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人体内脂肪组织分为白色脂肪组织(white adipose tissue,WAT)与棕色脂肪组织(brown adipose tissue,BAT)2种类型,WAT主要以三酰甘油的形式起保温和储存能量的作用;BAT可通过非战栗方式产热,是人体适应性产热的主要部位,也是目前研究代谢性疾病和肥胖的靶点[1-2]。在PET显像中,3.6%~9.1%的成年人的BAT可高摄取18F-FDG而显影[3-4]。成年人的BAT主要分布于锁骨上区、颈部、椎旁、纵隔及肾周区域[5],这些亦是肿瘤淋巴结转移的好发部位。18F-FDG PET/MRI和18F-FDG PET/CT在肿瘤的诊断、分期及疗效评价方面有重要作用,BAT的生理性摄取对二者的影像诊断均可产生不同程度的干扰。已有研究者对PET/CT显像中BAT摄取的影像学特点及规律进行了分析总结[6-7],但关于PET/MRI中BAT摄取特点的报道较少。本研究初步探讨18F-FDG PET/MRI中BAT摄取的规律和影像学表现。
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回顾性分析2017年7月至2020年1月于空军军医大学第二附属医院核医学科行18F-FDG PET/MRI检查的1 529名受检者的影像学资料,其中男性836名(54.7%)、女性693名(45.3%),年龄14~93(53.6±13.2)岁。BAT摄取18F-FDG阳性者31名(2.0%),其中男性13名(41.9%)、女性18名(58.1%),年龄16~61(33.3±11.6)岁。本研究符合《赫尔辛基宣言》的原则。
纳入标准:(1)临床资料完整;(2)在18F-FDG PET/MRI检查前签署了知情同意书。排除标准:(1)体内有金属植入物,如心脏起搏器、人工瓣膜或其他磁敏感金属异物;(2)患有幽闭恐惧症;(3)有严重外伤、陷入昏迷、高热。
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设备使用德国西门子公司的Biography mMR PET/MRI一体机(MRI 3.0T)。显像剂为18F-FDG(南京江原安迪科正电子研究发展有限公司),放射化学纯度>95%。受检者禁食6 h以上,空腹血糖水平<11.2 mmol/L,于安静状态下经肘静脉注射18F-FDG(3.7 MBq/kg),视听封闭休息40 min后行PET/MRI扫描,扫描范围从颅底至髋关节;采集方式为静态采集;扫描方式为容积扫描。PET与MRI同步扫描。MRI采集参数如下,(1)T1加权成像(weighted imaging,WI)三维容积内插体部检查轴位扫描:重复时间4.04 ms,恢复时间1.24 ms,层厚3.2 mm,层距0 mm,视野420 mm×320 mm,矩阵320×175;(2)T2WI轴位扫描:重复时间3000 ms,恢复时间89 ms,层厚6 mm,层距0 mm,视野400 mm×400 mm,矩阵256×256;(3)T2WI刀锋技术脂肪抑制频率衰减反转恢复序列(spectral attenuated inversion recovery,SPAIR)轴位扫描:重复时间2200 ms,恢复时间86 ms,层厚6 mm,层距0 mm,视野400 mm×400 mm,矩阵320×320;(4)弥散加权成像(diffusion weighted imaging,DWI)扫描:采用平面回波成像序列,b值为50、1000 s/mm2,重复时间6400 ms,恢复时间78 ms,层厚6 mm,层距0 mm,视野400 mm×300 mm,矩阵128×96,自动生成表观弥散系数图。PET采集4~5个床位,4 min/床位,衰减校正采用三维容积内插体部检查水脂分离技术,层厚2 mm,重建类型为高清PET,迭代次数2,矩阵172×172,放大倍数选择1,半峰全宽2.0,散射校正为相对校正。
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记录受检者的性别、年龄、身高、体重、体重指数(body mass index,BMI)、检查日期、检查当日的平均气温,18F-FDG PET/MRI图像由2名取得MRI大型设备上岗证及核医学大型设备上岗证的中级职称医师独立阅片,意见不同时与2名核医学科高级职称医师商议后确定最终结果。采用MIMvista 6.5后处理工作站(北京明维视景医疗软件开发有限公司)获得MRI、PET和PET/MRI融合图像。选取18F-FDG摄取增高部位(以周围肌肉组织的SUVmax为界值),当其在相对应的MRI图像上为脂肪信号时,判断为激活的BAT,采用三维勾画法勾画ROI,测量并记录SUVmax和平均标准化摄取值(mean standardized uptake value,SUVmean)。
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应用PASW Statistics 18.0软件进行统计学分析。符合正态分布的计量资料以
