CT与化学位移MRI鉴别重症肌无力胸腺瘤与非胸腺瘤性病变的定性和定量评估价值比较 ... ...

目的：评价CT和化学移位磁共振成像（MRI）在重症肌无力（MG）患者中鉴别胸腺瘤（thymoma，THY）与胸腺淋巴滤泡增生（thymic lymphoid hyperplasia，TLH）和正常胸腺（normal thymus，NT）的价值，并确定哪种方法更准确。

材料与方法：将83例全身性MG患者分为TLH/NT组（A；65例）和THY组（B组24例）。采用Fisher精确检验和Student's t-test，对各组间质性特征和定量数据（CT：Hounsfield单位的放射性密度；MRI:SII）的信号强度指数（SII]）进行了测试。对Logistic回归模型进行定性和定量分析。定量分析中，根据受试者工作特性曲线[area under the receiver operating characteristic (ROC) curve，AUROC]下的面积，计算最优截断值，确定鉴别能力。用McNemar试验比较CT和MRI的诊断准确性。

结果：定性评价MRI的准确性高于CT（96.4%，80/83，86.7%，72/83）。定量分析表明，两组间的放射密度和SII均有显著性差异（p<0.0001）。CT定量评价组间放射密度AUROC为0.904（最佳截止点20HU），准确率77.1%（64/83）。对于MRI检查，SII的AUROC为0.989（最佳截断值7.766%），准确率为96.4%（80/83），显著高于CT（p<0.0001）。对于定性诊断错误的患者，采用最佳截断值，CT（89.2%，74/83）和MRI（97.6%，81/83）的准确性均得到提高。

结论：定量分析有助于评价MG患者，在定性评价的基础上提高CT、MRI诊断的准确性。化学位移MRI在鉴别THYs与非胸腺瘤性病变方面较CT更可靠。

图1. 31岁男性，患有TLH。（a，上图）平扫CT显示主动脉弓和头臂静脉前有一个椭圆形小肿块（箭头），最大直径为2cm，软组织密度均匀（平均密度值，31HU），与肌肉相似，轻度对比增强（强化程度，6HU）。这个表现类似小的THY，类似于图2所示。（a，下排图像）化学位移MRI检测到与同期图像（箭头）相对的相位图像上组织的信号强度降低，与组织内的脂肪成分一致，提示肿块样的TLH的诊断。（b） 典型组织切片的镜下照片显示胸腺组织的极少量脂肪浸润，脂肪细胞（F）区域有限，许多淋巴滤泡（LF）密集排列，并以显著的生发中心为特征（苏木精和伊红染色，原始放大倍数×40）。

Figure 1. A 31-year-old man with TLH. (a, upper images) Unenhanced CT reveals an oval-shaped small mass (arrows) anterior to the aortic arch and brachiocephalic vein, measuring 2 cm in maximal diameter, which exhibits homogeneous soft-tissue attenuation (mean densitometric value, 31 HU), similar to that of muscles, and mild contrast enhancement (intensity of enhancement, 6 HU). This appearance mimics a small THY and is similar to that depicted in Fig 2. (a, lower images) Chemical-shift MRI detects signal intensity loss of the tissue on opposed-phase images relative to in-phase images (arrows), consistent with a fatty component within the tissue, which suggests the diagnosis of TLH with focal appearance. (b) Photomicrograph of a representative histological section demonstrates minimally fatty infiltration of the thymic tissue, with limited areas of fat cells (F), and many lymphoid follicles (LF) densely packed and characterised by prominent germinal centres (haematoxylin and eosin stain, original magnification ×40).

图2. 女性，49岁，早期THY。胸部平扫CT（上图）显示前上纵隔有一个2.5cm的圆形肿块（箭头），边缘光滑，界限清楚，呈软组织密度（平均CT值，45HU），与肌肉和血管的密度相似，注射造影剂后均匀强化（强化强度15HU），这种外观提示诊断为小的THY，在形态学上与图1a所示的病例相似。化学位移MRI（下排图像）显示，相对于同相位图（箭头），在反相图上肿块的信号强度没有变化，与组织内没有脂肪相一致。定量评估（SII = 0.62％）证实了这肿表现，并进一步加强了THY的假设。

Figure 2. A 49-year-old woman with early THY. Unenhanced chest CT (upper images) shows a 2.5 cm round mass (arrows) in the anterior superior mediastinum, with smooth and well-demarcated margins, which presents soft-tissue attenuation (mean densitometric value, 45 HU), similar to that of muscles and vessels, and homogeneous enhancement after contrast medium administration (intensity of enhancement, 15 HU), an appearance that suggests the diagnosis of small THY and is morphologically similar to the case depicted in Fig 1a. Chemical-shift MRI (lower images) demonstrates no change in signal intensity of the mass on opposed-phase images relative to in-phase images (arrows), consistent with the absence of fat within the tissue. This suggestion is confirmed at quantitative assessment (SII=0.62%) and strengthens the hypothesis of THY.

