Role of advanced magnetic resonance imaging in the assessment of malignancies of the mediastinum

The thymus: what's normal and what's not? Problem-solving with MRI

Anterior mediastinal masses can be difficult to characterise on computed tomography (CT) due to the wide spectrum of normal appearances of thymic tissue as well as the challenge of differentiating between benign and malignant pathologies. Additionally, attenuation of cystic mediastinal lesions can be misinterpreted on CT due to varying attenuation values. Anecdotally, non-vascular magnetic resonance imaging (MRI) of the thorax is underutilised across radiology departments in the UK, but has been shown to improve diagnostic certainty and reduce unnecessary surgical intervention. T2-weighted MRI is useful in confirming the cystic nature of lesions, whereas chemical shift techniques can be utilised to document the presence of macroscopic and intra-cellular fat and thus help distinguish between benign and malignant pathologies. In this review article, we present a practical approach to using MRI for the characterisation of anterior mediastinal lesions based on our clinical experience in a UK district general hospital.

Diffusion-weighted Imaging of the Chest: A Primer for Radiologists

Diffusion-weighted imaging (DWI) is a fundamental sequence not only in neuroimaging but also in oncologic imaging and has emerging applications for MRI evaluation of the chest. DWI can be used in clinical practice to enhance lesion conspicuity, tissue characterization, and treatment response. While the spatial resolution of DWI is in the order of millimeters, changes in diffusion can be measured on the micrometer scale. As such, DWI sequences can provide important functional information to MRI evaluation of the chest but require careful optimization of acquisition parameters, notably selection of b values, application of parallel imaging, fat saturation, and motion correction techniques. Along with assessment of morphologic and other functional features, evaluation of DWI signal attenuation and apparent diffusion coefficient maps can aid in tissue characterization. DWI is a noninvasive noncontrast acquisition with an inherent quantitative nature and excellent reproducibility. The outstanding contrast-to-noise ratio provided by DWI can be used to improve detection of pulmonary, mediastinal, and pleural lesions, to identify the benign nature of complex cysts, to characterize the solid portions of cystic lesions, and to classify chest lesions as benign or malignant. DWI has several advantages over fluorine 18 (18F)-fluorodeoxyglucose PET/CT in the assessment, TNM staging, and treatment monitoring of lung cancer and other thoracic neoplasms with conventional or more recently developed therapies. © RSNA, 2023 Quiz questions for this article are available in the supplemental material. Supplemental material and the slide presentation from the RSNA Annual Meeting are available for this article.

Diagnostic value of apparent diffusion coefficient in predicting pathological T stage in patients with thymic epithelial tumor

Purposes

This study aimed to evaluate the diagnostic capacity of apparent diffusion coefficient (ADC) in predicting pathological Masaoka and T stages in patients with thymic epithelial tumors (TETs).

Methods

Medical records of 62 patients who were diagnosed with TET and underwent diffusion-weighted imaging (DWI) prior to surgery between August 2017 and July 2021 were retrospectively analyzed. ADC values were calculated from DWI images using b values of 0, 400, and 800 s/mm². Pathological stages were determined by histological examination of surgical specimens. Cut-off points of ADC values were calculated via receiver operating characteristic (ROC) analysis.

Results

Patients had a mean age of 56.3 years. Mean ADC values were negatively correlated with pathological Masaoka and T stages. Higher values of the area under the ROC curve suggested that mean ADC values more accurately predicated pathological T stages than pathological Masaoka stages. The optimal cut-off points of mean ADC were 1.62, 1.31, and 1.48 × 10^–3 mm²/sec for distinguishing pathological T2-T4 from pathological T1, pathological T4 from pathological T1-T3, and pathological T3-T4 from pathological T2, respectively.

Conclusion

ADC seems to more precisely predict pathological T stages, compared to pathological Masaoka stage. The cut-off values of ADC identified may be used to preoperatively predict pathological T stages of TETs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40644-022-00495-x.