$\bar x \pm s $ 表示,计数资料以个数或百分比表示。计数资料的比较采用卡方检验,方差齐的计量资料的比较采用独立样本t检验,连续资料采用Pearson相关性分析,分类资料和等级资料采用Spearman相关性分析。P<0.05为差异有统计学意义。 -
31名BAT摄取18F-FDG阳性者的PET/MRI图像表现为颈部、锁骨上区、纵隔、脊柱两旁及肾上区等部位呈对称性分布的片状、结节状及串珠状的18F-FDG高摄取灶,MRI图像的T1WI、T2WI均呈高信号,SPAIR呈低信号(图1),DWI图像未见高信号。双侧颈部、锁骨上区摄取者30名(96.8%,30/31),脊柱两旁摄取者21名(67.7%,21/31),纵隔摄取者9名(29.0%,9/31),双侧肾上区摄取者5名(16.1%,5/31)。31名出现1~5个部位摄取的受检者数依次为2名(6.4%)、8名(25.8%)、11名(35.5%)、6名(19.4%)、4名(12.9%)。
图 1 18F-FDG PET/MRI显像中棕色脂肪组织的摄取图 受检者女性,29岁,因血清糖类抗原724升高行PET/MRI检查。A为最大密度投影,可见双侧颈部及锁骨上区对称性分布串珠状18F-FDG摄取增高灶;B为PET横断面,可见双侧锁骨上区对称性分布片状、结节状18F-FDG摄取增高灶,SUVmax=9.7;C为T1WI横断面,可见对应位置T1WI呈高信号;D为T2WI横断面,可见对应位置T2WI呈高信号;E为SPAIR横断面,可见对应位置SPAIR呈低信号;F为SPAIR与PET融合显像横断面,可见18F-FDG高代谢部位SPAIR呈低信号,局部未见肿大淋巴结。FDG为氟脱氧葡萄糖;PET为正电子发射断层显像术;MRI为磁共振成像;SUVmax为最大标准化摄取值;WI为加权成像;SPAIR为频率衰减反转恢复序列
Figure 1. Image of 18F-FDG uptake in brown adipose tissue by PET/MRI
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男性和女性BAT摄取18F-FDG的阳性率分别为1.6%(13/836)和2.6%(18/693),差异无统计学意义(χ2=2.07,P=0.15)。BAT摄取18F-FDG阳性者的年龄[(33.3±11.7)岁]低于阴性者[(54.1±13.5)岁],差异有统计学意义(t=−12.03,P<0.001);BAT摄取18F-FDG阳性者的BMI[(21.89±2.79) kg/m2]低于阴性者[(24.01±3.26 ) kg/m2],差异有统计学意义(t=−5.15,P<0.001)。BAT摄取18F-FDG阳性者检查当日的气温[(7.5±6.5 )℃]低于阴性者[(16.5±11.9)℃],差异有统计学意义(t=−8.97,P<0.001)。31名BAT摄取18F-FDG阳性者按检查月份1~12月的人数分布依次为5、4、2、2、1、0、0、0、0、4、6、7名,多分布于冬、春季节。
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31名BAT摄取18F-FDG阳性者的SUVmax为7.49±3.62(2.82~17.60)、SUVmean为2.92±1.53(0.91~6.47),将SUVmax和SUVmean分别与性别、年龄、BMI、检查当日平均气温及摄取部位BAT数量进行相关性分析,由表1可知,SUVmax和SUVmean均与摄取部位数量呈正相关(均P<0.001);与年龄和BMI呈负相关(均P<0.05);与性别及检查当日平均气温无明显相关性(均P>0.05)。
影响因素 (范围)$\bar x\pm s $ SUVmax SUVmean r值 P值 r值 P值 性别(男/女) − 0.07 0.69 0.01 0.93 年龄(岁) 33.3±11.7(16~61) −0.45 0.01 −0.41 0.02 体重指数(kg/m2) 21.89±2.79(15.50~26.45) −0.36 0.04 −0.40 0.02 检查当日气温(℃) 7.5±6.5(−4~19) 0.29 0.11 0.09 0.61 摄取部位数量(个) 2.9±1.5(1~5) 0.61 <0.001 0.59 <0.001 注:FDG为氟脱氧葡萄糖;PET为正电子发射断层显像术;MRI为磁共振成像;SUVmax为最大标准化摄取值;SUVmean为平均标准化摄取值。−表示无此项数据 表 1 31名棕色脂肪组织摄取18F-FDG阳性者PET/MRI显像的SUVmax和SUVmean与各因素的相关性分析
Table 1. Correlation analysis between maximum standard uptake value and mean standard uptake value by PET/MRI and various factors in 31 subjects with 18F-FDG uptake in brown adipose tissue
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PET/MRI作为一种新型多模态显像技术,具有空间分辨率和组织对比度高的优点,但是其更亦受运动伪影或磁敏感伪影的干扰,了解BAT的活动规律和分布,熟悉BAT摄取显像剂的PET/MRI影像学表现,有助于PET/MRI诊断医师及时作出准确判断。
