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Uvod: Eden najpogosteje uporabljenih slikovnih pripomočkov v nuklearni medicini je gama kamera. Le-ta se uporablja za zaznavanje posameznih gama fotonov, ki izhajajo iz radioaktivne snovi. Sestavljena je iz kolimatorja, kristala (NaI), fotopomnoževalk in elektronike. Za njeno ustrezno delovanje je potrebno redno izvajati teste kontrole kvalitete. Eden najpomembnejših testov je enakomernost, ki se mora izvajati vsak dan pred pričetkom dela. Merimo jo lahko intrinzično (brez kolimatorja s tehnecijevim izvorom) ali ekstrinzično (s kolimatorjem in kobaltovo ploščo). V obeh primerih jo merimo centralno in globalno na dva načina, in sicer integralno in diferencialno. Namen: Želeli smo ugotoviti, kako se enakomernost spreminja pri ekstrinzičnem testu, če pri tem spremenimo določene parametre, kot so število impulzov in velikost matrike. Poleg tega smo želeli ugotoviti, ali obstaja kakšna razlika v enakomernosti med avtomatskim in ročnim testom. Metode dela: Teste smo izvedli na SPECT/CT-ju (Siemens Symbia serije T2) na Kliniki za nuklearno medicino. Meritve smo izvedli tako, da smo kobaltovo ploščo neposredno postavili na detektor 2 in detektorja čim bolj približali. Nato smo izbrali ustrezen protokol, kjer smo spreminjali število impulzov (3 milijone, 4,5 milijonov in 10 milijonov) in velikost matrike (1024 x 1024, 512 x 512, 256 x 256 in 128 x 128). Tako so posamezne meritve znašale od 10 do 40 minut. Pri tem smo zbrali po 10 meritev za vsako spremenljivko. Enakomernost na posameznih slikah smo izračunali s pomočjo aplikacije IAEA v programu ImageJ. Podatke, ki smo jih dobili s kobaltovo ploščo pri izbranih parametrih 10 milijonov impulzov in velikostjo matrike 1024 x 1024, smo uporabili še za primerjavo z avtomatskim testom, pri katerem se uporabi linijski izvor (Gd-153), okrog katerega se zavrtijo detektorji gama kamere. Rezultati: Vse meritve smo statistično preverili s programom SPSS. Z ANOVO, pri kateri smo določili signifikantno mejo pri 0,01, smo ugotovili, da v vseh primerih vrednosti enakomernosti padajo z večanjem števila impulzov (p < 0,001). V večini primerov smo ugotovili, da velikost matrike ne vpliva na enakomernost (p > 0,01). Izjema se je pokazala pri integralni enakomernosti pri uporabnem vidnem polju na detektorju 1 (p = 0,001) in detektorju 2 (p < 0,001). Pri primerljivosti ročnega testa z avtomatskim smo uporabili Mann-Whitney test s signifikantno mejo pri 0,05. Ugotovili smo, da sta testa statistično značilno različna pri integralni enakomernosti, medtem ko se pri diferencialni enakomernosti ne razlikujeta. Razprava in zaključek: Z raziskavo smo ugotovili, da se integralna in diferencialna enakomernost pri centralnem in uporabnem vidnem polju izboljšuje z večanjem števila zajetih impulzov, saj se s tem manjša statistična variabilnost. Sama velikost matrike pa ne vpliva na enakomernost, saj število impulzov vedno pade na isto veliko površino detektorja ne glede na velikost matrike. Do odstopanja je prišlo zaradi naključnih dogodkov, do katerih pride v nuklearni medicini, ker uporabljamo radioaktivne vire, ki nenehno razpadajo in posledično nikoli ne dobimo enakih rezultatov. Zato tudi obstajajo meje, znotraj katerih je odstopanje še dovoljeno. Introduction: One of the most used imaging systems in nuclear medicine is gamma camera. It is used to detect single gamma photons emitted from radioactive substance. The main component parts of gamma cameras are collimator, crystal (NaI), photomultiplier tubes and processing electronics. If we want gamma camera to work correctly, we have to regularly perform quality control tests. One of the most important tests, that we have to perform daily before we start clinical work, is called uniformity. The uniformity can be measured intrinsically (without collimator and with technetium source) or extrinsically (with collimator and cobalt flood source). In both cases we measure it central, global, integral and differential. Purpose: We wanted to determine how uniformity varies with extrinsic test if we change specific parameters (number of counts and the size of matrix). We also wanted to determine if there is any difference in uniformity between automatic and manual test. Methods: The measurements were carried out on SPECT/CT (Siemens Symbia series T2) in Clinic of nuclear medicine, where we placed cobalt flood source directly on detector 2 and then moved the detectors closely together. Then we used the protocol for manual uniformity where we changed the number of counts (3 millions, 4.5 millions and 10 millions) and the size of matrix (1024 x 1024, 512 x 512, 256 x 256 and 128 x 128). The time duration of the tests was between 10 and 40 minutes. We have collected 10 measurements for each variable separately. Uniformity was calculated with the IAEA application in the program of ImageJ. Data that we collected with manual test where we used 10 million counts and the matrix size 1024 x 1024 was also used for comparison with automatic test, where it was used linear source (Gd-153) and the tomographic way of imaging. Results: The measurements were statistically tested with SPSS. One of the tests that we used was ANOVA, where we decided for the statistical significance to be at 0,01. ANOVA showed us that uniformity values are lower, if we use bigger number of counts (p < 0,001). In most cases was determined that the size of matrix does not affect uniformity (p > 0,01). Exception was shown at integral uniformity with useful field of view on detector 1 (p = 0,001) and detector 2 (p < 0,001). When we compared automatic test with manual we used Mann-Whitney test with statistical significance 0,05. We determined that significant difference does exist between the two tests at integral uniformity. Discussion and conclusion: With our research we have came to conclusion that both the differential and integral uniformities for useful and central field of view are improving as the number of acquired counts increase, because there are fewer statistical fluctuations. The size of the matrix does not affect uniformity. However there have been some deviations in relation to coincidentally events which happen in nuclear medicine, because we deal with radioactive sources. This sources are constantly decaying so we never get the same results. That is why our measurements have to be inside the written tolerance. |
