0 引言

       自身免疫性胰腺炎（autoimmune pancreatitis, AIP）是一种由免疫介导的特殊类型的胰腺炎症，有其独特的临床表现、病理改变、实验室血化验指标及影像学特征^[1]。AIP典型的CT和MRI表现为主胰管弥漫性不规则狭窄、胰腺实质弥漫性或局限性肿大、胰腺边缘光滑（腊肠样改变）及增强扫描早期呈不均匀斑片状或雪花样强化并随时间延长呈渐进性趋于均匀的延迟强化，30%~40%患者在胰腺周围出现假包膜^{[2, 3, 4]}。目前影像科医师对这些常见的影像特征已较熟悉，但对该疾病一些少见的影像表现尚认识有限。为了提高影像科医师对AIP病变征象的全面认识，本文着重介绍AIP少见的影像表现如下。

1 AIP伴主胰管扩张

       有关AIP胰腺病变的影像表现实际上包括胰腺实质和胰管两部分。主胰管病变的影像特征在AIP的诊断和鉴别诊断中扮演着重要的角色，其典型表现为主胰管较长节段的狭窄（大于1/3主胰管）或多节段狭窄。由于AIP胰管周围存在显著的淋巴-浆细胞浸润及纤维化，胰管病变更多表现为狭窄而非梗阻，因此非狭窄段胰管往往无扩张^{[5, 6]}。据一项对26篇文献706例中国AIP患者的综述报道，13.8%的AIP出现胰管扩张^[7]。在我们的研究中也发现^{[8, 9]}，12.7%（13/102）的AIP出现胰管扩张，扩张胰管的中位直径为4.0（3.5, 4.8）mm，仅4例超过5 mm。可见，少数AIP可以出现狭窄段之间或狭窄上游的胰管扩张，以轻度扩张为主（图1），这是鉴别AIP与胰腺癌的要点之一^[10]。个别AIP的狭窄段间胰管可形成囊状扩张，磁共振胰胆管水成像（magnetic resonance cholangiopancreatography, MRCP）表现为沿狭窄胰管分布的小囊灶，主胰管呈串珠状改变（图2）。

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图1  男，78岁，AIP主胰管扩张。1A：MRCP显示胰头区主胰管较长节段狭窄（红箭），其上游的胰管扩张（白箭），最宽处直径4 mm；1B、1C：轴面脂肪抑制FSPGR T1WI和FSE T2WI显示胰腺弥漫性肿大及信号异常，胰腺体尾部主胰管扩张（箭），同时见胰上段肝外胆管扩张及胆囊增大。
图2  男，55岁，AIP主胰管扩张。2A：MRCP显示胰头部主胰管较长节段狭窄（白箭），体尾部狭窄段间胰管不同程度扩张，部分扩张呈囊状（红箭）；2B、2C：轴面脂肪抑制FSPGR T1WI和FSE T2WI显示胰腺弥漫性肿大，体尾部胰管粗细不均匀（箭）。AIP：自身免疫性胰腺炎；MRCP：磁共振胰胆管水成像；FSPGR：快速扰相梯度回波；FSE：快速自旋回波。
Fig. 1  Male, a 78-year-old patient with AIP and dilated main pancreatic duct. 1A: MRCP shows a long segment stenosis (red arrow) of the main pancreatic duct in the pancreatic head area, with upstream pancreatic duct dilatation (white arrow), with a maximum diameter of 4 mm; 1B, 1C: Axial fat-suppressed FSPGR T1WI and FSE T2WI show diffuse enlargement and abnormal signal of the pancreas, with dilated main pancreatic duct in the pancreatic body and tail (arrow), as well as dilated extrahepatic bile duct in the upper segment of the pancreas and enlarged gallbladder.
Fig. 2  Male, a 55-year-old patient with AIP and dilated main pancreatic duct. 2A: MRCP shows a long segmental stenosis (white arrow) in the main pancreatic duct of the pancreatic head, and varying degrees dilatation of the pancreatic duct between the narrow segments of the body and tail, with some dilatation appearing cystic (red arrow); 2B, 2C: Axial fat-suppressed FSPGR T1WI and FSE T2WI show diffuse enlargement of the pancreas, with uneven thickness of the pancreatic duct in the body and tail (arrows). AIP: autoimmune pancreatitis; MRCP: magnetic resonance cholangiopancreatography; FSPGR: fast spoiled gradient recalled; FSE: fast spin echo.

