168
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1976
Fig. 3. Tone-gating circuit schematic.
30, and 40 dB attenuation factors. Tone gating under logic control used an integrator and two iight emitting diodecadmium sulfide cell pairs (Fig. 3). Tone rise/fall time was 25 milliseconds. Tone duration was 330 milliseconds. Within a tone train the duty factor was 50%. The interval between tone trains of different intensities was 1.8 seconds. The resultant hearing threshold levels from each ear are stored in bipolar memory for external interrogation by technician, automatic data logging device, or computer. EVALUATION named the Tone Count Audiosubsequently audiometer This metric Computer (TCAC) has undergone a favorable initial evaluation by both the Acoustical Sciences Laboratory, Naval Aerospace Medical Institute and Research Laboratory, Pensacola, Florida, and the Audiology and Hearing Conservation Function, Clinical Sciences Division, USAF School of Aerospace Medicine, Brooks AFB, Texas. ACKNOWLEDGMENT The authors would like to thank Francis A. Brogan for his help in the design of the tone attenuator section.
REFERENCES [11 G. Von Bekesy, "A new audiometer," Acta Otolaryngol. (Stockholm), 35, p. 411, 1947. [21 Problems in Military Audiometry: A CHABA Symposium, J. Speech Hear. Disord., 22, pp. 729-756, 1957. [31 H. Davis, D. Eldredge, and J. Usher, "The testing of hearing in the armed services," Nat. Res. Council Com. on Hear. and Bioacoustics, CHABA Rep. No. 5, Wash., D.C., 1955. [4] W. Hughson and H. Westlake, Manual for Program Outline for Rehabilitation of Aural Casualties Both Military and Civilian. Rochester, N.Y.: Am. Acad. of Ophthalm. and Otolarying., 1944. [5] H. Newhart and S. Reger, Manual for a School Hearing Conservation Program. Rochester, N.Y.: Am. Acad. of Ophthalm. and
Otolaryng., 1956. [6] R. Carhart and J. Jerger, "A preferred method for clinical determination of pure-tone thresholds," USAF School of Aviation Medicine, Brooks AFB, Tx., 1959. [7] M. Gardner, "A pulse tone technique for clinical audiometric threshold measurements," J. Acoust. Soc. Am., 19, p. 592, 1947.
[81 A. Glorig and R. Wilke, "A new automatic screening audiometer," J. Acoust. Soc. Am., 24, p. 450, 1952. [9] V. Bragg and F. Coffins, "Audiometer modifications and pulsetone technic for pure-tone threshold determination." SAM-TR-
68-91, USAF School of Aerospace Medicine, Brooks AFB, Tx., p. 2, 1968. [10] C. Meyer, H. Sutherland, Jr., and F. Brogan. "The tone count audiometric computer," SAM-TR, USAF School of Aerospace Medicine, Brooks AFB, Tx. (In preparation).
Formation of Hot Spots in Multilayer Spheres H. N. KRITIKOS, MEMBER, IEEE, AND H. P.
SCHWAN, FELLOW, IEEE
Abstract-A theoretical study of the distribution of the normalized heating potential resulting from a plane wave incident in a multilayered sphere simulating a human head with skin, fat, bone and brain tissue layers has been undertaken. It was found that for spheres of radii 10 cm and 5 cm a relative peak of the normalized heating potential occurs in the vicinity of the center of the sphere. For the case of the 5 cm sphere in the range of frequencies 400 MHZ to 2.76 GHZ the maximum value of the heating potential occurs at the center suggesting the possibility of a hot spot. A comparison between the multilayer and a single layer model shows that the maximum values of the heating potential are the same. Manuscript received February 18, 1975; revised May 27. 1975. This work was supported by the Office of Naval Research under Contract N00014-67-A-0216-0015, the National Institutes of Health under Grant 5 RO1 HL01253, and the National Science Foundation under Grant GK 40119. The authors are with the Electromedical Division, University of Pennsylvania, Philadelphia, PA.