BAT的摄取存在季节规律,寒冷的季节多发[8]。本研究结果显示,90.3%(28/31)的BAT摄取发生于寒冷的季节,且BAT摄取18F-FDG阳性者检查当日的平均气温明显低于BAT摄取阴性者,与文献报道一致。寒冷是BAT的重要激活因素[9],寒冷刺激时受检者的BAT显影率达70%~100%[2, 10]。寒冷刺激主要通过交感神经系统和瞬时受体电位通道介导,BAT细胞表面含丰富的β3受体,受到寒冷刺激时,去甲肾上腺素与甲状腺素协同上调解偶联蛋白(uncoupling protein, UCP)1的基因表达,增加适应性产热从而维持体温。BAT在婴儿体内的含量最高,占其体重的1%~5%,随年龄增长逐渐下降,成人经寒冷刺激后检测到可摄取18F-FDG的BAT含量仅几毫升至数百毫升不等[11]。在本研究中,BAT摄取18F-FDG阳性者较BAT摄取阴性者更偏年轻化。国内外研究结果显示,BAT摄取18F-FDG阳性在女性中的发生率高于男性[6-7],因雌激素及孕激素可通过上调UCP1基因转录增加BAT的代谢活性,而睾酮可抑制UCP1基因的转录。在本研究中,女性BAT摄取18F-FDG的阳性率高于男性,但差异无统计学意义,这可能与样本的构成有关。本研究结果显示,BMI较低者更易出现BAT摄取。活跃的BAT和肥胖之间的负相关意味着BAT在调节能量消耗方面发挥着重要作用,Crandall等[12]发现,活跃的BAT与BMI呈负相关,偏瘦者的BAT活性更强。基于此,利用BAT调节能量消耗有望成为人类对抗肥胖和治疗代谢性疾病的有效靶点[13]。
Fraum等[14]发现,18F-FDG PET/MRI检查中BAT摄取的最大瘦体重校正标准化摄取值变化范围较大,为2.5~17.8;在本研究中,18F-FDG PET/MRI检测到BAT摄取的SUVmax范围为2.82~17.60,与文献报道基本一致。我们还发现,SUVmax和SUVmean均与摄取部位BAT数量呈正相关,与年龄、BMI呈负相关,与性别及检查当日平均气温无明显相关性。即BAT显影的部位数量越多,激活范围越大,代谢程度越强。BAT的代谢活性随年龄增长而降低,具体机制尚不明确,可能与交感神经系统及甲状腺轴的活性有关。我们还发现,检查当日的平均气温与BAT活性无明显相关性,但具体机制尚不明确,可能是遇到寒冷刺激时, BAT一经激活即呈整体动员趋势,其代谢活性不会随温度降低而增加。Fischer等[15]报道,在亚低温环境中,UCP1基因轻度激活小鼠的BAT对葡萄糖的摄取增加,而持续低温暴露后则检测到较少的葡萄糖摄取,这可能是由于持续低温暴露后,BAT对脂肪酸的处理能力增强,而对葡萄糖摄取相对减少造成的,尚需后期前瞻性研究证实。
基于BAT的代谢特点,可在检查前采取一定的干预措施抑制BAT的活性,减少甚至消除其对影像诊断的干扰。如何减少PET/MRI显像中BAT对显像剂的摄取目前尚无统一标准,一般参考欧洲核医学协会制订的PET/CT肿瘤显像程序指南[16],在注射18F-FDG前30~60 min至检查过程中为受检者保温可有效减少BAT对18F-FDG的摄取,Crandall等[12]研制的抑制交感神经活性的药物也可抑制BAT的活性。
综上所述,在PET/MRI检查中,年轻、偏瘦的受检者较易出现BAT摄取,且大多发生在寒冷季节。在PET/MRI影像诊断过程中,结合BAT摄取的影像学表现、发生规律和代谢特点,可对BAT及时作出准确判断。
利益冲突 所有作者声明无利益冲突
作者贡献声明 惠金子负责研究的设计和实施、论文的撰写;李云波、施笑蕊负责图像数据的采集和处理;袁梦晖、魏龙晓负责论文知识性内容的审阅;许建林、施常备负责论文的指导与审阅
18F-FDG PET/MRI中棕色脂肪组织摄取的影像学表现和规律分析
Imaging characteristics and patterns of brown adipose tissues by 18F-FDG PET/MRI
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摘要:
目的 研究并分析18F-氟脱氧葡萄糖(FDG)PET/MRI中棕色脂肪组织(BAT)摄取的影像学表现和规律。 方法 回顾性分析2017年7月至2020年1月于空军军医大学第二附属医院行18F-FDG PET/MRI检查的1 529名受检者的影像学资料和临床资料,其中男性836名、女性693名,年龄14~93(53.6±13.2)岁;BAT摄取18F-FDG阳性者31名,其中男性13名、女性18名,年龄16~61(33.3±11.6)岁。分析BAT摄取18F-FDG的PET/MRI影像学表现,选取18F-FDG摄取增高部位[以周围肌肉组织的最大标准化摄取值(SUVmax)为临界值],采用三维勾画法勾画感兴趣区,计算SUVmax和平均标准化摄取值(SUVmean)。SUVmax和SUVmean与性别及摄取部位数目的相关性采用Spearman相关性分析,与年龄、体重指数(BMI)、检查当日平均气温的相关性采用Pearson相关性分析。组间计数资料的比较采用χ2检验;计量资料的比较采用独立样本t检验。 