Polimerik malzemelerin radyolojik özelliklerinin belirlenmesi ve tanısal radyolojide fantom malzemesi olarak kullanım yönünden değerlendirilmesi
Determination of the radiological properties of polymeric materials and evaluation of usage as phantom material in diagnostic radiology
Yazar: ERDİ ŞİRİN
Danışman: PROF. DR. NESRİN ALTINSOY
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı / Radyasyon Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Fizik ve Fizik Mühendisliği = Physics and Physics Engineering ; Nükleer Mühendislik = Nuclear Engineering ; Radyoloji ve Nükleer Tıp = Radiology and Nuclear Medicine
Dizin: Fantomlar = Phantoms ; Polimerik yapılar = Polymeric structures ; Radyasyon = Radiation ; X ışını = X ray

Özet:
X-ışınlarının keşfi tıp alanında hastalıkların tanısında önemli rol oynamış ve günümüzde vazgeçilmez hale gelmiştir. Radyasyonun tıp alanına girmesi ile bir çok görüntüleme ve tedavi yöntemi geliştirilmiştir. Bu alanda kullanılan cihazların doğru görüntü verebilmeleri için kalite kontrollerinin doku eşdeğeri malzemelerle yapılması gerekmektedir. Bu tür insan doku eşdeğeri malzemelerle yapılmış ürünler fiziksel fantom olarak adlandırılmaktadır. Fantom malzemelerinin elektron yoğunlukları, efektif atom numaraları ve radyasyonu zayıflatma özellikleri gibi radyolojik özelliklerinin insan dokularına benzemesi ve temsil ettiği dokuya yakın olması beklenmektedir.
Bu çalışmada, polimer malzemelerin tanısal enerji aralığındaki (15-150 keV) radyolojik özelliklerinin belirlenmesi ve doku eşdeğerlikleri yönünden irdelenmesi hedeflenmiştir. Malzemelerin ve ICRP 110 Erkek ve Kadın Yetişkin Hesaplamalı Fantom dokularının radyolojik özellikleri Phy-X PSD yazılımı ile hesaplanmıştır. Ayrıca, BT uygulamaları için önemli bir parametre olan Hounsfield Birimi (HU) değerleri deneysel ve teorik olarak incelenmiş ve literatürle karşılaştırmalı olarak değerlendirilmiştir. HU değerlerinin deneysel olarak belirlenmesinde poliamid (PA6), yüksek yoğunluklu polietilen (HDPE), ultra yüksek moleküler ağırlıklı polietilen (UHMWPE), polipropilen (PP), polivinil klorür (PVC), politetrafloroetilen (PTFE), polioksimetilen (POM), polietilen tereflatat (PET), poliüretan köpük (PU KÖPÜK) ve su ile oluşturulan fiziksel fantom kullanılmıştır. Element içeriği bilinen doku ve malzemelerin HU değerlerinin teorik olarak bulunabilmesi için, ölçülen HU değerleriyle, foton saçılması ve soğurmayı temsil eden ve katsayıları python kodu kullanılarak en küçük kareler yöntemi ile hesaplanmıştır. ICRP 110 Erkek ve Kadın Yetişkin Hesaplamalı Fantom dokularına ilişkin HUteorik değerleri, literatürle uyumlu olarak kemik doku, yumuşak doku ve akciğer doku olarak sınıflandırılmıştır. Doku denkliği incelenirken, yumuşak doku grubundan kas ve yağ doku için değerlendirme yapılmıştır.
Malzemelerin doku eşdeğerlikleri, ICRP 110 Erkek ve Kadın Yetişkin Hesaplamalı Fantom ile mukayeseli olarak değerlendirilmiştir. Radyolojik özellikler ve HU değerleri polimerlerin doku denklikleri yönünden irdelendiğinde, polietilen tereftalat (PET) ve polioksimetilen (POM) spongiosa kemik ile, polivinil klorür (PVC) ve politetrafloroetilen (PTFE) kortikal kemik ile uyum sağlamıştır. Kas doku için poliamid 6 (PA6), yağ doku için polipropilen (PP), ultra yüksek moleküler ağırlıklı polietilen (UHMWPE) ve yüksek yoğunluklu polietilen (HDPE) doku eşdeğeri malzeme olarak tespit edilmiştir. Poliüretan köpük (PU KÖPÜK), akciğer için doku eşdeğeri kabul edilebilecek polimer olarak öne çıkmıştır.

Abstract:
With the discovery of radiation, especially X-rays, and the understanding of its physical properties, nuclear sciences have developed and have been involved in human life in different fields, especially industry and medicine. Many devices and methods such as radiography, fluoroscopy and computed tomography have been developed and their use has become widespread and indispensable today in order to visualize anatomical disorders and diseases in humans with X-Rays. With the introduction of radiation into the field of medicine, many imaging and treatment methods have been developed. Quality controls of devices such as radiography and computed tomography used in this field should be carried out in order to give accurate images.