COMMUNICATIONS
169
TABLE I
Contributor
Model
Frequency
H. Schwan H. Kritikos/H. Schwan C. Johnson et al./ A. Guy J. Lin et al./A. Guy/ C. Johnson A. Shapiro et al. C. Weil Present contribution
100 MHZ -10 GHZ
multilayer plane model homogeneous sphere homogeneous sphere radii 3 cm and 6 cm homogeneous sphere radius 25.6 cm multilayer sphere radii 9 cm and 3.3 cm 100 MHZ to 10 GHZ multilayer sphere radius 6 cm 50 MHZ to 10 GHZ multilayer sphere radii 5 cm and 10 cm multilayer plane model
50 MHz - 10 GHz 918 MHZ and 2450 MHZ 1 MHZ, 10 MHZ, 20 MHZ 3 GHZ
E
K H
AI R
Fig. 1. The multilayer sphere model.
INTRODUCTION
The distribution of the heating potential inside specimens which are exposed to Electromagnetic Fields has been the subject of numerous contributions. It had become apparent from theoretical studies by Kritikos and Schwan [1] and Shapiro et al. [21 that under certain circumstances "hot spots" may be induced inside brain tissues by Electromagnetic Fields of only moderate intensity. Experimental work reported by Johnson et al. [31 with tissue phantoms and with cats supported this conclusion. There was an implication that the present long time exposure standard of 10 mw/cm2 might be somewhat high. However, Kritikos and Schwan [1], [6] pointed out that the development of hot spots is more pronounced for smaller head sizes, and the question arose how to extrapolate from animal experiments to man. For uniform spheres Kritikos and Schwan [ 1], [6] have investigated the formation of hot spots in the frequency region of 50 MHZ to 10 GHZ. Johnson et al. [ 31 reported results for the frequencies of 918 MHZ and 2450 MHZ. Lin et al. [4]
have examined the HF region. For a multilayer sphere, Shapiro et al. [2] have investigated the formation of hot spots for the frequency of 3 GHZ and Weil [5] has examined the region .1 GHZ to 10 GHZ. In this communication the formulation of the hot spots in multilayer spheres in the frequency region of 50 MHZ to 10 GHZ will be reported and a comparison with the plane model and homogeneous sphere will be made. Table I summarizes the reported theoretical contributions. THE MODEL The model is taken to be a four layer concentric sphere shown in Fig. 1. This is taken to be an ideafization of a head with skin, fat, and bone layers covering the brain core. Fat and bone have somewhat similar dielectric properties. So do muscle and brain tissues. Hence, the results of this study apply equally well to the head and other parts of the body where skin and subcutaneous fat cover muscular tissues. In each layer the propagation constant is
170
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1976
TABLE II Region p 1 2 -3 4
Tissue
(eoo)p
Brain Bone Fat Skin
10.00 1.60
2.
(eo - eoo)p
1.60 10.00
THICKNESS
(a00 - GO)p
1.0 0.09 0.09 1.00
55.00 0.155 0.155 0.55
37.00 3.40 3.40 37.00
RADIUS OF SPHERE
E
(Go)p
mho/m
mho/m
A-IOcm
SKIN Imm FAT-BONE 3mm
z Z
z
0
2.0.
w
En
(n
Cz
Ln 0
1.5-
EL
0
U,
m
c]
MAXIMUM HEATING OCCURS ALWAYS AT FRONT SURFACE
1.0.
w z
it
IL
z
w 0 w
N
.5. SKIN
0
z
BRAIN
^-BRAIN PLANE SLAB SKIN PLANE SLAB
20
30
40
50 60
80
100
200
.I 300
It
1i
400 500 600 800 1000
FR EQUENCY IN MHz
5
2000
3000
5000
Fig. 2. Normalized differential absorption cross section As vs frequency. The curve labeled skin and skin plane slab indicates As always at front skin surface. Curve labeled brain indicates maximum As in inner brain region. Curve labeled brain plane slab indicates As in the brain of the skull brain interface.
kp
VE
=
+ i
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1976
Fig. 3. Tone-gating circuit schematic.
