Rigorous dosimetry is a
key issue for ensuring the validity of results within a given RF study and
consistency of results between RF studies. This requires that a
well-characterized standard methodology and suite of tools be utilized for
the provision of dosimetry to research programs that are run within the
Centre. Further to this, concerns have been raised regarding the application
of compliance techniques in RF human exposure standards, based on adult
models, to children, and there are questions as to applicability of current
compliance techniques to the rapidly changing new communications
technologies. Therefore, apart from providing the necessary dosimetry for
ACRBR research programs, we will also undertake an in depth study of
fundamental dosimetry issues.
Current Projects
Specific Absorption
rates (SAR)
Compliance Methods
Project Leader: Mr. Ray McKenzie
NHMRC
Funded staff:
Maia Sauren (PhD student)
Background:
RF
transmitter devices are tested for compliance with exposure standards
through the measurement of deposited RF power in fiberglass human phantoms
filled with a tissue equivalent liquid. Computational modelling is
also rapidly developing into an alternative tool for human exposure
analysis.
Aims: To further advance knowledge in the identified areas and ensure
the highest possible standard of RF dosimetry, this project seeks to test
the accuracy and applicability of current and proposed exposure assessment
techniques for humans of all shapes, sizes, and tissue content.
The main topics considered will
be:
(i)
evaluation of measurement protocols, as
cited in regulations both nationally and internationally, given the
protocols differ from each other in a number of ways;
(ii)
review of the influence of phantom shapes
on energy deposition levels with a comparison of the Standard
Anthropomorphic Mannequin (SAM) phantom cited in many protocols (eg CENELEC
EN50361:2001) with others such as the phantom currently in use at TRL.
Further, analyse the influence of body shape and tissue content through the
comparison of computational human body models such as the Visible Human (US
National Library of Medicine), NORMAN (UK National Radiological Protection
Board), and Utah Man (University of Utah);
(iii)
investigation of the scalability of
results from adults to young teenagers and children (issues include the
change of anatomical features and tissue dielectrics with age, and, for
numerical models, the accuracy of results with changing voxel size);
(iv)
applicability of using homogeneous tissue
phantoms compared with a complex multi-tissue human;
(v)
general sensitivity analysis of the
effect of body size, shape and tissue type and the implications for
standards setting.
Specific hypotheses tested:
 |
What anatomical variations exist in human populations? |
|
Which anatomical variations affect the amount of RF energy absorbed by the
human body, and how? What are the dielectric properties of human tissues
and how important are they for SAR determination? |
|
What is an average human and can it be used as an accurate estimate for
modelling purposes? |
| Is
there a difference in SAR between males and females? Between racial
groups? Between children and adults? Between pregnant and non-pregnant
females? |
|
Should different models exist for various population subgroups? How would
such models be distinguished – by age, race, gender, other? |
| How
is energy absorption in children different to adults? What's the threshold
age between children & adults? |
| Can
children be modelled as small-scale adults? |
|
What approximations can be made in regards to population variations for
mathematical and physical SAR modelling without sacrificing accuracy? |
|
Which is more important – average SAR or maximum SAR? |
| Are
current phantoms an accurate representation of humans for the purpose of
compliance testing? |
| Are
whole-body models any better than head-only or head-and-shoulder ones?
|
|
Should models be homogeneous, i.e. model the body/head as consisting of
one tissue type, or heterogeneous? How much homogeneity can be considered
reasonable? |
| Is
it appropriate to assume that the brain is a 3-sphere model for modelling
purposes and research? |
| How
does SAR correlate to temperature rise in various tissues? |
Relevance to RF Bioeffects Research:
This study will allow faster, more efficient, more
accurate determination of compliance of radiofrequency technology with
existing radiation protection safety standards.
community benefits:
|
known exposure
levels from radio communications equipment use |
|
public confidence
in compliance processes and requirements |
industry benefits:
Methods:
Investigations will initially be limited to SAR inside the human head. Due
to the nature of available computational models based on either complex
realistic models such as Visible Human (Brooks Air Force) or the simple
single tissue SAM phantom model (specific Anthropomorphic Mannequin; IEEE)
this issue is not easily explored since the identified parameters are not
easily varied within the models. To overcome this deficiency, we have
proposed an alternative compromise model which includes a reduced set of
tissues in a semi-homogeneous, simplified geometry for which the key
parameters may be varied parametrically. A review of the current
literature is used to obtain an estimate of variations in a set of key
anatomical parameters affecting SAR - tissue dielectric properties,
thickness, relative location and tissue size. This information is used to
vary the model at the 5th, 50th and 95th percentiles of human anatomical
range for five key tissues: skin, skull, brain, eye and ear.
