Hi Spitz
From your clarification I conclude that it would be useful to start with
SEARCH TERM: extinction coefficient
e.g. this
http://en.wikipedia.org/wiki/Extinction_coefficient
You do not need to understand all the equations there, but this one
dI/dz = -alfa * I
Beer-Lambert law is essential (and not difficult to get).
It says that Intensity of electromagnetic wave (emg) in media decreases
according to exponential curve (same rule applies for light as for radio frequency)
That curve is shown here under heading
Exponential decay of intensity
http://www2.bioch.ox.ac.uk/~oubsu/ebjknight/tirtheory.html
The curve shows decrease of intensity with depth of penetration. Article also
describes other factors, angle of incidence, polarization, etc.
As I briefly mentioned before in my RFC, question 'how deep it
penetrates' depends on what is considered 'negligible intensity'.
Exponential never reaches zero: for function y= 10^(-x) we have
x y
2 .01
5 .00001
10 1.e-10
etc. So, if we consider .00001 to be 'really zero' penetration would be 5
but for some super sensitive effect it can be 10. So, to determine
penetration depth the threshold need to be specified. If effect of the
emg is heating, then threshold would be estimated from assuming
increase of 2 degree C to negligible.
Here, the length of exposure and also particular organ may be
relevant, as ability to carry away the heat (density of blood vessels)
will play a role in total
increase in temperature. (see BHP example below).
So, general conclusion is with present specification, you cannot
expect exact numbers.
Some specific numbers for different organs and frequencies are here:
http://privatewww.essex.ac.uk/~mpthak/A%20tour%20through%20the%20interaction%20between%20humans%20and%20electromagneti/node8.html
Note below Fig 2 and 3
' Between 10MHz and 1GHz both the quantities are complex. In the
GHz-THz band, a complex conductivity implies both significant
attenuation (from the real part of the conductance) and scattering of
EM waves with a gradual transition from attenuation to scattering as
frequencies increase. In the band between 10MHz and 1GHz, with both
the quantities being complex, it is difficult to solve the Maxwell's
equations to determine behavior because of the multiple cross terms. A
numerical simulation is probably an easier alternative...'
shows that in 'your range' behavior is complex (both absorbtion and transmission).
This work quotes authoritative source:
http://chppm-www.apgea.army.mil/DOHS/pgm24/Website/RF1.HTM#Direct
However, short duration exposure to RF-induced thermal load will
usually not cause damage and the heat will be dissipated. For this
reason, RF radiation exposure is not cumulative, unlike ionizing
radiation exposure. The biological effects of RF radiation are
thoroughly treated in textbooks such as CRC Handbook of Biological
Effects of Electromagnetic Fields.20-21
That handbook is expensive, but available in most college libraries.
http://www.amazon.com/Handbook-Biological-Effects-Electromagnetic-Fields/dp/0849306418
This critical report qutes considerable bibliography
http://www.brooks.af.mil/AFRL/HED/hedr/reports/bioeffects/1-0.htm
By the way 434mhz really means 434 milli Hertz, I assume you mean
434 MHz 434 Mega Hertz - part of FM radio spectrum (shown here)
http://www.emctech.com.au/emr/default.htm
Capitalization is relevant in SI units.
http://en.wikipedia.org/wiki/Hertz
I want to mention in conclusion one specific medical application of emg induced
hyperthermia . When temperature in a tissue exceeds 45C cells start dying and here
the duration of exposure is ,of course, significant. Typical treatment for BHP
lasts about 20 minutes.
Heating experiment using agar phantom showed the hot spots to be
distributed at 0.5-3 cm from the catheter surface. Heating experiment
using canine prostates demonstrated that an intraprostatic temperature
of > 48 degrees C could be achieved while the urethral and rectal
temperatures had not exceeded 36 and 40 degrees C respectively.
Histological examination immediately after the experiment showed the
urethral mucosa to be preserved while coagulation necrosis of the
periurethral prostate accompanied with congestion and hemorrhage of
small blood vessels were observed at 5-8 mm from the urethra.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7594814&dopt=Abstract
and image
http://www.images.md/users/explore_chapter.asp?ID=ACU0203-04&colID=ACU0203&coltitle=Prostate
Whole range of frequencies, from radio to microwave to UV can be used, as effect
depends on induced temperature, not the frequency itself.
Here is a bibliography of recent scholarly articles
http://scholar.google.com/scholar?q=Prostate+Electromagnetic+and+thermotherapy+mW&num=20&hl=en&lr=lang_en&client=opera&scoring=r&as_ylo=2001
I will stop here since I do not know which way to focuse. You can
ask for clarification or (preferably) post another question more still
more specific.
Are you interested in thermal ablation described above, for therapy, safety , ??
If you want me to continue the search, you may add 'for Hedgie' to
your second question. That method is preferable to me guessing the
cost of more detailed search. The more background of your interest and
more details (sketch of the
transmitter and application) outside of body, on the skin, inside the body, ...
and purpose you can describe, the easier it is to find relevant citations.
Hedgie |
