High-frequency induction heating of Ti-coated mild steel rod for minimally invasive ablation therapy of human cancer

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1 High-frequency induction heating of Ti-coated mild rod for minimally invasive ablation therapy of human cancer article info Article history: Received 4 November 211 Received in revised form 3 August 212 Available online 2 November 212 Keywords: High-frequency induction heating Ablation cancer therapy Computer simulation AC magnetic field Magnetic flux direction Eddy current loss abstract For application as a novel ablation therapy of human cancer, the heating property of a Ti-coated mild rod was studied in an AC magnetic field at 3 khz. When the Ti-tube thickness was as low as.1 mm, the specimen, when placed parallel to the magnetic flux direction (y¼1), exhibited a significant increase in temperature; however, its value gradually decreased with the increasing Ti thickness. In computer simulation images, the high magnetic flux concentration and concurrent large current density were observed around the interface between the mild rod and the Ti-tube. The heating property was drastically different at the three inclination angles (y¼1, 451, and 91) to the magnetic flux direction. However, the effect of the inclination angle was markedly reduced in the mild rod surrounded by a.3 mm thick Ti-tube, suggesting that the non-oriented heating property will be achieved for the prototype ablation needle coated with a Ti layer having the optimum thickness. & 212 Elsevier B.V. All rights reserved. 1. Introduction High-frequency induction is a technique of heating electrically conductive materials such as metals and alloys. It has been commonly utilized in process heating prior to metalworking, in heat treatment, welding, and melting. The number of industrial items which undergo induction heating during some stage of their production has been rapidly expanding [1]. In addition to such engineering fields, particular attention has been paid to the application of this technique as a novel thermotherapy of the deep-seated human cancer in the medical fields [2 4]. For primary liver cancer, there are different types of treatments such as surgical resection, radiotherapy [5 ], chemotherapy [8 1], and chemoembolization therapy [11,12] for patients with primary liver cancer, as shown on the left-side of Fig. 1. Moreover,thetwo current types of thermotherapy are listed on the right-side of Fig. 1. The radio-frequency ablation (RFA) therapy utilizing a high-frequency current of 4 khz has been widely carried out as a minimally invasive local treatment for primary liver cancer [13,14]. However, RFA possesses some problems attributable to the necessity to hold an electrode during treatment and difficulty in repeatability [4]. During the high-frequency dielectric heating therapy [15,16], body tissue is exposed to a temperature around 42 1C; however, it is too low to kill the liver cancerous cells. Thus, the establishment of a novel ablation therapy which solves these problems is expected for the treatment of primary liver cancer. Although a ferromagnetic mild rod is used as a component of the ablation needle, the encapsulation with a Ti layer is necessary to attain a superior biocompatibility in clinical use. In this thermotherapy, the pricking direction of the ablation needle seems to widely vary due to the tumor position. The non-oriented heating property to the magnetic flux direction is substantial for the rigid control of the treatment temperature. However, the shape magnetic anisotropy, which is closely associated with the value of the demagnetizing field coefficient, produces an undesirable effect on theheatingpropertyinanacmagneticfield[1]. In the present study, we attempted to evaluate the applicability of this novel ablation therapy utilizing high-frequency induction heating, and evaluating both the Ti-tube thickness surrounding the mild rod and inclination angle to the magnetic flux direction. Also, the computer simulation of the heat analyses was carried out to obtain the data concerning the origin of the heating property. 2. Materials and methods 2.1. Materials A 1 mm long mild rod was embedded into each Ti-tube which has the corresponding inner diameter. The magnetic

