Wireless Communication with Medical Implants: Antennas and Propagation

173 Pages·2004·3.03 MB·English

Wireless Communication with Medical
Implants: Antennas and Propagation
Anders J Johansson
June 2004
ii PREFACE
Abstract
With the increased sophistication of medical implants, there is a growing need
for flexible high-speed communication with the implant from outside the body.
Today the communication is done by an inductive link between the implant
and an external coil at a low carrier frequency. Extended range and commu-
nication speed are possible to achieve by increasing the carrier frequency and
the bandwidth. One frequency band that is available for this application is the
newly standardized 400 MHz MICS band, which has the benefit of being re-
served mainly for medical and metrological applications. In addition, the 2.45
GHz ISM band is a possibility, but has the drawback of being heavily used by
other applications, such as wireless computer networks and microwave ovens.
In order to assess the usability of wireless communication with medical im-
plants, we have investigated the design of implantable antennas to be used in
the body. Both theoretical limits and practical designs of the antennas are de-
scribed. The SAR levels of the implanted antennas have been calculated and
havebeenfoundtobeatasafelevel. Wehaveinvestigatedthewave-propagation
from the implanted antenna to the outside, and its dependence on the position
of the patient’s limbs and the size of the body. Full wave 3D-simulations of the
wave propagation are feasible, as the radio link between the patient and a base
station placed in the same room is very short in terms of wavelengths in the
MICS band. We have simulated the wave propagation in a furnished room and
compared the results with measurements of the same room. The results from
theseinvestigationsareevaluatedintermsoftheirimpactonthelinkbudgetfor
aprototypeMICSsystem. Fromthesecalculationsconclusionsonthenecessary
complexity of the transceivers are drawn, such as the need for both spatial and
polarization diversity to fully exploit the potential of the communication link.
iii
iv ABSTRACT
Acknowledgements
Without a lot of people this thesis would never have come to be. To name you
all and not to forget anybody is the hardest task I have in writing this thesis.
It is not only hard, I think it is impossible.
IwillbeginwiththankingYou,thereaderof thisthesis. Mostprobablyyou
are the one that, at one crucial point, gave me inspiration for yet another day
of pushing the boundaries of knowledge a little bit further out.
But still a few have to be named.
Ireallymustthankmyadvisors,ProfessorsAndersKarlssonandOveEdfors,
for their time, patience and inspiration.
Mymasterthesisstudents(inorderofappearance)LuzPicassoBrun,Patrick
Jansson,MartinKvistholm,VangelCukalevskiandMagnusSöderbergformak-
ing some of the more tedious parts of the research easier for me.
St. Jude Medical for their involment in the project and their financial sup-
port which made this project possible.
And here I could continue with a number of pages with names, but I will
refrain. They would have included, in no particular order:
Everyoneatthedepartment. Youhavegivengroundsupport,companionship
in despair and inspiration to go on. Thank you.
Myfamilyandallofmyfriends. WithoutwhomIwouldneverhavefinished
this task. And even more important: not even started it. Thank you.
TobeabletowriteaPhD-thesisistohavetravelledalongalongandwinding
road. Towritetheacknowledgementistotrytotellwhichsteppingstonesalong
the way that were the most important ones. Which is pointless, as they were
all used.
Thank You All.
Anders J Johansson
Lund
May 2004
v
vi ACKNOWLEDGEMENTS
Preface
This research has been performed within the Competence Centre for Circuit
DesignatLundUniversity. IthasalsobeensupportedbySt. JudeMedicalInc.
in Järfälla, Sweden. The workhas been done in cooperation between theRadio
Systems Group and the Electromagnetic Theory Group at the Department of
Electroscience at Lund University.
My main contributions to the field are the investigations of antennas for
medical implants, the simulations of theperformance of such an antenna in dif-
ferent body shapes and arm positions, and the simulation, measurement and
analysis of thespatiallyvariation of the400MHzchannel in anindoor environ-
ment. Fromtheseresults, thelinkbudget of amedical telemetrysystemcanbe
estimated, and some conclusions about the necessary complexity of the system
can be drawn. Furthermore, I have developed a hybrid model that facilitates
the formulation of tissue simulating liquids. Other data is taken as common
knowledge within the field, and is not referenced.
I have had great help from master thesis projects, which I have formulated,
specified and supervised, and which have helped me carrying out some parts of
the project.
Papers which are accepted or submitted :
Johansson, A. and Karlsson, A. ”Wave-Propagation from Medical Im-
•
plants - Influence of Arm Movements on the Radiation Pattern”
Proceedings of Radiovetenskaplig konferens (RVK’02), Stockholm, Swe-
den, 2002
Johansson,A.J,”Wave-Propagation from Medical Implants - Influence of
•
Body Shape on Radiation Pattern”
ProceedingsoftheSecondJointEMBS/BMESConference,Houston,TX,
USA 2002.
