Regional Variation within the Cerebral Cortex Evaluated by Diffusion-weighted Imaging and Apparent Diffusion Coefficients on 1.5T and 3T Magnetic Resonance Images
TW Yeung, HY Lau, YC Wong
Hong Kong J Radiol 2013;16:100-9
DOI: 10.12809/hkjr1313172
Objective: We aimed to determine whether cerebral cortices have regional variation on diffusion-weighted magnetic resonance imaging (DW-MRI) and corresponding apparent diffusion coefficient (ADC) maps. We also aimed to explore the effect of using 1.5T and 3T MRI with respect to such regional variation.
Methods:Axial 5 mm–thick DW-MR images at 1.5T and 3T of 49 neurologically normal adults (24 men and 25 women; mean age, 44 years; age range, 21-76 years) were evaluated retrospectively. The cortical signal intensities (SIs) at the frontal, parietal, occipital, and temporal lobes, as well as the cingulate gyri, insula, hippocampus, and amygdala were qualitatively categorised into five grades, relative to that of the right frontal lobe. Contrast ratios (CRs) on DW-MR images were compared for each area. ADC values for corresponding regions were also measured to analyse the diffusion change. With respect to male and female subjects and left and right hemispheres, CR difference and ADC values in each cortex were tested using one-way analysis of variance (ANOVA) for multiple comparisons. Once statistically significant differences were identified, post-hoc analysis by Tukey’s range test was performed. All statistical levels of significance were set at 5%. Results for patients imaged by 1.5T (n = 25) and 3T (n = 24) MRI machines were compared.
Results:Increased SI was demonstrated in the cingulate gyri and insula regardless of patient age, gender, or laterality-related difference. The visual grading of SI in the cingulate gyri and insula was higher than that of right frontal cortex in most subjects, more pronounced in 3T (cingulate: 100%; insula: 96%) than 1.5T (cingulate: 96%; insula: 82%) MRIs. ANOVA showed statistically significant differences among the means of various groups in both 3T and 1.5T groups of these subjects. The CRs measured at the cingulate gyrus and insula on both sides were significantly higher than that of other cortical areas in 3T MRI (p < 0.05). Although higher SIs at the cingulate gyri and insulae were found on visual grading, no consistent significant CR heterogeneity was demonstrated at these regions in 1.5T MRI. There was no significant apparent ADC difference on quantitative evaluation of the values at various cortices, indicating that the regional heterogeneity on DWI was caused by a T2 shine-through effect.
Conclusion:On DWI, high SIs at the cingulate gyri and insular cortices are frequently observed in neurologically normal adults. Absence of ADC map differences signifies that these findings are not related to restricted diffusion, but are caused by T2 shine-through effect. This reflects the basic cytoarchitectural difference between allocortex and periallocortex with the neocortex, as documented in previous literature. 3T MRI accentuates the depiction of physiological cortical heterogeneity on DWI. Awareness of normal patterns and special consideration of the variations of cortical appearance on DWI are necessary when evaluating cortex pathology, especially with the use of high-field MRI.
中文摘要
應用1.5T和3T磁共振彌散加權成像和表觀彌散系數評估大腦皮層 區域差異
楊子慧、劉顯宇、王耀忠
目的：確定大腦皮層在彌散加權磁共振成像（DW-MRI）和相應的表觀彌散系數（ADC）圖中是否 有區域差異，並探討使用1.5T和3T磁共振對此區域差異的影響。
方法：研究對象為49名神經功能正常的成年人（24男和25女；平均年齡44歲，介乎21至76歲），評 估他們軸向5毫米層厚的DW-MRI（1.5T和3T）圖像。以大腦右前額葉信號強度為參照標準，將額 葉、頂葉、枕葉、顳葉，以及扣帶回、腦島、海馬和杏仁核的皮層信號強度定性地分為5個等級。把 每個區域DW-MRI的對比度（CRs）進行了比較，並測量相應區域的ADC值來分析彌散變化。運用 單因素方差分析（ANOVA），多重比較男性與女性之間，左右半球之間各個皮層區的對比度差異及 ADC值。一旦達至統計學上的意義便以Tukey’s差距檢定的方法進行事後分析。統計顯著性水平設定 為5%。再比較分別用1.5T（n = 25）和3T（n = 24）MRI掃描得出的兩組數據。
結果：扣帶回和腦島皮層信號強度增強，且無年齡、性別或偏側差異。在大部份的參與者中，扣 帶回和腦島的目測信號強度高於右額葉皮層，並且3T（扣帶回：100%；腦島：96%）比1.5T（扣帶 回：96%；腦島：82%）的磁共振成像更明顯。ANOVA測試顯示這些研究對象中不同組別平均值有 顯著統計學差異，1.5T及3T磁共振成像均是如此。3T MRI中，雙側大腦半球扣帶回和腦島的CRs均 顯著高於其他皮層區（p < 0.05）。雖然發現扣帶回和腦島的目測信號強度較高，可是1.5T MRI中這 些皮層區並無對應的CR顯著差異。在對不同皮層區ADC值定量分析，結果並無顯著差異；提示DWI 上皮層信號的區域差異是由T2穿透效應導致。
結論：：神經功能正常的成年人的DWI圖像上，扣帶回和腦島皮質高信號強度很常見。但其ADC圖上 並無差異，提示該現象與彌散受限無關，而是由T2穿透效應導致。這反映了舊皮層、周圍舊皮層、 以及新皮層之間基本細胞結構的差異，如以往文獻所述。DWI圖像顯示了腦皮層的生理差異，這點 於3T MRI中尤為突出。評估腦皮層病理變化時，必須注意DWI圖像上腦皮層信號的正常表現，特別 考慮皮層信號的區域差異；尤其是使用高場強MRI時。