Fantom malzemelerinin doku denkliğinin deneysel ve  teorik olarak incelenmesi
Experimental and theoretical investigation of tissue equivalency for phantom materials
Yazar: İREM ERK
Danışman: PROF. DR. NESRİN ALTINSOY
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı / Radyasyon Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Radyoloji ve Nükleer Tıp = Radiology and Nuclear Medicine

Özet:
Radyasyonun endüstri, tıp ve bilim alanlarında kullanımının hızlı artışı, radyasyon dozimetrisinin önemini her geçen gün arttırmaktadır. Doku eşdeğeri malzemelerden üretilen fantomlar, dozimetri çalışmalarının temelini oluşturmaktadır. Malzemelerin doku eşdeğeri olduğunu belirlemek için fiziksel ve radyolojik bazı kriterleri sağlamaları gerekmektedir. Bu kriterler arasında malzemenin elektron yoğunluğu, etkin atom numarası ve kütle zayıflatma katsayısı sayılabilir. Bu çalışmada fiziksel yoğunluğu yumuşak dokuya yakın olan malzemeler (su, RW3, silikon, akrilik ve parafin) seçilerek radyolojik özellikleri deneysel ve teorik yöntemlerle tayin edilmiş ve ICRU'nun 44 numaralı raporunda verilen yumuşak doku ile uyumu irdelenmiştir.
Çalışmada fantom malzemelerinin lineer zayıflatma katsayıları dar demet geometrisi kullanılarak gama geçirgenlik tekniği ile bulunmuştur. Deneylerde 662 keV enerjili gama fotonları yayan Cs-137 radyoizotopu ve 1173 keV ile 1332 keV olmak üzere iki farklı enerjide gama fotonları yayan Co-60 radyoizotopu kullanılmıştır. Gama ölçümleri, NaI (Tl) sintilasyon dedektörü ve çok kanallı analizörden oluşan dijital gama spektrometre sisteminde gerçekleştirilmiştir. Fantom malzemelerin her bir kalınlığı ve 3 farklı gama enerjisi için ayrı ayrı ölçümler alınarak malzeme tarafından zayıflatılmış radyasyon şiddeti değerlerine ulaşılmıştır. Her kalınlık ve enerji için alınan ölçümler en az 3 kere tekrarlanmıştır. Deney düzeneğinde farklı malzeme kalınlıklarından alınan sayımlar, kaynak dedektör arasında malzeme olmadan alınan ilk sayımlara oranlanarak bağıl sayım sonuçlarına ulaşılmıştır. Orjin8 çizim programı kullanılarak bağıl sayım değerlerinin malzeme kalınlığı ile değişimini veren grafiklerden lineer zayıflatma katsayıları elde edilmiştir. Malzemelerin etkin atom numaraları ve elektron yoğunluklarını tespit etmek için XMuDat bilgisayar programından yararlanılmıştır. Deney sonuçlarını sınamak amacıyla XCOM, XMuDat ve GATE Monte Carlo programlarıyla fantom malzemelerinin ve ICRU yumuşak dokunun kütle zayıflatma katsayıları bulunmuştur.
Deneysel ve teorik sonuçların mutlak fark yüzdeleri değerlendirildiğinde, su ve RW3 için deney ve teorinin uyumlu olduğu ancak silikon ve parafin için mutlak farkların arttığı gözlenmiştir. Malzemelerin radyolojik özellikleri (etkin atom numarası, elektron yoğunluğu, kütle zayıflatma katsayısı) kullanılarak ICRU yumuşak doku ile denklikleri irdelendiğinde, çalışılan enerji değerleri için suyun yumuşak dokuya en yakın malzeme olduğu tespit edilmiştir.

Abstract:
Radiation is used  in many areas such as medicine, industry, agriculture etc. On the other hand it can cause damage to matter, particularly living tissue. Protection against ionizing radiation requires information on the absorbed doses in organs of the human body. Applications of dosimetric materials and tissue substitutes in radiological protection, medical, radiation physics and radiobiology are essential for exposure monitoring and estimation of the dose.
The basic principle of radiation protection, optimisation, also known as ALARA (as low as reasonably achievable) states that it is necessary to ensure minimal exposure to patients and staff while maintaining a good quality of diagnostic imaging. For optimisation studies, it is necessary to periodically monitor radiation doses received by patients. Physicists developed phantoms to simulate patients in order to make dosimetric measurements and to test the limitations of their systems.