30, and 40 dB attenuation factors. Tone gating under logic control used an integrator and two iight emitting diodecadmium sulfide cell pairs (Fig. 3). Tone rise/fall time was 25 milliseconds. Tone duration was 330 milliseconds. Within a tone train the duty factor was 50%. The interval between tone trains of different intensities was 1.8 seconds. The resultant hearing threshold levels from each ear are stored in bipolar memory for external interrogation by technician, automatic data logging device, or computer. EVALUATION named the Tone Count Audiosubsequently audiometer This metric Computer (TCAC) has undergone a favorable initial evaluation by both the Acoustical Sciences Laboratory, Naval Aerospace Medical Institute and Research Laboratory, Pensacola, Florida, and the Audiology and Hearing Conservation Function, Clinical Sciences Division, USAF School of Aerospace Medicine, Brooks AFB, Texas. ACKNOWLEDGMENT The authors would like to thank Francis A. Brogan for his help in the design of the tone attenuator section.
REFERENCES [11 G. Von Bekesy, "A new audiometer," Acta Otolaryngol. (Stockholm), 35, p. 411, 1947. [21 Problems in Military Audiometry: A CHABA Symposium, J. Speech Hear. Disord., 22, pp. 729-756, 1957. [31 H. Davis, D. Eldredge, and J. Usher, "The testing of hearing in the armed services," Nat. Res. Council Com. on Hear. and Bioacoustics, CHABA Rep. No. 5, Wash., D.C., 1955. [4] W. Hughson and H. Westlake, Manual for Program Outline for Rehabilitation of Aural Casualties Both Military and Civilian. Rochester, N.Y.: Am. Acad. of Ophthalm. and Otolarying., 1944. [5] H. Newhart and S. Reger, Manual for a School Hearing Conservation Program. Rochester, N.Y.: Am. Acad. of Ophthalm. and
Otolaryng., 1956. [6] R. Carhart and J. Jerger, "A preferred method for clinical determination of pure-tone thresholds," USAF School of Aviation Medicine, Brooks AFB, Tx., 1959. [7] M. Gardner, "A pulse tone technique for clinical audiometric threshold measurements," J. Acoust. Soc. Am., 19, p. 592, 1947.
[81 A. Glorig and R. Wilke, "A new automatic screening audiometer," J. Acoust. Soc. Am., 24, p. 450, 1952. [9] V. Bragg and F. Coffins, "Audiometer modifications and pulsetone technic for pure-tone threshold determination." SAM-TR-
68-91, USAF School of Aerospace Medicine, Brooks AFB, Tx., p. 2, 1968. [10] C. Meyer, H. Sutherland, Jr., and F. Brogan. "The tone count audiometric computer," SAM-TR, USAF School of Aerospace Medicine, Brooks AFB, Tx. (In preparation).
Formation of Hot Spots in Multilayer Spheres H. N. KRITIKOS, MEMBER, IEEE, AND H. P.
SCHWAN, FELLOW, IEEE
Abstract-A theoretical study of the distribution of the normalized heating potential resulting from a plane wave incident in a multilayered sphere simulating a human head with skin, fat, bone and brain tissue layers has been undertaken. It was found that for spheres of radii 10 cm and 5 cm a relative peak of the normalized heating potential occurs in the vicinity of the center of the sphere. For the case of the 5 cm sphere in the range of frequencies 400 MHZ to 2.76 GHZ the maximum value of the heating potential occurs at the center suggesting the possibility of a hot spot. A comparison between the multilayer and a single layer model shows that the maximum values of the heating potential are the same. Manuscript received February 18, 1975; revised May 27. 1975. This work was supported by the Office of Naval Research under Contract N00014-67-A-0216-0015, the National Institutes of Health under Grant 5 RO1 HL01253, and the National Science Foundation under Grant GK 40119. The authors are with the Electromedical Division, University of Pennsylvania, Philadelphia, PA.
