Microwave ablation system
This system (Fig. 1) consists of MWAC (UM-200, Beijing Sanhe Dingye Technology Co. China) with water-cooling circulation system and microwave ablation instrument (MWAI) (MC-1150, Beijing Sanhe Dingye Technology Co. China) (Fig. 2). The main parts are temperature-monitoring system and water-cooling circulation system. Peristaltic pump and its actuation control electricity provide power to water-cooling circulation system. It can help water fluid recycling in the cooling circulation catheter and accelerate the convection of the heat from MWAC into the air. Thermistor can measure the temperature of catheter and show the specific numbers. Water-cooling catheter consists of MWA inner catheter and exterior catheter, connect by Lure fitting.
Temperature test system
Semiconductor device (thermistor) (Fig. 3) can change along with the temperature, so MWAI adopt anti-interference 3-wire resistance temperature detectors (RTD) to measure the specific temperature of MWAC. It has the advantages of miniaturization, specific measurement and strong anti-interference ability.
Selection and installation of thermistor: Temperature measurement probe and microwave radiator are in the same catheter, the space left is limited, so we select a tiny, packaged thermistor with a diameter of 0.43mm. It packaged in glass to prevent inductive currents from being generated in microwave fields. The thermistor resist heat with high stability. Thermal time constant τ = 1S (in still air), temperature ranges from − 50°C to 250°C, dissipation constant: Approx. 0.25mW/℃ insulation resistance: Min. 10MΩ at DC 50V (between lead wire and glass).
The 3-wire RTD circuit (Fig. 4): Normally, RTD only uses one current source. When measure the voltage generated by the constant current source in the thermistor, it can be converted into the corresponding temperature parameters. The 3-wire RTD with two matched current sources are required for measuring the data more accurate. Current from current source (IOUT1) flows through lead resistor (RL1), thermistor (RTD), lead resistor (RL3), precision resistor (Rref) to earth. Current from current source (IOUT2) flows through lead resistor (RL2), thermistor (RTD), lead resistor (RL3), precision resistor (Rref) to earth. Assuming RL1equal to RL2 (same material and length), IOUT1 matches to IOUT2, error voltage from RL1 and RL2 are equal, and there is no error voltage between AIN1(+) and AIN1(-). Reference voltage is developed across precision resistor (Rref), the reference voltage REFIN = REFIN (+)-REFIN (-), there are no errors. AIN1 changes according to REFIN. Reference voltage compensated the error voltage caused by temperature drift of the RTD current source [12].
Lead resistances: RL1、RL2、RL3; Thermistor: RTD; Constant current sources: IOUT1、IOUT2; Sampling voltage: AIN1(+)、AIN1(-); Reference voltage: REFIN(+)、REFIN(-).
Figure 4. 3-wire RTD
Water-cooling circulation system
The water-cooling circulation system consists of peristaltic pump, cooling water circulation tube and inlet and outlet tube. The peristaltic pump (BT100, the pump head is YZ1515X) flow rate is 50ml/min, and the outlet pressure is 460Kpa. The principle of water-cooling circulation system designation: Peristaltic pump pushes the cooling water in the pump tube and flow into the MWAC. The cooling water absorbs the heat from the microwave coaxial cable. After the peristaltic pump rollers alternately extrudes the pump tube, the cooling water flows out of the tube and returns to the water bag (water container), and the low-temperature water flows back into the tube. In this way, the MWAC can be cooled.
The peristaltic pump and its drive control circuit provide the power source for the water-cooling circulation system. It transports cooling water by alternately extruding the pump tube (silicone tube). The peristaltic pump head is divided into two parts: the rotors and the pump shell. The pump tube (silicone tube) is fixed between the rotor and the pump shell. The rotors successively extrude the tube to cause negative pressure, and the cooling water flows into the tube (the blue part is the cooling water). Due to the elastic recovery of the rolled tube, a vacuum is formed in the tube (silicone tube), and then the cooling water flows. There is a distance between rollers, which makes the tube form a closed space (pump chamber). The volume of the tube chamber is related to the inner diameter of the tube and the rotary diameter and quantity of the rollers. The flow rate depends on the rotation speed of the pump head and the volume of the pump chamber. The water in the chamber is alternately extruded and transported to the MWAC by rollers (Fig. 5).
a b c d
1.pump tube (silicone tube) 2. pump shell 3. rotor 4. Cooling water 5. pump chamber (between the rotors)
Figure 5. The working principle of peristaltic pump.
