Transcript Diapositiva 1 - Centro Ricerche Frascati — it

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ABSORPTION AND DIFFUSION MEASUREMENT OF BIOLOGICAL
SAMPLES USING A FREE ELECTRON LASER
M. D’Arienzo, A. Doria, G.P. Gallerano, E. Giovenale, A. Lai, G. Messina, D. Piccinelli
ENEA C.R.Frascati, Via Enrico Fermi 45, 00044 Frascati (Italy)
Motivations of the Experiment
ENEA is involved in a large EU project, called THz-BRIDGE. Objective of the project is to set-up a spectroscopic database of biological materials in the THz region, test THz irradiation effects on biological
samples in vitro and provide safety procedures for people exposed to THz radiation.
So there is a great interest in investigating the optical properties of biological samples in the THz region, both to build a spectroscopic database and to obtain parameters that will allow us to correctly
calculate the absorbed dose when irradiating biological samples with THz radiation. Absorption Measurements have been carried out on Whole Human Blood, Serum, Plasma, Water, Saline Solution, Culture
medium, Lymphocytes, and other biological samples.
Theoretical calculations have been carried out to model the interaction of THz radiation with scattering elements within the various samples. In particular scattering of 120 GHz radiation with lymphocytes
and liposomes has been studied.
The Source
For our measurement the ENEA Compact FEL was utilized. ENEA FEL emits around 120 GHz
with a peak power exceeding 10 kW. Emission parameters are reported below.
The ENEA Compact FEL produces a "train" of micropulses of about 50 ps duration, with 330
ps spacing between adjacent pulses. The overall duration of the train (macropulse) is
several microseconds. Macropulses can be generated up to a maximum repetition frequency
of 20 Hz.
A Free Electron Laser (FEL) has been selected as a source to perform the transmission
measurements on biological samples. Advantages of FELs are the ability of producing radiation at
the desired wavelength, wide range tunability and high power emission.
In a FREE ELECTRON LASER radiation is produced
by a relativistic electron beam traveling through a
magnetic structure (undulator). FELs can produce
very short high power laser pulses. Emission
frequency is ruled by the electron beam energy,
the undulator parameter K (proportional to the
magnetic field) and the spatial period of the
undulator:
FEL EMISSION PARAMETERS
Electron beam energy: 2.3 - 5 Mev
Spectral Range: 90-150 GHz
Bandwidth : 7 %
Maximum peak power : 10kW
Average peak power (4 ms): 1.5 kW
CW power: ~ 30 mW
u
2
  2 (1  K )

It is then easy to realize a THz FEL, utilizing a low energy (few MeV) electron beam. Exploiting
new peculiarities of coherent radiation emission it is possible to realize a broad band FEL THz
radiator.
The Sample
Absorption Measurements have been carried out on Whole Human Blood, Serum, Plasma, Water, Saline Solution, Culture medium, Lymphocytes, and other biological samples. Absorption
measurements have been carried out also on materials to be used to hold the biological samples, like polystyrene and polyethylene. Polystyrene has been found to exhibit good optical properties in
the spectral region around 120 GHz, and was thus selected as material for spectroscopic cells
P o ly s t y r e n e ( d = 1 .1 6 5 m m )
3 .0
1.0
0 .0 1 0
Transmission
0.8
0 .0 0 8
2 .5
"
0.6
'
0.4
Radiation generated from the FEL was transported into
the control room by a light pipe and was delivered to the
biological sample, contained into a polystyrene Petri dish.
The transmitted radiation impinges on a beam splitter
and is finally collected by a pyroelettric detector.
'
T=300K
2 .0
0 .0 0 6
"
0 .0 0 4
1 .5
0.2
Polystyrene
d = 1.18 m m
0.0
0
10
10
0 .0 0 2
1 .0
1
10
2
10
3
10
0
4
2
4
6
8
10
12
F re q u e n c y (c m
-1
Frequency (cm )
-1
14
16
18
)
Transmission through polystyrene dishes and optical properties of polystyrene in the THz region.
Absorption Measurements
Serum
Whole blood
0,1000
0,0100
0,0010
Absorption coefficient values are close to the absorption
coefficient of water at room temperature. The value of
the absorption coefficient measured for whole blood
shows that less than 1% of the incident radiation
penetrates through 1 mm thickness.
Although weak scattering by blood cells does not cause a significant displacement from the exponential attenuation
law, it can be considered responsible of the difference in transmission between whole blood and physiologic solution.
0,0001
0,00
0,05
0,10
5
10
Absorption coefficient
4
10
3
10
-1
•Culture medium (not shown in the graph):  = 83 cm-1
•Saline Solution:  = 79 cm-1
•Whole blood:  = 75 cm-1
•Serum:  = 71 cm-1
Phys. Sol.
 (cm )
28 June 2001
file :b io w a ll.o p j
1,0000
2
10
1
10
0
10
-1
10
0
10
1
10
0,15
nominal thickness (cm)
• …which are the optical properties of lymphocytes.
• …if there is any resonant absorption at 120 GHz
• …which are the absorbing properties of lymphocytes.
• …which are the scattering properties of lymphocytes
(diffusive regime or ballistic?)
Diffusive regime
Ballistic regime
Lymphocytes in blood can be considered as scattering elements within an absorbing liquid solution (plasma).
The high absorption of the water content in the plasma (95% in volume) “hides” the optical properties of the
single particles. Thus, there is a need to find a THz transparent liquid solution as surrounding medium.
Alcoholic solutions showed weakly absorbing properties in the THz range of frequency.
Calculations for lymphocites in water
We can calculate the Rayleigh scattering and absorption cross sections
of lymphocytes in 2-propanol and in blood, considering for the latter
one the same optical properties of water (which is basically made of) :
 scatt  F
 abs

