Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Heart size is normal . The mediastinal and hilar contours are normal . The pulmonary vasculature is normal . 4 mm nodular opacity projecting over the left upper lobe , overlying the left fifth posterior rib , may be within the osseous structures or reflect a pulmonary nodule . Lungs are otherwise clear without focal consolidation . No pleural effusion or pneumothorax is seen . There are no acute osseous abnormalities .
    OUTPUT:
    
No acute cardiopulmonary abnormality . 4 mm nodular opacity projecting over the left upper lobe may be within the bone or within the lung parenchyma . Consider shallow oblique imaging or comparison with prior radiographs for further assessment .