MWAC with water-cooling circulation system
In the process of closuring the lower extremity vein, MWAC (radiation ends) continuously radiates microwave energy to vascular tissues, and its temperature rises rapidly. Generally, the temperature of the ablation area reaches 100–120℃ within 3–4 seconds [6], and the thermal microwave effect also expands. Under the effect of heat conduction, the heat at the ablation area spread from the radiation end of the catheter to along the tube, and the temperature gradually increases. The high temperature of the tube would hurt normal tissues and operators’ hands. Meanwhile, the temperature of the coaxial cable of the MWAC would increase, and it would aggravate the reflection and standing wave of microwave power. Then the actual output power would reduce and affect the closure of vessels. Therefore, we must adopt effective cooling method to reduce the temperature of the catheter and bring out the heat as quick as possible, so that the catheter can be used safely and effectively [10].
The MWAC with water-cooling circulation system (Fig. 6) includes radiant head, radiation window, copper sleeve, tube, operating handle, water inlet and outlet tube, high-frequency connector, temperature measuring probe, etc. In addition, there are coaxial cable, thermistor, water tank (temporarily collecting the circulating water carrying heat discharged from the tube and discharging it out of the tube), refrigerant tube (capillary stainless-steel tube) and PVC inlet and outlet tubes and other water-cooling circulation components inside (Fig. 7).
A: radiation head B: radiation window C: copper sleeve D: tube E: handle F: water tank G: temperature measuring probe H: High frequency connector I: laser coupling connector J: water inlet and outlet tube K: optical fiber L: refrigerant pipe M: coaxial cable
Figure 6. MWAC with water-cooling circulation system
Test and analysis
Test of temperature-monitoring system
Set the constant temperature water bath (CTWB) temperature, the temperature range is from 10℃ to 80℃, each test interval is 5℃, the MWAI is set to standby mode, fix the temperature probe of MWAI and mercury thermometer of 0.1℃ precision at the same temperature measuring position of CTWB, record the temperature of mercury thermometer and MWAI after the CTWB temperature is stable. Repeat the test for 10 times.
Test of water-cooling system
In order to confirm the effectiveness of the water-cooling circulation system of the MWAC, the catheter temperature was measured by two methods: non-water-cooling mode and water-cooling mode. Using normal saline of 25℃ as cooling water. MWA for VVs of lower extremities in-vitro simulation test was carried out. Setting the temperature measurement probes T1-T8 at the positions 5cm, 15cm, 25cm, 35cm, 50cm, 75cm, 90cm, 105cm away from MWAC head. The catheter was inserted into the high temperature resistant Teflon cannula (simulating human vein blood), the Teflon cannula was injected with 37 ℃ water (simulating human blood), the Teflon cannula was sealed at one end (simulating high ligation), and the 40cm head of the Teflon cannula was placed in the 37 ℃ CTWB (simulating the ablated blood vessel), the rest part is suspended. The experimental test parameters are shown in Table 1, recording the temperature of T1-T8. The actual layout of the experimental test is shown in Fig. 8.
Table 1
experimental test parameters
No. | Power | Time | Ablation interval | Water-cooling circulation system | Time | Speed of peristaltic pump (r/min) |
1 | 70W | 4S | 2s | Yes | 50 | 60 |
2 | 70W | 30S | 2s | Yes | 50 | 60 |
3 | 70W | 4S | 2s | No | 50 | 0 |
In ex vivo pilot study
In order to test the effect of MWAC temperature rise on in ex vivo ablation, using the MWAC with or without water-cooling circulation system on fresh pork tissues, setting the output power of MWAI as 70W and ablation time as 5S for both of them, 25 times in a row. Changing the pork tissue at the last ablation. Vernier calipers were used to measure the long and short diameters of the last ablation and record the data. Repeat the above operation for 10 times.
Recording and analysis
Draw the line graph of catheter temperature changing with ablation times. The other results were in the mean ± standard deviation (SD). Using T method to evaluate the difference between the two groups. SPSS software (version 23.0) was used for all statistical analysis. The difference was statistically significant when p was less than 0.05.