18 F-脱氧葡萄糖正电子发射断层显像（18F-FDG PET/CT）
18F-FDG PET/CT（18F-Fluorodeoxglucose Positron Emission Tomograph）的原理是恶性组织的葡萄糖代谢增强，细胞膜葡萄糖转运体高度表达。FDG作为葡萄糖的类似物，反映细胞的恶性特征。正常情况下，甲状腺轻度摄取FDG，但是低于血池的背景。
甲状腺摄取FDG分为3种形态，即局灶型、弥漫型和局灶型+弥漫型。三者的摄取率分别为0.1%～0.45%、0.1%～0.48%和0.2%～0.3%。局灶型见于恶性肿瘤，发生率为25%～50%；弥漫型见于自身免疫甲状腺炎，特别是桥本甲状腺炎。
局灶型摄取：恶性组织的摄取率平均19.8%（95%CI：15.3%～24.7%）。乳头状甲状腺癌摄取15.4%。摄取FDG越多，说明肿瘤的恶性度越高。因为肿瘤的恶性程度与细胞膜表达Glut-1密切相关。标准化提取值（Standardized Uptake Value，SUV）是一个半定量的指标，反映病灶的代谢活性。它有类似组织学的鉴定作用。分化好的组织SUV减低，分化差的组织SUV增加。SUV也可以出现重叠，例如甲状腺结节的代谢活性。
图15-12 甲状腺乳头状癌伴右侧颈部Ⅲ区、Ⅳ区淋巴结转移
A .冠状位T1WI右侧甲状腺增大，内见稍高信号结节，边界不清，形态不规则，信号不均匀。B.冠状位抑脂T2WI，右侧甲状腺病变呈混杂稍高信号。C.增强扫描明显欠均匀强化。右侧颈部Ⅲ区、Ⅳ区淋巴结肿大，淋巴结与右侧甲状腺病变信号相似。
弥漫型摄取：主要见于良性病变，例如桥本甲状腺炎、Graves病、结节性甲状腺肿。也有报告发生在原发性甲状腺淋巴瘤和硬化型甲状腺癌。桥本甲状腺炎47%～86%表现为弥漫型摄取，并且与TSH相关，与甲状腺抗体相关。为什么桥本甲状腺炎出现FDG摄取增加，机制尚不清楚，可能与炎症有关。Graves病发生率为30%～64%，可能与甲状腺血流加快、代谢增强有关。原发性甲状腺淋巴瘤（PTL）SUV显著增高。但是PTL与桥本甲状腺炎鉴别是困难的。PTL的迅速增长是一个鉴别要点。
甲状腺CT和MRI对于局限于甲状腺内的病变，其诊断价值有限，能够提供的信息与超声相似，甚至弱于超声检查，并且费用高和限制条件多，一定程度制约了临床的开展及应用。对于巨大病变，CT和MRI能够更好地显示胸骨后甲状腺肿的范围或者评价纵隔肿物。对于可疑的恶性病变，其能够准确地评价周围组织结构受侵情况，评估颈部淋巴结转移情况。对于甲状腺恶性肿瘤术后随访，在评价咽后淋巴结、下颈部Ⅵ区和纵隔淋巴结方面优于超声。
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