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Dear Filippo, From my tests (quite a lot of them, but at the moment not formalized and published), changes in injected dose and acquisition duration are completely irrelevant. I can't speak regarding the use of psycotropic substances instead, however I've never read them mentioned in the literature so _maybe_ it's safe to assume that they have small effects. (specifically I expect caffeine *at worse* to increase very mildly metabolism in areas unrelated to AD). However, what you definitely need to address is the difference in uptake time. Indeed I've never heard somebody acquiring images as early as 20 minutes after injection. The earliest I'm aware is 30 minutes, but I'd reccomend waiting at least 45 minutes, to be on the safe side. Nonetheless I've analyzed acquisition that were performed from 30 minutes post injection to 60 minutes. The spatial distribution of the tracer in neurological structures is extremely stable when comparing the 30-35 minutes frame to the 55-60 one. I've compared this in 100 subjects using paired t-tests, so the statistical power was very high. Nonetheless you might still encounter a problem. The uptake in non-neurological structures does vary consistently. So, if you are using global mean scaling you are going to introduce a large bias. Namely, early acquisitions are going to appear artifically hypermetabolic compared to late ones. Therefore you should either apply global mean scaling *after* masking non-neurological structures or use a reference region approach. Hope this can help Luca ________________________________________ Da: SPM (Statistical Parametric Mapping) <[log in to unmask]> per conto di Filippo A. <[log in to unmask]> Inviato: venerdì 29 aprile 2016 21.32.35 A: [log in to unmask] Oggetto: [SPM] PET acquisition difference and SPM study design Hi all, I’m here to request thoughts, comments ot tips from SPM experienced users/phisicians. I’m new to SPM and I’m retrospectively studing neurologic (MCI or AD) patients comparing them, with a 2 sample T Test, to “healtly subjects” underwent to a 18F-FDG PET/CT. The so-called “healtly-subjects” database came from oncologic patients undewent to PET/CT examination mainly for breast or lung tumors (TxNxM0). Those “healtly-subjects” patients had a slightly different acquisition protocol from neurologic patients. Brain scans for neurologic patients had a default acquisition time of 10 minutes, 20 minutes after about 185MBq FDG intravenous injection. Brain scans for oncologics patients had a default acquisition time of 6 minutes, about 50 minutes after a 300-400MBq (patient weight and glycemia driven) intravenous injection. Also, for neurologic patients psycotropics substances (like Tea or Cofee) withdrawal was mandatory. 1)Can those differences lead to inconsistent results ? 2)Did anyone already encountered this problem ? 3)Is there any “fix” (covariates or something) in the study design phase to minimize errors? Thanks in advance for your help Filippo Rispetta l’ambiente: non stampare questa mail se non è necessario. Respect the environment: if it's not necessary, don't print this mail. IL TUO 5XMILLE AL SAN RAFFAELE DI MILANO Devolvi il tuo 5 per mille all’Ospedale San Raffaele: perché al centro della Ricerca ci sei TU. CODICE FISCALE: 07636600962, nel riquadro RICERCA SANITARIA. Non c’è cura, senza ricerca. Non c’è ricerca, senza il tuo 5xmille. Scopri come su http://www.5xmille.org [http://www.5xmille.org/img/731x129.jpg]