An MRI-Based Clinical-Perfusion Model Predicts Pathological Subtypes of Prevascular Mediastinal Tumors

This study aimed to build machine learning prediction models for predicting pathological subtypes of prevascular mediastinal tumors (PMTs). The candidate predictors were clinical variables and dynamic contrast–enhanced MRI (DCE-MRI)–derived perfusion parameters. The clinical data and preoperative DCE–MRI images of 62 PMT patients, including 17 patients with lymphoma, 31 with thymoma, and 14 with thymic carcinoma, were retrospectively analyzed. Six perfusion parameters were calculated as candidate predictors. Univariate receiver-operating-characteristic curve analysis was performed to evaluate the performance of the prediction models. A predictive model was built based on multi-class classification, which detected lymphoma, thymoma, and thymic carcinoma with sensitivity of 52.9%, 74.2%, and 92.8%, respectively. In addition, two predictive models were built based on binary classification for distinguishing Hodgkin from non-Hodgkin lymphoma and for distinguishing invasive from noninvasive thymoma, with sensitivity of 75% and 71.4%, respectively. In addition to two perfusion parameters (efflux rate constant from tissue extravascular extracellular space into the blood plasma, and extravascular extracellular space volume per unit volume of tissue), age and tumor volume were also essential parameters for predicting PMT subtypes. In conclusion, our machine learning–based predictive model, constructed with clinical data and perfusion parameters, may represent a useful tool for differential diagnosis of PMT subtypes.

Diffusion Tensor Imaging in Characterization of Mediastinal Lymphadenopathy

OBJECTIVE

To determine the efficacy of diffusion-weighted MRI (DWI) and diffusion tensor imaging (DTI) in the characterization of mediastinal lymphadenopathy and the differentiation between malignant and benign lymph nodes (LNs).

METHODS

a retrospective evaluation of 58 patients with mediastinal lymphadenopathy that underwent DWI and DTI with calculation of apparent diffusion coefficient (ADC), fractional anisotropy (FA), and mean diffusivity (MD) values of LNs. Final diagnosis was made by the histopathology and proved metastatic (n = 21), lymphomatous (n = 14), granulomatous (n = 11) and reactive (n = 12) LNs.

RESULTS

Malignant mediastinal LNs had remarkably lower ADC and MD; (p = 0.001) and higher FA; (p = 0.001) than in benign LNs. The threshold of ADC, MD, and FA at (1.48, 1.32 × 10^-3 mm²/s), (1.31, 1.33 × 10^-3 mm²/s), (0.62, 0.52) to differentiate malignant from benign LNs has AUC of (0.89, 0.94), (0.96, 0.95), (0.72, 0.82), accuracy of (87%, 86%), (89%, 86%), (70%, 72%) by both observers respectively. The threshold of ADC, MD, and FA at (1.47, 1.32 × 10^-3 mm²/s), (1.31, 1.3 × 10^-3 mm²/s), (0.62, 0.67) used to differentiate metastatic from reactive LNs revealed AUC of (0.90, 0.94), (0.96, 0.96), (0.73, 0.77), accuracy of (87%, 81%), (87%, 81%), (72%, 66%) by both observers respectively. The mean ADC and MD values of metastatic LNs were statistically significant (p = 0.001) and (p = 0.002, 0.02) respectively when compared with that of lymphoma. The threshold of ADC, and MD (0.94, 0.97 × 10^-3 mm²/s) and (0.87, 0.91 × 10^-3 mm²/s) used to differentiates metastatic from lymphomatous nodes revealed AUC of (0.90, 0.91), (0.81, 0.74), an accuracy of (85%, 91%), (71%, 71%), by both observers respectively. The inter-class correlation between two observers for all nodes for ADC, MD and FA was r= 0.931, 0.956 and 0.885 respectively.

CONCLUSION

Using ADC, MD, and FA can help in the characterization of mediastinal lymphadenopathy noninvasively.

Role of diffusion MRI in diagnosis of mediastinal lymphoma: initial assessment and response to therapy

Background

Malignant lymphoma accounts for nearly 20% of all mediastinal neoplasms in adults and 50% in children. Hodgkin’s disease is the most common primary mediastinal lymphoma. In non-Hodgkin’s lymphoma, the two most common forms of primary mediastinal lymphoma are lymphoblastic lymphoma and diffuse large B-cell lymphoma. The aim of this study is to implement diffusion MRI in the algorithm of diagnosis of mediastinal lymphoma, differentiating Hodgkin's from non-Hodgkin's lymphoma and assessment of post therapeutic response.