2 T. Naohara et al. / Journal of Magnetism and Magnetic Materials 331 (213) Surgical Resection Thermal Ablation Magnetic flux direction Fiber-optic probe θ = θ =45 Radiotherapy Radio-frequency Ablation Therapy Chemotherapy Hyperthermia θ =9 Chemoemborization Therapy High-frequency Dielectric Heating Therapy Fiber-optic thermometer Copper pipe Fig. 1. Current therapies for primary liver cancer. Power source Specimen Specimen holder Table 1 Specimens used in the present study. Dimensions of the Ti-tubes used in the present study. Fig. 2. Setup used in the present study. Setup for measuring the heating property for various Ti-coated mild rods having different inclination angles to the magnetic flux direction. Specimen no. Inner diam. (mm) Outer diam. (mm) Thickness (mm) Ti-rod property of the mild is well known to be ferromagnetic, while that of Ti is nonmagnetic. The dimensions of all the Ti-tubes were a 1.8 mm outer diameter and a 1 mm length, while their inner diameters were varied from 1.6 mm to.8 mm. In addition, the 1 mm long and 1.8 mm diameter Ti-rod was employed as Specimen no. 6 in the present study. The values of inner diameter, outer diameter, thickness, and R (Ratio of inner diameter to outer diameter) of the Ti-tubes are summarized in Table 1. R Y Ti-tube O Z Y 25mm Air 3mm 5mm O rod X 5mm Coil X Y O.9mm θ= specimen 5mm Simulation conditions Specimen size : 1.8 mmφ 1 mm Frequency : 3 khz Current condition : 21.2 Relative magnetic permeability rod: 2 Ti-tube: 1 Turn number of coil : 8 Initial temperature : 25 C Computational mesh : 2(H) 2(W) X 2.2. Experimental procedure Fig. 3. Computer simulation model used in the present study. Fig. 2 shows the setup used for the measurement of the heating property in the AC magnetic field. The Ti-coated mild rod specimen was placed in a high-frequency induction coil at three different inclination angles (y¼1, 451, and 91) to the magnetic flux direction. The high-frequency induction coil was connected to a power supply through an impedance tuner [3]. The high-frequency output of 1 W at 3 khz corresponded to the AC magnetic field of 1.69 ka/m at the center of coil. A fiber-optic thermometer was used to directly measure the increase in temperature (DT) of these specimens in ambient air Computer simulation The heating property of the Ti-coated mild rods was investigated using the electromagnetic field analysis software, JMAG Studio, ver. 1. (JRI Solutions, Ltd.) [18]. The simulation model considering the physical parameters of the mild rod and Ti-tube as well as their size, frequency, current condition, and turn number of the coil is shown in Fig. 3. The relative magnetic permeability value of the mild rod used in the simulation was 2, and the center point of the specimens was determined as the origin. The present simulation was performed using a computational mesh of 2 2 for both the y¼1 and y¼91 specimens. 3. Results and discussion 3.1. Effects of R value on the heating property Fig. 4 shows the relationship between the increase in temperature (DT) and induction time for the Ti-coated mild rod having different R values in the AC magnetic field. These data were determined under the condition of y¼1, implying that the specimens were placed parallel to the magnetic flux direction in the high-frequency induction coil. It is noted that the R¼.89 specimen exhibited a marked increase in temperature (DT) during the initial stage of the magnetic induction. Their DT value was greater than 5 1C even after the induction time of 2 s. Such a large DT value found in the R¼.89 specimen should be associated with the effect of the shape magnetic anisotropy [1]. This specimen possesses a lower demagnetization field coefficient, because its longitudinal direction is parallel to that of the magnetic flux in the high-frequency induction coil. In the AC magnetic field, the hysteresis loss (P h ), and eddy current loss (P e ) given by Eqs. (1) and (2), respectively, significantly affect the difference in the heating property of the Ti-coated mild rod [19,2]. P h ¼ k h f B m 1:6 W m 3 ð1þ

