

ORIGINAL ARTICLE 

Year : 2020  Volume
: 16
 Issue : 5  Page : 11401147 

Thermal field study of ceramic slot microwave ablation antenna based on specific absorption rate distribution function
Yonggang Wang^{1}, Ronghua Jiang^{1}, Jie Yu^{2}
^{1} R and D Department, Nanjing Canyon Medical Inc., Nanjing, China ^{2} Department of Interventional Ultrasound, Chinese PLA General Hospital, Beijing, China
Date of Submission  11Jul2019 
Date of Decision  13Jul2020 
Date of Acceptance  02Aug2020 
Date of Web Publication  29Sep2020 
Correspondence Address: Yonggang Wang R and D Department, 4^{th} Floor, Building 03, Accelerator Phase II, No. 11 Yaogu Avenue, Jiangbei New District, Nanjing China
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/jcrt.JCRT_482_19
Objective: The objective was to investigate the law of threedimensional thermal field radiation of ceramic slot microwave (CSMW) ablation antenna. Materials and Methods: First, microwave ablation experiments were performed with phantom and temperature data were collected. Second, the specific absorption rate distribution function of microwave (MW) ablation antenna was fit. Third, the MW ablation thermal field morphology was simulated based on the rapid simulation method. In addition, to determine the thermal field simulation accuracy, comparative analysis on the ablation morphology of forty clinical patients with liver cancer receiving percutaneous treatment was conducted. Results: Regarding the ablation morphology, the CSMW ablation antenna had greater long and transverse diameters and ablation volume than the polytetrafluoroethylene slot microwave (PSMW) ablation antenna (P < 0.05). Compared with the actual ablation morphology in clinical practice, the error rate in long and transverse diameters of the simulated morphology of thermal field was up to 5% and the minimum was 1.2%, whereas the maximum volume error rate was up to 9.8%. Conclusion: The CSMW ablation antenna had a greater long diameter, transverse diameter, and volume regarding the ablation morphology than the PSMW ablation antenna, and the thermal field morphology obtained based on the rapid simulation algorithm had a high accuracy.
Keywords: Microwave ablation antenna, rapid simulation method, thermal field morphology
How to cite this article: Wang Y, Jiang R, Yu J. Thermal field study of ceramic slot microwave ablation antenna based on specific absorption rate distribution function. J Can Res Ther 2020;16:11407 
How to cite this URL: Wang Y, Jiang R, Yu J. Thermal field study of ceramic slot microwave ablation antenna based on specific absorption rate distribution function. J Can Res Ther [serial online] 2020 [cited 2020 Oct 26];16:11407. Available from: https://www.cancerjournal.net/text.asp?2020/16/5/1140/296436 
> Introduction   
Recently, microwave (MW) ablation has been increasingly used in minimally invasive tumor surgery.^{[1],[2],[3],[4],[5],[6]} Therefore, it is clinically significant to investigate the specific absorption rate (SAR) of the MW ablation antenna in human tissues to guide clinicians to properly set the group of MW ablation energy.^{[7],[8],[9]} In clinical applications, ceramic slot microwave (CSMW) ablation antenna (whose outside MW radiation end has a ceramic tube) was characterized by advantages such as better bending strength, and lower risks than polytetrafluoroethylene slot microwave (PSMW) ablation antennas, and studies on the threedimensional thermal field of MW ablation antennas mainly focused on PSMW ablation antenna.^{[10],[11],[12]} Notably, studies on the threedimensional thermal field of CSMW ablation antenna have not yet been considered.
