MR Imaging of Mediastinal Masses




The high soft tissue contrast of MR imaging enables superior tissue characterization of mediastinal masses, adding diagnostic specificity and often changing and benefiting clinical management. MR imaging can better discern cystic from solid content and can detect microscopic fat, hemorrhage, and fibrous content within lesions. In many cases, mediastinal MR imaging may prevent unnecessary diagnostic intervention. In other cases, MR imaging may indicate the optimal site for biopsy or the correct compartment for resection. Awareness of the efficacy of MR imaging with regard to mediastinal mass characterization and judicious MR imaging utilization should further improve patient care.


Key points








  • The high soft tissue contrast of MR imaging enables superior tissue characterization of mediastinal masses, compared with computed tomography, and often increases diagnostic specificity.



  • Appropriate use of mediastinal MR imaging may prevent unnecessary diagnostic intervention.



  • When intervention is needed, mediastinal MR imaging can direct interventionists toward the optimal site for biopsy or the correct compartment for resection.






Introduction


The high soft tissue contrast of MR imaging, relative to other imaging modalities including computed tomography (CT), enables superior tissue characterization of many lesions throughout the body, including those in the mediastinum. The result, in many cases, is added diagnostic specificity or virtual biopsy of the lesion. Much has been written about mediastinal masses and how their differential diagnosis can be narrowed by determination of their mediastinal compartment of origin. The premise of this method is that knowledge of the structures that normally reside in a given mediastinal compartment reduces the number of diagnostic possibilities.


This article describes how adding MR imaging to the diagnostic armamentarium yields further diagnostic precision, with potential to improve clinical management. Therefore, this article is organized not by mediastinal compartment but by MR findings that distinguish one mediastinal mass tissue type from another. The following categories are discussed in the context of their diagnostic significance, as they particularly highlight the value of MR imaging in the mediastinum:




  • Discernment of cystic from solid lesions




    • Corollary: discernment of solid tissue amidst hemorrhage and necrosis; guidance for diagnostic intervention




  • Detection of macroscopic and microscopic fat



  • Detection of lesion T2-hypointensity



  • Demonstration of a lesion’s dynamic contrast enhancement pattern



  • Demonstration of matching mediastinal lesions in terms of signal and enhancement in the same patient and its significance



  • Detection of low apparent diffusion coefficient (ADC) values



  • Determination of lesion invasiveness



  • Discernment of mediastinal from paramediastinal lesions



The new International Thymic Malignancy Interest Group (ITMIG) classification of mediastinal compartments, developed by a consensus of its members, is used when describing the compartment of origin of the mass under discussion in the article. Instead of basing lesion location on mediastinal compartments delineated by lines drawn on a lateral chest radiograph, this more modern classification bases lesion location on its relationship to 3 compartments delineated by cross-sectional imaging that extend from the thoracic inlet to the diaphragm: a prevascular (anterior mediastinal) compartment, including all structures anterior to the pericardium and proximal ascending aorta; a visceral (middle mediastinal) compartment, including all major mediastinal visceral structures extending from anterior pericardium posteriorly to a vertical line drawn 1 cm posterior to the anterior margin of the spine (both the trachea and the esophagus are therefore included in this middle mediastinal compartment); and a paravertebral (posterior mediastinal) compartment, including all mediastinal structures posterior to this vertical line. A list of mediastinal masses typically found in each of these compartments is provided in Table 1 .



Table 1

Mediastinal masses by compartment a
























































































Anterior Mediastinum or Prevascular Compartment a Middle Mediastinum or Visceral Compartment Posterior Mediastinum or Paravertebral Compartment
Lymphadenopathy Lymphadenopathy Lymphadenopathy
Thyroid and parathyroid lesions Thyroid lesions
Ascending aortic aneurysm
Aortic arch aneurysm
Dilated main pulmonary artery
Aberrant right/left subclavian artery
Descending aortic aneurysm
Descending aortic aneurysm
Thymic lesions (cyst, hyperplasia, thymic neoplasm, including lymphoma)
Paraganglioma and other neurogenic tumors Neurogenic tumors including neurofibroma, schwannoma, paraganglioma
Lateral meningocele
Germ cell tumors
Pancreatic pseudocyst Pancreatic pseudocyst
Pleuropericardial or mesothelial b cysts Mesothelial cysts Mesothelial cysts
Extramedullary hematopoiesis
Tracheal lesions
Esophageal lesions
Morgagni hernia Hiatal hernia Bochdalek hernia
Bronchogenic cysts (very rarely) Foregut duplication cysts Foregut duplication cysts
Abscess Abscess Abscess
Hematoma Hematoma Hematoma
Fibrosing mediastinitis Fibrosing mediastinitis Fibrosing mediastinitis
Hemangioma Hemangioma Hemangioma
Lymphangioma Lymphangioma Lymphangioma
Sarcoma Sarcoma Sarcoma

a Mediastinal compartments, as prescribed by the new ITMIG classification.


b Mesothelial cysts may be found anywhere in the body where mesothelium exists.