3 1 T. Naohara et al. / Journal of Magnetism and Magnetic Materials 331 (213) Increase in temperature / C Fig. 4. Effects of the Ti-tube thickness on the heating properties. Effects of R value on the temperature increase for the Ti-coated mild rod having the inclination angle of y¼1 to the magnetic flux direction in the AC magnetic field. Ti :.1mm Ti :.2mm Ti :.3mm (T ) Fig. 5. Effects of the Ti-tube thickness on the magnetic flux density in the y¼1 thickness on the magnetic flux density for the Ti-coated mild rod in the y¼1 specimens. (a) R¼.89, (b) R¼. and (c) R¼.66. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) where k h is a constant value, f is the frequency (Hz), and B m is the maximum magnetic flux density (T). P e ¼ k e ðb m t f Þ 2 W =r ð2þ m 3 where k e is a constant value, t is the thickness (m), f is the frequency (Hz), B m is the maximum magnetic flux density (T), and r is the electrical resistivity (O m). The Ti-coated mild rod with R¼.86 presumably possesses a higher B m, resulting in the high P h and P e values due to their lower demagnetizing field coefficient. However, the measured temperature continuously decreases with the increasing induction time for the Ti-coated mild rods having the lower R values. For instance, the DT value was at most C after 12 s in the R¼.56 specimen. The reduction in the R value produces a decrease in the cross-sectional area of the embedded mild rod. The small amount of magnetic flux passing through this reduced area leads to the lower effect of P h and P e on the DT value. On the other hand, no appreciable increase in temperature was determined over the induction time up to 12 s for the nonmagnetic Ti-rod specimen. The low DT value is attributable to only the effect of electromagnetically-induced eddy current Simulation analyses using the Ti-coated mild rod Fig. 5 shows the computer simulation images of the Ti-coated mild rods with different R values in the condition that placed at y¼1 to the magnetic flux direction. The color transition (blue-green-red) in the figure reflects the continuous increase in the magnetic flux density. For all the specimens shown in Fig. 5, the surface of the embedded mild rod is colored in red, suggesting that this region has a particularly high magnetic flux concentration. However, it is emphasized that the volume of the ferromagnetic mild rod decreases in the order of Fig. 5a, b, and c, resulting in the gradual decrease in the P h value. From a comparison of Fig. 5 with Fig. 4, the high DT value in the R¼.89 specimen originates from the high magnetic flux concentration around the surface of the mild rod having the largest volume among the y¼1 specimens. Fig. 6 shows the computer simulation images of the Ti-coated mild rods with different R values for the condition that placed at y¼91 to the magnetic flux direction. It is obvious that the magnetic flux is mainly concentrated at the right-side outer Steel Steel Steel Ti :.1mm Ti :.2mm Ti :.3mm (T ) Fig. 6. Effects of the Ti-tube thickness on the magnetic flux density in the y¼91 thickness on the magnetic flux density for the Ti-coated mild rod in the y¼91 specimens. (a) R¼.89, (b) R¼. and (c) R¼.66. surface of the embedded mild rod, where is opposite to the high-frequency induction coil. In addition, high magnetic flux concentration is observed in the horizontal surface of the embedded mild rod for all the y¼91 specimens. Computer simulation images taken from the Ti-coated mild rod are shown in Fig., showing that the Ti-tube thickness remarkably affects the current density in the AC magnetic field. In the all y¼1 specimens, the Ti-tube tends to have a greater current density in comparison to that of the embedded mild rod with different R values. However, it should be particularly noted that the R¼.89 specimen possesses an extremely high current density at the thin surface layer of the mild rod adjacent to the Ti-tube, as colored in red in Fig. a. These results are closely related to the magnetic penetration depth (d) factor given by Eq. (3) [21]. d ¼ 5:3 1 2 r 1=2 ½mŠ ð3þ ðmf Þ where r is the electrical resistivity (O m), m is the relative magnetic permeability (H/m), and f is the frequency (Hz). The d value is defined as the depth below the surface of the conductor at which the current density has decreased to approximately 3% of its value at the surface. When the m and r values are 2 and O m, respectively, for the mild rod, the calculated

