimate the size of the brain lesion, a finding that definitely should be taken into consideration when using the model prior to patient treatment.

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Figures

Figure 1

Three electrodes along with two guides that are used together with the stereotactic frame in order to guide the electrode to the target area. A magnification of the electrode tip is shown in the lower right corner. The bars in the upper left and lower right corners correspond to 50mm and 5mm, respectively.

Figure 2

The experimental setup. The heating process is controlled by the LNG. Data from LNG (temperature and power) and images from a digital camera were documented on a computer.

Figure 3

Measurement locations, A–L, where LDV measurements were carried out: (a) pertains to Fig. 6 and (b) pertains to Fig. 7

Figure 4

A principal view of the experimental setup for the LDV measurements. The two laser beams make up one dual beam; the other (horizontal) beam pair is not included in the figure. The figure is not to scale; the electrode is 2mm in diameter and the distance between electrode and optics is ∼120mm.

Figure 5

A lesion obtained in the albumin solution using the experimental model. The scale is in millimeters.

Figure 6

The component of the convective velocity in the “y” direction as a function of time in the locations shown in Fig. 3

Figure 7

The component of the convective velocity in the “y” direction as a function of time in the locations shown in Fig. 3

Figure 8

Arrows indicating the flow pattern in the proximity of the electrode for locations A–L which are given in Fig. 3, and the experimental setup is presented in Figs.  24. Solid arrows indicate the mean velocity between 20s and 60s (steady state). Dashed arrows indicate a velocity just prior to the time when the lesion grows into the measurement location, thus decreasing the velocity to zero which occurs after 10–17s. The velocity scale is given in the lower right corner.

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