> Materials and Methods   
Materials
(1) Materials for MW ablation: MW ablation instruments (H1, Canyon Medical Inc., Nanjing, China) with a transmission frequency of 2450 MHz, the power was adjustable ranging from 0 to 100 W. CSMW and PSMW ablation antenna (KY2450B1.918011, Canyon Medical Inc., Nanjing, China), the outer diameter of which is 1.9 mm, with an effective length of 180 mm; while CSMW ablation antenna had a slot width of 9 mm and a pinhead needle length of 11 mm, PSMW ablation antenna had 1.6 mm and 11 mm [Figure 1]. (2) Materials for temperature collection: The temperature collector was equipped with a 20channel multifunction data collector (34970A, Keysight Technologies, Santa Rosa, CA, USA), which was connected to the computer via the RS232 interface. To reduce the influence of the temperaturesensing element and its supporting structure on the thermal field of the MW ablation needle, a thermocouple (K type, omega, Norwalk, CT 06854, USA) with an outer diameter of 0.12 mm was used for the temperaturesensing element in this experiment. In the supporting structure, a fiberglass rod with an outer diameter of 1 mm (VIC Advanced Materials, Nanjing, China), which had good rigidity and no effect on the thermal field of ablation needle, was used. In addition, a PC with Agilent Benchink Data Logger 3.0 (Keysight Technologies, Santa Rosa, CA, USA) data acquisition software was required to be installed.  Figure 1: Polytetrafluoroethylene slot microwave ablation antenna and ceramic slot microwave ablation antenna; (a): (A) represents a polytetrafluoroethylene slot microwave ablation antenna; (b) represents a ceramic slot microwave ablation antenna; (B): (a) represents a schematic diagram of a polytetrafluoroethylene slot microwave ablation antenna, with the anterior pole (La1) measuring 11 mm in length (La1), the polytetrafluoroethylene slot measuring 1.6 mm in length (Lb1), and the antenna measuring 1.9 mm in diameter (Ф). (b) represents a schematic diagram of ceramic slot microwave ablation antenna, with the anterior pole (La1) measuring 6 mm in length (La1), the ceramic slot measuring 9 mm in length (Lb2), and the antenna measuring 1.9 mm in diameter (Ф)
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Methods
(1) In a container, 22 thermocouples (K type, Omega, Norwalk, CT 06854, USA) were precisely arrayed [Figure 2]. The phantom was poured, which was stirred to be uniform into the container for 12 h.^{[13]} (2) A MW ablation experimental platform was built [Figure 2]. Preliminary studies showed that the setting with a 50 W power has a higher equicircular rate and more widely used in clinical practice. In this study, five experiments were performed using the CSMW and PSMW ablation antennas, with 50 W as the ablation power (one experiment time exceeded 60 s), and the values at each point were averaged as the basis for SAR calculation. (3) According to the Pennes' bioheat transfer equation,^{[14]} neglecting the influence of heat conduction, blood flow, and metabolism, the SAR of each measurement point at the initial phase could be calculated as , where ρ, c, T, and t, represent tissue density, specific heat, temperature, and ablation duration, respectively. (4) According to the MW radiation law of MW ablation antenna in human tissues,^{[15],[16]} the calculated 22 SAR values were fitted by an exponential function in r (radial) direction, and used in the Z direction (axial direction), and a cubic polynomial in one variable was used for the fitting at the Z direction (axial direction). In addition, considering the sagittal asymmetry, the forward (Z > 0) nonwatercooled part and backward (Z < 0) watercooled part was subjected to the piecewise function for fitting. At a power of 50 W, forward and backward SAR distribution functions for the two specified antennas were obtained. (5) Based on the rapid simulation method of MW ablation thermal field calculated by the SAR threedimensional distribution function, a realtime simulation was performed on the thermal fieldsimulated morphology of MW ablation antenna through the Visual C++ software (Version 2017, Microsoft, USA), and the above experiment was repeated thirty times. (6) Using fresh pig liver ex vivo (rewarmed to 30°C) as experimental materials, we performed ablation thirty times using the MW ablation antennas of two specifications. (7) The results of the threedimensional thermal field simulation were evaluated using ex vivo ablation data and the data from clinical actual ablation.  Figure 2: Thermocouple layout scheme and data acquisition experimental platform. (a) Thermocouple layout scheme; (b) support frame after proper thermocouple arrangement; (c) data acquisition experimental platform
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> Results and Evaluation   
SAR distribution function
In this study, the temperature variation curve at each temperature measurement point (from 0 to 60 s) was plotted using Microsoft Excel software (Version 2017, Microsoft, USA), and the slope of the curve was obtained by the fitting based on the linear function. The obtained curvature value was the value at each measurement point at 50 W for CSMW and PSMW ablation antennas [Table 1] and [Table 2].  Table 1: Corresponding ∂T/∂t value at each temperature measurement point at 50 W (polytetrafluoroethylene slot microwave ablation antenna)
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 Table 2: Corresponding ∂T/∂t value at each temperature measurement point at 50 W (ceramic slot microwave ablation antenna)
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According to the MW radiation law of the MW ablation antenna in the liverlike phantom, the exponential and cubic polynomial in one variable was used for fitting at the rdirection and z direction during the SAR function fitting, and the z direction was divided into anterior (Z > 0) and posterior (Z < 0) segments, and the expressions of SAR at the forward and backward directions are defined as follows:
In the above formula, r represents the radial distance relative to the watercooled microwave antenna, taking the slot center as the center of a circle; z represents the axial distance relative to the microwave antenna; e represents the natural logarithm; and parameters a, b, c 1, c 2, c 3, c 4, c 5, c 6, and d are determined during the fitting.
