Piv Measurement of Fluid Flow Inside a Human Uterus Model for Cryoablation

PIV MEASUREMENT OF FLUID FLOW INSIDE A HUMAN UTERUS MODEL FOR CRYOABLATION

Xiaolong Luo1, Jim S.J. Chen1, Marla Wolfson2, Charles Philips2, Thomas Shaffer2

1Department of Mechanical Engineering, Temple University

2Temple University School of Medicine

Abstract - A new technique has been proposed that allows cryoablation to be achieved on the entire inner surface of the endometrium by circulating very cold perfluorochemical (PFC) fluid inside the human uterus. To understand the PFC flow field inside the uterus during cryoablation, a uterus model was constructed; a fluid delivery system including the fluid delivery probe was designed and built to simulate the PFC flow during cryosurgery. With particle image velocimetry (PIV), the fluid flow inside the uterus was investigated at room temperature to display a 2-D whole field velocity contour and vector plot.

Keywords: PIV, cryoablation, PFC, endometrium

I. INTRODUCTION

Menorrhagia is medically defined as excessive bleeding at menses, in duration or amount. A frequent cause of menorrhagia is uterine fibroid growth. Traditionally, hysterectomy is the main treatment for menorrhagia. Several cryosurgery methods such as localized cryoprobes [1] and balloon ablation [2] have been developed as alternatives to hysterectomy to treat the dysfunction of the endometrium. However, these are localized techniques and sometimes there is still proliferation of the endometrium. A new technique is proposed that allows cryoablation to be achieved on the entire inner surface of the endometrium using a PFC fluid [3]. The objective of this project is to develop an efficient treatment method for abnormal uterine bleeding by freezing the endometrium. It has been shown that a temperature of -30 C is required for destroy tissue necrosis [4].

The heat transfer process during cryoablation was investigated experimentally and numerically (1-D) with good agreement [5], in which a uniform fluid temperature boundary condition was used in the 1-D finite difference method to determine the temperature history in the human uteri of eight patients. This assumed that the fluid flow inside the uterus is uniform, repeatable and controllable. During the experiments it was found that the temperature difference at the inlet and outlet of the insertion probe varied from 10Ñ"C to 50Ñ"C. Inconsistent fluid temperatures may be caused by inadequate circulation of PFC liquid, i.e., recirculation within the uterus. The goal of this study is to understand the flow field inside the uterus cavity during cryoablation so that the desired flow field and uniform temperature field can be obtained, thus proposing an optimal insertion probe design so that minimum PFC liquid and surgery time is used.

II. MATERIALS AND METHODS

Particle Image Velocimetry (PIV), an instantaneous velocity field measurement over global domains, has gained widely utilization in fluid flow investigations. During the last few years, PIV has been expanding its extensive applications to the bioengineering and medicine field because it is a nonintrusive and whole field measurement with quantitative data. The PIV system used for this investigation is shown in Fig. 1. After the system was setup, it was calibrated within the PIV system and the velocity was verified at the outlet of an immerged pump in a water tank. PIV measurement results were in good agreement with four verification methods that includes the average velocity based on the flow rate, the velocity based on the maximum lift, the velocity determined by a Pitot tube, and the average velocity from a free jet.

A uterus model was constructed with three pieces of Plexiglas adhered together. The model is considered as 2-D because the side view of human uterus inner cavity is much more uniform than the front view. The middle piece (uterus cavity) was made based on the general uterus shape and average dimensions of 8 collected patient uteri (enclosed area of about 20 cm2).

Fig. 1 Setup of PIV system and fluid circulating system

The design was then drawn with the software IDEAS, programmed, and milled with a CNC milling machine. To meet the transparency requirement for the PIV laser beam through the Plexiglas, the middle piece was polished by a series of 220, 320, 400, 600, 800 sandpapers and finely polished by 5 µm alumina suspensions. The transparency was verified with good results before it was adhered with the cover and bottom pieces. Finally, the model was put on a 3-D movable machine table for experiments.

A fluid circulating system consisting of a pump, a relief valve, a flow meter and a pressure gage was designed with adjustable flow rate to simulate the PFC fluid circulation during cryoablation. For the study, a 97% Glycerin solution was chosen because of its very similar properties to PFC liquid at low temperature. This experimental fluid was necessary since the PFC liquids are very expensive currently (about $2,000 per liter). Note that