结果 (1)BAT摄取18F-FDG阳性者的PET/MRI图像表现为颈部、锁骨上区、纵隔、脊柱两旁及肾上区等部位呈对称性分布的片状、结节状及串珠状的18F-FDG高摄取灶,MRI图像的T1加权成像(WI)、T2WI均呈高信号,频率衰减反转恢复序列呈低信号,弥散加权成像未见高信号。(2)BAT摄取多发生于寒冷的季节,男性和女性BAT摄取18F-FDG的阳性率差异无统计学意义(χ2=2.07,P=0.15);BAT摄取18F-FDG阳性者的年龄、BMI和检查当日气温均明显低于阴性者[(33.3±11.7)岁对( 54.1±13.5)岁、(21.89±2.79) kg/m2对(24.01±3.26) kg/m2、(7.5±6.5)℃对(16.5±11.9)℃],且差异均有统计学意义(t=−12.03、−5.15、−8.97,均P<0.001)。(3)BAT摄取的SUVmax和SUVmean均与年龄、BMI呈负相关(r=−0.45~−0.36,均P<0.05),与摄取BAT部位数量呈正相关(r=0.61、0.59,均P<0.001),与性别和检查当日平均气温均无明显相关性(r=0.01~0.29,均P>0.05)。 结论 在18F-FDG PET/MRI显像中,BAT摄取存在特定的影像学表现和规律性,大多发生在寒冷的季节,年轻、偏瘦者较易出现。 -
关键词:
- 脂肪组织,棕色 /
- 正电子发射断层显像术 /
- 磁共振成像 /
- 氟脱氧葡萄糖F18
Abstract:Objective To investigate and analyze the imaging manifestations and rules of brown adipose tissue (BAT) uptake in 18F-fluorodeoxyglucose (FDG) PET/MRI. Methods The imaging and clinical data of 1 529 subjects (836 males and 693 females, aged 14–93(53.6±13.2) years) who underwent 18F-FDG PET/MRI examination in the Second Affiliated Hospital of Air Force Medical University from July 2017 to January 2020 were analyzed retrospectively. Thirty-one subjects, including 13 males and 18 females aged 16–61 (33.3±11.6) years, had positive BAT uptake of 18F-FDG. The PET/MRI imaging features of 18F-FDG uptake of BAT were analyzed, and three-dimensional sketching was adopted to select sites with increased 18F-FDG uptake to delineate the region of interest (with the maximum standardized uptake value (SUVmax) of surrounding muscles as the boundary value). SUVmax and the mean standardized uptake value (SUVmean) were calculated automatically by the software program. The correlation between SUVmax and SUVmean with variable factors(eg. gender, age, body mass index (BMI), mean temperature on the examination day and number of ingestion sites) were evaluated by Spearman or Pearson correlation analysis. Count data between groups was compared by χ2 test. Measurement data were compared by independent sample t test. Results (1) The PET/MRI images of subjects with positive BAT uptake of 18F-FDG showed symmetrical flaky, nodular and bead uptake in sucravicula, cervical regions, and paravertebral, mediastinal, and perirenal mediastinum. T1-weighted imaging (WI) and T2WI of the MRI images showed high signal. The frequency attenuation