前言：本文比較4種不同時間開始造影所獲得的影像是否有差異，我們以腦部灰質與白質的相對放射活性比值(ratio of average radioactivity count between gray and white matter regions of interest) 作為比較的依據。
方法：5位健康從注射後立即開始emission scan，持續收集資料120分鐘，這120分鐘分為4組30分鐘影像，分別為Scan 1：0至30分鐘的影像，Scan 2：31至60分鐘的影像、Scan 3：61至90分鐘的影像，Scan 4：91至120分鐘的影像。選取區 (regions of interest, ROIs) 的產生是從每個研究對象的15個橫切面 (tranverse slices) 之中挑選基底核 (basal ganglia) 最明顯的2個切面，手動操作 (manually) 選取每1個切面的兩測額葉 (frontal lobe)，顳葉上部(superior temporal) 及枕葉 (occipital lobe)的灰質，並且選取額葉的白質。腦部灰白質的相對放射活性比值為灰質選取區的平均放射活性計數與白質選取區的平均放射活性計數之比值。
結果：我們研究的放設活性比值結果介於1.68及1.91之間。Scan 1至Scan 4 的比值依序為1.72、1.68、1.87、1.92。最高值至最低值依序為Scan 4、Scan 3、Scan 1)、Scan 2。依據repeated measures ANOVA分析結果，Scan 1、Scan 2、Scan 3和Scan 4四者之間的灰白質相對放射活性比值有統計上的顯著差異(P<0.0001)。採用無母數統計檢定法佛里曼試驗及肯德爾試驗對四組影像的灰白質相對放射活性比值分析同樣也有統計上的顯著差異(P<0.0001)。我們再依據multiple paired t-test作兩個影像之間的分析結果，雖然顯示Scan 1與Scan 2的影像之間沒有統計上的顯著差異(P=0.305)；Scan 3與Scan 4的影像之間也沒有統計上的顯著差異(P=0.209)。採用無母數統計檢定法魏衛可遜配對組符號等分析也有同的結果(Scan 1與Scan 2的影像之間沒有統計上的顯著差異P=0.475及Scan 3的影像與Scan 4的影像之間也沒有統計上的顯著差異P=0.110)。但是Scan 1或Scan 2與Scan 3或Scan 4的影像之間有統計上的顯著差異 (Scan 1與Scan 2的灰白質相對放射活性比值之間有統計上的顯著差異P=0.005，Scan 1與Scan 4的灰白質相對放射活性比值之間有統計上的顯著差異 P=0.002，Scan 2與Scan 3的灰白質相對放射活性比值之間有統計上的顯著差異 P=0.000。Scan 2與Scan 4的灰白質相對放射沽性比值之間有統計上的顯著差異P=0.000)。無母數統計檢定法使用魏可遜配對組符號等級檢定分析也有相同的結果 (P<0.001)。這樣的結果顯示Scan 3或Scan 4可以得到比較好的影像品質。而Scan 1或Scan 2的影像品質比較差。結論：本研究的初步結果顯示延後造影可以得到比較好的影像品質，或許我們可以依據實際需要而彈性地提前或延後安排造影的開始時間。

Background: To determine the optimal scanning time for cerebral FDG PET scan, we compared 4 sequential 30- minute images using gray and white matter radioactivity ratio.
Methods: Emission scan was started immediately after injection of 10 mCi of 18F-FDG and lasted for 120 minutes in 5 healthy volunteer. The 120 minutes acquisition data were divided into four 30-minute images, Scan 1: image from 0 to 30 minute, Scan 2: image from 31 to 60 minute, Scan 3: image from 61 to 90 minute, and Scan 4: image from 91 to 120 minute. Regions of interest (ROIs) were generated from 2 transverse slices which showed most prominent basal ganglia in each subject. We manually draw ROIs in gray matter of bilateral frontal, superior temporal and occipital lobes, and white matter of bilateral frontal lobes. The gray and white matter radioactivity ratio was expressed as average radioactivity count of gray matter ROIs / average radioactivity count of white matter ROIs.
Results: The ratios of mean gray/white matter radioactivity were between 1 68 and 1.91. There was statistically significant difference (P<0.0001) between the 4 scans by repeated measures ANOVA. Multiple paired t-test showed no difference between Scan 1 and Scan 2 (P=0.305), and between Scan 3 and Scan 4 either (P=0.209). However, there are significant differences between Scan 1 and Scan 3 (P=0.005), Scan 1 and Scan 4 (P=0.O02), Scan 2 and Scan 3 (P=0,000), Scan 2 and Scan 4 (P=0.000). Conclusion: Our preliminary result showed that delayed initiating time could have better image quality, and the image acquired from earlier initiating time showed no difference in comparison with ordinary initiating time. Therefore, we can arrange the time to start image acquisition earlier and more flexibly.