4 T. Naohara et al. / Journal of Magnetism and Magnetic Materials 331 (213) Ti :.1mm Ti :.2mm Ti :.3mm d value is as low as 8. mm at 3 khz in accordance with the simulation image given in Fig. a. When compared to Fig. 5a, the heating property found in the R¼.89 specimen originated from the electromagnetic behavior near the surface of the embedded mild rod. It is appropriate to consider that the P e value given by Eq. (2) significantly contributes to the high DT value, leading to the localized heating in the R¼.89 specimen. For the nonmagnetic Ti-tube, on the other hand, the d value of approximately.63 mm is obtained using the m and r values of 1 and O m, respectively. Hence, the eddy current seems to completely penetrate into the Ti-tube with the maximum thickness of.3 mm, taking into account the simulation images shown in Fig.. The computer simulation images of the Ti-coated mild rod for the y¼91 specimens are shown in Fig. 8, showing the effect of the Ti-tube thickness on the current density behavior. As colored with red, the greater eddy current density is determined only at the right-side outer surface of the mild rod for all the specimens. Comparing Fig. 8 with Fig. 6, it seems likely that the high magnetic flux concentration results in the magneticallyinduced eddy current on the right-side outer surface of the embedded mild rod. However, the effect of the magnetic flux density and the concurrent eddy current density on the heating property are remarkably smaller in the y¼91 specimen, as compared with the y¼1 specimen. (A/m 2 ) Fig.. Effects of the Ti-tube thickness on the current density in the y¼1 thickness on the current density for the Ti-coated mild rod in the y¼1 specimens. (a) R¼.89, (b) R¼. and (c) R¼.66. Steel Steel Ti :.1mm Ti :.2mm Ti :.3mm ( A/m 2 ) Fig. 8. Effects of the Ti-tube thickness on the current density in the y¼91 thickness on the current density for the Ti-coated mild rod in the y¼91 specimens. (a) R¼.89, (b) R¼. and (c) R¼ Effects of inclination angle on the heating property in the AC magnetic field Fig. 9 shows the induction time dependence of the heating property for the Ti-coated mild rod (Specimen no. 1) with R¼.89 in the AC magnetic field. The specimens placed in the coil possessing the inclination angles of y¼1 exhibited a significant increase in temperature (DT) during the initial stage of the magnetic induction. The DT value is greater than 5 1C even after the induction time of 2 s as previously mentioned. For both the y¼451 and y¼91 specimens, the measured temperatures continuously increase with the increasing induction time, and the DT values reached 44. 1C and 2.2 1C, respectively, after 12 s. These results suggest that the Ti thickness of.1 mm is too low to reduce the effect of the shape magnetic anisotropy interpreted in terms of the demagnetizing field coefficient [22]. The heating property is remarkably different at the three inclination angles (y¼1, 451, and 91) to the magnetic flux direction; therefore, it is unsuitable to use a mild rod with a thin Ti coating for clinical testing. The induction time dependence of the heating property is shown in Fig. 1 for the Ti-coated mild rod (Specimen no. 3) with R¼.66 in the AC magnetic field. For the specimen of y¼1, the measured temperature continuously increases with the induction time and reaches the low value of DT¼23.6 1C after 12 s, exhibiting a tendency different from the data for the R¼.89 specimen given in Fig. 9. The temperature curve of the y¼451 and y¼91 specimens approaches that of the y¼1 specimen, reaching the DT values of 2.8 1C and C, respectively, after the induction time of 12 s. It is noteworthy that the effect of the inclination angle on the heating property is markedly reduced in the specimen with the higher Ti-tube thickness. Fig. 11 shows the relationship between the increase in temperature (DT) and induction time for the Ti-rod (Specimen no. 6) in the AC magnetic field. In the nonmagnetic Ti-rod, no significant increase in temperature is observed during the induction time. The DT value is C for the y¼91 specimen after 12 s, while those of the y¼1 and y¼451 specimens are at most 9.2 1C and C, respectively. It is important to note that the highest enlarged area perpendicular to the magnetic flux direction is obtained in the high-frequency coil for the y¼91 specimen. The enhanced DT value found in the nonmagnetic Ti-rod (Specimen no. 6) is probably associated with the electromagneticallyinduced eddy current flowing near the surface of the enlarged cross-sectional area. Increase in temperature / C Specimen No R.89 Fig. 9. Heating properties of the Ti-coated mild rod (R¼.89). Changes in temperature of the Ti-coated mild rod (R¼.89) having different inclination angles to the magnetic flux direction with the induction time in the AC magnetic field.