The selection of the appropriate materials is critical to the design and function of any type of phantom. In most cases, a phantom is meant to simulate some form of tissue, such as muscle, bone or lung. The simulated tissues have different properties, both physically and radiologically. The goal of the phantom materials is to represent these physical and radiological properties as accurately as possible. The radiological properties of a material are often highly dependent on the energy of the radiation incident upon it. Thus, a material may accurately simulate a tissue in a given energy range, but it could differ significantly in other energy ranges. 
There are a number of properties that can be used as a measure of the tissue equivalence of a phantom. The physical density (ρ) and effective atomic number (Zeff) can both be used as relatively crude assessments of a materials tissue equivalence. While these parameters provide insight into the physical properties of the material in question, they do little to describe the materials radiological properties. The electron density (ρe) of a material is a more detailed parameter that provides more insight into how a material will behave in a radiation field. The most widely used parameter to gauge tissue equivalence is the mass attenuation coefficient (µmass). 
The linear and mass attenuation coefficients for different materials such as alloys, biological samples, building materials, compounds, glasses, and soils were reported in many researches. Most of the obtained experimental data have been compared with the theoretical tabulations obtained using the XCOM program and there have been few attempts to apply Monte Carlo calculations for the determination of attenuation coefficients. 
Monte Carlo simulation is an effective tool to calculate radiation interaction parameters in different types of compounds and mixtures for shielding and energy deposition in human organs and tissues. Use of this method requires knowledge of the
chemical or elemental composition and density of the material and its physical characteristics as input information for performing these calculations. 
GATE (Geant4 Application for Emission Tomography) is an open source of Monte Carlo simulation program developed by the OpenGATE collaboration. GATE is a simulation platform improved for modelling planar scintigraphy, Single Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET). In addition to this not only modelling X-ray computed tomography and radiation therapy experiments feature but also dose calculation, simulation speeding up and optical physics modelling features are implemented to new version of GATE. 
In this study, gamma ray transmission measurements have been used to evaluate the five different phantom materials. Phantom materials considered in this study are water, solid water (RW3), silicon, acrylic and paraffin which are close to soft tissue density.  The mass attenuation coefficients were determined from the experimental study and compared with the GATE V6.2 code and computer programs (XCOM and XMuDat) results. 
In the experiments, Cs-137 and Co-60 gamma radioisotopes were used as gamma radiation sources. Cs-137 has a single gamma peak at 0.662 MeV and 30.1 years half life. Co-60 has two gamma peaks at 1.17 MeV and 1.33 MeV. Therefore Co-60 includes pair production (E > 1.02 MeV). Co-60 has 5.27 years half life. Gamma rays were detected by Canberra Model (802-2x2) NaI scintillation detector and digiBASE model PMT with integrated bias supply, preamplifier and digital multichannel analyzer, which was supplied with MAESTRO-32 MCA Emulation software combined system. The couting time was 15 min and were carried out three times for each material.
Gamma transmission technique is based on detection of radiation intensities with and without the material The gamma source and the detector are placed on opposite sides of the material on same axis. Experimental set up was prepared carefully to minimize scattering effects. The collimator which has 7 mm diameter hole was used to get narrow beam geometry. The detector was placed 10 cm away from the gamma sources. The detector counts the initial intensity (I0) which emitted from the source without material. Then, for all each material, the material is placed between the source and the detector, and the gamma ray intensities (I) are detected. 
The counted intensities are compared with the initial intensity to compute relative intensity (I/I0). Relative intensity values were carried out for the phantom materials at different thicknesses. Then relative intensity material thickness graphs were drawn. The linear attenuation coefficients were calculated from the graphs by using Origin 8 computer program. The mass attenuation coefficient (μmass) is obtained by dividing linear attenuation coefficient (μ) by the density (ρ) of the material.
Effective atomic numbers and electron densities of the materials were calculated from XMuDat program.  XMudat computer program is able to produce a single-valued effective atomic number data for compounds as well as mixtures. 
The theoretical mass attenuation coefficients of the materials were calculated from GATE V6.2 simulation code, XCOM and XMuDat computer programs. The physical properties of the experimental setup and the chemical properties of the materials were entered into the GATE V6.2 code and the relative count results were obtained. Theoretical  and experimental  results were evaluated and interpreted with  each other.
Theoretical and experimental results were evaluated and interpreted with each other. The differences between experimental and theoretical mass attenuation coefficient results were at their lowest for water and solid water (RW3). But the differences are increasing for silicon and paraffin materials.  This can be explained with material impurities and production conditions of these materials. 
Furthermore, soft tissue equivalency of the phantom materials were investigated in this study.  For this purpose, mass attenuation coefficients, effective atomic numbers and electron densities of the phantom materials and that of ICRU 44 soft tissue were evaluated. According to the results water has the most consistency with ICRU 44 soft tissue.