胸腺肿瘤CT与MRI的诊断价值比较

　　MR imaging is rarely the first imaging modality used for investigation of chest symptoms or an abnormality detected on chest radiography, because CT is generally considered to be superior to MR imaging in the evaluation and characterization of most anterior mediastinal masses.

　　When a cystic mass is suspected or is to be investigated, MR imaging is the most useful imaging modality because it is superior to CT in distinguishing cystic from solid masses (for instance, thymic cysts from thymic neoplasms) and in identifying cystic/necrotic components within solid masses. MR imaging is also useful in differentiating thymic hyperplasia from thymic tumors and in evaluating patients with an anterior mediastinal mass suspicious for thymoma in whom iodinated contrast is contraindicated.

　　Such patients should be imaged with MR imaging rather than an unenhanced chest CT because vessel and pericardial invasion (stage III) must be identified before surgery to enable neoadjuvant therapy in patients with thymoma. This invasion is readily demonstrated with MR imaging in such patients using black blood or white blood techniques.

目前潜在的优势：

1.MRI可以清楚显示肿瘤包膜完整性、瘤周脂肪水肿情况，无需对比剂即可显示肿瘤与血管的关系，在胸腺肿瘤的分期上较CT更具优势；

2.MRI在显示胸腺肿瘤内部成分，如纤维间隔、液化性质(hemorrhagic and proteinaceous components may result in T1 hyperintensity)、胶原纤维量上明显优于CT，有助于胸腺肿瘤的鉴别诊断；

3.双回波同反相位成像可有效鉴别胸腺增生与胸腺瘤(chemical shift imaging can differentiate between normal thymus and thymic involvement with tumor)；

4.DWI可初步判断肿瘤侵袭性和分型，有助于胸腺瘤的术前综合评估；

5.无需对比剂，无电离辐射。

　　The versatility of MR imaging may prove superior to that of CT in certain scenarios, which is elaborated on in the discussion of the differential diagnosis. In brief, chemical shift imaging can differentiate between normal thymus and thymic involvement with tumor, dynamic imaging can be used to distinguish thymoma from other anterior mediastinal tumors, and multishot spiral sequencing can be used for real-time imaging and evaluation of phrenic nerve involvement before surgery.

　　MR Imaging of Thymoma

　　The primary role of MRimaging in the evaluation of patients with thymoma is the staging of disease when allergy to contrast media and/or renal failure precludes the utilization of contrast-enhanced CT.

　　The typical appearance of thymoma is low to intermediate signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences.As the signal intensity of thymoma on T2-weighted images may approach that of fat, fat-suppression techniques may be used to distinguish thymoma from adjacent mediastinal fat. Cystic changes and necrosis typically manifest as low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.

　　Fibrous septa and intratumoral nodularity demonstrate low signal intensity. The signal intensity of intratumoral hemorrhage is highly variable and depends on its age. For instance, hemosiderin typically manifests as low signal intensity on T1- and T2-weighted sequences, whereas acute or subacute hemorrhage may demonstrate T1 hyperintensity.

　　As the main role of clinical staging is to identify local spread (ie, pericardium, vessels, and heart) as well as pleural spread, which is often resected,the entire chest should be imaged, not limiting the field of view to the anterior mediastinum. Intravenous contrast can be helpful in the identification of small pleural metastases as tumor enhances. The use of black blood or white blood techniques is helpful for evaluation of vascular invasion.Vessel distortion or tumor in the lumen of a vessel constitutes direct proof of stage III disease. Cardiac-gated black or white blood techniques are useful for better localization of tumors involving the heart, and perfusion studies are helpful when extensive cardiac muscle involvement is suspected for surgical planning.

　　Phrenic nerve involvement is important to recognize, not only because it constitutes stage III disease requiring neoadjuvant therapy but also for better planning for surgical resection. With multishot spiral sequences for real-time imaging, the diaphragmatic motion can be studied, obviating a separate sniff test. Paradoxic movement can be appreciated, and diaphragmatic movement span can be measured. 

　　Although the multiplanar capabilities of MR imaging may help in identifying pericardial invasion, which represents stage III as well, its presence is often impossible to identify by imaging, and it is only identified retrospectively microscopically.Some indirect MR imaging features of the primary tumor have been assessed to identify which primary tumor characteristics are less as well as more likely to be associated with local spread. A study demonstrated that 92% of stage II,III, and IV thymomas displayed heterogeneous signal intensity and 50% of these tumors exhibited lobulated internal features because of the presence of fibrous septa. All stage I tumors were heterogeneous in signal, but none demonstrated lobulation.

　　Another study as sessed enhancement patterns after the administration of contrast material. For instance, dynamic MR imaging has demonstrated delayed mean peak time in stage III thymomas when compared with early stage tumors. Although histologic classification of thymoma does not dictate therapy as much as staging does,some correlation of the primary tumor with MR imaging features were found. Specific findings such as visualization of the tumor capsule and fibrous septa within the tumor have been associated with less aggressive histologies（DOI:10.1016/j.ejrad.2006.05.003）（不一定，大部分情况下是错误的！，纤维间隔在低危或高危胸腺瘤的发生率并无显著差别）.Certain findings such as heterogeneous enhancement and predominant necrotic or cystic components suggest more aggressive histologies.

　　FOLLOW-UP IMAGING

　　The indolent nature of thymoma necessitates lengthy follow-up after initial treatment. Early identification of recurrence is crucial, because survival after complete resection of recurrent disease approaches that after initial resection, with reported 5-year survival rates of 65% to 80%.

　　The current recommendations from ITMIG regarding imaging follow-up of patients include yearly chest CT for the first 5 years after surgical resection, alternating chest CT and chest radiographs through year 11, and yearly chest radiographs thereafter. 当前ITMIG有关患者影像学随访的建议包括：手术切除后的前5年每年进行胸部CT扫描，到之后11年之前胸部CT和胸部X线交替进行，以及此后每年进行胸部X线复查。

　　For those patients with incomplete tumor resection or advanced-stage disease, follow-up chest CT every 6 months for the first 3 years is recommended. ITMIG suggests alternating between CT and MR imaging in these patients to decrease the cumulative radiation dose. However, there are currently no studies in the literature comparing the accuracies of these modalities for detecting tumor recurrence.　 对于那些肿瘤切除不完全或晚期疾病的患者，建议在头3年中每6个月进行一次胸部CT随访。 ITMIG建议这些患者交替行CT和MR成像，以减少累积辐射剂量。 但是，目前尚无文献比较这些方法检测肿瘤复发的准确性。

Carter BW, Benveniste MF, Truong MT, et al. State of the Art: MR Imaging of Thymoma[J]. Magnetic resonance imaging clinics of North America, 2015,23:165-177.