inversion recovery sequence showed low signal. The diffusion weighted imaging showed no high signal. (2) Most BAT uptake occurred in cold periods, and no significant difference was found in the positive rate of BAT uptake between males and females (χ2=2.07, P=0.15). The age, BMI, and the temperature on the day of examination of the subjects with positive uptake of BAT were significantly lower than those with negative uptake ((33.3±11.7) years vs. (54.1±13.5) years, (21.89±2.79) kg/m2 vs. (24.01±3.26) kg/m2, (7.5±6.5)℃ vs. (16.5±11.9)℃), and the differences were statistically significant (t=−12.03, −5.15, −8.97; all P<0.001). (3) SUVmax and SUVmean of BAT were negatively correlated with age and BMI (r=−0.45 to −0.36; all P<0.05) but positively correlated with the number of BAT depots (r=0.61, 0.59; both P<0.001). SUVmax and SUVmean of BAT had no significant correlation with gender or the average temperature on the day of examination (r=0.01−0.29; all P>0.05). Conclusion In 18F-FDG PET/MRI imaging, BAT uptake has specific imaging manifestations and regularity, which mostly occurs in cold season and is more likely to occur in young and low-BMI subjects. -
图 1 18F-FDG PET/MRI显像中棕色脂肪组织的摄取图 受检者女性,29岁,因血清糖类抗原724升高行PET/MRI检查。A为最大密度投影,可见双侧颈部及锁骨上区对称性分布串珠状18F-FDG摄取增高灶;B为PET横断面,可见双侧锁骨上区对称性分布片状、结节状18F-FDG摄取增高灶,SUVmax=9.7;C为T1WI横断面,可见对应位置T1WI呈高信号;D为T2WI横断面,可见对应位置T2WI呈高信号;E为SPAIR横断面,可见对应位置SPAIR呈低信号;F为SPAIR与PET融合显像横断面,可见18F-FDG高代谢部位SPAIR呈低信号,局部未见肿大淋巴结。FDG为氟脱氧葡萄糖;PET为正电子发射断层显像术;MRI为磁共振成像;SUVmax为最大标准化摄取值;WI为加权成像;SPAIR为频率衰减反转恢复序列
Figure 1. Image of 18F-FDG uptake in brown adipose tissue by PET/MRI
表 1 31名棕色脂肪组织摄取18F-FDG阳性者PET/MRI显像的SUVmax和SUVmean与各因素的相关性分析
Table 1. Correlation analysis between maximum standard uptake value and mean standard uptake value by PET/MRI and various factors in 31 subjects with 18F-FDG uptake in brown adipose tissue
影响因素 (范围)$\bar x\pm s $ SUVmax SUVmean r值 P值 r值 P值 性别(男/女) − 0.07 0.69 0.01 0.93 年龄(岁) 33.3±11.7(16~61) −0.45 0.01 −0.41 0.02 体重指数(kg/m2) 21.89±2.79(15.50~26.45) −0.36 0.04 −0.40 0.02 检查当日气温(℃) 7.5±6.5(−4~19) 0.29 0.11 0.09 0.61 摄取部位数量(个) 2.9±1.5(1~5) 0.61 <0.001 0.59 <0.001 注:FDG为氟脱氧葡萄糖;PET为正电子发射断层显像术;MRI为磁共振成像;SUVmax为最大标准化摄取值;SUVmean为平均标准化摄取值。−表示无此项数据 -
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