tube, was buried in a lossy medium, and specific absorp- tion rate (SAR) patterns .... heat shrink tubing with a relative permittivity of 2.1 was used to insulate the ...
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An Insulated Dipole Applicator for Intracavitary Hyperthermia SHIRA LYNN BROSCHAT, STUDENT MEMBER, IEEE, CHUNG-KWANG CHOU, KENNETH H. LUK, ARTHUR W. GUY, FELLOW,IEEE, AND AKIRA ISHIMARU, FELLOW,IEEE
SENIOR MEMBER, IEEE,
Abstract-SAR patterns for an insulated sleeve dipole applicator operating at 433 MHz were measured by means of infrared thermography. The applicator was modeled using classical transmission line theory, and experimental' and theoretical results were compared. In general, agreement between measured and calculated SAR values was good. However, at the antenna feedpoint, the measured values were appreciably higher than the calculated values. This is probably due to junction effects neglected in the model.
INTRODUCTION N recent years, there has been increasing interest in the use of hyperthermia to treat cancer (for example, see [ 11-[3]). Intracavitary hyperthermia has several advantages over other methods of hyperthermia. Since an intracavitary applicator can be placed in close proximity to the tumor, normal tissue exposure is minimized and there is less problem with reflection at tissue boundaries. It can be used to treat cancer of the prostate gland, esophagus, rectum, stomach, colon, pancreas, larynx, uterus, and bladder. This paper reports the results of an experimental study of a very simple intracavitary applicator designed to operate at 433 MHz. The applicator, comprised of a halfwavelength sleeve dipole antenna inserted into a latex tube, was buried in a lossy medium, and specific absorption rate (SAR) patterns were measured by means of infrared thermography. The applicator was modeled using classical transmission line theory, and experimental and theoretical results were compared. In general, agreement between measured and calculated SAR values was good. However, at the antenna feedpoint, the measured values were appreciably higher than the calculated values. This is probably due to junction effects neglected in the model.
I
THEORY The use of the transmission line model to solve the insulated dipole antenna problem is well-established in the Manuscript received June 6 , 1986; revised August 10, 1987. This work was supported in part by National Cancer Institute Grant 1 R01 CA3792301. S. L. Broschat and A. Ishimaru are with the Department of Electrical Engineering, University of Washington, Seattle, WA 98195. C.-K. Chou and K. H. Luk are with the City of Hope National Medical Center, Division of Radiation Oncology, Duarte, CA 91010. A. W. Guy is with the Bioelectromagnetics Research Laboratory, University of Washington, Seattle, WA 98195. IEEE Log Number 8718584.
Fig. 1. Insulated dipole in a lossy medium. Region 1 is the center-fed dipole antenna, Regions 2 and 3 are the dielectric layers, and Region 4 is the lossy medium corresponding to muscle tissue.
literature [4]-[ 113 and the derivation is not repeated here. For a comprehensive treatment, the reader is referred to [4]-[9]. In this paper, we simply state the final equations used to calculate the relevant quantities. For modeling purposes, the applicator system is divided into four regions and a cylindrical coordinate system is used as shown in cross section in Fig. 1. Region 1 is the center-fed dipole antenna consisting of a conductor with half-length h and radius a. The conductor is assumed to have a relative permittivity of e l = 1 and to have finite conductivity o1 and wavenumber k , = P1 ial. Region 2 is the first dielectric layer with radius b, relative permittivity e2, and wavenumber k2 = &. Region 3 is the second dielectric layer with radius c , relative permittivity e3, and wavenumber k3 = p 3 . Both dielectrics are assumed to be lossless and to extend far enough beyond the length
0018-9294/88/0300-0173$01.OO O' 1988 IEEE
+
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 3 5 , NO. 3, MARCH 1988
174
of the conductor so that they are effectively infinite in length. Finally, Region 4 is the lossy medium-that is, the body tissue that one is trying to heat-with permittivity e4, conductivity u4, and wavenumber k4 = p4 ia4. This region is assumed to be homogeneous and infinite in extent. Power absorption by biological tissue is quantified using the specific absorption rate (SAR). The average SAR in the lossy medium is given by I121
+
SAR = a4 lE4I2W/kg
(1)
2P
+
where E4 = PE44r is the electric field in Region 4 and p is the tissue density measured in kg/m3. The vectors P and 2 are unit vectors in the radial and axial directions, respectively. The electric field components are given by [71,[51 &r(~)=
-
47r
s” SI, 0
dz’
d4’(C,(z - z’) sin kL(h - 2’)
*
- C2 cos kL(h - z’))
- (c,cos 4‘ E 4 Z W
=
47r
s” I:, 0
R
1/R)
C,(r - r’ cos 4’))
-
(2)
d4’((C3 - C,) sin kL(h - z’)
dz’
C ~ (Z
-
- (ik4 -
2’) COS
kL(h
- 2’))
- q(c3- l / ~ ( i k ,- I / R ) ( C l ( r - r’ cos 4’) cos 4’
*
for 0
Iz’ Ih.