COMMUNICATIONS
169
TABLE I
Contributor
Model
Frequency
H. Schwan H. Kritikos/H. Schwan C. Johnson et al./ A. Guy J. Lin et al./A. Guy/ C. Johnson A. Shapiro et al. C. Weil Present contribution
100 MHZ -10 GHZ
multilayer plane model homogeneous sphere homogeneous sphere radii 3 cm and 6 cm homogeneous sphere radius 25.6 cm multilayer sphere radii 9 cm and 3.3 cm 100 MHZ to 10 GHZ multilayer sphere radius 6 cm 50 MHZ to 10 GHZ multilayer sphere radii 5 cm and 10 cm multilayer plane model
50 MHz - 10 GHz 918 MHZ and 2450 MHZ 1 MHZ, 10 MHZ, 20 MHZ 3 GHZ
E
K H
AI R
Fig. 1. The multilayer sphere model.
INTRODUCTION
The distribution of the heating potential inside specimens which are exposed to Electromagnetic Fields has been the subject of numerous contributions. It had become apparent from theoretical studies by Kritikos and Schwan [1] and Shapiro et al. [21 that under certain circumstances "hot spots" may be induced inside brain tissues by Electromagnetic Fields of only moderate intensity. Experimental work reported by Johnson et al. [31 with tissue phantoms and with cats supported this conclusion. There was an implication that the present long time exposure standard of 10 mw/cm2 might be somewhat high. However, Kritikos and Schwan [1], [6] pointed out that the development of hot spots is more pronounced for smaller head sizes, and the question arose how to extrapolate from animal experiments to man. For uniform spheres Kritikos and Schwan [ 1], [6] have investigated the formation of hot spots in the frequency region of 50 MHZ to 10 GHZ. Johnson et al. [ 31 reported results for the frequencies of 918 MHZ and 2450 MHZ. Lin et al. [4]
have examined the HF region. For a multilayer sphere, Shapiro et al. [2] have investigated the formation of hot spots for the frequency of 3 GHZ and Weil [5] has examined the region .1 GHZ to 10 GHZ. In this communication the formulation of the hot spots in multilayer spheres in the frequency region of 50 MHZ to 10 GHZ will be reported and a comparison with the plane model and homogeneous sphere will be made. Table I summarizes the reported theoretical contributions. THE MODEL The model is taken to be a four layer concentric sphere shown in Fig. 1. This is taken to be an ideafization of a head with skin, fat, and bone layers covering the brain core. Fat and bone have somewhat similar dielectric properties. So do muscle and brain tissues. Hence, the results of this study apply equally well to the head and other parts of the body where skin and subcutaneous fat cover muscular tissues. In each layer the propagation constant is
170
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1976
TABLE II Region p 1 2 -3 4
Tissue
(eoo)p
Brain Bone Fat Skin
10.00 1.60
2.
(eo - eoo)p
1.60 10.00
THICKNESS
(a00 - GO)p
1.0 0.09 0.09 1.00
55.00 0.155 0.155 0.55
37.00 3.40 3.40 37.00
RADIUS OF SPHERE
E
(Go)p
mho/m
mho/m
A-IOcm
SKIN Imm FAT-BONE 3mm
z Z
z
0
2.0.
w
En
(n
Cz
Ln 0
1.5-
EL
0
U,
m
c]
MAXIMUM HEATING OCCURS ALWAYS AT FRONT SURFACE
1.0.
w z
it
IL
z
w 0 w
N
.5. SKIN
0
z
BRAIN
^-BRAIN PLANE SLAB SKIN PLANE SLAB
20
30
40
50 60
80
100
200
.I 300
It
1i
400 500 600 800 1000
FR EQUENCY IN MHz
5
2000
3000
5000
Fig. 2. Normalized differential absorption cross section As vs frequency. The curve labeled skin and skin plane slab indicates As always at front skin surface. Curve labeled brain indicates maximum As in inner brain region. Curve labeled brain plane slab indicates As in the brain of the skull brain interface.
kp
VE
=
+ i
