Transcript Measuring the Attenuation Coefficient of Biological Materials

Measuring the Attenuation
Coefficient of Biological
Materials
Sarah DeLeo
Kyle Gautreaux
Shreya Purohit
Eric Richard
Introductions
Many uses for medical physics
– Radiation therapy, x-ray imaging, etc.
Attenuation occurs when photons pass
through a material.
– This value can change as the thickness and
density of different materials change.
– When plotted on a graph, (charge vs.
thickness) the attenuation coefficient can be
found based on the slope of the line.
Purpose:
– The purpose of this experiment was to find
the attenuation coefficient of a biological
material.
Materials tested:
– Polymethyl methacrylate – PMMA (plexiglass)
This was used to simulate water
– Bone (synthetic)
Procedure
CAMD Equipment
Synchrotron
Beam line
CAMD Equipment
“Hutches”
Beam line schematic, with
multiple gates for control.
Experiment Setup
Plates of plexiglass were setup
vertically on a stage
This allows the materials being
tested to move in and out of
the beam current
The readings are being taken
by an ionization chamber,
housed inside a plexiglass
plate
The experiment was run
several times for plates of
different thicknesses until a
desired depth was obtained
Experimental Setup
Experiment was
repeated for
thicknesses of
synthetic bone
The Ionization
chamber then
measures charge
differential created by
the induced current
within the chamber.
Experimental Setup
An Electrometer is
used to measure the
difference in electrical
charge within the
ionization chamber.
Computer uses LabU
program to control the
stage height and the
number of times the
sample is passed
through the photon
current
Synthetic Bone
Plexiglass (Water)
Results
Average Charge (10-8
Coulombs)
Average Current Corrected Charge (Charge
Depth (cm)
(mA)
per 100 mA)
-1.872
128.5
1.456809339
0.5832475
-1.6665
126.5
1.317391304
1.2474575
-1.281
125.125
1.023776224
2.5606375
-0.9445
123.5
0.764777328
3.8592125
-0.526
121.125
0.434262126
6.1750575
-1.7595
119.875
1.467778936
0.5832475
-2.2425
216
1.038194444
0.9191625
-1.654333333
209.5
0.789657916
1.2455525
-1.262
202.375
0.623594812
1.5757525
-0.959
198
0.484343434
1.9123025
-0.817
193.25
0.422768435
2.2501225
Plexiglass
1.6
Charge (10-8 Coulombs)
1.4
1.2
y = 1.7225e-0.2181x
R² = 0.9948
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
Penetration Depth (cm)
5
6
7
Figure 1: Charge (10-8 Coulombs) vs. Penetration Depth in
Plexiglass (polymethyl methacrylate – PMMA). The
exponential trend line shows the exponential decay of charge
with increasing penetration. Attenuation (absorbtion) coefficient
(µ) can be off the formula to be .2181 (1/cm).
-Density = 1.184756 g/cm3
- µ/ρ = 0.184089 cm2/g
Synthetic Bone
1.6
Charge (10-8 Coulombs)
1.4
y = 2.1166e-0.7509x
R² = 0.9843
1.2
1
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
Penetration Depth (cm)
Figure 2: Charge (10-8 Coulombs) vs. Penetration Depth in
synthetic bone. The exponential trend line shows the
exponential decay of charge with increasing penetration.
Attenuation (absorbtion) coefficient (µ) can be off the formula to
be .7509 (1/cm).
Conclusion
µ Plexiglass = .2181 (1/cm)
µ Synthetic Bone = .7509 (1/cm)
The mass attenuation coefficient of the
plexiglass was calculated by dividing the
density of the plexiglass and compared to
the calculated value on the NIST website.
The numbers did not correlate exactly but
there were multiple factors of error that
contributed to this inaccuracy.
http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/pmma.html