Plane wave excitation
is used as the source. The study may be expanded at a later stage to
include more tissues and other forms of excitation. Mathematical modelling
is performed using commercially available FDTD (finite-difference time
domain) and methods of moments software packages. Phantom studies will be
included for validation purposes. These will take place at the Telstra
Research Laboratories (TRL) using SAM head phantoms; the TRL whole body
phantom, which contains partitions for head, torso, arms and legs; and an
IEEE P1528 compliant flat torso phantom.
Results:
A simplified model of
the human head has been constructed. The model contains the five tissues of
interest previously noted, the properties of which will be varied over the
range of values for the human population as previously described. The
shapes in this model may be varied parametrically, greatly reducing the
effort required to undertake this extensive modelling task. We have
reviewed some of the available literature on the relevant tissue properties
which we have used to populate the new models. A preliminary study has been
conducted on the effect of adult cranial thickness on SAR (see Sauren et al
2005 and 2006), and work is now proceeding on the effects of skin thickness
on SAR. With further work and validation, it is hoped to develop a more
efficient and accurate model for determining compliance with SAR based
exposure levels for a range of RF devices.
Dosimetry support to other ACRBR studies
Project Leader: Mr. Ray McKenzie
NHMRC funded staff: Teddy
Kurniawan (PhD
student)
Aims: To provide Telstra research
Laboratory
(TRL) dosimetry support to current ACRBR investigations
Methods:
Rodent Stream
In Vitro
This work involves the
exposure of cells and tissue cultures using the TRL designed co-axial RF
exposure chamber. Dosimetry support in this case involves the provision of
signal generation and transmission equipment, chamber refinements and
ongoing dosimetry analysis. An additional study will consider analytic
modelling solutions to determine SAR in very thin layers such as cell
membrane interfaces.
In Vivo
At least some of this
work will involve the exposure of animals (mice or rats) using the Motorola
designed RF exposure carousel currently located at IMVS or other apparatus.
Modification of this apparatus for exposure of different animals or the use
of different exposure apparatus or regimes will require the provision of
some signal generation and transmission equipment and ongoing dosimetric
design and analysis.
Human Stream
RF-Sensitivity and
Hypersensitivity
This work requires the
use of modified mobile phone handsets featuring remote control of power
output by proprietary software, provided by TRL. Exposure assessment will
require computational dosimetry analysis, and, most likely, validation by
physical SAR measurement in one of TRL's phantoms.
RF-Effect Consistency
As for above
Heat Shock Proteins
Molecular Modelling of
RF/HSP Interaction
To validate the
molecular modeling results empirically, exposures of proteins in vitro
at known SAR's will be required to reproduce any potential Heat shock
protein (HSP) interactions predicted by the Resonant recognition model (RRM)
models. This will require provision of and dosimetry design for exposure
apparatus and ongoing dosimetric analysis.
Epidemiology
'Morpheous' Mobile
phone effects in teenage children
This study involves
the investigation of end points in subjects exposed from normal use of their
own handsets (i.e. not a controlled laboratory situation). To estimate
exposure for this study, analysis of actual handset typical exposures will
be conducted on the combined basis of self-reported usage, subscriber
records and handset type. Dosimetry analysis will be undertaken to
determine typical exposure from at least a few handset classes which may
reasonably represent the actual handsets used by subjects in this study.
Additional account will be taken of relevant network parameters relating to
location of use etc. Further validation using the Motorola Hardware and
Software Modified Phones (SMP, HMP) which function as a normal phone but
provide additional data which can be used for RF exposure assessment will
also be considered.
Results: Expected
December 2008