2 AIP继发胰腺囊性病变

       少数AIP可出现胰腺囊性病变、胰漏、胰源性腹水，发生率为9.7%~22.4%。AIP继发的胰腺囊性病变多见于胰体尾部，单发或多发，位于胰腺内或胰腺外（图3、4），类固醇激素治疗后可明显缩小或消失，但对直径大于55 mm的囊性病变往往需要手术干预^{[11, 12, 13]}。AIP胰腺囊性病变的形成可能与炎症的高活性状态和严重的胰管狭窄有关。胰管狭窄导致上游胰液潴留或形成潴留性囊肿，当囊壁周围纤维组织增生且壁内上皮细胞脱落时，潴留性囊肿转变为假性囊肿^{[14, 15]}。饮酒史、腹痛、血淀粉酶升高、脾静脉受累所致的胃底静脉曲张是AIP继发囊性病变形成的独立危险因素^{[16, 17]}。我们对102例AIP患者的回顾性研究显示^[9]，15例在病程中出现43个大于5 mm的囊性病变，其中，32个位于胰腺内，11个位于胰腺外。在这些囊性病变中，35个MR信号及CT密度均匀，1个囊液伴出血信号，4个在T2WI高信号的囊内见小灶低信号，3个囊内见线状分隔，均未见壁结节。文献报道AIP可与胰腺囊性肿瘤共存^[18]，壁结节是鉴别AIP继发囊性病变与囊性肿瘤的要点之一，当同时发现囊内分隔与壁结节时，肿瘤可能性更大。

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图3  男，72岁，AIP继发胰腺实质内囊性病变及其演变。3A、3B：初诊时轴面脂肪抑制FSPGR T1WI和FSE T2WI显示胰腺弥漫性肿大，胰腺尾部见一个类圆形囊性病变（箭），T1WI呈低信号，T2WI呈高信号，邻近的胰管扩张；3C：MRCP显示胰头及体部较长节段胰管狭窄（红箭），胰尾部囊性病变与扩张的胰管连接（白箭）；3D：同一患者类固醇治疗2个月后增强CT显示胰腺肿胀好转、体积缩小，胰尾部囊灶增大（箭）；3E：类固醇治疗36个月后轴面脂肪抑制FSPGR T1WI显示胰尾部囊灶明显缩小（白箭），同时见胰腺实质萎缩，胰管狭窄及扩张交替（红箭）；3F：类固醇治疗52个月后增强CT显示胰腺炎症复发，胰尾囊灶消失。
图4  男，67岁，AIP继发胰腺实质外囊性病变。4A、4B：轴面脂肪抑制FSPGR T1WI和FSE T2WI显示胰腺弥漫性肿大，胰周见低信号的假包膜（箭）；4C：较高层面的FSPGR T1WI显示胃大弯侧一个囊性病变呈稍高信号（箭）；4D：更高层面的轴面脂肪抑制FSE T2WI显示囊性病变内液-液平面，其上部呈高信号，下部呈稍低信号（箭），提示囊内出血；4E：冠状面SSFSE T2WI在胰尾（红箭）头侧及胃壁左侧见一个葫芦形不均匀高信号的囊性病变（白箭）；4F：MRCP显示主胰管全程弥漫性狭窄（红箭），胃壁左下方胰尾处见一个稍高信号的囊性病变（白箭）。AIP：自身免疫性胰腺炎；FSPGR：快速扰相梯度回波；FSE：快速自旋回波；MRCP：磁共振胰胆管水成像；SSFSE：单次激发快速自旋回波。
Fig. 3  Male, a 72-year-old man with AIP secondary to pancreatic parenchymal cystic lesions and its evolution. 3A, 3B: At the initial diagnosis, axial fat-suppressed FSPGR T1WI and FSE T2WI show diffuse enlargement of the pancreas, with a quasi-circular cystic lesion (arrow) in the pancreatic tail, which shows low signal intensity on T1WI and high signal intensity on T2WI, with adjacent dilated pancreatic ducts; 3C: MRCP shows long segment pancreatic duct stenosis (red arrows) in the head and body of the pancreas, and a cystic lesion in the pancreatic tail connected to the dilated pancreatic duct (white arrows); 3D: Enhanced CT after 2 months of steroid treatment shows improved swelling and reduced volume of the pancreas, with an enlarged cystic lesion in the pancreatic tail (arrow); 3E: After 36 months of steroid treatment, axial fat-suppressed FSPGR T1WI shows a significant reduction in the size of the cystic lesion in the pancreatic tail (white arrows), with concurrent atrophy of the pancreatic parenchyma, alternating pancreatic duct stenosis and dilation (red arrows); 3F: Enhanced CT after 52 months of steroid treatment shows recurrent pancreatic inflammation, with the cystic lesion in the pancreatic tail disappearing.
Fig. 4  Male, a 67-year-old patient with AIP secondary to extra-pancreatic parenchymal cystic lesions. 4A, 4B: Axial fat-suppressed FSPGR T1WI and FSE T2WI show diffuse enlargement of the pancreas, with a low-signal pseudocapsule (arrow) around the pancreas; 4C: Higher-level FSPGR T1WI shows a cystic lesion with slightly high signal intensity (arrow) on the side of the greater curvature of the stomach; 4D: Higher-level axial fat-suppressed FSE T2WI shows a fluid-fluid level within the cystic lesion, with high signal intensity in the upper part and slightly low signal intensity in the lower part (arrow), suggesting intralesional hemorrhage; 4E: Coronal SSFSE T2WI shows a gourd-shaped heterogeneous high-signal cystic lesion (white arrow) on the head side of the pancreatic tail (red arrow) and on the left side of the gastric wall; 4F: MRCP shows diffuse stenosis throughout the main pancreatic duct (red arrow), and a slightly high-signal cystic lesion (white arrow) is seen at the lower left pancreatic tail of the gastric wall. AIP: autoimmune pancreatitis; FSPGR: fast spoiled gradient recalled; FSE: fast spin echo; MRCP: magnetic resonance cholangiopancreatography; SSFSE: single-shot fast spin-echo.