Using the MATLAB curvefitting toolbox (Version 2017a, MathWorks, Natick, Massachusetts, USA), a fitting was performed to obtain the SAR spatial distribution the CSMW and the PSMW ablation antennas at 50 W [Figure 3] and [Figure 4] as well as the corresponding SAR distribution function [Table 3]. The corresponding goodness of fit (coefficient of determination R ^{2}_{max} = 0.9966, R ^{2}_{min} = 0.9899) showed that the degree of fitting was excellent.  Figure 3: Specific absorption rate spatial distribution of polytetrafluoroethylene slot microwave ablation antenna; (a) forward specific absorption rate spatial distribution (Z > 0); (b) backward specific absorption rate spatial distribution (Z < 0)
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 Figure 4: Specific absorption rate spatial distribution of ceramic slot microwave ablation antenna; (a) forward specific absorption rate spatial distribution (Z > 0); (b) backward specific absorption rate spatial distribution (Z < 0)
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Thermal field fast simulation algorithm
For the forward and backward SAR distribution functions shown in equations (1) and (2), z and r are the only two unknowns. If z is fixed, decreases in parallel with the increase in the radial distance. The r value that satisfies the SAR distribution function formulas (1) and (2) is unique, and this r value is a point of the ablation boundary. In the forward solution, if r is fixed, the cubic polynomial in one variable does not necessarily monotonically decrease as the z value increases, so if the forward maximum of the ablation boundary is to be found, it is necessary to obtain the axial maximum z of the ablation zone first.
The axial maximum value z of the ablation zone was obtained using the bisection method: Assuming that the initial solution interval was [z_{1}, z_{2}], if f (z) < 0.01, then z could be regarded as the root of the function f. For , take the derivative to find:
If the function discriminant was given by then < 0, that is, the f(z) at the forward direction increased monotonically in parallel with the increase in z; here, we selected the solution interval as (0, 50) according to the previous relevant experimental studies ^{[17],[18]} (i.e., z_{1} = 0, z_{2} = 50); if the function discriminant was given by the two real roots z_{1} and z_{2} (z_{1} < z_{1}) of were obtained; in the forward region, if z_{1} > 0, the solution interval was (0, z_{1}); if z_{1} ≤ 0< z_{2}, the solution interval was [0, z_{2}]; if z_{2} ≤ 0, the solution interval was (0, 50); the radial maximum z of the solution zone was obtained based on the solution interval obtained above using the bisection method.
In the backward solution, if r is fixed, the cubic polynomial in one variable does not necessarily monotonically decrease as the z value decreases, so if the backward minimum of the ablation boundary is to be found, it is necessary to obtain the axial minimum z of the ablation zone first.