Introduction


The high soft tissue contrast of MR imaging, relative to other imaging modalities including computed tomography (CT), enables superior tissue characterization of many lesions throughout the body, including those in the mediastinum. The result, in many cases, is added diagnostic specificity or virtual biopsy of the lesion. Much has been written about mediastinal masses and how their differential diagnosis can be narrowed by determination of their mediastinal compartment of origin. The premise of this method is that knowledge of the structures that normally reside in a given mediastinal compartment reduces the number of diagnostic possibilities.


This article describes how adding MR imaging to the diagnostic armamentarium yields further diagnostic precision, with potential to improve clinical management. Therefore, this article is organized not by mediastinal compartment but by MR findings that distinguish one mediastinal mass tissue type from another. The following categories are discussed in the context of their diagnostic significance, as they particularly highlight the value of MR imaging in the mediastinum:




  • Discernment of cystic from solid lesions




    • Corollary: discernment of solid tissue amidst hemorrhage and necrosis; guidance for diagnostic intervention




  • Detection of macroscopic and microscopic fat



  • Detection of lesion T2-hypointensity



  • Demonstration of a lesion’s dynamic contrast enhancement pattern



  • Demonstration of matching mediastinal lesions in terms of signal and enhancement in the same patient and its significance



  • Detection of low apparent diffusion coefficient (ADC) values



  • Determination of lesion invasiveness



  • Discernment of mediastinal from paramediastinal lesions



The new International Thymic Malignancy Interest Group (ITMIG) classification of mediastinal compartments, developed by a consensus of its members, is used when describing the compartment of origin of the mass under discussion in the article. Instead of basing lesion location on mediastinal compartments delineated by lines drawn on a lateral chest radiograph, this more modern classification bases lesion location on its relationship to 3 compartments delineated by cross-sectional imaging that extend from the thoracic inlet to the diaphragm: a prevascular (anterior mediastinal) compartment, including all structures anterior to the pericardium and proximal ascending aorta; a visceral (middle mediastinal) compartment, including all major mediastinal visceral structures extending from anterior pericardium posteriorly to a vertical line drawn 1 cm posterior to the anterior margin of the spine (both the trachea and the esophagus are therefore included in this middle mediastinal compartment); and a paravertebral (posterior mediastinal) compartment, including all mediastinal structures posterior to this vertical line. A list of mediastinal masses typically found in each of these compartments is provided in Table 1 .



Table 1

Mediastinal masses by compartment a
























































































Anterior Mediastinum or Prevascular Compartment a Middle Mediastinum or Visceral Compartment Posterior Mediastinum or Paravertebral Compartment
Lymphadenopathy Lymphadenopathy Lymphadenopathy
Thyroid and parathyroid lesions Thyroid lesions
Ascending aortic aneurysm
Aortic arch aneurysm
Dilated main pulmonary artery
Aberrant right/left subclavian artery
Descending aortic aneurysm
Descending aortic aneurysm
Thymic lesions (cyst, hyperplasia, thymic neoplasm, including lymphoma)
Paraganglioma and other neurogenic tumors Neurogenic tumors including neurofibroma, schwannoma, paraganglioma
Lateral meningocele
Germ cell tumors
Pancreatic pseudocyst Pancreatic pseudocyst
Pleuropericardial or mesothelial b cysts Mesothelial cysts Mesothelial cysts
Extramedullary hematopoiesis
Tracheal lesions
Esophageal lesions
Morgagni hernia Hiatal hernia Bochdalek hernia
Bronchogenic cysts (very rarely) Foregut duplication cysts Foregut duplication cysts
Abscess Abscess Abscess
Hematoma Hematoma Hematoma
Fibrosing mediastinitis Fibrosing mediastinitis Fibrosing mediastinitis
Hemangioma Hemangioma Hemangioma
Lymphangioma Lymphangioma Lymphangioma
Sarcoma Sarcoma Sarcoma

a Mediastinal compartments, as prescribed by the new ITMIG classification.


b Mesothelial cysts may be found anywhere in the body where mesothelium exists.