Results

Using Diffusion weighted magnetic resonance imaging DWI-MRI, there were statistic significant difference between ADC values in lymph nodes and mediastinal masses in Hodgkin and non-Hodgkin lymphomas. ADC range in non-treated Hodgkin lymphoma cases was 0.774 to 1.4, while ADC range in in non-treated non-Hodgkin lymphoma was 0.476 to 0.668. In this study, there was statistically significant difference of ADC values in lymphoma cases presented by mediastinal masses with and without chemotherapy.

Conclusions

Diffusion weighted magnetic resonance imaging DWI-MRI is a promising functional technique in diagnosis of Hodgkin's and non-Hodgkin's lymphoma and assessment of response to treatment with no need for special preparation, contrast injection or radiation exposure.

Mediastinal Masses in Contrast-Enhanced Ultrasound - Retrospective Analysis of 58 Cases

OBJECTIVES

To determine contrast enhancing features of mediastinal masses (ML) using transcutaneous contrast-enhanced ultrasound (CEUS).

METHODS

Retrospective analysis of n = 58 patients with histologically confirmed ML, which were examined in the period from October 2005 to February 2018 using transcutaneous B-mode ultrasound and CEUS. In n = 29 (50%) histological confirmation was performed by ultrasound guided core-needle biopsy. The lesions were evaluated using CEUS in regard to the enhancement pattern (hyper-, iso-, hypoenhancement, non-enhancement, homogenous, inhomogenous) compared to enhancement of the spleen as an in vivo reference.

RESULTS

N = 53 (91.4%) of ML were malignant (m) (lymphoma n = 36, metastasis n = 11, thymoma n = 2, teratoma n = 1, sarcoma n = 2, seminoma n = 1). In n = 5 (8.6%) cases there was a benign (b) histology (thyroid tissue n = 2, thymus residue n = 1, ganglioneurinoma n = 1, scar tissue: n = 1). In ultrasound, n = 53 (91.4%; (48 = m, 5 = b)) were hypoechoic, n = 5 (8.6%, (5 = m,0 = b)) hyperechoic. In CEUS, n = 35 lesions presented an arterial isoenhancement (60.3%; 33 = m, 2 = b). An arterial hypoenhancement had n = 21 (36.2%, (20 = m,1 = b)), and no enhancement showed n = 2 (3.5%, (0 = m, 2 = b) of the ML. A parenchymal isoenhancement was observed in n = 1 (1.7%, (1 = m, 0 = b)), a hypoenhancement in n = 54 (93.1%; 51 = m, 3 = b) of the patients and almost no enhancement in n = 3 ML (5.2%, (1 = m, 2 = b).) The enhancement was homogeneous in n = 26 (44.8%, (25 = m,1 = b)) cases, in n = 31 (53.5%, (28 = m,3 = b)) inhomogeneous and n = 1 (1.7%) benign lesion was exclusively cystic.

CONCLUSION

In CEUS, mediastinal tumor formations showed variable arterial enhancement, followed by parenchymal hypoenhancement (wash-out).

Thymic Epithelial Neoplasms: Radiologic-Pathologic Correlation

Thymic epithelial neoplasms, as classified by the World Health Organization, include thymoma, thymic carcinoma, and thymic carcinoid. They are a rare group of tumors and are often diagnosed incidentally in the work-up of parathymic syndrome, such as myasthenia gravis, or when mass effect or local invasion causes other symptoms. In each of these scenarios, understanding the radiologic-pathologic relationship of these tumors allows clinical imagers to contribute meaningfully to management decisions and overall patient care. Integrating important imaging features, such as local invasion, and pathologic features, such as necrosis and immunohistochemistry, ensures a meaningful contribution by clinical imagers to the care team.

Imaging of the Middle and Visceral Mediastinum

The visceral mediastinum contains important vascular and non-vascular structures including the heart, great vessels, lymph nodes, and portions of the esophagus and trachea. Multiple imaging modalities, including chest radiography, computed tomography, MR imaging, and nuclear medicine studies, can be used to detect, diagnose, and characterize masses in this compartment. Lymphadenopathy is the most common process involving the visceral mediastinum and can be seen with a wide variety of diseases. Less commonly seen entities include foregut duplication cysts, neoplasms and other lesions arising from the trachea and esophagus, paragangliomas as well as other mesenchymal tumors.

