Magnetic Resonance for Evaluation of the Thorax: Fibrosis
When evaluating patients following radiation therapy it is important to recall the time course of radiation-induced changes within the lungs. In the early stages, seen 6 to 12 weeks after radiation, there is an acute reaction termed radiation pneumonitis. The late or chronic phase of radiation change is associated with replacement of normal lung parenchyma by fibrosis. These changes have usually stabilized by 12 months after radiation therapy has been completed.
Another role that MR may play in the patient presenting with a recurrent mass is to aid in guiding the transthoracic biopsy. The areas of increased signal intensity within the mass on a T2 weighted sequence has been correlated with recurrent tumor while other areas with low signal intensity have been found to represent fibrosis.
On CT examinations of masses, there is no distinct differentiation between recurrent tumor and fibrosis.
In the evaluation of pulmonary air space disease it is often difficult to determine the precise etiology of a pathologic process as to whether the consolidation is secondary to blood, pus, edema or proteinaceous material. A recent study on MR showed it was not possible to differentiate between the various etiologies of pulmonary consolidation based upon the relaxation times known as T1 and T2. All types of pulmonary consolidation have intermediate intensity on a T1 weighted sequence and increased intensity on a T2 weighted sequence. Many times there is a necessity to differentiate between a proximal bronchogenic carcinoma from postobstructive lobar collapse.
In one study evaluating the capabilities of MR versus CT, it was found that MR and CT were equal in the identification of a contour abnormality. However, CT was more successful than MR for differentiating tumor mass from collapsed lung. Dynamic CT scanning differentiated tumor from collapsed lung in 80 percent of cases. MR demonstrated different signal intensities of tumor and collapsed lung in approximately 44 percent of cases. Other studies have postulated that by increasing the T2 weighting, MR may become more sensitive because the tumor has a relatively lower signal intensity than the increased signal intensity of fluid within the collapsed lung.