s”
&r(r) = 47r
-h
Similarly, for - h
E4ZW
(c1cos
-h
*
- (ik4
R
- 1/R)
4’ - C2(r - r’ cos 4’))
(4)
d4’((C3 - C , ) sin k L ( h + z’)
dz’
+ C ~ ( -Z 2’) *
Iz‘ I0
s” jIT
=
(3)
d+’(C,(z - z’) sin kL(h + z’)
dz‘
+ C2 COS kL(h + 2’)) *
+c~))
COS
kL(h
+ z’))
% ( C 3 - l/R(ik4 - 1 / R )
- ( C , ( r - r’ cos 4’) cos 4‘ - c2))
(5) where r = ( r , 4, z ) is a point in Region 4, the prime indicates a source coordinate located on the surface of the insulating cylinder, erkR
(6)
q=-R ’
R = [ r 2 + r r 2 - 2rr’
c1
= -
COS
i w p o c l ( 0 ) In ( . / a ) 27r sin kLh
4’
+ (Z
2 112
- 2’) ]
,
(7) (8)
HL1’and HI” are Hankel functions, po is the free space permeability, w is the radian frequency, and Vo is the input voltage at z = 0. Equations (2)-(5) were calculated numerically using Simpson’s composite rule and then used in (1). EXPERIMENT AND RESULTS A half-wavelength dipole antenna was built using RG178B/U coaxial cable and a sleeve dipole design. To simplify its construction, the experimental dipole was built asymmetrically-that is, the upper and lower halves of the conductor were of different sizes. The lengths were determined using (1 l)-( 14) together with the parameter values given in Table I. Results are shown in Table 11. A length of the coax jacket and outer conductor corresponding to a quarter wavelength was removed from the end of a strip of coax, exposing the inner conductor; this exposed inner conductor served as the top half of the dipole. A sleeve was fashioned from the outer conducting braid of the coaxial cable. This sleeve was then worked along the length of coax and soldered into place to form the bottom half of the dipole. The ends of the braid were then soldered and filed to make them as smooth as possible. The dipole antenna measured 17.76 cm in length. Teflon heat shrink tubing with a relative permittivity of 2.1 was used to insulate the entire dipole. This structure was inserted into a length of latex surgieal tubing 7.6 mm in diameter. The space between the Teflon insulating layer and the inner diameter of the latex tubing was filled with vaseline which is electrically identical to Teflon in the frequency and temperature range of interest [13]. Thus, the inner diameter of the tubing is equivalent to the diameter of the first insulating layer, and the outer diameter is eauivalent to the diameter of the sec-
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BROSCHAT et al. : DIPOLE APPLICATOR FOR INTRACAVITARY HYPERTHERMIA
TABLE I DIPOLE PARAMETERS
TABLE I1 EFFECTIVE WAVENUMBERS, WAVELENGTHS, AND ANTENNA LENGTHS
Frequency:
Upper Half f
=
kL = PL + iaL= 15.85 2u A, = - = 39.64 cm
433 MHz
Wavenumbers:
+
+ i1.65/m
PL
k , = 3.12 x lo5 i3.12 x l d / m k2 = 13.14/m k3 = 14.05/m k., = 71.74 i28.08/m
XL h == 9.91 cm 4
+
Lower Half kL =
Radii:
PL + iaL = 20.0
2u X, = - = 31.40 cm
a = 0.125 mm
(upper) 1 . 1 mm (lower) b = 2.15 mm c = 3.8 mm
+ i3.61/m
PL
=
XL h == 7.85 cm
4
SILICONE PLUQ
PETROLEUM JELLY
2.2mm d i m
\
JA~KET
OGER
CONDUCTOR
TEFLON HEAT SHRINK TUBINQ
TE~LON DIELECTRIC
INNER C ~ N D U C T O R 0 . 2 5 m m dlam
Fig. 2. Insulated sleeve dipole design for the half-wavelength dipole.