The axial minimum value z of the ablation zone was obtained using the bisection method: assuming that the initial solution interval was (z_{3}, z_{4}), if f(z) <0.01, then z could be regarded as the root of the function f. For f(z), take the derivative to find:
If the function discriminant was given by , then > 0, that is, the at the backward direction increased monotonically in parallel with the increase in z; here, we selected the solution interval as (−50, 0) similarly according to the previous relevant experimental studies ^{[18],[19]} (i.e., z_{3}= −50, z_{4} = 0); if the function discriminant was given by , the two real roots z_{3} and z_{4} (z_{3} < z_{4}) f(z) of were obtained; in the backward region, if z_{4} < 0, the solution interval was (z_{4}, 0); if z_{3} ≤ 0< z_{4}, the solution interval was (z_{3}, 0); if z_{4} ≥ 0, the solution interval was (−50, 0); the axial minimum z of the solution zone was obtained based on the solution interval obtained above using the bisection method. After obtaining the interval of the radial r and axial z in the ablation zone using the above algorithm, the 60°C isosurface was extracted as the simulated morphology of thermal field. The realtime simulation results of the threedimensional morphology of thermal field at 50 W could be realized by Visual C++ software (Version 2017, Microsoft, USA), and the realtime simulation results with an ablation power of 50 W could be obtained by time setting [Figure 5].  Figure 5: Threedimensional thermal field simulation of ceramic slot microwave ablation antenna and polytetrafluoroethylene slot microwave ablation antenna at 50W120S, 50W240S, and 50W360S
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> Evaluation of Thermal Field Simulation Results   
Using the same MW ablation instrument (H1, Kanyon Medical, Nanjing, China) and two kinds of MW ablation antenna (KY2450B1.918011, Kanyon Medical, Nanjing, China) and based on the ablation energy group described in [Table 4], the pig liver ex vivo (rewarmed to 30°C before the test) was subjected to the ablation test. After each test, the long and transverse diameters of the ablation morphology of the split pig liver were measured along the antenna insertion direction, and photographed for recording [Figure 6]. The thermal field simulation and ex vivo pig liver ablation tests were performed thirty rounds each (300 sets of data each), and the data were processed using the SPSS software package (Version 2017, IBM, Armonk, New York, USA) and converted into a line chart [Figure 7].  Figure 6: Ex vivo pig liver ablation test at 50W300S (ceramic slot microwave ablation antenna, with the ablation morphology of 33 mm × 24 mm)
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 Figure 7: Thermal field simulation of two kinds of microwave ablation antennas, and variation chart of long and transverse diameter of ex vivo ablation morphology (50 W)
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This study analyzed forty cases of liver cancer treated by two types of microwave ablation microantennas (KY2450B1.918011, Kanyon Medical, Nanjing, China) retrospectively in the department of Interventional Ultrasound at the PLA General Hospital between July 2016 and June 2018. In this study, 23 cases were treated by PSMW ablation antenna, while 17 were treated by CSMW ablation antenna, and they were all randomly selected single tumors. Between the two groups, no statistically significant difference (P > 0.05) based on gender composition, age (54 ± 10.7 years: 56.2 ± 9.8 years), child classification of liver function, proportion of patients with liver cirrhosis (19/23:13/17), and tumor size (3.46 ± 1.15:3.31 ± 1.08 cm) was observed. Each patient in the two groups was treated with a singleneedle singleshot MW ablation, and long and short diameters of the three directions of the ablation area were measured by ultrasound images at different times. The ablation volume was calculated according to the ellipsoid volume formula. A comparative analysis was conducted on long and transverse diameters and the volume of the clinical ablation morphology of the two types of MW ablation antennas [Table 5] and [Table 6] using the SPSS software package (Version 2017, IBM, USA), and the data were analyzed using the ttest (P < 0.05 indicates statistically significant difference). Besides, the postoperative complications of the two groups were mainly nausea, vomiting, abdominal pain, ascites, pleural fluid, etc., and no statistically significant difference was observed in the occurrence ratio (20/23:16/17) (P > 0.05).  Table 5: Long and transverse diameters of simulated and clinical ablation morphology of two kinds of microwave ablation antennas at different ablation time points at 50 W (x±s, mm)
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 Table 6: Volume of simulated and clinical ablation morphology of two kinds of microwave ablation antennas at different ablation time points at 50 W (x±s, cm^{3})