Johansson, A. J, ”Simulation and Verification of Pacemaker Antennas”
•
Proceedings of the 25th EMBS Conference, Cancun, Mexico 2003.
Johansson, A. J, Picasso, L. B. and Jansson, L. J. P. ”Indoor Wave-
•
propagationinthe403.5MHzMICSBand: simulationsandmeasurements”
Submittedforpublication. IEEETransactionsonBiomedicalEngineering.
vii
viii PREFACE
Johansson, A. J, ”Comparison between the MICS Standardized Phantom
•
and an anatomical Phantom”
Submittedforpublication. IEEETransactionsonBiomedicalEngineering.
Johansson, A. J, ”Performance of a Radiolink Between a Base Station
•
and a Medical Implant Utilizing the MICS Standard”
Submitted for publication. 26th EMBS Conference, SF, USA, 2004
Contents
Abstract iii
Acknowledgements v
Preface vii
1 Introduction 1
1.1 The pacemaker . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Existing communication methods . . . . . . . . . . . . . . . . . . 2
1.3 Radio communication . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3.1 Hospital checkup . . . . . . . . . . . . . . . . . . . . . . . 2
1.3.2 Home care. . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Telemedicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.5 Other implants . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.6 Percutaneous connections . . . . . . . . . . . . . . . . . . . . . . 4
2 Communication Methods 5
2.1 Electromagnetic methods . . . . . . . . . . . . . . . . . . . . . . 5
2.2 MICS standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 2.4 GHz ISM band . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Acoustic link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5 Optical link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.6 Phantoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Link Budget I 13
3.1 Fading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 ITU-R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.1 Uplink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.2 Downlink . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Wave Propagation into Matter 17
4.1 Maxwell’s equations . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1.1 Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2 Material data and measurements . . . . . . . . . . . . . . . . . . 19
ix
x CONTENTS
4.2.1 Tissue data . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2.2 Simulated Tissues . . . . . . . . . . . . . . . . . . . . . . 20
4.3 One-dimensional FDTD simulations . . . . . . . . . . . . . . . . 21
4.4 Analytic investigation of a layered structure . . . . . . . . . . . . 25
4.5 Two-dimensional simulations . . . . . . . . . . . . . . . . . . . . 27
4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5 Antenna Design 31
5.0.1 What is the antenna? . . . . . . . . . . . . . . . . . . . . 32
5.1 Antenna efficiency calculations in matter . . . . . . . . . . . . . . 32
5.2 Antennas in matter . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.3 Implantable antennas. . . . . . . . . . . . . . . . . . . . . . . . . 39
5.3.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.3.2 Wire antenna . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.3.3 Circumference antenna. . . . . . . . . . . . . . . . . . . . 48
5.3.4 Circumference plate antenna . . . . . . . . . . . . . . . . 52
5.3.5 Circumference PIFA . . . . . . . . . . . . . . . . . . . . . 56
5.3.6 Patch antenna . . . . . . . . . . . . . . . . . . . . . . . . 57
5.3.7 Magnetic antenna . . . . . . . . . . . . . . . . . . . . . . 63
5.4 Dependence on insulation thickness . . . . . . . . . . . . . . . . . 64
5.5 Dependence on surrounding matter . . . . . . . . . . . . . . . . . 65
5.6 SAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6 Influence of Patient 69
6.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.2 Gain variation from movement of the arms . . . . . . . . . . . . 70
6.3 Gain dependence on body size and shape . . . . . . . . . . . . . 81
6.4 Circumference antenna in phantoms . . . . . . . . . . . . . . . . 82
6.5 Validation of MICS phantom . . . . . . . . . . . . . . . . . . . . 84
6.5.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . 93
6.5.2 Placement sensitivity. . . . . . . . . . . . . . . . . . . . . 95
6.6 Linear polarization . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.8 Comments on commercial layered numerical phantoms . . . . . . 99
7 Channel Modelling 101
7.1 Wave propagation . . . . . . . . . . . . . . . . . . . . . . . . . . 101
7.1.1 Measurements in the MICS band . . . . . . . . . . . . . . 101
7.1.2 Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.1.3 Test of stationarity . . . . . . . . . . . . . . . . . . . . . . 103
7.2 Measurement results . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.2.1 Empty Room . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.2.2 Furnished Room . . . . . . . . . . . . . . . . . . . . . . . 108
7.3 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113