ond insulating layer. A cross section of the sleeve dipole and insulating layers is shown in Fig. 2. Phantom models were created by filling two rectangular acrylic boxes 15.5 cm deep, 12.7 cm wide, and 32.0 cm long with muscle phantom [141. The dipole was then positioned on the surface of one of the model halves and the other half was placed on top. An AGA Thermovision 680 system was used to take infrared pictures, or thermograms, of the SAR patterns of the dipole antennas. Images were processed on a PDP 11/34 minicomputer. Single line scans known as B scans were processed for both axial (z direction) and radial ( r direction) patterns. Figs. 3-7 show some of the resultant B scans after processing. The theoretical SAR values were obtained by assuming an input voltage of V, = 1 V, whereas the experimental values were obtained using an input power normalized to 1 W. In order to compare the two sets of values, it was necessary to multiply the calculated values by a constant of proportionality. This proportionality constant was found empirically, but the same value was used consistently for all comparisons. In Fig. 3, the feedpoint is located at z = 0; negative z values indicate the lower part of the dipole, and positive z values indicate the upper part. The asymmetry in the
10
-8 $ 6 I
2 4
v)
2
0
-10
-8
-6
-4
-2
0 1
2
4
6
8
10
rc.1
Fig. 3. Comparison of theoretical (circles) and experimental SAR pattern in the axial direction at r = 3.8 mm. The dipole feedpoint is located at z = 0.
distribution around z = 0 is due to the asymmetry of the antenna. Discrepancies between theoretical and experimental results occur near the feedpoint and in the upper half of the antenna. The former is due to neglected junction effects, and the latter is possibly due to the difficulty in centering the thin antenna in the insulator as well as ensuring its absolute straightness. Any bending, skewing, or eccentricity causes a certain amount of directivity [ 151, [161.
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 35, NO. 3, MARCH 1988
Fig. 4. Comparison of theoretical (circles) and experimental SAR pattern in the radial direction at z = -4.94 cm. The point r = 0 corresponds to the center of the antenna structure so that r = 3.8 mm is equivalent to the outer surface of the latex surgical tubing.
c4 5 3
Y
Fig. 5. Same as Fig. 4 but z = -2 cm.
Fig. 7. Same as Fig. 4 but
z
= 3.9 cm.
patterns were measured experimentally using infrared thermography. Theoretical and experimental results were compared and indicate that transmission line theory adequately, but not perfectly, models the large insulated sleeve dipole. Minor discrepancies between measured and calculated results are attributed to thermal diffusion which occurs during the thermography process and also to skewing of the antenna conductor in the dielectric jacket. However, a major discrepancy between theoretical and experimental SAR values at the feedpoint indicates that inclusion of junction effects would improve the model. ACKNOWLEDGMENT The authors would like to thank Prof. B. S . Trembly of Dartmouth College for his reading of the original Master’s thesis on which this paper is based and for his helpful comments. The authors would also like to thank J. A. McDougall, C. C. Sorensen, and A. M. Dong for their technical assistance.
REFERENCES
Fig. 6 . Same as Fig. 4 but
z
= 2 cm.