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Based on the experimental results and data analysis: (1) In the forward region (Z > 0), the radial ablation radius of the ablation morphology increases nonlinearly from the antenna tip to the slot site; in the backward region (Z < 0), the radial ablation radius of the ablation morphology first increases and then converges from the slot site to the needle bar site; the overall ablation morphology appears to be a regular “ellipsoid” [Figure 5] and [Figure 6]. (2) The variation of long and transverse diameters of thermal field simulation morphology and ex vivo pig liver ablation morphology over the time [Figure 7]: The two kinds of MW ablation antennas have similar variation trends in long and transverse diameters of the ablation morphology, all showing a variation law of rapid increase and then slowly increase the variation. (2) The CSMW ablation antenna had slightly greater long and transverse diameters than the PSMW ablation antenna, and long and transverse diameters of the thermal fieldsimulated morphology were larger than those of the ablation morphology in ex vivo ablation test. (3). The statistical table of long and transverse diameters of the thermal fieldsimulated morphology and clinical ablation morphology [Table 5]: The error rate in long and transverse diameters of the simulated morphology of the two MW ablation antennas and long and transverse diameters of the clinical ablation morphology at the corresponding ablation time point ranged from 1.2%–5.0%. (4) The statistical form of morphological volume of thermal field simulation and clinical ablation [Table 6] and [Figure 8]: the ablation volume of the CSMW ablation antenna was larger than that of the PSMW ablation antenna, and the difference was statistically significant (P < 0.05). The maximum error rate between the volume of the thermal fieldsimulated morphology and clinical ablation morphological volume of the two MW ablation antennas was 9.8%, and the minimum error rate was only 0.3% (the maximum simulation errors of the PSMW and CSMW ablation antennas were 9.1% and 9.8%, respectively).  Figure 8: Thermal field simulated volume, clinical ablation volume error, and volume versus time variation chart of two kinds of microwave ablation antennas (50 W)
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PSMW ablation antenna has been widely used in clinical practice, but the intraoperative emergence problems such as the blowingout of antenna pinhead caused by the deformation of polytetrafluoroethylene and the impedance matching affected the antenna pinhead bending, and the poor antibending properties of antenna can further affect the actual therapeutic effect. Despite that CSMW ablation antenna has eliminated the existing problems in PSMW ablation antenna, there are still no reliable data about its thermal field. Therefore, using only the thermal field data of PSMW ablation antenna to evaluate the thermal field of CSMW ablation antenna will be inappropriate. Consequently, studies on the distribution law of thermal field based on the rapid simulation method are required.
The results of this study showed that the ablation morphology of the CSMW ablation antenna and the PSMW ablation antenna was manifested as a relatively regular “ellipsoidal shape,” and the ablation of the former had greater long diameter, transverse diameter, and volume than the latter; the ex vivo and clinical ablation volumes were smaller than the thermal field simulation. The underlying reason was that after the CSMW ablation antenna performed ablation in the ex vivo pig liver and the patient's liver for a period of time, a certain degree of coking occurred in the peripheral area of the slot, resulting in changes in the electrical properties of the liver and causing a reflection in partial microwave power. The power that was actually transmitted to the ex vivo pig liver was reduced ^{[19],[20]} so that the actual ablation volume would be slightly smaller than the simulated volume obtained from the phantom thermal field test. A clinical data analysis showed that the long and transverse diameters of the actual ablation morphology had a significantly larger range of fluctuation than the simulated and ex vivo data, indicating that the metabolism, blood flow, heat conduction, and other factors would affect the ablation morphology during the actual ablation. During the study on the thermal field of the MW ablation antenna, the impact of the above factors should be considered critically.
The simulated solution of the thermal field morphology of MW ablation antenna is usually achieved via finite element analysis method using finite element analysis software such as ANSYS, ANSYS Workbench, and COMSOL Multiphysics but the calculation process is tedious and with a poor realtime performance (the calculation generally lasts more than 30 s). By analyzing and deriving the SAR threedimensional distribution function, the axial, and radial value interval of the thermal field range were obtained by the bisection method, followed by extraction of the 60°C isosurface. The rapid simulation of the thermal field morphology was realized by Visual C++ software. Therefore, the process was greatly simplified, the realtime performance was significantly improved (calculation lasted <1 s), and the maximum error rate of long and transverse diameters was only 5.0%, showing a high accuracy.
In this study, we ignored the influence of heat conduction and blood flow. Based on the theoretical analysis of the SAR threedimensional distribution function, the MW radiation law of CSMW ablation antenna was examined using the rapid simulation method of thermal field morphology, and the thermal field detection system is highly significant in intraoperative realtime navigation in clinical practice.
Acknowledgments
This study was supported by the National Key R and D Program of China (No. 2017YFC0112000). In addition, the authors gratefully acknowledge the assistance of Chen Rendong and Chen Gang for enlightening discussions.
Financial support and sponsorship
This study was supported by the National Key R and D Program of China (No. 2017YFC0112000).
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