3 AIP胰腺周围血管受累

       部分AIP可出现脾静脉受累，增强CT或MRI静脉期表现为脾静脉管腔狭窄或闭塞（图5A），有时见静脉内血栓，但在临床实践中这种直接征象常被忽视。约80%脾静脉受累的AIP患者伴发胃大弯侧静脉曲张（图5B），这种胰源性区域性门静脉高压表现与胃短静脉或胃网膜静脉的血流增加有关^{[19, 20]}，出现这种间接征象时，影像科医师应观察脾静脉是否受累。此外，我们的研究表明，AIP脾静脉受累与胰周的假包膜显著相关，推测是假包膜缩短了胰腺炎症与脾静脉的距离，从而增加了脾静脉受累的可能性^[21]，因此当AIP出现假包膜，尤其胰腺背侧假包膜形成时，影像科医师需观察脾静脉情况。早期识别AIP脾静脉受累，尽早使用类固醇治疗，可以减少甚至逆转区域性门静脉高压形成等并发症^{[20, 22, 23]}。

       AIP脾动脉受累CT或MRI表现为脾动脉边缘不规则，但管腔通常无明显狭窄（图5C）^[24]。偶见AIP继发脾动脉假性动脉瘤，被视为AIP患者潜在的致命并发症^[25]。

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图5  AIP胰腺周围血管受累表现。5A、5B：男，59岁，MRI增强扫描T1WI静脉期（5A）显示胰腺实质弥漫性肿大、强化减弱，胰周见相对弱强化的假包膜，胰腺背侧的脾静脉管腔明显狭窄，其内未见对比剂充盈（箭），较高层面的增强T1WI（5B）见胃大弯侧静脉迂曲扩张（箭），提示脾静脉受累。5C：男，65岁，CT增强扫描动脉期显示胰腺肿大、强化减弱，胰管轻度扩张（红箭），胰腺背侧的脾动脉管壁不光滑（白箭）。AIP：自身免疫性胰腺炎。
Fig. 5  The involvement of blood vessels around the pancreas in patient with AIP. 5A and 5B: Male, 59 years old. MRI enhanced scan T1WI venous phase (5A) shows diffuse enlargement of the pancreatic parenchyma, weakened enhancement, and a relatively weakly enhanced pseudocapsule around the pancreas. The dorsal pancreatic vein lumen is significantly narrowed, and no contrast agent filling (arrow) is seen inside. The enhanced T1WI at a higher level (5B) shows tortuous and dilated veins on the greater curvature side of the stomach (arrow), indicating involvement of the splenic vein. 5C: Male, 65 years old. CT enhanced scan arterial phase shows enlarged pancreas, weakened enhancement, and mild dilation of the pancreatic duct (red arrow). The wall of the splenic artery on the dorsal side of the pancreas is not smooth (white arrow). AIP: autoimmune pancreatitis.