Figs. 4-7 show radial patterns at different points along the z axis. The point r = 0 corresponds to the center of the antenna structure so that r = c = 3.8 mm is equivalent to the outer surface of the latex surgical tubing. In Figs. 4-7, the calculated results are higher than the measured results near the tubing and vice versa away from the tubing. This is probably due to thermal diffusion which occurs during the thermography process. Thermal diffusion is also thought to account for the difference between the calculated depth of penetration (1.4 cm) and the measured depth of penetration (1.7 cm). CONCLUSIONS In the preceding study, a large (7.6 mm diameter) insulated half-wavelength dipole antenna buried in a lossy medium was modeled using transmission line theory. SAR
[I] F. K. Storm, Ed., Hyperthermia in Cancer Therapy. Boston, MA: G. K. Hall, 1983. [2] R. K. Jain and P. M. Gullino, Eds., Thermal Characteristics of Tumors: Applications in Detection and Treatment. New York: The New York Academy of Sciences, 1980. [3] J. W. Strohbehn, T . C. Cetas, and G. M. Hahn, Eds., Special Issue on Hyperthermia and Cancer Therapy, IEEE Trans. Biomed. Eng., vol. BME-31, Jan. 1984. [4] R. W. P. King and G. S. Smith, Antennas in Matter. Cambridge, MA: MIT Press, 1982, ch. 8. [5] R. W. P. King, B . S. Trembly, and J. W. Strohbehn, “The electromagnetic field of an insulated antenna in a conducting or dielectric medium,” IEEE Trans. Microwave Theory Tech., vol. MTT-31, pp. 574-583, July 1983. [6] K. M. Lee, “Insulated linear antenna,” in Research Topics in Electromagnetic Wave Theory, J. A. Kong, Ed. New York: Wiley, 1981, ch. 1 1 . [7] T. T. Wu, R. W. P. King, and D. V. Giri, “The insulated dipole antenna in a relatively dense medium,’’ Radio Sci., vol. 8, pp. 5762, July 1973. [SI R. W. P. King, L. C. Shen, and T . T. Wu, “Embedded insulated antennas for communication and heating,” Electromagn., vol. 1, pp. 51-72, 1981. [9] K. M. Lee, T. T. Wu, and R. W. P. King, “Theory of an insulated linear antenna in a dissipative medium,” Radio Sci., vol. 12, pp. 195-203, Mar.-Apr. 1977. [lo] A . Ghods and K. M. Chen, “An insulated coaxial probe for EM local heating,” IEEE Trans. Biomed. Eng., vol. BME-32, pp. 418-427, June 1985. [ 1 I] J. P. Casey and R. Bansal, “The near field of an insulated dipole in a dissipative dielectric medium,” IEEE Trans. Microwave Theory Tech., vol. MTT-34, pp. 459-463, Apr. 1986
BROSCHAT et al.: DIPOLE APPLICATOR FOR INTRACAVITARY HYPERTHERMIA
[ 121 NCRP Rep. 67, Radiofrequency Electromagnetic Fields, National
Kenneth H. Luk was born in Yung District, Kwangsi Province, China, on March 19, 1945. He received the B.A. degree in zoology from the University of California, Berkeley, in 1966, and the M.D. degree in medicine in 1971 from the University of California, Los Angeles. From June through December 1972, he received his residency training in the field of radiation oncology at Cedars-Sinai Medical Center, Los Angeles, CA, and at the Claire Zellerbach Saroni Tumor Institute of Mount Zion Hospital and Medical Center, San Francisco, CA, from 1973-1975. He was also a Fellow at the M.D. Anderson Hospital and Medical Center in Houston, TX, from 1975 to 1976. His professional experiences include Assistant Chief, Department of Radiation Oncology, Claire Zellerbach Saroni Tumor Institute, Mount Zion Hospital and Medical Center, San Francisco, CA, 1976-1981; Assistant Professor of Radiology in Residence, University of California School of Medicine, San Francisco, CA, 1978-1981; Head, Division of Cancer Research, Department of Radiation Oncology. Mount Zion Hospital, and Medical Center, San Francisco, CA, 1978-1981; Clinical Director, Department of Radiation Oncology, University of Washington Hospital, Seattle, 1981-1983; Associate Professor of Radiation Oncology, University of Washington School of Medicine, Seattle, WA, 1981 to June 1985. In June 1985, he accepted the position as Chairman of the Division of Radiation Oncology at the City of Hope National Medical Center, Duarte, CA. Dr. Luk currently holds membership in the American College of Radiology, American Radium Society, American Society of Therapeutic Radiology and Oncology, Pacific Northwest Radiological Society, Radiological Society of North America, and the Radiation Research Society.
