Vpliv korekcije atenuacije na kvaliteto enofotonske emisijske računalniške tomografske slike : magistrsko delo

Frelih, Nina (author), Ležaič, Luka (mentor), Rep, Sebastijan (co-mentor)

Uvod: Atenuacija ima pomemben vpliv na podatke in s tem kvaliteto slike. Atenuacija fotonov pomeni znižanje števila impulzov iz telesa. Korekcija atenuacije korigira oslabitev žarkov v različnih debelinah. Pri SPECT/CT slikanju se za korekcijo atenuacije uporablja nediagnostičen, nizkodozni CT. Namen: Namen magistrske naloge je bilo ugotoviti, kako korekcija atenuacije pri SPECT/CT-ju vpliva na kvaliteto slike. Metode dela: Fantom NEMA IEC BodyPhantom smo napolnili z izotopom tehnecijem-99m in ga osemkrat poslikali, vsakič z drugačnim razmerjem specifične aktivnosti med ozadjem fantoma in sferami. Slike smo obdelali v Oasis programu. Obdelali smo jih s tremi različnimi rekonstrukcijami, filtrirano povratno projekcijo, nekorigirano iterativno rekonstrukcijo in s korekcijo atenuacije z uporabo CT-ja. Izmerili smo število impulzov v ozadju in v vseh šestih sferah, in nato primerjali kontrastnosti slik med različnimi metodami rekonstrukcije. Naredili smo opisno statistiko in repeated measures ANOVA. V Matlabu smo naredili parni Wilcoxonov test predznaka. Rezultati: Ozadje je na slikah, obdelanih z filtrirano povratno projekcijo in na slikah obdelanih z iterativno rekonstrukcijo, nehomogeno. Na korigiranih slikah z uporabo CT-ja, je število impulzov enakomerno po vsem ozadju fantoma, zato je ozadje homogeno. Statistična analiza je pokazala p < 0.001, kar pomeni, da je kontrastnost statistično značilno različna med različnimi metodami rekonstrukcije. Razprava in zaključek: Ugotovili smo, da število impulzov in s tem kontrastnost slike narašča z velikostjo sfere in z večjim razmerjem aktivnosti med ozadjem in sfero, pri vseh treh vrstah rekontrukcije. Najboljšo kontrastnost imajo CT-AC slike, sledijo slike z iterativno rekonstrukcijo in nato še slike obdelane s filtrirano povratno projekcijo. Na slikah rekonstruiranih z CT-AC je najmanj negativne kontrastnosti, medtem, ko je pri iterativni rekonstrukciji in FBP slikah negativne kontrastnosti precej več. Ker imajo slike z uporabo CT-AC-ja boljšo kvaliteto slik, v primerjavi z nekorigiranimi slikami, priporočamo uporabo CT-AC pri vseh preiskavah, vendar je za izključitev artefaktov zaradi korekcije atenuacije pomembno, da se vedno pogleda tudi nekorigirane slike.

• Language: Slovenian
• Keywords: atenuacija, korekcija atenuacije, kontrastnost
• Work type: Master's thesis/paper
• Organization: ZF - Faculty of Health Sciences
• Year: 2018
• PID: 20.500.12556/RUL-105806
• COBISS.SI-ID: 5556587
• Publication date in RUL: 18.12.2018

Secondary language

• Language: English
• Title: The influence of atteanuation correction on the image quality in single photon emission computed tomography : master's thesis

Abstract

Introduction: Attenuation has a strong influence on the data and consequently on the quality of the image. Attenuation of photons is the decrease in the number of counts coming from the body. Attenuation correction corrects the weakening of the rays in various depths. The non-diagnostic, low-dosage CT is used for the attenuation correction when the images are taken with the SPECT/CT. Purpose: The goal of this master’s thesis is to find out how the attenuation correction in the case of the SPECT/CT influences the quality of the image. Methods: The Phantom NEMA IEC BodyPhantom was filled with the isotope technetium-99m. Eight images were captured, each with a different ratio of the specific activity between the phantom background and the spheres. The images were reconstructed in the program called Oasis by three different reconstructions: the filtered back projection, the non-corrected iterative reconstruction, and with the attenuation correction using the CT. The number of counts in the background and in all six spheres was measured. This was followed by the comparison of the contrast in images that were reconstructed using different methods. A descriptive statistic and repeated ANOVA measure were conducted. The Wilcoxon signed-rank test was carried out in Matlab. Results: On the images that were processed by the filtered back projection or the iterative reconstruction, the background is not homogeneous. On the images that were corrected using the CT, the number of counts is evenly spread across the entire background of the phantom, thus making the background homogeneous. The statistical analysis showed that p < 0.001, meaning that, statistically, contrast is typically divergent among the different methods of reconstruction. Discussion and conclusion: It was discovered that the increase in the number of counts, and consequently the image contrast, is proportional to the size of the sphere, and to the increased ratio of activity between the background and the sphere with all three types of reconstruction. The CT-AC images have the best contrast; images with iterative reconstruction are second best; images processed by the filtered back projection are third. The images reconstructed with the CT-AC have the least negative contrast, while the negative contrast in the images with iterative reconstruction and FBP images is far more prominent. Because the CT-AC images are of better quality than the non-corrected images, it is recommended that the CT-AC be used for all tests; however due to the removal of artefacts caused by attenuation correction, it is also important to examine the non-corrected images every time.

• Keywords: attenuation, attenuation correction, contrast