5 12 T. Naohara et al. / Journal of Magnetism and Magnetic Materials 331 (213) Increase in temperature / C Taking Eqs. (1) and (2) into consideration, the effect of the inclination angle to the magnetic flux direction on the heating property is probably interpreted in terms of the B m value, because its value has a close relationship with the demagnetization field coefficient in the AC magnetic field [1]. For instance, the shape of the hysteresis curve is skewed by the increasing inclination angle (y) to the magnetic flux direction, resulting in a small remanence ratio [23,24]. The reduced B m gives rise to the small P h value as estimated from Eq. (1). It is inferred that the combined effects of P h and P e play an important role in achieving the non-oriented thermal property. Consequently, the effect of the shape magnetic anisotropy is significantly reduced using the mild rod surrounded by the Ti layer with the optimum thickness. 4. Conclusions Specimen No R Fig. 1. Heating properties of the Ti-coated mild rod (R¼.66). Changes in temperature of the Ti-coated mild rod (R¼.66) having different inclination angles to the magnetic flux direction with the induction time in the AC magnetic field. Increase in temperature / C Specimen No.6 R 2 θ 9 θ 45 1 θ Fig. 11. Heating properties of the Ti-rod (R¼). Changes in temperature of the Ti-rod (R¼) having different inclination angles to the magnetic flux direction with the induction time in the AC magnetic field. In order to clarify its applicability as a novel ablation therapy, the heating property of the Ti-coated mild rod was investigated in an AC magnetic field at 3 khz. At the measuring condition of y¼1, where the specimen is placed parallel to the magnetic flux direction, the DT value was greater than 5 1C even after 2 s for the R¼.89 specimen. However, its value gradually decreased with the increasing Ti thickness, and the R¼.56 specimen exhibited a DT value of C after the induction time of 12 s. Computer simulation images revealed that the high magnetic flux concentration and concurrent high current density are observed near the surface of the embedded mild rod in the y¼1 specimen. On the other hand, these characteristics were found at the right-side outer surface of the embedded mild rod in the y¼91 specimen. The heating property was remarkably different among the three inclination angles (y¼1, 451, and 91) to the magnetic flux direction in the specimens with the lower Ti-tube thicknesses, due to the effect of the shape magnetic anisotropy. On the other hand, the effect of the inclination angle on the heating property was considerably reduced in the R¼.66 specimen with the higher Ti-tube thickness. These results allowed us to expect that the non-oriented heating property to the magnetic flux direction is achieved for the prototype Ti-coated ablation needle having an optimum Ti thickness. Acknowledgments The present study was supported by a Grant-in-Aid from The Ministry of Education, Science, Sports and Culture of Japan (No : T. Naohara) and Comprehensive Support Programs for Creation of Regional Innovation, Research for Promoting Technological Seed Program in 29 (No : T. Naohara) from The Japan Science and Technology Agency. References [1] S Zinn, S.L. Seminatin, Elements of Induction Heating: Design, Control, and Applications, Electric Power Research Institute, Paro Alto, California, 22, p. 1. [2] T. Maehara, K. Konishi, T. Kamimori, H. Aono, T. Naohara, H. Kikkawa, Y. Watanabe, K. Kawachi, Heating of ferrite powder by an AC magnetic field for local hyperthermia, Japanese Journal of Applied Physics 41 (22) [3] T. Naohara, H. Aono, T. Maehara, W. Watanabe, H. Hirazawa, S. Matsutomo, Computer simulation of heat generation ability in AC magnetic field, in: Proceedings of the 6th International Symposium on Electromagnetic Processing of Materials, Dresden, 29, pp [4] Y. 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