Council on Radiation Protection and Measurements, Washington, chs. 1 and 5 . [I31 A. R. von Hippel, Dielectric Materials and Applications. Cambridge, MA: MIT Press, 1954. [14] C . K. Chou, G . W. Chen, A. W. Guy, and K. H. Luk, “Formulas for preparing phantom muscle tissue at various radiofrequencies,” Bioelectromagn., vol. 5 , pp. 435-441, 1984. [15] T. T. Wu, L. C. Shen, and R. W. P. King, “The dipole antenna with eccentric coating in a relatively dense medium,” IEEE Trans. Antennas Propagat., vol. AP-23, pp. 57-62, Jan. 1975. [I61 S . R. Mishra and R. W. P. King, “An experimental study of the circuit properties of the eccentrically insulated antenna,” IEEE Trans. Antennas Propagat., vol. AP-23, pp. 579-584, July 1975.
Shira Lynn Broschat (S’81) received the B.A. degree in applied linguistics and languages from the University of California, Santa Cruz, and the B.S. and M.S. degrees in electrical engineering from the University of Washington, Seattle. She is currently a Ph.D. degree candidate in electrical engineering at the University of Washington. Her present research interests include wave propagation and scattering, remote sensing, integrated optics, and inverse scattering. She has served as a Teaching Assistant in the DeDartment of Electrical Engineering at the University -of Washington and ‘has also worked as a student engineer for the Boeing Aerospace Company, Seattle. She is presently employed as a Research Associate by the Applied Physics Laboratory at the University of Washington where she is involved in research on wave scattering from rough surfaces. Ms. Broschat is a member of the Optical Society of America and Tau Beta Pi. She is a 1983 recipient of the Hewlett-Packard and American Electronics Association Faculty Development Award. From 1987 to 1988 she served as the student representative of the Puget Sound Section of the Optical Society of America.
Chung-Kwang Chou (S’72-M’75-SM’86) was born in Chung-King, China on May 11, 1947. He received the B.S. degree from the National Taiwan University in 1968, the M.S. degree from Washington University, St. Louis, MO, in 1971, and the Ph.D. degree from the University of Washington, Seattle, in 1975, all in electrical engineering. During his graduate study at the University of Washington, he had extensive training in both electromagnetics and physiology. He spent a year as a NIH postdoctoral fellow in the Regional Primate Research Center and the Department of Physiology and Biophysics at the University of Washington. He was a Research Associate Professor in the Center for Bioengineering and the Department of Rehabilitation Medicine, as well as Associate Director of the Bioelectromagnetics Research Laboratory until August 1985, engaged in teaching and research in electromagnetic dosimetry, exposure systems, biological effects of microwave exposure, and RF hyperthermia for cancer treatment. He is now the head of the Biomedical Engineering Section and the Associate Director of the Department of Radiation Research at the City of Hope National Medical Center, Duarte, CA. His main research there is in cancer hyperthermia. A consultant for the NCRP’s Scientific Committee 53 on the biological effects and exposure criteria for radio frequency electromagnetic fields, Dr. Chou has also served on the ANSI Subcommittee C95.4 since 1978, and is the Chairman of the 3 kHz-3 MHz working group. He was the Chapter Chairman of IEEE’s Seattle Section on Antennas and Propagation/Microwave Theory and Technique in 1981-1982. He was on the Board of Directors of the Bioelectromagnetics Society and is now the Associate Editor of the Journal of Bioelectromagnetics. He is also a member of the Bioelectromagnetics peer review group of the American Institute of Biological Sciences. In 1981, he received the Special Award for the Decade of the 1970’s for contributions in medical and biological research, and in 1985 he received the Outstanding Paper Award, both from the International Microwave Power Institute. He is a member of BEMS, AAAS, IMPI, the Radiation Research Society, North American Hyperthermia Group, Tau Beta Pi, and Sigma Xi.
