Measuring the Attenuation Measuring the Attenuation Coefficient of Biological Coefficient of Biological

MaterialsMaterials

Sarah DeLeoSarah DeLeoKyle GautreauxKyle GautreauxShreya PurohitShreya Purohit

Eric RichardEric Richard

IntroductionsIntroductions

Many uses for medical physicsMany uses for medical physics– Radiation therapy, x-ray imaging, etc.Radiation therapy, x-ray imaging, etc.

Attenuation occurs when photons pass Attenuation occurs when photons pass through a material. through a material. – This value can change as the thickness and This value can change as the thickness and

density of different materials change.density of different materials change.– When plotted on a graph, (charge vs. When plotted on a graph, (charge vs.

thickness) the attenuation coefficient can be thickness) the attenuation coefficient can be found based on the slope of the line.found based on the slope of the line.

Purpose: Purpose: – The purpose of this experiment was to find The purpose of this experiment was to find

the attenuation coefficient of a biological the attenuation coefficient of a biological material.material.

Materials tested:Materials tested:– Polymethyl methacrylate – PMMA (plexiglass)Polymethyl methacrylate – PMMA (plexiglass)

This was used to simulate waterThis was used to simulate water

– Bone (synthetic)Bone (synthetic)

ProcedureProcedure

CAMD EquipmentCAMD Equipment

Synchrotron Beam line

CAMD EquipmentCAMD Equipment

“Hutches”Beam line schematic, with multiple gates for control.

Experiment SetupExperiment Setup

Plates of plexiglass were setup Plates of plexiglass were setup vertically on a stagevertically on a stageThis allows the materials being This allows the materials being tested to move in and out of tested to move in and out of the beam current the beam current The readings are being taken The readings are being taken by an ionization chamber, by an ionization chamber, housed inside a plexiglass housed inside a plexiglass plateplateThe experiment was run The experiment was run several times for plates of several times for plates of different thicknesses until a different thicknesses until a desired depth was obtaineddesired depth was obtained

Experimental SetupExperimental Setup

Experiment was Experiment was repeated for repeated for thicknesses of thicknesses of synthetic bonesynthetic bone

The Ionization The Ionization chamber then chamber then measures charge measures charge differential created by differential created by the induced current the induced current within the chamber.within the chamber.

Experimental SetupExperimental Setup

An Electrometer is An Electrometer is used to measure the used to measure the difference in electrical difference in electrical charge within the charge within the ionization chamber.ionization chamber.Computer uses LabU Computer uses LabU program to control the program to control the stage height and the stage height and the number of times the number of times the sample is passed sample is passed through the photon through the photon currentcurrent

ResultsResults

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

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).

ConclusionConclusion

µ µ PlexiglassPlexiglass = .2181 (1/cm) = .2181 (1/cm)

µ µ Synthetic Bone Synthetic Bone = .7509 (1/cm)= .7509 (1/cm)

The mass attenuation coefficient of the The mass attenuation coefficient of the plexiglass was calculated by dividing the plexiglass was calculated by dividing the density of the plexiglass and compared to density of the plexiglass and compared to the calculated value on the NIST website. the calculated value on the NIST website. The numbers did not correlate exactly but The numbers did not correlate exactly but there were multiple factors of error that there were multiple factors of error that contributed to this inaccuracy. contributed to this inaccuracy. http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/pmma.html