4 AIP继发胰腺钙化和胰管结石

       由于对类固醇药物治疗反应良好，AIP曾被认为是一种非进展性疾病，但随后的研究发现，有些AIP患者在病程中可出现胰腺钙化和胰管结石^[26]。CT能准确评估这些病变。

       AIP胰腺钙化的发生率为4%~40%^{[26, 27]}。TAKADA等^[28]报道AIP病变的腺泡细胞内骨桥蛋白的发生率明显高于正常胰腺，这意味着AIP在长期的病程演变中发生钙化的可能性明显增加。胰腺钙化与胰头肿胀和胰头区胰管狭窄显著相关，亦与复发密切相关^[26]。胰腺钙化可为局限性，亦可弥漫分布（图6）^[29]。

       胰管结石是胰腺慢性炎症的重要表现，但在AIP并不常见。结石形成与胰管狭窄导致的胰液瘀滞有关，亦可能与AIP复发相关^{[30, 31, 32]}。ITO等^[33]对22个医疗中心的624例1型AIP患者随访发现，5%的AIP患者在中位随访时间7年（2547天）出现胰管结石。与普通慢性胰腺炎相比，AIP胰管结石的发生率低，且出现结石所需的时间更长。

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图6  男，59岁，AIP继发胰腺钙化CT表现。6A、6B、6C：上腹部平扫、增强扫描动脉期和静脉期CT显示胰腺头部肿大伴渐进性强化（箭）；6D：较低层面的平扫CT显示胰头散在点状钙化（箭）；6E：同一患者接受类固醇治疗1个月后CT随访显示胰头肿胀好转、体积缩小（箭）；6F：同一患者接受类固醇治疗36个月后CT随访显示胰头肿胀消退，钙化加重（箭）。AIP：自身免疫性胰腺炎。
Fig. 6  CT findings of pancreatic calcification secondary to AIP in a 59-year-old male. 6A, 6B, 6C: Upper abdominal plain scan, enhanced scan arterial phase and venous phase CT show enlargement of the pancreatic head with progressive enhancement (arrows); 6D: Lower-level plain CT scan shows scattered punctate calcifications (arrows) in the pancreatic head; 6E: CT follow-up scan of the same patient after 1 month of steroid treatment shows improved swelling and reduced volume of the pancreatic head (arrows); 6F: CT follow-up scan of the same patient after 36 months of steroid treatment shows regression of swelling and increased calcifications in the pancreatic head (arrows). AIP: autoimmune pancreatitis.

5 AIP类固醇治疗1年内显著胰腺萎缩

       AIP类固醇诱导缓解治疗后胰腺实质可发生显著萎缩^{[34, 35]}。以胰腺体部直径<10 mm作为判定胰腺萎缩的尺度，30.4%~38.7%的AIP患者可在类固醇治疗6个月或更长时间内发生胰腺萎缩（图7）^{[36, 37]}。我们对1组59例AIP患者类固醇治疗后随访观察，其中9例1年内胰腺实质显著萎缩，这种短期内胰腺显著萎缩的改变与普通慢性胰腺炎不同。后者的胰腺萎缩常发生于病程中晚期，是由于胰腺炎症的反复发作，胰腺腺泡逐渐萎缩甚至消失，纤维化延伸入小叶内并相互融合所致^[38]。少部分AIP患者接受类固醇治疗后胰腺短期内显著萎缩的原因尚不十分清楚，推测可能与类固醇治疗前病变胰腺的病理组织学改变差异（如不同程度的炎细胞浸润或纤维化）有关^[36]。临床研究表明，AIP类固醇治疗后胰腺萎缩与糖尿病加重密切相关，胰腺萎缩者糖尿病控制不佳，且新发糖尿病例数明显增多，因此建议更多关注胰腺萎缩，以评估胰腺内、外分泌功能不全的风险^{[36, 37, 39]}。