Device calibration, quality control and dosimetry studies on humans cannot be performed because of the risks and dose limitations of radiation. It is not possible to use live humans for occupational or patient protection and dose measurements, determination of radiological image quality and precise calculation of radiation dose. Therefore, quality controls should be carried out with tissue-equivalent materials. Many computational and physical phantoms have been made with different methods for dose calculations. Products made with such human tissue equivalent materials are called physical phantoms. The radiological properties of phantom materials, such as their electron densities, effective atomic numbers, and radiation-attenuating properties, are expected to resemble and be close to the tissue they represent. 
In this study, it was aimed to determine the radiological properties of polyamide (PA6), high density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), polypropylene (PP), polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), polyoxymethylene (POM) and polyurethane foam (PU FOAM) in the diagnostic energy range (15-150 keV) and to examine them in terms of tissue equivalences. The physical densities of the materials are taken from the manufacturers and the chemical contents are taken from the NIST database. 
The radiological properties of the materials and ICRP 110 Male and Female Adult Computational Phantom tissues were calculated with Phy-X PSD software. The Phy-X PSD software has been developed for the calculation of parameters such as linear and mass attenuation coefficients, half-tenths and tenths of the value thickness, mean free path, effective atomic number and electron density, related to shielding and dosimetry, which can be used online. In addition, HU values, which are an important parameter for CT applications, were examined experimentally and theoretically and evaluated in comparison with the literature. 
In determining the HU values by experimental measurements, a phantom formed with polymer materials and water based on the physical phantom CIRS 662 was used. The phantom body with a height of 5 cm, a length of 27 cm and a width of 33 cm is made of PA6 and consists of 17 holes with a diameter of 3 cm. In the center of the phantom, pure water and polymer materials in the other 16 holes were filled in two symmetrically. 
Experimental measurements were made with Siemens SOMATOM Edge CT device with tube voltage of 120 kVp and tube current of 640 mA. The reading of HU values was performed by taking the region of interest (ROI) of approximately 4.0 cm2 with the Onis program. In order to theoretically find the HU values of textures and materials whose element content is known, the measured HU values and the coefficients k1 and k2 representing photon scattering and absorption were calculated by the smallest squares method using python code. Coefficients were found as,   and , respectively.
The HU of the phantom materials were calculated with the coefficients found and compared with the experimental HU values and the consistency was observed. Experimental HU values were between -668±31 and 833±31, and theoretical HU values were between -758 and 913. The highest HU values were determined in PVC and PTFE, which have higher linear attenuation coefficients compared to other polymers, and the lowest values were determined in PU FOAM with a lower µ value.
Also, the theoretical HU values for ICRP 110 Male and Female Adult Computational Phantom tissues were calculated separately, compared with 6 different studies in the literature, and consistency was observed. ICRP computational phantom tissues are classified as bone tissue, soft tissue and lung tissue. While examining tissue equivalence, evaluation was made for muscle and adipose tissue from the soft tissue group.
The effective atomic numbers, electron densities and linear attenuation coefficients of the materials and the ICRP 110 computational phantom were calculated in the diagnostic energy range with the Phy-X PSD software. In addition, the ratios of effective atomic numbers, electron densities and linear attenuation coefficients in the diagnostic energy range of bone, muscle, fat and lung tissues were calculated and tissue equivalence was examined according to the ratios. Moreover, HU values obtained in studies on real tissues in the literature were compared.
When the ratio of the effective atomic numbers of the polymer materials to the values found for ICRP 110 Male Adult Phantom tissues was examined, PVC was compatible with bone at all energies and PTFE at 60 keV energy. When the equivalence is examined for muscle and fat tissues in the soft tissue group, POM, PE and PU foam in the region up to 50 keV seem to be compatible with PA6 from 50 keV. Tissues compatible with the lung are similarly listed as POM, PE, PU FOAM and PA6.
When the ratio of the electron densities of the polymers to the values found for ICRP 110 Male Adult Phantom tissues is examined, it is seen that PVC is generally more compatible with bone than other materials, and PTFE stands out as a bone equivalent polymer from 100 keV. When the compatibility with muscle tissue is examined, PA6 polymer stands out in general, but it is seen that other polymers also adapt in different energy ranges. In terms of adipose tissue, UHMWPE, HDPE and PP polymers are the prominent materials.
The tissue equivalencies of the materials were evaluated comparatively with ICRP 110 Male and Female Adult Computational Phantom. When the radiological features and HU values of polymers are examined in terms of tissue equivalence, PET and POM are compatible with spongiosa bone and PVC and PTFE with cortical bone. PA6 for muscle tissue and PP, UHMWPE and HDPE polymers for adipose tissue appear to be tissue equivalent materials. PU FOAM stands out as a polymer that can be considered a tissue equivalent for the lung.