A pulmonary nodule is defined as a more or less round lesion with a maximum diameter of 3 cm that is completely surrounded by normal lung (Figs. 6.1, 6.2, and 6.3) (Ost et al. 2003). Focal pulmonary lesions larger than 3 cm in size are generally considered indeterminate lung masses (Tan et al. 2003) and are much more likely to be malignant than smaller nodules (Winer-Muram 2006). Data obtained in larger CT screening populations suggest that, with a size threshold of 1–2 mm for the detection of pulmonary nodules, less than 5 % of all detected nodules with a diameter of up to 1 cm are malignant (Kim et al. 2002; Biederer et al. 2008). Based on the literature, it appears realistic to assume a threshold size of 3–4 mm for lung nodule detection with MRI performed at 1.5 T (The National Lung Screening Trial Research Team 2011). The sensitivities reported for MRI are greater than 80 % for pulmonary nodules measuring 5–10 mm and nearly 100 % for nodules larger than 10 mm (Girvin and Ko 2008; Feuerstein et al. 1992; Kersjes et al. 1997).

Even larger pulmonary masses may not exhibit morphologic features that allow them to be reliably classified as benign or malignant; however, there are several features that are helpful in arriving at a characterization. Unfortunately, some criteria that can be assessed on CT scans, attenuation values, for example, are not available to the radiologist interpreting MR images. Solid and nonsolid lesions cannot be distinguished with certainty. Patterns of calcification that indicate benign pulmonary lesions on CT scans are only rarely seen on MR images (Fig. 6.6). The fluid sensitivity of T2-weighted pulse sequences may, however, allow differentiation between cystic lesions and solid nodules. Some lesions have a characteristic imaging appearance. These include arteriovenous malformations, aspergillomas in preexisting cavities, rounded atelectasis, bronchoceles, and circumscribed mucus retention (Beigelman-Aubry et al. 2007). While a smooth, well-defined contour is suggestive of benignity (Ost and Fein 2000), 21 % of malignant pulmonary nodules, especially metastases, have well-defined margins (Erasmus et al. 2000). An ill-defined, irregular, or spiculated contour is typically associated with malignant growth, but it may also be seen in focal organizing pneumonia (FOP), some forms of vasculitis (e.g., Wegener and lymphomatoid granulomatosis), and other granulomatous diseases (tuberculosis) (Kohno et al. 1993; Zwirewich et al. 1991). Cavitation is more frequently seen in malignant lesions but is also typical of pulmonary abscess and other granulomatous inflammatory processes (Woodring and Fried 1983). Intralesional fat is diagnostic of a benign nodule. Most fat-containing masses are hamartomas; less common are fatty granuloma, lipoma, and lipomatoid pneumonia. Hamartomas, however, tend to be smaller than 25 mm (Siegelman et al. 1986).