> At the recent ISMRM meeting in Sydney, a few talks and posters were
> presented on the issue of SAR in high field systems.
> Some were simulations of the RF field distribution in the human body and
> paticular in the head.
> The simulations assumed (to the best of my knowledge) that particular organs
> (e.g. brain) have a particular permitivity and conductivity as a whole
> organ.
> I find this assumption to slimple.
It is not slimple..... it is complex ( r -j ).The wavelength of an
electromagnetic wave ( even in the near zone) is shortened by the square root of
the dielectric constant in reference to the air or vacuum. This is the same as
an index of refraction. The dielectric constant of water is about 70 . This
means the wavelength will be shortened by about a factor of 8. This takes the
quarter wavelength down at 4 Tesla to the dimensions of the organs in the human
body. There is a reactance. Energy is stored in both the magnetic and electric
field, it also takes time to propagate. There is a phase difference. Energy is
reflected .There are dielectric resonances. The computer simulations do match
the distortions of the RF B1 distribution observed by imaging. The induced RF
magnetic field is distorted
and there are standing waves. We have made reasonably efficient imaging coils by
filling a 25 CM diameter
fiberglass tube about 15 cm high with D2O and driving it with a small dipole. No
metal just basically bucket of water with uniform B1 distribution and a loaded Q
of 30. The B1 followed all TEM modes for cavity resonators. "Dielectric
Resonators" Artech House Publishing
> Each organ consist of individual cells .
Each dielectric consists of individual moleculesThey behave in bulk as a single
dielectric with a high loss. The resonance frequency and resonant
effects standing waves are established at the boundary / edges of the organs.
This has been fairly well characterized with numerous studies about
electromagnetic fields
and biological tissues.
" Antennas in Mater" R.P.King and
"Biological Effects of Electromagnetic Radiation"
IEEE Press J.M.Osepchuck
> with changes in conductivity and pemitivity at the boundary of each cell.
> A "standing wave" can only propagate in a homogenous medium (which is true
> for the assumption of an organ as a whole volume of constant permitivity and
> conductivity).
>
The Evaluation of Dielectric Resonators Containing H2O or D20 as Rf Coils for
High Field MR Imaging and Spectroscopy. Journal Of Magnetic Resonance B Vol 110
-01-07, 1996A.Scott Chesnick, Han Wen, R.S. Balaban,
> What would be a suitable phantom for descent SAR measurements ???
One that matches the average dielectric of the human body ~About 38
Regards
Scott Chesnick
NIH
Bethesda, MD