γ-氨基丁酸(GABA)是中樞神經系統中一種主要的抑制性神經傳遞物質，過往的研究指出GABA的濃度會依各種病理條件而在大腦中產生異常的現象。利用MEGA-PRESS的磁共振頻譜技術，能夠在活體上進行非侵入式的測量，得到人體腦中的GABA濃度。而利用MEGA-PRESS序列收取頻譜時，需要持續激發頻率在同一個頻率位置上，進行連續的掃描以編輯GABA的訊號。磁場均勻度和主磁場頻率的位移，將會影響GABA訊號量化的結果。

　　在這項研究中，我們收取24位受試者的磁共振頻譜，總計48筆資料。觀察磁共振頻率位移對GABA與Glx訊號量化結果的影響，其結果顯示GABA的量化與磁共振頻率位移為負相關，而Glx的量化與磁共振頻率位移為正相關，為減少量化受到頻率位移的影響，本研究另外設定了頻率位移門檻值來篩選頻率位移過多的頻譜，以觀察MEGA-PRESS的編輯效率，提供了一個不需要進行重新進行均勻度的校正，也能降低頻譜與頻譜自身位移相關性的方法。其結果表明，頻率位移門檻值在0.1~0.15ppm時，能有效的降低磁共振頻率位移對GABA與Glx等代謝物量化的影響，同時能保有一個良好的SNR(90.52%~95. 78%)。

GABA (γ-AminoButyric Acid) is a primary inhibitory neurotransmitter in the central nervous system (CNS). Previous studies showed that GABA concentrations in the brain may vary in various pathological conditions. MEGA-PRESS (MEscher–GArwood Point RESolved Spectroscopy) as a non-invasive magnetic resonance spectroscopy (MRS) method has been used to measure GABA. Because, MEGA-PRESS applied frequency-selective editing pulses at 1.9 ppm to edit GABA signal at 3 ppm, resonance frequency drift during successive scan will hamper the editing efficiency and thus influence the quantification of GABA concentrations.

In this study, we investigate the effect of resonance frequency drift on the quantification of GABA and Glx signal. GABA spectra were acquired from 23 subject in visual cortex. Our results showed a statistically significant negative correction (P<0.001) between resonance frequency shift and quantified GABA signal. To reduce the effect of frequency shift on the quantification of GABA, a frequency shift threshold is set to ensure relative consistency on the editing efficiency on GABA. Results showed the best threshold is set up in 0.1ppm~0.15ppm to avoid Influence on the quantification of metabolites using MEGA-PRESS method, and spectrum maintains higher SNR (90.52~95.78%).

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