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图7  男，72岁，AIP接受类固醇激素治疗后胰腺萎缩CT表现。7A、7B、7C：上腹部平扫、增强扫描动脉期和静脉期CT显示胰腺弥漫性肿大和渐进性强化（箭），胰体部直径测量为20 mm；7D、7E、7F：类固醇治疗后9个月平扫、增强扫描动脉期和静脉期CT随访显示胰腺实质明显萎缩（箭），胰体部直径测量为9 mm。AIP：自身免疫性胰腺炎。
Fig. 7  CT appearance of pancreatic atrophy in a 72-year-old male patient with AIP after steroid hormone treatment. 7A, 7B, 7C: Upper abdominal plain scan, enhanced scan arterial phase and venous phase CT show diffuse enlargement and progressive enhancement (arrows) of the pancreas, with a diameter of 20 mm in the pancreatic body. 7D, 7E, 7F: CT scans of the upper abdomen with plain and enhanced scans in the arterial and venous phases after 9 months of steroid treatment show significant atrophy of the pancreatic parenchyma (arrows), with a diameter of 9 mm in the pancreatic body. AIP: autoimmune pancreatitis.

6 AIP合并恶性肿瘤

       AIP合并恶性肿瘤的患病率为9.6%，后者可在AIP初诊时同时发现或在之后的病情演变过程中发生^[40]。文献报道AIP合并胰腺癌的患者94%为1型AIP，其中33%与AIP同时发生，67%在病程中位时间66.4个月发生^[41]。AIP伴发的胰腺外恶性肿瘤多样化，包括胃癌、肺癌、前列腺癌、胆管癌、结肠癌及肾癌等（图8）^[42]。AIP合并恶性肿瘤的机制可能与炎症反应和免疫系统紊乱有关，亦可能是一种副肿瘤综合征^[41]。MIYAGAWA等^[43]报道1例AIP合并早期胃癌患者，在内镜下治愈性切除伴有IgG4阳性浆细胞浸润的早期胃癌后，在未使用类固醇治疗的情况下，AIP出现长期的自发缓解，这个现象提示胃癌可能激活IgG4相关的免疫反应。由此，在临床实践中，影像科医师对AIP患者不仅需要关注胰腺及其他器官固有的炎症影像特征，还应仔细观察有无肿瘤性病变，以避免漏诊可能合并的恶性肿瘤。

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图8  男，67岁，AIP初诊时伴发右肾透明细胞癌。8A、8B：轴面脂肪抑制FSPGR T1WI和FSE T2WI显示胰腺体尾部肿大，周围见相对低信号的假包膜（箭）；8C、8D：轴面脂肪抑制FSPGR T1WI和FSE T2WI显示右肾上极一个类圆形混杂信号结节，直径2.1 cm，病灶周边环绕稍低信号带（箭），中心见点状高信号；8E：轴面FSPGR T1WI增强扫描动脉期显示右肾上极结节显著高强化（箭）；8F：冠状面FSPGR T1WI增强扫描延迟期显示右肾上极结节强化减弱，呈相对低信号（箭）。AIP：自身免疫性胰腺炎；FSPGR：快速扰相梯度回波；FSE：快速自旋回波。
Fig. 8  Male, a 67-year-old patient with AIP at initial diagnosis, accompanied by right renal clear cell carcinoma. 8A, 8B: Axial fat-suppressed FSPGR T1WI and FSE T2WI show enlarged pancreatic body and tail, with a relatively low signal pseudocapsule (arrow). 8C, 8D: Axial fat-suppressed FSPGR T1WI and FSE T2WI show a quasi-circular mixed signal nodule in the upper pole of the right kidney, with a diameter of 2.1 cm, surrounded by a slightly low signal zone (arrow) around the lesion, and a punctate high signal in the center; 8E: Axial FSPGR T1WI enhanced scan arterial phase shows significant high enhancement of the nodule in the upper pole of the right kidney (arrow); 8F: Coronal FSPGR T1WI enhanced scan delayed phase shows weakened enhancement of the nodule in the upper pole of the right kidney, with relatively low signal (arrow). AIP: autoimmune pancreatitis; FSPGR: fast spoiled gradient recalled; FSE: fast spin echo.

7 小结

       AIP少见的CT和MRI表现包括主胰管扩张、胰腺囊性病变、胰腺周围血管受累、胰腺钙化与胰管结石、类固醇激素治疗后短期内显著胰腺萎缩以及合并恶性肿瘤，这些改变对于评估AIP病情进展（严重程度）、选择（调整）治疗方案及判断预后有一定提示作用。充分认识这些AIP少见的影像表现，还有助于在临床实践中避免一些不必要的影像检查或介入手术治疗。