Magnetic Resonance for Evaluation of the Thorax: Normal Anatomy
On non-contrast CT studies, the aorta and pulmonary arteries are seen as gray attenuation that have a different appearance from fat. On MR, when there are masses within the mediastinum, the blood vessels are seen as low signal intensity, while the masses have an intermediate signal intensity. In contradistinction, on a noncontrast CT study, the vessels and masses will have a similar attenuation. On CT, in order to provide accurate differentiation between masses and blood vessels, it is important to provide an adequate bolus of intravenous iodinated contrast medium that will highlight the blood vessels separately from the surrounding masses. The bony structures on MR may be seen as regions of bright signal intensity due to the fat within the marrow or low intensity for cortical bone. On CT, the bony structures have a high attenuation (white appearance). The tracheobronchial tree is seen as low signal intensity similar to the appearance of the lungs as it is an air-containing structure. On CT, the tracheobronchial tree also has a low attenuation (black) appearance.
On standard gated MR studies of the thorax, it is possible to demonstrate the normal blood vessels, bronchi, mediastinal fat, and cardiovascular structures (Fig 1). canadian family pharmacy online
The differences between MR and CT relate to the imaging format of each. On MR, images are obtained over a period of time while the patient is breathing. As mentioned, MR images are ECG gated; therefore, there is good delineation of the blood vessels and cardiovascular structures. However, small lesions within the lung parenchyma may move in and out of the imaging plane on MR studies as a consequence of the patient quietly breathing during the study. With CT, images are obtained during suspended respiration; therefore, less difficulty arises with partial volume averaging of lesions within the lung parenchyma. Adequate delineation of the blood vessels on CT, as mentioned above, is related to the timing of the intravenous contrast administration. CT has the advantage of obtaining both lung and soft tissue windows. While the soft tissue windows are very similar to the gated MR studies for anatomic depiction, the lung windows are particularly important in CT for defining small pulmonary nodules that are more difficult to define on the MR study. This distinction will be discussed further in the section on lung parenchyma.
Figure 1. Normal thoracic anatomy. Gated coronal image of the thorax demonstrates signal void within the heart (LV = left ventricle, RA = right atrium) and great vessels (A = aorta, P = pulmonary artery) as a result of the appearance on MR of rapidly flowing blood. The myocardium (white arrow) has an intermediate intensity. The trachea (T) and lungs (L) are seen as low (black) intensity; the appearance of air on MR. Note fat (open arrow) has a high (white) intensity and muscle (curved arrow) intermediate intensity.