Mediastinal magnetic resonance protocol


High quality mediastinal MR protocols involve pulse sequences required to adequately characterize a lesion. These protocols generally include T1-weighted, T2-weighted, and T2-weighted fat-saturated pulse sequences, as well as pre-gadolinium and post-gadolinium three-dimensional (3D) ultrafast gradient echo (GRE) dynamic contrast-enhanced imaging. Ultrafast GRE in-phase and out-of-phase chemical shift MR imaging is recommended for T1-weighted sequences because lesion coverage for both phases can be acquired in a single 20-second breath hold and additional information is obtained regarding the presence or absence of microscopic or intravoxel fat within a lesion at no additional time cost. Supplemental diffusion-weighted and short tau inversion recovery imaging can be performed if the finding of low ADC values is anticipated to refine the differential diagnosis or if the detection of bone marrow edema or involvement is diagnostically or therapeutically critical, respectively.


Breath-hold imaging for all pulse sequences, including the 3-plane localizer, is strongly preferred to respiratory gating because it more reliably freezes respiratory motion and dispenses with associated respiratory motion artifact. Breath-hold imaging is almost universally successful in patients, from adolescent to elderly, provided the technologist rehearses breath-holding with the patient before the patient lies down on the table, primes the patient before each pulse sequence about the nature (length, frequency) of the breath holds, and provides MR-compatible 2-L nasal cannula oxygen when breath-hold difficulty is anticipated. If a patient requires higher amounts of oxygen therapy at home or in the hospital, the volume of oxygen delivery should be suitably adjusted.


When cardiac gating is used, electrocardiogram (ECG) gating is strongly preferred to peripheral gating because ECG gating more reliably freezes cardiac motion and virtually eradicates associated pulsatility artifact.


A suggested mediastinal MR imaging protocol is provided in Table 2 . For noninvasive thymic lesion evaluation, a shortened version of this protocol can be used ( Box 1 ).



Table 2

Mediastinal MR imaging protocol




























































































































Pulse Sequence GE Siemens Philips TR (ms) a TE (ms) b Flip Angle NEX Slice Thickness (mm)
BH axial and sagittal SSFP balanced gradient echo (GRE) FIESTA True FISP BFFE 3–270 1.2–1.5 (minimum full) 45–80 1 7
BH coronal ultrafast spin echo T2 SSFSE HASTE UFSE 900–1000 (minimum) 80–100 NA ≤1 8
BH sagittal UFSE fat-saturated T2 SSFSE HASTE UFSE 900–1000 (minimum) 80–100 NA ≤1 7
BH axial in-phase and out-of-phase (chemical shift) ultrafast GRE T1 (dual echo preferable) FSPGR TurboFLASH TFE 115–190 4.2–4.8/2.1–2.4 70–80 1 7
BH cardiac gated double IR T2 Double IR FSE T2 Double IR TSE T2 Double IR UFSE T2 1600–4600 110–140 NA 1 7
BH before and after 3D ultrafast GRE with automated subtraction (post-gadolinium imaging) acquired at 20 s (axial), 1 min (axial), 3 min (sagittal), and 5 min (axial) c upon administration of 20cc of IV gadolinium LAVA VIBE THRIVE 3–5 1.5–2.5 10–12 <1 5
Optional coronal/sagittal BH STIR Fast STIR Turbo STIR STIR TSE 2500 50 NA 1 7
Optional diffusion-weighted echo planar imaging eDWI DWI DWI 140 1–2 (minimum) NA 2 5–7
Optional BH cardiac gated double inversion recovery T1 Double IR prep FSE T1 Double IR prep TSE T1 Double IR prep UFSE T1 850–1100 30–40 NA 1 7
Optional BH cardiac gated sagittal double inversion recovery fat-saturated T2 Double IR FSE fat-saturated T2 Double IR TSE fat-saturated T2 Double IR UFSE fat-saturated T2 1600–4600 110–115 NA ≤1 7
Optional respiratory-triggered axial-driven equilibrium without and with fat saturation FRFSE RESTORE DRIVE 4000–6000 90 NA 4 7

Abbreviations: BH, breath hold; DWI, diffusion-weighted imaging; IR, inversion recovery; NA, not applicable; NEX, number of excitations; SSFP, steady state free precession; STIR, short tau inversion recovery; TE, echo time; TR, recovery time; UFSE, ultrafast spin echo.

Adapted from Ackman JB. A practical guide to non-vascular thoracic magnetic resonance imaging. J Thorac Imaging 2014;29(1):24; with permission.

a Sample TR: this parameter varies as a function of MR manufacturer, body habitus, desired coverage volume, and many other factors.


b Sample TE: this parameter varies as a function of MR manufacturer, body habitus, desired coverage volume, and many other factors.


c Option to add, for example, postgadolinium axial imaging 2 minutes and 4 minutes after injection. Imaging planes can be changed to optimize lesion characterization.


Sep 18, 2017 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on MR Imaging of Mediastinal Masses

Full access? Get Clinical Tree

Get Clinical Tree app for offline access