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1
Arthur W. Guy (S’54-M’57- SM’74-F’77) was born in Helena, MT, on December 10, 1928 He received the B S degree in 1955, the M.S degree in 1957, and the Ph D degree in 1966, all in electncal engineenng, from the University of Washington, Seattle From 1947 to 1950 and from 1951 to 1952, he served in the U.S. Air Force as an Electronics Technician Between 1957 and 1964 he was a Research Engineer in the Antenna Research Group, Boeing Aerospace Company, Seattle. While there, his field included research on broad-band and microwave devices, surface wave antennas, propagation through anisotropic dielectrics, and antennas buned in lossy media Between 1964 and 1966 he wai employed by the Department of Electncal Engineenng, University of Washington, conducting research on VLF antennas buned in polar ice caps At that time, he also served as Consultant to the Department of Rehabilitation Medicine, working on problems associated with the effect of electromagnetic fields on living tissue In 1966, he joined the faculty in the Department of Rehabilitation Medicine Presently, he is a Professor in the Center for Bioengineenng and has a joint appointment as Professor in Rehabilitation Medicine and Adjunct Professor in Electncal Engineering He is Director of the Bioelectromagnetics Research Laboratory in the Bioengineering Center and is involved in teaching and research in the area of biological effects and medical applications of electromagnetic energy. Dr Guy is a member of the AAAS, Bioelectromagiietics Society, the IEEE ANSI C95 Committee, ANSI C95 4 Subcommittee, and was Chairman of the 1970-1982 Subcommittee IV that developed the protection guides for human exposures to radiofrequency fields in 1974 and 1982 He is a member of NCRP, and Chairman of the Scientific Committee 53, responsible for biological effects and exposure cntena for radiofrequency fields, Chairman Elect of the IEEE Committee of Man and Radiation (COMAR), member of the U S National Committee of URSI Commission A, and past member of the EPA Scientific Advisory Board Ad Hoc Committee on Biological Effects of Radiofrequency Fields He also serves as a consultant of the NIEHS on the USSR-US Environmental Health Cooperation Program and was a member of the NIH Diagnostic Radiology Study Section 1979-1983 He is a member of the editorial boards of the Journal ON MICROWAVE THEORY AND of Microwave Power and IEEE TRANSACTIONS TECHNIQUES and is past President of the Bioelectromagnetics Society He holds memberships in Phi Beta Kappa, Tau Beta Pi, and Sigma XI
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Akira Ishimaru (M’58-SM163-F’73) received the B.S. degree in 1951 from the University of Tokyo, Tokyo, Japan, and the Ph.D. degree in electrical engineering in 1958 from the University of Washington, Seattle. From 1951 to 1952 he was with the Electrotechnical Laboratory, Tanashi, Tokyo, and in 1956 he was with Bell Laboratories, Holmdel, NJ. In 1958 he joined the faculty of the Department of Electrical Engineering of the University of Washington, Seattle, where he is currently a Professor of Electrical Engineering and an Adjunct Professor of Applied Mathematics. He has also been a Visiting Associate Professor at the University of California, Berkeley. His current research includes optical scattering
and propagation in the atmosphere, millimeter-wave propagation, ultrasound imaging, multiple scattering, waves in random media, remote-sensing and inverse problems, electromagnetic interference, and antennas. He is the author of a book, Wave Propagation and Scattering in Random Media (New York: Academic, 1978). Dr. Ishimam has served as a member-at-large of the U.S. National Committee (USNC) and is chairman (1985-1987) of Commission B of the USNC/International Union of Radio Science. He has served as editor (1979-1983) of Radio Science. He is a Fellow of the Optical Society of America. He was the recipient of the 1968 IEEE Region VI Achievement Award and the IEEE Centennial Medal in 1984. He was a Distinguished Lecturer of the IEEE Antennas and Propagation Society and an IRMARUNYON Distinguished Lecturer of Texas A&M University.
