Magnetic Resonance for Evaluation of the Thorax: Thoracic Inlet
MR is useful for evaluation of lesions that arise or extend into the thoracic inlet. Often with CT it is difficult to evaluate this region due to the streak artifacts caused by the surrounding bony structures. Additionally, unless an adequate bolus of contrast medium is given, the effect of the masses on the blood vessels within the thoracic inlet may not be adequately delineated by computed tomography. Ultrasound has often been used for lesions arising within the neck such as in the thyroid and parathyroid glands; however, limitations occur secondary to the lung apices and surrounding bony structure that prohibit adequate penetration of sound waves. The direct coronal and sagittal images available with MRI provide a large field of view, greater than that provided by sonography, which allows assessment of the substernal extension and the retrotracheal components of thyroid masses (Fig 3). itat on
Additionally, the sagittal and coronal images available with MR are not obtained with the same image quality by CT. These other planes allow demonstration of the neck, in particular the cervical thoracic junction and superior mediastinum on one or two sections. The sagittal and coronal planes allow composite visualization of the full extent of lesions within the neck, such as large thyroid malignancies and substernal goiters (Fig 3). The increased soft tissue contrast provided between neoplasms and normal muscle on MR is beneficial for assessing muscle invasion. Abnormalities involving muscle cause an alteration in the signal intensity. Thyroid neoplasms and goiters have a variable signal intensity. MR does not allow differentiation between either of these entities. As demonstrated in Figure 3, there is a large mass arising from the thyroid bed extending into the superior mediastinum. This mass has inhomogenous signal intensity. The region of bright signal intensity seen posteriorly within the thyroid gland was an area of colloid degeneration confirmed histopathologically.
Figure 3. Thyroid goiter with substernal extension. Sagittal MR image demonstrates enlargement of the thyroid (small arrows) extending retrosternal. The thyroid has inhomogeneous signal intensity secondary to a region of colloid degeneration (curved arrow). S = sternum.