Imaging features and differential diagnoses of non-neoplastic diffuse mediastinal diseases

Acute or chronic non-neoplastic diffuse mediastinal diseases have multiple causes, degrees of severity, and a wide range of management. Some situations require emergency care while others do not need specific treatment. Although the diagnosis may be suspected on chest X-ray, it is mainly based on CT. A delayed recognition is not uncommonly observed. Some findings may prompt the radiologist to look for specific associated injuries or lesions.

This pictorial review will successively describe the various non-neoplastic causes of diffuse mediastinal diseases with their typical findings and major differentials.

First, pneumomediastinum that can be provoked by extra- or intra-thoracic triggers requires the knowledge of patient’s history or recent occurrences. Absence of any usual etiological factor should raise suspicion of cocaine inhalation in young individuals.

Next, acute mediastinitis may be related to post-operative complications, esophageal perforation, or contiguous spread of odontogenic or retropharyngeal infections. The former diagnosis is not an easy task in the early stage, owing to the similarities of imaging findings with those of normal post-operative appearance during the first 2–3 weeks.

Finally, fibrosing mediastinitis that is linked to an excessive fibrotic reaction in the mediastinum with variable compromise of mediastinal structures, in particular vascular and airway ones. Differential diagnosis includes tumoral and inflammatory infiltrations of the mediastinum.

Surgical management of thymic epithelial tumors

Resection is the mainstay of treatment for thymic epithelial tumors (TETs), with complete removal of the tumor and involved organs being the ultimate aim. The choice of surgical approach plays a major role in defining treatment success, and the optimal choice of method should thus provide an adequate surgical view to achieve complete tumor resection. While median sternotomy is considered the gold standard for access to the mediastinum, several minimally invasive approaches to thymectomy have been described, including video-assisted robotic-assisted thymectomy, although the oncological outcomes of that procedure remain unclear. A multimodal approach incorporating chemotherapy or chemoradiotherapy followed by extended surgery may improve resectability and outcomes for patients with advanced TETs. Surgical debulking is also reportedly acceptable for invasive thymoma because of its potential for achieving favorable outcomes. Re-resection is an acceptable option for patients with recurrent thymoma after initial resection, and repeat resection for recurrent pleural dissemination seems effective. Here, the literature on current clinical practices in the surgical management and treatment of TETs is reviewed.

[Imaging of Mediastinal Tumors]

In computed tomography, the mediastinum is split into ventral prevascular, middle visceral and dorsal paravertebral compartments. Each compartment contains specific tumours which will be presented and discussed briefly. The focus is on image-based analysis of morphological signs to differentiate between entities using X-rays, ultrasound, CT and MRI. It is however difficult to differentiate between various types of lesions based on morphology and to estimate the grade of malignancy. For this reason, functional imaging techniques like PET and MRI are essential tools for detailed and non-invasive work-up. If a histological tissue examination is required, these tissue samples can be acquired using CT guided biopsies with high diagnostic yield and low complication rates.

Feasibility of using dynamic contrast-enhanced MRI for differentiating thymic carcinoma from thymic lymphoma based on semi-quantitative and quantitative models

AIM

To evaluate the value of using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) derived parameters to differentiate thymic carcinoma and thymic lymphoma based on semi-quantitative and quantitative models.

MATERIALS AND METHODS

Twenty-nine pathologically confirmed anterior mediastinum tumours in 29 patients were enrolled in this retrospective study, including 15 thymic carcinoma and 14 lymphoma patients. All the patients underwent pre-treatment mediastinum DCE-MRI. Both semi-quantitative and quantitative parameters were calculated and the volume transfer constant K^trans, the flux rate constant between extravascular extracellular space and plasma k_ep, the extravascular extracellular volume fraction v_e were obtained based on a modified Tofts model. DCE-MRI derived parameters were compared between thymic carcinoma and thymic lymphoma groups.

RESULTS

Thymic carcinoma had significantly lower k_ep (p=0.040) and higher v_e (p=0.018) than thymic lymphoma; however, there were no significant differences on K^trans and semi-quantitative parameters between the two groups. v_e had the highest area under the curve (cut-off value, 0.282; area under the curve, 0.748; sensitivity, 71.4%; specificity, 80%). The combination of k_ep and v_e could increase the diagnostic performance significantly (area under the curve, 0.752; sensitivity, 57.1%; specificity, 93.3%).

CONCLUSION

DCE-MRI derived parameters may have value in the differentiating thymic carcinoma and thymic lymphoma.