Pulmonary Parenchyma
As described previously, on MR the lungs are seen as regions of low (black) signal intensity. An abnormality within the lung parenchyma results in a region of increased signal intensity. Recently, a study evaluated the capabilities of MR in relation to CT for the detection of pulmonary nodules. This study showed that pulmonary nodules less than 1.3 cm were not demonstrated on either CT or MR. However, CT allowed better delineation of nodules close to the diaphragm, the pleura or to each other. In contrast, MR allowed better detection of nodules adjacent to the blood vessels. In general, most pulmonary nodules were seen using both CT and MR. However, CT usually allowed detection of more small nodules than MR while small nodules adjacent to the blood vessels were better displayed using MR. One of the reasons small lesions within the lung parenchyma are not as readily identified on MR compared to CT is related to the mode of acquisition.

MR employs a longer imaging time wherein patients breathe quietly throughout the acquisition of multiple images. Therefore, small lesions can suffer from partial volume averaging during an MR study as they move in and out of the imaging plane. In contrast, CT images are obtained during suspended respiration which decreases the affect of partial volume averaging of small nodules. Additionally, CT examinations using both soft tissue and lung windows allow increased identification of small pulmonary nodules.

Assessment of MR in relation to conventional and computed tomographic studies for evaluation of bronchogenic carcinoma has resulted in many different reports as to the capabilities of the MR technique.^ In one series, both MR and CT were found to be highly accurate in assessing the presence of hilar and mediastinal adenopathy with a sensitivity rate of 87 percent. However, neither method correlated well with mediastinal invasion. When there was mediastinal invasion, the sensitivity rate was 55 percent for CT and 64 percent for MR. Another study evaluating MR, computed tomography, and mediastinoscopy for preoperative assessment of mediastinal nodes in bronchogenic carcinoma found CT was associated with a sensitivity of 71 percent, specificity of 87.7 percent and overall accuracy of 82.1 percent. In this review MR did not provide any advantage over CT for the assessment of mediastinal nodes.