图3.女性，27岁，早期THY。肺动脉水平平扫CT（上图）显示弥漫性增大的三角形胸腺（箭头；胸腺叶厚度，2 cm），表现为箭头形和软组织密度（平均CT值，29.5 HU），与肌肉和血管相似，呈轻度均匀的强化（增强强度7.5HU）。这些发现提示TLH的诊断与软组织密度。化学位移磁共振成像（下排图像）显示，相对同相位图像（箭头），反相位图像上组织的信号强度没有降低，与没有脂肪浸润一致。在定量评估中，1.67%的SII证实了缺乏脂肪，并提出了三角形胸腺瘤的诊断假设。相反，与同相位图像（虚线箭头）相比，乳腺组织在反相位图像上呈现均匀性信号强度降低，这反映了腺体内的脂肪-水混合物。

Figure 3. A 27-year-old woman with early THY. Unenhanced CT (upper images) obtained at the level of the pulmonary arteries reveals a diffusely enlarged triangular-shaped thymus (arrows; lobe thickness, 2 cm) with arrowhead morphology and soft-tissue attenuation (mean densitometric value, 29.5 HU), similar to that of muscles and vessels, which presents slight and homogeneous contrast enhancement (intensity of enhancement, 7.5 HU). These findings suggest the presumptive diagnosis of TLH with soft-tissue attenuation. Chemical-shift MRI (lower images) shows no decrease in signal intensity of the tissue on opposed-phase images relative to in-phase images (arrows), consistent with the absence of fatty infiltration. At quantitative evaluation, the SII of 1.67% confirms the absence of fat and suggests the hypothesis of a triangular-shaped THY. Conversely, breast tissue presents homogeneous signal intensity loss on opposed-phase images compared with in-phase images (dotted arrows), which reflects the fat–water admixture within the gland.

Aim: To evaluate the usefulness of computed tomography (CT) and chemical-shift magnetic resonance imaging (MRI) in patients with myasthenia gravis (MG) for differentiating thymoma (THY) from thymic lymphoid hyperplasia (TLH) and normal thymus (NT), and to determine which technique is more accurate.

Materials and methods: Eighty-three patients with generalised MG who underwent surgery were divided into the TLH/NT group (A; 65 patients) and THY group (B; 24 patients). Differences in qualitative characteristics and quantitative data (CT: radiodensity in Hounsfield units; MRI: signal intensity index [SII]) between groups were tested using Fisher's exact test and Student's t-test. Logistic regression models were estimated for both qualitative and quantitative analyses. At quantitative analysis, discrimination abilities were determined according to the area under the receiver operating characteristic (ROC) curve (AUROC) with computation of optimal cut-off points. The diagnostic accuracies of CT and MRI were compared using McNemar's test.

Results: At qualitative assessment, MRI had higher accuracy than CT (96.4%, 80/83 and 86.7%, 72/83, respectively). At quantitative analysis, both the radiodensity and SII were significantly different between groups (p<0.0001). For CT, at quantitative assessment, the AUROC of the radiodensity in discriminating between groups was 0.904 (optimal cut-off point, 20 HU) with an accuracy of 77.1% (64/83). For MRI, the AUROC of the SII was 0.989 (optimal cut-off point, 7.766%) with an accuracy of 96.4% (80/83), which was significantly higher than CT (p<0.0001). By using optimal cut-off points for cases with an erroneous diagnosis at qualitative assessment, accuracy improved both for CT (89.2%, 74/83) and MRI (97.6%, 81/83).

Conclusion: Quantitative analysis is useful in evaluating patients with MG and improves the diagnostic accuracy of CT and MRI based on qualitative assessment. Chemical-shift MRI is more reliable than CT in differentiating THYs from non-thymomatous conditions.