4
c4

F V
c
16
2
V 2  scatt  3 10 cm
 abs  2 108 cm 2
10
to define an upper limit to the value of Lymphocytes
absorption cross section), so we can deduce that there
is
NO
RESONANT
ABSORPTION
OF
LYMPHOCYTES AT 120 GHz
0
1
6  10 5
2-Propanol (or Isopropyl alcohol) presented the lowest absorption
coefficient: (4.5±0.3) cm-1
The survival time of lymphocytes in 2-propanol has been measured and it
resulted to be about 15 minutes.
Thus we found that scattering is almost absent in the process.
All the electromagnetic energy is absorbed by the particle.
The absorbing properties are described by the complex
dielectric constant of the particle
1
mL
600
1
mm 3
BALLISTIC REGIME
Since the absorption coefficient is related to the
absorption cross section by the equation:
  N  abs
We can use the upper limit of a (0.5 cm-1 ) to estimate
the experimental absorption cross section of the process
 abs  8.5  107 cm 2
This value is very close to the geometric cross section,
being the medium radius of a lymphocyte 5 m m:
 abs  7.8  107 cm 2
Density of lymphocytes in 2-propanol:
0
200
400
600
800
1000
1200
Sample width (mm)
Theoretical approach: Mie simulations
Zone of interest for
lymphocytes at 120 GHz
1.5
CROSS SECTION (mm^2)
p r
2
Mie theory has been used to calculate scattering and absorption cross sections for
lymphocytes in blood and in propanol. Lymphocytes exhibit small scattering and absorbing
properties at 120 GHz, where the Mie parameter for lymphocytes is about 0.03. Higher
scattering contributions at shorter wavelengths is expected.

10
2
REDUCED CROSS SECTION
Transmission
10
4
10
CALCULATION PARAMETERS
• Lymphocytes radius ~5 mm
• 2-Propanol refractive index at 120 GHz ~2.3-0.53i
(dielectric constant ~1.4-1.5i, ENEA measurement)
• Water refractive index at 120 GHz ~3.4-1.9i (dielectric
constant ~9-1.4i)
Absorption coefficient measurements:
•2-Propanol: = (4.5±0.5) cm-1
Within the experimental error
it’ s impossible to
•2-Propanol + lymphocytes:  = (5±0.5) cm-1 distinguish the two curves (and thus it is only possible
3
Water absorption spectrum
Optical properties of Lymphocytes
What kind of information do we expect to find out from the experiment?
2
10
10
-1
Frequency (cm )
Extinction
Absorption
Scattering
1
0.5
0
0
0.5
1
1.5
2
2.5
2p r

REDUCED CROSS SECTIONS FOR LYMPHOCITES IN 2-PROPANOL
MIE PARAMETER
3
10
CONCLUSIONS
•
•
•
6
Extinction
Absorption
Scattering
7
5x10
0
0
•
Optical properties of blood are similar to water (high absorption at
120 GHz where rotational and vibrational states are excited).
Scattering contribution to extinction coefficient at 120 GHz is
negligible (evident exponential decay of transmitted signal).
Lymphocytes present poor absorption of radiation (low imaginary
part of the refractive index). In particular there are no resonances
around 120 GHz.
These results can be applied to the dose calculation when
irradiating human blood with THz radiation
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
RADIUS OF THE LYMPHOCITE (mm)
CROSS SECTION FOR LYMPHOCYTES IN BLOOD
This work has been carried out with financial support from the Commission of the European Communities, specific RTD programme
“Quality of Life and Management of Living Resources”, Key Action 4 “Environment and Health” - contract QLk4-2000-00129.